Nitrogen species distribution in the Taieri Plain aquifer, New Zealand

Abstract

Nitrogen contamination of drinking water has been linked to health risks including ‘blue baby syndrome’. In New Zealand, up to 51% of people use at least some groundwater for drinking, and there is growing concern that the rapid growth of intensive dairying may lead to aquifer contamination. The distribution of nitrogen species nitrate (NO3-), ammonium (NH4+), nitrite (NO2-) and organic nitrogen were measured in groundwater bores in the Taieri Plain aquifer, New Zealand, during March-November 2002. There was significant spatial variation in the concentration of nitrate and ammonium across the aquifer. Concentrations of nitrate were low in the confined aquifer in West Taieri, but high in unconfined bores in East Taieri and in bores in alluvial fans, i.e. where infiltration of surface runoff can occur. Ammonium showed the reverse pattern, i.e. low concentrations in the unconfined aquifer, and high concentrations in parts of the confined aquifer. Nitrite concentrations were low everywhere. Mean nitrate concentrations for confined and unconfined bores were well below the maximum limit for drinking water in New Zealand (11.3 ppm NO3-N), but in the unconfined zone some individual measurements approached or slightly exceeded this limit. Some individual measurements for ammonium species exceeded the aesthetic guideline for drinking water (1.2 ppm NH4-N). No temporal variations were seen in species concentrations over the relatively short time scale of the study, and there was little correlation between concentrations and bore depth. Fortuitously, the Waihola silt-sand unit protects the underlying aquifer and mitigates the potential for contamination in the areas where intensive dairying occurs on the Taieri Plain. The unconfined/semiconfined aquifer in East Taieri, and alluvial fan areas, are far more vulnerable to contamination from nitrogen species. Ideally, conditions and land use practices in these areas should be closely monitored and, perhaps, regulated.