The number of groundwater wells that exceed the US Environmental Protection Agency maximum contaminant level (MCL) has greatly increased with reduction in the MCL from 50 to10 ?g/L in 2006. The objective of this study was to evaluate the distribution, sources, and mobilization mechanisms of As in a semiarid, oxidizing system in the southern Ogallala aquifer (SOA), USA. The percentage of wells with groundwater As exceeding the MCL increased from 2% to 47% with the MCL reduction in the southern part of the SOA (SOA-S) (median As 9.4 ?g/L; range 0.3 – 164 ?g/L). In contrast, the percentage of wells exceeding the current 10 ?g/L MCL is much lower in the northern part of the SOA (SOA-N: 9%; median 4.1 ?g/L; range 0.2 – 43 ?g/L). The sharp contrast in As levels between the north and south coincides with a change in median values of aquifer saturated thickness from 21 m (north) to 14 m (south), water table depth from 63 m (north) to 25 m (south), and total dissolved solids (TDS) from 398 mg/L (north) to 885 mg/L (south). Arsenic is correlated with groundwater TDS (Spearman’s??=0.57). Arsenic is present as arsenate in this oxidizing system. High groundwater As concentrations in many oxidizing systems (e.g. Arizona and Nevada, USA; Argentina, S. Am.) are related to high TDS from evaporation; however, unenriched stable isotopes (?2H: -65 to -27; ?18O: -9.1 to -4.2) in the SOA do not support evaporation. The most plausible source of high TDS groundwater in the SOA-S is upward movement from subjacent aquifers, particularly the Triassic Dockum aquifer. However, subjacent Triassic and Cretaceous aquifers are not likely sources of As because they have low As concentrations (medians 2.8 and 6.6 ?g/L). The most plausible source of As is desoprtion from Fe-Mn (oxyhydr)oxides, similar to As sources in most semiarid, oxidizing systems. However, in contrast to known As contamination in other semiarid, oxidizing systems, As is not mobilized by increased pH as shown by the lack of correlation between pH and As (? -0.11) and near neutral pH values (median: 7.3; 10 – 90 percentiles, 7.0 – 7.6). The most plausible explanation of As mobilization in the SOA is the counter-ion effect caused by a change from Ca to Na type water as shown by the high correlation between As and the Na/Ca ratio (? = 0.45), which is also related to increased TDS. This counter-ion effect also likely mobilizes other anions and oxyanion forming elements that are correlated with As (F, ? =0.63; V, ? =0.88; Se, ? =0.54; B, ? =0.51; Mo, ? =0.46, SiO2, ? =0.41). The SOA case study contrasts with previously studied As contamination in semiarid, oxidizing systems by showing the lack of importance of evaporation and pH and significance of counterion mobilization.
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Figure 1. Distribution of a) arsenic and b) total dissolved solids (TDS) concentrations in the southern Ogallala aquifer (n=1220). Dark line represents the 500 mg/L TDS contour that defines the regional north/south water quality boundary. |
Scanlon, B. R., et al. (2009), Naturally occurring arsenic contamination in a semiarid oxidizing system, Southern High Plains Aquifer, USA, Applied Geochemistry.
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