• KALYAN ADHIKARI

      Articles written in Journal of Earth System Science

    • Early Permian transgressive–regressive cycles: Sequence stratigraphic reappraisal of the coal-bearing Barakar Formation, Raniganj Basin, India

      Biplab Bhattacharya Joyeeta Bhattacharjee Sandip Bandyopadhyay Sudipto Banerjee Kalyan Adhikari

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      The present research is an attempt to assess the Barakar Formation of the Raniganj Gondwana Basin, India, in the frame of fluvio-marine (estuarine) depositional systems using sequence stratigraphic elements. Analysis of predominant facies associations signify deposition in three sub-environments: (i) ariver-dominated bay-head delta zone in the inner estuary, with transition from braided fluvial channels (FA-B1) to tide-affected meandering fluvial channels and flood plains (FA-B2) in the basal part of the succession; (ii) a mixed energy central basin zone, which consists of transitional fluvio-tidal channels (FA-B2), tidal flats, associated with tidal channels and bars (FA-B3) in the middle-upper part ofthe succession; and (iii) a wave-dominated outer estuary (coastal) zone (FA-B4 with FA-B3) in the upper part of the succession. Stacked progradational (P1, P2)–retrogradational (R1, R2) successions attest to one major base level fluctuation, leading to distinct transgressive–regressive (T–R) cycles with development of initial falling stage systems tract (FSST), followed by lowstand systems tract (LST)and successive transgressive systems tracts (TST-1 and TST-2). Shift in the depositional regime from regressive to transgressive estuarine system in the early Permian Barakar Formation is attributed to change in accommodation space caused by mutual interactions of (i) base level fluctuations in responseto climatic amelioration and (ii) basinal tectonisms (exhumation/sagging) related to post-glacial isostatic adjustments in the riftogenic Gondwana basins.

    • Analysis of spatio-temporal trend in groundwater elevation data from arsenic affected alluvial aquifers – Case study from Murshidabad district, West Bengal, Eastern India

      RHITWIK CHATTERJEE SWETADRI SAMADDER DEBABRATA MONDAL KALYAN ADHIKARI

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      Fluctuation in groundwater level is a time-dependent stochastic process. It is also a function of various inflow and outflow components to and from the hydrologic system concerned. Depth to water level data are measured through a network of observation wells or hydrograph stations to ascertain the degree of fluctuation in groundwater level at the desired scale, on a long-term basis. Basically, these depths to water level data are point measurements, which can be regarded as random variables furnishing changes in groundwater storage over time. The intrinsic in-homogeneity in aquifer materials introduces variations like jumps, trends, and periodicities in such hydrologic time series data. Thus, trending results from certain gradual, natural and/or anthropogenic interventions in the hydrologic environment and analyses of their trends are imperative for assessment of groundwater level scenario in the area of interest. This in turn, is essential for strategic planning and management for exploitation of the precious groundwater resource in the same area. The area of interest in this article, i.e., Murshidabad is one of the nine arsenic affected districts of West Bengal. Here, contamination persist within, shallow, arseniferous, alluvial aquifers, which are otherwise widely exploited for irrigation purposes. According to many researchers working in this area, over-exploitation of groundwater is the root cause for the plummeting water level and the widespread arsenic contamination as well. The present study intends to detect and analyze the trends persisting in the depth to water level data measured over a period from 1996 to 2016, in Murshidabad district of West Bengal, India, amidst its complex and contrasting hydrogeologic set-up and interpret the results in terms of the hydrologic attributes of the Bengal basin as a whole. The non-parametric Mann–Kendall test and the Sen’s slope estimator have been used to identify the linear trend persisting in the time series pre- and post-monsoon groundwater elevation values. The analysis indicates statistically significant decline in water level across the study area especially during the post-monsoon season. This can be attributed to the recharge–discharge disparity within the hydrologic regime; brought through intense pumping over the study area. Its ill effect being particularly observed in the western part of river Bhagirathi. Findings of such study are crucial for assessment of dynamic groundwater resources of the district and subsequently can be utilized as a decision support tool for groundwater management at micro-level.

      $\bf{Highlights}$

      $\bullet$ Declining trend in ground water level elevation is indicative of startling water crisis over a region; however, assessment of such trend should be performed over a quantitative basis, as; fluctuation in groundwater level is a time-dependent stochastic process.

      $\bullet$ A well-knit methodology (including RL correction, spatial interpolation of data, analyses and quantification of trend present in the data) needs to be followed by the groundwater managers operating at the micro-level to keep an account of aquifer storage conditions.

      $\bullet$ Statistically significant declining trend in water level elevation is observed in the Arsenic affected shallow alluvial aquifers, on either sides of river Bhagirathi.

      $\bullet$ Lack of groundwater replenishment within recoverable recharge, coupled with over extraction causes such drop in groundwater elevation.

      $\bullet$ To combat similar situation groundwater extraction should be restricted within the limit of sustainability and should not exceed natural recharge potential.

    • Groundwater quality and hydrological stress induced by Lower Gondwana open cast coal mine

      UJJAL MAL KALYAN ADHIKARI

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      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.

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