IDENTIFYING GROUNDWATER RECHARGE PATHWAYS IN THE MOSCOW SUB-BASIN

Continual groundwater decline over the last 80 years in the Moscow-Pullman basin is motivating communities to explore a wide range of strategies ranging from reservoir development to direct injection to aquifers to ensure a sustainable regional water supply. Historic pumping records indicate the shallow Wanapum aquifer in the Moscow region does receive recharge however it is less certain that the deeper Grand Ronde aquifer is receiving any significant recharge. Moreover there is not a clear consensus in the region of the location of the major aquifer recharge flow paths. In this study we used both distributed hydrologic modeling based on detailed soil mapping and stable isotope tracers to explore and evaluate potential groundwater recharge pathways in the Moscow sub-basin. Modelling results indicate that subsurface water flow off the forested granitics in eastern margin of the sub-basin is likely a significant source of recharge. Biweekly water samples taken from 22 wells and 2 springs and high frequency streamflow and precipitation samples collected over a two year period throughout the Moscow sub-basin were analyzed for stable isotopes of hydrogen and oxygen. Frequency analysis of these stable isotope data suggest the some wells are receiving recharge. Furthermore many of these wells exist on the eastern margin of the sub-basin lending further support to the hypothesis that this region should be considered to be a critical ground recharge zone.