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U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
WATER-RESOURCES INVESTIGATIONS REPORT 96-4285

Delineation of Saltwater Intrusion in the Biscayne Aquifer, Eastern Dade County, Florida, 1995

By Roy S. Sonenshein

Introduction| Canal and Levee System| Hydrogeology of the Biscayne Aquifer
History of Saltwater Intrusion
Delineation of Saltwater Intrusion| Northeast| North-central| South-central| Southeast
Summary| References

INTRODUCTION

Saltwater intrusion is a major threat to the freshwater resources of the coastal areas in southeastern Florida. There are three primary mechanisms by which saltwater contaminates the freshwater reservoir in the unconfined, surficial aquifers of the region: (1) subsurface movement of seawater (lateral large-scale intrusion), (2) seepage of seawater from tidal canals and streams, and (3) upward movement of connate saltwater (water remaining from the time of deposition) from lower formations due to well-field withdrawals. All three mechanisms are driven by the lowering of freshwater levels in the surficial aquifers. However, the effects of the last two mechanisms (seepage and upward movement) generally are limited to relatively small regions, whereas lateral intrusion affects a much larger region of the aquifers.

Urbanization of the coastal area, construction of drainage canals, and development of municipal well fields have led to a lowering of water levels in the Biscayne aquifer, a designated sole-source unconfined aquifer, in Dade County. Increased chloride concentrations in water samples collected from monitoring and supply wells beginning in the early 1900's indicated the gradual inland movement of saltwater from the ocean into the freshwater part of the Biscayne aquifer in some areas along the eastern coast. Various methods to slow or reverse the inland movement of saltwater by raising water levels in the Biscayne aquifer have been implemented or are being considered. These methods include decreasing coastal withdrawals by the development of regional well fields farther inland, supplementing the water supply with water obtained from lower confined aquifers of poor water quality, increasing delivery of water to coastal canals from inland wetlands, and construction of additional surface-water control structures.

Water managers concerned with Dade County water supply need more information describing the inland extent and movement of saltwater in the Biscayne aquifer to determine which methods work best at controlling saltwater intrusion and the potential impacts of these methods on the ecosystem. The existing monitoring well network was not adequate to monitor the extent or movement of saltwater in the Biscayne aquifer. In some areas of Dade County, saltwater had reached or moved inland of the existing monitoring wells. There were no monitoring wells in other areas of Dade County. An improved network was needed to provide the information required by water-management officials to make decisions on changes in the operation of the well fields and surface-water controls in Dade County.

In October 1994, the U.S. Geological Survey, in cooperation with the Miami-Dade Water and Sewer Department and the Metro-Dade Department of Environmental Resources Management, began a 2-year study to: (1) determine the present location of the interface between freshwater and oceanic saltwater in the Biscayne aquifer along the eastern coast of Dade County, and (2) determine the historical and present rate of movement of the saltwater interface. This map report documents the movement of the saltwater interface at selected locations in the Biscayne aquifer in 1995 through the evaluation of previously published maps, chloride data, and geophysical data.

Canal and Levee System

A canal and levee system (fig. 1) is the primary means of controlling water levels in the Biscayne aquifer. Due to high hydraulic conductivity of the bottom and side materials of most of the canals and the high water-table altitude, canal stages generally reflect water levels in the Biscayne aquifer. Levees are used to impound water in the remaining areas of the Everglades in the western part of Dade County, which include Water-Conservation Areas 3A and 3B and Everglades National Park. Surface water from the water-conservation areas is moved by pumping and operation of control structures through a network of canals toward the coast to augment ground-water supplies during periods of low water. Similarly, this system is used to release excess water to the ocean during periods of high water.

Map of eastern Miami-Dade County, showing the locations of the saltwater interface in 1995 and 1984. Also shown are geophysical monitoring wells, chloride monitoring wells, and surface geophysics sites.

Figure 1.--Location of the saltwater interface in the Biscayne aquifer in 1995
and location of monitoring wells, major canals, levees, well fields, and surface geophysics sites
in Dade County.

