The Niobrara Formation as a challenge to water quality in the Arkansas River, Colorado, USA

Potential Climate and Human Water-Use Effects on Water-Quality Trends in a Semiarid, Western U.S. Watershed: Fountain Creek, Colorado, USA

“…A volumetrically small groundwater discharge with high selenium concentrations contributes much of the selenium observed in regional surface waters, with geologic units like the Niobrara Formation being the primary source [14,22]. In Fountain Creek, the spatial pattern of larger selenium concentration increases at the southern end of the watershed supports that idea because that is where the Niobrara Formation outcrops (Figures 1 and 12a,b).…”

“…The higher elevation terrain is underlain primarily by crystalline igneous intrusive and metamorphic rocks, and the Colorado Piedmont terrain is primarily underlain by sedimentary rocks of generally Cretaceous age [21]. Some of the sedimentary rocks, particularly the Niobrara Formation and the lower unit of the Pierre Shale, are known to negatively influence water quality via TDS and trace elements [22].…”

“…A perspective on the TDS concentrations in shallow groundwater can be gleaned from surface water flows into Fountain Creek south of the Fountain site, which are generally intermittent but may be more perennially connected to Fountain Creek in the subsurface via alluvium. The concentrations of TDS for two streams in the area (07105940 Little Fountain Creek near Fountain, CO; median = 1150 mg/L; range 161-4070 mg/L; n = 10 and 383325104424801 Sand Creek below Fort Carson near Wigwam, CO; median = 740 mg/L; range 301-1980 mg/L; n = 4) indicate that natural water entering Fountain Creek via surface or subsurface flow paths in that region could have high TDS concentrations [20] Elsewhere in the region, the Niobrara Formation is known to be a disproportionate source of TDS to surface water [22]. Niobrara Formation outcrops are present in the lower watershed, suggesting a role in the substantial TDS gains there (Figure 1).…”

“…Even where the Niobrara Formation does not outcrop at the surface, it is commonly overlain only by the lower units of the Pierre Shale (Figure 1). Both the Niobrara Formation and the lower units of the Pierre Shale (particularly the Sharon Springs Member) contain abundant pyrite which oxidizes to yield components of TDS [22] and sources therein. Near Pueblo, shallow emplacement of the Niobrara Formation along Fountain Creek indicates that it could be influencing the composition of groundwater that enters the creek (see Figures 4 and 5 in [14]).…”

Characterization of and temporal changes in groundwater quality of the Upper Black Squirrel Creek Basin, El Paso County, Colorado, 2018–20

“…Selenium and uranium are two trace elements of concern for water quality in much of Colorado because of the interaction between irrigated agriculture, a semiarid climate, and Cretaceous marine shales and limestones that weather to release these constituents to surface water and groundwater (Bern and Stogner, 2017;Bern and others, 2020a). Selenium concentrations greater than 6.0 µg/L were found at wells located along the main channel of the alluvium, most commonly at the southernmost end of the aquifer (fig.…”