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The state of our water environment

With all the pressures that people impose on New Zealand's surface waters and groundwaters, it is no surprise to find that water flows and quality have been widely affected. The natural character of many waterways has also been lost, something which is listed as a matter of national importance in the Resource Management Act.

Assessing the extent of these effects is difficult because most water data are collected by regional councils for local purposes and cannot be readily combined into national statistics. Some regional data have been used in preparing this chapter but other data have come from national surveys of rivers (Close and Davies-Colley, 1990; Smith and Maasdam, 1993; Maasdam and Smith, 1994), lakes (Livingston et al., 1986; Burns, 1994) and drinking water (Mattingley, 1992; Nokes, 1993; Walker, 1994; Ministry of Health, 1997) as well as published scientific papers. Two particularly useful sources have been NIWA's review of agricultural impacts on water quality (Smith et al., 1993) and the Hydrological Society's collection of papers on New Zealand's water resources (Mosley, 1992).

The State of Our Rivers and Streams

New Zealand rivers have very high quality water by international standards. They contain lower concentrations of dissolved material, and are low in nutrients when compared with rivers overseas. The Pupu Springs which feed the Waikoropupu River near Takaka in the South Island have been described as possibly the clearest freshwater in the world (D.G. Smith, 1993). The cold, turbulent, springs with their blue-violet water are very popular with divers. NIWA scientists discovered why when they measured the water's clarity. Visibility was so good that mirrors had to be used to increase the viewing range in the 30 metres wide by 7 metres deep main basin. The results showed that the water is optically almost indistinguishable from distilled water, with a horizontal clarity of 62 metres and a vertical clarity, as measured by a Secchi disk, estimated at 70-75 metres-a measurement surpassed only at a few sites in Antarctic sea waters.

Water quality varies with the geology of the catchment, the volume of stream flow, and, most significantly, the pattern of land use and associated human activities. In contrast to many overseas rivers, for example, New Zealand rivers are very short and steep. This gives them less time and distance in which to gather contaminants but also gives them high levels of sediment from erosion. As a result most New Zealand rivers are nowhere near as clear as Pupu Springs. In fact, in most of them, the black disk used for measuring clarity becomes invisible well before the 2 metre mark (see Table 7.9).

River water quality is generally best in sparsely developed areas, including many South Island rivers and the headwaters of major North Island rivers, but it deteriorates in the lower reaches of most rivers, particularly those in the North Island. The national river surveys do not test water quality in the many small streams around the country, but a national review of other studies has found that many streams and creeks in lowland areas tend to have poor water quality (Smith et al., 1993).

Particular water quality problems include faecal contamination from agricultural and urban run-off, excessive concentrations of nutrients, profuse aquatic plant growth, and high sediment concentrations. River survey results from the National Water Quality Network (NWQN) are summarised in Table 7.9 which shows the median or mid-point measurements from various water quality tests, as well as the range between the top 10 percent of water samples and the bottom 10 percent. Also included in the table are 'suitability thresholds' beyond which river water may be unsuitable for a specified use.

Suspended solids

Sediment from eroding land is the main source of suspended solids. It gives the water a turbid, or muddy, appearance, especially after heavy or prolonged rainfall on steep and deforested land. As a result, black disc visibility in half the rivers monitored by the NWQN is 1.3 metres or less, compared to the water clarity guideline requiring visibility of 1.6 metres or more for swimming and aesthetic quality (Ministry for the Environment, 1994). In the worst 10 percent of the sampled rivers, visibility was 17 centimetres or less, while in the top 10 percent it was 5.1 metres or greater (see Table 7.9).

Dissolved oxygen

Dissolved oxygen decreases at night and increases during the day in response to the respiration and photosynthesis of aquatic plants. When the concentration of oxygen falls below 80 percent, aquatic life can become stressed. None of the NWQN rivers fell below the 80 percent level at the time of measurement and more than half the NWQN rivers were fully saturated, with dissolved oxygen levels of 100 percent or greater. Because monitoring occurred during the day, when dissolved oxygen levels are at their highest, it is not known how many rivers fall below the 80 percent level after dark.

Table 7.9: Water quality in 77 rivers monitored by the National Water Quality Network, showing medians, ranges and suitability thresholds.
Parameter Median1 Range 10%-90% Suitability thresholds for particular uses

Suspended solids

Turbidity 2.15 NTU 0.45 - 31.3 NTU No more than 2 NTU Contact recreation (e.g. swimming)
Black Disc visibility 1.3 metres 0.17 - 5.10 metres No less than 1.6 metres Aesthetics
Biochemical oxygen demand (BOD 5)grams of consumed oxygen per cubic metre of water 0.45 g/m3 0.15 - 1.2 g/m3 No more than 12 g/m3(filtered water sample) Contact recreation
No more than 35 g/m3(unfiltered, total BOD5) Aesthetics
Dissolved reactive phosphorus (DRP) 4 mg/m3 220 mg/m3 No more than 1530 mg/m3 Contact recreation
Preventing algal growth
Dissolved inorganic nitrogen 2(DIN) Not known but likely to exceed threshold Not known No more than 40-100 mg/m3

Contact recreation

Preventing algal growth
Nitrate - nitrogen (NO3 -N) 105 mg/m3 10-620 mg/m3 No more than  
30,000 mg/m3for Stock water supply
10,000 mg/m3for Human consumption
Ammoniacal nitrogen3(NH4-N) 9 mg/m3 315.0 mg/m3 Suitability varies with temperature and pH Aquatic ecosystems
Dissolved oxygen (DO) 100.3% 93-108.2% No less than 80% Aquatic ecosystems
pH (acidity/alkalinity) 7.68 (mildly alkaline) 7.23-8.19 Acceptable within the range 6 - 9 (i.e. slightly acidic to moderately alkaline) Aquatic ecosystems
Water supply

Sources: Ministry for the Environment (1992; 1994), Smith et al. (1993), Smith and Maasdam (1994)

1 The median is a statistic which presents the middle score or measurement from all the samples. It is used in this instance to describe typical water quality. Unlike an average, the median is insensitive to changes in extreme values, making it a better indicator of normal water quality.

2 DIN is the sum of nitrate-nitrogen and ammoniacal nitrogen, but needs to be summed separately for each sample before the overall median can be calculated. Based on the nitrate-nitrogen component, however, DIN in more than half our rivers is likely to exceed the suitability threshold for contact recreation, aesthetic appreciation, and prevention of nuisance algal growth.

3 The toxicity of ammonia to aquatic life varies with pH and temperature. The threshold for long-term toxicity is a 4-day average concentration of 1,150 mg/m3 of ammonia at 20 ° C and pH 7.75.

Biochemical Oxygen Demand (BOD5)

Biochemical oxygen demand (BOD5) in uncontaminated waters is generally less than 1 g/m3. Most of the NWQN river sites have BOD5 concentrations well below this, indicating that these rivers are not polluted with organic material (Table 7.9). A guideline value of 2 g/m3 (for filtered water samples) is proposed by the Ministry for the Environment (1992) to prevent undesirable bacterial or slime growths in surface water.

Only four sites in the NWQN have BOD5 levels which exceed 2 g/m3 frequently, and they are all downstream of known organic waste discharges (e.g. processing works, pulp mill) (Smith and Maasdam, 1994). Excessive BOD5 levels are more common in small streams surrounded by cattle pasture (Smith et al., 1993).

Nutrient concentrations (nitrogen and phosphorus)

Half the NWQN rivers have a nitrate-nitrogen concentration of 105 mg/m3 or more (Table 7.9). This poses no threat to human or animal health but may make those rivers susceptible to nuisance algal growths. Nitrate concentrations appear to reflect the level of pastoral run-off, being higher in wet seasons and in rivers surrounded by pasture, such as the Mataura, Oreti (Southland), Waipa (Waikato - King Country), Tukituki (Hawke's Bay), Waingongoro (Taranaki), and Waihou (Hauraki Lowlands) (Smith and Maasdam, 1994). When low summer flows were monitored by the '100 Rivers' survey, the median nitrate-nitrogen concentration was only 40 mg/m3 (Close and Davies-Colley, 1990).

Nitrogen also enters water as ammonia. High concentrations can occur in swamps and other naturally low oxygen environments (such as geothermal areas) and downstream of point source discharges. Sustained high concentrations can be lethal to fish, though toxicity varies with pH and water temperature. The median ammonia concentrations in the '100 Rivers' and NWQN surveys were 13 and 9 mg/m3 respectively. Even with variations in pH and temperature, these levels pose little risk to fish life in most New Zealand rivers (Smith and Maasdam, 1994).

The combined nitrogen load from nitrates and ammonia is referred to as dissolved inorganic nitrogen (DIN). To prevent nuisance algal growths, the Ministry for the Environment (1992) has proposed that DIN concentrations be kept below 40-100 mg/m3.

Although direct measurements of DIN are not available, it is obvious from nitrate-nitrogen concentrations alone that many of our rivers probably exceed the guideline and are at risk of developing nuisance algae (see Table 7.9). However, although nitrogen levels are high enough to promote algal growth in over half the NWQN rivers, phosphorus levels are comparatively lower. In half the samples, concentrations of dissolved reactive phosphorus (DRP) were only 4 mg/m3 or less (see Table 7.9). This is well below the 1530 mg/m3 level which facilitates algal growth (Ministry for the Environment, 1992). In total, some 50 percent to 60 percent of river sites in the NWQN have sufficiently high nutrient levels to create a risk of nuisance algal growth. However, actual proliferations of algae in these rivers appear to be inhibited in many cases by the rivers' silty or mobile sandy beds and variable flows.

