View all publications

The state of our fresh water

In this section, we present information on the quality and quantity of our fresh water, and how it is changing over time.

The quality of water in New Zealand’s lakes, rivers, streams, and aquifers is variable, and depends mainly on the dominant land use in the catchment. Water quality is very good in areas with indigenous vegetation and less intensive use of land. Water quality is poorer where there are pressures from urban and agricultural land use. Rivers in these areas have reduced water clarity and aquatic insect life, and higher levels of nutrients (ie nitrogen and phosphorus) and E.coli bacteria.

Water clarity and nutrient levels

Some nutrient levels are higher but water clarity has improved overall

We report on three measures of nitrogen: total nitrogen, nitrate-nitrogen, and ammonia-nitrogen. Total nitrogen is all nitrogen forms found in waterways. Nitrate-nitrogen is highly soluble (dissolves in water) and can readily be used by plants and algae to help them grow. It can also leach through soils easily, particularly where the soils are sandy or porous, or after heavy rainfall (McDowell et al, 2008). In agricultural catchments, nitrate-nitrogen generally comes from nitrogen fertiliser and livestock urine.

At elevated concentrations, ammonia-nitrogen (ammonia in water) can be harmful to fish and other aquatic animals. Ammonia is commonly found in household bleach and is also a waste product from industry, humans, and animals. In the environment, it generally comes from point-source discharges of pollutants (rather than run-off), such as discharges from sewerage treatment plants, dairy sheds, and industrial operations.

Water clarity is a measure of underwater visibility, and is reduced by fine particles in the water like silt, mud, or organic material, mainly from soil erosion. Water clarity affects the suitability of waterways for recreational activities such as swimming and fishing, as well as affecting the habitat of fish and birds.

Between 1989 and 2013, water clarity improved overall. Total nitrogen and, to a lesser extent, nitrate-nitrogen increased (worsened) overall, while ammonia-nitrogen (ammoniacal nitrogen) decreased (improved) overall (see table 1). A trend could not be determined for total phosphorus, while dissolved reactive phosphorus (dissolved phosphorus) increased (worsened) overall. We could not determine a trend for the macroinvertebrate community index (MCI) at most sites.

Table 1:           Trends for water clarity, nutrients, and macroinvertebrate community index at NIWA sites, 1989–2013

Variable

Trend

Sites showing a statistically significant increase (%)

Sites showing a statistically significant decrease (%)

Sites showing an indeterminate trend (%)

Clarity

64

9

27

Total nitrogen

60

14

26

Nitrate-nitrogen

~

52

27

21

Ammonia-nitrogen

4

78

18

Total phosphorus

~

38

30

32

Dissolved phosphorus

51

14

35

Macroinvertebrate community index (MCI)

~

5

13

83

Source: Larned et al (2015)

Note: Trends for NIWA's National River Water Quality Network (77 sites). Data are for the period 1989–2013, except for the MCI (covering 462 NIWA and regional council sites) which are for the period 2004–13. Green arrows indicate improving water quality; red arrows indicate declining water quality. No trends could be determined for nitrate-nitrogen, total phosphorus, and the MCI. Percentages may not add to 100 percent due to rounding.

The increases in total nitrogen levels are likely to be due to an increase in nitrate leaching through soils, as a result of more intensive agriculture – especially from dairy farming expanding and intensifying in many regions. The improvement in concentrations of ammonia-nitrogen is likely to be due to improvements in the way discharges from sewage treatment plants, dairy sheds, and industrial operations are treated.

We report on two measures of phosphorus: total phosphorus and dissolved phosphorus. Total phosphorus in water is a measure of the phosphorus bound to sediment and phosphorus dissolved in the water itself. Elevated phosphorus levels in rivers promote the growth of slime and algae (periphyton), as long as there is enough sunlight, nitrogen, and a lack of flood events for periphyton to bloom (see figure 25).

Dissolved phosphorus levels increased (worsened) in the large rivers sampled by NIWA between 1989 and 2013, with 51 percent of the 77 monitored sites showing statistically significant increases. These sites contain low to moderate levels of phosphorus (median of 5.0 milligrams per cubic metre).

As dissolved phosphorus levels were significantly higher at regional council sites that are concentrated in pastoral areas (median of 13.6 milligrams per cubic metre), we also used these sites to assess phosphorus trends. Compared with the rivers sampled by NIWA, dissolved phosphorus levels decreased (improved) at the regional council sites between 1994 and 2013, with 48 percent of the 132 monitored sites showing statistically significant decreases.

