A Preliminary Study of the Alluvial Aquifer in Oakridge, OR

Abstract

Students in the University of Oregon’s Field Hydrology course investigated local groundwater flow in Oakridge’s alluvial aquifer, chemical variability in the groundwater, and groundwater availability.

Using ground penetrating radar (GPR), students performed imaging on the water table in several areas around the city. The imaging showed that the water table follows topography at a depth of about 20-25 feet, which is expected from an unconfined aquifer. The class performed chemical analyses at two well locations: one on the west end of the city and one at the foot of TV Butte to the east. The analyses showed that groundwater near the butte is more alkaline, has higher specific conductivity, less oxidation reaction potential, and higher levels of ferrous iron and sulfide. These results suggest groundwater interaction and mixing between TV Butte and the alluvial aquifer beneath Oakridge. These chemical interactions would likely manifest as dissolution of mafic (magnesium & iron-rich) minerals and weathering from the igneous butte. Students took an automatic pump test result from Well 2 of the municipal well field and compared it to the 1993 report. The result was a substantially lower level of transmissivity than those shown in the 1993 report. Lower transmissivity indicates that there is not as high a volume of water moving through the aquifer as there was in the past. Taking these results into mind, students recommend further study into the alluvial aquifer to better understand the implications of these findings on the Oakridge municipal water supply and sources.