Mass mixing approach for assessing effects of irrigation on water quality.

Predicting the magnitude of any increase in nutrient loads on surface waterand groundwater resources under future irrigation scenarios is a complex matter that necessarily relies on modelling approaches to simplify and approximate the key elements of the catchment(s). Our water quality assessment for the proposed Hunter Downs Irrigation Scheme (HDIS) in New Zealand utilised a mass mixing modelto predict nitrate concentrations within groundwater (and surface water) individual catchments. This approach was selected due to the hydrogeologyof the scheme area, which is characterised by poorly draining downland hill country intersected by permeable alluvial gravel aquifers which are interconnected with intermittent or perennial streams. This approach requires a robust understanding of the current hydrology, hydrogeology, existing and proposed landuse distribution and associated nutrient concentrations in soil drainage water. Our modelling was calibrated by comparing the output of the existing landuse simulations with the available current day groundwater and surface waterquality data, providing further confidence in the modelling output. The approach also allowed the effects of different landuse scenarios and management processes to be considered. The modelled effects on water quality varied across the scheme area due to differences in existing landuse and interactions between groundwater and surface waterthat affected the dilution of irrigation drainage. The predicted changes in nutrient concentrations for each catchment were used to assess the risk of increased algae blooms in rivers and coastal lagoon that is of significant ecological and cultural value to the local indigenous Maori people. In this paper we show how the use of a mass mixing modelhas provided a realistic basis for quantifying the potential adverse effects of irrigation on groundwater and surface waterquality, and evaluating the potential effectiveness of mitigation measures such as nitrification inhibitors.