Arsenic enrichment of ground water at two regions of the Chacopampean Plain, northwest Argentina

SRACEK O.; GARCIA, M.G.

London, England

Congreso; Annual International Conference 07 Royal Geographical Society; 2007

Royal Geographical Society

High concentrations of arsenic have been encountered in ground water of the Chacopampean Plain (CPP). Two regions have been studied: Santiago del Estero in semiarid central CPP and more humid Tucum¨¢n at western limits of CPP. In both regions concentrations of dissolved As may reach values above 1,000 ¦Ìg/l and high As concentrations are linked to shallow loessic sediments (upper 30 m of sedimentary sequence). There is a positive correlation between dissolved As and Na and HCO3 concentrations. Also, there is positive correlation between As and several oxyanion-forming elements including V, F, B, and Mo present in volcanic ash in loessic sequence. Primary source of arsenic in ground water cannot be determined unequivocally, but highly weathered glass present in volcanic ash is a principal candidate. Contents of ferric iron extracted by the oxalic extraction step are low because there is a limited amount of oxidizable ferrous iron minerals in volcanic ash of acid composition. Thus, adsorption capacity of solid phase for As is quite limited and may be further decreased by competition with other oxyanions for adsorption sites. Conditions in shallow aquifer are generally oxidizing and arsenic is present mostly as As(V). However, in the proximity of the Sal¨ª River in Tucum¨¢n As can be released as a consequence of the penetration of surface water contaminated by organic wastes and causing reductive dissolution of ferric mineral adsorbents in the neighboring aquifer. Similarly, high concentration of DOC with a potential impact on As mobilization were observed in the proximity of unlined irrigation canals at Santiago del Estero.In Santiago del Estero high As concentrations seem to be linked to slow ground water flow zones with long ground water residence times and high pH (up to 9.0) and TDS values. In Tucum¨¢n, As concentrations seem to increase from west towards east along with direction of flow, decreasing precipitation and longer residence time of ground water that recharges at the mountain front of Sierra del Aconquija at western limits of the region. Many of these findings are also applicable to other sites in Argentina with high dissolved As concentrations like La Pampa south of both studied sites.