A Columbia River Basalt Group Aquifer in Sustained Drought: Insight from Geophysical Methods

Abstract

Aquifers within the Columbia River Basalt Group (CRBG) provide a critical water supply throughout much of the Pacific Northwest of the United States. Increased pumping has resulted in water level declines in this region. Recharge into this aquifer system is generally not well understood. Recent suggestions of probable decades-long droughts in the 21st century add to this problem. We show that geophysical methods can provide useful parameters for improved modeling of aquifers in a primary CRBG aquifer located on the eastern edge of the Columbia Plateau. Groundwater models depend in part on the area, thickness, porosity, storativity, and nature of confinement of this aquifer, most of which are poorly constrained by existing well information and previous stress tests. We have made use of surface gravity measurements, borehole gravity measurements, barometric efficiency estimates, earth tidal response, and earthquake seismology observations to constrain these parameters. We show that the aquifer, despite its persistent drawdown, receives a great deal of recharge annually. Much of the recharge to the aquifer is due to leakage from overlying flows, ultimately tied to precipitation, an important result for future aquifer management in times of sustained drought.

Water Balance to Recharge Calculation: Implications for Watershed Management Using Systems Dynamics Approach

“…This probably implies that a significant amount of water might be entering as recharge to GR to balance the deficit. In a recent study, Piersol and Sprenke [61] indicated that about 93.5% ± 2.6% of water in GR is contributed by the recharge which agrees the finding this study (Figure 7). Figure 7 shows the contribution of recharge to GR as a percentage.…”

“…The answer might not be straightforward and probably the most up-to-date explanation can be considered to be the approach adopted by [45]. This study keeps on the paradigm that GR may be actively recharging based on the water balance model which is supported by some of the most current studies [61,62]. Figure 5 shows that with an increase of water demand in GR, recharge from WP to GR is also consistently increasing until it reaches the threshold.…”

“…The two closely located state universities of Idaho and Washington along with other entities are working to better understand the aquifer properties and hydrogeology of the basin. Studies show that storativity of PB aquifer basalt ranges approximately between 10´3 and 10´5 based on aquifer discharge tests [60], 1.5ˆ10´4˘0.2ˆ10´4 [61], 3ˆ10´5 to 3ˆ10´4 of GR [62] and 0.67ˆ10´3 of WP [63]. Moran [62] had compiled the storativity estimates from previous PB investigations.…”

“…With this assumption, any discrepancy in estimation of storativity or surface area of GR can affect the overall accuracy of the estimated recharge. Similar approach (Equation (8)) had been utilized to test how the system parameters affect the recharge estimates in GR [61,62]. The maximum recharge to GR (R GR (max) ) is limited to 4 cm/year not to surpass the recharge from water balance.…”

“…The aquifer recharging process can be less evident in GR because of the larger surface area, complex hydrology, and hydraulic and hydrologic connectivity among the other groundwater regions, but the inconsistency between the drawdown and extraction is indicating that GR might be actively recharging. Analyzing initial volume of aquifers, the current decline rate the aquifer and the long term pumpage, we utilized the storativity value of 10´3-10´4 [61] for the modeling purposes. Smaller storativity values imply less water stored in the specific saturated depth of aquifer as well as only a little amount of water that can be extracted from drawdown.…”