U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00-4099 

Analysis of Water-Quality Trends at Two

Discharge Stations − One within Big

Cypress National Preserve and One near

Biscayne Bay − Southern Florida, 1966-94

Prepared as part of the

U.S. Geological Survey Place-Based Studies Program

By A.C. Lietz

Tallahassee, Florida

2000
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS

The use of firm, trade, and brand names in this report is for identification purposes only and 

does not constitute endorsement by the U.S. Geological Survey.

For additional information

write to:

District Chief

U.S. Geological Survey

Suite 3015

227 N. Bronough Street

Tallahassee, FL 32301

Additional information about water resources in Florida is available on the

World Wide Web at http://fl.water.usgs.gov
Contents iii

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction ........................................................................................................................................................................... 2

Purpose and Scope....................................................................................................................................................... 2

Previous Studies........................................................................................................................................................... 3

Description of Study Area ..................................................................................................................................................... 3

Miami Canal System.................................................................................................................................................... 4

Tamiami Canal System ................................................................................................................................................ 7

Collection and Processing of Water Samples ........................................................................................................................ 9

Determining Trends in Water Quality ................................................................................................................................... 11

Seasonal Variation........................................................................................................................................................ 11

Streamflow Variation ................................................................................................................................................... 12

Criteria for Censored and Uncensored Data................................................................................................................ 13

Analysis of Water-quality Trends at the Miami and Tamiami Canal Stations ...................................................................... 13

Relation of Selected Water-Quality Constituents to Discharge................................................................................... 18

Major Inorganic Constituents and Physical Characteristics ........................................................................................ 19

pH and Dissolved Oxygen ........................................................................................................................................... 26

Suspended Sediment.................................................................................................................................................... 27

Nitrogen, Phosphorus, and Carbon Species................................................................................................................. 27

Trace Metals................................................................................................................................................................. 28

Bacteriological and Biological Characteristics............................................................................................................ 30

Summary................................................................................................................................................................................ 32

Selected References............................................................................................................................................................... 34

FIGURES

1. Map of southern Florida showing location of study sites, major canals, control structures, pumping 

stations, and land-use areas ................................................................................................................................... 4

2. Map showing physiographic provinces of southern Florida ................................................................................. 5

3. Graphs showing flow-duration curves for the Miami and Tamiami Canal Stations ............................................. 6

4. Photograph showing structure S-26 at the Miami Canal station ........................................................................... 7

5. Map showing Tamiami Canal Outlets ................................................................................................................... 8

6. Map showing southerly view of an outlet of the Tamiami Canal from Forty-Mile Bend to Monroe in 

the Big Cypress National Preserve ........................................................................................................................ 9

7-15. Graphs showing LOWESS lines of:

7. Concentration as a function of discharge for dissolved solids, suspended sediment, 

total nitrogen, and total phosphorus at the miami Canal station..................................................................... 20

8. Concentration as a function of discharge for dissolved solids, suspended sediment, 

total nitrogen, and total phosphorus at the Tamiami Canal station................................................................. 21

9. Selected major inorganic constituents and other characteristics as a function of time at the 

Miami Canal station........................................................................................................................................ 22

10. Selected major inorganic constituents and other characteristics as a function of time at the 

Tamiami Canal station .................................................................................................................................... 24

11. Total ammonia, total organic carbon, and total phosphorus as a function of time at the 

Miami Canal station........................................................................................................................................ 28

12. Total ammonia and total nitrite plus nitrate as a function of time at the Tamiami Canal station ................... 29

13. Barium and iron as a function of time at the Miami Canal station................................................................. 30

14. Barium and strontium as a function of the Tamiami Canal station ................................................................ 31

15. Fecal coliform and fecal streptococcus bacteria as a function of time at the Miami Canal station................ 32
iv Contents

TABLES

1. Period and frequency of data collection for water-quality constituents and groups at the Miami and 

Tamiami Canal stations........................................................................................................................................... 10

2. Criteria for selection of statistically significant trends based on uncensored data................................................. 13

3. Statistical summary and trends of selected water-quality constituents at the Miami Canal station ....................... 14

4. Statistical summary and trends of selected water-quality constituents at the Tamiami Canal station.................... 16

5. Number of statistically significant trends for selected constituents at the Miami and Tamiami Canal 

stations over time.................................................................................................................................................... 18

6. Summary of water-quality indicators showing improvement or deterioration at the Miami and 

Tamiami Canal stations over time .......................................................................................................................... 19

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F - 32)/1.8

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929

(NGVD of 1929)--A geodetic datum derived from a general adjustment of the first-order level

nets of both the United States and Canada, formerly called Sea Level Datum of 1929. 
Abstract 1

Analysis of Water-Quality Trends at Two Discharge 

Stations − One within Big Cypress National Preserve 

and One near Biscayne Bay − Southern Florida, 

1966-94

By A.C. Lietz 

Abstract

An analysis of water-quality trends was 

made at two U.S. Geological Survey daily discharge stations in southern Florida. The 

ESTREND computer program was the principal 

tool used for the determination of water-quality 

trends at the Miami Canal station west of Biscayne 

Bay in Miami and the Tamiami Canal station along 

U.S. Highway 41 in the Big Cypress National Preserve in Collier County. Variability in water quality caused by both seasonality and streamflow was 

compensated for by applying the nonparametric 

Seasonal Kendall trend test to unadjusted concentrations or flow-adjusted concentrations (residuals) determined from linear regression analysis.

Concentrations of selected major inorganic 

constituents and physical characteristics; pH and 

dissolved oxygen; suspended sediment; nitrogen, 

phosphorus, and carbon species; trace metals; and 

bacteriological and biological characteristics 

were determined at the Miami and Tamiami Canal 

stations. Median and maximum concentrations of 

selected constituents were compared to the Florida Class III freshwater standards for recreation, 

propagation, and maintenance of a healthy, wellbalanced population of fish and wildlife. The 

median concentrations of the water-quality constituents and characteristics generally were higher 

at the Miami Canal station than at the Tamiami 

Canal station. The maximum value for specific 

conductance at the Miami Canal station exceeded 

the State standard. The median and maximum 

concentrations for ammonia at the Miami and 

Tamiami Canal stations exceeded the State standard, whereas median dissolved-oxygen concentrations at both stations were below the State 

standard.

Trend results were indicative of either 

improvement or deterioration in water quality with 

time. Improvement in water quality at the Miami 

Canal station was reflected by downward trends in 

suspended sediment (1987-94), turbidity, (1970-

78), total ammonia (1971-94), total phosphorus 

(1987-94), barium (1978-94), iron (1969-94), and 

fecal coliform (1976-94). Deterioration in water 

quality at the same station was indicated by 

upward trends in specific conductance (1966-94), 

dissolved solids (1966-94, 1976-94, and 1987-94), 

chloride (1966-94), potassium (1966-94), magnesium (1966-94), sodium (1966-94), sulfate (1966-

94), silica (1966-94), suspended sediment (1974-

94), total organic carbon (1970-81), and fecal 

streptococcus (1987-94). The downward trend in 

pH (1966-94) was indicative of deterioration in 

water quality at the Miami Canal station.

