In central Alabama, the Mississippian-age Fort Payne Chert is an important aquifer for domestic and municipal water supplies. Geologic mapping shows the extent of Fort Payne Chert exposure at the ground surface, which is the recharge area for the aquifer. Planning for protection of the groundwater recharge from pollution requires information about the precise location of the chert at the land surface. Information from the geologic map enables careful planning for land uses that will protect critical aquifer recharge areas and avoid pollution of groundwater resources.
Defining the Problem
In central Alabama, the Mississippian-age Fort Payne Chert is an important aquifer for domestic and municipal water supplies. Rainfall recharges groundwater where the chert is exposed at the land surface. Where the chert has been quarried, many of the abandoned pits have become trash dumps containing potentially toxic material that can contaminate the water supply. Planning for protection of the groundwater recharge from pollution requires information about the precise location of the chert at the land surface.
The Geologic Map
The Fort Payne Chert, a fossiliferous cherty limestone, is resistant to erosion. The geologic map (Fig. 1) shows the extent of the chert (medium blue, Mfpm) and provides the basis for construction of a cross section to show the location and depth of the aquifer below the land surface (Fig. 2)1.
Figure 1 (top): The geologic map of part of the Pell City quadrangle, AL, accurately identifies the recharge area of the Fort Payne Chert aquifer (medium blue) that must be protected from pollution. Credit: W. Thomas
Figure 2 (middle): Geologic cross section (A-A on Fig. 2) showing the underground position of the Fort Payne Chert aquifer, the depth required for drilling water wells, and the location of abandoned chert pits. Credit: W. Ward 2002
Key to Figures 1 and 2 (bottom). Credit: W. Thomas
Applying the Geologic Map
The porous, fractured, thin-bedded chert is highly permeable and allows the free passage of groundwater. Water from rainfall percolates into the chert where it is exposed at the land surface (Fig. 3). In contrast, the overlying Floyd Shale is impermeable, and where the beds dip below the land surface, water in the Fort Payne Chert is confined beneath the Floyd Shale. Wells drilled through the shale into the chert find excellent supplies of water at relatively shallow depths.
The Fort Payne Chert is also an economically attractive source of road material. Quarries or pits in the chert can be opened easily, and production generally can begin using only a backhoe or bulldozer. Quarrying sites shift as road construction advances, and numerous pits are abandoned. Because of dumping of trash, water entering abandoned chert pits and reacting with waste can contaminate groundwater in the aquifer.
Figure 3: Where the fractured Fort Payne Chert is exposed, water entering cracks and moving along fractures recharges the aquifer. When this water becomes contaminated, the aquifer and its water resources are at risk. Credit: J DeAtley adapted from W Thomas
Geologic mapping shows the extent of Fort Payne Chert exposure at the ground surface this is the recharge area for this important aquifer. The recharge area is a relatively small proportion of the total ground surface; however, it is precisely the area that must be protected from pollution if groundwater resources are to be safeguarded. Information from the geologic map enables careful planning for land uses that will protect critical aquifer recharge areas and avoid pollution of groundwater resources.
1W. Ward, 2002, Geology of the Pell City 7.5-minute quadrangle, St.Clair and Talladega Counties, Alabama: Alabama Geological Survey
Case study authors: William A. Thomas (University of Kentucky), Willard E. Ward (Geological Survey of Alabama), and W. Edward Osborne (Geological Survey of Alabama)
Case study from: Thomas, W.A. 2004. Meeting Challenges with Geologic Maps, p. 58-59. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.