DIGITAL GEOLOGIC MAPPING OF AQUIFER RECHARGE ZONES IN THE UPPER COASTAL PLAIN OF GEORGIA

The Georgia Geologic Survey (GGS) has mapped 21 7.5 minute quadrangles during the past six years as part of the U.S. Geological Survey’s STATEMAP program. The mapping is focused on aquifer recharge zones in the Upper Coastal Plain of southwestern Georgia. Aquifers important to the agricultural industry and communities in the southwest Georgia include the Upper Floridan, Claiborne, Clayton and Upper Cretaceous. Mapping consists of classical field techniques supported by shallow core drilling. Outcrops are digitized directly on digital topographic bases in Arcedit and attributed in ArcView. Interpretive geologic maps are then compiled using Arcedit and ArcView software. In addition to more accurately locating the outcrop patterns of the stratigraphic units hosting the aquifers, geologic mapping indicates a considerably more complex geology that may significantly impact the aquifers. Besides regional depositional thinning, a series of erosional events represented by unconformities have locally and regionally impacted the thicknesses and, in some cases, the presence of the aquifers. Paleo - erosion has locally cut down through confining layers thereby allowing groundwater communication between aquifers. Surficial and subsurface groundwater alteration associated with the unconformities, also adversely affected some of the aquifers. Exposed limestones were dissolved, leaving clays and/or chert. Downward movement and concentration of clays have adversely affected porosity and permeability in sandstones. Precipitation of dissolved iron and silica in ground water formed relatively thin, but commonly impermeable layers or crusts of silica and/or iron oxides. Multiple episodes of structural deformation recorded as low-amplitude folds, normal faults and jointing locally reconfigured the aquifers, also.