Improvement in water quality at the Tamiami Canal station was reflected by downward 

trends in fluoride (1967-93), total ammonia (1970-

92), total nitrite plus nitrate (1975-85), and barium 

(1978-93). Deterioration in water quality at the 

same station was statistically significant by 
2 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

upward trends in specific conductance (1967-93), 

dissolved solids (1967-93), chloride (1967-93), 

sodium (1967-93), potassium (1967-93), magnesium (1967-93), strontium (1967-93), and suspended sediment (1976-93). The downward trend 

in dissolved oxygen (1970-93) was indicative of 

deterioration in water quality.

INTRODUCTION

Because of development and the disruption of 

natural drainage, the ecosystem of southern Florida has 

experienced a variety of environmental problems such 

as loss of soil, nutrient enrichment, contamination by 

pesticides, mercury buildup in the biota, fragmentation 

of landscape, loss of wetlands, widespread invasion by 

exotic species, increased algal blooms in coastal 

waters, seagrass die-off, and declines in fishing 

resources (McPherson and Halley, 1996, p. 2). The tremendous population growth from 1.3 million people in 

1960 to 3.3 million people in 1990 (South Florida 

Regional Planning Council, 1995, p. 11) and increased 

agricultural and urban water use in recent decades have 

increased the demand for water in southern Florida. 

This increased demand, together with changes in the 

hydrologic system, have caused a decrease in water 

supply and deterioration in water quality (McPherson 

and Halley, 1996, p. 2).

In southern Florida, water-quality trends may be 

directly influenced by urban and agricultural activities. 

Natural phenomena, such as physical and geochemical 

rock weathering, may result in increasing concentrations of some constituents over time. The implementation of best agricultural management practices or landuse changes in agricultural areas may result in concentrations of some chemical compounds decreasing over 

time. Additionally, the control of point and nonpoint 

source contamination in both urban and agricultural 

areas may also result in downward trends for specific 

constituents. Many substances found in atmospheric 

deposition, both wet and dry, can adversely affect water 

quality with time. Conversely, improvement in the control of toxic substances emitted in atmospheric discharges may result in temporal improvements in water 

quality.

A consensus was recently reached by governmental agencies and environmental groups for the need 

to restore the southern Florida and Everglades ecosystems to patterns similar to those of its predevelopment 

state. In 1996, the U.S. Geological Survey (USGS) 

conducted a study, as part of the South Florida PlaceBased Studies (PBS) Program, to analyze long-term 

water-quality trends in southern Florida areas critical to 

ecosystem protection and restoration. The USGS South 

Florida PBS Program was established for the purpose 

of providing physical and biological science data and 

information on which to base ecosystem restoration 

management decisions impacting the mainland of 

southern Florida, Florida Bay, and the Florida Keys and 

reef ecosystems.

Purpose and Scope

The purpose of this report is to describe longterm water-quality trends in selected ecosystem areas 

of southern Florida. Summary statistics and temporal 

trends of selected water-quality constituents were 

determined at two USGS daily discharge stations − one 

located within the Big Cypress National Preserve and 

the other located just west of Biscayne Bay along the 

southeastern coast of Florida. Comparisons between 

trends and median concentrations for selected constituents are also made based on the difference in land-use 

characteristics of the two stations. The period of record 

for constituents is 1966-94 (except for three nitrogen 

and phosphorus species, which were collected through 

1996), with most of the data having been collected 

under the auspices of the National Stream Quality 

Accounting Network (NASQAN) program. The objective of the NASQAN program is to determine the quality of the Nation’s streams on a temporal and spatial 

basis by routinely analyzing chemical constituents in 

the waters.

Descriptive statistics, including the minimum, 

maximum, mean, 25th, 50th (median), and 75th percentiles, are presented as well as temporal trends of 

major inorganic constituents and other related physical 

characteristics; nitrogen, phosphorus, and carbon 

species; trace metals; and bacteriological and biological characteristics. The ESTREND computer program 

was used to determine water-quality trends; it provides 

for the removal of extraneous variation in water-quality 

data caused by seasonality and discharge. The effects 

of seasonality and discharge on water quality and the 

criteria for selecting censored and uncensored data are 

addressed in this report. The specific statistical test 

used to compensate for seasonal variability in waterquality data is the Seasonal Kendall trend test. This 

nonparametric, distribution-free test compares relative 

ranks of data values rather than their specific 
Description of Study Area 3

magnitudes. The Seasonal Kendall Slope Estimator is 

the median slope of all pairwise comparisons from all 

seasons, and is presented in this report as changes per 

year, both in original units (depending on the constituent) as well as in percent. Locally Weighted Scatterplot 

Smoothing (LOWESS) plots are used to delineate local 

nonlinear variations in water-quality data over time 

where statistically significant trends exist.

Previous Studies

Previous studies involving water-quality trend 

analysis are numerous. Smith and others (1982) studied 

trends in total phosphorus measurements from 

NASQAN stations. Baldys and others (1995) determined summary statistics and trend analysis on waterquality data in New Mexico and Arizona. Sources of 

trends in water-quality data for streams in Texas were 

documented by Schertz and others (1994). Smith and 

others (1987) analyzed and interpreted water-quality 

trends in major U.S. rivers from 1974 to 1981. Trends 

in stream water-quality data in Arkansas between 1975 

and 1989 were determined by Peterson (1992). Trends 

in surface-water quality in Connecticut from 1968 to 

1988 were determined by Trench (1996), and associations between water-quality trends in New Jersey 

streams and basin characteristics from 1975 to 1986 

were determined by Robinson and others (1996). 

Miller and others (1999) documented water-quality 

trends for selected constituents in the southern Everglades and Big Cypress Swamp.

DESCRIPTION OF STUDY AREA

Southern Florida occupies an area of about 

10,000 mi2

 (square miles) and contains a complex 

hydrologic system of canals, control structures, pumping stations, and land-use areas (fig. 1). The eastern 

part of southern Florida, including Biscayne National 

Park and parts of Everglades National Park, represents 

a highly managed and controlled streamflow system 

that provides adequate flood protection during the wet 

season (May-October), water storage and groundwater replenishment during the dry season (NovemberApril), and retardation of saltwater intrusion. The western part of southern Florida is more representative of 

natural flow, especially through the Big Cypress 

National Preserve and adjacent parts of Everglades 

National Park (fig. 1). The Big Cypress National Preserve lies west of the water-conservation areas (fig. 1) 

and encompasses parts of Collier and Monroe Counties. Most of the preserve consists of swamps, ponds, 

and sloughs where flow moves slowly southward and 

southwestward through these broad strands and 

sloughs.

