Research papersGroundwater sustainability challenges revealed by quantification of contaminated groundwater volume and aquifer depletion in hard rock aquifer systems
Introduction
Groundwater is an integral component of the hydrological system. Recent investigations like hydrological modeling (Wada et al., 2014) and Earth observations (Rodell et al., 2009a, Rodell et al., 2009b, Famiglietti, 2014) have quantified alarming rates of groundwater depletion globally. The sustainable groundwater development is not only constrained by resource availability but also by quality deterioration (Morris et al., 2003). Wada et al‘s. (2010) study carried out in sub-humid and arid regions states that total global groundwater depletion during the period of 40 years (1960–2000) has increased from 126 (±32) km3/yr in 1960 to 283 (±40) km3/yr. Over-exploitation and excessive pumping can lead to groundwater depletion, where groundwater is extracted at a faster rate than it can be replenished. This eventually renders a groundwater system vulnerable to contamination, pollution, and degradation. The availability of groundwater and the suitability of its quality for different uses are inextricably intertwined (Alley et al., 1999). Many researchers have carried out considerable work on groundwater sustainability and related factors like quantification of sustainability metrics (Pandey et al., 2011, Gleeson et al., 2012a) global groundwater assessments, and stress indices (Richey et al., 2015). The impacts of groundwater withdrawals on global groundwater storage (Gleeson et al., 2012b, Döll et al., 2012) and temporal changes in groundwater storage influence the complex interactions in global aquifer systems (Thomas et al., 2017). However, limited research has been carried out to study the integrated effects of groundwater depletion and groundwater contamination on groundwater sustainability and renewability. The objective of our study is to quantify spatio-temporal aquifer depletion through hydraulic head monitoring and groundwater modeling at a watershed scale as well as quantify contaminated groundwater volume through volume–concentration relationship to investigate virtual groundwater loss from the aquifer system. We introduce a new concept of virtual groundwater loss through contamination. Our concept of virtual groundwater loss through contamination is different from the definitions provided by (Allan, 1998; Hoekstra & Chapagain, 2008; Hoekstra and Mekonnen, 2012) which state that virtual water and water footprint is the amount of water ‘embedded’ in an agricultural product and covers the entire food supply chain from production to delivery including pollution produced in the process between the interconnected regions involving exchange. However, our concept of virtual groundwater loss considers the volume of the contaminated groundwater physically available in the aquifer but not fit for use. We use the term virtual groundwater loss as this volume is present in the aquifer but cannot be considered a part of groundwater resource as it is contaminated beyond the permissible limit. This is an entirely new concept and the novelty and uniqueness of the study is its ability to quantify the contaminated groundwater volume using the volume concentration relation. This research generates an idea about the degraded volume of groundwater in an aquifer, aquifer depletion through ground-based observations like hydraulic head monitoring and numerical modeling, effects of dilution from rainfall as well as solutes from irrigation return flow. The scope of the study is an overview and new insights to understand the integrated effects of groundwater depletion and aquifer contamination as well as advance our understanding of groundwater renewability and sustainable groundwater resource management. This study was performed on the southern India watershed (Maheshwaram) as this region is witnessing alarming declines in groundwater resources as well as fluoride contamination and data time series of 10 years was used. The watershed consists of a hard rock aquifer system and semi-arid climate.
Section snippets
Site description
The Maheshwaram watershed (53 km2) (Fig. 1) in Telangana, India is a hard rock formation composed of Archean granites of the Hyderabad group. The mean annual precipitation is 750 mm which occurs as rainfall in the monsoon season (June–October). The potential evapotranspiration is 1800 mm. The watershed has a nearly flat topography with elevations ranging from 590 to 670 m AMSL (meters above mean sea level) (Sarah et al., 2014). Paddy is the predominant cultivation of the area consuming the
Compartmentalization of the watershed and its effects on hydrochemistry
The work carried out by (Guihéneuf et al., 2014, Sarah et al., 2014) highlighted the aquifer connectivity under different water table conditions. (Guihéneuf et al., 2014) report that the aquifer moves from regional to local flow conditions under varied water table fluctuations e.g. under the low water table conditions which are usually noticed during the pre-monsoon period, lateral compartmentalization is induced by the smaller lateral extensions of deeper fractures. The changes in the
Depletion estimates by groundwater modeling and quantification of total aquifer volume through a change in storage.
Maheshwaram watershed is modeled with Visual Modflow flex software developed by Schlumberger. This was used to build a conceptual model of the watershed and then numerical modeling was carried using MODFLOW (Harbaugh, 2005). The watershed is taken as a two-layer system, the first one as porous formation representing the weathered layer, the aquifer in the weathered layer is dry during the simulation period with comparatively low variability in hydraulic conductivity. The first layer is
Total groundwater volume estimation
The total volume of groundwater was quantified with 10-year hydraulic head data (2000–2010) using the height of the water column, Area of the watershed, and specific yield. The height of the water column was calculated both in dry and wet seasons by subtracting the known average depth to bedrock elevation (interpreted from the Vertical Electrical Sounding and lithologs of the drilled borewells) from the water level above mean sea level.
The method used here is specific yield (Sy) or fillable
Contaminated groundwater volume (VC)
The set of equations (Eqs. (3)–(6)) is used to quantify the contaminated groundwater volume (Vc). The volume of contaminated water Vc is a state indicator that has been useful to know the present status and future trends in groundwater quality and has also helped to analyze and visualize groundwater quality contamination both in space and time. The dilution from rainfall controls the natural attenuation of fluoride in groundwater also visible in Pre-monsoon (PRM) and Post-monsoon (POM) fluoride
Conclusion
The integrated studies of the spatio-temporal quantification of contaminated groundwater volume and groundwater depletion through volume concentration relationship, hydraulic head monitoring, and groundwater modeling respectively advances our understanding of groundwater contamination coupled with groundwater depletion. The study indicates that hard rock aquifers having discontinuities do not behave as single units rather different units dominantly governed by local heterogeneities. The series
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are thankful to CSIR-NGRI and UPMC, Paris for providing the facilities. The first author is thankful to EC, French Embassy in India, and CSIR-NGRI for providing the fellowship to carry out this research. Dr. Helene Beucher of Centre de Géoscience, Fontainebleau, France is kindly acknowledged for the productive discussions, anonymous reviewers whose reviews helped to improve the quality of the manuscript) and Mr. Waseem Shah for helping with figures. The Department of Earth sciences,
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