LATERAL EXTENT AND EFFECTIVENSS OF SHALE AND SILTSTONE AQUITARDS IN RIDGETOPS CONTAINING PENNSYLVANIAN STRATA

Ridges in the Appalachian and Eastern Interior Coal Basins are composed of interbedded sandstone, siltstone, shale, and limestone units and coal seams. These sedimentary units are often irregular in shape, heterogeneous, and discontinuous. In many cases, the shale and siltstone units can act as significant aquitards and, as a result, cause most water infiltrating the upper surface of a ridge to move laterally through sandstones and coal seams and discharge along the sides of the ridges as small springs or diffuse seepage. Only a small proportion of recharge water actually migrates vertically downward through the core of a ridge (in unmined ridges). Groundwater investigations currently being conducted on several ridges in Indiana are showing that sandstones within the middle and lower portions of the ridges are partially to almost completely dry. Vertical hydraulic gradients are in the range of 0.7 to 1.0 foot/foot, indicating large head losses across shales and siltstones within the core of the ridges. Contaminant plumes that are contained in shallow sandstones (e.g., < 40 feet deep) have migrated several hundred feet to one km laterally through the sandstones, which is evidence that the sandstones are at least moderately permeable. Contaminants are restricted to the shallowest sandstone in each ridge and are not being detected in the deeper sandstones. Hence, the shale and siltstone aquitards seem to be extremely effective in preventing downward groundwater flow.

Groundwater appears to be seeping out along the sides of the ridges as seeps or diffuse seepage; however, visual evidence of this has not been readily apparent. Several possible fates exist for the discharging groundwater. The groundwater may be taken up and transpired by oaks and other vegetation in the heavily forested hillsides, discharged directly into gullies that are deeply incised into the sides of the ridges, or continue to migrate downslope through a thin mantle of soil and weathered bedrock. Contaminants are moving laterally to the edge of the ridges and some contaminant mass is reaching the intermittent streams at the base of the ridges. Modeling and mass budget calculations show that evapotranspiration can account for a large portion of water and contaminant mass reaching the hillsides.