Georgia Geologic Survey Information Circular 100
The Girard hydrogeologic test site (Girard site), in southeastern Burke County, Ga., was constructed during 1992-95 to better characterize the geologic, hydrogeologic, and water-quality characteristics of a multi-aquifer system in Coastal Plain sediments. Data presented herein include the depth, thickness, geologic properties, hydrologic properties, paleontology, and water quality of the Coastal Plain aquifers at the Girard site. During March and April 1992, continuous, 2-inch diameter core was collected from land surface to a total depth of I ,385 feet using the wire-line coring method. The core penetrated Coastal Plain sediments and bottomed in pre-Cretaceous basement rock.
Paleontologic data provide geologic age, and when combined with lithologic data indicate the environment of deposition for several geologic units at the Girard site. Twenty-five samples were examined for dinoflagellates, pollen, benthic foraminifera, and calcareous nanofossils. Eleven of the samples yielded age-diagnostic assemblages, ranging from middle Eocene to the Late Cretaceous.
Water-bearing units at the Girard site were related to previously defined hydrogeologic units by comparing borehole data collected at this site to interpreted borehole data from nearby sites. This comparison indicates that several equivalent hydrogeologic units are present at the Girard site. In descending order, these are the Upper Three Runs aquifer, Gordon aquifer, Millers Pond aquifer, the upper and lower Dublin aquifers, and the upper and lower Midville aquifers.
Selected core samples were analyzed to determine vertical hydraulic conductivity and porosity. Laboratory analyses indicate that the vertical hydraulic conductivity of confining units is less than 2 x 10-9
Three test wells were completed at the Girard site. Clemson University personnel conducted aquifer tests in two of the wells to determine transmissivity, horizontal hydraulic conductivity, and to detect any interaquifer leakage. Horizontal hydraulic conductivity estimated from these tests indicates that the lower Dublin aquifer is much more productive than the lower Midville aquifer and that both aquifers are confined at the Girard site.
Horizontal hydraulic conductivities for several of the aquifers at the Girard site were estimated by applying logarithmic regression models to borehole resistivity data. These values are comparable to tho se derived from aquifer pumping tests and are within the range of error reported for the method. Average hydraulic conductivity values from TW-2 ranged from 26.93 feet per day using the logarithmic regression model to 177 feet per day using TW-2 aquifer pumping tests. In addition, average horizontal hydraulic conductivity values for TW-3 ranged from 24.74 feet per day using the logarithmic regression model to a value of 8.9 feet per day using TW-3 aquifer pumping tests.
Continuous water-level recorders were installed in each test well to monitor water-level fluctuations and trends. Water-level data also were used to determine the vertical distribution of hydraulic head in the water-bearing units. A statistical comparison of ground-water levels, stream stage, and precipitation was performed using Spearman's rank correlation coefficient. Based on this statistical analysis, two significant correlations are apparent. A mass loading water-level response in the lower Dublin aquifer occurs in response to recharge of the Upper Three Runs aquifer. In addition, some interaquifer leakage from pumping of the lower Dublin aquifer may be affecting the water level in the Upper Three Runs aquifer.
Water samples were collected from two of the test wells to determine the physical and chemical characteristics of water from the screened water-bearing zones. Trace-element chemistry shows significant differences in water quality between the lower Midville and lower Dublin aquifers. Of the 11 trace elements tested, barium, iron, and strontium have disparate values between the two aquifers. Values for these elements are significantly higher in the lower Dublin aquifer than in the lower Midville aquifer; the value for iron (1,600 micrograms per liter) exceeds the U.S. Environmental Protection Agency drinking-water standard by 1.300 micrograms per liter. This high value for iron would limit the usefulness of the lower Dublin aquifer to agricultural purposes unless extensive pretreatment is utilized.
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Purpose and scope
Description of study area
Well construction and coring
Hydraulic conductivity of core samples
Hydraulic conductivity estimated from borehole resistivity
Summary and conclusions
Appendix- lithologic description ofGirard hydrogeologic test site, Burke County, Georgia