Noble gases, dead carbon, and reinterpretation of groundwater ages and travel time in local aquifers of the Columbia River Basalt Group

Abstract

Groundwater noble gas concentrations are used to model the temperature of aquifer recharge but also may be used to understand geologic influences on water chemistry. Aquifers in the South Fork Palouse River Basin are hosted within the fractured basalts of the Columbia River Basalt Group and the interbedded sediments of the Latah Formation. The travel time of groundwater, and the associated recharge rate, in this multiple aquifer system is of primary interest to water resource managers because of declining water levels. Prior investigations of the deeper groundwater indicated low values of percent modern carbon and elevated alkalinity and δ13C values compared to shallower groundwater. The groundwater travel time suggested by the uncorrected carbon-14 ages (up to 31,000 BP) implies a recharge rate much slower than hypothesized from identified recharge pathways, low storativity values, and relatively large groundwater withdrawals. No sources of dead carbon were previously identified, but the disconnect between old groundwater and likely recharge pathways suggest input from a dead carbon source. Groundwater collected for this study contained elevated alkalinity, δ13C, He, and 3He/4He values deeper in this aquifer system located in this young flood basalt province. Elevated alkalinity and δ13C values correlate with a mantle CO2 source that is reflected in oversaturated He and elevated 3He/4He (R/Ra) ratios. The flood basalts are Miocene expressions of the modern Yellowstone hotspot, which exudes relatively large concentrations of He (with elevated 3He/4He (R/Ra) ratios) and CO2. A deep and 14C-free geologic source of CO2 helps to explain the low percent modern carbon in the deeper groundwater. Correction of groundwater ages through incorporation of this dead carbon source produced ages up to 64% smaller than previous age estimates; although, greater age corrections may be necessary because of differences in sample and analysis methods for alkalinity, δ13C, and 14C that influence detection of the dead carbon input.