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 m$^{3}$/d will ingress into the excavation of 2.03 km$^{2}$ 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 km$^{2}$ area of the mine cavity. The irrigation canal passing through this region, will lose 1473 m$^{3}$/d water during future extension of mine.

$\bf{Highlights}$

$\bullet$ 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.

$\bullet$ 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).

$\bullet$ 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.

$\bullet$ 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 m$^{3}$/d and 24163 m$^{3}$/d respectively. Water loss from the aquifer will cause lowering of water table in the surrounding areas of mine.

$\bullet$ 4–5 m lowering of water table may take place around 2.86 km$^{2}$ area of the mine cavity within 15 years. The irrigational canal may loss water of 1473 m$^{3}$/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.

The metallurgical processes in production of iron and steel from ores produce slags (blast furnace slag and basic oxygen furnace slag), which is usually dumped as solid waste by various steel plants around the world. The present study assessed and estimated the impact of slag dump piling up for the last 60 years on the hydrogeological environment in the industrial city of Durgapur in eastern India. Groundwater survey resembles the unconfined nature of shallow aquifer with elevated concentrations of Ca and Mn in the study area. A factor analysis of groundwater physicochemical parameters suggests natural rockwater interaction dominantly controls groundwater composition in the study area. Saturation index of groundwater samples defines supersaturated conditions of aragonite, calcite, dolomite and rhodochrosite near the slag dump. A leaching study of slag samples demonstrates high concentration of Mn in the leachate of the slag dump. Laboratory-scale soil column experiment supplemented with ${HYDRUS 1D }$model was used to evaluate Mn transport through the vadose zone. ${HYDRUS 1D}$model predicts that Mn attain complete breakthrough for 2m thick soil column in the 40th year. Elevated concentrations of Mn and Ca in groundwater samples near the slag dump and the results of leaching study vouch for progressive contamination of groundwater in the area with time.

$\bf{Highlights}$

$\bullet$ Slags, the obvious by-product of iron and steel making, have been piled up in different steel plants. Blast furnace (BF) slag finds almost 100% use but there are issues with complete utilisation of steel slag. As a result of which, slags are still getting dumped as solid waste in different plants. The hillocks of slags are vulnerable to weathering and may generate leachate of adverse chemical quality containing metals, non-metals, heavy metals, which may eventually pollute nearby soil regime and groundwater. A case study has been carried out to assess and quantify the groundwater pollution through slag dump derived leachate.

$\bullet$ Elevated concentrations of Mn and Ca are present in groundwater at close proximities of slag dump. Weathering reactions of primary and/or secondary carbonate minerals present in slag dump result in Mn and Ca contamination in groundwater.

$\bullet$ Column experiment and numerical simulation reveal that the concentration of Mn in groundwater near slag dump will be increasing with time as the resilience of vadose zone will be nullified with time due to regular leaching of Mn-laden contaminants from overlying slag dump. The Mn plume forming below the slag dump has been spreading following groundwater biogeochemical dynamics of the local groundwater system. If leaching continues, the expanding plume may affect larger areas involving new localities with greater implications in future.