Environmental factors controlling arsenic mobilization from sandy shallow coastal aquifer sediments in the Mannar Island, Sri Lanka

Abstract

Elevated levels of arsenic (As) have been reported in groundwater from sandy aquifer systems in the Mannar Island, located in the northwest of Sri Lanka. As a result of serious health concerns, this study aims to investigate groundwater aquifers in order to trace the different As sources and mechanisms involved in its release from aquifer sediments. A piezometer was constructed in an As contaminated area and groundwater and sediment samples were retrieved from the well for further analyses. Water samples were also collected from wells in the vicinity of the piezometer and As species and other trace elements were determined using voltammetric and ICP-MS methods. Groundwater samples showed total As concentrations ranging from 6.5 μg/L up to 43.8 μg/L. All wells with high As concentrations (>25 μg/L) showed near-neutral pH conditions. Fe and Ba concentrations in groundwater indicated their coexistence with As. Among the different elements detected in sediments, Na, K, Ca, Mg, Al, Fe, As, Ba and organic matter content showed an increasing trend towards the groundwater table. Sediment analysis depicted very high affinities of both As and Fe, particularly in the finer grain size. Desorption experiments indicated that most effective release of As occurred between pH 5.0 and 6.0, the average release of As being 61 mg/L and 54 mg/L per kg of aquifer material, respectively. The FTIR analyses indicated As-bearing metal-OH complexes in sediment coatings. When rain water infiltrates the aquifer, oxygen becomes depleted at or near the surface due to the decay of organic matter, and hence, dissolution of carbonate minerals may increase with subsequent increase of groundwater pH to near-neutral levels. Such conditions are favored for mobilizing As into groundwater from As-adsorbed Fe-oxyanions in sediments.