(figure 1 is available in other sizes: full publication (4.8mb), large , and small)

Hydrogeology of the Biscayne Aquifer

The Biscayne aquifer in Dade County consists of a wedge-shaped sequence of sedimentary deposits of the upper, most permeable zones of the surficial aquifer system (fig. 2). The Biscayne aquifer, as defined by Fish and Stewart (1991, p. 11-12), is composed primarily of limestone and sand, with hydraulic conductivities commonly exceeding 10,000 ft/d (feet per day). The Biscayne aquifer is thickest along the eastern coast of Dade County. In southern and central Dade County, thicknesses along the coast range from 80 to 120 ft (feet). Near the coast in the north, the aquifer is between 160 and 200 ft thick Fish and Stewart (1991, fig 16) . Water in the Biscayne aquifer is unconfined. Water levels respond rapidly to stresses on the ground-water system, including drainage and recharge from canals, recharge from rainfall, evapotranspiration, and pumpage from supply wells.

West-to-east geologic cross-section of Miocene Age and later strata underlying eastern Miami-Dade county

Figure 2.--Geologic formations, aquifers, and confining units of the surficial aquifer system
in central Dade County. (from Fish and Stewart, 1991).

HISTORY OF SALTWATER INTRUSION IN DADE COUNTY

The problem of saltwater intrusion in Dade County has been addressed by the U.S. Geological Survey since 1932 through the collection and evaluation of water-level and salinity data. Beginning in 1904 to present time, the history of saltwater intrusion in central Dade County has been well documented including research done by Parker and others (1955) through 1950, Leach and others (1972) through 1969, and Klein and Waller (1985) through 1984.

In 1904 (prior to any human-induced drainage), The saltwater interface was estimated to be at or near the coast because of the very high water levels which occurred naturally in the Everglades. Freshwater was reported to seep from the Biscayne aquifer offshore into Biscayne Bay in sufficient quantities to be used as a supply of freshwater for ships. Beginning in 1909 with the extension of the Miami River and continuing through the 1930's, construction of drainage canals (with no control structures) and pumpage from coastal well fields resulted in the lowering of water levels in the Biscayne aquifer, thereby inducing the inland movement of saltwater into the aquifer. Additionally, seawater driven by tides flowed inland in the drainage canals, resulting in the seepage of saltwater into the Biscayne aquifer from the canals. By 1946, salinity-control structures had been installed in all primary canals as far seaward as possible. These controls prevented saltwater driven by tidal changes from moving upstream in the canals beyond the controls. The controls also served to backup freshwater which maintained higher water levels in the Biscayne aquifer near the coastline. These water levels are higher than those that occurred during the period of uncontrolled drainage. The inland migration of saltwater in northern Dade County slowed or reversed in some areas as a result of the effects of these controls on water levels.

In the early 1960's, the existing canal system in southern Dade County was expanded to provide for flood control. The canals were equipped with flow-regulation structures both near the coast and inland, allowing water levels to be stepped down from structure to structure to prevent excessive drainage. However, the design and operation of this system lowered freshwater levels in the Biscayne aquifer, especially near the coast, allowing for the inland movement of saltwater during the drought years of 1970 and 1971. In 1976, additional water was routed to southern Dade County, raising water levels along the coast and slowing or reversing the inland movement of the saltwater interface Klein and Waller (1985).

Since 1984, additional events have occurred which have affected water levels in the Biscayne aquifer and, hence, the movement of the saltwater interface. Among these events are the initial operation of the Northwest Well Field and a consequent reduction in pumping from the Hialeah-Miami Springs Well Field, expansion of the Southwest Well Field, and changes in the delivery schedule of water to southern Dade County and Everglades National Park (fig. 1). Future changes in water levels might occur as a result of changes in the management of the ecosystem of south Florida. These changes will be based on the results of studies being conducted as part of the U.S. Geological Survey South Florida Ecosystem Program (McPherson and others, 1995) and other studies.

DELINEATION OF SALTWATER INTRUSION IN THE BISCAYNE AQUIFER

The inland subsurface movement of oceanic seawater into the Biscayne aquifer is of primary concern in coastal Dade County. The saltwater, being more dense, moves inland below the freshwater, forming a wedge-shaped interface with freshwater in the Biscayne aquifer (fig. 3). This interface is dynamic, moving laterally in response to changes in water levels. Generally, there is a time lag in the movement of the interface in response to changes in water levels, with regional long-term changes having a greater effect than seasonal changes (Parker and others, 1955, p. 611).

