BOTTOM AND SUB-BOTTOM GPR IMAGING WITHIN SHALLOW WISCONSIN LAKES: BENEFITS AND PROBLEMS

Using geophysical methods to image the bottom/sub-bottom of shallow lakes has been a challenge, particularly in ones that have significant amounts of industrial waste. Half Moon Lake, a former entrenched oxbow of the Chippewa River in Eau Claire, WI, has been used as a log reservoir for major sawmills. Throughout this time period waste from the mills was directly deposited into the Lake which has resulted in the present day environmental problems (e.g. algae blooms, weed growth). In over 60 years of operation at Rib Lake, Wisconsin, the Rib Lake Lumber Company processed over 1.4 billion board feet of lumber. The associated lake was the millpond as well as an industrial waste site. This practice added 3 to 10 m of organic sediment into the lake, including what is presumed a significant number of sunken logs. As a result, both lakes contain an unknown amount of phosphorus-rich organic sediment, such as bark from logs that were once stored in it and sawdust from mills that once operated along its' shores. Plans by officials to rehabilitate the lakes presently focus on ways to reduce phosphorus cycling from the lake bed into the water column, but for such efforts to be effective, the volume and distribution of organic sediment within the lake should be known. To this end, noninvasive investigations of lakebed sediments using ground penetrating radar (GPR) were undertaken. Results from the waterborne and iceborne surveys on both lakes indicate: 1) the depth of the lake were mapped accurately; 2) internal stratigraphy and thickness of organic waste thickness were imaged to a thickness of ~2.0 m; 3) geomorphic structures were imaged (including those now buried by the organic sediment; 4) diffraction patterns were observed and are interpreted as submerged logs. These findings provide a basis for more detailed GPR surveys and a vibracoring program of lake-bed sediments.