Nitrate in groundwater

Nitrate is an essential plant nutrient and a natural constituent of any soil. It is an important source of nitrogen for plant growth.

However, if the soil contains more nitrate than plants can use, then the excess nitrate can be leached from the soil and contaminate groundwater.

Most nitrate leaching occurs over the winter months, when plant growth is slow, soils are wet and rainfall is plentiful. As a result, nitrate concentrations in groundwater are highest in the winter and springtime.

In the summer and autumn, plants are growing and taking nitrate and water from the soil, so nitrate leaching rates are lower. As a result, nitrate concentrations in groundwater generally decrease in summer and autumn.

For maps of Environment Canterbury's nitrate-monitoring stations and the latest nitrate measurements, see below.

Nitrate concentrations in drinking water and groundwater

High concentrations of nitrate in drinking water can pose a health risk for certain people, particularly bottle-fed babies who drink formula made with the water. For this reason the Ministry of Health has set a “Maximum Acceptable Level” (MAV) equal to 11.3 mg/L [milligrams per litre] for nitrate-nitrogen (see table below for an explanation of units). More information on the health risks from nitrate in drinking water.

The Ministry of Health has set the MAV at 50 mg/L for the concentration of the nitrate ion. Environment Canterbury records these concentrations in units of "nitrate nitrogen", in other words, we only record the nitrogen portion of the nitrate ion. A concentration of 50 mg/L nitrate ion is equal to a concentration of 11.3 mg/L of nitrate nitrogen.

Across most parts of Canterbury where groundwater is widely used, nitrate concentrations in groundwater vary within the general range of about 5 to 10 mg/L. In some wells and at some times, the concentrations in the groundwater exceed the MAV of 11.3 mg/L. In some wells, the concentrations exceed the MAV at all times, whereas in some other wells, the exceedences only occur during part of the year. In many wells, the concentrations have never exceeded the MAV.

Why do the concentrations vary so much? The nitrogen system in soil is complex, and there are many factors that affect the amount of nitrogen that is leached from the soil, including:

•     Rates and timing of fertiliser applications
•     Rates, timing and methods of irrigation
•     Cultivation practices
•     Wastewater disposal
•     Weather.

We don't fully understand how all of these factors affect groundwater quality. Environment Canterbury is supporting research into the effects of land use change on groundwater quality, and into ways that farmers and land managers can decrease nitrate leaching losses from their land.

Groundwater nitrate monitoring

There are currently 97 wells in Environment Canterbury's quarterly nitrate monitoring programme. These wells are located throughout the region in areas where groundwater is widely used.

Map of the well locations (pdf 64kB)

Most of the wells are relatively shallow; 72 of them are less than 30 metres deep. Nitrate concentrations are generally highest in shallow wells, although we have measured concentrations above the MAV in wells as deep as 50 metres.

Nitrate concentrations have exceeded the MAV in groundwater from many of these wells at some time in the past.

Map of the maximum concentrations measured in the wells between 1996 and 2006 (pdf 157 kB)

For the 2006 winter, results are available for 73 of the programme wells (results are not yet available for the Ashwick Flat and Levels Plain areas). Since the autumn sampling round, nitrate concentrations have increased in 50 of the wells, compared to 23 wells where concentrations have decreased or remained the same. The increases are scattered across the region.

Map of the programme wells that have shown an increase in nitrate concentrations over the winter (pdf 102 kB)

The concentrations in the samples from 18 of the programme wells have exceeded the MAV this winter.

Map showing the wells where nitrate concentrations have exceed the MAV this winter (pdf 149.23 kB)

Again, these wells are scattered across the region.

The 97 wells in the quarterly monitoring programme represent only a small fraction of all wells used for drinking-water supply in Canterbury. For any well not in the programme, it is impossible to predict whether the nitrate concentration has increased or decreased, or whether it exceeds the MAV.

The recent data provides a reminder that people who rely on private wells for their drinking water need to be vigilant.  Environment Canterbury strongly recommends that people test their well water regularly for nitrate, and that if they fall into one of the at-risk groups for nitrate, that they don’t drink the water until testing indicates it is safe.