In Palm Beach County, the Surficial aquifer extends from near ground surface to depths in excess of 165 ft below land surface (bls). Deposition of the shallow water marine units in south Florida was regulated by eustatic sea-level fluctuations associated with glacial and interglacial stages of the Pleistocene (Perkins, 1977). Climatic instability, and glacial retreat and advance during the Pleistocene caused sea level to repeatedly recede and advance over large areas of south Florida. Many of the sediments appear lithologically similar but often represent different depositional events (Perkins, 1977). Sediment deposition during this period resulted from dune building, near-shore progradation of the coastline, and soil development.

The Quaternary and Tertiary deposits of south Florida are dominated by shallow water marine carbonates and siliciclastic materials deposited as part of reef systems, tidal flats, and coastal barrier/bar complexes. Interbedded within these units are indications of subaerial exposure, including paleosols and freshwater limestones (Perkins, 1977). The stratigraphy of these Quaternary sediments is described briefly below following from Parker and others (1955), Brooks (1968), and Perkins (1977).

Lake Flirt Marl Formation

The Lake Flirt Marl is Pleistocene in age. According to Schroeder and others (1958), it is up to 6 ft thick, relatively impermeable, and composed primarily of calcareous mud with some areas of dense limestone. The Lake Flirt Marl underlies the organic (peat) soils throughout much of the Everglades and coastal marshes (Parker and others, 1955). Reese and Cunningham (2000) found the Lake Flirt Marl in southwestern Palm Beach County to be composed of silty marl or quartz sand with a marl matrix. The areal distribution of the Lake Flirt Marl and lithographic textures are consistent with deposition in freshwater lakes (Reese and Cunningham, 2000).

Fort Thompson Formation

The Fort Thompson Formation consists of alternating beds of marine, brackish and freshwater limestones similar to those found at the type locale along the Caloosahatchee River. The formation overlies the Caloosahatchee Marl and is Pleistocene in age (Parker and Cooke, 1944; Mitterer, 1975). The thickness of the formation is about 40 ft in eastern Miami-Dade, Broward and Palm Beach Counties (fig. 1) where it makes up the highly productive zone of the Biscayne aquifer (Fish and Stewart, 1991). This formation covers the greatest geographical expanse of all Quaternary formations in southern Florida. The depositional environment of this formation can be linked to late Quaternary sea level fluctuations. The discontinuity surfaces of the Fort Thompson Formation can include dense, well-indurated laminated crusts (Giddings, 1999). Core samples collected during this study are primarily from the Fort Thompson Formation.

Anastasia Formation

The Anastasia Formation consists of alternating offshore bar, beach ridge, and dune system deposits and may be at least 39 ft thick along the coast (Perkins, 1977). The age of the formation is estimated to be late Pleistocene and is considered to be contemporaneous with the Fort Thompson Formation (Parker and others, 1955). This formation can be divided into two distinct facies: a coquina facies and a shell rock facies (Lovejoy, 1983). The coquina facies represents a high-energy environment typical of an offshore bar complex and generally is aligned with the present coastline. The shell rock facies is found behind the coquina facies, between the offshore bar complex and the shoreline. The shell rock facies is characterized by a diverse molluscan fauna with minimal damage to the fossils in a fine-grained quartz matrix, suggesting a shallow bay origin (Lovejoy, 1983). The Anastasia Formation is seen in areas east of the study sites.

Caloosahatchee (Marl) Formation

The transition between the Tertiary Pliocene and the Quaternary Pleistocene occurs in the upper members of the Caloosahatchee Formation (Enos, 1977). The Caloosahatchee Marl is composed of sandy marl, clay, and silt-size particles interbedded with shell beds (Land and others, 1973). Parker and others (1955) describe the Caloosahatchee Marl as sandy marl, clay, and silt with interbedded layers of sand and shell beds. Hydraulic conductivities range from 1 ft/d to 10 ft/d (Scott, 1977). Parker and others (1955) state that in many places the Caloosahatchee Marl is thinner because of erosion and solutioning, whereas in some places it is absent or appears only in isolated patches.

Tamiami Formation

The Tamiami Formation underlies the Caloosahatchee Formation and generally is described as cream, white, and greenish-gray clayey marl, silty and shelly sands, and shell marl that may be hardened locally into limestone (Schroeder and others, 1958). In Broward County, the formation grades from hard, sandy limestone interbedded with calcareous sandstone to green marly silt (Parker and others, 1955). Fish (1988) describes the Tamiami Formation in Broward County as greenish clay marl, silty and shelly marl with calcareous marl, locally hardened to impure limestone. The Tamiami Formation is estimated to be about 6 million years before present (Hoffmeister, 1974). Parts of the upper Tamiami are cavity-riddled and hydraulically similar to the Anastasia Formation (Russell and Wexler, 1993). In this study, the formation was found only at ENR site MP3-A.

Hawthorn Group

The Hawthorn Group underlies the Tamiami Formation and is described by Scott (1988) as highly complex. In south Florida, the upper layer of the Hawthorn Group consists of phosphatic siliciclastic sediments of fine- to coarse-grained quartz sand, quartz silt, and clay minerals (Scott, 1992). According to Miller (1987), impermeable and semi-permeable marls (calcareous clays) of the Hawthorn Group form the base of the Surficial aquifer. Because of the phosphatic sediments, the Hawthorn Group in south Florida generally has a higher gamma-ray signature than underlying or overlying layers (Scott, 1988). In this study, these sediments were seen only at the bottom of the borehole at site MP3-A.

A summary of the chronostratigraphy of the Surficial aquifer is shown in table 1. The age of each formation, its average thickness, primary lithology, and Q unit designations are shown in table 2.