J. Earth Syst. Sci. (2020) 129:228 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12040-020-01489-8 (0123456789().,-volV)(0123456789().,-volV)

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

1, 2 3 RHITWIK CHATTERJEE * ,SWETADRI SAMADDER ,DEBABRATA MONDAL 1 and KALYAN ADHIKARI 1Department of Earth and Environmental Studies, National Institute of Technology, , India. 2Department of Mathematics, Fakir Chand College, University of Calcutta, , India. 3Department of Geography, Centenary College, University of Kalyani, Lalbagh, Murshidabad, India. *Corresponding author. e-mail: [email protected]

MS received 17 February 2020; revised 10 June 2020; accepted 6 August 2020

Fluctuation in groundwater level is a time-dependent stochastic process. It is also a function of various inCow and outCow 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 Cuctuation 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 aAected districts of . 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 Mur- shidabad 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 eAect 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. 228 Page 2 of 15 J. Earth Syst. Sci. (2020) 129:228

Keywords. Mann–Kendall test; Sen’s slope estimator; groundwater elevation; water level Cuctuation; hydrograph; arsenic.

1. Introduction within the complex and contrasting hydrogeologic framework of . Results of such The use of groundwater at the global scale has raised study have been interpreted in terms of the hydro- by approximately 300% since 1950s (Doll€ et al. 2012). logic characteristics of the Bengal basin as a whole. Aquifer systems react to hydraulic stresses (recharge In the context of the present article, trend refers to and discharge from the system) through changes in to the long-term movement (steady increase or groundwater level with respect to time (and/or sea- decrease) in time series without Bxed or regular son) and space. Such Cuctuation in water level ele- eAects. In the case of a hydrologic time series, per- vation is a time-dependent stochastic process, which sistence of trend can be attributed to impacts of varies as a function of various inCow and outCow changes in climate, land use and demand (ex- components to and from the system. Hence, ploitation) pattern and can be established through groundwater level can indicate the groundwater statistical analyses. For the detection of trend in availability, Cow and physical characteristics of the hydrologic time series, various statistical methods entire hydrogeologic system of an area. The temporal are available which ranges from simple linear variability in the Cuctuation in groundwater level is regression analyses to advanced parametric and utilized for estimation of changes in aquifer storage. non-parametric methods (Kendall 1938, 1955, 1975; At the same time, in-homogeneity in aquifer prop- Brophy 1968; Mantegna 1999; Mantegna and erties like transmissivity and storativity make that Stanley 2000; Bouchaud and Potters 2001; Pro- aquifer to respond differently, which in turn intro- khorov 2001; Wilcox and Gebbie 2004; Thakur and duces variations like trends, jumps, periodicities, etc. Thomas 2011; Samadder et al. 2015). In the present (Thakur and Thomas 2011). In India, variability in study, the authors have attempted to detect, ana- monsoon is a crucial factor in introducing seasonality lyze and quantify the trend persisting in the time in the hydrologic time series data (Hanson et al. 2004) series water level data from Murshidabad district of by augmenting recharge through rainfall and stream- West Bengal and to pin-point the major fac- aquifer interface. Modelling of natural hydrologic tor(s) controlling the attributes. Hence, a non- systems using time series observations involves cer- parametric test has been preferred in the study to tain uncertainties; which are explicable in terms of overcome the problem caused by data skewness the geologic and/or hydrogeologic parameters of the (Smith 2000). Here Mann–Kendall test has been concerned systems. Plenty of studies have been con- considered as a robust statistics to identify the ducted in the subject during recent past (Ahmadi and monotonic trend in a time series data. One advan- Abbas 2007;Kumar2007; Angelo et al. 2010;Panda tage of this test is that this test does not require any particular distribution of the data (Mann 1945; et al. 2012; Alexander and Likens 2012; Narany et al. Kendall 1975; Gan 1998; Zhang et al. 2001; Burn 2014; Patle et al. 2015;Rahmanet al. 2016;Vish- and Elnur 2002;Xuet al. 2003; Yang et al. 2004; wakarma et al. 2018). However, all such studies differ Mondal et al. 2012) and is weakly sensitive to sud- from the present endeavor in terms of their scale and den break of inhomogeneous time series observa- approach. Majority of the previous studies, have tions (Tabari et al. 2011). The test compares the addressed similar problems to decipher the inCuence relative magnitude of the data instead of their of factors like climatic variability. The Murshidabad absolute values (Gilbert 1987; Hirsch et al. 1991). district of West Bengal is underlain by alluvial Magnitude of the trend has been estimated by aquifers which have gained notoriety for proliBc Theil–Sen’s slope estimator (Theil 1950; Sen 1968). arsenic contamination. At the same time, it repre- sents a major agricultural hub for the region. It is not perceived as water stressed in general. It is impera- tive to frame out a scientiBc protocol for assessment 2. Hydrogeologic setup of the study area of its water sub-soil water resource situation, which is the prime objective of the study. In the present The Murshidabad district of West Bengal, eastern endeavor trends persisting in the time series India, is located at between 87848050.0300– groundwater level elevation data have been analyzed 8884404700E and 2384305.700–24851015.600N covering J. Earth Syst. Sci. (2020) 129:228 Page 3 of 15 228 an area of 5324 km2 (Bgure 1). It lies on the west- Bagri found in the eastern side of Bhagirathi River, ern part (Bhagirathi sub-basin) of the Bengal is characterized by light alluvial fertile soil com- Basin to the south of Ganga River and occupies the prising of the huge thickness of recent to sub-recent central plain of Gangetic West Bengal (GWB). sediments of Ganga River system. These recent The district comprises 26 community development Cuviatile sediments consist of a succession of clay, blocks (herein after referred as blocks) divided into silt, sand and gravel and are characterized by grey Bve administrative sub-divisions (Ghosh 2007). sediment aquifers containing high arsenic, total iron Bhagirathi, a branch of Ganga River bifurcated and low manganese (Sankar et al. 2014). The general the triangular district in a manner so that two broad inclination of the portion of the district lying to the geographic regions of almost equal area with striking west of Bhagirathi is from northwest to southeast; difference in their geology, soil types and hydrogeo- but in the track east of Bhagirathi, the main rivers logical characteristics are juxtaposed (Bgure 2). The and the lines of drainages seem irregular and do not Rarh found in the western side of the Bhagirathi take this direction uniformly. River is a continuation of the sub-Vindhyan region Within the study area, construction of Farakka of lateritic clay and calcareous nodules. This part is Barrage marks a massive human intervention into covered by the older alluvium of Pleistocene age, the hydrologic regime through diverting water deposited by the Ajoy–Damodar– from the (average discharge 1046 m3sÀ1) system. This older alluvium is characterized by the (Bandyopadhyay et al. 2014; Rudra 2014). argillaceous sediments (clay and calcareous mate- According to Adel (2005), such upstream diversion rial). The clays are very stiA in nature. This region is of water imparts deleterious eAects like depletion of characterized by brown sediment aquifers. The surface water resources, drying and desiccation of

