Application of Field Scale Infiltration and Heat Transport Methods to Estimate the Hydraulic Conductivity of Coarse Grained Vadose Zone Materials

Storm water management in the Missoula Valley of Western Montana utilizes over 6,000 Class V injection wells to dispose of storm water runoff. The vadose zone is predominantly very coarse-grained boulder, gravel, and sand fluvial sediments, with limited amounts of silt and clay. Percolation through the coarse-grained vadose zone, and controlling hydraulic conductivities, were assessed using a large-scale field tracer and infiltration test. Strings of thermistors were installed into the base of the gravel-bottomed storm drains to monitor the temperature of infiltrating water. Datalogging pressure transducers were placed at the bottom of the sump to record ponding levels. Infiltration experiments were conducted using a water truck or hydrant as an artificial source of storm water. Flux rates into the vadose zone at the base of the sump were determined by calculating volume changes with falling head data and drain geometry information. A set of five datalogging thermistors are installed and isolated vertically in a steel 3 cm diameter by 1m long sandpoint well installed in the base of the sump. Additional temperature data were obtained by placing thermistors into 4-meter long PVC tubes (3.5 cm diameter) below the base of the drain. In addition to field infiltration testing and temperature monitoring, infiltration was examined using cross borehole radar tomography. The stratigraphy was determined using detailed geologic logging of cores obtained using rotosonic drilling. Data were analyzed using standard falling head test analyses, one-dimensional models for heat transport with VS2DHI software, and standard geophysical methods. With known infiltration rates, the field measurements and model results allow estimates of the saturated hydraulic conductivity. These data will support development of more complex two-dimensional unsaturated flow models and be used to assess storm water management scenarios. 6-3-2008-->