OF-25-06 Baseline Radiological Study Year 4: Lower Arkansas River Area, Colorado

The Colorado Geological Survey (CGS), a department of the Colorado School of Mines, has been funded through a grant from the Colorado Department of Public Health & Environment (CDPHE) to conduct a 5‐year study of baseline naturally occurring radionuclides and metals in groundwater obtained from privately owned residential water wells throughout Colorado. This report presents the methodology and available results from the fourth year of the study (2025) that focused on the Lower Arkansas River Area of southeast Colorado, specifically including portions of Pueblo, Crowley, Otero, Kiowa, Bent, and Prowers counties. Digital PDF download. OF-25-06D

From the report:

The geology of the study area is composed of surficial Quaternary alluvial deposits that infill the Lower Arkansas River valley, overlying a bedrock sequence of Upper Cretaceous marine sediments. These marine units, deposited between about 100 and 76 million years ago (within the Western Interior Seaway), consist primarily of shale, limestone, and sandstone. Near surface bedrock units consist of predominantly younger Cretaceous formations on the northern side of the valley compared to the south. Mapped units include Late Cretaceous formations (Pierre Shale, Fort Hays Limestone Member of the Niobrara Formation, and Carlile Shale of the Benton Group) and the early Late Cretaceous formation (Dakota Sandstone). The Dakota Sandstone is present almost exclusively on the southern side (Figure 1). Cretaceous marine shales, including several of those present in the Lower Arkansas Valley, are recognized sources of naturally occurring uranium throughout the western United States (Barkmann and others, 2022; Bern and Stogner, 2017).

Beyond lithological variability, land use practices also play a significant role. Agricultural irrigation activities are primarily concentrated near the Arkansas River and its tributaries, which can enhance the mobilization of naturally occurring uranium in oxic waters. Irrigation can contribute to oxidizing conditions, which result in a mobilized uranium that remains mobilized (Barkmann and others, 2022; Zielinski and others, 1995). In semi-arid regions (such as the study area), where the Cretaceous marine formations underlie irrigated areas, elevated concentrations of dissolved uranium in both surface and subsurface waters have been documented (Qurban and others, 2025). Notably, uranium and radium demonstrate an inverse mobility regulated by redox conditions, wherein oxidizing environments facilitate uranium dissolution while anoxic conditions allow for radium mobility (Felmlee and Cadigan, 1979; Faraja and others, 2020).