Cross-section (vertical axis is depth in feet to 140 ft below sea level; horizontal axis is distance in feet from Biscayne Bay, decreasing west to east from 12,000 feet), showing the estimated lines of equal chloride concentration in the Biscayne Aquifer

Figure 3.--Cross section through the Silver Bluff area showing the zone of diffusion (from Kohout, 1960).

The boundary between the freshwater and saltwater is not a sharp interface, but a transition zone with a width controlled, in part, by the hydraulic conductivity of the aquifer. In the Biscayne aquifer, this zone is generally thin. Chloride concentrations greater than 100 mg/L (milligrams per liter) are generally considered to be evidence of contamination with seawater, which has a chloride concentration of about 19,000 mg/L.

The approximate location of the saltwater interface was defined through the evaluation of a previously published 1984 map of the interface Klein and Waller (1985), chloride data (both current and historical), and geophysical data. Chloride data were obtained from 24 wells in the existing U.S. Geological Survey monitoring well network (table 1). The depths of the wells were considered in the evaluation because some of the wells were not screened at the base of the Biscayne aquifer. Vertical chloride profiles were obtained during the drilling of 16 new wells, with water samples collected about every 10 ft. The wells were drilled and screened below the saltwater interface, if present, or at the base of the Biscayne aquifer, if no saltwater was encountered (table 2).


Table 1. -- Inventory data and chloride concentrations in 1995 for existing monitoring wells
[? indicates top of open interval is unknown]

Local well number

USGS site
identification
number

Latitude

Longitude

Open interval
(feet below
land surface)

Chloride concentration (milligrams per liter)

F-45

254943080121501

254943

0801215

?-85

200

F-279

255315080111501

255315

0801115

?-117

2,800

G-354

254828080161501

254828

0801615

88-90

40

G-432

254335080170501

254335

0801705

98-100

2,300

G-576

254849080154802

254849

0801548

91-97

40

G-894

255350080105801

255350

0801058

74-76

42

G-896

254107080165201

254107

0801652

60-74

760

G-901

254201080173001

254201

0801730

95-96

2,300

G-939

253652080183701

253652

0801837

57-60

2,100

G-1009B

254106080174601

254106

0801746

99-100

34

G-1179

252944080233401

252944

0802334

0-51

4,800

G-1180

252947080235301

252947

0802353

0-67

34

G-1264

252532080244301

252532

0802443

6-59

72

G-1351

254813080161501

254813

0801615

100-103

24

G-1604A

254020080183101

254020

0801831

91-92

26

G-3162

253202080232601

253132

0802325

82-92

1,100

G-3164

252519080261101

252519

0802611

75-85

150

G-3166

252506080282201

252506

0802822

65-75

180

G-3224

255222080123001

255222

0801230

93-95

42

G-3226

254923080120201

254923

0801202

111-111

510

G-3229

254457080160301

254457

0801603

?-85

600

G-3235A

252824080250601

252824

0802506

72-82

50

G-3344

252334080280101

252334

0802801

55-58

84

G-3348

252502080254301

252502

0802543

59-62

210



Table 2. -- Inventory data and depth to saltwater interface in 1995 for geophysical monitoring wells

[Fresh indicates saltwater interface not found at well; < indicates less than the value]


Local well
number

USGS site
identification
number

Latitude

Longitude

Open interval
(feet below
land surface)

Approximate
depth to saltwater
interface in 1995

(feet below land surface)