There has been no clear trend for total phosphorus over the 25-year period, although levels decreased (improved) from 2004 to 2013.

Phosphorus mostly enters waterways as part of eroded sediment. Improvements in phosphorus levels and water clarity in rivers over the past 20 to 25 years were likely due to a number of factors. These include the management of erosion along river banks and surroundings, tree planting near waterways, reduced effluent discharges from industry, and a decrease in phosphorus fertiliser use from 2004 to 2014. We are currently collecting further information about these factors to inform future reports.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality trends: clarityRiver water quality trends: nitrogen, and River water quality trends: phosphorus.

Water quality and land use

Nitrogen is higher in urban and agricultural catchments

Nitrogen levels are higher in urban and pastoral lowland sites (see figures 23 and 24). The elevated levels of nitrogen are mainly due to an increase in nitrate-nitrogen from nitrogen fertiliser and untreated effluent. The contribution from ammonia-nitrogen from sewage treatment plants, dairy sheds, and industrial operations is relatively minor in comparison.

The greatest impact of excessive nitrogen in New Zealand rivers is nuisance slime and algae (periphyton) growth. This growth can reduce oxygen in the water, impede river flows, block irrigation and water supply intakes, and smother riverbed habitats. About 49 percent of monitored river sites currently have enough nitrogen to trigger nuisance periphyton growth (see figure 23), as long as there is enough sunlight, phosphorus, and a lack of flood events for periphyton to bloom.

High levels of nitrogen can also be harmful to fish, but less than 1 percent of monitored river sites in New Zealand have nitrate-nitrogen levels high enough (>6,900 milligrams per cubic metre) to affect the growth of multiple fish species (see figure 24).

For more detail see Environmental indicators Te taiao AotearoaRiver water quality trends: nitrogen and Geographic pattern of nitrogen in river water.

Figure 23:


Click image to view full size

Note: The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The maximum value for pastoral is 15,000 mg/m(which is beyond the range of this figure). The blue line (at 496 mg/m3) represents the median nitrogen threshold (Larned et al, 2015) required to meet the periphyton minimum standard in the National Policy Statement for Freshwater Management 2014. The threshold varies with climate and the source of flow (Larned et al, 2015). Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

This graph shows the median, upper and lower quartiles, and highest and lowest values for total nitrogen between 2009 and 2013 at monitored river sites under urban, pastoral, indigenous vegetation, and exotic forest land covers across New Zealand. Visit the MfE data service for the full breakdown of the data.

Figure 24:


Click image to view full size

Note: The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The maximum value for pastoral is 13,500 mg/m3 (which is beyond the range of this figure). The blue line (at 2,400 mg/m3) represents the nitrate-nitrogen level that has some growth effects on up to 5 percent of fish species (National Policy Statement for Freshwater Management 2014). The maximum acceptable limit is 6,900 mg/m3 – this is the level that has some growth effects on up to 20 percent of sensitive fish species. Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

This graph shows the median, upper and lower quartiles, and highest and lowest values for nitrate-nitrogen between 2009 and 2013 at monitored river sites under urban, pastoral, indigenous vegetation, and exotic forest land covers across New Zealand. Visit the MfE data service for the full breakdown of the data.

Phosphorus is higher in exotic forest, urban, and agricultural catchments

Phosphorus mostly enters waterways as part of eroded sediment and from phosphorus-based fertiliser. Phosphorus levels are higher in exotic forest, urban, and pastoral lowland sites (see figure 25). It is likely that the higher level of phosphorus in catchments dominated by exotic forest is connected with the geology of many of these sites. About half of the 14 exotic forest sites are in the Volcanic Plateau in the central North Island, where levels are naturally higher because the underlying rock is rich in phosphorus (Timperley, 1983).

Elevated phosphorus levels in rivers promote the growth of slime and algae (periphyton). About 32 percent of monitored river sites have enough phosphorus to trigger nuisance periphyton growth (see figure 25), as long as there is enough sunlight, nitrogen, and a lack of flood events for periphyton to bloom.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality trends: phosphorus and Geographic pattern of phosphorus in river water.

