Groundwater contaminants

Dieldrin was regularly used in sheep dips for parasite control during the 1950-1960s.  Although it has not been used for over 30 years, dieldrin is still contaminating groundwater decades later. This highlights the persistence and the long-lasting nature of this pollutant.

It is estimated that there are several thousand old dieldrin dip sites in the region. Unfortunately many old sheep dip sites are located beside farm water supply wells.  Find out more about these sites and what we've been doing about it.

Excessive nitrate and iron are the most common groundwater contaminants in the region.

Iron occurs naturally, but high nitrate levels indicate contamination from human activities. Nitrate levels are generally highest in shallow groundwater aquifers.

Nitrate contamination of groundwater is a concern for human health and the environment. High nitrate concentrations have been linked to a blood disorder in bottle-fed babies, known as ‘blue baby’ syndrome (or methaemoglobinaemia).

The effect of contaminated groundwater on our rivers and streams is also important. In the Hamilton Basin, up to 85 percent of the base flow of small streams comes from groundwater. Much of this water is already enriched with nutrients including nitrate.

Nitrate and land use

Excessive nitrate concentrations in groundwater are related to land use activities, including:

• pastoral farming
• market gardening
• application of nitrogenous fertilisers
• industrial and sewage waste disposal (for example, septic tanks).

Studies have shown that high use of nitrogen fertilisers on pasture can make the groundwater unsuitable for human consumption.

Where nitrate is a problem

Levels of nitrate in groundwater vary a lot across the region. Areas where nitrate levels most commonly exceed drinking water guidelines are:

• Hamilton-Mangaouna – an area of intense farming.
• Pukekohe - an area of mainly market gardening.

We have few records to indicate long-term nitrate trends. But long-term information from some schools supplies (since 1950s) indicates a steady increase in nitrate at these sites.

It is important to note that for healthy ecosystems nitrate concentrations need to be much lower than the drinking water guideline, possibly as low as 13 g/m³ (3 g m^-3 as nitrate –N).¹

You can help

Remember that it is much easier to prevent groundwater contamination than to fix it.

Look after your well

• Prevent contaminants from entering your well by sealing the top - for example with a concrete pad around the casing.
• Fence around your well or bore to keep animals away.
• Don’t store, mix or use chemicals (such as fertilisers) too close to a well or bore.

Be careful with nitrogen fertiliser

• Avoid excessive nitrogen fertiliser use. Use nitrogen fertiliser on pasture to supplement nitrogen produced by clover - not to replace it.
• Use nutrient budgets to work out a sale level of nitrogen fertiliser use.
• Avoid heavy use of nitrogen fertiliser during winter when there is higher rainfall and slower pasture growth, and a greater likelihood of leaching to groundwater.
• Apply nitrogen fertiliser in split dressings, rather than all at once, so pasture can use it for growth (reduces the amount lost to leaching and is more cost effective).

Growing commercial vegetables

Reduce the amount of nitrogen leaching when growing vegetables commercially:

• Break the application of nitrogen fertiliser for commercial vegetable growing into several smaller applications over the growing season (tailored to the crop development stage) - reducing leaching while maximising yield.
• During non-cropping times, cover crops such as oats and mustard can be cultivated and ploughed into the soil. This adds organic matter and traps unused nitrogen in the soil, making it available for the next crop.

Remember that most Waikato soils are over-fertilised. You should also follow the guidelines on pesticide use in Growsafe, the New Zealand Standard for Agrichemical Users' Code of Practice.

More information

• Footnote 1: Drinking water standard for nitrate is 50 g/m³ (11.3 g m^-3 as nitrate-N).
  From: Ministry of Health. 2000: Drinking Water Standards for New Zealand. Ministry of Health, Wellington.

Pesticides are contaminating some of our region’s groundwater supplies. Pesticides detected are usually at low concentrations, well below drinking water guidelines. The types of pesticide found reflect local land uses, both past and present. Careful pesticide selection and management is required to avoid contaminating groundwater.

A growing problem

Pesticide use in New Zealand has grown significantly in recent years. A total of 669 pesticides containing 302 different active ingredients were registered for use in New Zealand in 1995. The same year about 4,000 tonnes of pesticides were used across the country.

Large quantities of pesticides (including herbicides, insecticides and fungicides) are used in the Waikato region. From 1985 to 1987 about 500 tonnes of pesticide were used in the Waikato region annually.

Pesticide contamination of soils and waterways affects:

• our health
• our environment
• access to overseas markets (many of our international markets want to know their food is produced in an environmentally responsible way).

Pesticides on the move

Where pesticides end up in our environment depends on many factors:

• the pesticide’s chemical properties
• the type of soil (for example, the amount of organic matter)
• the local climate (sunlight and rainfall)
• how people use pesticides (when, where, how often).

