Abstract Woodchip denitrification walls are a tried and tested groundwater nitrate remediation concept in shallow sandy aquifer conditions. There are however no published case studies of them having been applied in heterogeneous, fast-flowing gravel aquifers. Such a pilot study is being made in a shallow alluvial gravel aquifer on the Canterbury Plains, New Zealand, as part of an assessment of whether denitrification walls represent a viable edge-of-field nitrate mitigation option for the New Zealand hydrological landscape. Hydrogeological conditions at the field study site were characterised using a suite of investigative methods, the results from which informed design and placement of an experimental woodchip denitrification wall that was installed in November 2018. The average specific flux in the target gravel aquifer is estimated at 2.7 m/d, and 3.1 m/d through the woodchip wall itself, owing to its hydraulic efficiency. These groundwater fluxes are significantly higher than conditions reported for pre-existing denitrification wall case-studies. Monitoring of the groundwater chemistry over the first year of the denitrification woodchip wall's operational life has shown how the woodchip initially leached labile dissolved organic carbon and created a redox plume in which methanogenic conditions existed. Even though dissolved organic carbon concentrations have restored to background levels, the woodchip wall remains effective at nitrate reduction. The measured nitrate removal rate of between 4.2 and 5.4 g N removed/m3 wall/d is higher than what had previously been predicted from controlled lab-scale studies of the wall media and ranks towards the higher end of published removal rates for denitrification walls. Whilst there is direct evidence that heterotrophic denitrification is contributing to the observed nitrate removal, on the basis of chemical indicators, it is assumed other reactive process, such as dissimilatory reduction to ammonia, anammox, and possibly nitrate-reducing Fe(II)-oxidising reduction reactions may also be contributing to the overall removal of nitrogen in the system. Indications are the woodchip wall is enhancing emission of methane gas, albeit at rates less than what is typically reported for constructed wetlands that are an alternative nitrate-remediation option. Emission of the more potent greenhouse gas nitrous oxide from the woodchip denitrification wall has so far been immeasurably low. Longer-term study of the woodchip denitrification wall is continuing.

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木屑反硝化墙技术在浅层冲积砾石含水层试验

摘要 木屑反硝化墙是一种在浅沙含水层条件下久经考验的地下水硝酸盐修复概念。然而，没有已发表的案例研究表明它们已应用于非均质、快速流动的砾石含水层。此类试点研究正在新西兰坎特伯雷平原的浅层冲积砾石含水层中进行，作为评估反硝化墙是否代表新西兰水文景观的可行的田间硝酸盐缓解方案的一部分。现场研究地点的水文地质条件使用一套调查方法进行了表征，其结果为 2018 年 11 月安装的实验性木屑反硝化墙的设计和放置提供了信息。目标砾石含水层中的平均比通量估计为 2.7米/天，由于其水力效率，3.1 m/d 通过木屑墙本身。这些地下水通量显着高于先前存在的反硝化墙案例研究报告的条件。在反硝化木片墙运行寿命的第一年对地下水化学进行监测表明，木片最初是如何浸出不稳定的溶解有机碳并产生氧化还原羽流的，其中存在产甲烷条件。即使溶解的有机碳浓度已恢复到背景水平，木片壁在硝酸盐还原方面仍然有效。测得的硝酸盐去除率在 4.2 和 5 之间。4 g N 去除/m3 壁/天高于先前从壁介质的受控实验室规模研究中预测的值，并且在已公布的反硝化壁去除率中排名靠前。虽然有直接证据表明异养反硝化有助于观察到的硝酸盐去除，但根据化学指标，假设其他反应过程，例如异化还原为氨、厌氧氨氧化和可能的硝酸盐还原 Fe(II)-氧化还原反应也可能有助于系统中氮的整体去除。迹象表明，木屑墙正在增加甲烷气体的排放，尽管其排放速率低于人工湿地的典型报告，后者是替代硝酸盐修复的选择。迄今为止，木屑反硝化墙排放的更强效的温室气体一氧化二氮低得无法估量。木屑反硝化墙的长期研究仍在继续。