Abstract

Woodchip denitrification walls are an in situ groundwater nitrate remediation concept that has been successfully demonstrated for shallow sandy aquifer systems. Some of the earliest experimentation was conducted here in New Zealand (NZ), in the Waikato region (e.g. Schipper and Vojvodić-Vuković, 1998). We perceive woodchip denitrification walls to be a potentially useful edge-of-field nitrate-mitigation practice for addressing the challenge of farming within catchment nutrient limits. Aquifers composed of outwash gravels represent the most common and important groundwater systems in NZ, particularly on the South Island where there are plentiful examples of them having exceeded their capacity to naturally assimilate nitrate leached from intensive land-use. There are no published cases of woodchip denitrification walls ever having been emplaced in gravel aquifer systems. To address this limitation and assess whether woodchip walls are a viable edge-of-field N-mitigation practice, we are undertaking a pilot study of a woodchip denitrification wall applied in a shallow gravel aquifer setting. The experimental woodchip denitrification wall at Silverstream Reserve, North Canterbury, measures 25 m long x 5 m wide and was built in November 2018. It is entrenched through highly permeable gravel outwash, deposited by the Waimakariri River. Being 3 m deep, it penetrates about 2.5 m below the water table. We estimate that somewhere between 179 and 195 m 3 of groundwater flows through the wall each day, under the natural hydraulic gradient of 0.002. This flux is significantly more than any reported for other woodchip wall studies, hence our wall is ageing faster than other examples and rapid depletion of reactive organic carbon is evident in the time-series water chemistry data we have been collecting. Over its first year, the wall has demonstrated effective nitrate removal of between 93 and 100%, for influent concentrations that have ranged from 7.1 to 8.8 mg NO3-N/L. It is too early to make reliable predictions of the longevity of the denitrification wall, yet our initial calculations made using findings from our field study tend to suggest that within the suite of known N-mitigation practices, woodchip denitrification walls rank as a relatively cost-effective mitigation option.

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