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Determining the hydraulic properties of wood/gravel mixtures for use in denitrifying walls

Abstract

New Zealand's alluvial gravel aquifers are particularly vulnerable to nitrate impacts from intensified land-use because they are ineffective at naturally attenuating nitrate. The general limiting factor is a lack of available organic carbon to stimulate nitratereducing microbial-catalysed reactions in the groundwater system. Denitrifying walls are engineered permeable reactive barrier systems designed to overcome this carbon-limitation problem and enhance natural attenuation of nitrate in situ, in shallow groundwater settings. These reactive barriers operate as passive groundwater treatment systems and offer a potential nitrate mitigation measure for managing nitrate impacts, in particular those in New Zealand alluvial gravel aquifers. Successful design of a permeable reactive barrier requires an understanding of the hydraulic and chemical properties of the aquifer target, and the hydraulic properties of the barrier medium, together with some estimate of its capacity to denitrify. A set of permeameter tests were conducted to determine the hydraulic conductivity and porosity of potential reactive barrier fill material consisting of mixtures of wood-chip and gravel. The study used aggregate with a broad particle size distribution, characteristic of alluvial gravel outwash, and which is procurable in bulk during the excavation works for any reactive barrier installed in an alluvial gravel aquifer. The limitations of using conventional mathematical models based on fractional packing concepts to predict the hydraulic properties of wood-chip/gravel mixtures are examined. A wood-chip/gravel mixture with hydraulic characteristics well suited to a New Zealand alluvial gravel aquifer and which can promote denitrification reactions at an effective rate (estimated to be about 3.2-3.6 mg N/L/d) was identified. When combined, wood-chip and gravel follow a largely occupational packing arrangement and hydraulic conductivity was found to correlate negatively with the porosity of the mixture. Because of the complex shape and rough surface characteristics of wood particles, the hydraulic conductivity of the mixed media could not be predicted using a modified version of the Kozeny-Carmen equation. This work has established some useful initial permeable reactive barrier design parameters that are applicable to New Zealand alluvial gravel aquifers. Investigation of how the hydraulic and reactive properties of the barrier media degenerate with time under field conditions would best be achieved through an in situ field trial, and an assessment needs to be made of the civil engineering requirements and economics of constructing denitrifying walls in such a challenging environment.

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