Potential Mobilization of Arsenic and Other Solutes in the Vadose Zone through Land Use Change in the Southern High Plains, USA

Many studies have shown that the vadose zone can act as a reservoir for chloride and nitrate that can be mobilized under changing land use. The purpose of this study was to assess whether the vadose zone also stores arsenic and perchlorate and to determine if these contaminants can be mobilized by changing land use to cause groundwater contamination. A total of 20 boreholes were drilled in different land use categories (natural rangeland ecosystems and irrigated and dryland cultivated ecosystems) in the Southern High Plains of Texas. Soil cores were analyzed for arsenic, chloride, nitrate, sulfate, perchlorate, and phosphate by leaching the samples. Chloride data are used to estimate water fluxes and recharge rates to the aquifer. Pressure head data are used to estimate direction of water movement in the vadose zone. Arsenic concentrations in soils are variable and high in many profiles, with median concentrations in individual profiles ranging from 2.6 to 23 ug/kg. Highest arsenic levels are found in a natural rangeland ecosystem (77 ug/kg at 6.4-m depth), indicating that there is a reservoir of natural arsenic in the soil profile. Arsenic profiles in cultivated areas are variable. Some profiles have highest arsenic concentrations near the surface that may be related to arsenical pesticide application. Other profiles have highest arsenic levels at depth, suggesting natural sources. Chloride profiles have high concentrations beneath natural rangeland areas. Chloride is flushed out to >4 m in dryland cultivated areas, whereas chloride is concentrated at shallow depths by evapotranspiration in many irrigated sites. Chloride concentrations indicate that water fluxes in the vadose zone change from upward in natural rangeland systems caused by evapotranspiration to downward beneath cultivated areas. The change in flow direction caused by cultivation is also supported by pressure head data. Therefore, land use change from natural rangeland to cultivated ecosystems can potentially mobilize arsenic and other water-soluble contaminants from the unsaturated zone to the underlying aquifer. Artificial recharge in these regions could also mobilize these contaminants. A comprehensive approach should be developed to study the impacts of land use change on the water cycle that assesses reservoirs of potential contaminants in the vadose zone and mobilization resulting in contamination of underlying aquifers.