Vulnerability of Coastal Aquifers Due to Nutrient Pollution from Agriculture: Kalpitiya, Sri Lanka

This study focuses on spatial and temporal nutrient pollution of groundwater in the unconfined sandy aquifers of Kalpitiya peninsula, Sri Lanka, where agricultural activities are intense. The study covers two consecutive dry and rainy seasons during the period from 2008 to 2010. Nitrate is the dominant nutrient pollutant in groundwater. The values of Nitrate-N contents ranged from 0.60 to 212.40 mg/L in the dry seasons and 0.20–148.50 mg/L in rainy seasons. Phosphate in groundwater ranged from 0.20 to 5.70 mg/L in dry seasons and 0.04–10.35 mg/L with few exceptions in rainy seasons. About 50% of the studied water samples had Nitrate-N concentrations above WHO drinking water guideline values both in dry and rainy periods. These high concentrations were recorded from wells in agricultural lands. Although there is a slight decrease in the Nitrate-N concentrations at random in rainy seasons, an increasing trend of average concentrations became evident over the study period as a whole, probably indicating building up of Nitrate-N in groundwater in the vegetable growing areas. The spatial distribution of Nitrate-N too shows a good match of high Nitrate-N bearing zones with vegetable cultivated areas indicating intensive leaching from application of excessive chemical fertilizers. High Nitrate-N zones also showed fairly steady lateral distribution indicating slow lateral mobility of Nitrate-rich groundwater probably due to low hydraulic gradients. Low phosphate concentrations in both groundwater and surface soils either indicates their less use in the area or that the available phosphate is leached and removed from the aquifer water and (sandy) soil solutions and probably adsorbed in clayey deeper horizons. Low concentrations of major cations (especially K, Ca, and Na) indicate less impact on cation concentrations in groundwater by the fertilizer application or sea water intrusions/up-coning.