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.