SCALE-DEPENDENT AQUIFER TEST AT UNIVERSITY OF MINNESOTA'S HYDROGEOLOGY FIELD SITE

The Department of Geology and Geophysics at University of Minnesota started to offer hydrogeology summer field course in 1995. The field site was jointly supported by U.S. Geological Survey's Interdisciplinary Research Initiative (IRI) site and the University of Minnesota. Six monitoring wells and one pumping well were drilled for single and multiple aquifer tests. The aquifer system in the field site was initially assumed to be a homogeneous and isotropic confined aquifer. Based on results of a series of slug tests, the hydraulic conductivity near the top of the aquifer is between 0.001 ft/s and 0.0001 ft/s. However, results from pumping tests showed that the overall hydraulic conductivity in the field site is at least one order of magnitude higher than that from slug test. In addition, the multiple well pumping tests showed horizontal variations of hydraulic conductivity, transmissivity, and storativity observed from different observation wells. Therefore, the aquifer system in the field is heterogeneous both horizontally and vertically. The vertical heterogeneity is more dramatic and the hydraulic conductivity increases approximately two orders of magnitude higher from the top to the bottom of the aquifer. This scale-dependent variation can be explained by well log records which recorded silty and fine sands near the top of the aquifer and grain size increases to coarse and well-sorted sands near the bottom of the aquifer. Two deep wells were drilled in the field site in 2004 and a more comprehensive aquifer test will be conducted in July 2005 to further investigate the heterogeneity of the aquifer system. This heavily instrumented site is an excellent place for field based hydrogeology course. A teaching package of this study is posted at the "Teaching Hydrogeology in the 21st Century" website (http://serc.carleton.edu/NAGTWorkshops/hydrogeo/activities.html).