Draining the Landscape: How Do Nitrogen Concentrations in Riparian Groundwater and Stream Water Change Following Milldam Removal?
Draining the Landscape: How Do Nitrogen Concentrations in Riparian Groundwater and Stream Water Change Following Milldam Removal?
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DOI：
10.1029/2021jg006444
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发表时间：
2021-07
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通讯作者：
Evan Lewis;S. Inamdar;A. Gold;K. Addy;T. Trammell;D. Merritts;M. Peipoch;P. Groffman;Johanna Hripto;M. Sherman;J. Kan;R. Walter;E. Peck
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作者：
Evan Lewis;S. Inamdar;A. Gold;K. Addy;T. Trammell;D. Merritts;M. Peipoch;P. Groffman;Johanna Hripto;M. Sherman;J. Kan;R. Walter;E. Peck
美国各地拆除大坝的数量正在增加，宾夕法尼亚州目前是全美拆除大坝数量最多的州。虽然大多数大坝拆除是出于水生栖息地和公共安全考虑，但我们对大坝拆除如何影响水质和河岸带过程知之甚少。大坝的拆除降低了河流的基准面，这导致了河岸地带的脱水。我们假设，这种河岸带的脱水会增加硝化作用，减少反硝化作用，从而导致河岸带的氮(N)泄漏。这一假设在1.5米高的水坝拆除中得到了验证。河流、土壤水和地下水氮浓度监测超过2年。测定了土壤氮浓度、过程速率和δ15N值。河岸沉积物的反硝化速率和土壤δ15N值下降支持了我们的假设，但硝化作用没有显著变化。虽然表层土壤水硝酸盐- N浓度很高（中位数为4.5 mg N L−1），但河岸地下水硝酸盐- N值很低（中位数为0.09 mg N L−1），表明硝酸盐- N泄漏最小。我们将低地下水硝酸盐- N归因于较低、更动态的地下水界面的反硝化损失和/或异化硝酸盐还原为铵（DNRA）。溪流中的硝酸盐- N浓度很高（中位数为7.6 mg N L−1），与我们的大坝拆除假设相反，显示出归因于区域水文变化的流域范围内的下降。这项研究首次揭示了大坝拆除如何影响河岸带氮循环过程及其对水质和流域管理的影响。
Dam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over 2 years. Soil N concentrations and process rates and δ15N values were also determined. Denitrification rates and soil δ15N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate‐N concentrations were high (median 4.5 mg N L−1), riparian groundwater nitrate‐N values were low (median 0.09 mg N L−1), indicating that nitrate‐N leakage was minimal. We attribute the low groundwater nitrate‐N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate‐N concentrations were high (median 7.6 mg N L−1) and contrary to our dam‐removal hypothesis displayed a watershed‐wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management.