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Carbonate and shale rock core samples from the St. Francois confining unit

Carbonate and shale rock core samples from the St. Francois confining unit

If lead–zinc mining is permitted in the new exploration area, the upper part of the Bonneterre Formation near mine tunnels and shafts would be dewatered. There is concern that mine dewatering could lower ground–water levels in the overlying Ozark aquifer and decrease the flow of springs and streams that receive ground water from the Ozark aquifer. The USGS with the cooperation of the Missouri Department of Natural Resources; Geological Survey and Resource Assessment Division (formerly known as Division of Geology and Land Survey) is evaluating the the vertical hydraulic conductivity of the St. Francois confining unit to determine if mine dewatering in the St. Francois aquifer would be likely to lower water levels in the Ozark aquifer. The investigation includes laboratory measurements of the vertical hydraulic conductivity of rock core samples from the St. Francois confining unit collected from exploration boreholes by mining companies, as well as mapping of the thickness, altitude of top, and shale content of the St. Francois confining unit (Kleeschulte and Seeger, 2000; Kleeschulte and Seeger, 2001). The estimated vertical hydraulic conductivity of 10-12 to 10-14 ft/sec (foot per second) determined for carbonate and shale rocks that form the St. Francois confining unit suggests that the unit forms an effective barrier to ground–water flow. However, the investigation does not include an assessment of the occurrence or hydraulic properties of vertical fractures that may extend through the St. Francois confining unit. A pilot study is being conducted to investigate the value of using audio magneto-telluric surveys to determine the presence of vertical fractures in the confining unit.

For more information contact:

David C. Smith, Hydrologist

U.S. Geological Survey

Missouri Water Science Center

1400 Independence Rd., MS 100

Rolla, MO 65401

Telephone: (573) 308-3675

Fax:(573) 308-3645

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