GENERIC MODELLING OF THE IMPACT OF IRRIGATION ON ARSENIC TRANSPORT IN SHALLOW GROUNDWATER IN THE ALLUVIAL AQUIFERS IN THE SANTIAGO DEL ESTERO PROVINCE, ARGENTINA

Arsenic is one of the most dangerous inorganic contaminants, distributed widely in natural environments. An increased dependence on groundwater for drinking and irrigation has resulted in several millions of people worldwide being exposed to arsenic. In Argentina the arsenic contamination problem affect 1.2 million people in the rural areas. The worst problems were reported in the province of Santiago del Estero. This province uses groundwater in the Río Dulce alluvial cone, which has a high concentration of arsenic since the sedimentary aquifers comprise volcanic ash present as distinct layers as well as dispersed within the loessic and alluvial sediments. The objective of the present study is to reveal the impact that irrigation has on the release of arsenic from the volcanic ash layer in the shallow aquifer sediments. Volcanic ash is believed to be the primary source of arsenic in the area. Irrigation leads to variably saturated flow conditions during very short time periods, implying that the water chemistry and, subsequently, the conditions for the mobility of arsenic will change under equally short time periods. The conceptual model of arsenic transport includes arsenic release from volcanic material, and its adsorption on ferric oxide and hydroxides in zones with Ca-HCO₃-type groundwater with a neutral pH. In contrast, in zones of Na-HCO₃-type groundwater, with high pH values, the mobility is high. The factors responsible for the formation of high pH, Na-HCO₃-type groundwater are not completely clear, but it seems that both cation exchange, coupled with dissolution of carbonates and silicates are occuring. Modelling has been done using COMSOL multiphysics software. The chemical reactions and processes are expressed by means of the Chemical Reaction Engineering Lab and then exported to the Earth Science Module. The Earth Science Module is used to perform a time and space dependent analysis of the fate of the arsenic for some generic scenarios. The study is preliminary in its character and based on simplified assumptions.