Integrating Electrical Geophysical Surveys And Subsurface Probing To Locate Karst Aquifer Recharge Features, Northern Illinois

Sinkholes, solutionally enlarged bedrock fractures, soil pipes, and swallow holes are important<br>recharge features for shallow karst aquifers. These features also may transmit significant quantities of soil<br>water, sediment and waste material within the vadose zone. In many cases, however, sinkholes are filled<br>with sediment or debris and have no topographic expression. In this study, geophysical methods are<br>employed to noninvasively identify and characterize buried sinkholes associated with soil pipes, enlarged<br>bedrock fractures and conduits. Geophysical surveys suggest a 500-m long soil pipe, or networks of pipes<br>underlie a portion of the Perry Farm Park in Bourbonnais, Illinois. Boring and electrical resistivity<br>soundings suggest Silurian dolomite bedrock lies at a depth of approximately 4 m across most of the site,<br>and soil pipes are probably related to hydraulically active fractures within the bedrock. Electrical resistivity<br>pseudosections suggest an undulating bedrock surface. Buried sinkholes appear in inverted pseudosections<br>as 4-8 m wide low conductive zones. Electromagnetic conductivity profiling identified high conductivity<br>anomalies over the locations of buried sinkholes, over soil pipes and topographically low areas. Values as<br>high as 33 mS/m were recorded over the sinkholes and over portions of the pipe system, whereas the average<br>background conductivity is about 22 mS/m. A linear zone of elevated conductivity trends WSW from the<br>sinkholes, and may indicate a soil pipe linked to other sinkholes in a ravine. Some reaches of the pipe<br>system also exhibit small sand "volcanoes", suggesting subsurface liquefaction and upward movement of<br>sediment. Probing data indicate drill stem drops of 0.3 m close to a known buried sinkhole. These drops<br>may be due to cavities in the soil or weathered dolomite bedrock.