Description of Study Area:
Hydrochemistry

The regional quality of ground water within the surficial aquifer system can be characterized by the hydrochemical facies of the water and its dissolved-solids concentration. Hydrochemical facies are determined by the dominant anions and cations in the ground water. For example, calcium bicarbonate water is one in which calcium ions are more than 50 percent of the total cations in the water, and bicarbonate ions are more than 50 percent of the anions.

Figure 11. Hydrochemical facies within the surficial aquifer system in southeastern Florida. Modified from data in the files of the U.S. Geological Survey and sections shown in Scott and others (1977), Howie (1987), and Sontag (1987). [larger image]

Calcium bicarbonate waters are dominant in shallow parts of the surficial aquifer system in most of the tri-county area (fig. 11). In western Miami-Dade and Palm Beach Counties, however, calcium bicarbonate waters are limited in extent, and mixed ion (sodium bicarbonate) and sodium chloride water occur at shallow depths. This combination is attributable to lower permeability within rocks that comprise the surficial aquifer system and the incomplete flushing of relict seawater by meteoric water. Mixed ion (sodium bicarbonate) and sodium chloride waters, however, mostly dominate deeper parts of the surficial aquifer system. A general comparison of the distribution of hydrochemical facies and permeability within the surficial aquifer system (fig. 11) suggests that calcium bicarbonate waters tend to have the greatest vertical extent in areas where permeability is high. Sodium chloride water is also common along the coast where the surficial aquifer system contains seawater.

The specific conductance and concentration of dissolved chloride can be used in southeastern Florida to evaluate the relative quality of ground water. Specific conductance is a measure of the ability of water to carry an electric current and provides an approximate estimate of the quantity of dissolved ions in solution. Dissolved chlorides can be used to provide insight to the potable nature of ground water. The standard for potable water set forth by the U.S. Environmental Protection Agency (1999) is 250 mg/L.

Specific conductance has been mapped at selected depths in the tri-county area (fig. 12) to determine where: (1) potable water and relict seawater exist, and (2) seawater has intruded landward (Howie, 1987; Sonntag, 1987). Specific conductance (and chloride concentration) is generally less than 1,000 µS/cm in areas with high transmissivity, except along the coast where the surficial aquifer system has been subject to saltwater intrusion. Low specific conductance within the surficial aquifer system is reported in eastern Palm Beach County, eastern Broward County, and much of central Miami-Dade County. With the exception of areas where saltwater intrusion has occurred, such as coastal Miami-Dade County, high specific conductance measurements are considered representative of relict seawater. Relict seawater is inferred to underlie an area that extends southeast of Lake Okeechobee into southwestern Broward County.

Figure 12. Distribution of dissolved chloride and specific conductance at depths of (A) 50, (B) 100, and (C) 150 feet within the surficial aquifer system in southeastern Florida. Modified from Scott and others (1977). Howie (1987), Sonntag (1987), and data in the files of the U.S. Geological Survey. [larger image]

< Previous: Hydrogeology | Next: Changes in Land Use & Population Trends >