Groundwater Flow Patterns
Determination of Groundwater Flow Patterns from Cave Exploration in the Woodville Karst Plain, Florida
(Reprinted from Florida Geological Survey Special Publication No. 46)
Woodville Karst Plain Project
The Woodville Karst Plain (WKP), located in the panhandle of Northern Florida, is characterized by a layer of unconsolidated sediments 3-20 m thick, predominantly sands, with shell and clay, overlying an extensive sequence of carbonate deposits, 150-600 m thick. The surface of this area is distinguished by the presence of numerous sinkholes, karst windows, sinking streams, and large springs. There is over 42 km of surveyed underwater cave passages present in several large systems within the WKP. These include Indian Spring, Sally Ward Spring, Wakulla Springs, Shepherd Spring and the Leon Sinks Cave System. Several physical controls are observed to operate, with varying degree, in cave passage development, including lithology, stratigraphy, regional and local groundwater flow patterns, and water table elevation directly influenced by sea-level fluctuations. These parameters are considered in the context of cave-system development presently charted in the WKP. Regional groundwater flow extends from southern Georgia through the WKP and south to the Gulf of Mexico. The orientations of cave passages within the WKP suggest a nearly parallel alignment with regional flow. Current drainage patterns primarily transport groundwater south through these conduits toward the Gulf. However, notable exceptions to this trend occur within Wakulla Spring cave and Indian Spring cave. Geomorphic features, cave passage orientation, current branchwork drainage patterns and flow directions suggest paleoflow directions during conduit formation in the above mentioned caves were most likely in contrast to present day observations.
The exploration and survey of underwater cave systems south of Tallahassee by the Woodville Karst Plain Project (WKPP) has significantly improved the ability of hydrologists to understand the complexity of groundwater flow in a multiple-porosity medium such as the carbonates of the WKP. The improvement in understanding has been hampered in the past by the lack of qualified scientists to visit the remote and hostile environment of deep underwater caves. There is no substitute for detailed observations in improving the quality and quantity of information needed to advance scientific understanding within this multifaceted system of fluid flow.
While the recent observations are an important glimpse into the groundwater flow regime, it should be noted that they represent a significantly limited data set. The small size of the data set and its relation to the regional groundwater flow pattern may be restricted, but this does not mean that it is insignificant. Reasonable assumptions may be made, such that a greater overall understanding of the evolution of this complex groundwater drainage basin may emerge. This paper is an endeavor to add accurate and thorough scientific observations to the current body of knowledge. By using reasonable assumptions in conjunction with detailed observations, it is expected that significant conclusions may be drawn which aid in the understanding of this unique resource.
Regional geology and hydrology
The WKP developed in Leon, Wakulla, and Jefferson Counties, Florida, is characterized by a thin veneer of unconsolidated and undifferentiated Pleistocene quartz sand and shell beds, overlying a thick sequence of relatively horizontal carbonates (Hendry and Sproul, 1966). The WKP is a gently sloping topographic region of low sand dunes and exposed carbonates rising northward from the Gulf of Mexico to approximately 20 m in elevation within Leon County, terminating at the Cody Scarp. The loosely consolidated Pleistocene sands, being very porous and permeable, allow rapid infiltration of precipitation. Important to our study, the St. Marks and Suwannee limestones, underlying the unconsolidated
Figure 1. Plan view map of the western Woodville Karst Plain illustrating of the Leon Sinks cave system (comprising northernmost Sullivan sink and Big Dismal sink to southernmost Turner sink), Chips Hole cave, Indian Springs cave, Sally Ward Spring cave, McBride’s Slough Spring cave and Wakulla Springs cave system.
sands, comprise hydrostratigraphic units of the Upper Floridan Aquifer System (FAS). These limestones, being very porous, permeable and soluble, have undergone considerable dissolution from groundwater movement (Hendry and Sproul, 1966). Consequently, the topography is karstic in nature, with numerous sinkholes, karst windows, sinking streams, and large springs (Rupert & Spencer, 1988).
The St. Marks is a predominately fine to medium-fine grained, partially recrystallized, silty to sandy limestone that has undergone degrees of secondary dolomitization (Hendry and Sproul, 1966). It also contains extensive shallow conduits in portions of the Leon Sinks cave system, Chips Hole cave and Indian Springs cave. It pinches out against the Suwannee limestone in southwestern Jefferson County and reaches a maximum thickness of approximately 60-m in western Wakulla County.
The Suwannee limestone, Oligocene in age, reaches a maximum thickness of 160 m at approximately 30-150 m below land surface within Leon and Wakulla Counties (Davis, 1996). The thickest portion of the Suwannee is found south at the Gulf of Mexico and the thinnest is located near the Georgia border (Hendry and Sproul, 1966). It consists of two types of permeable rock: (1) a crystalline tan, highly fossiliferous limestone and (2) a white to cream, finely crystalline limestone containing foraminifer with micritic limestone pellets (Davis, 1996). The Suwannee limestone is the principal lithology transporting much of the groundwater of the Upper FAS within the WKP. The majority of dissolution conduits within the WKP are primarily developed in the Suwannee limestone.
The regional recharge area for WKP extends north of the Georgia border for over 80 km and covers portions of over five Georgia Counties (Davis, 1996). The regional groundwater flow pattern, taken from piezometric contour maps, shows overall south trending flow lines (Davis, 1996; Fig. 32 Scott et. al., 1991). There is an interesting feature of the piezometric contour maps which show a saddle or potentiometric low area extending well into the WKP. This indicates a convergent flow line pattern toward the south-central region of the karst plain. In this area, there are several first-order magnitude springs including Wakulla Springs, St. Marks Springs and Spring Creek Springs. The regional convergence of flow is thought to originate from the fact that the WKP confining unit is absent. Thus, flow through Leon county, being confined by the Miccosukee and Hawthorn Formations, converges in the WKP, where the lack of confining units allows groundwater transport to result in artesian flow at the surface.
We will turn our attention to the south-central region within the WKP. Within this region, where the majority of springs discharge and the majority of caves have been explored, there are several interesting features. The first is the numerous sinkholes. These depressions are readily observed on 7