NATURALLY OCCURRING ARSENIC CONTAMINATION IN A SEMIARID OXIDIZING SYSTEM, SOUTHERN HIGH PLAINS AQUIFER, USA

Groundwater arsenic contamination greatly increased in the US with reduction in the maximum contaminant level (MCL) from 50 to 10 μg/L, effective in 2006. The objective of this study was to evaluate the distribution, sources, and mobilization mechanisms of arsenic in a semiarid, oxidizing system in the southern High Plains (Ogallala) aquifer, USA. The proportion of wells with groundwater arsenic contamination increased with the MCL reduction from 4% to 52% in the southern half of the southern High Plains (median As 12 μg/L; range <1 to 561 μg/L). In contrast, arsenic concentrations in the northern half of the southern High Plains are much lower (median As 4.3 μg/L; range <1 to 43 μg/L), with only 8% exceeding the new MCL. The sharp contrast in arsenic levels between the northern and southern parts of the southern High Plains coincides with a change in median values of aquifer saturated thickness from 21 m (north) to 14 m (south) (a 33% decrease), water table depth from 63 m (north) to 25 m (south) (a 60% decrease) and TDS from 385 mg/L (north) to 901 mg/L (south) (a 134% increase). The most likely source of arsenic is adsorption onto Fe-Mn (oxyhydr)oxides, similar to arsenic sources in most semiarid, oxidizing systems. However, in contrast to known arsenic contamination in other semiarid, oxidizing systems (e.g. Arizona, Nevada, Argentina), (1) TDS variations in the southern High Plains aquifer are not related to evaporation but to mixing with water from subjacent aquifers and (2) arsenic is not mobilized by increased pH (pH near neutral, r=0.1 for As versus pH) but by the counterion effect caused by a change from Ca to Na type water (r=0.53, As versus (Na/Ca)) that is related to increased TDS. Until now this mechanism has only been documented in laboratory experiments. This counterion effect also mobilizes other oxyanions that are correlated with arsenic (V, r=0.85; Mo, r=0.51; B, r=0.57; Se, r=0.48). The southern High Plains case study demonstrates the lack of importance of evaporation and importance of counterion mobilization, which contrasts with previously studied arsenic contamination in semiarid, oxidizing systems.