Box 7.6: Changing water quality in the Waikato River

The Waikato River, which originates in Lake Taupo, is the longest and most used river in New Zealand. It is 425 kilometres long with a catchment area of 1,114,000 hectares. The catchment includes agricultural land, electric power stations, planted exotic forests, mining and manufacturing industries, townships, and a major city (Hamilton). Until recently, the river was also New Zealand's representative waterway in UNEP's Global Environmental Monitoring Survey (GEMS).

A survey of the catchment's status between 1972 and 1978 showed that the river was under stress (Ministry of Works and Development, 1979). Large amounts of fertiliser and around 30 billion litres of animal wastes, were deposited annually in the catchment. The river also received wastes from 12 dairy factories, two abattoirs, one wool scour, one pulp and paper mill, several open cast coal mines, one sulphur mine, one ironsand mine and 13 urban sewage treatment plants. Further pressures came from 13 power stations, nine of which were hydro powered, two thermally powered and two geothermally powered. The stations drew off substantial amounts of cooling water and also discharged effluent into the river. Discharges from a geothermal station were high in toxic elements such as arsenic, fluoride and borate (Glasby, 1991).

In the years since the original survey, the Waikato Catchment Board and its successor, the Waikato Regional Council (or Environment Waikato), have systematically monitored the river. By 1988, the river was considered to be of very high quality along most of its length (Zuur, 1989). Improvements in waste treatment had significantly reduced bacteria numbers and BOD5 levels. Dissolved oxygen levels had increased, and nutrient levels, though still higher than in many other New Zealand rivers, were declining as a result of cutbacks in fertiliser use and stock numbers. Six years on, the improvements were still being sustained, with a slight increase in dissolved oxygen, a continuing decline in nutrient concentrations, and no significant changes in bacterial numbers, BOD5 levels or water clarity (Environment Waikato, 1995).

However, the improvements have not been evenly spread along the river. The lower reaches drain intensively used farmland, as well as swamps and peatlands, and are in far worse condition than the upper reaches of the river. Turbidity and faecal bacteria in the lower river commonly exceed the recommended guidelines for recreational waters. The black disc, which is used to measure water clarity, is visible for less than half a metre in the lower river compared to a visibility depth of 10 metres in Lake Taupo. Concern has also been expressed that the boom in dairy farming may reverse the Waikato River's declining trend in nutrient levels as more nitrogen fertiliser is used on pasture (Environment Waikato, 1995).

Bacterial concentrations

When disease-causing bacteria and protozoans get into water through animal waste and human sewage, they pose a risk to swimmers and shellfish eaters (see Box 7.7). The risk is greater for shellfish eaters because the shellfish are filter feeders which accumulate large amounts of bacteria from the water and become heavily infected themselves.

Two recent studies have investigated the risk to swimmers at coastal bathing beaches (Bandaranayake et al., 1996) and river swimming spots (McBride, et al., 1996). The river study is still in progress, but the coastal study has been completed. It compared the symptoms of 3,887 people at three different types of beach which we could label as: 'rural' (i.e. beaches exposed to some pastoral run-off), 'town' (beaches exposed to some sewage from oxidation ponds), and 'ideal' (beaches exposed to neither of these).

All beaches had relatively low bacterial levels, and the overall rate of pollution and illness was low by international standards. Nevertheless, small but statistically significant differences were found. Those who spent more than half an hour in the water at either the rural or town beaches had a slightly higher rate of stomach bugs and chest infections. Paddlers at the town and rural beaches had a slightly higher rate of chest infections but not of stomach bugs. Those at the ideal beaches, and those who stayed out of the water at all beaches, had no increase in rates of illness (Bandaranyake, et al., 1995).

Although the national river surveys do not monitor bacteria, many regional authorities do. The Bay of Plenty Regional Council, for example, surveys bathing water quality in its region every two years. In its 1995 survey, it found a significant improvement over the previous survey with all marine beaches and 96 percent of lake shores meeting the Ministry of Health guidelines. However, only 38 percent of the river sampling holes came up to standard, reflecting land use patterns (Ogilivie, 1995c).

A selection of unpublished regional council microbiological data was mapped in 1994 by the Ministry for the Environment to show average bacterial concentrations at swimming sites around the country (see Figures 7.18a and 7.18b). Smith et al. (1993) also consulted regional microbiological data in assessing the effects of agriculture on stream and river quality. From these data it seems that the upper reaches of many streams and rivers are suitable for contact recreation (e.g. swimming and other activities where there is a risk of swallowing water or transferring it from hands to food) but that, further downstream, some rivers become quite unsuitable. In some areas sewage and industrial discharges contaminate waterways, and streams surrounded by dairy pasture are particularly prone to faecal and nutrient contamination. Sometimes, they may even be unsuitable for sheep and cattle to drink (Smith et al., 1993).

Overall river and stream quality

In general, the upper reaches of most rivers have high water quality. The middle and lower reaches of South Island rivers also have high water quality, but the middle and lower reaches of North Island rivers tend to have elevated turbidity and nutrient levels (Maasdam and Smith, 1994; Smith et al., 1993). In most cases contamination is from pasture run-off, but contamination also occurs downstream of towns and industrial sites with significant sewage and stormwater discharges (see Box 7.6).

Box 7.7: Microbiological contamination of surface waters

Micro-organisms are single-celled creatures that are invisible to the naked eye. The smallest are bacteria and the larger ones, algae and protozoans, are collectively called protists. While most are harmless to humans, some micro-organisms can cause serious health problems. The Campylobacter bacterium, for example, is the most common source of food and waterborne illness in New Zealand, causing stomach ailments in people and abortions in sheep and cattle (Public Health Commission, 1994). Other harmful bacteria which can occur in water are Salmonella and Shigella (which cause severe stomach upsets and dysentery) , Meningococcus (which causes a type of meningitis) and Rickettsia (which causes typhus). Harmful protists include Giardia (see Box 7.11) , Cryptosporidium (which causes diarrhoea), Naegleri fowleri (an amoeba which causes meningitis and which can be caught in geothermal pools) and several types of toxic algae (e.g. Gymnodinium) which may cause nausea, diarrhoea, disorientation or even paralysis (see Box 7.13).

It is not practical to monitor water for all potentially harmful micro-organisms. Instead, most authorities monitor for a few common micro-organisms on the assumption that, where these are rare, harmful ones are also likely to be rare. The most commonly monitored micro-organisms are ordinary bacteria which live inside humans and other mammals and usually exit with their faeces. These are the enterococci bacteria, which normally live in the intestine, and the faecal coliform bacteria, which normally live in the colon or bowel. Faecal coliforms were used to define the water quality classes in the Water and Soil Conservation Act 1967, and enterococci were used in the Department of Health 1992 guidelines for bathing and shellfish gathering. If these bacteria are present at all, water is classified as unfit for human consumption. As bacterial density increases, water is progressively classed as unfit for shellfish harvesting, contact recreation and sheep and cattle consumption.

For this report, a number of regional councils and health authorities provided bacterial data from a selection of swimming areas, including rivers, lakes and beaches. Where possible, data were from sites which had been monitored at least five times at regular intervals, though in some cases sites had only been monitored once. Averages were calculated for the sites that had been monitored more than once. The averaged figures were graded into four water quality classes using published standards and recommendations. These are presented in Figures 4.18a and 4.18b (Department of Health, 1992; Water and Soil Conservation Act, 1967; ANZECC, 1992).

Coastal sites had fewer bacteria than river sites, and the upper reaches of rivers were less contaminated than the lower reaches. However, these averages mask significant variations. Pastoral run-off yields higher bacterial counts at times of high rainfall, usually winter, while point sources, such as sewage outfalls, yield higher counts in summer. Also, these data come from swimming sites, rather than sites exposed to run-off and outfalls. In estuaries and harbours, for example, water quality varies markedly. Advice from health authorities should be sought before shellfish are taken from harbours which are near ports or urban areas.

Figure 7.19: Trends in Waikato River water quality 1988-94 (Narrows Bridge site, Hamilton)
Textual description of figure 7.19

The Waikato river quality at Narrows Bridge has fluctuated greatly over the period 1988 to 1994. Overall, the nutrient concentrations have decreased from around 200 milligrams per cubic metre in 1988, to around 140 milligrams per cubic metre in 1994. Over the same time period faecal coliforms have increased slightly.

The State of our Lakes

Both the number and size of our lakes have been affected by human activities. Hydroelectric development has increased the size of some of the deep lakes, and has created 16,000 hectares of new lakes. On the other hand, land drainage has reduced the size of many shallow lakes. Total lake area has probably undergone little net change, though local changes have been quite substantial. The creation of Lake Dunstan, for example, flooded the gorge and half the old town of Cromwell, while the development of farmland around Lake Ellesmere reduced the lake from a pre-European area of some 30,000 hectares to around 18,000 hectares (Stephenson, 1986).

Water quality in our lakes ranges from the apparent high quality of the large deep lakes to the very poor quality of some small shallow lakes. Because of the difficulty and expense of monitoring the deep lakes, details of their water quality are relatively unknown, but visible water quality problems seem rare. Most of the hundreds of shallow lakes are also unmonitored. In those for which data exist, however, eutrophication is a frequent problem.

Eutrophication occurs when too many nutrients enter a lake or stream and cause excessive growth of weeds and photosynthesising algae (phytoplankton). These can suffocate the oxygen-breathing organisms in the lake. Eutrophic lakes have low levels of dissolved oxygen, poor water clarity, nuisance algal blooms and fewer fish. Many small lakes, particularly in the North Island, exhibit some or all of these problems. Nutrient-enriched run-off from surrounding pastoral catchments is the main cause (White, 1982). Although small urban lakes are usually eutrophic too, as a result of drainage, groundwater seepage, large duck populations and intensive recreational use, urban sewage has not been a contributing factor except in two lakes - Lake Rotorua and Levin's Lake Horowhenua (White, 1982).

