Irrigation impact on water quality and aquifer vulnerability in Kala Oya basin, Sri Lanka

The impact of precipitation and irrigation return flow on water quality and groundwater vulnerability was investigated in the Kala Oya basin, Sri Lanka using the Weighted Arithmetic Water Quality Index, DRASTIC Index, and isotope hydrology. Groundwater and surface water were analyzed for water quality parameters and stable isotopes (¹⁸O and ²H) in dry and wet seasons. In both seasons, neutral to alkaline water was observed. Electrical conductivity (EC) and major ions were higher in shallow (EC: 718.3 ± 323.8 μS/cm; Ca²⁺: 49.54 ± 32.99 mg/L) and deep groundwater (EC: 708.1 ± 321.5 μS/cm; Ca²⁺: 37.06 ± 11.21 mg/L) than in surface water (EC: 371.7 ± 154.4 μS/cm; Ca²⁺: 29.66 ± 11.64 mg/L), while similar values of EC and major ions were reported in both shallow and deep groundwater samples. Ca–Mg–HCO₃ was the dominant groundwater type which was derived from silicate and carbonate rock weathering. Observations in the wet season revealed δ²H ranged from −47.97 to −16.71‰ and δ¹⁸O ranged from −7.56 to −2.87‰, while in the dry season, δ²H ranged from −44.90 to −14.06‰, and δ¹⁸O ranged from −7.69 to −2.08‰. The enriched δ¹⁸O and δ²H groundwater values were mainly the result of progressive evaporative concentration of the heavier isotopes before water infiltration and groundwater recharge. The solute concentrations were enriched in groundwater by agricultural irrigation returns and associated evaporation indicating the influence of irrigation return flow on groundwater. It is suggested that agricultural practices increase groundwater vulnerability, which recommends special attention to long-term groundwater quality and quantity monitoring, hydrological modelling, land-use planning, and the development of sustainable policies in the basin.