U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3117

September 2004

Novel geophysical and geochemical techniques used to study 

submarine groundwater discharge in Biscayne Bay, Florida

INTRODUCTION

Submarine groundwater discharge 

(SGD) is a problem of major proportions on a world-wide scale. The 

ubiquitous nature of SGD along 

varied coastlines and its importance 

to coastal water and geochemical budgets have recently been thrust into the 

global spotlight [(Moore, 1996, and 

colleagues (cf. Burnett et al., 2003, 

and references therein)]. For example, 

the discharge of nutrient-enriched 

groundwater into coastal waters may 

cause nutrient imbalances that can 

lead to eutrophication (Bokuniewicz, 

1980; Giblin and Gaines, 1990) or 

near-shore micro-organism blooms 

(Valiela and DʼElia, 1990; Laroche 

et al., 1997). Similarly, SGD can 

also directly affect threatened coastal 

freshwater resources and impact fragile coastal ecosystems, such as coral 

reefs. 

Recently, much effort has been 

devoted to developing and adapting 

new tracer techniques and methods for 

the identification and quantification 

of SGD. As the discharge of coastal 

groundwater most often occurs as 

diffuse seepage rather than through a 

single vent feature (Swarzenski et al., 

2001), assessing SGD has remained 

difficult for both oceanographers 

and hydrologists alike. Burnett 

and colleagues have developed a 

systematic approach to investigate 

SGD by using a combination of both 

physical seepage measurements and 

a suite of naturally occurring isotopic 

tracers in the U/Th decay chain 

– 222Rn and 223,224,226,228Ra. Manheim 

et al. (2002) further extended SGD 

investigations by adapting geophysical 

resistivity techniques to examine 

fine-scale change in conductivity 

In this paper, we report on the use of 

streaming resistivity profiling, continuous water-column 222Rn mapping, 

and the deployment of electromagnetic seepage meters to identify and 

quantify submarine groundwater disFigure 1. Landsat TM image of south Florida showing Biscayne Bay and Miami. Water-column 222Rn and streaming resistivity survey track lines (A-A’ and B-B’) as well as the electromagnetic 

seepage meter site (•) at Cutler Ridge are identified. 

fields within coastal sediments. Such 

streaming resistivity profiling has 

been successfully applied to identify 

sites of SGD (Belaval et al., 2003), 

as well as the dynamic position of the 

fresh water/saltwater interface.

Reef Tract

Biscayne Bay

80o30'0"W 80o15'0"W

25o30'0"N

Everglades

Seepage

study site Figure 3

boundary

Cutler 

Ridge

Black Point

Mowry Canal

Miami

A

B B'

A'

Legend

Streams/Canals

Biscayne National Park

0 5 10 20

Kilometers

Florida
charge at select sites in Biscayne Bay, 

FL. Such data support and validate 

variable-density modeling results, and 

provide insight into the mechanisms 

and scales of SGD in Biscayne Bay.

BISCAYNE BAY

Biscayne Bay is an estuarine lagoon 

that is ~ 61 km long and 18 km wide, 

located just south of the Miami-Dade 

County metropolitan area (Figure 

1). Several rivers and canals on the 

western shore discharge surface water 

into the bay. On the seaward side of 

Biscayne Bay, coral reef structures 

make up the northern extent of the 

Florida reef tract. Most regions of the 

bay have been variably impacted by 

agricultural, municipal, and industrial 

activities. For almost 100 years, the 

natural hydrogeologic regime adjacent 

to the bay has been altered through an 

extensive network of dredged waterways and drainage canals (Parker, 

1955). Numerous retention ponds 

and lakes store water and modulate 

surface runoff. Infiltration of organic 

and inorganic pollutants into the 

groundwater from such storage sites is 

likely enhanced by the highly porous 

and transmissive Biscayne Aquifer 

limestone. Submarine groundwater 

discharge into the bay has been prominently observed (Kohout, 1960) and 

more recently modeled (Langevin, 

2001, 2003). Noted declines in adjacent offshore coral reef health and 

overall ecological stress may be 

linked to alterations to the groundwater and surface water flow paths, 

groundwater and surface water pollution, or other large-scale factors such 

as sea-level fluctuations.