The climate of southern Florida ranges from 

tropical to subtropical marine in the west and east 

coasts to tropical maritime in the Florida Keys; the climate is characterized by long, warm summers with 

abundant rainfall and mild, dry winters. The average 

monthly rainfall during the wet season is 6.24 in. 

(inches) and 1.97 in. during the dry season, and the 

average annual rainfall ranges from about 40 to 65 in. 

(South Florida Regional Planning Council, 1995, 

p. 23). Rainfall comes mostly from local showers, 

which usually are of short duration and high intensity, 

and is more abundant along the east coast and in the 

Florida Keys than on the west coast.

The five distinct physiographic provinces of 

southern Florida (fig. 2) are the Atlantic Coastal Ridge, 

Sandy Flatlands, Everglades, Big Cypress Swamp, and 

the mangrove and coastal glades (Klein and others, 

1975, p. 2). The Atlantic Coastal Ridge is densely populated, with an average width of 5 mi and elevations 

that range from 8 to 24 ft (feet) above sea level (Klein 

and others, 1975, p. 7). The Sandy Flatlands lie 

between the Atlantic Coastal Ridge and the Everglades 

on the east coast (where elevations range from 6 to 20 ft 

above sea level) and north of the Big Cypress Swamp 

on the west coast (where elevations range from 5 to 

40 ft above sea level). The Everglades extends from 

Lake Okeechobee southward to the mangrove and 

coastal glades, with much of the area developed in 

Miami-Dade and Broward Counties. Vast regions of 

the Everglades are inundated seasonally during natural 

conditions, with elevations that range from 14 ft above 

sea level near Lake Okeechobee to sea level at Florida 

Bay. Big Cypress Swamp is characterized by flat, 

poorly drained marshes and tree islands, with elevations usually less than 15 ft above sea level. The mangroves and coastal glades consist of tidal streams, bays, 

lagoons, and small islands that lie along the gulf coast 

of Florida.

The two daily discharge stations selected for this 

study are:

Station 02288600 Miami Canal at N.W. 36th 

Street, Miami, Fla., and

Station 02288900 Tamiami Canal Outlets, 40-

Mile Bend to Monroe, Fla.
4 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

Station 02288600 is located in Miami upstream 

from salinity-control structure S-26 and about 5 mi 

upstream of Biscayne Bay. Station 02288900 is located 

on U.S. Highway 41 in the Big Cypress National Preserve in Collier County, 54 mi west of Miami. For identifications purposes only, station 02288600 will be 

referred to as the Miami Canal station in this report, and 

station 02288900 will be referred to as the Tamiami 

Canal station. The station locations are shown in 

figure 1.

Miami Canal System

The Miami Canal traverses 81 mi through agricultural, water-conservation, and urban areas from 

Lake Okeechobee southeastward to Biscayne Bay 

(fig. 1). Flow in Miami Canal is highly regulated with 

many pump stations, levees, and control structures situated throughout the canal reach. Flow at the northern 

end of the canal is commonly reversed (into Lake 

Okeechobee) as a result of pumpage from agricultural 

MONROE COUNTY

COLLIER COUNTY

COLLIER COUNTY

HENDRY COUNTY

PALM BEACH COUNTY

DADE CO

BROWARD COUNTY

Tamiami Canal BROWARD CO

Station

Miami 

Canal 

Station

(at S-26)

Miami

LAKE

25°00′

26°00′

27°00′

83°00′ 82°00′ 81°00′ 80°00′

0 25 50 MILES

0 25 50 KILOMETERSATLANTIC OCEAN

FLORIDA 

KEYS

3A

3B

2B

1

2A

Everglades Agricultural Area

Big Cypress National Preserve

Biscayne National Park

Fakahatchee Strand State Preserve

Everglades National Park

Water-conservation area and number

Indian Reservation Lands

Station location and name 

(this report only)

Canal or levee

Control structure or pumping 

station and number

3A

EXPLANATION

S-31

TAMIAMI CANAL

FLORIDAS TURNPIKE

Naples

L-28

L-67C

L-30

L-67A

Biscayne 

Bay

GULF OF MEXICO

S-3

S-354

S-8

S-339

S-340

S-344 S-151

S-343A

S-343B

S-333

S-31

S-32

S-32A

710

846

833

833

Florida Bay

OKEECHOBEE

1

1

1

98

27

27

41

27

95

75

Figure 1. Southern Florida showing location of study sites, major canals, control structures, pumping stations, 

and land-use areas.
Description of Study Area 5

lands into the canal after heavy rainfall and operation 

of pump station S-3 or because of gravity flow at structure S-354 when negative head exists (fig. 1).

Pump stations S-8 and S-151 and structures 

S-31, S-32, S-32A, S-339, and S-340 are located along 

Miami Canal near or within Water Conservations Areas 

3A and 3B. Pump station S-8, the northernmost station 

along the middle reach of the canal and just outside of 

the water-conservation areas, is used to discharge water 

(to the south) into Water Conservation Areas 3A and 

3B for storage. Structures S-339 and S-340, located 

within Water Conservation Area 3A, are used to prevent overdrainage of the northern part of the waterconservation area by: (1) forcing flows into the marsh 

when the three manually operated slide gates are 

closed, and (2) transferring water along Miami Canal to 

urban areas when the gates are opened. Pump station 

S-151 discharges water to Levee 67A (L-67A) to 

convey water southward to structure S-333 for discharge eastward through Tamiami Canal and eventually 

25°00¢

26°00¢

27°00¢

83°00¢ 82°00¢ 81°00¢ 80°00¢

0

0 25

25

50 KILOMETERS

50 MILES

FLORIDA

KEYS

ATLANTIC

COASTAL

RIDGE

GULF OF MEXICO

ATLANTIC OCEAN

MANGROVE AND

COASTAL GLADES

Biscayne

Bay

Florida Bay

Charlotte

Harbor

Lake

Okeechobee

SANDY

FLATLANDS

BIG CYPRESS

SWAMP

EVERGLADES

LAKE

WALES

RIDGE

Figure 2. Physiographic provinces of southern Florida (modified from Parker and others, 1955).
6 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

southward to Everglades National Park through culverts along the reach from Levee 30 (L-30) to L-67A 

(fig. 1). Water is discharged from L-30 through structures S-31, S-32, or S-32A to Miami Canal (fig. 1) for 

ground-water replenishment to the Biscayne aquifer 

(the sole source of potable water for southeastern Florida) during dry periods and for retardation of saltwater 

intrusion.

