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Hydrogeology of Southeastern Florida

Aquifer Properties

Characterization of three different types of aquifer properties is required for a study of ground-water flow in a coastal environment. These properties can be categorized as transmissive, storage, and dispersive properties. Transmissive properties include three related properties: intrinsic permeability, hydraulic conductivity, and transmissivity. Intrinsic permeability (or simply permeability) is a measure of how easily any particular fluid will flow through aquifer material. Permeability is strictly a measure of the aquifer medium and does not depend on fluid properties. Hydraulic conductivity is related to permeability with the following equation:

K = k

g µ

,

(1)

where

K is hydraulic conductivity [L / T],
k is intrinsic permeability [L²],
is fluid density [M / L³],
g is the acceleration due to gravity [L / T²], and
µ is the dynamic viscosity of the fluid [M / LT].

The g / µ term in equation 1 represents properties of the fluid, whereas the k term (intrinsic permeability) represents the aquifer medium. Hydraulic conductivity, therefore, is a property of the fluid and the medium. In the field, transmissive properties commonly are measured with ground-water pumping tests. These tests typically estimate transmissivity, which is the average hydraulic conductivity multiplied by the thickness of the aquifer. Fish and Stewart (1991, fig. 17) present a contour map of transmissivity for the Biscayne aquifer. Their map clearly illustrates the spatial variability in aquifer transmissivity, which must be accurately represented in a numerical model of ground-water flow. Within Miami-Dade County, values of transmissivity range from 0 to 1.9 x 10⁵ m²/d (square meters per day) as reported by Fish and Stewart (1991).

Submerged ground-water springs in Biscayne Bay probably can be explained by the presence of preferential flow pathways, or conduits, within the Biscayne aquifer. Conduits are commonly found in limestone units because aggressive recharge waters preferentially dissolve the rock matrix along fractures and bedding planes. These solution cavities can affect ground-water flow by increasing the overall hydraulic conductivity of the aquifer.

Aquifers are typically referred to as confined or unconfined, depending on the storage properties of the aquifer. A confined aquifer is bounded on the top by a confining or semiconfining unit; when a well is installed in a confined aquifer, the water level in the well will rise above the base of the overlying confining unit. The elevation of the water level in the well is a measure of the potentiometric surface. When a well is pumped, the potentiometric surface declines in response to the pumping. The storage coefficient provides a measure of how quickly the potentiometric surface declines (or rises) in response to a hydrogeologic stress. For an unconfined aquifer such as the Biscayne, the water level in a well marks the position of the water table. Withdrawals from an unconfined aquifer lower the water table by dewatering pores. Conversely, pores are resaturated when the water table rises. The rate at which the water table rises and falls depends on the storage coefficient. For unconfined aquifers, the storage coefficient is usually referred to as specific yield. Merritt (1996a) performed an analysis of specific yield for a monitoring well in Miami-Dade County. He estimated a specific yield value of 0.2 by analyzing water-table fluctuations in response to heavy rainfall events.

Chemical constituents dissolved in ground water will disperse as they flow through an aquifer. There are two main processes that cause dispersion--molecular diffusion and mechanical dispersion (Fetter, 1993). Molecular diffusion reduces concentration gradients by redistributing dissolved constituents from areas of high concentration to areas with lower concentration. Molecular diffusion slowly occurs over long periods of time and generally is considered negligible compared to mechanical dispersion, when mechanical dispersion occurs to a significant degree. Mechanical dispersion refers to the spreading of a plume or solute as a result of heterogeneities in the aquifer. These heterogeneities occur at many different scales (microscopic, macroscopic, local, and regional). The parameter that is used to describe the effects of heterogeneities on mechanical dispersion is called dispersivity and is dependent on the scale of the problem (Gelhar, 1986).

For the Biscayne aquifer, few studies have addressed dispersivity at the regional or county scale. In his simulation of a brackish water plume, Merritt (1996b) calibrated a numerical model with values of 76 m [250 ft] and 0.03 m [0.1 ft] for longitudinal and transverse dispersivity, respectively. Kwiatkowski (1987) used 1.5 m [5 ft] and 0.15 m [0.5 ft] for longitudinal and transverse dispersivity, respectively, for a numerical model of saltwater intrusion at the Deering Estate near Miami, Fla. The differences between these values of reported dispersivity most likely result from issues of scale; the study by Merritt (1996b) encompassed a much larger area than the study by Kwiatkowski (1987).