Smith et al. (1993) reviewed data on the trophic (or nutrient) state of 177 lakes (see Table 7.10). Seventy-one of the lakes were eutrophic or hypertrophic (extremely eutrophic). This represents 40 percent of their sample and 10 percent of all New Zealand's shallow lakes. Because the sample was not selected on a random basis, but on the basis of data availability, it would be wrong to conclude that 40 percent of all New Zealand shallow lakes are eutrophic. What can be concluded is that at least 10 percent are, and that the true figure is probably considerably higher.

More than 90 percent of the eutrophic lakes are in the North Island, while a similar proportion (86 percent) of the high quality oligotrophic (low nutrient) lakes are in the South Island. The largest of the eutrophic lakes are Wairarapa in the North Island and Ellesmere in the South Island. Both of these lakes have predominantly agricultural catchments, although Lake Ellesmere also receives wastes from several town sewerage systems.

Evidence that run-off from pastoral land is the main cause of eutrophication comes from the strong relationship between catchment type and trophic state (see Table 7.10). Only 6 percent of the oligotrophic lakes in the sample are in pasture-dominated catchments (i.e. catchments with half or more of the land in pasture as opposed to forest or tussock), but pasture-dominant catchments surround 36 percent of the mesotrophic lakes, 66 percent of the eutrophic lakes and 92 percent of the hypertrophic lakes.

We cannot say whether our lakes are getting better or worse. A 1976 review of 72 lakes, reported that 11 (15 percent) were eutrophic, 30 (42 percent) were mesotrophic, and 31 (43 percent) oligotrophic (White, 1976). Comparisons cannot be made, however, as the study was not based on a random selection of lakes and its main purpose was classification. However, the author subsequently reported that: "Eutrophication is here, it will worsen insidiously, it will require control in places, but rapid and widespread environmental degradation will not occur" (White, 1982).

Table 7.10: Trophic (nutrient) state, catchment status and water quality of 177 New Zealand lakes.
  Oligotrophic lakes (low nutrient levels) Mesotrophic lakes (moderate nutrient levels) Eutrophic lakes (high nutrientlevels) Hypertrophic lakes (very high nutrients)
Number in sample 63 43 39 32
Percent of sample 36% 24% 22% 18%
% in North Island 14% 72% 90% 94%
% in South Island 86% 28% 10% 6%
% of lakes in pasture-dominant catchments 6% 35% 66% 92%
Phytoplankton biomass as chlorophyll a (mg/m3 ) less than 2 2-5 5-30 more than 30
Water clarity (metres) more than 10 5-10 1.5-5 less than 1.5
Oxygen status of bottom waters in summer Well oxygenated Moderately depleted Depleted Severely depleted
Total phosphorus (mg/m3) less than 10 10-20 20-50 more than 50
Total nitrogen (mg/m3) less than 200 200-300 300-500 more than 500

Adapted from Smith et al. (1993)

Figure 7.18a: Microbiological quality of some North Island swimming areas
Textual description of figure 7.18a

Acceptable concentration levels of bacteria for different uses of water (median per 100 millilitres):

  • Shellfish gathering: faecal coliforms 0 to 4; enterococci not specified.
  • Swimming: faecal coliforms 15 to 200; enterococci 0 to 35.
  • Livestock watering: faecal coliforms 201 to 1000; enterococci not specified.
  • None of the above: faecal coliforms 1000 plus; enterococci not known.

Areas safe for shellfish gathering:

  • Bay of Islands
  • Whangarei Harbour
  • Orewa
  • North Shore
  • Waiheke Island
  • Maraetai
  • Coromandel, west and east coast
  • Bay of Plenty, western, central and eastern beaches
  • Okere Falls (Kaituna River)
  • Taupo
  • Breakwater (Port Taranaki)
  • Northern Hawkes Bay
  • Napier

Areas safe for swimming:

  • Waitemata Harbour
  • Tamaki Estuary
  • Manukau Harbour
  • Narrows and Tuakau Bridges (Waikato River)
  • Waikato Lakes
  • Paengaroa (Kaituna River)
  • Rotorua Lakes
  • Waiohou and Murupara (Rangitaiki)
  • Whakapapa-Owhanga
  • Poverty Bay
  • South of Napier
  • Ngamotu (Port Taranaki)
  • Upokongaro (Whanganui River)
  • Waipawa River
  • Tukituki River
  • Foxton Beach
  • Kapiti Coast
  • Porirua
  • Petone
  • Wellington Harbour
  • Eastbourne
  • Wellington South Coast

Areas safe for livestock watering:

  • Maukoro Landing and Kiwitahi (Piako River)
  • Paeoa - Tahuna Road (Waihou River)
  • Huntly - Tainui Bridge (Waikato River)
  • Te Tumu (Kaituna River)
  • Thornton (Rangitaiki River)
  • Te Maire
  • Lower Taranaki streams
  • Manawatu River Estuary

The Whanganui River Estuary is not fit for any of the above activities.

Figure 7.18b: Microbiological quality of some South Island swimming areas
Textual description of figure 7.18b

Acceptable concentration levels of bacteria for different uses of water (median per 100 millilitres):

  • Shellfish gathering: faecal coliforms 0 to 4; enterococci not specified.
  • Swimming: faecal coliforms 15 to 200; enterococci 0 to 35.
  • Livestock watering: faecal coliforms 201 to 1000; enterococci not specified.
  • None of the above: faecal coliforms 1000 plus; enterococci not known.

Areas safe for shellfish gathering:

  • Golden Bay
  • Tasman Bay
  • Whites Bay
  • Pegasus Bay Beaches
  • Sumner area
  • Hores Bridge (Taieri River)
  • Otago Harbour
  • Lake Waihola
  • Oreti Beach
  • Jacobs River Estuary

Areas safe for swimming:

  • Tasman Rivers
  • Ashley River
  • Slewyn River
  • Avon-Heathcote
  • Lyttelton Harbour
  • Akaroa Harbour
  • Opihi River
  • Timaru Beaches
  • Kakanui River
  • Outram, Allanton and Henley Ferry (Taieri River)
  • St Clair and Brighton beaches (Dunedin)
  • Colac Bay
  • Ocean Beach
  • Awarua Bay

Areas safe for livestock watering:

  • Wairau River
  • Lower Waimakariri River
Figure 7.20: The decline of New Zealand's freshwater wetlands, inferred from soil types.
Textual description of figure 7.20

The number of wetlands in New Zealand has significantly decreased throughout the country between pre-1840 and 1990. Prior to 1840 wetlands were particularly prominent on the west and south coasts of the South Island and also through the Southern Alps and around Christchurch. In the North Island, wetlands were prominent in the north, the southern west coast and the Bay of Plenty.

Source: Landcare Research

The State of our Wetlands

The vast majority of New Zealand's wetlands have been drained or irretrievably modified for coastal land reclamation, farmland, flood control, and the creation of hydroelectricity reservoirs. This occurred mostly between 1920 and 1980 but still continues to a limited degree in some areas. The rainwater which would normally pond and seep slowly into the surrounding waterways is now swiftly carried to rivers, reservoirs and lakes by hundreds of kilometres of ditches and channels. Cattle now graze where water birds once waded, and weeds, eutrophication and pollution have reduced the biodiversity of many surviving wetlands. Only the South Island high altitude wetlands have escaped this process.

Several attempts have been made to estimate the extent of the wetlands' decline. Soil type is a useful guide to the original wetland area because many vanished wetlands left distinctive soil types. In an analysis commissioned for this report, Landcare scientists used soil maps to estimate that the original area of freshwater wetland was approximately 672,000 hectares (see Figure 7.20). An earlier estimate, which included saltwater wetlands, such as estuaries and salt marshes, put the original wetland area at over one million hectares (Stephenson, 1983).

The area of wetland which remains today cannot be simply calculated from maps because it has been shrinking yearly (see Box 7.8). Between 1954 and 1976, surveys by the former Wildlife Service found that 263,000 hectares were losta rate of nearly 12,000 hectares per year. Resurveys of sample areas in Northland indicated that between 1978 and 1983 approximately 15 percent (3,175 hectares) of the remaining wetland had been drained. These surveys ceased in the mid1980s when the Wildlife Service became part of the newly established Department of Conservation. The Department subsequently set up a wetland inventory (known as WERI), which lists about 3,000 wetlands. However, the inventory's focus is on ecologically and regionally significant wetlands rather than general trends in wetland loss or restoration and, in any case, it is not systematically updated.

Estimates of the amount of freshwater wetland that still remained in the mid1970s range from 89,000 hectares (Newsome, 1987) to 265,000 hectares (Stephenson, 1983). The higher estimate includes 'developed' wetlands (partially drained pasture). The Landcare Research estimate for this report put the figure at around 100,000 hectares, suggesting that the original freshwater wetlands have declined by about 85 percent since European settlement. The working party convened by Stephenson (1983) put the decline at 90 percent. Both estimates exclude the 45,000 hectares of pakihi heathland mostly in the western South Island and Stewart Island. The pakihi area, with its low fern, scrub, rush and moss plants on poordraining podzol soils, does not appear to have changed since preEuropean times.

The national estimates of wetland loss disguise wide regional variations. While the trend has been downward in all regions, it has been more marked in some than in others, depending on both the extent of the original wetland cover and the degree of agricultural and urban development. For example, unmodified wetlands have been reduced to about 37 percent of their original area in Southland, about 15 percent in the Waikato, 2 percent on the Rangitikei Plains, and less than 1 percent in the Bay of Plenty (Cromarty and Scott, 1996). The national estimates of loss also disguise wide variation between different types of wetlands. For example, ephemeral wetland systems (eg, in dunelands) have been impacted more than lacustrine (standing, open water) wetlands.

