GroMoPo Metadata for Bengal Basin regional model

Quantitative evaluation of management strategies for long-term supply of safe groundwater for drinking from the Bengal Basin aquifer (India and Bangladesh) requires estimation of the large-scale hydrogeologic properties that control flow. The Basin consists of a stratified, heterogeneous sequence of sediments with aquitards that may separate aquifers locally, but evidence does not support existence of regional confining units. Considered at a large scale, the Basin may be aptly described as a single aquifer with higher horizontal than vertical hydraulic conductivity. Though data are sparse, estimation of regional-scale aquifer properties is possible from three existing data types: hydraulic heads, C-14 concentrations, and driller logs. Estimation is carried out with inverse groundwater modeling using measured heads, by model calibration using estimated water ages based on C-14, and by statistical analysis of driller logs. Similar estimates of hydraulic conductivities result from all three data types; a resulting typical value of vertical anisotropy (ratio of horizontal to vertical conductivity) is 10(4). The vertical anisotropy estimate is supported by simulation of flow through geostatistical fields consistent with driller log data. The high estimated value of vertical anisotropy in hydraulic conductivity indicates that even disconnected aquitards, if numerous, can strongly control the equivalent hydraulic parameters of an aquifer system.针对孟加拉盆地含水层（印度与孟加拉国）的长期安全饮用水地下水源供应管理策略开展定量评估，需先估算控制地下水流的大尺度水文地质参数。该盆地由层状非均质性沉积序列构成，其中分布有可局部分隔含水层的隔水层，但现有证据并不支持区域隔水单元的存在。若从大尺度视角考量，该盆地可被恰当地描述为水平水力传导率高于垂向的单一含水层。尽管实测数据较为匮乏，但通过三类现有数据即可实现区域尺度含水层参数的估算：地下水头、碳14（C-14）浓度以及钻井日志。本次参数估算通过三种方法完成：基于实测水头开展地下水逆向模拟、依托碳14推算的地下水年龄进行模型校准，以及对钻井日志进行统计分析。三类数据得到的水力传导率估算结果高度一致；最终得到的垂向各向异性（水平传导率与垂向传导率的比值）典型值为10^4。该垂向各向异性的估算结果，可通过与钻井日志数据匹配的地质统计场的水流模拟得到验证。水力传导率的垂向各向异性估算值偏高，这表明即便隔水层互不连通，只要其数量足够多，便可对含水层系统的等效水力参数产生极强的调控作用。