A ~75 km survey of Biscayne Bay 

(Figure 2) for surface water 222Rn 

activities and streaming resistivity 

profiling was conducted during June 

7-9, 2004. Simultaneous GPS positions, depth soundings, salinity, and 

temperature were obtained using a 

Lowrance echo sounder and an In Situ 

profiler, respectively. 

 

RADON-222

Radon-222, an inert gas produced by 

the decay of 226Ra in sediments, is 

typically present in groundwater at 

much greater activities than in surface waters. Its short half-life of 3.8 

Figure 3. Continuous water-column 222Rn (t

1/2

= 3.8 d) activities as a tracer for identifying 

sites of enhanced submarine groundwater discharge. Note elevated activities (~ 11 dpm L-1) 

at Cutler Ridge relative to mid-bay background 

activities (~ 2-3 dpm L-1). 

Figure 2. Operational configuration of the continuous 222Rn and streaming resistivity equipment 

deployed on a 25-ft-long pontoon boat. 

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Cutler Ridge

80o15'0"W

1.8

2.0

3.3

4.2

5.6

5.2

3.7

4.3

11.3

4.7

2.8

2.5

3.1

2.4

6.0

5.7

25o30'0"N

Background (1.8–3.1 dpm L

-1)

Radon Activities 

Intermediate (3.3–6.0 dpm L

-1)

Highest (>11 dpm L

-1)

0 2.5 5 10

Kilometers
the receiver injects current in the first 

two electrodes and then measures 

eight voltage potentials in the trailing 

electrode pairs. Streaming resistivity 

data were collected once every ~3 sec. 

Post-processing of the resistivity data 

involves several inverse modeling 

iterations. 

In addition, continuous surface 

salinity, pH, temperature, and depth 

soundings were recorded to support 

post-processing of the resistivity data. 

Interpretations of the streaming resistivity data confirm enhanced freshened subsurface water masses at sites 

of increased 222Rn activities (Figure 4). 

ELECTROMAGNETIC SEEPAGE 

METER DEPLOYMENTS

The USGS has been developing and 

utilizing electromagnetic (EM) seepage 

meters to study groundwater/surface 

exchange (Rosenberry and Morin, 

2004) and submarine groundwater discharge into coastal waters (Swarzenski 

et al., 2004). Such EM seepage meters 

were deployed at a site by Cutler 

Ridge in Biscayne Bay during March 

2004. Electromagnetic seepage-rate 

data collected at this site show distinct 

and continuous discharge of groundwater. The rate of exchange across the 

sediment/water interface ranged from 

10 to 50 cm day-1, with an average of 

23.2 cm day-1 (Figure 5). It appears that 

tidal forcing at least partially controls 

the pattern of submarine groundwater 

discharge. These data were collected 

during the south Florida dry season 

and therefore such seepage rates would 

most likely increase during periods of 

higher rainfall (July-November). The 

average seepage rate (23.2 cm day-1) 

observed in this study corresponds very 

closely to modeled fluxes of groundwater into the bay (Langevin, 2001, 2003). 

SUMMARY

Near-continuous excess 222Rn measurements in the surface waters of 

Biscayne Bay show some striking 

anomalies that suggest enhanced 

submarine groundwater discharge 

days and its conservative geochemical nature make this isotope ideal to 

study exchange processes across the 

sediment/water interface over daily 

to weekly timescales. Burnett et 

al. (2001) modified a commercially 

available continuous radon-in-air 

monitoring system to accurately measure excess 222Rn activities in coastal 

surface waters. To obtain near-continuous (~5 min. data updates) watercolumn 222Rn activities in Biscayne 

Bay, we utilized six RAD7 (Durridge, 

Co., Inc.) systems fed simultaneously from one air-water exchanger, 

where the 222Rn present in water is 

allowed to equilibrate with the Rn in 

air. By applying a temperature and 

solubility coefficient correction, one 

can calculate the activity of 222Rn in 

water, as the Rn in air will equilibrate 

with the seawater flowing through the 

exchanger.