Figure 25:


Click image to view full size

Note: The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The maximum values for urban and pastoral are 270 mg/m3 and 290 mg/m3, respectively (which are beyond the range of this figure). The blue line (at 33.2 mg/m3) represents the median dissolved phosphate threshold (Larned et al, 2015) required to meet the periphyton minimum standard in the National Policy Statement for Freshwater Management 2014. The threshold varies with climate and the source of flow (Larned et al, 2015). Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

This graph shows the median, upper and lower quartiles, and highest and lowest values for dissolved phosphorus between 2009 and 2013 at monitored river sites under urban, pastoral, indigenous vegetation, and exotic forest land covers across New Zealand. Visit the MfE data service for the full breakdown of the data.

E.coli levels are higher in urban and agricultural catchments

Like nutrients, levels of E.coli are higher in urban and pastoral lowland sites (see figure 26). E.coli in rivers or lakes comes from animal or human faeces. Higher levels of E.coli are indicative of higher risks of infection from pathogens like Campylobacter while swimming, wading, or boating. Median E.coli levels in New Zealand rivers meet acceptable standards for wading and boating at 98 percent of monitored sites (see figure 26). The 2 percent of sites that exceed acceptable levels for wading and boating (>1,000 E.coli per 100 millilitres) are in urban and pastoral areas in Auckland, Canterbury, Southland, and Wellington.

Figure 26:


Click image to view full size

Note: The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The maximum values for pastoral and urban are 1,750 E.coli /100 mL and 2,300 E.coli /100 mL, respectively (which are beyond the range of this figure). The blue line (at 1,000 E.coli per 100mL) represents the maximum acceptable limit for wading or boating (National Policy Statement for Freshwater Management 2014). Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

This graph shows the median, upper and lower quartiles, and highest and lowest values for E.coli concentrations between 2009 and 2013 at monitored river sites under urban, pastoral, indigenous vegetation, and exotic forest land covers across New Zealand. Visit the MfE data service for the full breakdown of the data.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: bacteria (Escherichia coli).

Water clarity is better in areas with indigenous land cover

Water clarity is mainly affected by land cover, and is lower (worse) in pastoral lowland sites and higher (better) in hilly or mountainous areas covered by indigenous vegetation (see figure 27). The Australian and New Zealand guidelines for fresh and marine water quality (ANZECC 2000 guidelines) recommend a trigger value of 0.6 metres for lowland rivers and 0.8 for upland rivers. The guidelines recommend that rivers with water clarity below this level be actively managed.

Figure 27:


Click image to view full size

Note: The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The maximum value for indigenous is 12.3 metres (which is beyond the range of this figure).The ANZECC 2000 guidelines recommend a minimum trigger value of 0.6 metres for lowland rivers and 0.8 metres for upland rivers. The guidelines recommend that rivers with water clarity below these levels be actively managed. Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

This graph shows the median, upper and lower quartiles, and highest and lowest values for water clarity between 2009 and 2013 at monitored river sites under urban, pastoral, indigenous vegetation, and exotic forest land covers across New Zealand. Visit the MfE data service for the full breakdown of the data.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality trends: clarity and Geographic pattern of river water clarity.

Macroinvertebrate levels are lowest in agricultural and urban catchments

The presence of macroinvertebrates (aquatic animals such as insects, freshwater crayfish, snails, and worms) in rivers is a good indication of stream health. Macroinvertebrates play an important role in stream ecosystems by feeding on plants and other aquatic life, and are a food source for fish. Rivers in New Zealand are scored based on the abundance of species sensitive to pollution using an index called the macroinvertebrate community index (MCI) (Stark & Maxted, 2007). A high index value (>100) generally indicates good river health.

Macroinvertebrates were assessed at 512 river sites between 2009 and 2013. The best MCI values were in catchments with predominantly indigenous vegetation in hilly areas. Most pastoral sites (in lowland and hilly areas) had MCI scores classed as fair to good. Fifty-five sites (about 11 percent) had a poor MCI value. All these sites were in urban and pastoral areas (see figure 28).

Figure 28:


Click image to view full size

Note: The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). A macroinvertebrate community index (MCI) greater than 119 indicates excellent river health, 100–119 indicates good health, 80–99 indicates fair health, and below 80 indicates poor river health (Stark & Maxted, 2007). Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

This graph shows the median, upper and lower quartiles, and highest and lowest values for the macroinvertebrate community index between 2009 and 2013 at monitored river sites under urban, pastoral, indigenous vegetation, and exotic forest land covers across New Zealand. Visit the MfE data service for the full breakdown of the data.

For more detail see Environmental indicators Te taiao AotearoaRiver water quality: benthic macroinvertebrates.

