Journal of Earth System Science ( IF 1.3 ) Pub Date : 2021-02-05 , DOI: 10.1007/s12040-020-01486-x Ujjal Mal , Kalyan Adhikari
Abstract
Open cast coal mine (OCCM) may impose adverse effects on groundwater quality and on its flow regime. Evaluation of water quality often becomes difficult and confusing due to presence of multiple criteria and index based on different parameters. A novel approach has been introduced which speaks about a single index revealing the effect of all important parameters to determine the quality for a particular use of groundwater. Physico-chemical parameters of groundwater are grouped according to their toxicity and the weightage is permitted to each group using analytic hierarchy process (AHP). Overall groundwater quality is found suitable for drinking (DWQI = 1.5–2) and irrigation (IWQI = 2 to 3) purposes in Barjora area. Effect of acid mine drainage (AMD) is not significant in controlling the quality of groundwater in the study area. Carbonate and silicate weathering are the dominant hydro-geochemical processes occurring in the study area. Saturation index derived through Phreeqc Interactive software shows that calcite and aragonite are at supersaturated condition in close proximities of OCCMs. Buffer reactions of carbonate minerals may attenuate the effect of AMD in the study area. The hydrological stress induced by OCCM has been evaluated in OCCM 2 through numerical modelling using MODFLOW software. Evaluation has been performed in three stages of mine development – pre-mining condition, present mining condition and future extension of mine. It is estimated that groundwater of almost 24,163 m3/d will ingress into the excavation of 2.03 km2 area with maximum depth around 200 m from ground level. Model predicted 4–5 m lowering of water table, as an impact, around 2.86 km2 area of the mine cavity. The irrigation canal passing through this region, will lose 1473 m3/d water during future extension of mine.
Highlights
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Use of different parameters and indices to determine water quality often becomes confusing. The water quality index (WQI) introduced in this paper is all encompassing based on the relative weightage of various parameters of drinking water quality index (DWQI) and irrigation water quality index (IWQI) assigned through analytic hierarchy process (AHP). Each single index value for DWQI and IWQI derived through this process eliminate confusion.
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The spatial distribution map of DWQI and IWQI specifies groundwater in this region is overall suitable for drinking and irrigation purpose with an exception of water from wells in close proximity of the open cast coal mines (OCCMs).
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Effect of acid mine drainage (AMD) is not observed in the study area. Spatial distribution map of saturation index(SI) displays higher degree of saturation of carbonate minerals (aragonite, calcite and dolomite) in the groundwater at close proximities of OCCMs. These minerals may buffer the low pH of AMD which renders adsorption and precipitation of various metals in the soil/rock and consequently resist the transport of many toxic metals to the groundwater.
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Groundwater flow model provided a significant idea about the groundwater budget in the study area. Well (OB2) close to OCCM 2 with lowering of water table from 2011-2014 during operating stages of mine substantiates the negative impact of mining and associated dewatering process on the aquifer. Model predicts that in the stage 1 and 2 groundwater seepage into the mine will be 3120.6 m3/d and 24163 m3/d respectively. Water loss from the aquifer will cause lowering of water table in the surrounding areas of mine.
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4–5 m lowering of water table may take place around 2.86 km2 area of the mine cavity within 15 years. The irrigational canal may loss water of 1473 m3/d from its stream inflow on enhancement of mine in north-west direction, which may impact on production of agricultural crops in the downstream areas. Present mining operation will not impact much on the flow condition of the Damodar River.
