ARSENIC OCCURRENCE IN THE CENTRAL OKLAHOMA AQUIFER, OKLAHOMA, USA

Changes in national drinking water standards for arsenic have fueled renewed interest in factors affecting arsenic levels in municipal groundwater wells in the Central Oklahoma Aquifer in and around Norman, Oklahoma. Hypotheses explaining elevated concentrations range from various types of water-rock interaction, water residence time effects, influences of pumping schedules or well construction and completion techniques. To discern among these hypotheses a study combining water chemical analyses, GIS, well log and core sample analysis and hydrological/geochemical modeling is being conducted on a portion of the aquifer system. Major and trace elemental analyses, including a rapid colorimetric technique for arsenic determination (sensitive to 4 ppb and adaptable for well-head analysis) is being conducted and examined for As correlations with variables such as alkalinity, redox potential, temperature, dissolved oxygen, pH, etc. New geological mapping by the Oklahoma Geological Survey is being used to develop a 3-D GIS combining aquifer geology, water table/potentiometric surfaces, well location and perforated intervals, and major, minor and trace groundwater chemical changes through time. Well log and core sample analysis is being conducted to examine patterns of arsenic occurrence with lithological variation and to determine any influence made by water well construction and completion techniques. Finally, hydrological/geochemical modeling is being used to simulate existing pumping schedules, determine groundwater residence times and to predict arsenic desorption as a function of space and time in the aquifer system. The overall goal of this study is to provide a comprehensive view of how and why elevated arsenic concentrations exist in this aquifer and to suggest additional aquifer management methods needed to ensure drinking water standard compliance.