Figure 1. Location map of Murshidabad district along with the block boundaries and SWID monitored PHSs (permanent hydrograph topographic elevation). The red dots showing the location stations. 228 Page 4 of 15 J. Earth Syst. Sci. (2020) 129:228

Figure 2. Geology, major rivers and wetlands of Murshidabad district adopted from Mondal (2017). soil surfaces, plummeting water table, expansion in Groundwater in the western part of Bhagirathi depth and spread of the vadose zone, etc., in the River occurs under semi-conBned to conBned con- downstream. However, the seasonally Cuctuating ditions (CGWB 2009). West bank aquifer sediments Cow in the Padma and other drainages from the are oxidized and contain abundant ferric oxide region makes the district immensely prone to coatings on mineral grains (Datta et al. 2011), which inundation from the excess Cow from the main are sometimes separated by lenticular clay beds at channel of Bhagirathi and its western tributaries, varying depths resulting into regionally connected along with the spillover from reservoirs and water- artesian conditions. Here Cowing artesian condi- logging due to embankments. Such Cooding at tions are found in shallow tube wells which are times reaches as far as 5 km from the central seasonal in nature. The area is hard crusted with channel (Bandyopadhyay et al. 2014; Mollah abundant calcareous nodules and ferruginous 2016). concretions (Ghosh 2007). Groundwater in the area occurs within a thick Hydro-dynamically, the groundwater system of zone of saturation in the alluvial aquifers deposited Murshidabad district follows the pattern of the by the aforementioned river systems. The aquifers Bengal basin as a whole ( 2018). Studies consist of different grades of sand and gravel, on groundwater Cow patterns are scarce especially extending down to a depth of 90–350 m bgl in the at the district level and only regional scale data are east and 140–150 m bgl in the west of Bhagirathi available. Michael and Voss (2009) opined that the River, respectively. Groundwater generally occurs Bengal basin as a whole consists of stratiBed, under water table condition in the east; the shallow heterogeneous sequence of sediments with aquitard aquifers at places contain high arsenic groundwater. separating aquifers only locally, but evidences do J. Earth Syst. Sci. (2020) 129:228 Page 5 of 15 228 not support existence of regional scale conBning (table 2). However, the groundwater development units. Considered at a regional scale, the basin can status is not uniform throughout the district. It is be considered as a single aquifer system with extensive in the eastern part. Nearly 90% of total greater horizontal conductivity than vertical one. groundwater structures are located in this area, The SE monsoonal wind originating from the Bay with private individuals having major stake. of Bengal brings heavy rainfall in the region from Whereas the stage of groundwater development is mid-June to mid-October. The groundwater Cow in comparatively lower in the Rarh region. As per the the GWB is strongly inCuenced by that. The guideline of Groundwater Estimation Committee annual rainfall of the area ranges from about (GEC 1997), Government of India, 10 numbers of 125 cm in the north-central part of the basin to blocks of the district are categorized under safe and *160 cm near Bay of Bengal (Mukherjee et al. 16 blocks are under the semi-critical stage of 2007a, b, 2009). In general, everywhere on the delta groundwater development (SWID 2009). plain, evapo-transpiration is exceeded by the pre- cipitation (Allison 1998). Different Bgures have been cited by various authors as estimates of 3. Materials and methods annual potential recharge (PR), as shown in table 1. Depth to water level data for various blocks of Due to such heavy monsoonal precipitation, Murshidabad district are measured on a quarterly factually most of the PR happens during monsoon basis by State Water Investigation Directorate with little or no recharge during pre- and post- (SWID) under Water Resources Investigation & monsoon. Substantial part of this recharge is Development Department (WRIDD), Government abstracted for irrigation which subsequently get of West Bengal, through a network of 114 perma- back to the groundwater system in the form of nent hydrograph stations (PHS) dispersed all over irrigation return Cow (annual mean; 0.75 mÀ2 the district (Bgure 1). Conventionally, the data dayÀ1). For Bengal basin as a whole, the total measured during the months of April and Novem- annual recharge accounts for only a small fraction ber is termed as pre- and post-monsoon depth to of the total groundwater resource of the area. The water level data, respectively. Such data have been mean annual rainfall of Murshidabad district for collected for all the 26 blocks of Murshidabad dis- the year 2017 was close to the annual average for trict from SWID authority for all the 114 PHS for the country, but is marginally less compared to the period from 1996 to 2016. Coordinates of the that for the state of West Bengal (table 2). PHS were recorded using hand-held GPS (E-trex). The district is typically not perceived as water The collar reduced levels (RL) of the PHS have stressed and groundwater based irrigation have been measured using technique suggested by Ho remained a popular and perennial option for the et al. (2013) in Arc GIS environment. Firstly, peasants especially on or after 1970s compared to biased free digital surface model has been gener- rain-fed agriculture. Groundwater is generally used ated within a moving window 270 9 270 m from to make up the irrigational deBcit under draught raw Shuttle Radar Topography Mission (SRTM) conditions (Zhang et al. 2004), imparting a resilient digital elevation model (1 arc-sec). It inherently response to precipitation variability like many consists error due to overestimation above other parts of the GWB. The overall stage of settlement and vegetated area. To get rid of the groundwater development in the district is around error, point elevation of each well has been 79%, which is substantially higher than the extracted using ‘zonal statistics’ of ‘Arc Tool’. national average and the state average as well Subsequently, all the DTWL (both pre- and

Table 1. Annual potential recharge scenario of Bengal basin.

Sl. no. Source Estimated potential recharge (PR) 1 BGS/DPHE 2001 0.3 to 5.5 mm/day 2 Basu et al. (2001), Dowling et al. (2003) Avg. 1.6 mm/day 3 SWID (1998) Less than or equal to *223 mm/yr (15% of the mean annual pptn) 4 Rangarajan and Athavale (2000) 198 mm/(13.6% of mean annual pptn) 5 Mukherjee et al. (2007a, b) 587 mm/yr or 1.61 mm/day (39% of the annual pptn) 228 Page 6 of 15 J. Earth Syst. Sci. (2020) 129:228

Table 2. Mean annual rainfall vs. stage of groundwater development at Murshidabad district.