Results from the near-continuous 

222Rn survey (Figure 3) show greatest activities (> 11 dpm L-1) at Cutler 

Ridge, a site where F. Kohout worked 

on freshwater/saltwater dynamics 

(Kohout, 1960). Such elevated 222Rn 

activities can easily be discerned from 

background Biscayne Bay surfacewater Rn activities (~2-3 dpm L-1). 

STREAMING RESISTIVITY 

PROFILING

A SuperSting Marine Logging System 

(AGI, Inc.,) was used for the streaming resistivity profiling measurements 

in Biscayne Bay. This system consists of a 50-m cable and an eightchannel resistivity receiver. The 

streaming resistivity cable contains 

two current electrodes and nine potential electrodes, and is towed across 

the waterʼs surface at a speed of ~3 

knots. Operating in continuous mode, 

Background activities

Background

activities

South North

Mowry Canal Black Point Cutler Ridge

0.0

3.7

7.3

11.0

14.7

Depth (meters) 222Rn (dpm/L)

Time

9:30 9:45 10:00 10:15 10:30 10:45 11:00 11:15 11:30 11:45 12:00 12:15 12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15

Seafloor

0

2

4

6

8

10

12

Streaming Resistivity Profile

Streaming Resistivity Profile

Continuous 222Radon Profile

222Rn (dpm/L)

Continuous 222Radon Profile

East West 0.0

3.3

6.5

9.8

13.1

Seafloor

0

1

2

3

4

5

15:18:30 15:33:30 15:48:30 16:03:30 16:18:30 16:33:30 16:48:30 17:03:30 17:18:30 17:33:30 17:48:30 18:03:30 18:18:30 18:33:30

Time

A A'

B B'

Shore–parallel trackline

Shore–perpendicular trackline

20

5.0

1.2

0.31

0.08

Resistivity

(ohm-m)

seawater brackish freshened

Figure 4. A comparison of two modeled streaming resistivity profiles (top) and simultaneous 

water-column 222Rn activities from transects A – A’ and B – B’ (see Figure 1) in Biscayne Bay. 

Darker hues in the streaming resistivity interpretation correspond to freshened subsurface water 

masses (i.e., at Cutler Ridge). 
at discrete sites within the bay. 

Interestingly, at Cutler Ridge – the 

well-known site of Kohoutʼs work 

on freshwater/saltwater dynamics 

– water-column 222Rn activities are 

highest and indicate the most active 

submarine groundwater discharge. 

This is also supported by the streaming resistivity profiling data, which 

indicate greater freshened water 

masses in this region. Such data 

confirm the utility of these two techniques in identifying sites of SGD 

and provide direct evidence in support of ongoing modeling efforts on 

freshwater/saltwater interface processes in Biscayne Bay. The electromagnetic seepage-meter data provide the 

first continuous record of exchange 

rates across the sediment/water interface at Cutler Ridge and similarly 

support recent modeling predictions. 

The use of trade, firm and brand 

names is for identification purposes 

only and does not constitute endorsement by the U.S. Government.

References

Belaval, M., Lane, J.W., Jr., Lesmes, D.P. and 

Kineke, G.C., 2003, Continuous-resistivity 

profiling for coastal groundwater investigations: three case studies. In, SAGEEP 

Proceedings, Texas, 14 p.

Bokuniewicz, H., 1980, Groundwater seepage 

into Great South Bay, New York. Estuarine, 

Coastal Marine Science, v. 10, p. 437-444.

Burnett, W.C., Kim, G. and Lane-Smith, D., 

2001, A continuous monitor for assessment 

of 222Rn in the coastal ocean. J. Radioanal. 