Taranaki region has a particularly good inventory of its remaining wetlands. As in other areas, most of Taranaki's wetlands were drained or filled for pasture development, but many small ones still remain in valleys and hollows. In fact, the regional council has recently identified 717 surviving wetlands, five times more than the 139 identified in the region by the Department of Conservation's Protected Natural Areas Programme and recorded on the WERI database, although the council's tally also includes artificial ponds (Taranaki Regional Council, 1996).

The council considers that 61 percent of the WERI wetlands are threatened by pollution from agricultural run-off, 58 percent are threatened by grazing animals, and a quarter (27 percent) are still threatened by ongoing drainage. Virtually half of the wetlands (48 percent) appear to have suffered some degree of modification since they were first recorded by the Department of Conservation 10 years ago, many having significantly decreased in size and one having disappeared completely since being included in the WERI database (Taranaki Regional Council, 1996).

Box 7.8: Wetlands what remains

Thousands of wetlands remain, but their total area is small (Stephenson, 1983, 1986; Cromarty and Scott, 1996). Although some span thousands of hectares, most are only a few hectares. The WERI database held by the Department of Conservation contains data on about 3,000 wetlands. It is not comprehensive, but includes all the important wetlands. The Taranaki Regional Council has identified five times more small wetlands in its region than are recorded on the WERI database, but this figure also includes artificial ponds.

Among the known survivors are more than 900 mountain tarns and small lakes (e.g. Lewis Pass Tarn, Lakes Heron and Alexandrina), a number of coastal lagoons (e.g. Ellesmere, Wairarapa, Okarito in Westland, Waituna near Invercargill), many small dune lakes (e.g. along the Northland and Manawatu coasts), several peat wetlands (e.g. Kopuatai and Whangamarino in the Waikato), numerous swampy valleys (e.g. Taupo Swamp near Wellington), saltmarshes (e.g. Pauatahanui inlet near Wellington), mangrove estuaries (e.g. Waitangi in the Bay of Islands), and braided rivers (e.g. the large Canterbury rivers). To these should be added a small number of constructed wetlands specially created on farms and the margins of hydro lakes for duck shooting, bird watching or waste water disposal. Different ends of the range can be illustrated by two examples: the large peat wetlands of the Waikato, and the tiny Pukepuke lagoon in the Manawatu dunelands.

Example 1: The Waikato peat wetlands (Kopuatai and Whangamarino)

By the 1970s, Waikato's low-lying wetlands had been reduced to less than 15 percent of their original area, and are probably even smaller today (Ogle and Cheyne, 1981). However, two of the surviving wetlands, Kopuatai and Whangamarino, still retain enough of their grandeur and natural character to have been designated as wetlands of international importance for wildfowl under the Ramsar Convention (Cromarty and Scott, 1996). Although they are the two largest wetlands in the North Island, they were once much larger. Kopuatai originally covered the Hauraki Plain from the Firth of Thames as far inland as Matamata, and just ten kilometres to the west, on the other side of the Hapuakohe Range, Whangamarino was almost as vast.

Kopuatai Peat Dome is a remnant of the raised peat bogs which were once a feature of this area. Several rare or threatened plants dwell here, including the endemic greater jointed rush ( Sporadanthus traversii), several threatened birds, and the black mudfish (Neochanna diversus). The wetland is also important for what lies beneath its surface. The peat layers represent various stages in the formation of coal from ancient forests. They also contain pollen, plant, macrofossils, and evidence of past seismic activity, sea level fluctuations and climate change. Like most peatlands, Kopuatai is extremely vulnerable to fire, and is also threatened by such invaders as crack willow (Salix fragilis) on the more fertile margins.

Whangamarino, which has a road through its centre, is an accessible and wellknown wetland, lying within 100km of Auckland and Hamilton. It has four peat domes and provides habitat for 56 bird species, including such threatened species as the weweia or dabchick (Poliocephalus rufopectus) and brown teal (Anas aucklandica chlorotis) and perhaps 30 percent of the known New Zealand population of bitterns (Botaurus poiciloptilus). The Reao arm of the swamp is a vital habitat for fernbirds ( Bowdleria punctata) and spotless crakes (Porzana tabuensis) though marsh crakes (P. pusilla) have not been seen here for the past 15 years or more. The wetland's 18 fish species include the threatened black mudfish, while the invertebrates include New Zealand's only species of moth with aquatic larva (Nymphyla niteris).

Although only a small range of plant species live in the peat bog itself (e.g. wire rush and manuka) they include the threatened orchid, Corybus carsei, for which the Reao arm is its last remaining site. Away from the peat, the surrounding swampland contains nearly 240 wetland plant species, 60 percent of which are indigenous. Several of these are threatened, including the large water milfoil or yarrow, Myriophyllum robustum. The Department of Conservation's management plan for Whangamarino seeks to control the threats currently posed by low water levels, fires, stock grazing and invasions of willows and the exotic grass, Glyceria maxima, around the margins.

Example 2: Pukepuke Lagoon in the Manawatu dunelands

Located north of Levin in the Manawatu sand dune country, Pukepuke is not a wetland of international importance, having been greatly modified by drainage and introduced plants and animals. Its fate is more representative of the many small wetlands around our coasts affected by livestock, rabbits, and weeds, and by lowered water tables from surrounding land uses such as pine plantings and drainage. In 1870 the Pukepuke wetland was around 480 hectares, consisting of a lagoon of 160 hectares and swampland of 320 hectares (Ogden and Caithness, 1982). A hundred years later, the total area is barely 100 hectares (one square kilometre), and the lagoon itself, 15 ha (see Figure 7.21). Even so, it is probably the largest and least modified dune lake in the Manawatu coastal region - a region whose wetlands once spanned thousands of hectaresand one of the few formally protected such areas.

Before agricultural development, the area was stable, with the surrounding dunes held in place by the native plants, raumoa (spinifex) and pingao. For the local Māori tribe, Ngati Apa, the area was once the site of a pa (fortified settlement) and a significant source of eels, harakeke (flax), and wildfowl. The Ngati Apa owners sold the land to the Crown in 1958 but retained fishing rights. Cattle, sambar deer and rabbits were introduced before the turn of the century. By 1906 cattle had destroyed the raumoa on the foredune causing extensive mobile dunes to drift inland, covering much of the lagoon and swampland. The drifting dunes also blocked off the outlet drain, causing flooding of the surrounding farmland. This led to efforts to drain the lake and to stabilise the dunes by planting introduced marram grass, tree lupins, pine trees and even the native reed, raupo. The dunes continued to drift, however, splitting the lagoon in two and reducing its area to about 50 hectares. Dairy farming commenced in the 1920s and, with it, the first applications of phosphate fertilisers which contributed to eutrophication, and caused raupo to spread at the expense of other plants. More drainage during the 1930s and 1960s helped further reduce the area until, by 1970, the swamp had shrunk to 90 hectares and the lagoon was less than 15 hectares.

The lagoon dried up in 1956, 1961, and again in 1970. The Wildlife Service dug six artificial ponds between 1970 and 1974 and maintained the wetland as a wildlife reserve.

Scientists found that, by the late 1970s, twothirds (120) of the 176 vascular plants at Pukepuke were introduced species. One native species, raupo, had proliferated into a virtual weed because of pollution from cattle and fertilisers (Ogden and Caithness, 1982). The scientists also found dramatic changes in the native duck populations. Scaup (Aytha novaeseelandiae) and brown teals (Anas aucklandica chlorotis), once abundant, had disappeared, and grey ducks (Anas s. superciliosa) had been replaced by the introduced mallard (Anas p. platyrhynchos). Banded rails (Rallus philippensis) had also not been seen for many decades. Since the late 1970s attempts have been made to reestablish the indigenous duck populations, including New Zealand shovelers (Anas rhyncotis variegata ), and today the lagoon provides habitat for them as well as small numbers of other rare or declining bird species, such as fernbirds, dabchicks, bitterns, and marsh crakes. The extent to which the coastal wetlands have been lost is revealed in the fact that Pukepuke, despite its drastic changes, is now considered one of the best of those remaining.

The State of our Groundwaters

Both the quantity and quality of groundwater may be affected by human activities. Excessive draw-off or land drainage may reduce the supply available for irrigation or urban use. Near the coast, excessive use can actually reduce groundwater levels below sea level so that sea water intrudes into the aquifer, making the groundwater salty and unusable. Groundwater is also affected by the activities on the land above it. Nutrients, faecal bacteria and toxic chemicals can all leach down from the land surface degrading the quality of the groundwater. On the evidence to date, such effects have been relatively minor, though sea water intrusion has occurred in some coastal areas, and a number of aquifers, particularly in dairying areas, show elevated levels of nitrate-nitrogen. Careful management of groundwater supplies has prevented serious depletion in most parts of the country. However, depletion of geothermal fields is a more serious issue (see Box 7.9).

Quantity of Groundwater

Groundwater levels have fallen in most of New Zealand's flood plains through a prolonged programme of unchecked land drainage. Farmers themselves consider some areas to be 'over-drained'. For example, the groundwater below Christchurch declined progressively from the 1890s to the 1950s. Since then it has stabilised, with levels dropping in summer, then returning to normal in winter. However, dry seasons can sometimes disrupt the process of winter replenishment, causing longer term declines in groundwater. For example, a series of dry winters in Hawke's Bay from 1981 to 1984, led to a decline in the groundwater beneath the Heretaunga Plain. The amount of water entering the aquifer fell below the 320,000 m3being removed for agricultural, industrial and domestic purposes. To counteract this, trenches were constructed on the river flood plain to artificially recharge the aquifer by 54,000 m3 per day (Mosley, 1993).

Figure 7.21: The changing shape of Pukepuke Lagoon and wetland
Textual description of figure 7.21

In 1872 the Pukepuke wetland was around 480 hectares with a lagoon of 160 hectares and swampland of 320 hectares. The area decreased over the following century with a total area of less than 100 hectares and a lagoon of 15 hectares.