Murshidabad West Bengal India Mean annual rainfall (2017) (mm)* 1187 1570 1127 Stage of groundwater development** 79% 42% 44.60% *Source: IMD website. **Source: Groundwater Year Book 2017 (CGWB).

post-monsoon) data have been converted to water Xn level elevations through RL correction. Conse- z à ðÞ¼u0 kizðuiÞþk0; quently, block-wise average water level elevations i¼1 Xn were calculated for both pre- and post-monsoon k ¼ 0 and k ¼ 1; seasons for statistical treatment of the data. 0 i i¼1

where z à ðÞu0 is the krigged value at location u , 3.1 Interpolation of water level elevation 0 zuðÞi is the sample value at location u , k is the and cross-validation i i weight factor, and k0 is a constant. Results of Precise mapping of the spatial variability of spatial interpolation of the groundwater elevation groundwater level requires interpolation of spatially data across the study area over the study period dispersed data. For the present study, all the data have been presented in the form of thematic raster were processed using Arc GIS 10.3 software to gen- maps. Selected maps are shown in Bgure 3. These erate water level elevation maps to visualize the thematic raster maps have been re-classiBed into changes in water level scenario in the district over six classes of equal interval to compare the degree the study period. In the process, a comparison of of spatial variability in groundwater elevation deterministic interpolation methods, i.e., inverse across Murshidabad district. distance weight (IDW) with stochastic methods, i.e., Ordinary Krigging (OK), Universal Krigging (UK), Empirical Bayesian Krigging (EBK), Simple 3.2 Mann–Kendall test Krigging (SK) have been made, using time series Let ðÞt ; x ; ðÞt ; x ; ...; ðÞt ; x be a set of observa- water level elevation data from 3 years, viz., 1996, 1 1 2 2 n n tions of the joint random variables T and X, 2006 and 2016 (table 3). This also helped to evaluate respectively, such that all the values of t and x are the performance of the methods over the given i j unique. The Mann–Kendall statistic S is given by dataset and to choose the unbiased and best-Bt method for estimation of water level elevation for the XnÀ1 Xn non-measured area over the district and to cross S ¼ signðxj À xkÞ validate the degree of agreement between the esti- k¼1 j¼kþ1 mated and observed values of groundwater elevation where data (Kholghi and Hosseini 2009; Sun et al. 2009). As shown in table 3, comparison of the methods so signðxj À xkÞ¼1ifxj À xk [ 0 mentioned, have been done on the basis of calculated 0ifx x 0 1 root mean square error (RMSE) and the correlation ¼ j À k ¼ ð Þ coefBcient (r) values. The comparison revealed ¼À1ifxj À xk\0: consistently similar results for OK and UK, which To compute probability associated with Mann– means that these two methods can be used reliably Kendall statistic, Kendall (1975) proposed a nor- and interchangeably for instant purpose. Accord- mal approximation test when n [ 10. Variance of S ingly, OK, also known as a partial interpolation, is calculated as: linear weighted-average technique have been used P for unbiased spatial interpolation of the water level g nðn À 1Þð2n þ 5ÞÀ tpðtp À 1Þð2tp þ 5Þ elevation values at unsampled locations and to VarðSÞ¼ p¼1 ; generate the spatial maps. The estimation for the 18 linear Krigging is given by the equation: ð2Þ J. Earth Syst. Sci. (2020) 129:228 Page 7 of 15 228

Table 3. Comparison of spatial interpolation methods on the basis of RMSE and r values.

1996 2006 2016 Pre-monsoon Post-monsoon Pre-monsoon Post-monsoon Pre-monsoon Post-monsoon Interpolation methods r RMSE r RMSE r RMSE r RMSE r RMSE r RMSE EBK 0.649 2.53 0.752 2.01 0.636 3.06 0.745 2.75 0.885 2.86 0.889 3.15 OK 0.662 2.50 0.783 1.89 0.604 3.35 0.711 2.80 0.890 2.80 0.894 3.08 UK 0.662 2.50 0.783 1.89 0.604 3.35 0.711 2.85 0.890 2.80 0.894 3.08 SK 0.674 2.50 0.770 1.90 0.649 2.94 0.643 3.08 0.885 2.82 0.930 2.46 IDW 0.623 2.64 0.742 2.03 0.641 3.07 0.723 2.82 0.873 3.00 0.895 3.15

