Fish and Stewart (1991) described the hydrostratigraphy of Miami-Dade County, which encompasses the eastern half of the study area. Reese and Cunningham (2000) present a cross section of the surficial aquifer system (Figure 2) that is located just north of our study area. The surficial aquifer system is composed of four units (see Figure 2). From the surface downward these are: 1) the Biscayne aquifer, composed of limestones and quartz sands; 2) the upper semi-confining unit, composed of quartz sand, sandstone, and mudstone; 3) the Gray limestone aquifer, a pelecypod, lime rudstone and float stone, pelecypod-rich quartz sand, and moldic quartz sandstone; and 4) the lower semi-confining unit, a moldic pelecypod-rich quartz sand or sandstone. The total thickness of the surficial aquifer system ranges from 55 to 75 m (180-240 ft).

The Biscayne aquifer is the principal aquifer in eastern Miami-Dade and Broward Counties. Composed of several Pleistocene, Pliocene, and late Miocene age sedimentary units, the Biscayne aquifer is defined on the basis of being a highly permeable zone of at least 3 m (10 ft) thickness and horizontal hydraulic conductivities of 3.5 mm/s (1000 ft/day) or more (Fish and Stewart, 1991). Hydraulic conductivities in excess of 3.5 cm/s (10 000 ft/day) are not uncommon for this aquifer owing to the well developed secondary porosity.