Introduction

Purpose
The purpose of this paper is to demonstrate the potential magnitude and rate of direct river water intrusion to the Floridan aquifer in regions where the Floridan aquifer is unconfined and dissected by surface streams. Furthermore, the mechanisms responsible for this exchange process are outlined. Finally, it is the intent of this paper to demonstrate that, in such regions, there is no clear distinction between ground water and surface water.

Figure 1. Ground water and surface water mixing at the entrance to the Devil's Ear cave system. View is looking up at the surface of the Santa Fe river. The tannin stained surface water appears brown whereas the ground water is clear. Photo by Wes Skiles of Karst Productions. Copyright Wes Skiles.

Problem
Standard hydrogeological data such as potentiometric surface maps and discharge measurements from stream gauging stations are not capable of documenting ground water / surface water interactions taking place across short reaches of a river or stream. On the other hand, the hazards of diving in turbid water have spurred cave-divers in Florida to document significant seasonal water clarity reductions that occur in their saturated cave systems. For years, they have attributed increased turbidity to river water intrusion which introduces dark tannin stained water into the otherwise crystal clear ground water. But, because they are diving for sport, their observations have not been noted by the hydrologic and regulatory communities. This investigation capitalizes on cave-diving techniques to collect water samples from large saturated karst conduits beneath a river that could be measured for geochemical tracers to identify and quantify river water intrusion to the aquifer.

Study Area
The field area lies in the western Santa Fe river basin of north-central Florida along a 1.5 km reach of the Santa Fe river between Rum Island and July springs. The investigation focused on the exchange of water between the Santa Fe river and the huge underwater conduits that comprise the Devil's Ear cave system underlying this reach of the river. Figure 2 is a reduced section of the US Geological Survey High Springs SW topographic quadrangle showing the location of the field area and the position of the Devil's Ear cave system relative to the Santa Fe river.

Figure 2. Topographic map showing the position of the Devil's Ear cave system relative to the Santa Fe river, north-central Florida. Topography from the U.S. Geological Survey High Springs SW 7.5 Minute Topographic Quadrangle. Copyright Todd R. Kincaid

Significance
Ground water is a vital natural resource in Florida providing 87% of the public water supply and 94% of the dispersed private supply (Fernald and Patton, 1984). Florida has promulgated environmental regulations with the specific intent of protecting the present and future most beneficial uses of ground water to insure continued availability and utility (Florida Environmental Health and Safety Regulations, 62-520). The regulations continue to specifically outline the permissible levels of known contaminants for aquifers and surface waters. Under the regulations, aquifers are classified as: class-1, 2, and 3 where class-1 is the most protected. Surface waters typically have the least stringent regulations. Definitions at the beginning of Chapter 62-520 of the Florida Environmental Health and Safety Regulations specifically outline the criteria for classifying a body of water as ground water or surface water.

“Ground Water means water beneath the surface of the ground within the zone of saturation, whether or not flowing through known or definite channels...

Surface Water means water upon the surface of the earth, whether contained in bounds created naturally or artificially or diffused. Water from natural springs shall be classified as surface water when it exits from the spring onto the earth's surface.”

(Florida Environmental Health and Safety Regulations, 62-520.200)

Much of the land in north Florida and particularly in the western Santa Fe river basin is used for agriculture and industrial dairy farms. Pesticides , fertilizers, and high-nitrate sewage effluent associated with these land uses either infiltrate directly to underlying aquifers or discharge to rivers and streams via overland runoff. The continued growth of the dairy industry has caused increased public concern for the protection of water quality in local springs, rivers, and the Floridan aquifer from organic pollution. Andrews (1992) showed that ground water and surface water in the vicinity of dairies commonly contains large quantities of nitrate, ammonia, and soluble organic nitrogen species. Water quality data compiled from nine dairy farms in north Florida demonstrated that where unconfined, the Floridan aquifer is vulnerable to contamination from wastes applied to the land surface and that nearby surface waters receive large quantities of nutrient-rich runoff (Andrews, 1992).

Forty-four percent of all the ground water consumed in Florida and 100% of the water used for private consumption in the field area comes from the Floridan aquifer which is designated as a class-1 aquifer (Fernald and Patton, 1984). In the western Santa Fe river basin, many wells used for private water consumption are located within tens of meters of the Santa Fe river. Here and in other similar areas, river water intrusion poses a significant threat to water quality in the upper Floridan aquifer. Pollutants carried to the river by overland runoff from area farms and dairies will enter the Floridan aquifer via river water intrusion and pose a threat to human health.