where g is the number of the tied groups and tp is The median of these N values of Mi is the number of data points in the pth tied group. represented as Theil–Sen’s slope estimator and is The test statistic ZC is given by given by S À 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M Nþ1 if N is odd ZC ¼ if S [ 0 2  VarðSÞ 1 Q ¼ MN þ MNþ2 if N is even; ¼ 0ifS ¼ 0 ð3Þ 2 2 2 S þ 1 provided M s are ranked from smallest to largest. ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi if S\0: i VarðSÞ Negative value of Q signiBes rising/upward (falling/downward) trend in the time. A 100ð1 À ZC follows a standard normal distribution. A Àve aÞ% conBdence interval is computed by non- value of Z signify a rising (falling) trend. A signif- parametric technique based on normal distribution. icance level a is used for testing the significance of This is valid for n [ 10 (Salmi et al. 2002;Drapela and the monotonic trend (two tailed test). If jjZC [ Za, 2 Drapelov ap2011ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi). For this purpose, Ca is computed as then the trend is considered significant. Ca ¼ Z1 a VarðSÞ.Next,L ¼ ðÞN À Ca =2andU ¼ À2 ðÞN þ Ca =2 are calculated. The lower and upper limits of conBdence interval, Qmin and Qmax are the 3.3 Theil–Sen’s slope estimator test Lth largest and (U+1)th largest of the N ordered This estimator can be computed eDciently, and is slope estimates Mi.IfL and/or U are not integers, the insensitive to data outliers. It can be significantly respective limits are interpolated. more accurate than simple linear regression for To estimate b in the trend line, the n values of hetero skedastic and skewed data. It performs differences xi À mti, i =1,2,…, n are calculated. better than ordinary linear regression in case of Estimation of b is given by the median of these normally distributed data. The slope estimator is values (Serois 1998). For calculation of conBdence the median of the set of slopes jointing pair of interval for estimation of b similar procedure can distinct points. The conBdence interval is also be followed. determined by two order statistics of this set of slopes. If the trend of the time series is given by x = mt+b, the magnitude of the trend m obtained 4. Results and discussion in Mann–Kendall analysis is estimated by Theil–Sen’s slope estimator. Theil–Sen estimator is 4.1 Spatial variation of water level elevation a linearly Btted line to (t ,x), i = 1,2, , n, by i i … Decline in the water level, inherently, corresponds choosing the median of the slopes of all the lines to loss in storage within the concerned aquifer through the pair of points. The slope (M ) of all i system; which in turn reCects the dominance of data pairs is computed as (Sen 1968): extraction rates over natural recharge and capture x À x rates (Wada et al. 2012a, b; Russo and Lall 2017). M ¼ j k for i ¼ 1; 2; ...N; i j À k Generally, within the study area, the water level elevations are shallower in the Holocene Bagri where N is the number of data pairs where region compared to that of the Pleistocene Rarh nðnÀ1Þ xj [ xk ¼ 2 : tract. Over the study period, significant drop in 228 Page 8 of 15 J. Earth Syst. Sci. (2020) 129:228

