OF-24-09 Geologic Salinity Sources in the South Platte River Basin within Colorado

Potential groundwater salinity contributions into the South Platte River sourced from sedimentary bedrock were studied from the Colorado Front Range to the Nebraska state line. Eleven geologic bedrock formations or groups located at or near the ground surface spanning the Pennsylvanian-Permian-Triassic (P-T) to Neogene were sampled for salinity constituents. Sixty weathered rock outcrop samples were field collected, and ten unweathered core samples were obtained from the USGS Core Research Center in Lakewood, Colorado. Additionally, 10 alluvial groundwater samples were included in the study. Funding was provided by the Colorado Water Conservation Board (CWCB) with match provided by the CGS.

This study is considered “Part B” to a prior “Part A” study published in 2023 by Neirbo Hydrology, Geologic Salinity Sources in the South Platte River Basin, funded by the Colorado Water Conservation Board (CWCB) and the Colorado Geological Survey both through labor and financial contributions. Part A focused on surface water and the hyporheic zone whereas this Part B study focuses primarily on bedrock contributions. Digital PDF download. OF-24-09D

From the abstract:

For this Part B Study, salinity focused chemical composition data were collected from 10 alluvial aquifer wells for comparison with the prior Part A study results and from the basin’s primary bedrock geologic formations to identify which of the various geologic formations could potentially contribute salinity to the basin’s groundwater and surface water. Since chemical composition in near surface weathered and deeper unweathered rock may differ, both were analyzed.

Rock: Geologic formations or groups of formations present at or near the ground surface along the South Platte River and tributary Cache la Poudre, Big Thompson, Saint Vrain rivers in the South Platte River Basin were studied from the Front Range canyon exits to the Nebraska state line. This region was selected to match the prior study area of elevated salinity. From generally upstream to downstream and oldest to youngest, these included:

• Pennsylvanian-Permian-Triassic mixed formations (mostly sandstones),
• Jurassic Morrison Formation,
• Cretaceous Dakota Group,
• Cretaceous Colorado Group (Niobrara Formation),
• Cretaceous Pierre Shale,
• Cretaceous Fox Hills Sandstone,
• Cretaceous Laramie Formation,
• Cretaceous/Paleogene Denver Group (combined Denver & Arapahoe Formations),
• Paleogene White River Group (Brule and Chadron Formations),
• Paleogene Arikaree Formation, and
• Neogene Ogallala Formation.