GEOCHEMISTRY AND OCCURRENCE OF ARSENIC IN THE RUSH SPRINGS SANDSTONE, CADDO COUNTY, OKLAHOMA

Naturally occurring arsenic is a known health risk and occurs in detectable concentrations under certain conditions in aquifers around the world. Arsenic is considered highly toxic; ingestion causes toxic reactions ranging from skin lesions to cancer. Groundwater in some public supply wells within the Rush Springs Sandstone aquifer in Caddo County, Oklahoma contain arsenic concentrations that exceed the maximum contaminant level (MCL) of 10 parts per billion (ppb) established by the U.S. Environmental Protections Agency. The Rush Springs Sandstone is a Permian-age redbed sandstone that outcrops in western Oklahoma. Iron oxide, which is prevalent in the Rush Springs Sandstone, has been associated with arsenic and its potential release into the groundwater system. It has been shown that pH of the groundwater is a primary control of arsenic concentration and pH values of 8.5 or higher will initiate arsenic release. Preliminary work has shown that the pH of groundwater in the Rush Springs Sandstone is less than this threshold value (6.1-7.8) and that another process is likely at work. Compounds coming from the groundwater and interacting with the rock could be a factor controlling the arsenic release. Dissolution due to the presence of organic carbon or desorption resulting from the introduction of competing adsorbents such as phosphate will cause arsenic levels to increase in the groundwater. To detect possible compounds that drive desorption, concentrations of metals, nonmetals and radioactive elements will be measured in outcrop samples and cores using a handheld x-ray fluorescence analyzer. Comparison of these data with arsenic concentrations in the groundwater will be used to determine if a relationship exists that will help explain the processes controlling the release of arsenic in the Rush Springs aquifer.