Evaluation of major-ion chemistry for Niobrara River watershed, Nebraska, USA

Abstract Examination of historical (1968–1991) aqueous geochemistry data [major cations and anions, pH, specific conductance, and total dissolved solids (TDS)] for 500 water samples provided by the Water Quality Portal maintained online by the US Environmental Protection Agency for the Niobrara River, a National Scenic River, and six tributary creeks, a reservoir, and two tributary rivers, located in northwestern and northcentral Nebraska, revealed that the mineralization (TDS) is relatively low. Calculated median TDS for the Niobrara River is 192 mg/L (n = 322); calculated median TDS for the tributary rivers and creeks is 129 mg/L (n = 181). Only a few sample analyses are not the Ca-HCO3 hydrochemical facies. The mineralization (salinity) level of Niobrara River water is essentially constant from upstream to downstream. Hydrochemical equilibrium modeling of the Niobrara River watershed (NRW) reveals that most samples occurred in an aqueous environment characterized by slight calcite and dolomite supersaturation and gypsum undersaturation; low supersaturation index values comprise a line of evidence for the thermodynamic equilibrium or near equilibrium of the NRW waters. Statistical analysis (one-way ANOVA) and graphical methods (e.g., Stiff diagrams, scatter plots) are used to characterize and interpret the hydrogeochemistry of the NRW. Scatter plots of meq/L concentrations of (Ca + Mg) versus (HCO3 + SO4) reveal the impact of silicate minerals (e.g., feldspar) weathering on the aqueous geochemistry throughout the watershed. These ubiquitous feldspar minerals most likely originated along the eastern slope of the Front Range during Late Cretaceous and Tertiary time (Laramide Orogeny).