Characterising groundwater-surface water connectivity in the lower Gandak catchment, a barrage regulated biodiversity hotspot in the mid-Gangetic basin
Characterising groundwater-surface water connectivity in the lower Gandak catchment, a barrage regulated biodiversity hotspot in the mid-Gangetic basin
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DOI：
10.1016/j.jhydrol.2020.125923
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发表时间：
2021-03
影响因子：
6.4
通讯作者：
D. Lapworth;B. O. Dochartaigh;T. Nair;J. O'Keeffe;G. Krishan;A. MacDonald;M. Khan;N. Kelkar;S. Choudhary;J. Krishnaswamy;C. Jackson
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作者：
D. Lapworth;B. O. Dochartaigh;T. Nair;J. O'Keeffe;G. Krishan;A. MacDonald;M. Khan;N. Kelkar;S. Choudhary;J. Krishnaswamy;C. Jackson
印度-恒河盆地（IGB）的冲积含水层系统是世界上最重要的淡水资源之一，维持着人类和河流生态系统。了解地下水的补给过程以及与大气水和地表水的联系，对于有效管理水资源以满足人类和更广泛的生态需求是必要的。与印度西北部地区相比，横跨北方邦东部和比哈尔邦的恒河盆地中部部分地区的特点是地下水位长期稳定，年降雨量高，历史上地下水使用量有限。在本文中，我们使用的环境示踪剂和水文观测相结合的补给源和地下水-地表水的相互作用，采用横断面的方法横跨流域的甘达克河，一个主要的barbal调节的恒河支流。稳定同位素结果表明，浅层（0-40 m bgl）全新世和下伏更新统含水层系统（>40 m bgl）中地下水补给的主要来源是当地降雨。浅层全新世含水层还得到了来自河流和渠道渗漏以及集水区上部和中部灌溉回流的局部补给。这些观测结果得到了详细的地下水水文和盐度观测证据的证实，表明局部运河，河流和湖泊与地下水的连通性。在中下游集水区，在枯水期拦河坝关闭时，河流流量主要由地下水基流控制，这有助于为濒危的河豚和长吻鳄提供生态流量。地下水停留时间示踪剂表明，活跃的现代补给在整个集水区的浅层冲积含水层系统。在浅层全新世含水层中，砷（As）、铁（Fe）和锰（Mn）的含量在少数地点超过了世界卫生组织的饮用水指南，铀（U）和氟（F）的浓度接近但不超过世界卫生组织的指南值。这些观察结果在整个流域变化，较高的As，Fe和Mn在上游和中游流域和较高的U在下游流域。地下水盐度通常在500至1000 μS/cm之间，个别较高的盐度是由于受蒸发影响的洪泛区湿地和湖泊的补给造成的。目前，甘达克集水区的降雨量相对较高，抽取量较低，这使地下水位保持稳定，从而在旱季保持流入河流的基流。由于对集水区基流变化的敏感性，未来对地下水资源的潜在威胁以及河流生态可能受到当地季风降雨量减少、水管理做法改变和地下水使用增加的驱动。
The alluvial aquifer system of the Indo-Gangetic Basin (IGB) is one of the world’s most important freshwater resources, sustaining humans and river ecosystems. Understanding groundwater recharge processes and connections to meteoric and surface water is necessary for effective water resource management for human and wider ecological requirements. Parts of the mid-Gangetic Basin, across eastern Uttar Pradesh and Bihar, are characterised by stable long-term groundwater levels, high annual rainfall, and limited historical groundwater use compared to parts of Northwest India for example. In this paper we use a combination of environmental tracers and hydrograph observations to characterise sources of recharge and groundwater-surface water interaction using a transect approach across the catchment of the River Gandak, a major barrage-regulated tributary of the River Ganga. Stable isotope results show that the dominant source of groundwater recharge, in the shallow (0–40 m bgl) Holocene and underlying Pleistocene aquifer system (>40 m bgl), is local rainfall. The shallow Holocene aquifer is also supplemented by local recharge from river and canal seepage and irrigation return flow in the upper and mid parts of the catchment. These observations are corroborated by evidence from detailed groundwater hydrographs and salinity observations, indicating localised canal, river and lake connectivity to groundwater. In the middle and lower catchment, river discharge is dominated by groundwater baseflow during the peak dry season when barrage gates are closed, which contributes to ecological flows for endangered river dolphins and gharial crocodiles. Groundwater residence time tracers indicate active modern recharge in the shallow alluvial aquifer system across the catchment. In the shallow Holocene aquifer elevated arsenic (As), iron (Fe), and manganese (Mn) exceeded WHO drinking water guidelines in a minority of sites, and uranium (U) and fluoride (F) concentrations approach but do not exceed the WHO guideline values. These observations varied across the catchment with higher As, Fe and Mn in the upper and mid catchments and higher U in the lower catchment. Groundwater salinity was typically between 500 and 1000 μS/cm, and isolated higher salinity was due to recharge from flood-plain wetlands and lakes impacted by evaporation. At present, the Gandak catchment has relatively high rainfall and low abstraction, which maintains stable groundwater levels and thus baseflow to the river in the dry season. Potential future threats to groundwater resources, and therefore river ecology due to the sensitivity to changes in baseflow in the catchment, would likely be driven by reductions in local monsoon rainfall, changes in water management practices and increased groundwater use.