Sources of pollution
The main sources of pollution in rivers, lakes and groundwater are industry, towns and roads, livestock farming, crops and human effluent disposal. These are identified as point-sources and non-point-sources.
Point sources
Point-sources refer to discharges of pollutants from a single facility at a known location (for example, a wastewater treatment plant).
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. Pollution from point-sources has dropped significantly as a result of these measures.
Non-point sources
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).
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.
Sources in different catchment land uses
The largest factor influencing nutrient levels in our rivers is land use.
Urban land use
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 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.)
Agricultural land use
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).
In rural environments, agricultural fertilisers, stock manure and urine are the major non-point-sources of nitrogen and phosphorus. These nutrients can enter water bodies relatively quickly if they are carried across the land surface by rainfall run-off, particularly if there are drains such as the mole and tile drains (below-ground channels constructed to drain surplus water) that are common on farmed pasture in Otago and Southland.
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.
Horticultural and arable land use
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), herbicides and pesticides leaching through soils may still pollute freshwater in some areas (particularly those where market gardening is common).
Plantation forestry
The pressure on fresh water from plantation forestry is comparatively low. Nutrient yields from plantation forestry are very similar to those from native forest. However, when forest is being harvested the sediment dispersed to waterways, particularly from roadways and landings, can increase.
Want to know more?
About the proportion of land uses in New Zealand.
This information has come from the latest state of the environment report, Environment New Zealand 2007.
Last updated: December 2007