G-3313E

253831080180206

253831

0801802

32-114

95

G-3600

255626080093201

255626

0800932

195-200

120

G-3601

255358080114101

255358

0801141

185-190

180

G-3602

255116080120601

255116

0801206

155-160

70

G-3603

254908080125201

254908

0801252

155-160

Fresh

G-3604

254722080152201

254722

0801522

115-120

95

G-3605

254629080143101

254629

0801431

105-110

100

G-3606

254341080174001

254341

0801740

115-120

Fresh

G-3607

254156080172101

254156

0801721

115-120

Fresh

G-3608

254108080170601

254108

0801706

95-100

Fresh

G-3609

254005080171601

254005

0801716

80-85

70

G-3610

253819080183201

253819

0801832

105-110

Fresh

G-3611

253710080184701

253710

0801847

95-100

100

G-3612

253457080195501

253457

0801955

56-61

55

G-3613

253214080215401

253214

0802154

55-60

<40

G-3615

253024080231001

253024

0802310

75-80

65

G-3616

252812080244301

252812

0802443

90-95

Fresh


Borehole induction logs (McNeill and others, 1990) were used for 1 existing well and for the 16 new wells to locate the depth to the saltwater interface (table 2). Induction logs were obtained from these wells at two different times in 1995 and 1996, 6 to 10 months apart, to detect any movement of the saltwater interface. No differences were noted in the induction logs for any of the monitoring wells over that time period. Examples of induction logs for two wells (G-3603 and G-3604) are shown in figure 4. The location of the saltwater interface is very distinct in the logs, when present (fig. 4, well G-3604). The small distance for the change from low to high conductivity, generally less than 10 feet, is an indication of the relatively thin transition zone between freshwater and saltwater. Surface-geophysical measurements were obtained at 11 sites using techniques prescribed by McNeill (1980) to determine the depth to the saltwater interface in areas where monitoring well data were not available (fig. 1).

Example conductivity and resistivity plots from wells G-3603 and G-3604

Figure 4.--Borehole induction logs for wells G-3603 and G-3604.

Northeastern Dade County

The canal system is the major factor influencing ground-water levels and the movement of the saltwater interface north of Little River Canal in northeastern Dade County. Snake Creek, Biscayne, and Little River Canals (fig. 1) are used to drain the area, maintaining a very flat water table between the upper reaches of the Miami Canal in Dade County and the coast (Sonenshein and Koszalka, 1996). Prior to the installation of coastal control structures in the 1940's on the Snake Creek, Biscayne, and Little River Canals, tidal saltwater movement occurred inland in the canals as far as the Red Road Canal (fig. 1) during extreme dry periods (Parker and others, 1955, p. 632). Thus, there was seepage of saline water into the Biscayne aquifer throughout much of the area during this time period. Due to the installation of the control structures, seepage is no longer a concern in this area, but the location of the coastal saltwater interface is being monitored.

Chloride concentrations in water samples collected from existing monitoring wells in northeastern Dade County have indicated little, if any, inland movement of the saltwater interface since 1984. In fact, chloride concentrations decreased in the vicinity of the North Miami Eastside Well Field, after the well field was shut down between 1977 and 1982 (Sonenshein and Koszalka, 1996). Data collected from existing monitoring wells and three new monitoring wells (fig. 1 and table 2, wells G-3600, G-3601, and G-3602), and a map depicting the location of the saltwater interface in southeastern Broward County (Koszalka, 1995), were used to define the location of the saltwater interface in northeastern Dade County. The saltwater interface in northeastern Dade County is shown in approximately the same location as in 1984 (Klein and Waller, 1985), with differences due to more information being available and not to any perceived movement of the saltwater interface.

North-Central Dade County

The canal system and pumpage from the Hialeah-Miami Springs and Northwest Well Fields are major factors influencing ground-water levels and the movement of the saltwater interface in north-central Dade County, between the Miami and Little River Canals (fig. 1). Saltwater intrusion is considered a major threat to the Hialeah-Miami Springs Well Field, with evidence of contamination as early as 1939 (Klein and Ratzlaff, 1989). Two possible sources of saltwater to the well field are nearby tidal reaches of the Miami and Tamiami Canals and the coastal interface. Coastal control structures have been installed in these canals to prevent inland migration of tidal canal water. Pumpage from the Hialeah-Miami Springs Well Field was significantly reduced from 1984 to 1992 because of industrial contamination in the supply wells. Variations in chloride concentrations in water at monitoring wells near the Hialeah-Miami Springs Well Field indicate the possible effects of the installation of the control structures and the changing pumpage rates (Sonenshein and Koszalka, 1996).