Source: Ogden and Caithness (1982)

Lowered aquifer levels have led to sea water intrusion in a number of places, including quite extensive aquifers on the Heretaunga and Waimea Plains, and small aquifers situated beneath rural seaside communities such as those on the Coromandel Peninsula. In 1990, for example, seawater intruded 600 metres into a shallow gravel aquifer at Lower Moutere near Nelson after irrigation caused groundwater levels to drop. In this case, water use restrictions and improved rainfall allowed the aquifer to recover, but in some areas contamination can last for many years. To prevent such occurrences in the Christchurch area, yearly draw-off limits, or 'safe yields', have been set for the artesian aquifers (Mosley, 1993).

Box 7.9: The state of our geothermal aquifers

Although only a fraction of our useable geothermal energy is currently exploited, the effects of extraction have been dramatic in the geothermal systems that have been developed. Of the 24 high-temperature geothermal fields in New Zealand, many, including Wairakei and Ohaaki, Tauhara, Tokaanu-Waihi, Rotoiti, Kawerau and Rotorua are exploited for industrial, commercial and domestic purposes, and for electricity generation.

For centuries, Māori communities in the central North Island had used geothermal waters for cooking, washing, warmth and healing. In the 1930s drilling began for domestic heating supplies in Rotorua and continued on a large scale in the 1940s. In 1956, the Tasman Pulp and Paper Mill began piping steam from the Kawerau geothermal field for timber drying and to generate electricity for the plant. In 1958 a geothermal power station was commissioned at Wairakei. It reached full production in 1963. Glover (1977) monitored the Geyser Valley around Wairakei and found that the development of the field caused the geothermal activity to change from mainly geysers and flowing springs to steam-heated pools, fumaroles and steaming ground. Some changes were also noticed as far away as Taupo (10 km from Wairakei), with some springs recording an increase in temperature, as a result of fluid withdrawal at Wairakei (Environment Waikato, 1993).

The number of active geysers in New Zealand diminished from 130 in 1950, to only 11 by 1990. Of these, 50 were lost when the geyser field at Orakeikorako was drowned for hydroelectric development. The rest were destroyed by excessive draw-off for power generation, industrial and domestic uses. Many natural features have been destroyed or altered by the development of land in and around geothermal fields. Houghton et al. (1980 and 1989) reviewed the country's geothermal systems and recommended five areas for complete preservation: White Island, Waimangu and Waiotapu (south of Rotorua), Ketetahi (near the Tongariro National Park), and the Rotorua field. Most of the surface features are protected within reserves, but the Resource Management Act does allow regional councils to grant consent to draw from contributing fields. Any person may apply for a Water Conservation Order over a geothermal field. To date, none are protected.

At present, some 520 megawatts (MW) of energy are extracted from geothermal aquifers, half as electricity, half as direct heat. The total resource is estimated at 2,100 to 4,700 MW of energy, or 21,000 to 43,000 petajoules (PJ) of useable heat. This compares to a total coal resource of some 37,000 PJ and gas/oil/condensate resources of 5,000 PJ, making it a vast energy resource (Hunt and Bibby, 1992; Ministry of Commerce, 1992; Cave et al., 1993).

Quality of Groundwater

Groundwater quality varies markedly, depending on the sources of the incoming water (e.g. rainfall, rivers, run-off and leachate), as well as the geology, soils and land use of the surrounding catchment, and the chemical characteristics of the aquifer itself. The potential for groundwater pollution is greatest where the aquifers are closest to the surface, where the overlying land is permeable, where human activity is significant, and where there is a high recharge (refill) rate from rainfall, irrigation or liquid waste disposal. In contrast to surface waters, groundwater movement, and the chemical processes that go on in groundwater, are very slow. Once aquifers are contaminated they may take several to hundreds of years to cleanse themselves of pollution.

In the opinion of the water management experts surveyed by Hoare and Rowe (1992), Sinner (1992) and C.M. Smith (1993) the greatest contamination threats to groundwater are from the leaching of nitrates, and to a lesser extent, pesticides, from agricultural soils. Other threats to groundwater quality include leachate from landfills and dumps, contaminated sites, and waste disposal facilities, including the disposal of wastewater onto land.

Nitrate in groundwater

Nitrate contamination of groundwater is of concern because, at quite low concentrations, nitrate can be toxic to humans. The Ministry of Health has set a maximum allowable nitrate limit in drinking water of 50 g/m3, which is equivalent to 11.3 g/m3of nitrate- nitrogen. (The nitrogen makes up only a small part of the total nitrate molecule, NO3 ).

Table 7.11: Nitrate nitrogen concentrations in some New Zealand groundwaters.
Location Type of aquifer Concentration (g/m3 nitrate-N) Exceeds MAV1 Land use above aquifer Data sources

South Island

Waimea Plain, Confined - Yes Crops, Thomas (1995)
Tasman Unconfined Yes Effluent disposal
Christchurch Unconfined 4-8 g/m3 - Pasture, Crops, Bowden (1986)
Confined <1 g/m3 Point sources
Lincoln Unconfined - Yes Pasture, Crops Adams et al. (1979)
Ashley Catchment Unconfined - Yes Pasture, Crops Bowden et al. (1982)
Ashburton Unconfined 4-8 g/m3 - Pasture, Crops Burden (1984)
Clutha Valley (1988) - 0.2-5.3 g/m3 - Pasture Close and McCallion
North Island
Northland - <0.01-3.5 g/m3 - Pasture Smith et al.(1993)
Bay of Plenty (48 wells sampled) - <4.3 g/m3 - Pasture O'Shaughnessy and Hodges (1992)
Pukekohe, Onewhero (129 wells sampled) Confined & unconfined <0.1-23.5 Yes (7 wells) Pasture, Market gardening Ringham et al., (1990)
Hamilton Lowlands Unconfined - Yes Pasture, Dairy Hoare (1986)
Hauraki Lowlands - - Yes (5 wells) Pasture Environment Waikato (1993)
Takapau Plains - about 10 g/m3 Yes Pasture, Effluent disposal

Willoughby and Dravid (1992)

Heretaunga Plains Unconfined 10-20 Yes Pasture Burden (1980)
Tokoroa Confined - Yes Pasture, Effluent disposal Bird (1987)
Taranaki - <10 g/m3 - Pasture, Effluent disposal Smith et al. (1993)
Manawatu <30 m 313 g/m3 Yes Pasture Brougham et al. (1985)
>30 m

<2 g/m3

Wairarapa Confined <7.5 g/m3 - Pasture O'Dea (1980)

Adapted from Roberts et al. (1995)

1 A tick in this column means that at least one of the wells in each sample exceeded the Maximum Acceptable Value (MAV) for nitrate-nitrogen stipulated by the New Zealand Drinking Water Standards (Ministry of Health, 1995c).

The MAV is much lower for groundwaters discharging into nitrogen-limited lakes in the central North Island.

Generally, significant nitrate concentrations are only found in shallow unconfined aquifers. Deeper confined aquifers, fine-grained aquifers and aquifers containing high concentrations of dissolved iron, rarely have elevated nitrate concentrations. The most widespread sources of the nitrates which leach into groundwater are animal urine, clover-based dairy pastures, and nitrogen fertilisers used on dairy farms, orchards, and market gardens. Point sources of nitrate from waste disposal or waste irrigation systems may also be significant in some areas.

Table 7.11 shows groundwater nitrate concentrations from a variety of aquifers around New Zealand. Confined aquifers rarely have nitrate-nitrogen concentrations exceeding 1 g/m3, except where their water comes from contaminated unconfined aquifers, or other contaminated sources. This is the case with the two major confined aquifers of the Waimea Plains which regularly exceed the maximum allowable values for nitrate in drinking water (Thomas, 1995).

Most of the major unconfined aquifers in New Zealand also have elevated nitrate levels and some exceed the maximum allowable limit for drinking water. For example, groundwater beneath the Oreti Plain in Southland has recently been found to have excessive nitrate levels which may take years to recover. Groundwater sampling of five sites in 1982, had found only one with excessive nitrate levels (Robertson, 1992). Southern Taranaki is another area where nitrate levels have risen (Taranaki Regional Council, 1996).

Particularly high nitrate concentrations occur around meat and dairy effluent disposal systems. For example, Hoare (1986) reported nitrate-nitrogen concentrations of up to 67 g/m3 around a dairy factory effluent disposal area at Cambridge in the Waikato region. Large increases in nitrate concentrations have also been observed in some unconfined aquifers below irrigated pastures. Nitrate-nitrogen concentrations beneath the Ashburton-Lyndhurst irrigation scheme in Canterbury increased from 1.6 g/m3 to 7.5 g/m3 from 1961 to 1976 (Mosley 1993).

Leachate in groundwater

Substances leaching from landfills, rubbish dumps, effluent disposal areas and contaminated sites are a potentially serious threat to groundwater. New Zealand has an estimated 7,800 potentially contaminated urban and industrial sites (see Chapter 8). The groundwaters beneath some of these are currently being investigated by local authorities, but results from a representative number of sites are not yet available.

Pesticides in groundwater

Some 3,000-4,000 tonnes of pesticide ingredients were applied to New Zealand farms, orchards, market gardens, commercial forests and household gardens in 1989 in the midst of a farming downturn (McIntyre et al., 1989; Wilcock, 1989; Wilcock and Close, 1990). Most of this (75 percent) was in the North Island. Similar, or greater, quantities are probably being applied now. Given public concern about the potential impact of pesticides on the environment, a number of groundwater investigations have been undertaken during the past decade (Close, 1994).

