From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Austin Geological Society, Austin, Texas, February 6, 2006
Arsenic in Groundwater
Lowering the Federal standard for arsenic in drinking water from 50 to 10 ug/L greatly increases the number of groundwater sites in Texas where contamination exceeds the new maximum contaminant level (MCL). The objective of this reconnaissance study was to (1) determine the distribution of arsenic in groundwater; (2) assess the potential of arsenical pesticides as a source of arsenic; and (3) evaluate geologic sources of arsenic in the Southern High Plains, Texas. Anthropogenic sources of arsenic, such as arsenical pesticides, were examined using GIS overlay analyses and soil sampling. Geologic sources of arsenic were evaluated using relationships between arsenic concentrations and different geologic units and relationships between arsenic concentrations and other ions, particularly oxyanions. Groundwater arsenic contamination is widespread in Texas. Approximately 6% of wells exceed the MCL of 10 ug/L.
Contamination is focused in the Southern High Plains (32% of wells exceed the MCL) and the southwestern Gulf Coast (29% of wells exceed the MCL). The Southern High Plains (SHP) region was subdivided into two areas: a northern area (SHP-N) characterized by low total dissolved solids (TDS <500 mg/L) and a southern area (SHP-S) characterized by high TDS (>500 mg/L). Arsenic contamination is much greater in the SHP-S region (51% of wells >10 ug/L; 2% >50 ug/L) than in the SHP-N region (7% of wells >10 ug/L). Regional analyses of groundwater arsenic concentrations do not support a surficial source of arsenic contamination. Arsenic concentrations are not correlated with land use, with cotton production, with distance from cotton gins, or with nitrate concentrations. Results of drilling and sampling 18 boreholes in the Southern High Plains indicate that the distribution of arsenic is not related to the distribution of cotton production. High arsenic concentrations in a rangeland profile (peak 77 ug/kg) indicate that background levels of water-soluble arsenic are high in soils. Arsenic levels in cultivated areas are variable. Some profiles have highest arsenic levels near the surface, which are correlated with nitrate and phosphate that may suggest a fertilizer or arsenical pesticide source. These data indicate that arsenic related to arsenical pesticides is probably restricted to the near-surface zone. Other profiles have peak concentrations in the middle of the profile or at depth. It is unlikely that arsenical pesticides associated with cotton production would have reached the water table. The unsaturated zone data indicate a widespread source of water-soluble arsenic in soils in the Southern High Plains that may contribute to groundwater arsenic contamination.
Groundwater arsenic contamination occurs in generally oxidizing conditions in the High Plains. Correlations between arsenic and other constituents (vanadium, r2 0.65; fluoride r2 0.30; molybdenum r2 0.18; boron r2 0.17; selenium r2 0.14) suggest a geologic rather than an anthropogenic source. Arsenic concentrations are highest in the Ogallala aquifer and much lower in the Dockum aquifer. Potential sources of arsenic include volcanic ash beds in the Ogallala, black shales in the Cretaceous (Kiamichi Shale), and saline lakes. Additional studies will be required to assess geologic sources, including geophysical logging and stratified sampling.