August 18, 2006

Nitrate levels in well water on the rise due to recent wet weather

Recent measurements show that nitrate nitrogen concentrations in groundwater from many wells across Canterbury have increased over the winter, say Environment Canterbury scientists. Concentrations in some wells are exceeding Ministry of Health drinking water standards for the first time in several years.

“The increases are not unexpected,” says Carl Hanson, ECan groundwater quality scientist. “Nitrate concentrations in groundwater are commonly higher over winter when there is a lot of water that drains through the soil. Given the currently wet winter, the peaks this year are higher than they have for the past few years.”

High concentrations of nitrate in drinking water can pose a health risk for certain people. For this reason the Ministry of Health has set a “Maximum Acceptable Level” (MAV) of 11.3milligrams per litre for nitrate-nitrogen.

The greatest risk is to bottle-fed babies less than six months old and to babies in the womb (through the food and water their mothers consume). Some adults with specific, rare metabolic disorders (deficiency of glucose-6-phosphate dehydrogenase or methaemoglobin reductase) may also be at risk.

Common household filters, boiling water and chlorine treatments will not remove nitrate from water, says Carl Hanson. “There are some treatment systems, such as reverse osmosis and ion exchange, that can remove nitrate from water.”

Common sources of nitrates include fertiliser application, cultivation, grazing animals and the discharge of nitrate-rich wastewater. “Good land-use practices such as nutrient budgeting ensure chemical fertilisers and effluent are applied in optimal quantities and only when soil-testing shows it is necessary. This helps to reduce the amount of nitrate reaching both ground and surface water,” Mr Hanson says.

As well as the risk of increased nitrate leaching, the wet conditions may also increase the risk of water-borne bacteria reaching shallow wells. This may affect anyone who drinks the water and result in symptoms similar to food poisoning.

“This recent data provides a reminder that people who rely on private wells for their drinking water need to be vigilant,” says Mr Hanson. “Environment Canterbury strongly recommends that people test their well water regularly for nitrate and faecal bacteria , and that they don’t drink the water until testing indicates it is safe.”

For more information on nitrates in drinking water: See “What’s New” on the ECan web front page, for maps etc – www.ecan.govt.nz or http://www.ecan.govt.nz/Our+Environment/Water/Groundwater/Nitrates/
or request a factsheet from ECan customer services, (03) 353 9007 or 0800 EC INFO. For ECan laboratory information: http://www.ecan.govt.nz/Our+Environment/Water/Laboratory/lab-cost-list.htm

ENDS

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