Restoration and management of the south Florida ecosystem will be guided by hydrologic models that simulate water flowing through the wetlands and shallow subsurface aquifers beneath them. The restoration of the ecosystem is, essentially, the restoration of the natural hydrologic system. As surface water is re-diverted from manmade canals to its more natural state as overland flow, several changes are predicted to occur. First, because water flowing over land moves more slowly than in canals, overland flow should remain in the wetland ecosystem for a longer period each year. Second, as the flowing water spreads out over the wetlands, recharge to the shallow aquifers should increase as more of that water infiltrates into the ground. The U.S. Corps of Engineers and the South Florida Water Management District (SFWMD) will use hydrologic models to anticipate the consequences of these proposed restoration plans. This research program is designed to provide essential subsurface data to improve hydrologic models for land and water managers in southwest Florida where subsurface information is lacking. Obtaining hydrogeological data requires core drilling, corehole testing, and rock and sediment analysis.

The objective of this project is to provide to hydrologic modelers a three-dimensional database of the geologic and hydrologic properties of the sediments and rocks of the surficial aquifer system in southwest Florida, in Collier and Monroe Counties. Emphasis will be placed on the geologic framework of the aquifer.

Figure 1. Project study area.

Nearly all sediment and rock in the subsurface of south Florida contains ground water; however, much of that water is not accessible for use. Zones from which economically significant quantities of water can be withdrawn are called aquifers. The most important properties of an aquifer are porosity and permeability. Porosity is the amount of void space (pores) in sediment or rock that water can occupy, and permeability is the rate at which water can flow between those pores. The quantity and flow rate of ground water in aquifers are determined primarily by differences in porosity and permeability. These important sediment and rock properties were determined in the geologic past by a combination of original depositional processes, by past and present dissolution of the rock, and by the precipitation of new minerals. In other words, the rocks of an aquifer system have a history, and understanding that history will enable us to construct more realistic hydrologic models and to approximate aquifer properties between sampling sites. In southwest Florida, the surficial aquifer system (from the water table down to a depth of about 200 feet) is the primary source of freshwater. This system includes the water-table aquifer, the lower Tamiami aquifer, and the confining unit (rock and sediment of low permeability) that separates them. Surface water that flows through the wetlands is in the water-table aquifer.

Understanding the geologic history of the sediments and rocks of the aquifer system is necessary to place the hydrologic properties of that system into a geologic framework. Most of the sediments in the surficial aquifer system were deposited in the last 5 million years, when quartz silt and sand, clay, carbonate minerals, and shells of invertebrate organisms accumulated in the shallow marine environment that covered south Florida. During that period, the sea level rose to cover south Florida numerous times. Each time the sea level rose, more carbonate minerals, shells, quartz sand, and clay accumulated. Each time the sea level fell, quartz sand and clay were transported into the marine environment, and newly exposed carbonate sediments were subjected to weathering. Some of those sediment surfaces exposed to weathering in the past may have developed into highly porous zones forming conduits for ground-water flow today, or they may have become tightly cemented to form confining units that separate aquifers. Two independent methods will be used in this study to estimate the age of the aquifer rocks and sediments. Samples from cores will be examined for fossil dinoflagellate cysts, pollen, mollusks, foraminifers, and ostracodes, and their age determined by correlation to other distant sites that have been dated isotopically. Age also will be estimated by the isotopic composition of strontium in unaltered shells. The ratio of the stable isotopes of strontium in the oceans has varied over geologic time such that, in the last 40 million years, there has been a unique relation between age and isotopic composition. Marine invertebrates incorporate the strontium isotopic ratio of the ocean into their shells as they grow, thereby preserving evidence of their age.