Water Quality in the Lower Bengal Basin: Risks and Opportunities

Mohasin, Piya (2024) Water Quality in the Lower Bengal Basin: Risks and Opportunities. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Piya Mohasin (13IP037)) 13IP037.pdf - Submitted Version Restricted to Repository staff only Download (28MB)

Abstract

Detection of high pesticide concentrations in sediments and water often leads to prioritizing a site as being ‘at risk’. However, the risk depends not on pesticide concentration alone, but on other site-specific characteristics. Therefore, an indicator was developed that identifies the ‘Level of Concern’ by integrating five such characteristics: (i) pesticide concentrations in surface and groundwater causing risks to ecological health, (ii) impacts on human health, (iii) water scarcity, (iv) agricultural production, and (v) biodiversity richness. This framework was applied to an agricultural region of the Lower Bengal Basin in West Bengal, India, where organochlorine pesticides (OCPs) were measured in surface and groundwater within an 8 km² area. Of 20 banned and restricted OCPs, 11 were causing high risk to ecological health, and 10 were at concentrations above the Accepted Carcinogenic Risk Limit (ACRL) for humans. Although the OCP concentrations were below the statutory guideline values (100 ng/L for pesticides and 500 ng/L for the residue mixture), the study area is at high risk of OCP pollution, as indicated by the framework. The study area is also at high risk of water scarcity, with steady groundwater levels declining to meet the primary demand for irrigation. To measure the changes in subsurface characteristics that may be associated with falling groundwater levels, the electrical resistivity survey revealed that the thickness of the unsaturated zone increased from pre-monsoon to post-monsoon. While groundwater potential and potential of natural recharge decreased from pre-monsoon to post-monsoon, contributing to a seasonal decline in the groundwater level. Therefore, a small-scale, cost-effective Managed Aquifer Recharge (MAR) system was implemented to replenish the aquifer with rainwater or agricultural runoff to address this groundwater depletion. Despite being in its early stages, the MAR system has raised groundwater levels (about 1 m) in the study area and improved water quality, ensuring sustainable groundwater supply.

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