The location of the saltwater interface is difficult to determine in north-central Dade County because of the lack of monitoring wells near the coastal saltwater interface and because of the presence of tidal canal water. Elevated chloride concentrations in the monitoring wells nearest to the Hialeah-Miami Springs Well Field appear to be due to seepage from tidal canals (Sonenshein and Koszalka, 1996). Surface-geophysical data at six locations and data from existing and four new monitoring wells (fig. 1 and table 2, wells G-3602 to G-3605) were used to define the location of the saltwater interface in north-central Dade County. The inland bulge of the saltwater interface (fig. 1) is due to drainage of ground water by the tidal reaches of the Miami Canal. The saltwater interface in this area is shown farther seaward than in 1984 (Klein and Waller, 1985), partly because more information is now available and partly as a result of the possible seaward migration of the saltwater interface due to the reduced pumpage from the Hialeah-Miami Springs Well Field. However, there is still uncertainty as to the exact location of the saltwater interface, as indicated by the dashed line in figure 1. Additional wells are needed in this area to better define the position of the saltwater interface and to monitor any possible movement toward the nearby Hialeah-Miami Springs Well Field.

South-Central Dade County

The canal system and pumpage from the four large well fields (Alexander Orr, Snapper Creek, Southwest, and West Well Fields) are major factors influencing ground-water levels and the movement of the saltwater interface in south-central Dade County (fig. 1). The Alexander Orr Well Field is the well field of primary concern in considering movement of the saltwater interface because it is located nearest to the coast. There was a decline in water levels in the center of the Alexander Orr Well Field, as a result of increased pumpage beginning in 1988 (Sonenshein and Koszalka, 1996).

The location of the saltwater interface is well defined in south-central Dade County, with data available from existing and six new monitoring wells (fig. 1 and table 2, wells G-3606 to G-3611). For comparison with the location of the saltwater interface in 1984 (Klein and Waller, 1985), the area is divided by the Snapper Creek Canal into two regions. North of the Snapper Creek Canal, the saltwater interface has moved inland (Sonenshein and Koszalka, 1996). The cause of this movement has been attributed to one of two possible causes, or a combination of the two: (1) a decline in water levels at the Alexander Orr Well Field, located north of the Snapper Creek Canal; and (2) a lowering of water levels in the Coral Gables Canal as a result of the reconstruction of the tidal control structure (fig. 1). Additional research is required to determine the exact reason for the movement of the saltwater interface.

In the area south of the Snapper Creek Canal, the saltwater interface was farther inland on the 1984 map (Klein and Waller, 1985) than on the 1995 map (fig. 1). The difference is due to more information being available and not to any perceived movement of the saltwater interface. Elevated chloride concentrations in one monitoring well, G-1604, located just west of well G-1604A (fig. 1), were previously attributed to saltwater intrusion. However, based on data from the new monitoring wells, it is now believed that the elevated chloride concentrations are from another unknown source.

Southeastern Dade County

The canal system is the major factor influencing ground-water levels and the movement of the saltwater interface in southeastern Dade County (fig. 1). The initial canal system, completed in 1967, was designed not only to prevent flooding but also to prevent excessive drainage, allowing for the inland movement of the saltwater interface. Subsequently, the primary changes to the system have been the addition of control structures and pump stations in response to changes in needs for water-level controls. There are numerous small municipal well fields in the area (fig. 1), but due to the low pumpages and high hydraulic conductivities of the Biscayne aquifer, they appear to have little effect on regional water levels.

The location of the saltwater interface is difficult to determine in southeastern Dade County. Surface-geophysical data at four locations and data from existing and four new monitoring wells (fig. 1 and table 1, wells G-3612, G-3613, G-3615, and G-3616) were used to define the location of the saltwater interface in this area. Preliminary results from a saltwater intrusion study in Everglades National Park (Fitterman, 1996) were used to determine the location of the saltwater interface at the southern end of the study area. The saltwater interface is shown in the same general location as in 1984 (Klein and Waller, 1985), except for the central part of this area between the Black Creek and Mowry Canals (fig. 1). This farther inland location in 1995 is due to more information being available and to movement of the saltwater interface detected between 1984 and 1990 (Sonenshein and Koszalka, 1996). In the southern half of the study area, elevated chloride concentrations in water samples collected from some of the monitoring wells might be due to seepage from tidal saltwater. Additional data are needed in these two areas of southeastern Dade County to better define the position of the saltwater interface.

SUMMARY AND CONCLUSIONS

This map report depicts the approximate location of the saltwater interface in the Biscayne aquifer in 1995. The saltwater interface was defined through the evaluation of previously published maps of the interface, chloride data (both current and historical), and surface and borehole geophysical data. Sixteen wells were drilled to determine the location of the saltwater interface where additional data were needed. The location of the saltwater interface in 1995, compared to the location in 1984, was found to be approximately the same throughout much of Dade County, with most differences attributed to more information being available and not to any perceived movement of the saltwater interface. The canal system is the major factor influencing the movement of the saltwater interface throughout the county. Pumpage from the large well fields in central Dade County also influence the movement of the interface.

