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

Abstract

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 −, CO3 2−, SO4 2−, 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 (Código Alimentario Argentino) standards (10 μg/L) for As (arsenic) and 9% exceeded the WHO standard (1.5 mg/L) for F−.Groundwater highly contaminated with As (max. 1,073 μg/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), SO4 2− (max. 11,862 mg/L), Na+ (max. 3,158 mg/L), Cl− (max. 10,493 mg/L) and electric conductivity greater than 33.3 μS/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.