Change in Groundwater Nitrate Concentration in a Large River Floodplain: Denitrification, Uptake, or Mixing?
Change in Groundwater Nitrate Concentration in a Large River Floodplain: Denitrification, Uptake, or Mixing?
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大河流漫滩地下水硝酸盐浓度的变化：反硝化、吸收还是混合？
DOI：
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发表时间：
1998
影响因子：
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通讯作者：
F. Gazelle
中科院分区：
文献类型：
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作者：
G. Pinay;C. Ruffinoni;S. Wondzell;F. Gazelle
我们研究了法国西南部加龙河7级河段不同河岸植被(天然河岸森林、3年生和15年生杨树人工林，以及湿润草甸)对非点源氮污染的缓冲能力。对河漫滩含水层内的51口井和加龙河水中的地下水硝酸盐浓度进行了为期1.5年的月度测量。根据测得的氯化物浓度，使用二端元模型估算了河水和地下水的混合程度。地下水中硝酸盐浓度沿穿过河岸地带的地下水流动路径显著降低。富含硝酸盐的地下水和贫硝酸盐的河水的混合是地下水流动路径上硝酸盐浓度变化的主要原因。水井中河水的比例从冲积阶地的泛滥平原边缘向河流增加，平均80%的河水来自天然河岸森林。然而，观测到的硝酸盐浓度总是小于或等于单独混合时的预期浓度，这表明了生物效应。反硝化速率在所有季节都低于地下水位深度或为0，这表明反硝化作用不是一个重要的硝酸盐汇。硝酸盐浓度的剩余变化可能是植被吸收和土壤微生物固定的结果，而混合不能解释这一变化。事实上，沿地下水流动路径的硝酸盐浓度(70μg N L-1 m-1)的最大降幅是在生长季节的天然河岸森林下测得的。由于混合不是一个保留过程，如果不考虑地下水和河水混合的重要性，大片河流边缘的氮保留可能被高度高估。然而，我们的结果证实，减少非点源污染的努力应该集中在湿地和小溪沿岸植被繁茂的河岸地带。
We examined the buffering capacities of different riparian vegetation (natural riparian forest, 3- and 15-y-old poplar plantations, and a wet meadow) on non-point-source nitrogen pollution along a 7th-order reach of the Garonne River in southwest France. Groundwater nitrate concentration was measured monthly for 1.5 y in 51 wells installed within the aquifer of the river floodplain and in River Garonne water. The mixing of river water and groundwater was estimated using a 2-end-member model based on measured concentrations of chloride. Nitrate concentrations in groundwater decreased significantly along groundwater flow paths crossing the riparian zones. Mixing of nitrate-rich groundwater with nitrate-poor river water accounted for most of the change in nitrate concentration along groundwater flow paths. The fraction of river water in wells increased from the margin of the floodplain with an alluvial terrace (31% river water) to the river; an average of 80% river water occured in the natural riparian forest. However, observed concentrations of nitrate were always less than or equal to the concentration expected from mixing alone, indicating biological effects. Denitrification rates were low or 0 below the depth of the water table in all seasons, suggesting that denitrification was not an important nitrate sink. The remaining change in nitrate concentration not accounted for by mixing was likely the result of vegetation uptake and soil microbial immobilization. Indeed, the largest decrease in nitrate concentration (70 μg N L-1 m-1 of groundwater flow) along a groundwater flow path was measured under the natural riparian forest during the growing season. Because mixing is not a retention process, nitrogen retention in large river margins can be highly over-estimated if the importance of mixing between groundwater and river water is not accounted for. Nonetheless, our results confirm that the effort to reduce non-point-source pollution should be concentrated on wetlands and vegetated riparian zones along small streams.