Figure 3. Changes in groundwater level scenario at Murshidabad district; selective results from 1996, 2006 (a, b: pre-monsoon, a0, b0: post-monsoon) and selective results from 2016 (c: pre-monsoon, c0: post-monsoon). J. Earth Syst. Sci. (2020) 129:228 Page 9 of 15 228 water level has been observed in the Bagri region emphatic than the climatic factors, at least up to with statistically significant declining trend. the scale of the present study. Assessment of the raster maps for pre- and post- Maximum groundwater recharge occurs when monsoon period points that although both the (i) soils have high conductivity, (ii) water table lies pre- and post-monsoon season depict statistically at shallow depth, (iii) soil is relatively wet, and (iv) significant drop of water level elevation, for the the water input rate is low and lasts for a relatively same zones (Bgure 3) the variability is more in the longer interval of time (Freeze and Cherry 1969). post-monsoon period. This portrays the persistent However, for the present study area, the eAect of recharge–discharge disparity over the study area. rainfall to the monsoonal recharge of the local Normally, groundwater depletion is triggered by groundwater system seems insignificant. This is two factors; viz., climatic factors and human due to the fact that prolonged and unsustainable intervention into the hydrologic regime through pumping for irrigation since 1970s (globally 1950s pumping subject to the availability of water for after the introduction of cheap drilling technology replenishment of groundwater reserves. Hence, it is at the public domain) and continuous upstream likely that arid regions are more prone to the crisis. water diversion since the inception of Farakka However, Aeschbach-Hertig and Gleeson (2012; Barrage in the year 1975, dryness in soil have been their Bgure 3) opined that instances of groundwa- inducted in a widespread manner especially during ter depletion is most common in semi-arid and summer (Adel 2005). Besides, due to little or sus- humid regions of the world; where over-extraction tained intermittent rainfall especially during pre- pre-dominates over climatic factors controlling and/or post-monsoon period, monsoonal rainwater natural recharge. Thus, evidences from these fails to recharge the sub-soil aquifers eAectively. As observations suggest that climate-induced changes a consequence, the impact of intense pumping upon in recharge rates may inCuence rates of ground- district seems readily discernable when the water depletion. However, the surveys of global observed patterns are interpreted in terms of the literatures anticipate that the eAects of such basin scale hydrodynamics, complex sub-surface changes to aquifers have so far been small com- geology and aquifer geometry. pared with non-climate drivers (Kundzewicz et al. Regional hydro-stratigraphic analyses by 2007; Bates et al. 2008). Mukherjee et al. (2007a, b, 2018) reveals the per- Response of groundwater systems to climatic sistence of a single semi-conBned aquifer (termed as variability varies with local geology (Chen et al. Sonar Bangla aquifer or East bank aquifer (EBA)) 2004), land use land cover (LULC) (Scanlon et al. underneath the Bagri region of Murshidabad. Here 2005) and other factors aAecting inBltration and the aquifer material is medium to Bne sand with recharge rates. Russo and Lall (2017) have shown occasional gravel and channel lag deposits. Here that changes in groundwater correspond to selected the annual rainfall recharge is around 1200– global climate variability and annual precipitation. 