Hydrogeology of the Surficial Aquifer System in Southwest Florida

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. Two independent methods are 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. Geophysical logs provide a continuous downhole record of the properties of the rocks that form the aquifer. They are especially valuable in providing physical and chemical properties of the corehole where particular intervals of core recovery are poor. Also, they allow extension of hydrologic test data from discrete samples to the rest of the core. Geophysical logs, combined with aquifer water properties and flow measurements, will be used to relate large-scale ground-water circulation to the distribution of hydrologic properties of the aquifer. For example, flowmeter logs can confirm that the most permeable intervals, as inferred from core measurements, coincide with the intervals that conduct the most flow in the vicinity of test wells. Geophysical logs also will indicate which confining units act to separate the aquifer system into discrete aquifers having different water quality and hydraulic head.

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 sstate and overland flow, several changes are predicted to occurr. First, because water flowing over land moves more slowly than in canals, overland flow should remain in the wetlnad ecosystem for a longer period each year. Second, as 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 (USACE) and the South Florida Water Management District (SFWMD) will use hydrologic models to anticipate the consequences of these proposed restoration plans. This reseaerch project 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 involves core drilling, corehole testing, and rock and sediment analysis. 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.