Upstream from Biscayne Bay along the lower 

reach of the Miami Canal is structure S-26 (fig. 1, 

Miami Canal station). For this study, water samples 

were collected upstream of this site at the bridge at 

N.W. 36th Street and N.W. 42nd Avenue. Structure 

S-26 has two vertical lift gates, which when closed, 

prevent saltwater intrusion during dry periods, and 

when opened, discharge excess water during heavy 

rainfall. The reach from structure S-26 to Biscayne Bay 

is commonly referred to as the Miami River. Based on 

a 38-year period of record, summary statistics indicate 

a mean discharge of 251 ft3

/s (cubic feet per second), a 

maximum daily mean discharge of 1,570 ft3

/s, and a 

minimum daily mean discharge of -279 ft3

/s (Price and 

others, 1998). A flow-duration curve for the Miami 

Canal station is shown in figure 3. A photograph showing structure S-26 on the Miami Canal is shown in 

figure 4.

1,600

1,500

1,400

1,300

1,200

1,100

1,000

900

800

700

600

500

400

300

200

100

0

6,500

6,000

5,500

5,000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80 90 100

PERCENTAGE OF DAYS DISCHARGE

WAS EQUALED OR EXCEEDED

PERCENTAGE OF DAYS DISCHARGE

WAS EQUALED OR EXCEEDED

DISCHARGE, IN CUBIC

FEET PER SECOND

DISCHARGE, IN CUBIC

FEET PER SECOND

Miami Canal Station

Tamiami Canal Station

Figure 3. Flow-duration curves for the Miami and Tamiami Canal 

stations. Valid only for positive discharges.
Description of Study Area 7

Tamiami Canal System

The Tamiami Canal traverses in an east-west 

direction from Miami along the southeastern coast to 

Naples along the southwestern coast (fig. 1). East of the 

Big Cypress National Preserve, flow is highly regulated along the reaches of the Tamiami Canal. The 

direction of flow is to the south through 19 outlets from 

Monroe to Carnestown, through 29 outlets and structure S-14 from Forty-Mile Bend to Monroe, through 

4 control structures (S-12A, B, C, D) from L-67A to 

Forty-Mile Bend, and through 19 outlets and structure 

S-12E from L-30 to L-67A (fig. 5). East of Monroe, the 

Tamiami Canal crosses the highest elevation in its 

reach (about 8 ft above sea level). Although flow 

diminishes during the dry season, ground-water seepage occurs until the water table recedes beneath the 

streambed. The canal water eventually evaporates and 

stagnant pools of nonflowing water also develop during the dry season.

The section of the Tamiami Canal that is of principal interest in this study is Forty-Mile Bend to Monroe, which consists of 29 bridges over a distance of 

about 15 mi in an east-west direction. For this study, the 

entire 15-mi reach was considered to be one site (fig. 1, 

Tamiami Canal station). Discharge at the station is 

computed from a stage/discharge rating based on a single stage recorder at bridge 105. Two control structures, S-343A and S-343B (figs. 1 and 5), may 

influence flow in the easternmost part of this section. 

Generally, the gates of both structures are only operated during high stages in Water Conservation Areas 

3A and 3B. Another nearby structure (S-344 along 

L-28) also may influence flow in the Tamiami Canal 

from Forty-Mile Bend to Monroe. The gates at structure S-344 are rarely opened. Based on a 35-year period 

of record, summary statistics for Tamiami Canal from 

Forty-Mile Bend to Monroe (fig. 1, Tamiami Canal station) indicate a mean discharge of 363 ft3

/s, a maximum daily mean discharge of 6,110 ft3/s, and a 

minimum daily mean discharge of 0.0 ft3

/s (Price and 

others, 1998). A flow-duration curve for the Tamiami 

Canal station is shown in figure 3. A southerly view of 

the Tamiami Canal outlet from Forty-Mile Bend to 

Monroe in the Big Cypress National Preserve is shown 

in figure 6.

Figure 4. Structure S-26 at the Miami Canal station.
8 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

STATION 02289040

EVERGLADES NATIONAL PARK

CONTROL

STRUCTURES

12

B C A D

STATION 02289060

S-12 E

41 40-MILE BEND

41

29

41

94

STATION 02288800 MONROE TO CARNESTOWN

CARNESTOWN

MONROE

STATION 02288800

41

MONROE

40-MIL

S-343A

S-343A

S-343B

S-343B

E BEND

94

STATION 02288900

STATION 02288900 40-MILE BEND TO MONROE

S-1

S-14

4

STATION 02288800

MONROE

94

EVERGLADES NATIONAL PARK

STATION 02288900 STATION

02289040

STATION

02289060

41

41

CARNESTOWN

CONSERVATION AREAS

3 A 3 B

40-MILE BEND

CONTROLS

EXPLANATION

DIRECTION OF FLOW

CANALS

LEVEES

STATION 02289040 LEVEE 67A TO 40-MILE BEND STATION 02289060 LEVEE 30 TO LEVEE 67A

FLOW IS TO SOUTH THROUGH

FLOW IS TO SOUTH THROUGH

FLOW IS TO SOUTH THROUGH

4 CONTROL STRUCTURES 19 OUTLETS AND S-12 E

29 OUTLETS AND S-14

LEVEE 28

LEVEE 67 A

LEVEE 67 A

LEVEE 30

LEVEE 28

ATLANTIC OCEAN

TAMIAMI CANAL

LEVEE 28

LEVEE 67 A

LEVEE 30

MIAMI CANAL MIAMI

29

FLOW IS TO SOUTH THROUGH

19 OUTLETS

GULF OF MEXICO

TAMIAMI CANAL

TAMIAMI CANAL

Figure 5. Tamiami Canal Outlets (from Price and others, 1998, p. 203).
Collection and Processing of Water Samples 9

COLLECTION AND PROCESSING OF 

WATER SAMPLES

Representative water samples were collected 

from the Miami and Tamiami Canal stations. Sample 

processing and preservation techniques were such that 

chemical and biological contamination and degradation were minimized. The methodology employed for 

the study was similar to that used throughout the 

Nation for water-quality sampling under the NASQAN 

program; minor exceptions were made to accommodate the unique hydrology of southern Florida. Prior to 

the NASQAN program, sample collection and processing procedures were not well documented. The use of 

nonstandard procedures for analysis of certain constituents and changes in laboratory analytical procedures 

during the sampling program may have resulted in variation in water-quality results over time. The waterquality constituents and groups sampled at the Miami 

and Tamiami Canal stations and the period of frequency of data collection are presented in table 1.

