PRELIMINARY RESULTS OF STORM WATER INFILTRATION STUDIES THROUGH A COARSE-GRAINED VADOSE ZONE, MISSOULA VALLEY, MONTANA

The capacity of sand, gravel and cobble dominated vadose zones to treat the water quality of storm water runoff is poorly understood in western alluvial valleys. A storm water infiltration study has been initiated in the Missoula Valley of Western Montana to assess the impact of the use of storm water drainage sumps on water quality in a sole source, unconfined receiving aquifer. The vadose zone in the Missoula Valley is comprised of coarse grained sand, gravel and cobbles, with little clay or organic materials. This system allows for relatively rapid percolation of water though the unsaturated zone. The USEPA classifies storm sumps as Class V Injection Wells, used for disposal of wastewater, with more than 6,000 cataloged wells in the Missoula area. Sump instrumentation comprises shallow sandpoint wells with a string of thermistors, data logging water level sensors, gypsum block sensors at multiple depths, and suction lysimeters. Preliminary data from the gypsum block sensors show the expected drying of the vadose zone with time since the last significant rainfall and the onset of the summer hot and dry period. Planned activities for the late summer of 2007 include rotary sonic drilling to collect complete stratigraphic cores to assess geologic controls to percolation characteristics. In addition, wells will be constructed for geophysical borehole tomography studies of infiltration coupled with infiltration tracer tests. Data will be utilized to support development of infiltration models to understand the hydraulics of storm water infiltration from point sources through coarse-grained vadose zone materials