Hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from La Pampa, Argentina

Groundwaters from Quaternary loess aquifers in northern La Pampa Province of central Argentina have significant quality problems due to high concentrations of potentially harmful elements such as As, F, NO3-N, B, Me, Se and U and high salinity. The extent of the problems is not well-defined, but is believed to cover large parts of the Argentine Chaco-Pampean Plain,. over an area of perhaps 10(6) km'-. Groundwaters from La Pampa have a very large range of chemical compositions and spatial variability is considerable over distances of a few km. Dissolved As spans over 4 orders of magnitude (<4-5300 mug l(-1)) and concentrations of F have a range of 0.03-29 mg l(-1), B of 0.5-14 mg l(-1), V of 0.02-5.4 mg l(-1), NO3-N of <0.2-140 mg l(-1), Mo of 2.7-990 mug l(-1) and U of 6.2-250 mug l(-1). Of the groundwaters investigated, 95% exceed 10 mug As l(-1) (the WHO guideline value) and 73% exceed 50 mug As l(-1) (the Argentine national standard). In addition, 83% exceed the WHO guideline value for F (1.5 mg l(-1)), 99% for B (0.5 mg l(-1)), 47% for NO3-N (11.3 mg l(-1)), 39% for Mo (70 mug l(-1)), 32% for Se (10 mug l(-1)) and 100,% for U (2 mug l(-1)). Total dissolved solids range between 730 and 11400 mg l(-1), the high values resulting mainly from evaporation under ambient semi-arid climatic conditions. The groundwaters are universally oxidising with high dissolved-O-2 concentrations. Groundwater pHs are neutral to alkaline (7.0-8.7). Arsenic is present in solution predominantly as As(V). Groundwater As correlates positively with pH, alkalinity (HCO3), F and V. Weaker correlations are also observed with B, Mo, U and Be. Desorption of these elements from metal oxides, especially Fe and Mn oxides tinder the high-pH conditions is considered an important control on their mobilisation. Mutual competition between these elements for sorption sites on oxide minerals may also have enhanced their mobility. Weathering of primary silicate minerals and accessory minerals such as apatite in the loess and incorporated volcanic ash may also have contributed a proportion of the dissolved As and other trace elements. Concentrations of As and other anions and oxyanions appear to be particularly high in groundwaters close to low-lying depressions which act as localised groundwater-discharge zones. Concentrations up to 7500 mug l(-1) were found in saturated-zone porewaters extracted from a cored borehole adjacent to one such depression. Concentrations are also relatively high where groundwater is abstracted from close to the water table, presumably because this zone is a location of more active weathering reactions. The development of groundwaters with high pH and alkalinity results from silicate and carbonate reactions, facilitated by the arid climatic conditions. These factors, together with the young age of the loess sediments and slow groundwater flow have enabled the accumulation of the high concentrations of As and other elements in solution without significant opportunity for flushing of the aquifer to enable their removal. (C) 2002 NERC. Published by Elsevier Science Ltd. All rights reserved.