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Runoff and leaching

It’s not just factories and sewage treatment plants that affect water quality in the Waikato River. Studies show that many of the contaminants in the river come from rural and urban runoff, which are described as non-point source discharges.

Stormwater discharge

On this page: Non-point sources of pollution, Urban runoff, Rural runoff and leaching, Reducing rural runoff and leaching, How you can reduce contaminated runoff to rivers

Non-point sources of pollution


Non-point source discharges are sources of contaminated water that don’t have a single identifiable source or specific outlet. For example, nutrients and sediment from farm land are washed into waterways by rainfall runoff or leach through the soil into ground water.


Change in influence on water quality since 1955

Ground water contributes much of the water to many of the streams that drain into the Waikato River, so its quality affects the river. In urban areas, debris and contaminants from streets and industrial areas are washed away by rainfall runoff into stormwater drains, which often empty straight into waterways.

The pictograph shows that while point sources such as factories were once the main source of contaminants in rivers, today non-point sources have the most influence on water quality.

Waste water treatment has improved considerably over the past two decades, so that many waste water discharges are now treated to a high standard. Runoff and ground water flow to rivers, however, are difficult to isolate, let alone to treat and control.

Urban runoff

Stormwater from towns and cities contains debris from urban and industrial areas, including substances such as:

  • oil and other hydrocarbons from roads
  • heavy metals
  • chemicals
  • sediment.

Most stormwater drains empty directly into waterways, including the Waikato River and its tributaries.

City and district councils are responsible for most stormwater drains and are working with Waikato Regional Council to minimise environmental effects. It is important for everyone to be aware that stormwater drains empty into waterways and to be careful what we wash into them.

Sources of nitrogen in the lower Waikato River during summer

Rural runoff and leaching

Rural runoff and leaching (non-point source discharges) are the main sources of the nutrient, nitrogen, in the lower Waikato River during the summer. Around 25 per cent of the non-point source nitrogen (see pie graph) is estimated to be natural. The rest probably comes from pasture – mostly from cow urine, which leaches into ground water and eventually flows into the river. Nitrogen promotes the growth of nuisance plants and algal blooms in the river, especially during summer.

Studies show that most of the bacteria (a health risk indicator) found in the lower Waikato River are from non-point source discharges. View a pictograph of e.coli bacteria levels in the river.

The three major point source discharges in this part of the river (Hamilton sewage, a meat-works and Ngaruawahia sewage) only generate five per cent of the total bacteria levels. Scientists believe the remaining bacteria in the river come from sources like rural and urban runoff. Only a small amount of all animal waste deposited in paddocks would be needed to account for the remaining bacteria.

Farming is probably the main non-point source of contaminants to the Waikato River, followed by cities and towns. Scientists estimate that the waste generated by the 3,000 dairy herds in the Waikato River catchment is equal to the waste from about five million people or nearly 50 cities the size of Hamilton. Increasing herd sizes are likely to result in higher amounts of nutrients and bacteria entering waterways through runoff and leaching.

The use of fertiliser and agrichemicals and spreading wastes onto land can also contaminate runoff, so these activities need to be carried out with care.

Reducing rural runoff and leaching

Sediment rich tributaries empty into the Waikato River

Planting of riverbank margins or riparian strips can be an effective way to reduce the impact of runoff on waterways.

  • Planted strips act as buffers, soaking up runoff before it reaches the river and filtering out nutrients such as nitrogen.
  • Planting also helps stabilise riverbanks, preventing erosion and therefore sediment runoff to the river.
  • Riverbank plants also shade the water and keep its temperature lower, which makes a better habitat for the plants and animals living in the river.

How you can reduce contaminated runoff to rivers

  • Never tip hazardous substances down stormwater drains, such as paint washings, used oil, harsh detergents or chemicals.
  • Install stormwater detention areas in new subdivisions to improve water quality.
  • Wash your car on the grass so the water soaks into the ground.
  • Form a StreamCare group with your neighbours to discuss and implement better land management practices to protect your local river or stream.
  • Establish and maintain vegetation on riverbanks to stabilise banks and trap contaminants before runoff reaches waterways.
  • Prevent stock access to riverbanks.
  • Protect wet areas, seeps and bogs from stock, to allow them to filter contaminants from runoff before it reaches waterways.
  • Use fertilisers and chemicals responsibly. Check out the New Zealand Fertiliser Manufacturer’s Research Association Code of Practice (Code of Practice) for fertiliser use, especially when close to waterways.
  • Make sure dairyshed effluent irrigators are operating effectively and are moved frequently to prevent ponding and runoff.
  • Avoid break-feeding or mob-stocking close to riverbanks, especially in wet weather.