The arsenic concentration in groundwater from the Abbey Arms Wood observation borehole, Delamere, Cheshire, UK:

A 150 m observation borehole was drilled in Abbey Arms Wood, Delamere, Cheshire, UK in order to explore the local hydrogeological conditions and to understand better the source of the high concentrations of arsenic in some of the local groundwaters. The borehole was located on an outcrop of the Helsby Sandstone Formation (part of the Sherwood Sandstone Group) and was cored into the underlying Wilmslow Sandstone Formation. The aquifers in the area are unconfined and give rise to low-Fe groundwaters with As concentrations in the 10–50 µg l–1 range. The chemical composition of the sediments is quite uniform down to 150 m. The total arsenic content is in the range from 5 to 15 mg kg–1 and averaged 8 mg kg–1 (n = 60). There is no trend in sediment As concentration with depth, but pore water centrifuged from the core steadily increased in As concentration with depth. The As concentration ranges from 8 µg l–1 at 10 m (unsaturated zone) to 30 µg l–1 at 150 m. The source of the dissolved As remains unclear but the lack of evidence for discrete high-As minerals or zones of mineralization suggests that it is probably derived by desorption from rock-forming minerals in the sandstones, e.g. iron oxides. This may be in response to slightly higher pH (up to 8.0 at depth). If this trend applies throughout the area, restricting the screened interval for abstraction boreholes to the uppermost parts of the saturated zone may reduce As concentrations, but is likely to reduce yields and may also risk encountering groundwaters with high nitrate concentrations

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

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