Old Water – New Issues and Challenges

Development in the Hangman Creek watershed south of Spokane Washington could jeopardize groundwater resources based on recent groundwater age data. The Columbia River Basalt Group outcrops are found in river canyons, underlie most of the watershed, and contain the principal aquifers. This study was conducted to gain a better understanding of the groundwater resources in the basalt aquifers and to initiate long-term groundwater monitoring. Results will provide information to facilitate sustainable groundwater development. Hundreds of well logs were reviewed and used to characterize the subsurface and understand the hydrogeology. Five monitoring wells were drilled, tested, sampled, and analyzed for routine constituents and naturally occurring environmental isotopes. Data from drilling and testing indicate that aquifers are limited to fractured or vesicular zones. These zones have limited lateral extent, and are generally discontinuous across the watershed. Significant upward gradients exist in isolated areas of the watershed with flowing artesian conditions occurring. Carbon-14 apparent age of groundwater samples indicate ages from 7,670 to 16,880 years old in the deep aquifer.  The deep groundwater aquifer has a lighter stable isotopic signature than the shallow, indicating distinct recharge sources for the different aquifers. The apparent ages of the deeper water are consistent with aquifer recharge from a slow regional flow system originating in the Rocky Mountain foothills to the east. Alternatively, groundwater may have been emplaced from Glacial Lake Columbia, 12,000-20,000 years ago. Regardless of the recharge mechanism, groundwater ages prescribe cautious groundwater development to achieve sustainable supply.