Columbia Plateau Regional Aquifer System Research Articles

Monitoring the Ambient Seismic Field to Track Groundwater at a Mountain–Front Recharge Zone

The heterogeneity of the fractured-basalt and interbedded-sediment aquifer along the eastern margin of the Columbia Plateau Regional Aquifer System has presented challenges to resource managers in quantifying recharge and estimating sustainable withdrawals. Previous studies indicated recharge pathways in alluvial sediments atop a mountain–front interface upgradient of the basalt flows. In this sedimentary zone, six seismic stations were deployed for one year to detect velocity changes in low-frequency seismic waves that could be correlated to changes in groundwater recorded by a well transducer near the center of the seismic station network. Waveforms in the 1−5 Hz range were recorded at each station to determine changes in wave velocities between station pairs and correlate these velocity changes to changes in groundwater levels. The velocity–groundwater relation allowed for estimation of daily groundwater levels beneath the seismic station network. Existing hydrogeologic information was used to estimate hydraulic gradients and hydraulic conductivities, which allowed for the calculation of the daily volume of recharge passing beneath the seismic stations and into the confined aquifer system. The daily recharge volumes across the seismic station network were summed for comparison of the total annual recharge calculated from the change in seismic wave velocities (154,660 m3) to a flow model calculation of recharge based on areal precipitation and infiltration (26,250 m3). The 6× greater recharge estimated from the seismic wave velocity changes for this portion of the recharge zone is attributed to preferential pathways of high hydraulic conductivity and greater depth associated with paleochannels beneath the seismic station network.

A Basin-Scale Groundwater Flow Model of the Columbia Plateau Regional Aquifer System in the Palouse (USA): Insights for Aquifer Vulnerability Assessment

Bedrock aquifers are vulnerable to contamination due to the preferential movement of pollutants via rock discontinuities and porous layers. In this research, we propose an approach to assess vulnerability in three dimensions by combining stable isotope values and particle tracking in a vertically anisotropic aquifer of basaltic-fluvial origin at the basin-scale. A steady-state flow and particle tracking model is presented for the Columbia River Basalt aquifer in the South Fork Palouse Basin. Backward particle analysis combined with the distribution of δ2H, δ18O and 4He values vs. depth shows how the aquifer is characterized by two separated zones. A shallow ( 150 mBGL) aquifer zone is characterized by much higher particle travel times as well as a distinctive isotopic fingerprint. At such depths, penetration of particles is partially impeded by the low hydraulic conductivity of the sedimentary layers and recharge preferentially occurs in correspondence of the basin margin. Along this margin, the vulnerability is higher for the contaminants to enter the aquifer system and reach the pumping wells. Thus, following this research, we envisage efforts to combine stable isotope techniques with particle tracking analysis in three dimensions to define areas exposed to contamination risk in fluvio-volcanic bedrock aquifers. These research efforts can represent an approach to integrate with two-dimensional GIS tools that are commonly used to assess aquifer vulnerability.

Understanding heat and groundwater flow through continental flood basalt provinces: insights gained from alternative models of permeability/depth relationships for the Columbia Plateau, USA

AbstractHeat‐flow mapping of the western USA has identified an apparent low‐heat‐flow anomaly coincident with the Columbia Plateau Regional Aquifer System, a thick sequence of basalt aquifers within the Columbia River Basalt Group (CRBG). A heat and mass transport model (SUTRA) was used to evaluate the potential impact of groundwater flow on heat flow along two different regional groundwater flow paths. Limited in situ permeability (k) data from the CRBG are compatible with a steep permeability decrease (approximately 3.5 orders of magnitude) at 600–900 m depth and approximately 40°C. Numerical simulations incorporating this permeability decrease demonstrate that regional groundwater flow can explain lower‐than‐expected heat flow in these highly anisotropic (kx/kz ~ 104) continental flood basalts. Simulation results indicate that the abrupt reduction in permeability at approximately 600 m depth results in an equivalently abrupt transition from a shallow region where heat flow is affected by groundwater flow to a deeper region of conduction‐dominated heat flow. Most existing heat‐flow measurements within the CRBG are from shallower than 600 m depth or near regional groundwater discharge zones, so that heat‐flow maps generated using these data are likely influenced by groundwater flow. Substantial k decreases at similar temperatures have also been observed in the volcanic rocks of the adjacent Cascade Range volcanic arc and at Kilauea Volcano, Hawaii, where they result from low‐temperature hydrothermal alteration.