Aquifer Segmentation as a Tool for Sustainable Drinking Water Supply and Reverse-Osmosis Waste Management in the Dry Zone of Sri Lanka’s North Central Province

Rural communities in Sri Lanka’s North Central Province (NCP) Dry Zone face a drinking water security problem with two linked dimensions: physical scarcity, driven by low rainfall, high evapotranspiration and near total dependence on a thin, fractured bedrock aquifer; and poor water quality, with widespread hardness and elevated fluoride closely associated with an epidemic of Chronic Kidney Disease of unknown etiology (CKDu). The principal response over the last two to three decades has been the installation of several hundred small, community operated reverse osmosis (RO) treatment stations. These stations reliably remove hardness, salinity and fluoride from feed groundwater, but recover only 19–50% of the water they treat, and a large majority discharge their concentrated reject brine directly onto open soil, creating a second, comparatively unstudied threat to shallow groundwater and soil quality. This study asks whether subsurface structural information makes both problems more tractable. We monitored water chemistry at nine tube wells, one hand dug well and one seasonal tank in the Rambewa and Medawachchiya divisional secretariat divisions over approximately two years (July 2016–February 2018), and constructed a three dimensional groundwater flow model from 384 well logs. Piper diagram classification identified two distinct hydrochemical facies within close proximity: a dilute, meteoric Ca-Mg-HCO3 facies at two locations and a more evolved, higher TDS Na-HCO3 facies elsewhere, with no evidence of mixing during the study period. Cross sections through the flow model show a bedrock topographic high coincident with the facies boundary, consistent with compartmentalisation of the aquifer into discrete isolated segments rather than a single well-mixed system. Groundwater model is presented as a supporting hydrogeological interpretation rather than as quantitative proof of hydraulic separation. We propose that mapping this kind of segmentation at the scale of individual RO stations offers two low-cost management tools: siting supply wells within lower TDS compartments to reduce the volume of brine generated, and directing that brine toward already higher TDS, isolated compartments. Both proposals require on site testing: including a site specific risk assessment of the proposed disposal compartment before either is adopted operationally.

aquifer segmentation; groundwater quality; reverse osmosis reject water; Dry Zone of Sri Lanka; chronic kidney disease of unknown etiology

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