OF-19-11 Natural Sources of Mobile Uranium in the Downstream Reach of Colorado’s Arkansas River Valley and Evaluation of Best Management Practices for Mitigation


This CGS report examines geologic sources of elevated uranium (U) concentrations present in surface water and groundwater in the section of the Arkansas River Valley between the John Martin Reservoir and the Colorado-Kansas border. A multi-agency study was conducted by the CGS and Colorado State University (CSU) Department of Civil and Environmental Engineering for the Colorado Department of Public Health and Environment (CDPHE). The CGS evaluated natural sources of U contributing to return flows in the stream-aquifer system. CSU has developed and is applying computational models for assessing best management practices (BMPs) to mitigate human activities that may be contributing to mobilization of that naturally-occurring U. Modeling efforts are ongoing at CSU and there are additional reports available from the CSU Colorado Water Center. Digital PDF download. OF-19-11D

From the Introduction:

The CDPHE set out to investigate possible natural geologic sources of U that may be contributing to elevated dissolved concentrations in irrigation return flows to the Arkansas River downstream of John Martin Reservoir. Elevated concentrations in the shallow groundwater return flows along this reach are of concern to Colorado because they often exceed allowed drinking water concentrations. Also of concern is the total mass of U transported through the shallow groundwater system contributes to uranium load in surface water flowing out of Colorado into Kansas.

The Arkansas River has incised its course through Upper Cretaceous marine sediments, including the Pierre Shale, Niobrara Formation, and Benton Group. The Benton Group contains the Carlile Shale, Greenhorn Limestone, and Graneros Shale. Some of these marine sediments are known to contain naturally occurring U. Agricultural development along the river has occurred on Quaternary sediments deposited on those marine sediments. Fields in this reach are irrigated with water diverted from the main stem of the Arkansas River and distributed via a series of canals, as well as with water pumped from shallow alluvial wells. Subsurface return flows from excess irrigation tend to occur along the interface of the permeable Quaternary sediments on the weathered surface of the underlying marine sediments. This shallow return flow groundwater system eventually discharges directly to the surface flow of the Arkansas River and its tributaries.

The ultimate goal of this effort is to develop BMPs for water conveyance and irrigation, along with land BMPs like reduced fertilizer application and enhanced riparian buffers, to mitigate high U concentrations in the Arkansas River system. Understanding the underlying geology of the area and the distribution of naturally occurring U in the strata of the irrigated area is necessary to develop, simulate, and evaluate BMP alternatives using computational models of flow and reactive solute transport.