An aquifer map shows the regional location of various aquifers and the type of aquifers present in those regions. Types of aquifers include carbonate-rock aquifers, sandstone aquifers, igneous or metamorphic rock aquifers and various types of unconsolidated or semiconsolidated gravel aquifers.
On an aquifer map, each type of aquifer is generally represented by a unique color, with similar shades referring to various individual aquifers in the area. For example, on the United States Geological Survey aquifer map, all unconsolidated and semiconsolidated sand and gravel aquifers are represented with a shade of blue, except for coastal plain aquifer systems located in semiconsolidated sand such as those located throughout the Gulf of Mexico's Texas and Louisiana coastline.
In addition to carbonate-rock aquifers and sandstone aquifers, combination sandstone and carbonate-rock aquifers exist and are typically given special designations on maps. On the United States Geological Survey aquifer map, these aquifers are represented with purple shades. They include the Edwards-Trinity aquifer system located in Texas, the Valley and Ridge aquifers located throughout the Appalachian Mountains, Mississippian aquifers, and Paleozoic aquifers located throughout the upper Midwest.
Aquifer maps also designate rocks that are minimally permeable but that may contain locally productive aquifers. These areas are typically represented with a unique shade that differentiates them from known, named aquifers.