Most pesticides break down at the ground surface or in shallow soil. However, some are reaching and contaminating our groundwater. Pesticides most likely to contaminate groundwater:

• last a long time - they don’t break down very easily
• dissolve easily in water
• travel easily through soil.

Shallow aquifers with only limited protection from soil are most vulnerable to contamination.

Contamination sources

Most pesticide contamination of groundwater is a result of poor management practices. Contamination may result from:

• excessive pesticide use
• storing or mixing pesticides too close to a well
• chemical use on neighbouring properties
• pesticides used many years ago that have not broken down (persistent).

Aquifers located under land used for cultivation are more likely susceptible to be contaminated. Cultivated soils often have:

• high levels of pesticides
• less organic matter to slow down the movement of pesticides to groundwater
• less bacteria present to break down pesticides.

What we have found

From 1996 to 1999 Waikato Regional Council studied the extent of pesticide contamination in the region’s groundwater. Samples were taken from 35 wells in two areas where pesticides are regularly used:

• Pukekohe/Pukekawa
• Hamilton Basin/southern Hauraki Plains.

The study focused on shallow aquifers lying under free draining soil, which are vulnerable to pesticide contamination, and where pesticides are in common use. This meant that we were studying a ‘worst case’ scenario.

Our study found pesticide residues in nearly three-quarters of the 35 wells sampled. A total of 20 different compounds were detected. The most common were:

• atrazine
• alachlor
• diuron
• procymidone.

Residue concentrations were generally well below the maximum accepted values (MAV) set for drinking water in New Zealand and Australia. A notable exception was dieldrin, which was much higher than the MAV.

Our study showed only a small number of all pesticides used in the region were actually found in the groundwater. This suggests that most, particularly modern, pesticides do not pass through the soil into the aquifers below.

What's being done

Waikato Regional Council is working with Crown Research Institutes, looking at how pesticides travel through different soil types and the likelihood they will contaminate groundwater supplies.

Many industry groups are working to reduce pesticide use. Some apple and kiwifruit growers are monitoring pest numbers in their orchards and only using pesticides when pest numbers increase past a certain number. Reduced pesticide use saves money and is better for the environment.

For policy information on pesticides check out our Regional Plan and our Regional Policy Statement.

How you can help

Remember, it is much easier to prevent groundwater contamination than to fix it later!

• Don’t risk contaminating your water supply by storing, mixing or using chemicals too close to a well or bore.
• Make sure your well is sealed (for example with a concrete pad around the casing) to prevent contaminants entering into your aquifer.
• Always follow the recommended application rates and methods, and storage and disposal practices. Efficient pesticide use avoids waste and can save money.

Guidelines on pesticide use are contained in Growsafe, the New Zealand Standard for Agrichemical Users’ Code of Practice.

Microbial contamination of groundwater can happen when wells, septic tanks and offal holes are poorly constructed. Small communities using septic tanks are especially vulnerable to microbial contamination of their drinking water. Community groundwater supplies are routinely monitored for drinking water standard compliance and are generally free of contamination.

The most common health risk associated with drinking water is microbial contamination from disease-causing micro-organisms, such as bacteria, viruses and protozoa.

Gut bacteria from warm-blooded animals (E-coli) pose a threat to human health. Drinking water guidelines state that there should be no bacteria (E-coli) present in drinking water¹. E-coli are measured in drinking water as indicators of microbial contamination.

Groundwater is better protected from the risk of microbial contamination than surface water because of the soil which acts as a barrier above it. However, direct contamination can occur from:

• poorly constructed groundwater wells
• septic tanks
• offal holes.

Localised microbial contamination of groundwater is a problem in many coastal areas where small communities rely on septic tanks for sewage disposal. On the Coromandel Peninsula, monitoring at Cooks Beach in 1990 showed about 40 percent of 17 wells sampled contained faecal bacteria. In 1997, monitoring showed about 10 percent of 19 wells sampled at Whangapoua and just over 50 percent of 15 wells sampled at Te Puru contained faecal bacteria.

At present there is little information on the extent of microbial contamination in individual rural groundwater supplies. However, a study of 40 wells in Matangi (near Hamilton) found 5 (12.5 percent) were contaminated with faecal coliforms.

Routinely monitored community groundwater supplies are generally free of faecal contamination.

What you can do

• Fix any leaking septic tanks or pipes.
• Seal the top of your well (for example with a concrete pad around the casing).
• Fence around your well or bore to keep animals away.
• Don’t drill offal holes closer than one metre above the water table.
• Don’t place offal in offal holes with water in the bottom of them.

Check out the national drinking water standards on the Ministry of Health website (online publications).

Footnotes

1. Ministry of Health. 2000: Drinking Water Standards for New Zealand 2000. Ministry of Health, Wellington.