Depth-integrated samples were collected under 

the NASQAN program by the equal-width-increment 

method. This method divides the stream cross section 

into equally spaced intervals, with sampling verticals 

established at the middle of each interval. The first 

sampling vertical is located at half the distance of the 

first interval from the bank, with subsequent verticals 

located equidistant from the first vertical at the midsection of each of the remaining intervals. During every 

sampling event, water samples were collected from at 

least three verticals in the cross section. Because of the 

low velocities, less than 2 ft/s (feet per second) that 

usually predominate in the canals of southern Florida, 

the weighted-bottle method was used to collect water 

samples. With this method, a 1-L (liter) open-mouth 

polyethylene bottle is placed in an epoxy-coated, 

weighted basket sampler that is raised and lowered 

through each vertical at a constant transit rate. Thus, 

the bottle is not overfilled when returned to the surface. 

During the study, the same transit rate was maintained 

throughout each of the sampling verticals. Because 

flow moves through 29 outlets at the Tamiami Canal 

station and concentration or volume may vary spatially 

depending on the season and the amount of streamflow 

present, equal-width-increment sampling was conducted at no less than three bridges in which flow 

existed in the canal reach for each sampling event and 

composited as one water sample.

Figure 6. Southerly view of an outlet of the Tamiami Canal from Forty-Mile Bend to Monroe in the Big 

Cypress National Preserve.
10 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

 

The equal-width-increment method was used to 

collect water samples from the Miami Canal station. 

The water samples were collected from at least three 

verticals in the cross section per sampling event and 

composited in a churn splitter for processing and determination of major inorganic constituents and other 

related physical characteristics, nitrogen and phosphorus species, and trace metals. Water samples for total 

organic carbon and bacteriological characteristics were 

collected as grab samples from midstream using a sterilized glass bottle. Before sampling, the churn splitter 

was washed with a nonphosphate detergent, tap water, 

and deionized water; the basket sampler was rinsed 

with native water. When used for trace metal compositing, the churn splitter was soaked with a 5-percent 

hydrochloric acid solution and rinsed with tap water 

and deionized water.

The churn splitter was used to withdraw water 

samples at a churning rate of about 9 in/s (inches per 

second). This process was carefully executed to ensure 

that the churn disc did not break the surface, resulting 

in aeration of the water sample and subsequent changes 

in water quality. Water samples were initially withdrawn for total (or total recoverable) and suspended 

determinations. Water samples for all of the dissolved 

constituents were filtered using 0.45-micron filters. 

Caution was taken with water samples for filtered constituents; water samples were not withdrawn when the 

level in the churn splitter was less than 1.5 in. above the 

spigot intake.

Various methods were used for the preservation 

of nutrients, trace metals, and bacteriological and biological characteristics during sampling. Water samples 

for nutrient analysis were collected in amber polyethylene bottles and chilled to 4.0 °C (degrees Celsius). For 

a period of time, mercuric chloride was used for sample 

preservation. To prevent cross contamination, the nutrient samples were placed in a separate ice chest that was 

shipped immediately for laboratory analysis. For trace 

metals, water samples were collected in acid-rinsed 

bottles and treated with nitric acid to prevent sample 

degradation. Water samples for fecal coliform and fecal 

streptococcus were chilled to 4.0 °C and analyzed at the 

Miami Subdistrict laboratory within 4 to 6 hours of 

sample collection. The field and analytical methods 

that were used to collect and analyze the water samples 

are described in publications by Guy and Norman 

(1970), Goerlitz and Brown (1972), Greeson and others 

(1977), Wershaw and others (1987), and Fishman and 

Friedman (1989).

Table 1. Period and frequency of data collection for water-quality constituents and groups at the Miami and Tamiami Canal 

stations

[Individual major inorganic constituents and other related physical characteristics, except where noted below, are given in tables 3 and 4]

Water-quality

constituent or groups

Miami Canal station Tamiami Canal station

Period

of data

collection

Frequency

Period

of data

collection

Frequency

Major inorganic constituents and 

physical characteristics1,2

1966-74

1975-80

1981-94

Intermittently

Monthly

Quarterly

1967-75

1976-80

1981-93

Intermittently

Monthly

Quarterly

Specific conductance 1966-80

1981-94

Monthly

Quarterly

1967-75

1976-80

1981-93

Intermittently

Monthly

Quarterly

pH and dissolved oxygen

1966-74

1975-80

1981-94

Intermittently

Monthly

Quarterly

1967-75

1976-80

1981-93

Intermittently

Monthly

Quarterly

Suspended sediment 1974-80

1981-94

Monthly

Quarterly

1975-80

1981-93

Monthly

Quarterly

Nitrogen and phosphorus species

1970-74

1975-80

1981-94

1995-96

Intermittently

Monthly

Quarterly

Intermittently

1968-75

1976-80

1981-93

Intermittently

Monthly

Quarterly

Total organic carbon 1970-74

1975-81

Intermittently

Quarterly

1970-75

1976-80

1981-85

Intermittently

Monthly

Quarterly

Trace metals

1969-74

1975-80

1981-94

Intermittently

Monthly

Quarterly

1967-75

1976-80

1981-93

Intermittently

Monthly

Quarterly

Fecal coliform and fecal streptococcus 1976-80

1981-94

Monthly

Quarterly

1978-80

1981-93

Monthly

Quarterly

Phytoplankton 1974-81 Quarterly 1975-81 Quarterly

1

Includes all of the individual constituents given in tables 3 and 4, except for specific conductance.

2

Chloride also sampled from 1966 to 1973.
Determining Trends in Water Quality 11

DETERMINING TRENDS IN 

WATER QUALITY

A trend in water quality is defined as a monotonic change in a particular constituent with time. As 

previously mentioned, the ESTREND program 

(Schertz and others, 1991) was used in this study to 

determine trends of water-quality constituents for the 

Miami and Tamiami Canal stations. This program 

employs parametric and nonparametric tests that are 

designed to deal with characteristics unique to waterquality data.

Water-quality data possess unique characteristics 

that may require specialized approaches to statistical 

testing. Data sets have a base limit of zero due to positive values only and can contain censored (less than) 

values, outliers, multiple detection limits, missing values, and serial correlation. These characteristics commonly present problems in the use of conventional 

parametric statistics based on normally distributed data 

sets. The presence of censored data, non-negative 

values, and outliers may lead to an asymmetric or nonnormal distribution instead of a normal, symmetric, or 

bell-shaped (Gaussian) distribution, which is common 

for many data sets. These skewed data sets may require 

use of specific nonparametric statistical procedures for 

their analysis. The use of nonparametric statistical procedures also is preferred when determining trends of 

many constituents at multiple stations. Additionally, 

nonparametric statistical tests are more powerful when 

applied to non-normally distributed data, and almost as 

powerful (under certain conditions) as parametric tests 

when applied to normally distributed data (Helsel and 

Hirsch, 1992, p. 102).

