Arsenic has long been known for its toxicity, but it has not always been included as a standard parameter in water quality analyses. Arsenic is a natural geogenic groundwater contaminant found in many aquifers around the world and the consumption of groundwater from aquifers with arsenic mobilisation has led to serious health problems in many locations. In Vieira de Leiria – Marinha Grande coastal aquifer in Portugal, the local water authorities have had long standing concerns regarding high arsenic, iron and manganese concentrations in the multilayer porous aquifer used for drinking water supply in the region. Groundwater abstracted from the aquifer frequently shows concentrations of arsenic above the Portuguese drinking water limit (DWL) of 10 μg L-1 As. In addition, concerns about salinization near the coast have been raised due to a steady increase of chloride concentrations during the summer months. Groundwater is the main source of water in the area but its quality is currently constraining its use for public supply.
No hydrogeological or hydrogeochemical research of the aquifer has yet been reported by the scientific community. Consequently, this is the first study of stratigraphic, geochemical and hydrological controls on chloride and arsenic concentrations in the Vieira de Leiria-Marinha Grande multilayer aquifer and the implications for water management and human health.
Groundwater sampling was done in wells for public water supply and in private boreholes. On site measurements in deep boreholes included pH, water temperature (T), dissolved oxygen (DO), redox potential (Eh), specific electrical conductance (SEC, 25°C) and total alkalinity. Measurements were done using an in-line flow cell to prevent aeration. Acidification of sample aliquots ensured preservation of arsenic species. ICP-MS was used to analyse samples for arsenic species, total arsenic as well as major-, minor- and trace elements, while ion chromatography was used to analyse anions. The water quality data was thereafter examined using geostatistical methods and geochemical modelling to understand the origin of arsenic and major geochemical patterns.
The results indicate a geogenic origin of arsenic in groundwater and were used to calculate potential health risks (carcinogenic and non-carcinogenic). Arsenic, iron and manganese values were used to map areas with water quality constraints for public water supply and are used as planning tool for future drilling in the aquifer. EC values, chloride results and chemical ratios were used to identify the areas of saline intrusion risk.