Topical Area: Water Quality

William C. Burnett, Jeffrey P. Chanton, D. Reide Corbett, and Kevin S. Dillon, Department of Oceanography, Florida State University, Tallahassee, FL; James Fourqurean, Florida International University, Department of Biological Sciences, Miami, Florida; Steven L. Miller, NOAA National Undersea Research Center, Key Largo, FL

Submarine groundwater discharge (SGD) has been recognized as an important source of nutrients to some coastal environments. We hypothesize that SGD may be responsible for some excess nutrient loading to Florida Bay. If correct, then groundwater inflow may be related to the deterioration of water quality and the observed ecological changes which have occurred in the bay over the past several years. This inflow may be associated with either the tidally-driven discharge of subsurface fluids along the keys and/or related to a buried paleo-channel filled with coarse-grained sand. This channel, often referred to as the "River of Sand," may act as a conduit for groundwater flow from areas further north.

SGD is by nature patchy, temporally variable, and difficult to measure. How can one assess the quantitative importance of such a process? Our group has had reasonably good success in other coastal environments by application of natural tracers to this problem. The radioisotopes ²²⁶Ra (t_1/2 = 1600 y) and ²²²Rn (t_1/2 = 3.8 d), and the trace gas CH₄ are good tracers of groundwater flow into surface waters because they are: (1) greatly enriched in groundwater relative to seawater; (2) conservative or nearly so; and (3) easily measured, even at very low concentrations.

We have conducted four extensive surveys of Florida Bay including three in 1997 (June 24 - July 4, August 18-22, and Dec. 3-8) and one thus far in 1998 (February 23-28). We occupied several dozen stations during each survey and collected water samples for analysis of ²²⁶Ra, ²²²Rn, and CH₄. In addition, samples of attached macroalgae were collected from some stations for analysis of ¹⁵N. Many of these stations were also sampled simultaneously by our collaborators at the University of Miami (Larry Brand and Zafer Top, P.I.’s) for analysis of ³H and helium isotopes.

All water samples were collected using a peristaltic pump with a collection hose positioned directly over the bay bottom. Radon and radium samples were collected in evacuated 4-liter bottles and the analyses were conducted in a field laboratory (either at NURC or at the Flamingo Lodge) using helium to sparge radon, cryogenic trapping, and counting via alpha scintillation cells. Methane samples were collected in triplicate and measured using a head-space equilibration technique and a gas chromatograph in the field laboratory within 24 hours after collection.

Nutrient samples and chlorophyll were collected using standard protocols followed by Florida International University (FIU) and NURC. All nutrient, chlorophyll, and some ¹⁵N analyses were performed at FIU.

One difficulty in the application of natural tracers is that they seldom have a unique source. Radon and methane exist at some concentrations almost anywhere, including the atmosphere. It is thus necessary to examine patterns closely and consider all possible input terms. Figure 1 shows an example of the distribution of ²²²Rn in December.