The development of a conceptual model for arsenic mobilization in a fluvio-eolian aquifer using geochemical and statistical methods

Abstract

The studied area is part of the sandy plain of the South of Córdoba (Argentina). The objective of this work was to better understand the controls on groundwater geochemistry in the unconfined aquifer, especially in relation to arsenic occurrence and mobilization in groundwater, to provide baseline information for water managers. The aquifer shows a varied spatial geochemical pattern with fresh-to-salty water (0.4?10.0 g/L) and sodium bicarbonate-to-sodium chloride geochemical types, in some places of mixed anionic type. The groundwater dynamics and the geochemical features are influenced by changes in lithology and by large changes in relief at both a regional and a local scale. Numerical modeling suggested that the dissolution of carbonate minerals, cation exchange, and mineral hydrolysis are the main geochemical processes that support the chemical groundwater composition. Although the variation in As concentrations does not follow clear trends associated with major ion chemistry, laboratory batch tests, and geochemical modeling supported by statistical analysis indicated, As solubility is favored under alkaline and high pH conditions. Thus, the As values would be closely related to the desorption from the surfaces of oxides and hydroxides, due to the change in their net surface charge from positive to negative as the pH increases. The Eh?pH conditions show that the main species in solution are HAsO4 2−. The results suggest a need to carry out epidemiological and toxicological studies which are scarce or even absent in the South of Córdoba province.