Nitrate concentrations in many New Zealand groundwater systems continue to increase, as a response to land-use intensification. Concerns over degrading water quality have prompted changes to freshwater management and raised interest in nitrate mitigation tools that includes woodchip denitrification walls to target removal of nitrate in shallow groundwater. While denitrification walls are a tried and tested concept in sandy aquifers (e.g. Schipper et al., 2000; Schmidt and Clark, 2012), there are no reported examples of them having being applied in alluvial gravel aquifers, such as constitute the most important aquifer systems in New Zealand.
We are piloting a denitrification wall in a shallow alluvial gravel aquifer impacted with nitrate. The project aims to assess whether such passive, in situ groundwater remediation systems are a viable nitrate-mitigation option for New Zealand farmers. The permeable wall, constructed using a 50/50 (v/v) woodchip/gravel mix, measuring 25 m long x 5 m wide, and entrenched 2.5 m below the water table, was built in November 2018. The changes the remediation system has had on groundwater chemistry, groundwater ecology and hydraulic function of the shallow gravel aquifer are being monitored. Electrical resistivity tomographic methods have been used to acquire knowledge of the subsurface conditions and to inform design of the wall. Also to evaluate the treatment performance of the wall. Results to date show the woodchip denitrification wall is proving effective at reducing groundwater nitrate in the shallow aquifer. Notable pollution-swapping phenomena that have been quantified include arsenic mobilisation from the aquifer sediments, and greenhouse gas production. Our study demonstrates first-hand the technical challenges of installing a denitrification wall in an alluvial gravel aquifer, such as addressing the issue of complex heterogeneity, and associated preferential flow phenomena.
References
Schmidt, C.A. and Clark, M.W. (2012). Evaluation of a denitrification wall to reduce surface water nitrogen loads. J. Environ. Qual. 41:724–731
Schipper, L., and Vojvodic-Vukovic, M. (1998). Nitrate removal from groundwater using a denitrification wall amended with sawdust; field trial. J. Environ. Qual. 27:664–668