U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 03-4329
By: M. J. Kleeschulte, U.S. Geological Survey, and C. M. Seeger, Missouri Department of Natural Resources, Geological Survey and Resource Assessment Division
The confining ability of the St. Francois confining unit (Derby-Doerun Dolomite and Davis Formation) was evaluated in ten townships (T. 31–35 N. and R. 01–02 W.) along the Viburnum Trend of southeastern Missouri. Vertical hydraulic conductivity data were compared to similar data collected during two previous studies 20 miles south of the Viburnum Trend, in two lead-zinc exploration areas that may be a southern extension of the Viburnum Trend. The surficial Ozark aquifer is the primary source of water for domestic and public-water supplies and major springs in southern Missouri. The St. Francois confining unit lies beneath the Ozark aquifer and impedes the movement of water between the Ozark aquifer and the underlying St. Francois aquifer (composed of the Bonneterre Formation and Lamotte Sandstone). The Bonneterre Formation is the primary host formation for lead-zinc ore deposits of the Viburnum Trend and potential host formation in the exploration areas.
For most of the more than 40 years the mines have been in operation along the Viburnum Trend, about 27 million gallons per day were being pumped from the St. Francois aquifer for mine dewatering. Previous studies conducted along the Viburnum Trend have concluded that no large cones of depression have developed in the potentiometric surface of the Ozark aquifer as a result of mining activity. Because of similar geology, stratigraphy, and depositional environment between the Viburnum Trend and the exploration areas, the Viburnum Trend may be used as a pertinent, full-scale model to study and assess how mining may affect the exploration areas.
Along the Viburnum Trend, the St. Francois confining unit is a complex series of dolostones, limestones, and shales that generally is 230 to 280 feet thick with a net shale thickness ranging from less than 25 to greater than 100 feet with the thickness increasing toward the west. Vertical hydraulic conductivity values determined from laboratory permeability tests were used to represent the St. Francois confining unit along the Viburnum Trend. The Derby-Doerun Dolomite and Davis Formation are statistically similar, but the Davis Formation would be the more hydraulically restrictive medium. The shale and carbonate values were statistically different. The median vertical hydraulic conductivity value for the shale samples was 62 times less than the carbonate samples. Consequently, the net shale thickness of the confining unit along the Viburnum Trend significantly affects the effective vertical hydraulic conductivity. As the percent of shale increases in a given horizon, the vertical hydraulic conductivity decreases.
The range of effective vertical hydraulic conductivity for the confining unit in the Viburnum Trend was estimated to be a minimum of 2 x 10-13 ft/s (foot per second) and a maximum of 3 x 10-12 ft/s. These vertical hydraulic conductivity values are considered small and verify conclusions of previous studies that the confining unit effectively impedes the flow of ground water between the Ozark aquifer and the St. Francois aquifer along the Viburnum Trend.
Previously-collected vertical hydraulic conductivity data for the two exploration areas from two earlier studies were combined with the data collected along the Viburnum Trend. The nonparametric Kruskal-Wallis statistical test shows the vertical hydraulic conductivity of the St. Francois confining unit along the Viburnum Trend, and west and east exploration areas are statistically different. The vertical hydraulic conductivity values generally are the largest in the Viburnum Trend and are smallest in the west exploration area. The statistical differences in these values do not appear to be attributed strictly to either the Derby-Doerun Dolomite or Davis Formation, but instead they are caused by the differences in the carbonate vertical hydraulic conductivity values at the three locations.
The calculated effective vertical hydraulic conductivity range for the St. Francois confining unit at each location is: 2 x 10-13 to 3 x 10-12 ft/s for the Viburnum Trend; 3 x 10-14 (minimum reporting level) to 1 x 10-12 ft/s for the west exploration area; and 3 x 10-13 to 2 x 10-12 ft/s for the east exploration area. Based on the calculated vertical hydraulic conductivity ranges, the St. Francois confining unit is considered ‘tight’ at all locations. However, in relation to each other, the west exploration area is the tightest, and the most conductive area is the Viburnum Trend. No apparent large cones of depression have developed in the potentiometric surface of the Ozark aquifer as a result of mining activity in the Viburnum Trend. Therefore, using similar mining practices as those along the Viburnum Trend, no large cones of depression in the Ozark aquifer would be expected in the exploration areas, unless preferred-path secondary permeability has developed along faults or fractures or resulted from exploration activities.
TABLE OF CONTENTS
Purpose and Scope
Exploration Borehole Data
Thickness and Net Shale Thickness of the St. Francois Confining Unit
Vertical Hydraulic Conductivity
Evaluation of the St. Francois Confining Unit along the Viburnum Trend
Evaluation of the St. Francois Confining Unit along the Viburnum Trend and Exploration Areas
Summary and Conclusions
1. Map showing location of the Viburnum Trend, exploration areas, and study area
2. Stratigraphic column for an exploration hole in Reynolds County, Missouri, and a general lithologic description of formations in the Viburnum Trend
3. Geohydrologic section from southeastern Crawford County along the Viburnum Trend to west-central Reynolds County
4. Borehole-numbering system used for this report
5.–10. Maps showing:
5. Location of exploration boreholes
6. Structure of the top of the Bonneterre Formation
7. Structure of the top of the Davis Formation
8. Structure of the top of the Derby-Doerun Dolomite
9. Thickness of the St. Francois confining unit
10. Net shale thickness of the combined St. Francois confining unit and the upper Bonneterre Formation
11.–12. Boxplots showing:
11. The vertical hydraulic conductivity and porosity of the St. Francois confining unit and upper Bonneterre Formation plotted by formations and rock types along the Viburnum Trend
12. The vertical hydraulic conductivity of the St. Francois confining unit and rock types along the Viburnum Trend and in the exploration areas
1. Viburnum Trend core log analysis data
2. Adjacent property core log analysis data
3. Porosity, vertical permeability, and vertical hydraulic conductivity data
4. Summary of Kruskal-Wallis statistical test p-values
5. Summary of vertical hydraulic conductivity values for the St. Francois confining unit along the Viburnum Trend
Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). Altitude, as used in the report, refers to distance above or below NGVD 29. NGVD 29 can be converted to the North American Vertical Datum of 1988 (NAVD 88) by using the National Geodetic Survey conversion utility available at URL http://www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html.
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Last modified: Friday, September 16 2005, 04:23:27 PM