Limestone beds of Eocene, Oligocene, and lower Miocene age, called the principal artesian aquifer, are the chief source of ground water for Colquitt County. Because streams are small, undependable and relatively inaccessible to most users, ground water is the most important source for increased industrial and agricultural water use. Southeast of Moultrie the principal artesian aquifer is very productive, and has transmissivity in excess of 145,000 feet squared per day (13,500 meters squared per day). A structural or paleogeographic feature called the Suwannee strait traverses the county from near the southwest corner to the northeast corner. In this strait, limestone in the principal artesian aquifer is partly replaced by fine-grained clastic sediment, impairing transmissivity and making wells hard to construct. Transmissivity is much lower northwest of the strait than it is southeast, probably because of facies changes in the aquifer.

In the belt of the hypothesized Ochlockonee fault, water containing greater-than-usual concentrations of dissolved solids is produced. This anomaly could be the result of the fault having acted as a conduit when the vertical hydraulic gradient was upward from a deeper aquifer, or be the result of the movement of ground water from a sulfate-rich source in the sediments of the Suwannee strait. In south-central Colquitt County, clay-capped permeable beds extending above potentiometric surfaces create conditions favorable for breathing wells.

Predominantly clastic beds of Miocene age overlie the Suwannee Limestone. These beds have a transmissivity of about 2,280 feet squared per day (215 meters squared per day), but they are of comparatively little importance because larger yields can be obtained from the underlying principal artesian aquifer. Moreover, wells are difficult to construct in the poorly consolidated clastic sediments, and water from these shallow beds is likely to be depleted during droughts.

The ground water is generally hard, but is otherwise of good quality. One exception is near the hypothesized Ochlockonee fault where water contains higher-than-usual concentrations of various ions, especially sulfate. Another exception occurs along the axis of the Suwannee strait where clay minerals in the channel facies may soften the water by ion exchange.