Abstract Introduction Study Area Aquifer Framework and Definitions Transmissivity, Hydraulic Conductivity and Storage Coefficient Hydrogeology >Ground-Water Flow System > Present System - Predevelopment System Summary References Cited PDF Version

The principal sources of recharge to the surficial aquifer system in east and south-central Broward County are infiltration of rainfall through surface materials to the water table, and to a lesser extent, water imported from the west or north that seeps through canals to the aquifer or is withdrawn from canals from irrigation. Under the water-conservation areas of west north-central Broward County, recharge is by infiltration of direct rainfall, of water backpumped from south-central Broward County or from west Palm Beach County, or of water drained by canal from Hendry County. Soil types have significant control on the ease of infiltration. As shown in figure 5, the most rapid drainage occurs in east Broward County on medium to fine sand or on rocky coastal areas. Drainage is much slower over most of Broward County where peat and muck or marl and fine sand predominate. Infiltration also varies seasonally. Recharge by rainfall is greatest during the wet season from June to November, and recharge by canal seepage is greatest during the dry season from December to May. A highly generalized water budget for the Biscayne aquifer in east Broward County (fig. 39) indicates that about 37 inches, or 62 percent, of the 60-inch average rainfall reaches the water table. In addition, a little more than 1 inch of the 2.5 inches pumped by supply wells returns to the water table.

Discharge from the surficial aquifer system is by: (1) evapotranspiration (about 20 inches of the 37 inches that infiltrate the aquifer, fig. 39); surficial aquifer system in adjacent counties; and (3) pumping wells for municipal, industrial, domestic, and agricultural supplies. Evapotranspiration and ground-water discharge are greatest during the wet season when water levels, temperature, and plant growth rates are high. Reports by Sherwood and others (1973) and Leach and others (1972) provide much quantitative information on the hydrology of the canal system and its effects on ground-water levels, temperature, and plant growth rates are high. Reports by Sherwood and others (1973) and Leach and others (1972) provide much quantitative information on the hydrology of the canal system and its effects on ground-water levels, which will not be included here. Although pumpage (about 2.5 inches in 1973, fig. 39) is only a small part of the total discharge from the aquifer, its effect is amplified because it is greatest during the dry season when recharge and aquifer storage are smallest. Also, because pumpage has increased over the amount shown in the water budget and because many pumping centers are near the coast or near uncontrolled canal reaches inland from the coast, saltwater intrusion is a serious concern.

Figure 39. Generalized water budget for the Biscayne aquifer in east Broward County (from Sherwood and others, 1973). [larger image]

Surface-water bodies and the ground-water system in Broward County are sufficiently well connected so that surface-water levels and adjacent ground-water levels are comparable under most circumstances. Surface water and ground water are considered the visible and hidden components of a continuous water body. Therefore, the hydraulic head at the upper limit of the ground- water body may be mapped both from water-table wells and from surface-water staff gages or recorders.

Ground-water level maps for the surficial aquifer system at the end of wet seasons and dry seasons are shown in figure 40 and figure 41, respectively. The maps represent 9-year monthly water levels for September (wet season) or for April (dry season). The maps portray average hydraulic head in the surficial aquifer system and the sustained hydraulic gradients available for moving ground water during the extreme seasonal conditions. Also, some areas of recharge and discharge and generalized directions of flow may be interpreted from the maps, as shown by arrows in figure 40.

Figure 40. Contours of average water table altitude in Broward County for September (wet season) 1974-82 and interpreted directions of ground-water movement. [larger image]

Figure 41. Contours of average water table altitude in Broward County for April (dry season) 1974-82. [larger image]

Download the PDF version of the report plates (Figures 15-22, 40 & 41, 23.3 MB). PDF files require the FREE Adobe Acrobat Reader ® to be read.

