http://sofia.usgs.gov/publications/ofr/94-276/index.html\
Eugene A. Shinn, Ronald S. Reese and Christopher D. Reich
USGS Coastal Geology Center, St. Petersburg, Florida
USGS Water Resources Division Office, Miami, Florida
US Department of the Interior, US Geological Survey OFR 94-276 1994
EXECUTIVE SUMMARY
I. Twenty four wells (21 locations) were core drilled into the limestone beneath the Keys, reef tract, and outer reefs to determine if sewage effluents injected in class V wells onshore are reaching offshore reef areas via underground flow.
These wells were fitted with PVC casings and well screens and were sampled every three months for a period of one year.
Analyses showed consistent hypersalinity in most wells and a marked increase in nitrogen (as ammonia) in offshore ground water.
Other forms of nitrogen (NO2 and NO3) and phosphorus were not particularly elevated in offshore ground water but were above the levels found in surface marine water.
The highest levels of nitrogen (NO2 NO3) and phosphorus were in shallow onshore groundwaters.
Sources for the nutrients in the shallow onshore groundwater consist of septic tanks and cesspools (@ 24,000 and 5,000 in the Florida Keys respectively), agricultural fertilizers, and natural vegetation.
Ammonia concentrations were low in shallow ground waters beneath the Florida Keys, probably because of oxidizing conditions.
II. Tidal pumping is particularly active, especially nearshore. Hydraulic heads sufficient to elevate well water as much as 7 cm above sea level during falling tides were detected in all nearshore wells.
During rising tides, the situation was reversed and water flowed into the wells. Tidal pumping implies considerable water movement both in and out of the upper few meters of limestone.
Tidal pumping is a likely mechanism for mixing and transferring nutrient-rich ground water into the overlying marine waters.
Although tidal pumping should cause rather complete mixing and dilution of any freshwater-based effluents entering the limestone via the more than 600 disposal wells in the Florida Keys, the ground waters in the 30 to 40 ft depth range (9-12m) nevertheless remained slightly hypersaline relative to sea water throughout the year.
III. Rock analyses of material from our cores do not prove or disprove the hypothesis that limestone beneath the Keys or reef tract is serving as a sink for phosphorus and other nutrients.
The data, however do not rule out phosphorus uptake by limestone adjacent to disposal sources.
For the purposes of this study, monitoring wells were not positioned sufficiently close to injection wells to determine if uptake phosphorus is taking place.
Ground waters were found to contain more dissolved solids that could be accounted for if hypersalinity resulted from simple evaporation of sea water.
These data indicate that ground waters in the vicinity of our wells are dissolving solids from the rock rather than precipitating material within the rock framework; however, as mentioned above, our wells were not positioned sufficiently close to disposal wells to determine if localized uptake is occurring.
IV. Examination of rock cores from these wells revealed a general distribution of reef-and grainstone-facies belts. The Upper and Middle Keys are composed of a thin coral faces that extends only a few hundred feet seaward of the Keys.
Reef facies give way to mudstone facies within a few hundred feet seaward of the Keys.
Reef facies give way to mudstone facies within a few yard of shore on the Florida Bay side of the Keys.
On the seaward side of the Keys, beneath Hawk Channel and white bank, the Pleistocene limestone is a mixed grainstone, packstone, and wackstone facies.
Corals are rare or absent. The Pleistocene limestone beneath the outer reefs 4 to 5 miles offshore, however, consists of reef facies with the same coral fauna as that found on Key Largo.
This pattern of two major reef-facies belts separated by a 2-to 4-mile-wide belt of grainstone facies may have as yet undetermined effects on groundwater circulation beneath the Florida Reef Tract. Grainstone is approximately an order of magnitude less permeable than the coralline Key largo facies.
V. The Q3 surface, a major subsurface unconformity thought to form an effective confining zone elsewhere in south Florida, was not detected in wells drilled more than one mile from shore.
This unconformity, however, was detected in all wells drilled on or near the Keys.
What was found to be a more effective and widespread conforming layer is the Holocene sediment deposited on the Pleistocene limestone during the past 6,000 to 7,000 years.
These relatively impermeable sediments are extensive, forming a belt up to 5 miles wide beginning about 0.5 miles offshore.
Holocene sediments generally consist of low-permeability lime mud just above the Pleistocene surface, overlain by more permeable carbonate sands and reefs.
Leakage of ground water by tidal pumping is not likely to occur through lime-mud-dominated areas such as Hawk Channel but is likely to occur through isolated porous and permeable Holocene reefs situated on Pleistocene limestone highs, and in places where Holocene sediment does not cover the limestone bedrock.
Leakage is therefore limited to: 1) a shallow- water 0.5-mile-wide nearshore belt of exposed Key Largo limestone. 2) Holocene patch reefs, which grow on mud-free topographic rock highs, and 3) along the seaward side of the outermost reef in 35 to 65 ft (10-20m) of water, where Holocene reef and sediment accumulations are thin or absent.
IV. This study did not address direct measurements of lateral groundwater movement or a hydrologic mechanism for transplanting hypersaline groundwater away from the Florida Keys.
More recent work, however (Halley et al., 1994), shows that sea level in Florida Bay is higher on the Atlantic side of the Keys more than 50% of the time.
Higher sea level on the bay side of the Keys provides a potential groundwater flow towards the Atlantic most of the time.
Use of tracers (dyes or harmless bacteriological tracers) injected into the center of tightly spaced clusters of monitoring wells is a simple way to ascertain the net direction and rate of groundwater movement is needed for prediction and modeling efforts in the future.