Integrating stable isotopes and hydrogeochemical indices for vulnerability assessment of extensively irrigated Kala Oya basin, Sri Lanka.

This study investigates the role of irrigation on groundwater quality, agricultural viability, and groundwater management in the Kala Oya River basin. Utilizing the DRASTIC-Land Use (DRASTIC-LU) model in conjunction with geographic information systems, the research assesses groundwater vulnerability and suitability for irrigation through a comprehensive analysis of chemical, isotopic, and irrigation quality indices. Seven irrigation quality indices (Sodium Adsorption Ratio, Percent Sodium, Permeability Index, Kelly's Ratio, Magnesium Hazard Ratio, Soluble Sodium Percentage, and a novel Global Index (GI)) were analysed. The GI is a comprehensive metric that integrates hydrochemical parameters (major ion concentrations and water quality indices) with isotopic signatures of oxygen and hydrogen to provide a holistic assessment of groundwater suitability for irrigation. The DRASTIC-LU assessment identified moderate vulnerability across the watershed, with increased susceptibility during the wet season due to irrigation return flow. According to the results, both shallow and deep groundwater in the basin is suitable for irrigation, supporting agricultural needs year-round. Principal component analysis of chemical and isotopic parameters revealed two principal components explaining 70.45 % and 72.51 % of the variance in wet and dry seasons, respectively. The first component reflects water-rock interactions influencing chemical composition, while the second highlights isotopic variations linked to evaporation processes. The developed GI, classified most groundwater samples as Excellent Water (75 % during the wet season and 67 % during the dry season), with no samples falling into unsuitable quality categories. The findings underscore the basin's overall suitability for sustained agricultural use, while also highlighting the need for ongoing monitoring and management to safeguard groundwater resources. The study concludes with recommendations for continued water quality assessments to ensure the long-term sustainability of agricultural production in the Kala Oya basin.