The results of these investigations show that the vast majority of groundwater, including all surveyed public drinking water supplies, are free from pesticide contamination (Close, 1994; Nokes, 1992). Even among high risk sites (i.e. those with shallow groundwater, leachable soils, and high rates of pesticide use) less than 20 percent have detectable pesticide levels and nearly all of these are well within acceptable limits (Close, 1994 and 1995). In a small number of cases where excessive levels have been found, they have generally been in association with a landfill or other point source discharge. The main pesticide studies to date are summarised below, and results of some of these are outlined in Table 7.12.

The Ministry of Health's monitoring of public drinking water supplies found no pesticides in either the 60 groundwater-based supplies sampled between 1987 and 1991 (Nokes, 1992) or the 290 sampled during 1993 and 1994 (Close, 1994). Districts covered in these surveys included Whangarei, Gisborne, Hamilton, Napier, Palmerston North, New Plymouth, Lower Hutt, Greymouth, Christchurch, Timaru and Dunedin.

Table 7.12: Concentrations of pesticides in groundwater where detectable.
Region Pesticide Concentration (mg/m3) MAV1 (mg/m3) Class of pesticide
Poverty Bay2 Atrazine 37 2 Triazine
Diazinon 0.03 10 Organophosphate
Chlorpyriphos 0.03 70 Organophosphate
Te Puke2 2,4-D 0.05-0.1 30 Phenoxy
Pukekohe2 Procymidone 1.7 700 Dicarboximide
Unidentified Trace   Organochlorine
Motueka2 Unidentified Trace   Organophosphate
Pirimiphos methyl 0.06 100 Organophosphate
Oamaru2 Unidentified Trace   Organophosphate
Canterbury3 Simazine 0.01-0.78 2 Triazine
Atrazine 0.01-0.31 2 Triazine
Terbuthylazine 0.07 20  
Marlborough4 Simazine 1.3-1.6 2


Mecoprop 2.4-3.2 10  
Phenylphenol 0.1-2.3    
Clorophene 2.1    
Taranaki5 Metalaxyl 2.4 200  
Simazine 0.1-0.3 2


1 Maximum Acceptable Values (MAV) stipulated by the Ministry of Health (1995c)

2 Wells sampled by Close (1991)

3 Wells sampled by Canterbury Regional Council (1995)

4 Wells sampled by Marlborough District Council (Close, 1994)

5 Wells sampled by Taranaki Regional Council (Evans, 1995)

The first National Assessment of Pesticides in Groundwater was carried out in 1990 and 1991. It focused on 17 areas where the risk of contamination seemed highest because of high pesticide usage, permeable soils and shallow groundwater (Close, 1993a and 1993b). A total of 82 wells were sampled of which only nine (11 percent) had detectable pesticide levels and only one (1 percent) exceeded the maximum acceptable value (MAV) for drinking water (Ministry of Health, 1995c). The contaminated well, which had nearly 20 times the allowable concentration of atrazine, was shallow and adjacent to a maize field. Three of the other wells where pesticides were detected had such low concentrations that the pesticide could not be identified and two had a pesticide which was identifiable (2,4-D), but at levels that were too low to measure accurately. The districts surveyed were: Pukekohe, Te Puke, Poverty Bay, Motueka, Ashburton, and Oamaru.

The second National Survey of Pesticides in Groundwater was carried in 1994. Again the shallower, more vulnerable, aquifers were targeted, and the results were very similar to the first survey. This time, out of a total of 79 wells, 13 (17 percent) had detectable pesticide levels, most of which were well within acceptable limits (Close, 1995). Because these were the high risk groundwaters, it is likely that the percentage of other groundwaters with detectable pesticides is very low indeed. This conclusion has been borne out in several regional studies.

Canterbury Regional Council has carried out the most extensive monitoring for pesticides, beginning in 1988. However, pesticides have only been detected in one part of the region-the Levels Plain and Temuka area which cover about 10,000 hectares (Smith, 1993a and 1993b). Virtually all samples were well within acceptable limits, and the only high reading was attributed to point source contamination (Smith, 1994). Unlike other parts of Canterbury, the district has a unique combination of features which predispose it to pesticide leaching. These include: thin soils; a thin unconfined aquifer close to the surface; numerous pits and irrigation channels; the application of pesticides just before the irrigation season; and irrigation water forming the dominant water source for the aquifer.

Marlborough District Council surveyed groundwater quality in the Wairau Plains in May 1994 and again in June 1994 and found that only 2 wells (6 percent) of the 33 wells sampled had detectable pesticide levels on both occasions. Both were within the acceptable levels. One was beneath a vineyard and the other was near a landfill. Two other wells had detectable pesticide levels in one of the two surveys, and these were well below the acceptable levels (Close, 1994).

Taranaki Regional Council surveyed 30 sites in 1995 and found only one (3 percent) with detectable pesticide residues (Evans, 1995). Two other Taranaki sites were included in the national survey of high risk groundwaters and one of these had detectable pesticide residues. The low result is not too surprising, as Taranaki has the lowest pesticide use of any region in New Zealand (Wilcock, 1989).

The State of our Drinking Water

Probably the most immediate water quality concern for New Zealanders is the quality of their drinking water. Since 1960 when the former Board of Health began grading New Zealand drinking-water supplies, the safety of many supplies was found to be suspect. In 1991, a quarter of the supplies surveyed failed to meet the Department of Health's microbiological standards (Walker, 1993). In 1992 and 1993, several small communities were advised to boil water because of microbiological contamination of their supplies (Public Health Commission, 1994). In 1994, at least 8 percent of the population were served by unsafe water supply systems while a further 35 percent had supply systems of unknown status because they were inadequately monitored (Ministry of Health, 1995a and 1995b).

The Ministry of Health grades water supplies by assessing the quality of the original water source and the ability of the treatment system and water pipes to prevent contamination. The results are published four times a year in the Register of Community Drinking Water Supplies in New Zealand which is available at public libraries. The latest survey shows that New Zealand has 1,638 community drinking water supplies, serving 85 percent of the population (Ministry of Health, 1997). Of these supplies, 7 percent (serving 54 percent of the population) are considered safe and are graded A or B. A further 2 percent (serving 5 percent of the population) are of borderline safety and are graded C.

However, 19 percent of supplies (serving 18 percent of the population) provide an unsatisfactory level of protection against contamination and are graded D or E. The D and E gradings do not mean the supplies are actually contaminated, merely that the risk is high. Most of these high risk water supplies serve small communities, though four serve cities of over 20,000 people (Dunedin, Timaru, Nelson and Wanganui). The remaining 71 percent of community water supplies (serving 8 percent of the population) have not been graded because they are in communities of less than 500 people. Approximately 15 percent of the population are not connected to community supplies.

The Ministry of Health gradings focus on the adequacy of the supply system rather than the actual quality of drinking water as it comes out the tap. However, some data on drinking water quality does exist. Reviews of data gathered in the 1980s show that concentrations of some common chemicals exceeded the standard guidelines in a significant percentage of water supplies. One review found that 82% of samples failed to comply with at least one guideline value (Mattingley, 1992; Nokes, 1992; Public Health Commission, 1994). Common failings were excessive levels of: aluminium (in 20% of supplies), copper (in 31%), turbidity (25%), and potentially carcinogenic trihalomethanes (26% of chlorinated supplies) and all were a result of poor water treatment. Two thirds were outside the recommended pH range of 7.4 to 8.5 (Public Health Commission, 1994).

One chemical that is excessively low in many water supplies is fluoride. Nutritionists recommend a daily fluoride intake of 0.9 to 1.1 parts per million (ppm) to minimise tooth decay. Because New Zealand waters have unusually low fluoride concentrations (ranging between 0.1 and 0.3 ppm), many local authorities add fluoride to water supplies to improve dental health. In the early 1990s, some 84 water supplies, serving around 50% of the population, were fluoridated but nearly half these (44%) were below the recommended level (Public Health Commission, 1994).

Local authorities and other public water suppliers, are required under the Health Act to monitor the state of their water to ensure that it is safe to drink. Several of the largest authorities carried out monitoring programmes which met or exceeded the requirements of the former Health Department's 1984 Drinking Water Standards, but monitoring by many of the smaller authorities was quite inadequate. To supplement the authorities' monitoring, the Ministry of Health carried out a surveillance programme which drew on microbiological data from Crown Health Enterprises but also contracted the Institute of Environmental Science and Research (ESR) to monitor the chemical and physical quality of drinking water.

Since 1992 the Ministry of Health has undertaken a programme to improve water quality management. This has involved the review of management procedures and of legislation relating to the public health aspects of drinking water. It has also involved the revision of public health grading procedures for community drinking water supplies, the development of an accessible national drinking water database and the publication of Guidelines for Drinking-Water Quality Management, Drinking Water Standards for New Zealand 1995 and the Register of Community Drinking-Water Supplies in New Zealand.

The 1995 Drinking Water Standards include a number of features that were not in the previous standards. Under these new procedures, community water supplies are graded according to the degree to which they can show that both their drinking water and their treatment systems are safe from a public health point of view and will continue to remain so. Because the previous grading criteria were based principally on the quality of the source water and the nature of treatment and management, the grading given did not always reflect the actual quality of the tap water. The programme has significantly improved the quality of information on New Zealand's drinking-water supplies and stimulated many water supply authorities to upgrade their monitoring and their treatment systems.

Box 7.10: Giardia in New Zealand waters

Giardia lamblia is a single-celled protistan parasite that lives in the intestines of warm-blooded animals, including humans, and is passed on through water, hands or food that have been contaminated by faeces. Once expelled from the body, it survives by enclosing itself in a protective cyst until swallowed by another host (Ryan, 1991). Giardia is of interest to scientists for two reasons. First, it is the most primitive living organism after the bacteria and may be the common ancestor of all higher organisms (Day, 1994). Second, in our particular species of higher organism, it can cause a nasty illness (giardiasis) which may last for months. Although up to 80 percent of infected people show no symptoms, a minority experience severe diarrhoea, stomach cramps, bloating, dehydration, nausea and weight loss. The illness is most common in children under five years and those in close contact with them.

