New Zealand has 425,000 kilometres of rivers and streams, almost 4,000 lakes that are larger than 1 hectare (Ministry for the Environment, 2006c), and about 200 groundwater bodies (aquifers) (White, 2001).
Freshwater is among our most valuable natural assets. New Zealand’s rivers, streams, and lakes are a focal point of our national identity and outdoor way of life. They are highly valued for recreational activities such as swimming, boating, and fishing, and are the centrepiece of some of the country’s best-known tourist destinations.
Clean and plentiful water provides us with a safe drinking supply, and also sustains the natural ecosystems that are home to many of New Zealand’s native species.
In addition, freshwater is a vital part of the New Zealand economy: it is used to irrigate crops and pastures, dispose of or dilute trade wastes and sewage, and produce hydro-electric energy (see chapter 5, ‘Energy’).
Water is also a fundamental taonga (treasure) for Māori. Waterways are considered the arteries of Papatūānuku (Mother Earth) (Ministry for the Environment, 2005b). Māori have cultural, historical, and spiritual links with many of the country’s springs, wetlands, rivers, hot pools, and lakes. They also value having healthy water bodies for mahinga kai (customary food and resource gathering).
By international standards, freshwater in New Zealand is both abundant and clean. Rainfall, which is the source of replenishment for our streams, rivers, lakes, and groundwater, is generally plentiful. With a population of just over 4 million and limited heavy industry, New Zealand’s human pressures on freshwater are relatively light compared with the pressures on such resources in many other developed countries.
However, protecting the country’s freshwaters is a growing challenge. With land-use practices becoming more intensive, particularly in farming, there is greater demand for water now than ever before, and evidence is building that its quality is declining in many water bodies.
New Zealand is a narrow, mountainous country characterised by relatively small catchments and fast-flowing rivers and streams. Half of its 425,000 kilometres of rivers and streams are small headwater streams.
Of New Zealand’s total length of rivers and streams, 51 per cent lies in catchments with predominantly natural land cover, such as native bush or alpine rock and tussock. The remaining 49 per cent of river length is in catchments that have been modified by agriculture (43 per cent), plantation forestry (5 per cent), or urban settlement (1 per cent).
New Zealand has 3,820 lakes that are larger than 1 hectare. Of these, 229 have an area greater than 50 hectares (Ministry for the Environment, 2006b). About 40 per cent of all lakes are in catchments in which the predominant land cover is pasture. Less than 2 per cent of lakes are in towns and cities (Ministry for the Environment, in press c).
The underground areas in which groundwater collects are known as aquifers. In New Zealand, the largest aquifers are porous gravels. Examples include the Heretaunga Plains in Hawke’s Bay and the Wairarapa, Manawatū, Canterbury, and Southland Plains. Other forms of aquifer include the fractured basalts of the Auckland region and the Coromandel Peninsula’s coastal sand aquifers.
Three main natural factors influence the quantity and quality of freshwater in New Zealand: climate, topography, and geology.
Rainfall patterns vary across the country and between seasons. Generally, rainfall is much higher on the western side of both the North and South Islands, because the prevailing westerly winds pass over mountains that form the backbone of much of the country.
Rainfall is also higher in the winter than in the summer. This seasonal variation is more extreme on the east coast of both main islands, where summers are relatively dry, compared with on the west coast. Not only does rainfall control the amount of water that flows in rivers and aquifers, it can also affect water quality by carrying pollutants from the surface of the land to water bodies. The pattern of rainfall in New Zealand is expected to change in the future; the anticipated affects on freshwater quantity and quality are summarised in the box ‘More about climate change and freshwater’.
Current research suggests that New Zealand will experience changes in the frequency of droughts, rainfall patterns, and evaporation rates, which are likely to change water flows and worsen existing problems with water availability. Irrigation needs will increase in the east of both main islands, where pressure on available water resources is already significant. At the same time, water quality is likely to deteriorate in some areas because of lower flows in rivers and streams. Algal blooms may occur more frequently because of higher water temperatures.
The shape and geology of New Zealand’s catchments also strongly influence water quality. Catchments that are steep-sided (such as those in mountainous regions) or made up of soft sediments are more prone to natural erosion. As a result, rivers draining from these catchments may carry high levels of sediment.
The type of rock and soil the water moves past and the time over which this interaction takes place also determine the characteristics of our freshwater. For example, groundwaters moving through volcanic rocks or geothermal areas are more likely to contain higher concentrations of minerals (such as sulphates and/or chloride) and metals (such as arsenic) than are fast-moving river waters.
The main pressures on freshwater quantity and quality are the growing demand for water to meet society’s various needs and pollution resulting from human activities on land.
Demand for freshwater resources is increasing as New Zealand’s population grows and more intensive forms of land use, particularly farming, become increasingly widespread. This is especially noticeable in drier regions, such as Canterbury, where relatively high volumes of water are needed to irrigate pasture.
