In recent years, considerable interest has been focused on deep saline aquifers as reservoirs for the disposal of liquid waste. In general, the water in such aquifers is already in motion, being controlled by three sets of gradients: the hydraulic-pressure gradient, the geothermal gradient, and the salt-concentration gradient. In thick aquifers, interaction of these gradients may induce gravity convection currents which are not generally present in shallow constant-density fluid systems. The fate of the waste liquids entrained in such aquifer systems will depend, among other things, upon the state of motion in the aquifer before injection and the modification of this state by the injection process.

The Floridan aquifer underlying peninsular Florida is more than 2,000 ft (610 m) thick and provides a field situation for comparison with mathematical and hydraulic models. The aquifer is exposed to cold seawater where truncated by the deep trenches of the Gulf of Mexico and the Florida Strait. Natural upwellings of warm saline water and observations of temperature and salinity in wells suggest that the seawater flows inland at depth then upward into shallow parts of the aquifer, and, after mixing with fresh water, it flows seaward again to form a large, geothermally heated, convective flow cycle.

To develop predictive techniques for injection of waste into the deep part of this massive aquifer, a hydraulic sand model was built to simulate a saline aquifer, a geothermal source, freshwater recharge, and waste-injection wells. The studies show stream lines, velocities, and temperature/salinity distributions. The governing equations--namely, the hydraulic-flow equation, the diffusion equation for salt and injected contaminants, and the heat-diffusion equation--are solved simultaneously on a high-speed digital computer. Obtaining theoretical solutions comparable to the model data requires choosing correct empirical values of coefficients of salt and heat diffusion.

At the time of this writing, theoretical solutions have been obtained for the hydraulic model which qualitatively bear a remarkable correspondence to observed distributions of temperature and salinity in the Floridan aquifer. Work is progressing to obtain quantitative correspondence to the field observations and to extend the predictive techniques to solutions of practical problems of waste injection.