This study is a hydrogeological exploration to determine recharge connections in the Moscow sub-basin to the underlying basalt aquifers, by using isotopic and caffeine tracers, and a Soil Moisture Routing model in combination with a GMS ModFlow model.
Identifying hydrologic recharge connections in the Moscow sub-basin
Supervisors: Roel Dijksma & George Bier
During the last century groundwater levels have been declining with rates of 30 cm yr-1 or more in the Moscow sub-basin (150 km2). This sub-basin is located on the Washington-Idaho border (USA). The groundwater systems are fairly complex, with multiple basalt flows underlain by protruding basement metamorphic/granitic ridges, paleo-valleys and interwoven with sand/gravel inter-beds. Previous studies found vertical infiltration is limited in the area. Nevertheless, recharge to Wanapum and Grande Ronde aquifer might occur through preferential flow-paths, called paleo-channels. Knowing the source locations of recharge to the basalt aquifers in the Moscow sub-basin is vital for understanding the long-term sustainability of its water resources and for developing solutions which enhance recharge and protect the drinking water sources for future generations. In this study, presenting field observations and groundwater modelling, such recharge locations were identified. Samples of 12 wells downstream the Moscow waste water treatment plant have been sampled for caffeine. Here the main stream, the Paradise Creek, runs directly over the basalt. Caffeine is known to be conservative and often present in human waste water, so it can be used as a tracer. One well contained enough detectable caffeine, suggesting infiltration occurs west of Moscow. Discharge measurements in the Paradise Creek also suggested infiltration occurs west of Moscow from the Paradise Creek directly into the Wanapum basalt aquifer. In addition, field results of 9 months of long-term, high-frequency isotopic sampling of 24 wells (depth 23 - 213 m) northeast of Moscow show large variations over time, up to 1.4±0.1 ‰ in 18O. This presents a relatively fast (± 2-3 weeks) response to the precipitation isotopic signature. Temperature measurements of these wells show an average drop in temperature of 8.4ºC from July (2013) until January (2014). These large seasonal temperature fluctuations are in line with the characteristics of an infiltration area with a low porosity and are also indications of fast recharge pathways. The isotopic signature, temperature and EC data of these wells show strong indications of recharge in the proximity of the Moscow Mountain. It is suggested by the data that recharge occurs in lateral flow paths from the Moscow Mountain, since the sampled wells reached below thick, impermeable layers. The Moscow Mountain being the recharge location is verified by a Soil Moisture Routing (SMR) model. This one layer grid based model performs best in the Moscow sub-basin’s soil characteristics; shallow soils above impermeable fragipan layers. The SMR model calculated the average daily infiltration rate for the period 2001-2008. The infiltration rate is highest at the Moscow Mountain (up to 2.0 mm d-1). This output was used as upper boundary of a groundwater model: GMS ModFlow. First a steady state model was constructed based on 207 borehole-descriptions. This steady state model was able to reconstruct the estimated total average discharge of 0.7 m3 s-1 with an accuracy of 0.002 m3 s-1. With this model the ranges and sensitivity of physical properties were tested. The model showed that all net groundwater fluxes to the Wanapum Aquifer originate from the Moscow Mountain and mainly flows through weathered, decomposed granite. Also a transient GMS ModFlow model was constructed using monthly average infiltration rates calculated by SMR for the period 2001-2008. Although this model was not able to show the 30 cm yr-1 decline of groundwater in the Moscow sub-basin, it showed a clear response to precipitation peaks in the potentiometric head in the lower aquifer, which also suggest fast recharge pathways exist.