1600 mm, with rejected recharge during the sum- According to them, shallow groundwater (\30 m) mer monsoon Cooding. The otherwise shallow sustains river base-Cow and ecosystem functions water table undergoes steepest drawdown during (Fan et al. 2013) and climatic factors impact the May–June (up to *10 m bgl with an annual Cuc- shallow water levels of such unconBned aquifers tuation of 3–6 m) (Mukherjee et al. 2007a, b). only nominally at sub-annual to annual time scales Simulations in regional scale groundwater-Cow (Healy and Cook 2002). Whereas, the water level of model by Mukherjee et al. (2007a, b) identiBed deep conBned aquifers varies as a function of cli- southward Cow within the study area parallel to matic factors only with respect to longer timescales the topography, with several waterlogged areas to due to physical constraints like the recharge signal the east of Bhagirathi River, without intensive travel time. While the heavy withdrawal from the pumping (pre-1970s). Subsequently, after the onset sub-surface storage imparts immediate local of indiscriminate pumped irrigation, such Cow impact on groundwater mass changes even in deep patterns have been perturbed significantly. CGWB aquifers (Bredehoeft 2011; Sophocleous 2012). (2016) has reported a major groundwater trough at Following these principles, it can be emphasized Bhagwangola II block which may be due to exces- that over the present study area, the impact of sive withdrawal of the same for irrigation purpose. irrigational pumping on the monitored aquifers The matter is also supplemented by the fact that of 30–150 m depth range (identiBed on the basis of all the public water supply schemes established screen depths of the monitoring wells) are more here are solely based on groundwater. According to 228 Page 10 of 15 J. Earth Syst. Sci. (2020) 129:228

Table 4. Mann–Kendall Trend test (Theil–Sen’s slope estimate and significance of the test) result on pre-monsoon depth to water level values.

Z test Sl. Mann–Kendal statistics P Sen Sen no. Name of the block coefBcient value value slope intercept Remarks 1 Lalgola À0.13 À0.79 0.43 À0.05 19.68 No conspicuous trend observed 2 Bhagwangola-I À0.42 À2.66 0.008 À0.075 19.25 Falling trend observed 3 Bhagwangola-II À0.46 À2.90 0.004 À0.088 17.15 Falling trend observed 4 Raninagar-I À0.02 À0.13 0.90 À0.003 16.37 No conspicuous trend observed 5 Raninagar-II À0.33 À2.05 0.04 À0.08 18.45 Falling trend observed 6 Domkol À0.19 À1.21 0.23 À0.035 19.23 No conspicuous trend observed 7 Jalangi À0.22 À1.39 0.16 À0.045 17.71 No conspicuous trend observed 8 Murshidabad–Jiaganj 0.005 0.00 0.98 0.00013 18.54 No conspicuous trend observed (M–J) 9 Nowda À0.28 À1.75 0.08 À0.05 14.42 No conspicuous trend observed 10 Farakka 0.31 1.93 0.053 0.067 20.42 No conspicuous trend observed, but trend may be rising as p value is marginally above 0.05 11 À0.58 À3.62 0.00 À0.18 17.69 Falling trend observed 12 Beldanga-I À0.24 À1.45 0.15 À0.086 15.70 No conspicuous trend observed 13 Beldanga-II À0.081 À0.48 0.62 À0.0271 15.71 No conspicuous trend observed 14 Hariharpara À0.27 À1.69 0.09 À0.05 16.56 No conspicuous trend observed 15 Raghunathganj-I 0.08 0.48 0.63 0.02 20.73 No conspicuous trend observed 16 Raghunathganj-II 0.16 0.97 0.33 0.04 20.23 No conspicuous trend observed 17 Nabagram À0.93 À5.87 0.00 À0.46 19.93 Falling trend observed 18 Sagardighi À0.91 À5.74 0.00 À0.34 24.12 Falling trend observed 19 Kandi À0.83 À5.26 0.00 À0.40 15.58 Falling trend observed 20 Burwan À0.81 À5.07 0.00 À0.53 21.18 Falling trend observed 21 Bharatpur-I À0.73 À4.59 0.00 À0.55 15.65 Falling trend observed 22 Bharatpur-II À0.81 À5.07 0.00 À0.39 14.90 Falling trend observed 23 Khargram À0.88 À5.56 0.00 À0.46 17.21 Falling trend observed 24 Samserganj 0.02 0.13 0.90 0.006 19.38 No conspicuous trend observed 25 Suti-I 0.06 0.36 0.72 0.03 20.42 No conspicuous trend observed 26 Suti-II 0.14 0.84 0.40 0.04 20.36 No conspicuous trend observed

