Real-time groundwater monitoring of managed recharge dispersion of groundwater at Hinds, Canterbury

Abstract

The Hinds Plains is underlain by a glacial outwash aquifer that has been subject to nitrate-nitrogen accumulation under intensive agriculture for at least the last 40 years. Canterbury Water and a number of associated parties are in the process of undertaking a pilot trial of managed aquifer recharge by applying up to 500 litres per second of very low nitrate content race water to an infiltration basin. The author intends to provide characterisation of the displacement and dispersion of native, high nitrate groundwater in the shallow water bearing layers down-gradient of the infiltration basin. The means of characterisation utilizes locally developed optical nitrate sensors suspended in the slotted interval of a series of monitoring bores immediately beneath the pre-trial water table. The initial time-series of high frequency nitrate measurements is capable of batch telemetry to the investigation base offices, allowing superior data-validation and capture.

Having a network of continuous and periodically sampled bores, the investigation was able to assess the effect on down-gradient groundwater nitrate concentration in the lateral and vertical planes. The influence of open framework gravels will be examined as they affect differential passage of the low nitrate concentration peak following the onset of injection. Continuous nitrate monitoring also serves to support the assessments of environmental impacts of the pilot project in the wider Hinds Plains, including domestic water bores and spring-fed creeks or drains.