Geologic Division

Groundwater Seepage in the Florida Keys

Project Leader:
E. A. Shinn
600 Fourth Street South, St. Petersburg, FL 33701-4846
E-mail: eshinn@cfcg.er.usgs.gov
Phone: (813) 893-3100 ext. 3030
Fax: (813) 893-3333

The Issue

The Florida Keys contain 25,000 septic tank systems, approximately 5,000 cesspools, and 1,000 class 5 injection wells. Depth of injection wells ranges from 10 to 30 m. Excessive algal growth, coral diseases and both marine grass and sponge mortality is perceived to be caused by sewage nutrients leaking from groundwater on both sides of the Florida Keys. Determining the volume and composition of groundwaters seeping into the marine environment from the sea floor is vital to management decisions in the area. The data can be used for:

planning future disposal systems
modeling the hydrology of Florida Bay
understanding the distribution of benthic flora and faunna

Clients include State of Florida DEP, EPA, NOAA (this is a NOAA Sanctuary) ENP, and Monroe County. Monroe County is planning a multi-billion dollar sewage disposal system and has been following our work very closely. The PI meets regularly with Monroe county and NOAA Sanctuary management and serves on the Technical Advisory Panel (TAP) for the Florida Keys Marine Sanctuary Water Quality Advisory Committee.

How the USGS is addressing this issue

The purpose of this study is to determine the volume and composition of groundwaters seeping upward from the sea floor into Florida Bay and the coral reef tract. The method involves the use of seepage meters installed on the porous rock bottom of Florida Bay. The volume (expressed as l/day/m2) is used to calculate monthly and yearly additions to the overlying marine environment. Analysis of nutrients in the collected waters are needed to model nutrient loading. These studies are necessary to determine:

if the level of nutrients and other contaminants is rising (i.e. baseline data)
to provide data for modeling efforts
explain the mortality of certain benthic organisms such as coarls and seagrass

1998 Activities

This project consists of reinstalling newly designed seepage meters at sites where more than 50 have been already installed. The new meters consist of a circular (1m2 surface area) flexable heavy gauge clear vinyl sheet cemented to the rock bottom with hydraulic cement. Each meter has a central port to which a plastic volume collection bag is attached. The volume of water collected in the bag is measured on-site. Because of irregularities in the surface and variable internal porosity and permeability of the limestone, 2 to 4 meters are installed at each site. These new meters produce reliable seepage estimates not possible with the inflexable fiberglass meters already installed. The "hard" meters were found, in the presence of waves, to actually pump water from the limestone.

Continue installing the new design meters and make modifications as needed.
Sample meters biannually for nutrient and other chemical analyses.
Calculate annual seepage volume of ground water into Florida Bay and the Reef tract.
Begin synthesis of results (Shinn, Reich Hickey and Tihanaky)

Accomplishments to Date

Fifty five fiberglass seepage meters were installed on rock surfaces in Florida Bay and the reef tract and measurements of seepage were made for 6 months. Data collection ceased when tests indicated results were bogus. After much redesign and testing we developed a new meter which will be installed in the latter part of 1997 and early 1998.

Our studies show that conventional seepage meters made from the ends of 55 gallon oil drums, produce bogus results in areas subject to wave pulsing. Oil drum based seepage meters have been used to obtain results reported in dozens of peer reviewed publications. Our tests indicate that in the presence of waves, any "hard" meter design acts as a one-way pump and tends to over estimate the actual volume of seepage. This effect was tested in-situ by attaching various meter designs to plywood sheets isolated from the bottom. Under field conditions, the "hard" meters filled the volume collection bags at a rapid rate, even when it was impossible for groundwater to enter.

As a result of these tests we developed and tested two new seepage meter designs. One meter is a modification of the conventional oil drum type. To make these the end is removed and a clear flexable vinyl end is installed. The unit resembles a large snare drum. A port is positioned in the center for attaching the removable volume colection bag. These kinds of units are designed for insertion into sediment which is the application in all previously published seepage studies. The second design, for installation on hard rock surfaces, consists of a sheet of clear vinyl plastic cemented directly to the rock with hydraulic cement. A port for the volume collection bag is attached to the center. The unit flexes with each passing wave such that the rock beneath the meter "feels" the same wave-induced positive and negative pulses as the surrounding bottom. Net outflow is channeled into the volume collection bag. The recently completed sediment facies map of Florida Bay (see proposal by Ellen Prager) shows that more than 70 percent of the bottom in eastern Florida bay consists of Pleistocene rock. Preliminary results indicate that seeping groundwater may replace all of the water in eastern Florida Bay in less than 1 year. This information has not been incorperated any hydraulic modeling attempts.

In our outreach efforts, a USGS video, titled "The Edge of the Continent" highlights this work as does a video on the Geology of Florida, produced by the State Geological Survey. The study has also been featured on PBS television.

USGS Cooperators

USGS
Reich, Hickey and Tihansky
Army Core of Engineers
Our results and ongoing monitoring activities will provide data needed by those who make hydraulic and nutrient models of South Florida.
South Florida Water Management
Florida Marine Research Institute and University of South Florida
Florida State University
University of Miami
Department of Environmental Protection
National Park Service
Park service biologists are interested in the delivery of nutrients from below, especially from the so-called "river of sand" that underlies the area.
Other Environmental Organizations and Businesses
The results of this study could impact management of the Florida Keys marine sanctuary and thus, future development and tourism in the Florida Keys

Products

Shinn E. A., Reich C. D., Hickey, D. T., Bohlke, J. K., Plummer, L. N., Coplen, T. B., Busenbeberg, E., Chanton J., Burnett, W., Dillon, K., Corbett, R., 1996, Assessing the Origin and Fate of Ground Water in the Florida Keys, Florida Bay Science Conference, Program and Abstracts.
Reich, C. D., 1996, Diver-operated manometer simple device for measuring hydraulic head in underwater wells, Journal of Sedimentary Research Vol. 66, No.5, p.1032-1034.
Shinn, E. A., Reich, C. D., Hickey, T. D., Tihansky, A. B. 1997, Geology and tidal pumping in the Florida Keys, AAPG annual meeting Dallas TX, abstracts p 106-107.
Shinn, E. A., Reich C. D., Halley, R. B., Reese, R. S. 1995, Hydrogeologic aspects of sewage disposal in the Florida Keys GSA annual meeting New Orleans, abstract.

U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
This page is: http://coastal.er.usgs.gov/projects98/7242-37657.html
Address questions and comments about this page to: webmaster@cfcg.er.usgs.gov
Updated: 18 March, 1998@02:13:02 PM (THF)