Geophysical investigation of dewatering in Lolo Creek, Southwest Missoula, Montana, USA

Interest in finding the causes of reoccurring late summer dewatering of Lolo Creek, Montana, led to geophysical and hydrogeologic studies to understand the cause and effect relationship between surface water uses, groundwater uses and stream flow, critical to water management decisions. Electrical Resistivity Tomography (ERT), Spontaneous Potential (SP), Multichannel Analysis of Surface Waves (MASW) and Seismic Refraction Tomography methods were used to provide information on potential seepage pathways leading to dewatering of Lolo Creek and the subsurface lithology of the area. Results from the MASW and seismic refraction tomography studies showed the bedrock at depths of about 35 m close to Highway 93 and 27 m at the end of the 288 m East-West seismic line running through Lewis and Clark Drive. ERT results showed a shallow, high- resistive layer underlain by low resistive layers. The Eocene bedrock (mylonite) was found to have a resistivity between 180 and 400 Ωm. A geologic fault was inferred in a 360 m East-West ERT profile. SP measurements around the inferred fault indicate a zone of infiltration, with SP values between −18 mV to −2 mV. A 1D-depth velocity profile obtained from MASW survey located close to East-West ERT profile, indicates a low velocity subsurface between 0 and 39 m, which is interpreted to be fractured bedrock. This fractured, shallow bedrock and associated fault work as a seepage path from the shallow to deep aquifer and could possibly account for dewatering of the Creek during low-flow periods in the late summer.