Modeling Assessment of Aquifer Contamination by Sulfate and Uranium in the Upper Arkansas River Corridor in Kansas

Abstract Groundwater in the upper Arkansas River corridor has been contaminated by elevated concentrations of dissolved solids in Arkansas River water. Most of the dissolved solids originate from the soils and bedrock in eastern Colorado, where river water is diverted for irrigation and storage systems, and evapotranspiration significantly increases the salinity of return flow to the river. Dissolved solids contents in low flows of the Arkansas River can sometimes exceed 4000 mg/L at the Colorado–Kansas border. To assess how sulfate and uranium in the river have contaminated the aquifer system in the corridor, which includes several municipal wellfields, a groundwater flow and transport model was developed. Due to its largely conservative characteristics, sulfate was directly simulated by the model; uranium, however, was estimated based on empirical relations between sulfate and uranium concentrations in groundwater samples. The comparison between the simulated and observed sulfate concentrations demonstrated a high level of agreement, affirming the accuracy of the model for assessing various water management scenarios. Our results indicate that river contaminants enter the aquifer through streambed infiltration and ditch diversion of river water for irrigation. Ditch irrigation and lateral groundwater movement have caused significant spreading of contaminants in the aquifer on both the north and south sides of the river. Any contamination treatment of the river water will likely take centuries to make a significant impact on the drinking water supply wells in the area due to the large amounts of contaminants already in the subsurface and the slow rate of lateral groundwater flow. Site‐specific remediation measures in the immediate vicinity of wellfields will be needed to reduce contaminant concentrations in municipal supply wells.