Modelling the movement of leachate from a Marlborough Vineyard through soil to groundwater and re-emerging as spring flow

Abstract

Aims Marlborough District Council as a water regulator needed to know the nitrate leaching rate of vineyard as this was the predominant irrigated crop overlying the important Wairau Aquifer used for water supply and drinking purposes for most Wairau Plain residents. MDC also wanted to know what the likely increase in nitrate concentration in groundwater and downstream aquifer fed springs would be associated with a change from vineyard to dairying or cropping. Method Plant & Food Research measured the leaching rate of irrigated vineyard at Giffords Road overlying the Wairau Aquifer starting in 2012. Leachate was measured using 12 drainage pipes buried several metres below the land surface in the inter-rows and under the grape plants. Quantifying the leaching rate of nutrients from the land surface downwards to the groundwater was the first part of the travel path through the Wairau Plain. A MODFLOW model was built by Lincoln AgriTech to describe the subsequent movement of the nutrients as they were entrained by groundwater flowing horizontally eastwards towards the coast and eventually breaking the surface as part of the belt of naturally occurring Wairau Aquifer fed springs. The flow model was calibrated based on knowledge of aquifer properties and observations of aquifer level at several MDC monitoring wells and Spring Creek flows. Once a good fit was achieved, the MT3D contaminant transport model was then used to superimpose drainage events with known nitrate-nitrogen concentrations from the leachate array, versus values in surface or groundwater within the stream tube. Results Average concentrations of 3 g/m3 nitrate-nitrogen were measured in the drainage lysimeters which is relatively low when compared to similar results from dairy or cropping farmland. The median measured concentrations of nitrate-nitrogen were: 0.067 g/m3 in Wairau River recharge water, 0.26 g/m3 in Wairau Aquifer water downstream of the leachate array at Wratts well 3009, and 0.22 g/m3 in in Spring Creek water at the floodgates at the end of the flow path. The transient MODFLOW/MT3D model was run at a weekly time step and accounted for most of the variation in observed nitrate-nitrogen. The model was then used to predict the likely median nitrate-nitrogen concentrations of Spring Creek water if current vineyard was converted to irrigated pasture or arable crops. The model demonstrated the importance of the diluting effect of groundwater throughflow on land surface contaminants as well as crop type.