Mukherjee et al. (2018) such composite cones of Pleistocene tracts of the Rarh region are known for depression, induced by irrigation pumping can high yielding paddy (Boro) cultivation. Such an be as large as 20 km in diameter,with a vertical agro-practice is extremely water intensive and gradient up to *0.25 m/m at present, which is unabated groundwater abstraction for these leads projected to be *0.36 m/m in 2021. According to to further rapid decline of water level in the region. Mukherjee et al. (2011), the groundwater Cow systems are different on the either banks of the 4.2 Temporal variation of depth to groundwater River Bhagirathi. Amidst the existing groundwater abstraction scenario, deeper circulation of shal- The non-parametric Mann–Kendal test as well as low/surface water is observed below the thick Theil–Sen slope parameter reveal significant sands of the EBA with the development of local declining trend in water level elevation data during Cow systems. However, evidences of such circula- both the season for majority of the blocks (shown tion are lacking in the west-bank aquifer (WBA) of in tables 4 and 5) at 5% level of significance. In pre- Rarh region owing to the dominance of clay and silt monsoon season, most of the blocks of Bagri region intercalations in the local hydrostratigraphy. depict no conspicuous trend, whereas declining Rather in this region, groundwater replenishment trend have been observed in blocks of Rarh region through rainfall inBltration is almost absent due to (table 4 and Bgure 4a). In the case of Farakka, no hard clay blanket over older alluvium. Also these conspicuous trend observed, but the same may be J. Earth Syst. Sci. (2020) 129:228 Page 11 of 15 228

Table 5. Mann–Kendall Trend test (Theil–Sen’s slope estimate and significance of the test) result on post-monsoon depth to water level values.

Z test Sl. Name of the Mann–Kendal statistics Sen Sen no. block coefBcient value P value slope intercept Remarks 1 Lalgola À0.36 À2.26 0.02 À0.12 22.38 Falling trend observed 2 Bhagwangola-I À0.61 À3.83 0.00 À0.12 21.63 Falling trend observed 3 Bhagwangola-II À0.59 À3.71 0.00 À0.15 20.55 Falling trend observed 4 Raninagar-I À0.37 À2.32 0.02 À0.10 19.42 Falling trend observed 5 Raninagar-II À0.59 À3.71 0.00 À0.13 20.89 Falling trend observed 6 Domkol À0.49 À3.05 0.002 À0.12 21.87 Falling trend observed 7 Jalangi À0.54 À3.41 0.00 À0.14 21.05 Falling trend observed 8 Murshidabad–Jiaganj À0.51 À3.23 0.001 À0.06 21.03 Falling trend observed (M–J) 9 Nowda À0.47 À2.93 0.003 À0.13 17.89 Falling trend observed 10 Farakka À0.46 À2.87 0.004 À0.15 25.48 Falling trend observed 11 Berhampore À0.57 À3.59 0.00 À0.20 19.89 Falling trend observed 12 Beldanga-I À0.51 À3.23 0.001 À0.11 18.31 Falling trend observed 13 Beldanga-II À0.53 À3.35 0.00 À0.10 18.62 Falling trend observed 14 Hariharpara À0.59 À3.71 0.00 À0.10 19.79 Falling trend observed 15 Raghunathganj-I À0.51 À3.23 0.001 À0.18 26.84 Falling trend observed 16 Raghunathganj-II À0.19 À1.18 0.24 À0.06 24.18 No conspicuous trend observed 17 Nabagram À0.84 À5.28 0.00 À0.56 22.39 Falling trend observed 18 Sagardighi À0.86 À5.40 0.00 À0.56 27.69 Falling trend observed 19 Kandi À0.83 À5.22 0.00 À0.59 20.02 Falling trend observed 20 Burwan À0.81 À5.10 0.00 À0.77 28.73 Falling trend observed 21 Bharatpur-I À0.81 À5.10 0.00 À0.69 22.26 Falling trend observed 22 Bharatpur-II À0.84 À5.28 0.00 À0.57 20.43 Falling trend observed 23 Khargram À0.88 À5.53 0.00 À0.78 24.71 Falling trend observed 24 Samserganj À0.58 À3.65 0.00 À0.17 24.35 Falling trend observed 25 Suti-I À0.11 À0.69 0.49 À0.03 23.98 No conspicuous trend observed 26 Suti-II À0.40 À2.51 0.01 À0.11 24.34 Falling trend observed