Nuc. Chem., 249 (1), 167-172.

Burnett, W.C., Bokuniewicz, H., Huettel, M., 

Moore, W.S. and Taniguchi, M., 2003, 

Groundwater and pore water inputs to the 

coastal zone. Biogeochemistry, 66, 3-33.

Giblin, A.E. and Gaines, A.G., 1990, Nitrogen 

inputs to a marine embayment: The importance of groundwater. Biogeochemistry, 10, 

309-328.

Kohout, F.A., 1960, Cyclic flow of saltwater 

in the Biscayne aquifer of southeastern 

Florida, Journal of Geophysical Research, 

65, 2133-2141.

Langevin, C.D., 2001, Simulation of groundwater discharge to Biscayne Bay, southeastern Florida. USGS Water Resources 

Investigation Report 00-4251, pp. 137.

Langevin, C.D. 2003, Simulation of submarine ground water discharge to a marine 

estuary: Biscayne Bay, Florida. Ground 

Water 41, no. 6: 758-771.

Laroche, J., Nuzzi, R., Waters, R., Wyman, 

K., Falkowski, P.G. and Wallace, D.W.R., 

1997, Brown tide blooms in Long Islandʼs 

coastal waters linked to inter-annual variability in groundwater flow. Global Change 

Biology, 3, 397-410.

Manheim, F.T., Krantz, D.E., Snyder, D.S. 

and Sturgis, B., 2002, Streamer resistivity surveys in Delmarva coastal bays. In, 

SAGEEP Proceedings, Las Vegas, NM, p. 

18.

Moore, W.S., 1996, Large groundwater inputs 

to coastal waters revealed by 226Ra enrichments. Nature, 380, 612-614.

Parker, G.G. et al., 1955, Water resources of 

southeastern Florida, with special reference to the geology and ground water of 

the Miami area. USGS Water Supply Paper 

1255, pp. 965.

Rosenberry D. O. and Morin, R.H., 2004, 

Use of an electromagnetic seepage meter 

to investigate temporal variability in lake 

seepage. Groundwater, 42, 68-77.

Swarzenski, P.W., Reich, C.D., Spechler, 

R.M., Kindinger, J.L. and Moore, W.S., 

2001, Using multiple geochemical tracers 

to characterize the hydrogeology of the 

submarine spring off Crescent Beach, 

Florida. Chemical Geology, 179, 187-202.

Swarzenski, P.W., Charette, M. and Langevin, C., 2004, An autonomous, electromagnetic seepage meter to study coastal 

groundwater/surface water exchange, U.S. 

Geological Survey, Open File Report 2004-

1369.

Valiela, I. and DʼElia, C., 1990, Groundwater 

inputs to coastal waters. Special Volume, 

Biogeochemistry, 10, 328.

Figure 5. Ten-minute time-averaged electromagnetic (EM) seepage-meter results from Cutler Ridge 

site (see Figure 1). The EM seepage meter was likely exposed to air during a low-tide event at ~8:00 

pm. 

-20

-10

0

10

20

30

40

50

60

0

10

20

30

40

50

60

Sun 28 12:00 Sun 28 20:00 Mon 29 4:00 Mon 29 12:00

Seepage Flux (20 to 30 m offshore)

Surface Water Level (i.e., tide)

Average Flux = 23.2 cm/d

Surface Water Level (cm)

Seepage Flux (cm/d)

Dome exposed at low tide

Cutler Ridge, Biscayne Bay

March 2004

For more information, 

please contact:

Peter Swarzenski

600 4th Street South

St. Petersburg, FL 33701

phone: 727-803-8747 x3072

fax: 727-803-2030

pswarzen@usgs.gov

Peter Swarzenski1, Bill Burnett2, Chris Reich1, 

Henrieta Dulaiova2 , Richard Peterson2 and 

Jeff Meunier3

1

USGS, St. Petersburg, FL

2

Florida State University, Tallahassee, FL

3

Express, Reston, VA