Georgia fall line, but also in the Valley and Ridge region in the northwestern part of the state. In the intervening Piedmont and Blue Ridge regions, the crystalline bedrock and overlying saprolite typically do not provide significant amounts of groundwater. The biggest demand for groundwater in Georgia comes from agriculture, which pumps as much as 1.5 billion gallons of groundwater per day at the peak of the growing season.
South Georgia is underlain by a thick sequence (up to 4,400 feet) of sand, silt, clay, and limestone beds that thicken toward the south and east and are gently inclined in those directions. Sandy layers and limestones make up the Coastal Plain aquifers. From oldest to youngest, they are named the Cretaceous, Clayton/Dublin, Claiborne/Gordon, Floridan, and Brunswick aquifers. An aquifer is a formation that contains sufficient saturated, permeable material to yield a significant amount of water to wells and springs.
The extremely productive Floridan aquifer system, a series Flint River basin of southwest Georgia, the Floridan aquifer is semiconfined. There, the aquifer recharges annually with seasonal rainfall from November to April. Where the Floridan aquifer is at or near the surface, many springs can be found. Most of the groundwater used by agriculture comes from the lower Flint River basin because the Floridan aquifer is shallow and productive.
Other aquifers in south Georgia are not as productive or widespread as the Floridan. Closest to the fall line, the Cretaceous aquifer system consists of sands and gravels deposited on ancient beaches. This aquifer is mostly used in a narrow band through the middle of the state.
Above the Cretaceous aquifer, the Clayton/Dublin aquifer is a limestone that can produce large amounts of groundwater. Unfortunately, the Clayton aquifer has a very small recharge area. From the late 1970s until the early 1990s, agricultural and municipal pumping greatly exceeded recharge, and water levels dropped precipitously. No new withdrawals are being permitted for the Clayton aquifer in southwest Georgia, and the water-level decline has greatly slowed.
Above the Clayton aquifer is the Claiborne/Gordon aquifer. Because it is sandy and thinner than the Cretaceous aquifer, it is not as productive as the state's other major aquifers.
In the coastal counties of Georgia, sands and limestones of the Brunswick aquifer are thickest in the counties along the Altamaha River. This aquifer is being used more than in the past as an alternative to the Floridan aquifer.
In northwest Georgia, folded and faulted Paleozoic limestones underlie the intermontane valleys. These limestones are extremely productive aquifers that, like the Floridan aquifer, recharge annually. The limestone aquifers of north Georgia are associated with many springs. The combination of recent drought and heavy municipal and industrial pumping, however, has created localized sinkholes that have caused significant property damage.
Across Georgia, groundwater levels typically rise and fall throughout the year in a predictable cycle. In normal rainfall years, groundwater levels are highest in March and April after being recharged by winter and early spring rains. Levels decline through natural discharge to rivers and springs and from pumping until they reach their lowest levels in late October and early November. They gradually rise thereafter until the following spring.
In some parts of south Georgia east of the Flint River basin, there have been long-term declines in the Floridan aquifer, indicating that withdrawals are exceeding recharge. Heavy industrial and municipal pumpage of the Floridan aquifer along the coast of Georgia, especially in Savannah and Brunswick, has resulted in large cones of depression in which hydraulic pressure has been significantly reduced. This has led to saltwater intrusion in Brunswick and Hilton Head, South Carolina, and the cessation of many flowing artesian wells.