Two major causes of variation in water-quality 

data are the effects of seasonality and discharge. Both 

need to be compensated for in order to discern the specific anthropogenic or natural processes that affect 

water quality over time. The effects of seasonality and 

streamflow on water quality, as well as methods of 

dealing with both censored and non-censored data are 

addressed in the subsequent sections.

Seasonal Variation

Water-quality data may exhibit seasonal variation (Schertz and others, 1991, p. 12). This variation 

may be the result of a variety of conditions, including 

specific agricultural land-use practices, biological 

activity, or sources of streamflow or sediment. As an 

example, precipitation-induced discharge may predominate during specific months of the year, whereas 

base flow (due to ground-water seepage) may be dominant at other times of the year. Another example is the 

increase in biological activity that occurs during summer months because of warmer temperatures as 

opposed to decreased activity during winter months, 

which might cause seasonal variation in nutrient concentrations. 

The statistical approach that is used to compensate for seasonal variability in water-quality data is the 

distribution-free, nonparametric Seasonal Kendall 

trend test. This test, modified from the Mann-Kendall 

test (Helsel and Hirsch, 1992, p. 338), compares relative ranks of data values from the same season. For 

example, January values are compared to January values, February values are compared to February values, 

and so forth. No comparisons are made across seasonal 

boundaries. Plus values are recorded if the subsequent 

value in time is higher, and a minus is recorded if the 

subsequent value in time is lower. If pluses predominate, a positive trend exists and if minuses predominate 

a negative trend results. No trend is the result of pluses 

and minuses being equal. The null hypothesis is that the 

concentration of the water-quality constituent is independent of time (Smith and others, 1982, p. 5). The test 

assumes that the data are independent and from the 

same statistical distribution. The Seasonal Kendall test 

statistic is the summation of the Mann-Kendall test 

results from all the seasons. The attained significance 

level (or p-value) is the probability of incorrectly 

rejecting the null hypothesis of no trend when actually 

there is a trend. The Seasonal Kendall slope estimator 

is computed according to the method of Sen (1968); it 

is the median slope of all the pairwise comparisons 

from all of the seasons expressed as rate of change per 

year in original units (usually in milligrams per liter 

depending on the constituent) and in percent per year.

Because all data may not have been collected at 

the same frequency for the duration of a project or 

monitoring program, specific seasonal definitions are 

needed to prevent bias in the trend results. For example, data collected under the NASQAN program were 

collected monthly for the first few years of the program 

and then quarterly thereafter. Use of a monthly seasonal definition would unduly bias the trend results in 

favor of the beginning years of the record. The 

ESTREND program provides a mechanism for determining the appropriate seasonal definitions for specific 

constituents at each station. The program allows the 
12 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

user to divide the record into 2, 3, 4, 6 or 12 seasons. 

The specific length and dates of each season are specified by the user. An automated procedure in the 

ESTREND program determines the degree to which 

the sampling frequency of a record supports userdefined seasons. This is determined by dividing the 

record into beginning, middle, and ending periods. The 

beginning and ending periods each consist of one-fifth 

of the record, whereas the middle period consists of 

three-fifths of the record. Seasonal pairwise comparisons are made according to the seasonal definitions 

with the beginning and ending periods of the record 

and also with the middle period of the record. For each 

seasonal definition, the ratio of observed pairwise comparisons to the maximum possible number of pairwise 

comparisons is computed (Schertz and others, 1991, 

p. 17) for the beginning and ending, as well as the middle periods of the record for each season. Generally, 

seasonal selection is based on the lowest sampling frequency for the beginning and ending periods of the 

record.

Streamflow Variation

Removing unwanted variation in water-quality 

data caused by extraneous variables is a necessary step 

in understanding the anthropogenic processes that may 

have affected water quality over time. These extraneous variables often represent natural processes such as 

rainfall, temperature, or streamflow (Helsel and Hirsch, 1992, p. 330). Water-quality constituents may be 

affected by streamflow in various ways. Major inorganic constituents are usually inversely related to 

streamflow due to the effects of dilution as streamflow 

increases with time. Nutrients tend to be directly 

related to discharge through the process of washoff, 

which results in increases in concentrations of nitrogen 

and phosphorus species due to runoff from agricultural 

and urban areas and from streambank erosion. Some 

constituents may be affected by both dilution 

and washoff.

The ESTREND program is used to determine 

water-quality trends by removing flow-related variation based on the development of a time series of flowadjusted concentrations (residuals) from concentration/discharge regression models. The Seasonal Kendall trend test is then applied to the flow-adjusted 

residuals. One of the two methods (for using concentration/discharge regression models to reduce flowrelated variability) utilizes linear models to regress 

concentration with various functional forms of flow 

that include linear, inverse, hyperbolic and log transformations (Schertz and others, 1991, p. 22). The mathematical expression for this method is:

, (1)

where

C is the estimated concentration;

a is the intercept;

b is the slope; and

Q is instantaneous discharge or mean 

daily discharge.

f(Q) represents functional forms of flow, such as: 

Q linear, ln (Q) log (natural), 1/(1 + BQ) hyperbolic where B is one of eight possible coefficients 

based on range of discharge, and 1/Q inverse.

The above method is used for most conservative 

water-quality constituents. Selection of the best model 

is based upon the lowest Prediction Error Sums of 

Squares (PRESS) statistic. However, for nonconservative constituents (suspended sediment, total organic 

carbon, nitrogen and phosphorus species, bacteria, turbidity, and phytoplankton), a log-log multiple linear 

regression model is preferred (Smith and others, 1987). 

The mathematical expression is:

exponential. (2)

The second method of flow adjustment is nonlinear and involves fitting a concentration/discharge or 

a log concentration/log discharge relation by generation of a locally weighted scatterplot smoothing line, 

known as LOWESS (Cleveland, 1979). This method 

uses weighted least squares regressions by weighting 

observations both as to distance from the fitted line and 

magnitude of residuals; the method is applicable for 

many of the nonconservative constituents mentioned 

above.

Reduction of flow-related variability in waterquality data requires that discharge be unaffected by 

human influence. If this is not possible, the effect must 

be constant over the period of record (Helsel and 

Hirsch, 1992, p. 332). Because discharge at the Miami 

Canal station is highly influenced by anthropogenic 

activities and water-management and land-use 

C a bf Q = + ( )

lnC a b1 Q b2( ) lnQ 2 = + ln +
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 13

practices (and because this influence may not have 

remained constant over the period of record resulting in 

changes in the probability distribution of the discharge 

data), only trends on unadjusted concentrations are 

reported for the Miami Canal station (shown later).

