RAPID INFILTRATION THROUGH A COARSE GRAINED VADOSE ZONE

The surficial geology of the Missoula Valley of Western Montana is characterized by a very coarse grained vadose zone overlying a highly permeable unconfined aquifer. EPA has designated the aquifer as a sole source system, representing the only potable water source for local residents. Source water protection studies classify the aquifer as highly vulnerability to contamination from surface sources. The vadose zone ranges in thickness from 25 feet near the west flowing Clark Fork River in the central part of the valley, to 70 feet or more along bedrock defined valley margins. Infiltration rates were evaluated at two sites with geophysical cross borehole tomography, using ground penetrating radar between well pairs centered around a storm drain during controlled infiltration events. The storm drains are dry wells or sumps, classified by EPA as Class V Injection Wells designed as a storm water management method using infiltration into the subsurface. The initial experiments were completed at each site during April 2008, with a second set completed in August 2008. The infiltration rates estimated from the geophysics studies were confirmed using a series of tracer tests at the same sites during Summer 2009. The tracer tests were completed by adding fixed concentrations of sodium chloride at elevated concentrations into known volumes of water infiltrating into the drains. After infiltration of the sodium chloride solution was complete, an additional volume of low conductivity water, obtained from the public water supply mains, was directed into the sumps to maintain infiltration rates through the system. The conductivity of the infiltrating water in the sump, and in the geophysical test monitoring wells adjacent to the sump, were monitored using datalogging instruments which recorded water level, temperature and specific conductivity at regular intervals. The data show infiltration rates through the majority of the coarse grained vadose zone are approximately 6-8 ft/hr. The data were collected from field studies for use in development and calibration of unsaturated flow models of infiltration into a coarse grained vadose zone. The rapid infiltration rates characterize the high vulnerability of the aquifer.