Evaluation of spatial and temporal patterns in groundwater quality in the Kalpitiya peninsula

Abstract

The study investigates the spatial and temporal variation of groundwater quality within the unconfined sandy aquifers of the Kalpitiya Peninsula, Sri Lanka, focusing on four Grama Niladari (GN) divisions: Thethapola, Nawakkaduwa, Narakkaliya, and Kandakuliya Kudawa. These divisions were strategically selected to represent different land-use practices, with groundwater samples collected from 40 agricultural and drinking water wells between October 2023 and January 2024. Selected water quality parameters were assessed using standardized analytical methods. The impact of spatial location and seasonal variation on these parameters was statistically evaluated using the General Linear Model (GLM), followed by Tukey’s pairwise comparison. The results reveal significant spatial variations in groundwater quality across the studied GN divisions, with the highest concentrations of several key parameters observed in Nawakkaduwa. Specifically, Nawakkaduwa exhibited the highest electrical conductivity (EC) (1948.40 ± 5.99 µS/cm), total dissolved solids (TDS) (900.52 ± 12.27 mg/L), salinity (1.01 ± 0.01 ‰), total hardness (1026.78 ± 5.61 mg/L), sulfate (77.58 ± 0.21 mg/L), and nitrate (58.36 ± 1.47 mg/L). These elevated levels are likely attributable to seawater intrusion, evaporation, and mineral deposits in the region. Thethapola displayed the lowest values for several parameters, including EC (544.05 ± 5.48 µS/cm), TDS (247.85 ± 15.35 mg/L), salinity (0.24 ± 0.02 ‰), total hardness (327.75 ± 2.76 mg/L), chloride (34.95 ± 0.64 mg/L), and sulfate (1.50 ± 0.05 mg/L). This suggest that Thethapola's groundwater is characterized by lower salinity and fresher water quality, possibly due to the area's geological characteristics, and reduced influence of seawater intrusion. Kandakuliya Kudawa exhibited the highest total alkalinity (297.38 ± 23.63 mg/L), indicative of a moderate buffering capacity against pH changes, while Narakkaliya had the lowest total alkalinity (69.33 ± 10.85 mg/L). Additionally, Kandakuliya Kudawa recorded the highest chloride concentration (274.52 ± 4.87 mg/L). Narakkaliya displayed the highest phosphate concentration (1.38 ± 0.01 mg/L) and relatively high salinity, which could be associated with agricultural runoff and fertilizer use in the area. The alkaline conditions (pH > 7) observed in Nawakkaduwa and Narakkaliya, alongside the higher salinity and total hardness in Nawakkaduwa, suggest that groundwater in these areas is more prone to contamination from seawater intrusion and dissolved minerals. Thethapola and Kandakuliya Kudawa, with their slightly acidic to neutral pH and lower salinity, reflect freshwater characteristics with less influence from coastal processes. The study highlights the importance of ongoing monitoring and targeted management to address groundwater quality variations in the Kalpitiya Peninsula, emphasizing the need to understand local hydrogeology and land use for sustainable resource management.