Criteria for Censored and Uncensored Data

Specific guidelines are established by the 

ESTREND program for the determination of waterquality trends using the Seasonal Kendall trend test on 

both censored and uncensored data. If less than 5 percent of the record is censored, all censored data values 

are recoded to half the reporting limit, provided that the 

required specific criteria are met. These criteria are as 

follows: (1) at least 5 years of record must exist; (2) the 

minimum number of observations in the record must be 

at least three times the designated number of seasons 

and greater than or equal to 10; and (3) a minimum percentage (recommended 40 percent) of the total number 

of observations must be present at the beginning and 

ending periods of the record. Criteria for the selection 

of statistically significant trends for uncensored data 

are listed in table 2. The use of this test incorporates 

flow adjustment on residuals from concentration/discharge regressions and slope estimation.

The Seasonal Kendall trend test can also be 

applied when more than 5 percent of the record is censored at a single reporting limit or for multiple-censored data when few values are above the minimum 

detection limit. The previously mentioned criteria also 

apply; the exception, at least one observation annually 

must be present at the beginning and ending fifths of 

the record. In this case, all detected and nondetected 

data values below the reporting limit are considered 

tied. Therefore, the trend is determined only on data 

that exceed the reporting limit. Use of the Seasonal 

Kendall trend test on censored data does not result in 

removal of flow-related variability, and trend slopes 

may not be reliable and were not reported due to the 

amount of censored data. For data censored at more 

than one reporting limit, a parametric censored regression test called Tobit also may be used (Schertz and 

others, 1991, p. 31). Criteria for use of this test is the 

same as for the Seasonal Kendall trend test on censored 

data, except a minimum percentage (recommended 

20 percent) of the total observations in the record must 

be detected values. This test uses a maximum likelihood estimation method based on a linear model and 

does not incorporate flow adjustment.

ANALYSIS OF WATER-QUALITY TRENDS 

AT THE MIAMI AND TAMIAMI CANAL 

STATIONS

Statistical summaries and spatial and temporal 

trends of selected major inorganic constituents and 

physical characteristics; pH and dissolved oxygen; suspended sediment; nitrogen, phosphorus, and carbon 

species; trace metals; and bacteriological and biological characteristics were determined for the Miami and 

Tamiami Canal stations. Statistical summaries presented in tables 3 and 4 include the number of observations, mean, minimum, maximum, median (50th 

percentile), and interquartile range (25th and 75th percentiles). A log probability regression of concentration 

values on normal quartiles is used to predict the magnitude of values for constituents with censored data values. The mean, median, and 25th and 75th percentiles 

are then estimated from predicted values in place of 

censored values (Schertz and others, 1991, p. 31). 

Median concentrations of selected constituents also are 

compared to the Florida Department of Environmental 

Protection Class III freshwater standards for recreation, 

propagation, and maintenance of a healthy, well 

balanced population of fish and wildlife (Florida 

Department of Environmental Protection, 1993).

Table 2. Criteria for selection of statistically significant 

trends based on uncensored data

[p-value less than 0.10 is statistically significant]

Best trend Criteria for best trend

Unadjusted

concentration (U)

No statistically significant correlation of 

concentration with discharge, or statistically significant correlation of concentration with discharge and statistically 

significant correlation of flow-adjusted 

concentration with discharge, or statistically significant correlation of concentration with discharge and no statistically 

significant flow-adjustment model

Flow-adjusted 

concentration (F)

Statistically significant correlation of 

concentration with discharge and no statistically significant correlation of flowadjusted concentration with discharge 

and a statistically significant flowadjustment model
14 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

Table 3. Statistical summary and trends of selected water-quality constituents at the Miami Canal station

[Only trends in unadjusted concentrations reported. N, total number of observations; n, number of observations used for trend determination; mg/L, milligrams per liter; Pt-Co units, platinum-cobalt units; NTU, nephelometric turbidity units; μS/cm, microsiemens per centimeter; t/acre-ft, tons per acre feet; 

°C, degrees Celsius; μg/L, micrograms per liter; mL, milliliter]

Constituent Period of 

record N Mean Minimum

Percentile

Maximum n

Rate of increase 

or decrease

pvalue1

25 50 75

Units 

per 

year

Percent 

per 

year

Major Inorganic Constituents and Physical Characteristics

Calcium, in mg/L 05-66 to 09-94 158 76.3 45.0 73.0 77.0 81.0 95.0 55 -0.07 -0.09 0.47

Chloride, in mg/L 05-66 to 09-94 249 72.5 12.0 58.0 70.0 82.0 290.0 141 .89 1.23 .00

Fluoride, in mg/L 05-66 to 09-94 156 .3 .0 .2 .3 .3 .5 54 .00 .00 .12

Magnesium, in mg/L 05-66 to 09-94 132 10.4 2.1 8.6 9.9 11.0 26.0 93 .13 1.20 .00

Potassium, in mg/L 05-66 to 09-94 157 2.1 .4 1.6 1.9 2.6 5.2 54 .07 3.35 .00

Silica, in mg/L 05-66 to 09-94 168 6.8 1.0 5.7 6.9 7.8 16.0 54 .06 .82 .03

Sodium, in mg/L 05-66 to 09-94 156 45.5 7.2 38.0 44.5 54.0 86.0 115 .70 1.54 .00

Sulfate, in mg/L 05-66 to 09-94 157 7.7 .0 3.4 5.8 9.8 43.0 54 .37 4.74 .00

Color, in Pt-Co units 05-66 to 05-78 52 53.4 30.0 41.3 50.0 60.0 80.0 19 1.25 2.34 .17

Hardness, as CaCO3, in mg/L 05-66 to 09-94 157 234.0 160.0 220.0 230.0 245.0 300.0 54 .00 .00 .27

Turbidity, in NTU 05-70 to 04-78 91 1.6 .1 .5 1.0 2.0 11.0 72 -.04 -2.37 2.06

Specific conductance, in μS/cm 05-66 to 09-94 263 649.0 455.0 580.0 640.0 700.0 1,350.0 139 5.70 .88 .00

Dissolved solids, in mg/L

05-66 to 09-94 156 392.0 213.0 369.0 396.0 419.0 520.0 54 1.62 .41 .01

1976-94 121 398.0 213.0 379.0 398.0 420.0 520.0 89 1.50 .38 .01

1987-94 33 391.0 213.0 382.0 416.0 427.0 462.0 25 3.25 .83 2.09

Dissolved solids, in t/acre-ft

05-66 to 09-94 155 .5 .3 .5 .5 .6 .7 54 .00 .45 .01

12-74 to 09-94 101 219.0 .0 91.8 215.0 315.0 1,020.0 70 -3.70 -1.69 .22

pH, Dissolved Oxygen, and Suspended Sediment

pH, in standard units 05-66 to 08-94 166 7.6 6.7 7.4 7.6 7.8 8.6 54 -.01 -.17 .05

