Data from: Predicting nitrate retention at the groundwater- surface water interface in sandplain streams

Groundwater-surface water ecotones present opportunities for nitrate retention because changes in organic carbon availability, redox potential and nitrate demand often occur at these locations. Although there have been many measurements of nitrate retention and denitrification at the groundwater-surface water interface few investigators have quantitatively predicted where nitrogen transformation occurs in this zone. Our main objective was to describe and predict spatial variation in nitrate retention and removal in shallow groundwater among and within streams in the central sand plains of Wisconsin. We predicted that nitrate transformation rates would be positively related to nitrate and particulate organic carbon availability, a negative nonlinear function of dissolved oxygen availability and would peak at intermediate rates of groundwater discharge. Five sites on 4 streams were chosen that spanned an order of magnitude in groundwater nitrate concentration. Nitrate retention and N2 production were quantified by determining how nitrate and N2 fluxes changed along nominal 60 cm groundwater flow paths. Nitrogen transformation was widespread but highly variable within the study sites. Partial least squares regression models explained a much larger amount of variation in nitrate retention and N2 production at the local scale than at the regional scale. Dissolved oxygen (DO) availability and ground water discharge were the most consistent predictors of nitrate retention at both scales. We conclude that nitrate retention at the groundwater-surface water interface can be predicted but that more research is necessary to determine if nitrate transformation at this ecotone can be modeled at broader scales.

地下水-地表水交错带（groundwater-surface water ecotones）为硝酸盐滞留提供了契机，因该区域往往伴随有机碳可利用性、氧化还原电位与硝酸盐需求的动态变化。尽管学界已针对地下水-地表水界面的硝酸盐滞留与反硝化作用开展了大量观测，但鲜有研究者对该区域内氮转化发生的具体位置进行定量预测。本研究旨在描述并预测威斯康星州中部砂质平原各溪流间及溪流内部浅层地下水中硝酸盐滞留与移除过程的空间变异特征。我们假设，氮转化速率与硝酸盐及颗粒态有机碳的可利用性呈正相关关系，与溶解氧（dissolved oxygen, DO）可利用性呈负相关的非线性函数关系，并会在中等地下水排泄量条件下达到峰值。研究选取了4条溪流上的5个采样点，其地下水硝酸盐浓度跨度达一个数量级。通过测定沿标称60 cm地下水径流路径的硝酸盐与氮气通量变化，本研究量化了硝酸盐滞留与氮气产生量。研究区域内氮转化现象普遍存在，但空间异质性极强。相较于区域尺度，偏最小二乘回归（partial least squares regression）模型在局地尺度上可解释更多的硝酸盐滞留与氮气产生量的变异信息。在两个研究尺度下，溶解氧（DO）可利用性与地下水排泄量均为预测硝酸盐滞留最稳定的自变量。本研究表明，地下水-地表水界面的硝酸盐滞留过程可被预测，但仍需开展更多研究以明确该交错带内的氮转化过程是否可在更大尺度上进行建模。