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Groundwater Characterization and Assessment of Contaminants in Marine Areas of Biscayne National Park


Geologic Setting
References Cited
Tables and Figures

Water Analyses

Results of analyses of surface- and groundwater samples are tabulated in Appendix A and shown graphically in Appendix B. Here we show the results for salinity, dissolved oxygen, pH, nutrients, metals, and wastewater indicators.

Basic Characterization

Salinity is arguably the most obvious indicator of Biscayne (salinity near 0 parts per thousand, ppt) or Floridan Aquifer (salinity about 2 ppt) water entering seawater (salinity about 35 ppt). Salinity of coastal surface water can also be affected by precipitation, evaporation, and surface-water runoff. The range of surface-water salinity encountered during the study period (Figure 4A and Appendices A2 and B1) was consistent with the known variability of salinity in the bay and offshore as shown by surface-water quality monitoring sites (http://serc.fiu.edu/wqmnetwork/). The lowest salinity and greatest variability were observed at the near shore sites, which are most affected by rainfall and runoff. Variability diminishes greatly offshore to normal seawater salinity of the reef tract that is maintained primarily by the salinity of the Gulf Stream. Groundwater salinity ranges are shown in Figure 4B. Samples from an onshore well (G-3613) in the shallow Biscayne Aquifer are shown for comparison. Only the Black Point Inshore (BPI) well consistently exhibited a pronounced and consistent low salinity of about 21 ppt, indicating possible dilution by Biscayne Aquifer water. The offshore Black Point (BkP) and Petrel Point wells showed slight decreases in salinity (32- 33 ppt), perhaps reflecting some brackish-water mixing from the Biscayne Aquifer and the lens beneath Elliott Key, respectively.

The range of surface-water temperatures (Figure 4C) reflected seasonal temperature change, also moderated by the temperature of the Gulf Stream to the east. Maximum ranges were recorded in the western bay, and minimum variation occurred on the reef tract. Groundwater wells all showed an expected decrease in seasonal temperature variation, but their variation was greater than that of the onshore well (G- 3613, Figure 4D).

statistical box plot of (A) surface-water salinity, (B) groundwater salinity, (C) surface-water temperature, and (D) groundwater temperature for the five sampling rounds
Figure 4. Statistical box plot of (A) surface-water salinity, (B) groundwater salinity, (C) surface-water temperature, and (D) groundwater temperature for the five sampling rounds. [larger image]

DO, DOC, and Nutrients

Dissolved oxygen (DO) was depleted in groundwater relative to that of overlying seawater. Surface waters were generally near saturation with respect to oxygen, but groundwater generally exhibited only a fraction of a percent saturation, nearing 2-3% in a few samples.

Dissolved organic carbon (DOC) in surface water was concentrated near the western shore of the bay (Appendices A1 and B2). In groundwater samples, DOC was also greatest along the western shore of the bay, with a secondary enrichment at Petrel Point. At the other well sites, surfacewater and groundwater values were similar.

With the exception of near shore sites, surface water contained very little soluble silicate (SiO2). Groundwater typically contains an order of magnitude more silica, perhaps as a result of groundwater interactions with quartz sand, than that observed in surface waters. Similarly, the onshore well exhibited high concentrations of nitrate, nitrite, dissolved inorganic nitrogen, and total soluble phosphorous. In contrast, relatively little ammonium exists in Biscayne Aquifer water compared to some surface-water samples from the Black Point Inshore site. Farther offshore in the bay and on the reefs, surface-water nutrient concentrations were low (compared to near shore values), but groundwater was consistently elevated relative to overlying seawater.


Of the 64 elements analyzed, 19 were found to be above detection limits. The distributions of these elements in groundwater are listed in Appendices A1 and A2 and shown graphically in Appendix B3. Also shown is an average value from ocean water from Millero (1996). Some obvious differences occurred in the nearshore wells as a result of mixing seawater with the Biscayne Aquifer. These included low values of boron, calcium, lithium, magnesium, sodium, potassium, strontium, and vanadium in the Black Point wells. Farther offshore, these metals have similar values in seawater and groundwater, with a slight tendency toward higher values in surface water, perhaps as the result of surface evaporation.

Wastewater Compounds

Results of analyses for wastewater compounds are listed in Appendices A3 and A4. Of the suite of compounds analyzed, none were found to occur consistently at any sample site. Only three compounds (DEET, acetophenone, and total para-nonylphenol) were encountered above the method-reporting limits (MRL) during this study. All of these compounds were also encountered in field blank samples (de-ionized water samples that have undergone similar collection procedures as ground and surface water samples).

Radium and Radon Isotopes

During two field efforts in August 2002 and June 2003, we analyzed several groundwater and surface-water samples from select sites within BNP for radium-223 (223Ra) and excess radium- 224 (xs224Ra) as well as water-column radon-222 (222Rn) activities (Appendix A5). During August 2002, average groundwater activities of 223Ra and xs224Ra were 113.6 and 633.3 disintegrations per minute (dpm) 100L-1, respectively, while the average groundwater xs224Ra/223Ra-activity ratio was 10.2. In contrast, surface waters had expectedly much lower xs224Ra and 223Ra activities (10.9 and 24.1 dpm 100L-1 respectively) and an activity ratio (xs224Ra/223Ra) of 2.8. The xs224Ra/223Ra ratio value is in close agreement with an average Biscayne Bay surface water xs224Ra/223Ra activity ratio of 2.0 in water collected during a subsequent submarine groundwater investigation of Biscayne Bay (Swarzenski and others, 2004).

In August 2002, excess 222Rn activities were determined in select groundwater samples from wells within BNP. From five offshore wells, the average excess 222Rn activity was 256.8 dpm L-1, whereas an onshore well had an activity of 939.2 dpm L-1. From a recent 2004 surface-water radon survey, Biscayne Bay had an average background surface-water 222Rn activity of 2-3 dpm L-1 (Appendix A5; Swarzenski and others, 2004).

Strontium Isotopes

Strontium-87/86 for 19 water samples was determined (Appendix A6). One sample was collected from an approximately 1500-ft-deep well on Elliott Key that supplies a BNP reverse-osmosis plant with water from the Upper Floridan Aquifer. The other 18 samples are from the onshore-to-offshore transect of shallow wells. Plotting the 87/86Sr of these samples against salinity (Figure 5) shows how low-salinity samples fall along a mixing line between Biscayne Aquifer water and seawater. No samples were encountered that have low salinity as a result of mixing with Floridan Aquifer water.

plot of Strontium 87/86 versus salinity from well samples
Figure 5. Most well samples had 87/86Sr typical of seawater (red circle). Water samples from nearshore wells fell along a mixing line between seawater and Biscayne Aquifer water (green line). Samples indicating mixing between Floridan Aquifer water and seawater (blue line) were not found. [larger image]

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