The Hydraulic Evolution of Groundwater-Fed Pit Lakes After Mine Closure.

Ground water Pub Date : 2024-11-01 Epub Date: 2024-06-03 DOI:10.1111/gwat.13419
Birte Moser, Peter G Cook, Anthony D Miller, Shawan Dogramaci, Ilka Wallis
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Abstract

Open pit mining frequently requires regional water tables to be lowered to access ore deposits. When mines close, dewatering ceases allowing the water table to recover. In arid and semi-arid mining regions, the developing pit lakes are predominantly fed by groundwater during this recovery phase and pit lakes develop first into "terminal sinks" for the surrounding groundwater system. With time, the re-establishment of regional hydraulic gradients can cause pit lakes to develop into throughflow systems, in which pit lake water outflows into adjacent aquifers. In this study, we use numerical groundwater modeling to aid process understanding of how regional hydraulic gradients, aquifer properties, net evaporation rates, and pit geometry determine the hydraulic evolution of groundwater-fed pit lakes. We find that before the recovery of the regional water table to its new equilibrium, pit lakes frequently transition to throughflow systems. Throughflow from pit lakes to downstream aquifers can develop within two decades following cessation of dewatering even under low hydraulic gradients (e.g., 5 × 10-4) or high net evaporation rates (e.g., 2.5 m/year). Pit lakes remain terminal sinks only under suitable combinations of high evaporation rates, low hydraulic gradients, and low hydraulic conductivities. In addition, we develop an approximate analytical solution for a rapid assessment of the hydraulic status of pit lakes under steady-state conditions. Understanding whether pit lakes remain terminal sinks or transition into throughflow systems largely determines the long-term water quality of pit lakes and downstream aquifers. This knowledge is fundamental for mine closure and planning post-mining land use.

矿山关闭后地下水滋养坑湖的水力演变。
露天采矿经常需要降低区域地下水位,以便开采矿藏。矿场关闭后,排水工作停止,地下水位得以恢复。在干旱和半干旱矿区,正在形成的矿坑湖泊在这一恢复阶段主要由地下水提供水源,矿坑湖泊首先发展成为周围地下水系统的 "终端汇"。随着时间的推移,区域水力梯度的重新建立会使矿坑湖泊发展成为贯流系统,矿坑湖泊的水会流向邻近的含水层。在这项研究中,我们利用地下水数值建模来帮助理解区域水力梯度、含水层特性、净蒸发率和矿坑几何形状如何决定地下水注入矿坑湖的水力演变过程。我们发现,在区域地下水位恢复到新的平衡点之前,矿坑湖泊经常会过渡到直流系统。即使在低水力梯度(如 5 × 10-4)或高净蒸发率(如 2.5 米/年)的情况下,坑湖也能在停止脱水后的二十年内形成向下游含水层的贯通流。只有在高蒸发率、低水力梯度和低水力传导性的适当组合下,坑湖才能保持终端汇。此外,我们还开发了一种近似分析方法,用于快速评估稳态条件下的坑洼湖泊水力状况。了解矿坑湖泊是保持终端汇还是过渡到通流系统在很大程度上决定了矿坑湖泊和下游含水层的长期水质。这些知识对于矿山关闭和规划采矿后的土地利用至关重要。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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