Intermediate to high levels of arsenic and fluoride in deep geothermal aquifers from the northwestern chacopampean plain, Argentina

GARCIA, M.G; MORENO, C.; FERNÁNDEZ D.S.; GALINDO M.C; SRACEK, O; HIDALGO M. DEL V

Mexico DF, Mexico

Congreso; International congress of Natural arsenic in groundwaters of Latin America; 2006

UNAM

High levels of natural occurring arsenic and fluoride in groundwaters from the Chacopampean plain have been assigned to the presence of volcanic shards spread within the loess matrix. The primary source of these elements has not been determined yet but there is almost a clear understanding about the mechanisms that promote their mobilization in the aquifers. Geochemical evidence suggests that, after being released into groundwater, the concentration of arsenic in solution is controlled by pH. Arsenic is preferentially scavenged by adsorption on Fe (hydr)oxide coatings under acidic to neutral pH conditions. The concentration of fluoride depends on the fluorite solubility and also on pH-dependent adsorption with adsorption minimum at high pH values. In the province of Tucuman, the loessic layer is restricted to the first 30 meters of the Quaternary sequence. As a consequence, most shallow groundwater is contaminated by high levels of As and F-. Groundwater in deep confined aquifers is considered to be suitable for human consumption. However, in the southern part of the province several wells show from intermediate to high concentrations of As (between 10 to 79 ¦Ìg L-1) and high concentrations of F- (between 0.6 and 6.0 mg L-1). These wells that penetrate saturated layers as deep as 500 mbs, show ground water temperatures above the annual average in the region. Some authors proposed that the heat is supplied from a basaltic layer located 7000 mbs. The geochemistry of As and F- in deep aquifers shows certain characteristics that are not completely coincident with those described in the rest of the Chacopampean plain. Unlike in shallow groundwaters, the concentrations of As increase with increasing depth and temperature. The same trend is observed for F-, but the relation with depth is not such clear. Furthermore, As shows direct linear correlation with sulphate and reverse correlation with bicarbonate and calcium. F- is poorly correlated with arsenic, but highly correlated with chloride and sodium. It also shows reverse correlation with calcium. Concentrations of F- increase at increasing pH and decreasing Eh, but this trend is less evident for As. The primary source of As and F- in the deep confined aquifers can be associated with volcanic Tertiary sediments that are supposed to be in the deepest part of the sedimentary sequence. The up-flow of geothermal fluids through structural conduits is considered negligible because ground water chemistry matches those of volcanic sediments. The mobilization of As does not seem to be controlled only by the pH, but also by other factors such as the presence of As-rich primary source sediments. The concentration of F- is not affected by the precipitation of fluorite as its supersaturation is never reached due the removal of Ca2+ by the precipitation of calcite and/or cation exchange. The primary source of As and F- in the deep confined aquifers can be associated with volcanic Tertiary sediments that are supposed to be in the deepest part of the sedimentary sequence. The up-flow of geothermal fluids through structural conduits is considered negligible because ground water chemistry matches those of volcanic sediments. The mobilization of As does not seem to be controlled only by the pH, but also by other factors such as the presence of As-rich primary source sediments. The concentration of F- is not affected by the precipitation of fluorite as its supersaturation is never reached due the removal of Ca2+ by the precipitation of calcite and/or cation exchange. The geochemistry of As and F- in deep aquifers shows certain characteristics that are not completely coincident with those described in the rest of the Chacopampean plain. Unlike in shallow groundwaters, the concentrations of As increase with increasing depth and temperature. The same trend is observed for F-, but the relation with depth is not such clear. Furthermore, As shows direct linear correlation with sulphate and reverse correlation with bicarbonate and calcium. F- is poorly correlated with arsenic, but highly correlated with chloride and sodium. It also shows reverse correlation with calcium. Concentrations of F- increase at increasing pH and decreasing Eh, but this trend is less evident for As. The primary source of As and F- in the deep confined aquifers can be associated with volcanic Tertiary sediments that are supposed to be in the deepest part of the sedimentary sequence. The up-flow of geothermal fluids through structural conduits is considered negligible because ground water chemistry matches those of volcanic sediments. The mobilization of As does not seem to be controlled only by the pH, but also by other factors such as the presence of As-rich primary source sediments. The concentration of F- is not affected by the precipitation of fluorite as its supersaturation is never reached due the removal of Ca2+ by the precipitation of calcite and/or cation exchange. The primary source of As and F- in the deep confined aquifers can be associated with volcanic Tertiary sediments that are supposed to be in the deepest part of the sedimentary sequence. The up-flow of geothermal fluids through structural conduits is considered negligible because ground water chemistry matches those of volcanic sediments. The mobilization of As does not seem to be controlled only by the pH, but also by other factors such as the presence of As-rich primary source sediments. The concentration of F- is not affected by the precipitation of fluorite as its supersaturation is never reached due the removal of Ca2+ by the precipitation of calcite and/or cation exchange.