School of Environmental Science and Engineering, Indian Institute of Technology Kharagpur, West Bengal, India.
Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, West Bengal, India.
J Environ Manage. 2021 Apr 15;284:112026. doi: 10.1016/j.jenvman.2021.112026. Epub 2021 Feb 1.
The coastal aquifers of Sundarbans, an UNESCO world biodiversity heritage site, are highly vulnerable due to changing climatic conditions, intensification and increasing frequency of extreme climate events and uncontrolled abstraction of groundwater. The exchange of solutes between hydraulically connective shallow and deep aquifers, the seawater intrusion and the role of growing population are poorly understood in the Sundarbans. This study aims to address the solute exchange (Cl, Sr, and salinity) process between surface water and groundwater (SW-GW) at local to regional scale under variable hydraulic head conditions, where annual rainfall is declining and population density is increasing [population 573 (1991) to 819 (2011)/Km]. Electrical resistivity tomography (ERT) in combination with salinity and δO data was used to address the exchange of solutes between SW-GW in a hydraulic continuation. The results revealed that regionally, the Cl concentration of Sundarbans shows an increasing trend (average 329-351 mg/L) with declining groundwater levels (⁓3 m). Local, depth-dependent study depicting there is a predominant exchange of Sr between shallow depth [D1: 14-25 and D2: 30-50 m below ground level (m bgl)] with seawater (Sr: 30-85 μM), which is possibly absent at greater depths (D3:115 and D4: 333 m bgl). The recorded Sr content ranged from 25 to 102 and 16 to 78 μM for shallow depth D1 and D2, respectively, whereas, the Sr concentrations ranged from 1.4 to 6.8 and 1.2 to 5.7 μM for D3 and D4, respectively. The ERT data showed progressively increasing resistivity with increasing depth, similar to high salinity and enriched δO at shallow depths and depleted δO with low salinity at higher depth reflects the continuous distribution of solutes, which is possibly a result of local downward migration of contaminated shallow brackish water within this physically disconnected zone. The lateral and vertical transportation of solutes in variable hydraulic head conditions would be a measure of drinking water threat in present-day and in imminent future for millions of inhabitants near the coastal area.
孙德尔本斯的沿海含水层是教科文组织世界生物多样性遗产地，由于气候变化、极端气候事件的加剧和频率增加以及地下水的无控制开采，这些含水层非常脆弱。在孙德尔本斯，水力连通的浅层和深层含水层之间的溶质交换、海水入侵以及不断增长的人口的作用都知之甚少。本研究旨在解决在水力连通条件下，当地到区域尺度上地表水和地下水（SW-GW）之间的溶质交换（Cl、Sr 和盐度）过程，该地区的年降雨量在下降，而人口密度在增加[1991 年人口为 573（人/平方公里），2011 年人口为 819（人/平方公里）]。电阻抗层析成像（ERT）与盐度和 δO 数据相结合，用于解决水力连续条件下 SW-GW 之间的溶质交换。结果表明，从区域上看，孙德尔本斯的 Cl 浓度呈上升趋势（平均 329-351mg/L），地下水水位下降（约 3m）。局部深度依赖研究表明，浅层（D1：地面以下 14-25 米，D2：地面以下 30-50 米）与海水（Sr：30-85μM）之间主要发生 Sr 交换，而在更深层（D3：115 米和 D4：333 米）则可能不存在这种交换。记录的 Sr 含量在浅层 D1 和 D2 分别为 25 到 102 和 16 到 78μM，而在 D3 和 D4 分别为 1.4 到 6.8 和 1.2 到 5.7μM。ERT 数据显示，随着深度的增加，电阻率逐渐增加，与浅层的高盐度和富集 δO 以及深层的低盐度和贫化 δO 相似，这反映了溶质的连续分布，这可能是由于在这个物理上不连续的区域内，受污染的浅层微咸水向下迁移所致。在可变水力头条件下，溶质的侧向和垂直运移将是衡量当前和即将到来的沿海地区数百万居民饮用水威胁的一个指标。
