Seasonal spatial distribution of groundwater suitability in the Cabo Frio shallow coastal aquifer, Rio de Janeiro, Brazil: applying groundwater quality index and GIS approaches

Significant demographic growth, lack of basic sanitation, and scarcity of surface water sources have led to groundwater stress around Cabo Frio city in the semi-arid region of Rio de Janeiro State, Brazil. Therefore, groundwater is potentially contaminated and unfit for human consumption. Hydrogeochemistry tools were applied to evaluate groundwater quality seasonal variation in the year 2019. The Groundwater Quality Index for Human Consumption (GWQIHC) was used to evaluate groundwater samples collected in shallow wells during rainy and dry season campaigns. Sampled wells were located in distinct socio-environmental contexts. Hydrogeochemical analyses demonstrated groundwater is in non-equilibrium conditions with the aquifer minerals, suggesting recent external inputs and influences. Samples were mostly classified as sodium chloride and magnesium chloride types in the rainy and dry periods, respectively. The bulk of groundwater samples were ranked as suitable for irrigation purposes by the US Salinity Laboratory classification. The GWQIHC was applied to both non-toxic and toxic parameters, revealing samples that do not conform to established standards for drinking water quality regarding physical-chemical parameters (pH, electrical conductivity, hardness, total dissolved solids), total coliforms, major ions (Ca2+, Mg2+, Na+, Fe, K+, NO3-, Cl-, and SO42-), and most of the potentially toxic metals (Ba, Se, As, B, Al, and Cr). All samples were classified as improper or in need of treatment before human consumption, according to the GWQIHC. The significant amount of non-compliant parameters can cause multiple health problems for the entire local community. Parameters with concentrations close to the limit have a high risk of worsening water quality and the health of residents. Spatial distribution demonstrated that the western (farthest from the coast), and southern portions of the areas had the poorer ratings, despite oscillations. The largest area with poor groundwater quality was observed in the dry season by GWQIHC indexed with toxic parameters. Therefore, the government and associated agencies have to guarantee that measures aimed at safeguarding the population's health are implemented, such as expanding access to treatment for water-borne diseases and the sewage collection and treatment network.