Giardia received a great deal of attention during the early 1990s, when the first test results showed it to be widespread in New Zealand waters (Ampofo et al., 1991). Monitoring in 1990, found cysts in 33 percent of water samples (135 of 412). Concentrations ranged from 1 cyst per 10 litres in some streams to 450 cysts per 10 litres in the worst infected water. However, it takes about 10 cysts to infect a person, so the risk in most New Zealand waters appears to be quite low. Although the parasite has been identified in some high-use parts of national and forest parks, and several trampers have contracted giardiasis, the highest infection rates seem to be in urban regions where, ironically, water supplies are most thoroughly protected against Giardia (e.g. Auckland) . This suggests that infection is spread more through person to person, and animal pet to person contact, than through water.

Giardiasis was first detected among returning servicemen in the 1940s. Like other food and waterborne diseases, it now seems to be on the increase. By 1993, the number of laboratory-identified cases had risen to 2,882, or 85 per 100,000 people, which is high compared to the reported rates for other western countries (van Duivenboden and Walker, 1993). Because it is not a notifiable disease, many cases probably go unreported. The most common notifiable food and waterborne diseases are campylobacteriosis (8,101 cases in 1993) and salmonellosis (1,340 cases in 1993) which are both caused by bacteria.

Cryptosporidium is another disease-causing micro-organism. It is a slightly more advanced protist than Giardia, and may turn out to be a more significant health risk. The extent of its distribution in New Zealand waters and water supplies is unknown because the cysts are small and difficult to detect. The infective dose of Cryptosporidium is not known, but may be as low as one cyst. Public water supplies are not permitted to contain even low concentrations of bacteria and infectious protists (Ministry of Health, 1995c). As a result, most (but not all) urban water supplies have treatment systems which remove organisms such as Giardia and Cryptosporidium . Most supplies with substandard treatment are currently being upgraded.

The State of our Coastal and Marine Waters

The harbours, estuaries and coastal waters around New Zealand receive the outflow of hundreds of rivers, the stormwater run-off from many towns and cities, and sewage from both rivers and coastal outfalls. They also receive spillage and effluent from vessels. Coastal water monitoring by regional councils is generally confined to bathing and shellfish gathering areas and, in general, the results show that our coastal water quality is high. Harmful micro-organisms are relatively uncommon and shellfish from most locations can be safely eaten.

Coastal swimming areas have better water quality than many river sites (see Box 7.7). However, coastal water near river mouths, in some harbours and estuaries, and near outfall pipes is unsuitable for shellfish gathering (e.g. Wellington's Moa Point), and in rare cases (e.g. the Whanganui River estuary) may be unsuitable for bathing. Nutrient enrichment of coastal areas from land-based sources is likely to be significant. The impacts of this are largely unknown in coastal water, but recent blooms of sea lettuce (a native seaweed) in Tauranga Harbour, have been attributed to increased nutrient inputs in combination with other factors such as favourable substrate, and temperature (Hawes, 1994). The outbreak of toxic algal blooms may also be partly nutrient- related. In recent years, these blooms have become a recurrent problem in some coastal areas (see Box 7.11).

The coastal waters that have probably been most affected by pollution and human activities are the estuaries. Most estuaries have people living on or near them and six have cities in excess of 80,000 people (Burns et al., 1990). Estuaries have also been affected by the management of surrounding land and tributary rivers. For example, a small number have been converted from freshwater to saline coastal estuaries (e.g. Maketu in the Bay of Plenty) or brackish lagoons (e.g. Matata and Whakaki) by the removal of their rivers of origin for flood control purposes. Most of these are now the subject of expensive rehabilitative works. Many other estuaries have had water quality problems arising from the surrounding land uses.

No recent assessment of the nation's estuaries has been undertaken, but an impressionistic survey of estuarine pollution status was undertaken by the New Zealand Ecological Society 20 years ago through a questionnaire sent to local authorities (McLay, 1976). Of the 162 estuaries for which replies were received: 62 (38 percent) were described as 'clean'; 67 (41 percent) as 'slightly polluted'; 26 (16 percent) as 'moderately polluted'; and 7 (4 percent) as 'grossly polluted'.

The problem estuaries included Waitemata Harbour, Manukau Harbour, Kaipara Harbour, Tauranga Harbour, Porirua Harbour, Wanganui River mouth, Ahuriri Estuary, Pauatahanui Inlet, Wellington Harbour, Waimea Inlet (Nelson), Brooklands Lagoon and the Waimakiriri River mouth, Avon-Heathcote estuary, Lyttelton Harbour and the New River Estuary (Invercargill).

In the decade following the Ecological Society survey, contamination levels in a number of estuaries were scientifically measured (Smith, 1986). Heavy metals and toxic organic chemicals were found near several large urban areas, though levels were generally well below those found overseas (e.g. Manukau, Waitemata and Tamaki near Auckland, the Waiwhetu Stream near Petone, Christchurch's Avon-Heathcote estuary and also Whangarei). Since then, point source discharges have been considerably improved in many areas and measures have been taken to restore water quality in many harbours, such as Whangarei (see Box 7.13). However, no national assessment has been made of estuarine water quality.

The most comprehensive recent studies of estuarine contamination are probably those in the Auckland region which have found that, although there has been a steady fall in contamination from pesticide residues, such as DDT and chlordane, there has been a steady increase in concentrations of heavy metals (specifically lead, zinc and copper) and hydrocarbons (specifically polycyclic aromatic hydrocarbons, or PAHs). Heavy metal levels in the sediment of some estuaries and harbours exceed North American criteria for the protection of aquatic life. In fact, about half of Auckland's 3,500 hectares of coastal sediment have excessive concentrations of lead, zinc and copper, with circumstantial evidence of reduced animal diversity, elevated contaminant levels in shellfish and crustaceans, and changes in their growth or behaviour.

The worst affected areas are estuaries or upper harbours that are sheltered from ocean currents (Auckland Regional Council, 1995).

The main source of these increasing contaminants is the motor vehicle. The heavy metals come from leaded petrol, tyres (zinc), and vehicle wiring (copper). PAHs come from vehicle exhausts. Like lead, they bind to particles of dust and sediment which are then washed into the stormwater system. At present, PAHs are not at levels which threaten aquatic life, but they are increasing (Wilcock, 1994). The banning of leaded petrol will probably lead to a slow decline in lead pollution, but if the other contaminants continue to be generated at present day rates, the proportion of contaminated estuarine sediment will expand from 50 percent to 70 percent by the year 2021 (Auckland Regional Council, 1994).

Waste disposal from boats is a problem in some areas. In 1994, for instance, the Tasman Bay oyster fishery had to be closed because of raw sewage discharges from Russian trawlers which were not covered by New Zealand regulations. Fishing industry sources estimate that approximately 45 percent of the total fish catch is waste material, much of which is thrown overboard. For example, some 50,000 tonnes of hoki offal are dumped into the sea each year by vessels fishing on the continental slope off the South Island West Coast. This has raised concerns that the decomposing waste could locally deplete oxygen levels (Livingston and Rutherford, 1988). A preliminary assessment confirmed that enough waste reaches the sea floor to alter the species composition (Grange, 1993).

Oil spills are also a frequent problem. In 1996 the Maritime Safety Authority received 84 reports of marine oil spills, most of them small. Of these, 18 (21 percent) were false alarms and only 12 involved spills of more then 100 kilograms. Only two of the reported spills released more than a tonne of oil, with the largest being 6.5 tonnes of marine diesel and lube oil. Despite the false alarms, these numbers probably under-represent the true frequency of oil spills, particularly small ones. The rate of spill reporting varied considerably between the first half of the year when only 15 spills were reported nationwide and the second half when 69 were reported at a rate of about 11 per month.

The dramatic change in spill reporting followed the completion of the Maritime Safety Authority's Marine Oil Spill Response Strategy, which formalised a national reporting procedure. The improved reporting rate was also associated with the development of regional and national oil spill contingency plans. Adjusting for under-reporting in the first half of the year, and false alarms in the latter, a truer minimum estimate of the number of spills in 1996 is probably around 104. However, even this figure is likely to be an under-estimate as many small spills still go unreported.

Non-biodegradable litter (e.g. plastic bags, wrappers, strapping and containers, aluminium cans, glass bottles, wire, synthetic ropes and nets), is a widespread problem, particularly near large urban areas (Gregory, 1991). Plastic items are by far the most common. The steady build up of plastic litter on the sea floor can inhibit gas exchange between sediments and overlying waters, thereby reducing oxygen levels and killing organisms that normally dwell there (Goldberg, 1995). Plastic items can also entangle or be swallowed by marine mammals, seabirds and turtles. In fact, they may be a greater cause of death among the world's marine mammals than oil spills, heavy metals, or other toxic materials. Plastic bags were found in the guts of three rare whales which stranded on the New Zealand coast in 1994, and Department of Conservation staff at Kaikoura get 20 to 30 callouts each year to seals caught with plastic strapping around their necks (Island Care New Zealand Trust, 1995).

Although boats are an important source of marine debris, a recent year-long study of Auckland's stormwater discharges found that 28,000 pieces of litter per day, or over 10 million items per year, mostly plastic, pour from Auckland's stormwater drains into Waitemata Harbour. This does not include small plastic granules which escaped the survey nets. Beach surveys in Auckland and Canterbury have found that marine litter increases near cities, confirming that stormwater is a major source. Compared with many other countries, New Zealand's marine debris contains less plastic and more cardboard and paper, fewer bottles but more food containers and wrappers, less boating waste and sewage waste but more plastic sheeting and strapping. It is predicted that, as coastal populations increase, marine debris is likely to worsen (Island Care New Zealand Trust, 1995).

