Baseline evaluation of legacy hydrochemical data for the Republican River watershed, southcentral Nebraska, USA

Abstract

The objective of this study was to comprehensively evaluate and interpret hydrogeochemically the extensive legacy surfacewater data for the expansive Republican River watershed in southcentral Nebraska. Historical (1968-1991 hydrochemistry data [major cations and anions, total salinity [specific conductance (SC)] and pH supplied by the Nebraska Department of Environmental Quality and the US Geological Survey were hydrogeochemically and statistically analyzed. Interpretation of this large legacy dataset (315 sample analyses at 12 Republican River sampling stations) revealed that the Republican River's range in SC is relatively low (200-1020 µS/cm), with a river-wide median of 587 µS/cm (~ 380 mg/L). The range in SC for 10 tributaries (204 sample analyses) is 190-2910 µS/cm with a median value of 550 µS/cm (~ 360 mg/L). The Ca-HCO 3 hydrochemical facies dominates throughout the watershed followed by the Na-HCO 3 facies. Hydrochemical equilibrium modeling reveals that most samples occurred in an aqueous environment characterized by gypsum undersaturation but slight calcite and dolomite supersaturation; low supersaturation values comprise a line of evidence for the thermodynamic equilibrium or near equilibrium of the studied surface waters. Scatter plots of meq/L concentrations for selected anions and cations reveal the impact of silicate mineral (e.g., feldspars) weathering on the aqueous hydrochemistry throughout the watershed. These ubiquitous feldspar minerals most likely originated along the eastern slope of the Front Range during Late Cretaceous time (Laramide Orogeny) and were subsequently transported into the study area. The relatively high concentrations of sulfate and sodium in numerous samples collected from the Arikaree River probably originate in the underlying Pierre Shale, rich in S-bearing minerals.

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

“…To evaluate and enhance major-ion data quality, concentrations expressed in milliequivalents per liter (meq/L) for the major ions, cation-anion balance (CAB) errors, and numerous quality-assurance algorithms [e.g., SC (in µS/cm)*0.65 = calculated TDS (in mg/L), (0.01)*(SC) = sum of anions in meq/L, (0.01)*(SC) = sum of cations in meq/L], were used to evaluate and enhance major-ion data quality (Hem 1985;Atkinson 2011Atkinson , 2018Atkinson , 2021. The USGS considers ion balances within the ±6% range to indicate major-ion analyses of good quality; however, ion balances up to 12% are considered acceptable (Bartos and Ogle 2002).…”

“…The USGS considers ion balances within the ±6% range to indicate major-ion analyses of good quality; however, ion balances up to 12% are considered acceptable (Bartos and Ogle 2002). For NRW analyses that exceeded 5% CAB errors, ion concentrations were revised using computerbased algorithms (Atkinson 2011(Atkinson , 2021, and revisions were constrained by measured TDS concentrations, SC values, and several empirical water-quality ratios as described above.…”

“…It consists primarily of argillaceous chalk, limestone, and shale aggregating a thickness of about 30 m (100 ft) (Souders1976). Thin layers of gypsum (calcium sulfate) commonly occur at the top of the unit (Atkinson 2021).…”

“…Engberg (1984) noted that, in general, most groundwater in Nebraska is of the Ca-HCO3 type. Like groundwater, most surface-water bodies in Nebraska also are the Ca-HCO3 type with low to moderate (i.e., < ~700 mg/L) TDS content (Atkinson 2021). to the 95th percentile (66% for SC, 75% for HCO3) compared with that percent difference in concentrations between the 5th and 50th percentiles (34% for SC, 25% for HCO3).…”