Arsenic associations in sediments from the loess aquifer of La Pampa, Argentina

摘要: Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concentrations As (up to 5300 μg L−1) as well other potentially toxic trace elements such F, B, Mo, U, Se and V. Total in 45 samples collected have a range 3–18 mg kg−1 mean 8 kg−1. These values are comparable world-average sediment concentrations. Five rhyolitic ash area 7–12 Chemical analysis included sediments extracted porewaters two specially cored boreholes. Results reveal large porewater concentrations, being generally higher horizons highest The displaced ranging up 7500 L−1 exceptionally some oxyanion species, including V 12 L−1. found borehole located topographic depression, which is zone likely groundwater discharge enhanced residence time. Comparison data does not unequivocally sources As, but selective extract (acid-ammonium oxalate hydroxylamine hydrochloride) suggest that much (and V) associated Fe oxides. Primary oxides magnetite ilmenite may be partial given weathered nature many sediments, secondary oxide minerals probably more important. Extract compositions also Mn an source. groundwaters region oxidising, dissolved O2, NO3 SO4 normally present As(V) usually dominant species. Under conditions, solubility low mobilisation strongly controlled by sorption–desorption reactions. Desorption facilitated relatively high-pH conditions (7.0–8.8) potential competitors (e.g. V, P, HCO3). PHREEQC modelling suggests presence at observed Pampean can suppress sorption hydrous Fe(III) (HFO) order magnitude. Bicarbonate had comparatively small competitive effect. Oxalate been used provide upper estimate amount labile sediments. A near-linear correlation between oxalate-extractable one boreholes investigated approximate Kd value for 0.94 L This indicates unusually affinity As.