Stratigraphy and vertical hydraulic conductivity of the St. Francois confining unit in townships 25-27 N. and ranges 01-02 W., southeastern Missouri
The St. Francois confining unit (DerbyDoerun Dolomite and Davis Formation) lies beneath the Ozark aquifer (Jefferson City Dolomite to the Potosi Dolomite) and impedes the circulation of water between the overlying Ozark aquifer and the underlying St. Francois aquifer (Bonneterre Formation and Lamotte Sandstone). The Bonneterre Formation is the potential host formation for lead-zinc deposits in the area. There is concern that mine dewatering in the Bonneterre Formation could lower water levels in the Ozark aquifer. To address this concern, the vertical hydraulic conductivity of the St. Francois confining unit in six townships (T. 25-27 N. and R. 01- 02 W.) of Oregon, Carter, and Ripley Counties of southeastern Missouri was evaluated by describing the stratigraphy and measuring the vertical hydraulic conductivity of core samples. The Davis Formation is an intrashelf basin facies consisting of a series of shales interbedded with shaley limestones, shale-free limestones, and local dolostones, and ranges from 24 to 320ft (feet) thick, but typically the thickness is 100 to 200 ft. Shale-dominant sequences can be tens of feet thick, and contain as much as 90 percent shale. Carbonate-dominant zones may be 70 ft thick or greater. The top of the Davis Formation (based on 56 data points) ranges from 620 to 2,022 ft deep and ranges in altitude from 40ft below sea level in the northern part of the study area to 1, 182 ft below sea level in the southern part of the study area. The Derby-Doerun Dolomite represents a pair of superimposed carbonate ramp cycles. Where present, the basal shaley sequence represents a transition with the Davis Formation. The formation (based on 50 data points) ranges from 50 to 386ft thick, but typically is 120 to 180ft thick in the study area. The top of the DerbyDoerun Dolomite ranges from 495 to 2,020 ft deep (based on 53 data points), and ranges in altitude from 85 ft above sea level to 94 7 ft below sea level. The St. Francois confining unit is thickest in the central and southern parts of the study area. The thickness, as determined by 51 core logs that completely penetrate the unit, ranges from less than 200ft in the northwestern and east-central parts of the study area to 411 ft in the central part, but typically ranges from 270 to 340 ft. The net shale thickness of the confining unit (based on 29 data points) ranges from 1. 7 ft in the east -central part of the study area to 89 ft in the southwest part. These net shale thickness values include the cumulative shale thickness of rock from the top of the Derby-Doerun Dolomite to the base of the False Davis. Vertical hydraulic conductivities of 35 rock core samples from the St. Francois confining unit in the study area range from 7.6 x 10 -15 to 2.1 x 10 -10 ft/s (foot per second). The logarithmic transformed vertical hydraulic conductivities of the Derby-Doerun Dolomite and Davis Formation are similar (p-value = 0.073) using the statistical twosample t-test; however, this p-value approaches the level of significance value of 0.05. The vertical hydraulic conductivity of the Derby-Doerun Dolomite is larger and less variable than the Davis Formation. When grouped by rock type, the vertical hydraulic conductivity of samples that contain carbonate, shale, or both carbonate and shale, are similar. A comparison on the ranked data using the Mann-Whitney test shows the confining unit in the study area is statistically different (p-value = 0.020) from the confining unit in the prospecting area (west and adjacent to the study area). The median value of the vertical hydraulic conductivity data from the study area (6.7 x 10 -13 ft/s) is three times larger than the median vertical hydraulic conductivity value for the prospecting area (2.2 x 10 -13 ft/s ). The interquartile range shows that the variability of the study area data spans one order of magnitude (2.0 x 10 -13 to 2.2 x 10 -12 ft/s) and that the corresponding data from the prospecting area spans nearly two orders of magnitude (3.2 x 10 -14 to 1.1 x 10 -12 ft/s). The ranked vertical hydraulic conductivities of the Derby-Doerun Dolomite in the two areas are statistically similar (p-value = 0.514). The median vertical hydraulic conductivity of the study area data ( 1.2 X 10 -12 ft/s) is about three times greater than the median value of the prospecting area data (4.4 x 10 -13 ft/s). The variability of the data, as shown by the interquartile range, is less in the study area (5.5 x 10 -13 to 2.2 x 10 -12 ft/s; spanning less than one order of magnitude) as compared to the prospecting area (3.2 x 10 -14 to 6.3 x 10 -10 ft/s; spanning over four orders of magnitude). The ranked vertical hydraulic conductivities of the Davis Formation in the two areas show these data sets are statistically similar (p-value = 0.076). The median vertical hydraulic conductivity value of study area samples ( 4.5 x 10 -13 ft/s) is three times greater than the median value of the prospecting area data (1.6 x 10 -13 ft/s). The interquartile range of the study area data spans one order of magnitude (1.2 x 10 -13 to 1.4 x 10 -12 ft/s) and the corresponding data from the prospecting area spans nearly 1.5 orders of magnitude (3.2 x 10 -14 to 7.4 x 10 -13 ft/s). The Mann-Whitney test shows the ranked vertical hydraulic conductivities of each rock type from the study area are statistically similar to the same rock type in the prospecting area [carbonates (p-value = 0.225), shales (p-value = 0.668), and carbonates and shales (p-value = 0.227)]. However, in each of the three cases the study area samples have larger median values and less variability than the prospecting area samples. Because the vertical hydraulic conductivity of the various rock types of the confining unit in the study area are statistically similar, the entire carbonate-shale thickness is the primary factor determining the effectiveness of the confining unit. The range of effective vertical hydraulic conductivity of the St. Francois confining unit in the study area using appropriate minimum and maximum thickness, net shale thickness, and vertical hydraulic conductivities is 3 X 10 -13 to 2 X 10 -12 ft/s. The vertical hydraulic conductivity of the confining unit is small, and the confining unit effectively impedes the ground-water flow between the Ozark aquifer and the St. Francois aquifer, unless preferred- path secondary permeability has developed along faults and fractures that extend through the confining unit.