DISTINGUISHING SUB-AQUIFERS IN A REGIONAL FRACTURED BASALT AQUIFERS THROUGH ISOTOPIC TRACERS AND LONG-TERM VARIATIONS IN GROUNDWATER LEVELS

The Eastern Snake Plain Aquifer (ESPA), located in southeastern Idaho is one of the world’s largest and most productive aquifers. Changes in irrigation practices, groundwater pumping, and extended periods of drought have resulted in large groundwater level decreases across the ESPA. To help mitigate these losses, the State of Idaho, through the Idaho Water Resource Board (IWRB) has developed a managed recharge program. Since 2014, this program has recharged over 1,000,000 acre-feet across the aquifer. The largest managed recharge site in the IWRB’s program, the Mile Post 31 (MP31) site is responsible for 440,000 acre-feet of that recharge. Water levels in the immediate area have increased by as much as 30 feet. However, other nearby water levels have not been increasing as dramatically.

Previous authors have identified a distinct geochemical and isotopic signature in the region south of MP31. δ¹⁸O values are depleted and nitrate concentrations are elevated compared to the surrounding areas. This has been attributed to agricultural water leaking from irrigation canals into the aquifer. However, other nearby areas have similar amounts of irrigation and do not exhibit this behavior. In addition to this distinct geochemical and isotopic signature, groundwater levels have also demonstrated a different response pattern. The winter of 2017 was an exceptionally wet season throughout Idaho. Unlike many wells across the ESPA, wells in the region south of MP31 showed no direct response to the large snow season. Despite all these differences, well logs and geophysical surveys show no clear geological difference between these regions.

δ¹⁸O, tracer tests, and water level measurements were used to identify a separate perched aquifer overlying the larger regional aquifer. Because the ESPA consists of a series of numerous overlying lava flows, these perching conditions may be caused by only slight differences in the geologic conditions. This makes it extremely difficult to detect these perching conditions through pure geological investigation, and shows how combing isotopic tracers and groundwater levels is extremely important to properly characterizing groundwater flow in aquifers. This information is crucial in assessing the effectiveness of the managed recharge program in a highly complex aquifer.