IWRA Proceedings

This study assesses the distribution of fresh and saline groundwater and its seasonal dynamics in response to rainfall, storm surges, and abstractions on a small coral-limestone island in Sri Lanka, within a semi-arid setting. Similar to other coral-limestone islands, the groundwater in the study area, named Delft Island, occurs as a lens of freshwater overlaying seawater in a highly permeable aquifer. Short-term growth in population and tourism, in combination with long-term sea-level rise, is expected to affect the availability of groundwater on the island, and the current study further looks at solutions towards optimizing groundwater abstractions. Field assessments, involving well inventories, sampling for stable water isotopes and hydrochemistry, interviews with the local population, and 1D vertical electrical soundings (VES), were combined with steady-state analytical solutions and numerical modelling using MODFLOW & MT3DMS to evaluate the spatial distribution of fresh and saline water and the response of the aquifer to recharge and abstractions. Results reveal a thin and irregularly shaped freshwater lens overlying seawater with a relatively thin transition zone, as well as small-scale heterogeneity in the aquifer and localised upconing below some pumping wells. Estimated recharge is high, in particular in the elevated (3-6 m +msl) parts of the island covered by sand deposits. Findings from δ18O and δ2H analyses suggest the meteoric origin of surface water and groundwater, with salinization mainly caused by mixing with seawater and evaporation. The very shallow occurrence of seawater is mostly a result of high aquifer transmissivity, low elevation and low hydraulic heads, as well as the presence of lagoons in the centre of the island which are inferred to be in hydraulic continuity with the ocean. Discharge is mostly towards the sea and rates are high. High alkalinities and CO2 pressures in saline groundwater near the coast further suggest the possible role of infiltration of saline water from storm surges and subsequent percolation through the soil zone. Elevated nitrate concentrations in both groundwater and surface water in some areas reveal anthropogenic contamination from sewage and agricultural runoff. Steady-state simulations highlight that the freshwater lens and transition zone thickness are highly sensitive to recharge and mechanical dispersion. Solutions towards increasing groundwater availability for abstraction therefore include managed recharge in the sandy aquifer during the rainy season and recovery through horizontal abstraction techniques. These techniques are currently being studied in more detail, which should ultimately result in a pilot employing these techniques on the island.