considered rising as p value is marginally above district, coupled with minimal natural recharge than 0.05. However, in the post-monsoon, falling through rainfall have resulted in declining trend in trend has been observed in all the blocks except sub-surface water-levels of this region. According Raghunathganj-II and Suti-I at 5% level of signif- to Santra (2017), in this particular part of the icance (table 5 and Bgure 4b), which means that lower Gangetic delta, the principal source of climatic factor like monsoonal rainfall imparts a arsenic is the arsenic sulphide minerals deposited little impact upon the rise or fall of water level at with the clay in the reducing environment. Here, the study area. The scale of the present study overexploitation of groundwater during summer limits its scope to discern the larger impact of cli- for Boro cultivation not only lowers the ground- mate or cropping pattern but, it definitely reCects water level beyond the limit of sustainability, but that alike many other parts of the GWB, the also leads to aeration and oxidation of arsenic human intervention into the hydrologic regime sulphides. This ultimately leads to the leaching of through pumping can readily be held responsible arsenic in ground water. Pumping beyond sus- for the declining trend of groundwater level within tainable limit since 1970s has disrupted the regio- the study area. nal groundwater Cow pattern within the entire According to Pal and Akoma (2009), overex- GWB, including Murshidabad district as well ploitation of groundwater resources, for irrigation through development of several local to interme- of Boro crops in various parts of Murshidabad diate-scale Cow systems (Sikdar et al. 2001; 228 Page 12 of 15 J. Earth Syst. Sci. (2020) 129:228

Figure 4. Interpolated raster surface map showing the average rate of changes of groundwater elevations from wells with statistically significant trends (p \ 0.5) observed between 1996 and 2016. (a) pre-monsoon and (b) post-monsoon.

Mukherjee et al. 2007a, b, 2009; Banerjee and recent indiscriminate usage of the water acts as a Mitra 2017). The regional hydraulic gradients got bane in intensely irrigated areas. The major aqui- impacted by the pumping centers and aquifer fers from the arid and semi-arid regions, world- architecture. Incidentally, being an agricultural wide, are Bnding hard to survive the menace of hotspot of whole GWB Murshidabad, happens to rapid groundwater depletion. Decline in water be one major pumping centre. As a consequence, level, inherently, corresponds to loss in storage associated to the dramatic growth in irrigated area, within the concerned aquifer system; which in turn aquifers from the region have experienced decline implies that extraction has exceeded natural in their storage vis-a-vis water level elevation as recharge. Similar scenario has been observed in the revealed by the present study. In fact, despite present study area as well. Murshidabad does not being located within a water proliBc Cuvio-deltaic resemble typical water-stressed regions of the environment, the statistically significant fall in country. However, the present studies are warrants water level elevations seems comparable to that of of startling the water crisis in the region. So, the the popularly known water-stressed areas of the future groundwater resources planning should be country; primarily due to insignificant natural oriented accordingly. The hydrological response of recharge through rainfall. aquifer systems of Murshidabad, as revealed by the present study, in terms of declining water level elevation should be considered for the subsequent 5. Conclusion control, to attain sustainability in terms of avail- ability of groundwater resources. EAective water The non-parametric Mann–Kendal test as well as resource management initiative should be thrived Theil–Sen slope parameter reveals that over major from all levels to arrest the problem before it is too portion of the study area, significant declining late. The present study intends to frame a technical trend in water level elevation is observed during protocol (including RL correction, spatial interpo- both the season. Lack of groundwater replenish- lation of data, analyses and quantiBcation of trend ment within recoverable recharge, coupled with present in the data) for micro-level (block/district) overextraction causes such drop of groundwater groundwater resource managers to keep vigil upon elevation on either side of Bhagirathi River. The the changes in aquifer storage conditions. J. Earth Syst. Sci. (2020) 129:228 Page 13 of 15 228

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