890-901 Gw N-d 04

Quantifying surface water/ground water exchanges has become an important component of water resources management because of the increase in the conjunctive use of water. Reducing uncertainty in models used to select an optimal operation management alternative requires proper identification of the spatial and temporal variations in physical parameters such as the hydraulic conductivity of the streambed and aquifer. Recently, heat as a tracer has been demonstrated to be a robust method for quantifying surface water/ground water exchanges in a range of environments—from perennial streams in humid regions (Lapham 1989; Silliman and Booth 1993) to ephemeral channels in arid locations (Constantz and Thomas 1996; Constantz et al. 2001; Constantz et al. 2002; Stonestrom and Constantz 2003). In these studies, diurnal temperature profiles were measured and analyzed to quantify streambed fluxes and hydraulic conductivities. Diurnal temperature variations typically occur over a shallow depth in the range of 0.2 to 2 m (Constantz et al. 2003). At greater depths, weekly and seasonal temperature variations may be observed where the temperatures vary over several days or months rather than during a day. These temperature profiles provide estimates of conductivities over a larger spatial and temporal scale (Lapham 1989; Bartolino and Niswonger 1999; Mihevc et al. 2001) compared to those obtained from diurnal temperature profiles. In addition to quantifying surface water/ground water exchanges, temperature has also been used as a tool for estimating ground water fluxes and recharge rates in aquifers and wetlands. Steady-state temperature-depth profiles have been used to estimate these parameters in aquifers and in wetland systems (Boyle and Saleem 1979; Hunt et al. 1996; Abstract Well water temperatures are often collected simultaneously with water levels; however, temperature data are generally considered only as a water quality parameter and are not utilized as an environmental tracer. In this paper, water levels and seasonal temperatures are used to estimate hydraulic conductivities in a stream-aquifer system. To demonstrate this method, temperatures and water levels are analyzed from six observation wells along an example study site, the Russian River in Sonoma County, California. The range in seasonal ground water temperatures in these wells varied from < 0.2°C in two wells to ~8°C in the other four wells from June to October 2000. The temperature probes in the six wells are located at depths between 3.5 and 7.1 m relative to the river channel. Hydraulic conductivities are estimated by matching simulated ground water temperatures to the observed ground water temperatures. An anisotropy of 5 (horizontal to vertical hydraulic conductivity) generally gives the best fit to the observed temperatures. Estimated conductivities vary over an order of magnitude in the six locations analyzed. In some locations, a change in the observed temperature profile occurred during the study, most likely due to deposition of fine-grained sediment and organic matter plugging the streambed. A reasonable fit to this change in the temperature profile is obtained by decreasing the hydraulic conductivity in the simulations. This study demonstrates that seasonal ground water temperatures monitored in observation wells provide an effective means of estimating hydraulic conductivities in alluvial aquifers.