Salt Flushing, Salt Storage, and Controls on Selenium and Uranium: A 31‐Year Mass‐Balance Analysis of an Irrigated, Semiarid Valley

Abstract

Salinity, selenium, and uranium pose water‐quality challenges for the Arkansas River in southeastern Colorado and other rivers that support irrigation in semiarid regions. This study used 31 years of continuous discharge and specific conductance (SC) monitoring data to assess interannual patterns in water quality using mass balance on a 120‐km reach of river. Discrete sampling data were used to link the SC records to salinity, selenium, and uranium. Several important patterns emerged. Consumptive use reduced discharge by a median value of 33% and drove corresponding increases in salinity and uranium concentrations. Increased water availability for irrigation from rainfall and upstream snowpack in 1995–1999 flushed additional salinity and uranium into the river in 1996–2000; average annual total dissolved solids (salinity) concentrations increased 25%, and loads increased 131%. Smaller flushing events have occurred, sometimes lagging an increase in water availability by about one year. The pattern indicates flushing of salts temporarily stored, evaporatively concentrated, or of geologic origin. Mobilization of selenium from the reach was minor compared to salinity and uranium, and net selenium removal from the river was suggested in some years. Several processes related to irrigation could be removing selenium. The results provide context for efforts to improve water quality in the Arkansas River and rivers in other semiarid regions.

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

“…The decreases in flow-normalized TDS concentration of 134 mg/L and load of about 16,000 mg/yr at Pueblo (Table 2) were important because they indicated decreased TDS contributions from Fountain Creek to the Arkansas River, where TDS presents a major concern [18,19]. The upward trends in TDS concentrations for Monument Creek at Bijou Street and Fountain Creek at Colorado Springs (Table S3) may have been related to effluent discharge from the JD Phillips facility.…”

“…The 46% decrease in irrigation diversions between 1980 and 2022 could have driven decreased TDS-bearing return flow in the lower watershed. Irrigation-driven flushing of salts from soils in the Arkansas River Valley, downstream from Fountain Creek, mobilizes substantial fractions of TDS [18]. However, unlike the Arkansas River Valley, much of the irrigated agriculture along Fountain Creek occurs on modern alluvium that may contain less salt than upland soils due to recent fluvial re-working [21,65].…”

“…Flow-normalized concentrations of both filtered and unfiltered selenium were generally above that value (Figure 12a,b). However, the likely downward trends in unfiltered and filtered selenium concentrations and loads were important because of the issues selenium presents downstream [18,19,71].…”

“…However, bedrock in that watershed is highly mineralized, and drying was understood to expose more pyrite to oxidation, liberating more trace elements and sulfate through mineral weathering [75]. In Fountain Creek, by contrast, both the trace elements and TDS would seem to exist in forms ready for mobilization, as precipitated salts in the unsaturated zone, ions sorbed on surfaces, or already dissolved in groundwater, and so annual fluxes of natural recharge could be a strong influence on mobilization [18].…”

“…Stormwater management has been a challenge as the watershed has urbanized, and efforts to improve it are underway [17]. Farther downstream, constituents like total dissolved solids (TDS), a measure of salinity, and selenium are substantial concerns in the Arkansas River, to which Fountain Creek is a tributary [18,19].…”