The highest water levels in Broward County are maintained in the water conservation areas, particularly in north-central Broward County where the September average is 13.5 to 14 feet above sea level, and in the northeast were water pumped from Hillsboro Canal infiltrates from several canals to from a ground-water mound at 12.5 to 14 feet above sea level (Roy Reynolds, Broward County Water Resources Management Division, oral commun., 1984). These are originating areas of flow systems, wherein ground water moves from recharge zones where levels are high to discharge zones where levels are low. Water levels of less than 4 feet occur in most of south-central, southeast and coastal northeast Broward County. In these low-water areas, especially in sandy areas, infiltration of rainfall is the principal source of recharge, although ground-water mounds do not appear at the scale of mapping and contour interval in figure 40. The lowest water levels are within the cones of depression formed around the major municipal well fields. The largest drawdown cones are those of the Fort Lauderdale Prospect well field and the Pompano Beach well field (fig. 40). Water levels in April (near the end of the dry season) range from 0 to 2 feet lower than those in September (end of the wet season) over most of the area, but declines greater than 2 feet occurred in the Fort Lauderdale Prospect well field and the Pompano Beach well field.

Ground-water movement in the Biscayne aquifer has been usually considered in terms of generalized areal flow directions as shown in figure 40. However, the three-dimensional nature of ground-water circulation and the effects of canals on water levels and circulation patterns should be considered.

Prior to canal construction, most ground water in east or south-central Broward County probably move downward from the water table through sand (and perhaps some less-permeable beds) to cavernous limestones and sandstones at depths of 40 to 100 feet below land surface and 30 feet to more than 100 feet thick (fig. 16, fig. 17, fig. 18). Infiltrating water that reached this very highly permeable zone of the Biscayne aquifer, which is 100 to 1,000 times more permeable than overlying sands, could move long distances horizontally, perhaps as much as 20 miles, with relatively small head loss. Water would then move upward through sand or other materials to areas of discharge in the ocean, the Intracoastal Waterway, sloughs, or low swamp areas. This circulation pattern would likely be more efficient hydraulically than flow paths through the shallow materials of moderate to low permeability.

Since canal construction, canals have had a major effect on ground-water levels and movement. Their prime function is drainage of ground water, although some canals, especially near well fields (for example, the Dixie and Prospect well fields, fig. 40), have become sources of infiltrating to the aquifer system. In most water-control districts (fig. 6), canals lessen peak water levels by providing lower ground-water levels prior to storms and by providing efficient surface-water and ground-water drainage. The multiplicity of canals effectively "short circuits" much of the natural ground-water flow by allowing shorter flow paths, local discharge, and rapid movement away by canal rather than slower movement through the ground to more distant discharge zones. The water budget (fig. 39) shows that for an estimated 14.5 inches of water involved in ground-water flow, about 13.5 inches returned to canals for discharge to the ocean, but only 1 inch is discharged directly from the ground to the ocean.

As an aid to understanding the effects of canals on the aquifer, interpreting ground-water movement, and planning site investigations, canal-aquifer relations in Broward County may be conceptually divided into the following possible classes:

• Losing–Canals or reaches of canals that lose (recharge) water to the surficial aquifer system.

  - Partially penetrating–Recharge is added to ground-water flow that passes underneath the canal.

  - Fully penetrating–The canal is a line source for ground-water movement away from the canal in both directions; there is no regional underflow.

• Gaining–Canals that collect water discharge from the surficial aquifer system and carry the water away.

  - Partially penetrating– Canals that intercept only part of the ground-water flow; some ground water passes underneath the canal to more distant discharge areas.

  - Fully penetrating–The canal is a line sing to ground-water flow. Although canals usually penetrate a small part of the surficial aquifer system, there is no underflow, and all flow is captured.

• Cross flow–Canal or ditches usually blocked or controlled, which have a negligible effect upon the surficial aquifer system; ground water passes across or under the canals.

However, except where the aquifer is highly stressed either by well fields in northeast Broward County or by ponding (water-conservation areas), water-level gradients are often so low and water levels of deep zones unavailable that classification of individual canal sites is difficult without further investigation. The peripheral canals of the water-conservation areas, which collect large amounts of seepage, are examples of sites where it is important to determine the ground-water flow pattern in more detail. Some underflow in the very highly permeable zone of the Biscayne aquifer is inferred in figure 40.