Role of water table dynamics on stream nitrate export and concentration in agricultural headwater catchment (France)
Role of water table dynamics on stream nitrate export and concentration in agricultural headwater catchment (France)
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DOI：
10.1016/j.jhydrol.2007.10.005
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发表时间：
2008-01
影响因子：
6.4
通讯作者：
J. Molénat;C. Gascuel-Odoux;L. Ruiz;G. Gruau
中科院分区：
文献类型：
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作者：
J. Molénat;C. Gascuel-Odoux;L. Ruiz;G. Gruau
这项工作的目的是确定水文过程控制硝酸盐出口和基流浓度在一年的规模在农业源水集水区流喂养的潜水含水层。这项研究是基于水文和水化学监测的溪流和浅层地下水的三个源头集水区（0.1- 5平方公里）在法国西部超过三至五个水年。结果表明，在年尺度上，三个流域硝态氮的输出是一个输运受限的过程。溪流硝酸盐通量取决于溪流中有多少水流动，而不是流量在一年中的分布。在水年浓度的季节变化是复杂的。变化是不同的，为一个给定的一年从一个集水区到另一个，也不同的一个给定的集水区从一年到另一个。我们发现，季节性变化是由地下水位深度动态沿着山坡与空间分布的地下水中的硝酸盐浓度。地下水中硝酸盐含量较高，在6 ~ 22 mgN-NO3-/L之间，主要分布在高地和洼地的深层。这种高浓度的持久性是由于含水层的地球化学和矿物学特性造成的，含水层由古老的、高度氧化的和强烈风化的物质组成，因此任何初级电子供体都已被沥滤。在河岸带，由于有机物氧化的反硝化作用，浓度接近于零。在冬季，溪流硝酸盐浓度受来自高地的富含硝酸盐的地下水流的控制。在夏季，来自高地的地下水流量减少，河流浓度受底部陆地水文和地球化学过程控制。冬季和夏季控制之间的转换取决于沿着山坡的地下水位动态。只要高地的地下水位仍然很深，夏季控制就占上风。一旦地下水位上升，在高地，水力梯度和地下水流量从这个区域增加，导致流硝酸盐浓度增加。从夏季到冬季控制的转变可以被认为是高地硝酸盐丰富的地下水和河流之间的连接的结果，连接由高地地下水位上升触发。
The objective of this work is to identify hydrological processes controlling nitrate export and base flow concentration at the year scale in agricultural headwater catchment streams fed by an unconfined aquifer. The study is based on the hydrological and hydrochemical monitoring of the stream and shallow groundwater of three headwater catchments (0.1–5km2) in Western France over three to five water years. Results show that at the year scale nitrate export from the three catchments is a transport-limited process. The stream nitrate flux depends on how much water flows in the stream and not on the distribution of the flow over the year. Seasonal variations of concentration over the water year were complex. Variations were different for a given year from one catchment to another, and also different for a given catchment from one year to another. We show that the seasonal variations are controlled by water table depth dynamics along hillslope associated with spatially distributed nitrate concentration in the groundwater. The groundwater displays high nitrate concentrations, from 6 to 22mgN–NO3-/L, in upland and in the deeper zones of bottom lands. Persistence of such high concentrations results from the geochemical and mineralogical properties of the aquifers that consist of old, very oxidised and strongly weathered material, such that any primary electron donors have been leached. In riparian zones, concentrations are close to zero due to denitrification with oxidation of organic matter. In winter, stream nitrate concentration is controlled by the nitrate rich groundwater flow from upland. In summer, groundwater flow from upland decreases and stream concentration is controlled by bottom land hydrological and biogeochemical processes. The shift between winter and summer control depends on the water table dynamics along hillslopes. As long as the water table remains deep in upland, summer controls prevail. As soon as water table rises in upland, hydraulic gradient and groundwater flow from this zone increase, leading to an increase in the stream nitrate concentration. The shift from summer to winter control can be considered as the result of a connection between the upland nitrate rich groundwater and the stream, connection triggered by upland water table rise.