Damming and diverting water to meet needs for power generation, irrigation storage, and human consumption can deplete flows in rivers and reduce groundwater levels. As well as having effects on water quality (described below), flow depletion can lead to insufficient water being available to meet the needs of downstream users.
Draining land to improve farming productivity or enable urban development also reduces the size of water bodies. The Waikato region’s shallow peat lakes are examples of lakes that have shrunk in size and number as the surrounding farmland has been drained.
Figure 10.1 summarises the main sources of pollution in rivers, lakes, and groundwater. These are identified as point-sources and non-point-sources. Point-sources refer to discharges of pollutants from a single facility at a known location (for example, a wastewater treatment plant). Non-point-source pollutants do not have a single point of origin (for example, they may include pollutants that have run off wide areas of disturbed or developed land after rainfall).
Source: Ministry for the Environment.
Until the 1970s, the major cause of deterioration in water quality in New Zealand was the discharge of poorly treated sewage, stock effluent, and other wastes from primary production and industry directly into water bodies. These discharges came from both urban and rural point-sources. However, stricter controls on discharge practices were introduced with the Water and Soil Conservation Act 1967 and the Resource Management Act 1991. Wastewater treatment systems have been upgraded and there has been a continuing trend towards applying effluent to land, rather than discharging it into waterways (see Figure 10.2). Pollution from point-sources has declined significantly as a result of these measures.
These percentages are averaged from resource consent data provided by regional councils and unitary authorities in Auckland, Hawke’s Bay, Manawatū–Wanganui, Wellington, and Marlborough.
Data source: Ministry for the Environment.
This bar graph shows the percentage of consents which discharge to water compared with the source of effluent (municipal, dairy and piggery) for 1995 and 2005. The number of consents for discharge of effluent to water has decreased between 1995 and 2005 for dairy, municipal and piggery activities.
While sewage and wastewater discharges from point-sources are still a significant influence on water quality in some areas, the effects of non-point-sources of pollution on streams, rivers, and lakes have been identified as the most serious freshwater management challenge in New Zealand today (Hill Young Cooper, 2006).
Urban land use affects the quality of our freshwater; 86 per cent of people live in towns and cities and produce large amounts of different types of pollution. The most significant source of bacteria and nutrients (nitrogen and phosphorus) in urban streams is human wastewater and sewage leaking from broken sewer pipes, or being discharged into stormwater systems through faulty connections. In addition, run-off to streams from paved surfaces, gardens, and disturbed land commonly has high levels of sediments (see the photo below) and can contain pollutants, including bacteria from animal faecal matter, herbicides, pesticides, detergents, and other household chemicals.
Run-off from busy roads carries pollutants such as metals (particularly zinc, copper, and lead), and hydrocarbons. (These pollutants come from the road itself, through asphalt wear, and from vehicles using the road, through exhaust emissions, brake linings, and tyre wear.)
As the dominant land use in New Zealand, agriculture has the most widespread impact on water quality. Agricultural pasture makes up almost 40 per cent of New Zealand’s total land area and occupies about four times the area of planted forestry and all other modified types of land cover combined (that is, horticultural, viticultural, industrial, and other urban land uses). (See chapter 9, ‘Land’.)
In recent years, the impact of agricultural land use on water quality has grown as a result of increased stocking rates and use of nitrogen fertilisers. Within the agricultural sector, there has also been a move away from low-intensity to high-intensity land use (for example, converting from sheep farming to dairy or deer farming). The net effect of most intensified land use is to increase the amount of nutrients, sediment, and animal effluent dispersed into water bodies (Davies-Colley et al, 2003).
Pollution officer sampling sediment run-off from a residential subdivision as it enters an urban stream.
Source: Courtesy of Greater Wellington Regional Council.
Horticultural and arable land use occupies a small proportion of New Zealand land (less than 2 per cent of total land area) compared with the land occupied by agricultural farming. Most of the land producing arable, vegetable, and fruit crops in New Zealand has flat to gently rolling terrain. As a result, the surface run-off is low. However, nutrients (from fertiliser application) and herbicides and pesticides leaching through soils may still pollute freshwater in some areas (particularly those where market gardening is common).
The pressure on freshwaters from plantation forestry is comparatively low. Nutrient yields from plantation forestry are very similar to those from native forest (Davies-Colley et al, 2003). However, when forest is being harvested, the sediment dispersed to waterways, particularly from roadways and landings, can increase.
Other forms of land use can affect water quality. As noted earlier under ‘Water quality’, the damming of rivers can change a river’s natural flow and cause increased sedimentation, higher water temperatures, and reduced oxygen concentrations. Algae and other nuisance plants may proliferate downstream from dams because the high flows that regularly flush the river system have been reduced.