Sediment washed down from eroding hillsides and riverbanks is believed to be having an impact on the water and marine life of harbours and estuaries. In the South Island, the loss of reef sponges, kelp forests, weed beds, and the disappearance of fish nursery grounds has been linked to increased coastal sedimentation (Royal Society of New Zealand, 1993). The disappearance of seagrasses in harbours and estuaries has also been attributed to declining water clarity, which is caused by sedimentation (Turner, 1995).

Deforestation by early Māori communities appears to have led to a three- or four-fold increase in estuarine sedimentation in some areas (Hume and McGlone, 1986). Modern patterns of land use have continued the process. In the past hundred years, intertidal sediments have been accumulating at a rate of 36 millimetres per year in sandy estuaries (e.g. Nelson Haven, Pauatahanui Inlet, and Tairua Harbour) and 2-5 mm per year in muddy ones (e.g. Waitemata and Manukau Harbours) (Burns et al., 1990). This amounts to a sediment layer 20-60 cm deep deposited in our estuaries.

Another source of sediment is coastal erosion, which is also a problem in populated coastal areas because it creates a hazard for houses and roads that have been built too close to the shoreline. Although most of the coastline is static, an estimated 20 percent is eroding, while about 15 percent is extending, or accreting, into the sea (Gibb, 1984; Hicks, 1990) (Figure 7.22). The accreting areas are near river mouths where large sediment flows accumulate. The eroding areas tend to be on exposed beaches of sand and gravel with no sources of new sediment. Though largely a natural hazard, coastal erosion has been assisted in some areas by such human activities as aggregate mining of seabed, beaches and rivers, and by the removal or destabilisation of coastal foredunes. Threatened homeowners and beach communities often contribute to the problem by erecting sea walls and barriers of concrete and boulders whose reflected waves can intensify the scouring action.

On a global scale, the human-influenced 'greenhouse effect' may be influencing marine water quality in several ways. In the past century, average temperatures have risen by half a degree and New Zealand's sea level has risen by 15 centimetres, or an average of 1.5 millimetres per year (Hamilton, 1992). This may be contributing to the spread of invasive seaweeds and toxic algae, altering the distribution and composition of marine life around the coasts, and exacerbating coastal erosion by causing sea levels to rise (Hicks, 1990).

In summary, although a variety of human-induced pressures are known to have localised impacts on our coastal waters, pervasive widespread impacts have not been documented to date, though some are suspected. In general, our coastal waters are of high quality by international standards but are under stress in some areas, particularly near the larger estuarine towns and cities. Beyond the coastal zone, very little is known about the status of the sea water environment.

Box 7.11: The toxic algae menace

New Zealand's first outbreak of shellfish poisoning occurred in the summer of 1992 and lasted through the early months of 1993 (Jasperse, 1993). It began when two cats in Whangarei showed symptoms of poisoning after eating shellfish. Their owners, who had also eaten shellfish, showed similar symptoms. More cases came to light. By the end of January more than 130 cases had been diagnosed, most of them in the top half of the North Island, and the nation's entire coastline was closed to shellfish gathering. The shellfish industry suffered no long-term economic effects, but was brought to a temporary halt with around 1,000 workers laid off for some months. By May the final tally of human poisonings had risen to 187, none of them fatal.

Although most concern focused on the impacts on human health and commercial shellfish, other environmental effects were also reported, though not all could be definitely attributed to toxic algae. In Northland, thousands of dead shellfish were washed up along five kilometres of coast north of Mangawhai Heads. Dead paua, kina, octopus, whelk, sea slug and chiton were found at Bream Bay. In Hawke Bay, paua, cat's-eye and paddle crab were found dead at Kairakau Beach. Dead pipi were reported from New Plymouth, and dead cockles, tuatua, and turret shells from Te Horo, near Wellington. Paua and pipi were found dead at Gore Bay and Robinson's Bay near Kaikoura. In Southland dead toheroa were washed up at Oreti beach. Little blue penguin deaths were reported from various coastal locations, mostly from East Cape to Northland. Poisoned gulls and shags were found in both the North and South Islands. In May, 300 sooty shearwaters were found dead near Christchurch.

The shellfish poisoning had been triggered by a spate of 'algal blooms' in our coastal waters. The blooms contained billions of marine algae-microscopic organisms which are normally invisible to the naked eye-whose populations had exploded to densities of more than 100,000 per litre, to form red, brown and green patches in the sea. The prime offender was identified as a species of dinoflagellate called Gymnodinium breve. This algae produces a substance called brevetoxin which causes Neurotoxic Shellfish Poisoning (NSP). Symptoms include respiratory problems, diarrhoea, muscular weakness and changes in skin sensitivity which sometimes cause a reversal of temperature sensations so that hot water seems freezing and cold water seems hot.

The toxin had entered the food chain when algae were ingested by filter-feeding shellfish such as mussels, oysters, scallops and clams and by algal grazers such as paua. Humans became infected by eating the shellfish or simply by inhaling sea spray which contained broken cells of Gymnodinium breve. Gymnodinium breve was not found south of New Plymouth or the Bay of Plenty. Other illnesses reported south of there may have been caused by some other type of shellfish toxicity, or may have been unrelated to the consumption of shellfish. Extensive blooms of other toxic algae, including species from another dinoflagellate genus, Alexandrium, were found around the lower North Island and off the north, south and east coast of the South Island at this time, but it is not known whether they caused the reported illnesses.

The outbreak caught the country by surprise. The only monitoring being done was a small industry-funded programme set up just six months earlier in the four main commercial shellfish areas. The need for more extensive monitoring had not been recognised because, although there had been occasional reports of fish kills and slime outbreaks going back to 1860 (including a serious outbreak in 1982 and 1983 from Northland to the Bay of Plenty), shellfish poisoning of humans was unknown or unrecognised in New Zealand. During and after the crisis, many theories were put forward to explain the sudden surge of toxic blooms. Some blamed nutrient pollution. Others blamed ballast water discharged from foreign ships (Baldwin, 1993). Shortly before the crisis in 1992, for example, in a report on the threats posed by ballast waters, the Ministry of Agriculture and Fisheries noted that toxic dinoflagellate algae were currently unknown in our waters, but that Gymnodinium and Alexandrium had recently been found in Australian waters (Ministry of Agriculture and Fisheries, 1992). The implication that these species may have arrived in foreign ships cannot be proven. Some scientists argue that the algae may have lain here unnoticed all the time and that their sudden blooming was caused by the unusual weather conditions associated with El Niño (Chang, 1993).

Whatever the explanation, the crisis was not an isolated event. Less extensive blooms occurred in subsequent summers and throughout the year in parts of Northland (Mackenzie, 1994; Mackenzie et al., 1995). The blooms will continue to occur whenever temperature and currents are favourable. Worldwide, in fact, blooms appear to be increasing (Anderson, 1994). Although less than two dozen of the world's 27,000 formally identified species of marine algae are known to be toxic to humans, they pose a constant threat to coastal marine life and to people who gather and eat shellfish. To reduce the risk, shellfish and phytoplankton are now monitored regularly around our coastline.

Box 7.12: Cleaning up Whangarei Harbour

Like many other harbours in New Zealand, Whangarei Harbour used to receive raw and partially treated wastewater, uncontrolled stormwater, and industrial wastes. Also, like many other harbours, there has been a prolonged and concerted effort by the Northland Catchment Commission and then the Northland Regional Council, to improve the quality of discharges to the harbour, or to divert them wherever possible. Between Whangarei City and the Whangarei Heads there are many beachfront settlements, such as Parua Bay and McLeod Bay. Other than parts of Parua Bay, these settlements are unsewered and the on-site wastewater treatment and disposal systems they contain sometimes contribute to the faecal contamination of the harbour waters. In 1994, the District Council commissioned a study which identified options for wastewater disposal over the next 25 years for 13 communities or sets of communities in the district. As a result, it is expected that this source of contamination will be significantly reduced.

In the late 1980s, the Whangarei Main Wastewater Treatment Plant was upgraded by the construction of additional secondary treatment stages, ultraviolet (UV) disinfection, and the country's largest wetland treatment system. As a consequence, water quality in the vicinity of the discharge from the plant has been significantly improved. For example, there has been a 10-fold reduction in median faecal coliform concentration of the waters in this area of the harbour. The closure of the Onerahi Wastewater Treatment Plant, the upgrading of he Okara Park pumping station to reduce overflows, and a sewer rehabilitation programme, have also had a beneficial effect on harbour water quality. The Marsden Point Oil Refinery wastewater collection and treatment system is quite sophisticated and is monitored closely. The cement works at Portland used what was called 'the wet manufacturing process'. In 1983, this was changed to 'the dry process', which ceased the discharge of 106,000 tonnes per annum of limestone washings into the Portland area of the harbour. This discharge had reduced water clarity over a large area of the harbour. Smothering of some of the shellfish beds and the disappearance of extensive areas of eelgrass were also attributed to the effects of the discharge. Since the conversion to the dry process, water clarity in the Portland area of the harbour has markedly improved and growths of eelgrass have re-established.

Recently, the Regional Council's attention has turned to controlling other discharges into the harbour, including those from slipways and boat maintenance areas, and the discharge from the main storage area at Port Whangarei. The Regional Council's farm wastes programme has reduced the impacts of dairy farm wastes on receiving water quality throughout the region, although there are only a few dairy units in the harbour catchment. It is expected that the result of all these activities has improved the harbour's water quality. A few years ago the clarity of the water was quite poor; boaties often could see only 1 metre into the water, but today it is possible to see the bottom through 10 metres. (Dall and Ogilvie, 1996).