Natural contamination with arsenic and other trace elements in groundwater of the Central-West region of Chaco, Argentina

This study covered the central agricultural region of the Chaco province, which lacks a permanent river networks. However, during the rainy period there is localized groundwater recharge. About 84 groundwater samples were taken during the period April-December 2007. These groundwater samples were collected from two different depths: 62 samples from shallow wells (4 to 20 m) and 24 samples from deep wells (20 to 100 m). Chemical variables were determined: pH, specific conductance, total dissolved solid, hardness, alkalinity, HCO3-, CO32-, SO42-, Cl-, NO3-, NO2-, NH4+, F-, As-(tot), Na+, K+, Ca2+, Mg2+, Fe, Cu, Ni, Pb and Zn. The chemical composition of groundwater in the study area is dominantly sodium bicarbonate and sodium chloride bicarbonate, comprising more than 60% (52/86) of shallow and deep groundwater samples. Of the 86 analyzed groundwater samples, 88% exceeded the WHO (World Health Organization) and CAA (Codigo Alimentario Argentino) standards (10 mu g/L) for As (arsenic) and 9% exceeded the WHO standard (1.5 mg/L) for F-. Groundwater highly contaminated with As (max. 1,073 mu g/L) and F- (max. 4.2 mg/L) was found in shallow aquifer. The contaminated groundwater is characterized by high pH (max. 8.9), alkalinity (max. HCO3- 1,932 mg/L), SO42- (max. 11,862 mg/L), Na+ (max. 3,158 mg/L), Cl- (max. 10,493 mg/L) and electric conductivity greater than 33.3 mu S/cm. Other associated elements (Ni, Pb, Cu and Zn) are present in low concentrations, except for Fe that in 32% of samples exceeded the guideline value of 0.3 mg/L suggested by the CAA.