中文翻译：
下冈瓦纳露天煤矿引起的地下水质量和水文应力
摘要
露天煤矿（OCCM）可能会对地下水水质及其流态产生不利影响。由于存在多个基于不同参数的标准和指数，对水质的评估通常变得困难而混乱。引入了一种新颖的方法，该方法谈论单个指数，该指数揭示了所有重要参数对确定特定地下水使用质量的影响。地下水的物理化学参数根据其毒性进行分组，并使用层次分析法（AHP）对每组的重量进行加权。发现巴乔拉地区的地下水总体质量适合饮用（DWQI = 1.5–2）和灌溉（IWQI = 2至3）目的。酸性矿山排水（AMD）的影响在研究区域的地下水质量控制中并不重要。碳酸盐和硅酸盐的风化作用是研究地区发生的主要水文地球化学过程。通过Phreeqc Interactive软件获得的饱和指数表明，方解石和文石在OCCM接近时处于过饱和状态。碳酸盐矿物的缓冲反应可能会减弱AMD在研究区域的作用。在OCCM 2中，使用MODFLOW软件通过数值建模对OCCM引起的水文应力进行了评估。在矿山开发的三个阶段进行了评估-开采前的状况，当前的开采状况和矿山的未来扩展。据估计，地下水将近24,163 m 通过Phreeqc Interactive软件获得的饱和指数表明，方解石和文石在OCCM接近时处于过饱和状态。碳酸盐矿物的缓冲反应可能会减弱AMD在研究区域的作用。在OCCM 2中，使用MODFLOW软件通过数值建模对OCCM引起的水文应力进行了评估。在矿山开发的三个阶段进行了评估-开采前的状况，当前的开采状况和矿山的未来扩展。据估计，地下水将近24,163 m 通过Phreeqc Interactive软件获得的饱和指数表明，方解石和文石在OCCM接近时处于过饱和状态。碳酸盐矿物的缓冲反应可能会减弱AMD在研究区域的作用。在OCCM 2中，使用MODFLOW软件通过数值建模对OCCM引起的水文应力进行了评估。在矿山开发的三个阶段进行了评估-开采前的状况，当前的开采状况和矿山的未来扩展。据估计，地下水将近24,163 m 在矿山开发的三个阶段进行了评估-开采前的状况，当前的开采状况和矿山的未来扩展。据估计，地下水将近24,163 m 在矿山开发的三个阶段进行了评估-开采前的状况，当前的开采状况和矿山的未来扩展。据估计，地下水将近24,163 m3 / d将进入2.03 km 2区域的开挖，其最大深度为距地面200 m左右。该模型预测，作为影响，地下水位将下降4-5 m，约为矿井腔2.86 km 2区域。通过该地区的灌溉渠在矿山的未来扩建期间将损失1473 m 3 / d的水。
强调
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使用不同的参数和指标来确定水质经常变得令人困惑。本文介绍的水质指数（WQI）涵盖了通过层次分析法（AHP）分配的饮用水水质指数（DWQI）和灌溉水水质指数（IWQI）各个参数的相对权重。通过此过程得出的DWQI和IWQI的每个索引值消除了混淆。
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DWQI和IWQI的空间分布图表明，该区域的地下水总体上适合饮用和灌溉，但露天煤矿（OCCM）附近的井中的水除外。
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在研究区域未观察到酸性矿井排水（AMD）的影响。饱和指数（SI）的空间分布图显示，OCCM接近时，地下水中碳酸盐矿物（文石，方解石和白云石）的饱和度较高。这些矿物质可以缓冲AMD的低pH值，从而降低土壤/岩石中各种金属的吸附和沉淀，从而阻止许多有毒金属向地下水的迁移。
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地下水流模型为研究区域的地下水预算提供了重要的思路。井（OB2）接近OCCM 2，2011年至2014年矿山运营阶段的地下水位降低，证实了采矿及相关脱水过程对含水层的负面影响。模型预测，在第一阶段和第二阶段，渗入矿井的地下水将分别为3120.6 m 3 / d和24163 m 3 / d。含水层的水分流失会导致矿井周围地区地下水位降低。
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在15年内，矿井周围2.86 km 2的区域内地下水位下降4-5 m 。在西北方向增加矿山量的作用下，灌溉渠可能会因流失而损失1473 m 3 / d的水，这可能会影响下游地区的农作物产量。目前的采矿作业不会对Damodar河的水流状况产生太大影响。
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