Water quantity and availability

New Zealand has plentiful freshwater but not always where demand is greatest

New Zealand’s average annual precipitation is about 550 billion cubic metres – enough to fill our largest lake (Taupō) nine times over. However, rain does not fall uniformly across the country, and this influences the amount of water flowing through our waterways.

The volume of water is generally lower in rivers and streams in the north and on the east coasts of the North and South islands (except for the larger rivers like Rakaia and Rangitata that flow from the Southern Alps).

For more detail see Environmental indicators Te taiao AotearoaGeographic pattern of natural river flows.

The largest volume of groundwater is stored in Canterbury aquifers

Aquifers are underground layers of water-bearing rock or sand from which groundwater can be extracted. They store water that can feed some lakes and rivers. Groundwater is also pumped directly from aquifers through wells and bores. Aquifers lie under 26.3 percent of New Zealand’s land surface. About 200 aquifers have been identified nationally (see figure 29).

The volume of water stored in our aquifers is estimated to be about 711 billion cubic metres. Canterbury has an estimated 519 billion cubic metres of water in its aquifers (73 percent of New Zealand’s total groundwater volume). Waikato has the next largest volume of groundwater (4.9 percent), followed by Bay of Plenty (4.4 percent), and Taranaki (3.5 percent).

Between 1994 and 2014, groundwater volumes varied by less than 2 percent, providing a relatively stable source of fresh water. About 20 percent of the water allocated for irrigation, drinking water, stock water, and hydroelectricity generation is estimated to be extracted from groundwater (Aqualinc Research, 2010).

Figure 29:


Click image to view full size

Note: Identification of aquifers is limited by a number of factors, including different methods for identifying aquifer boundaries, and the scale at which mapping has been undertaken.

This map illustrates the location and extent of aquifers across New Zealand in 2014. Visit the MfE data service for the full breakdown of the data.

For more detail see Environmental indicators Te taiao AotearoaLocation and extent of New Zealand’s aquifers and Water physical stocks: precipitation and evapotranspiration.

Trends for groundwater quality are unclear

Groundwater and the water in rivers, lakes, and wetlands are part of a single hydrological system. However, it can sometimes take decades for water (and any contaminants it contains) to cycle from Earth’s surface through the ground to aquifers, and then back into surface water systems.

From 2004 to 2013, there were no overall trends for groundwater quality. Over the 10-year period, 86 groundwater sites were analysed for nitrate trends. Nitrate concentrations increased at 22 of the sites (26 percent), but decreased at 13 sites, resulting in no overall trend. There was also no overall trend for dissolved phosphorus.

For more detail see Environmental indicators Te taiao AotearoaGroundwater quality: phosphorus and Groundwater quality: nitrogen.

Lake water quality is lower in urban and agricultural catchments

Nutrients can accumulate in lakes and, above certain levels, cause them to become murky and green with algae. The lake trophic level index (TLI) indicates the health of a lake based on its concentration of nutrients. In general, a higher TLI means poorer water quality. Lakes with extremely poor water quality are rarely suitable for recreation and provide poor habitats for aquatic species. Four percent of lakes are currently monitored, but this proportion includes many of the largest and most popular lakes close to urban areas.

Between 2009 and 2013, the median TLI score for 65 monitored lakes was 3.6. This score reflects moderately nutrient-enriched conditions. The TLI is lower (better) in deep lakes in hilly country or mountainous areas dominated by indigenous vegetation. It is higher (worse) in lowland shallow lakes fed by urban or agricultural catchments. Trends were assessed for 30 lakes, mainly located in Northland and the Bay of Plenty. During 2004–13, the TLI significantly increased (worsened) for 11 lakes (37 percent) and significantly decreased for four lakes (13 percent). A trend could not be determined for 15 lakes (50 percent).

For more detail see Environmental indicators Te taiao AotearoaLake water quality: trophic level index.

Wetlands occupy about 10 percent of their original extent

Wetlands perform a variety of functions, often described as ‘ecosystem services’. They filter nutrients and sediment from water; absorb floodwaters; and provide habitat for plants, fish, and other animals.

In 2008, wetlands occupied approximately 250,000 hectares (or 1 percent) of New Zealand’s land area – only about 10 percent of their original extent. The West Coast has the greatest extent of wetlands (84,000 hectares), followed by Southland (47,000 hectares), and Waikato (28,000 hectares).

For more detail see Environmental indicators Te taiao AotearoaWetland extent.