Hydrochemical processes controlling arsenic and heavy metal contamination in the Elqui river system (Chile)

Severe arsenic poisoning from drinking water has been documented in Northern Chile. However, the Elqui River, which provides water for approximately 200,000 people in this region, is poorly studied and no data on contaminants have been published to date. In this study, trace elements and the main aqueous constituents were monitored for approximately 2 years in the entire river system. Aqueous species of trace elements were determined via thermodynamic equilibrium calculations, and two operationally-defined suspended fractions were analyzed. Chalco- and arsenopyrite deposits in the upper Andes, in conjunction with mining and geothermal activity, were identified as exclusive point sources of heavy metals and arsenic. The annual input to the river system was approximately (t year(-1)): Fe 600, Mn 110, Cu 130, Zn 35 and As 2.0. The confluence with pH-buffered waters in the upper river system caused collapse of iron hydroxide colloids and coprecipitation of all heavy metals, e.g. dissolved copper concentrations decreased from approximately 100 to approximately 0.2 micromol l(-1), which is still of ecotoxic concern. The heavy metal enriched suspended solids settled only in the lower Elqui River. Arsenate did not adsorb to suspended solids and behaved strictly conservatively, exceeding the WHO guideline value for drinking water (0.13 micromol l(-1)) in the entire river system. Decontamination may be accomplished with reasonable efforts upstream in direct vicinity to the sources via coprecipitation, settling and appropriate pH adjustment for arsenate adsorption.