Groundwater denitrification - can we afford to ignore it?

Abstract

New Zealand’s freshwater management system is undergoing a comprehensive process of reform to establish a limits-based approach to addressing the effects of diffuse nutrient pollution from intensive agricultural land uses. As a result, farmers in many areas are likely to be farming to nutrient limits in future that will require them to modify existing management practices, and which will limit development options. Currently, the estimation of nutrient losses from farmland is most commonly undertaken using the OVERSEER model, which estimates nutrient losses only from the root zone, without accounting for biogeochemical and physical processes that determine the extent to which the groundwater in the catchment assimilates nitrogen. To demonstrate the importance of accounting for groundwater denitrification, the potential economic impacts of failing to do so when estimating nutrient losses from farming in the area of the proposed Ruataniwha Water Storage Scheme were investigated. Estimation of nitrate leaching from farms on the basis of root zone losses alone was found to impose unnecessarily high mitigation costs on many properties in the catchment under proposed nitrate limits. Nitrate loss estimates for the whole catchment were almost halved by accounting for groundwater denitrification, and the distribution of losses by area significantly altered.