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.}