Abstract
Over-exploitation of groundwater in India’s fastest-growing metropolis, Bengaluru, has resulted in wells being bored to unprecedented depths in a crystalline-rock aquifer. However, key questions about sustainability of this extraction process remain unaddressed due to the complexity of monitoring. Using primary surveys, this study looks at the spatio-temporal evolution of the wells on a city scale, finding that catchments with deficient water infrastructure have deeper wells. To maintain yields, well with depths >400 m are drilled, especially since 2000, leading to unsustainable groundwater extraction. Camera inspections in 54 wells at Electronic City in 2016 and 2017 revealed that water levels in the majority of the wells remained lower at depths <100 m, although some wells had deeper water levels at depths >250 m. Analysis of δ18O and δ2H signatures of groundwater samples at all depths followed the local meteoric water line indicating recent recharge, implying that drilling deeper only increases the borehole volume and does not tap into newer water sources. Water levels in deeper wells may stabilize at lower depths, are subject to high spatial variability, density of drilling, and high connectivity in upper zones. Given the interconnectedness between shallow and deeper aquifers, our research shows that increasing borewell depths could be a good indicator for falling aquifer water levels. This study fills an important gap in peri-urban, intermediate-scale aquifer conceptualizations across different land uses and provides further evidence for the difficulties of reliable groundwater monitoring in the over-exploited hard-rock aquifers of Bengaluru city.
Highlights
Across Asia, growing urbanization puts pressure on groundwater resources, in the cities of Delhi, Shanghai, Karachi, and Jakarta [1]
The metropolitan can be divided into six sub-catchments: the Upper Arkavathy Catchment (UAC) and the Vrisbhavathy Catchment (VVC), which are tributaries to the Arkavathy river on the western side of the city; and the Yellamallappa Chetty (YMC) Valley, Western Hoskote Catchment (WHC), the Kormangala-Challaghatta Catchment (KCC), and the Bommansandara-Attibele
We argue that the main characteristics of the borewell evolution of the region are captured in our dataset, since the dataset covers a large area, significant trends could be detected and the heterogeneity typical for hard-rock aquifers is included in the camera inspections
Summary
Across Asia, growing urbanization puts pressure on groundwater resources, in the cities of Delhi, Shanghai, Karachi, and Jakarta [1]. Schwartz et al [3] deplored the myths of groundwater sustainability in Asia that erroneously claim its use without depletion of aquifers. Gleeson et al [4] suggested that, under such conditions, the governance of groundwater is more important for its sustainable extraction than resource depletion. The debate on estimating the availability of groundwater resources based on recharge and management strategies can be traced back to Bredehoeft [5]. He argued that there is a great need to better understand groundwater capture based on an analysis of preferred flow paths to assess well recharge and productivity.
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