In northeastern Dade County, the saltwater interface has moved seaward near the north Miami Eastside Well Field, which was shut down between 1977 and 1982, with no evidence of any additional movement elsewhere in the area. In north-central Dade County (in the vicinity of the Hialeah-Miami Springs Well Field), the saltwater interface has also moved seaward, possibly in response to decreased pumpage at the well field from 1984 to 1992. In south-central Dade County, the saltwater interface moved inland north of the Snapper Creek Canal due to either increased pumping from the Alexander Orr Well Field or to a lowering of water levels in the Coral Gables Canal, or a combination of the two. South of the Snapper Creek Canal, the saltwater interface is not as far inland as was previously believed. In southeastern Dade County, the saltwater interface moved inland between the Black Creek and Mowry Canals from 1984 to 1990. Additional data are needed, especially in north-central and southern Dade County, to better determine the location of the saltwater interface.

REFERENCES CITED

Fish, J.E., and Stewart, Mark, 1991, Hydrogeology of the surficial aquifer system, Dade County, Florida: U.S.Geological Survey Water-Resources Investigations Report 90-4108, 50 p.

Fitterman, D.V., 1996, Geophysical mapping of the freshwater/saltwater interface in Everglades National Park,Florida: U.S. Geological Survey Fact Sheet 173-96, 2 p.

Klein, Howard, and Waller, B.G., 1985, Synopsis of saltwater intrusion in Dade County, Florida, through 1984: U.S. Geological Survey Water-Resources Investigations Report 85-4104, 1 sheet.

Klein, Howard, and Ratzlaff, K.W., 1989, Changes in saltwater intrusion in the Biscayne aquifer, Hialeah-Miami Springs area, Dade County, Florida: U.S. Geological Survey Water-Resources Investigations Report 87-4249, 1 sheet.

Kohout, F.A., 1960, Cyclic flow of saltwater in the Biscayne aquifer of southeastern Florida: Journal of Geophysical Research, v. 65, no. 7, p. 2133-2141.

Koszalka, E.J., 1995, Delineation of saltwater intrusion in the Biscayne aquifer, eastern Broward County, Florida, 1990: U.S. Geological Survey Water-Resources Investigations Report 93-4164, 1 sheet.

Leach, S.D., Klein, Howard, and Hampton, E.R., 1972, Hydrologic effects of water control and management of southeastern Florida: Florida Bureau of Geology Report of Investigations no. 60, 115 p.

McNeill, J.D., 1980, Applications of transient electromagnetic techniques: Technical Note TN-7, Geonics Limited, Mississauga, Ontario, Canada, 17 p.

McNeill, J.D., Bosnar, M., and Snelgrove, F.B., 1990, Resolution of an electromagnetic borehole conductivity logger for geotechnical and ground water applications: Technical Note TN-25, Geonics Limited, Mississauga, Ontario, Canada, 28 p.

McPherson, B.F., Higer, A.L., Gerould, Sarah, and Kantrowitz, I.H., 1995, South Florida Ecosystem Program of the U.S. Geological Survey: U.S. Geological Survey Fact Sheet 134-95, 1 sheet.

Parker, G.G., Ferguson, G.E., Love, S.K., and others, 1955, Water resources of southeastern Florida with special reference to the geology and ground water of the Miami area: U.S. Geological Survey Water-Supply Paper 1255, 965 p.

Sonenshein, R.S. and Koszalka, E.J., 1996, Trends in water-table altitude (1984-93) and saltwater intrusion (1974-93) in the Biscayne aquifer, Dade County, Florida: U.S. Geological Survey Open-File Report 95-705, 2 sheets.


For More Information

Roy S. Sonenshein
U.S. Geological Survey
3110 SW 9th Avenue
Ft. Lauderdale, Florida 33315
954-377-5924
sunshine@usgs.gov


This report was prepared in cooperation with the
Miami-Dade Water and Sewer Department and the

Metro-Dade DERM logo

Metro-Dade Department of Environmental Resources Management