The Choctawhatchee River flows through the Dougherty Karst Plain District which encompasses the northern portions of Bay and Calhoun counties, all of Jackson County, most of Washington and Holmes counties, and portions of Walton County. The Floridan Aquifer is recharged through the leaky confinement of the Intermediate System and discharges to springs and rivers throughout the Dougherty Karst Plain. The semi-confined condition of the Floridan Aquifer across the Dougherty Karst Plain allows for large amounts of local recharge, but also makes the Floridan Aquifer especially vulnerable to contamination from activities occurring on the land surface. The Apalachicola and Choctawhatchee rivers and Holmes and Econfina creeks all gain water from the portion of the Floridan Aquifer that is recharged through the Dougherty Karst Plain (Pratt et al. 1996). The physiography of the Choctawhatchee River basin in the Dougherty Karst Plain was originally described by R.O. Vernon as the River Valley Province, defined by an area of low elevations and wetlands, referred to as bays in this area, created by erosion and dissolution activity in the karst plain (Vernon, 1942). The region is now more commonly known as the Marianna Lowlands (Pratt et al. 1996).

In Holmes, Walton and Washington counties, the Floridan Aquifer is comprised of the Chattahoochee Formation, the undifferentiated Marianna/Suwannee Limestone, and the Ocala Limestone (Scott 1993 and Campbell 1993). In this region the Floridan Aquifer itself is relatively thin, with a thickness of approximately 150 feet near the Alabama border, where it is composed of the Ocala Limestone only (Moore 1955). Continuing south, the Floridan plunges to approximately 700 feet in thickness with the addition of the younger Marianna, Suwannee, and Chattahootchee Formations. The Floridan Aquifer, though relatively thin and only semi-confined in this area, is the primary source of water for consumptive use (i.e. public supply, domestic supply, irrigation, etc.).

The region is characterized by a thin and inconsistent confining Intermediate System, composed of the Alum Bluff Formation. The Intermediate System generally thickens to the west and south through the Choctawhatchee River basin. Continuing south, the Alum Bluff Formation grades into the more carbonate Intracoastal Formation (Schmidt, 1984). The Alum Bluff is composed of sands, clays and shell beds. Contained within the Intermediate System may be a thin, minimally water-bearing surficial aquifer or minor, confined water-bearing zones. Because of the carbonate components of the Intermediate System, these water bearing zones at times may appear to have the characteristics of the carbonate Floridan Aquifer.

In the study area along the Choctawhatchee River, the top of the Floridan Aquifer lies approximately 100 feet above mean sea level and is thinly confined, if at all. Carbonates comprising the Floridan System are frequently exposed in the channel of the Choctawhatchee River near Alabama. As the river heads south, it becomes characterized by massive sandbars and exposed limestone becomes rare. The Floridan Aquifer potentiometric surface elevation drops from approximately 200 feet above sea level near the northern-most springs to approximately 10 feet above sea level at southern extent of the study area near the mouth of the Choctawhatchee (Mahoney, et al., 1998). It is logical to expect a ground water contribution to the Choctawhatchee River as the potentiometric surface elevation is fairly commensurate with topographic elevations in much of the Choctawhatchee River basin.

A ground water contribution zone for the Choctawhatchee River basin interpreted from the 1998 NWFWMD potentiometric surface map is depicted in Figure 3 but is subject to change under different hydrologic conditions. It is bounded by ground water contribution zones for Holmes Creek and Econfina Creek to the east and a regional down-gradient ground water flow to the west.