IMPACTS OF WELL PUMPING ON TRACE METALS IN THE CENTRAL OKLAHOMA AQUIFER, NORMAN, OK

Due to increasing global demand for fresh water, it is increasingly necessary to understand how aquifer pumping affects groundwater chemistry. Water quality may be affected by mobilizing naturally occurring salts or trace metals via water-rock interactions, mixing with deeper saline groundwaters, increased recharge containing anthropogenic inputs, etc. Comprehensive predictive relationships between pumping and groundwater quality have yet to be developed, although many variable results have been shown in several studies. Pumping in the Central Oklahoma Aquifer (COA) has increased drastically since the 1980’s, especially around Norman, OK. By studying how pumping change has affected the high concentrations of uranium, arsenic, and chromium in the COA, we can provide unique insights into how trace metals will respond to increased pumping in Norman, OK over the next few decades.

Averages obtained from hierarchical clustering of groundwater samples from the USGS NWIS database, in conjunction with previous literature on dominant reactions in the COA, suggest that ion exchange and mixing with an Na-Cl brine largely affect the groundwater chemistry and chromium concentrations in the deep COA. Changing hydrologic conditions from pumping in the aquifer could potentially affect both of these mechanisms. Chromium concentrations generally decrease both north to south and west to east towards the center of Norman’s cone of depression, which provides some circumstantial evidence that pumping in Norman, OK may induce mixing with brine, thus lowering trace metal concentrations.

We obtained pumping and geochemical sampling data from the City of Norman well field through 2012-2019 to determine if the same trends were seen on a well-by-well basis and to avoid sampling bias in the data. Although several wells did show lowered chromium or arsenic during periods of increased pumping, and vice versa, many other wells had no correlation, not enough samples, or extremely variable pumping rates. This suggests that, even within the small zone of study, there may be many other potential factors influencing local trace metal concentrations. As the study progresses, we will integrate the City of Norman data with other databases and use PHREEQC inverse modeling to identify reactions and mixing along specific groundwater flow paths.