Geophysical response of bedrock fractures, karst conduits, and associated surficial features, Bourbonnais, Illinois

Sinkholes, swallow-holes, and soil pipes are important groundwater recharge points for shallow karst aquifers. They are also sites of subsidence and may indicate deeper hydraulically-active conduits. The thickness of the protective soil cover, macropores, and openings within the soil cover and the nature of the weathered bedrock surface all influence infiltration. In the Perry Farm Park study site, some sinkholes are filled with sediment or debris and have no topographic expression. In other areas, swallow-holes and soil pipes are clearly visible at the surface. This dissertation tests the hypothesis that geophysical techniques may be used to characterize conduits and fractures that act as groundwater recharge pathways for surface waters and sediments entering the karst bedrock aquifer.; Seismic refraction and electrical resistivity soundings suggest Silurian dolomite bedrock lies at a depth of approximately 4 m across most of the park, beneath alluvial sands, silts, and clays. Electromagnetic conductivity profiling identified high conductivity anomalies over the locations of buried sinkholes in the eastern part of the park, possibly due to infilling by clayey sediments or elevated soil moisture. A zone of elevated conductivity trends WSW from the sinkholes and may indicate a conduit linked to other sinkholes in a ravine approximately 0.5 km away. Ground-penetrating radar (GPR) profiles also identified an epikarstal air-filled cave adjacent to Bourbonnais Creek and shallow soil pipes in the western part of the park. Soil borings confirmed the resistivity surveys' prediction of a 3–5 m unconsolidated sediment thickness overlying dolomite bedrock. Borings also confirmed a heavily weathered and enlarged bedrock fracture, initially imaged by 2D resistivity tomography, that contains a clay-filled cave lying below filled sinkholes in the eastern portion of the park. Soil cores from borings also indicate topsoil is transported to depths of over 7 m in this location, suggesting an active natural plumbing system connects the surface to openings in the bedrock.; Integrating geophysical surveys with results from borings and water-level measurements provide a comprehensive approach to delineating these karst recharge features (and contaminant pathways) in three-dimensions. The proposed hypothesis has thus been confirmed by these measurements at Perry Farm Park.