Dissolved oxygen, in percent 

saturation 05-68 to 08-94 59 48.6 .0 18.0 47.0 75.0 119.0 50 -.16 -.33 .80

Dissolved oxygen, in mg/L

05-68 to 08-94 147 3.7 .0 1.6 3.0 5.9 10.4 88 .04 1.10 .15

1987-94 33 3.8 .4 1.5 3.3 6.0 8.4 25 -.13 -3.44 .38

Suspended sediment, in mg/L

12-74 to 09-94 107 6.7 .0 1.0 3.0 6.0 92.0 81 .24 3.56 2.09

1976-94 97 7.0 .0 2.0 3.0 6.0 92.0 76 .10 1.42 .25

1987-94 33 8.6 .0 2.0 3.0 8.5 48.0 25 -1.25 -14.44 2.09
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 15

Nitrogen, Phosphorus, and Carbon Species3

Total ammonia, in mg/L 12-71 to 09-94 153 0.3 <0.01 0.1 0.2 0.5 1.2 83 -0.02 -5.63 .01

Total nitrite, in mg/L 12-71 to 12-92 94

 

.03 <.01 .01 .02 .03 .2 24 .00 .00 .48

Total nitrate, mg/L 12-71 to 12-92 93

 

.1 .01 .1 .2 .2 .4 23 .00 -.27 .76

Total nitrite plus nitrate, in mg/L 12-74 to 12-92 90

 

.2 .01 .1 .2 .2 .5 51 .00 .00 .96

Total organic nitrogen, in mg/L 08-70 to 12-96 164

 

1.2 .2 1.0 1.2 1.4 2.6 70 .00 .00 .41

Total ammonia plus organic 

nitrogen, in mg/L 11-74 to 11-90 141

 

1.5 .2 1.2 1.5 1.8 2.7 75 .00 .00 .68

Total organic carbon, in mg/L 08-70 to 10-81 62

 

21.5 4.0 15.8 20.0 24.0 87.0 49 .61 2.83 .03

Total nitrogen, in mg/L

06-73 to 10-96 127

 

1.7 .5 1.4 1.7 2.0 3.8 54 .01 .54 .36

1976-94 104

 

1.7 .5 1.4 1.7 2.0 3.8 72 .00 -.13 .58

1987-94 24

 

1.7 1.0 1.4 1.5 2.0 2.6 16 .08 4.51 .13

Total phosphorus, in mg/L

12-71 to 12-96 162 .02 <.01 .01 .02 .03 .3 85 .00 .00 .95

1976-94 122 .02 <.01 .01 .02 .03 .3 88 .00 .00 .42

1987-94 32 .02 <.01 .01 .02 .03 .1 24 .00 -12.94 2.09

Trace Metals

Barium, in μg/L 04-78 to 09-94 64 36.1 5.0 29.0 31.0 45.8 66.0 58 -1.39 -3.86 .00

Iron, in μg/L 12-69 to 09-94 93 112.0 .0 41.0 100.0 175.0 300.0 70 -2.71 -2.43 2.06

Bacteriological and Biological Characteristics

Fecal coliform, in colonies per 

100 mL

12-76 to 09-94 82 1,241.0 2.0 130.0 230.0 470.0 43,000.0 71 -47.70 -3.84 2

.07

1987-94 28 415.0 5.0 102.0 150.0 290.0 5,700.0 23 26.22 6.32 .32

Fecal streptococcus, in colonies 

per 100 mL 1987-94 28 188.0 5.0 38.5 125.0 198.0 1,000.0 23 86.19 45.86 2

.08

Phytoplankton, in cells per mL 12-74 to 10-81 36 6,503.0 14.0 400.0 840.0 2,650 160,000 36 163.00 2.50 .72

1Attained significance level. Value less than 0.10 is statistically significant as shown in red.

2Not statistically significant at an alpha level of 0.05.

3Total organic nitrogen, total nitrogen, and total phosphorus at the Miami Canal station were the only constituents sampled through 1996 in 

this study.

Table 3. Statistical summary and trends of selected water-quality constituents at the Miami Canal station (Continued)

[Only trends in unadjusted concentrations reported. N, total number of observations; n, number of observations used for trend determination; mg/L, milligrams per liter; Pt-Co units, platinum-cobalt units; NTU, nephelometric turbidity units; μS/cm, microsiemens per centimeter; t/acre-ft, tons per acre feet; 

°C, degrees Celsius; μg/L, micrograms per liter; mL, milliliter]

Constituent Period of 

record N Mean Minimum

Percentile

Maximum n

Rate of increase 

or decrease

pvalue1

25 50 75

Units 

per 

year

Percent 

per 

year
16 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

Table 4. Statistical summary and trends of selected water-quality constituents at the Tamiami Canal station

[N, total number of observations; n, number of observations used for trend determination. Best trend: F, flow-adjusted concentrations; U, unadjusted concentrations. Other abbreviations: mg/L, milligrams per liter; mL, milliliter; Pt-Co units, platinum-cobalt units; NTU, nephelometric turbidity units; μS/cm, 

microsiemens per centimeter; t/d, tons per day; t/acre-ft, tons per acre feet; °C, degrees Celsius; CD, censored data; TS, trend slopes not reliable due to 

censored data]

Constituent Period of 

record N Mean Minimum

Percentile

Maximum n

Rate of increase or 

decrease

pvalue1

Best 

trend

25 50 75 Units per 

year

Percent 

per year

Major Inorganic Constituents and Physical Characteristics

Calcium, in mg/L 05-67 to 09-93 113 58.7 34.0 49.0 58.0 66.0 110.0 42 0.32 0.54 0.18 F

Chloride, in mg/L 05-67 to 09-93 113 20.1 7.8 13.0 17.0 24.0 74.0 42 .21 1.07 .04 F

Fluoride, in mg/L 05-67 to 09-93 113 .1 -- .1 .1 .2 .7 113 TS TS .08 U

Magnesium, in mg/L 05-67 to 09-93 112 2.8 .8 1.9 2.5 3.2 12.0 42 .04 1.48 .00 F

Potassium, in mg/L 05-67 to 09-93 112 1.1 .0 .5 .6 .8 18.0 42 .02 1.34 .00 F

Silica, in mg/L 05-68 to 09-93 112 3.3 .2 1.5 2.8 4.4 14.0 42 .07 2.15 .19 F

Sodium, in mg/L 05-67 to 09-93 111 13.6 5.2 8.7 12.0 16.0 63.0 42 .20 1.45 .01 F

Sulfate, in mg/L 05-67 to 09-93 113 2.3 -- .3 1.0 3.3 17.0 113 .00 .00 .63 U

Color, in Pt-Co units 05-67 to 11-85 55 30.8 10.0 20.0 30.0 40.0 100.0 19 .25 .80 .79 F

Hardness, as CaCO3,

in mg/L

05-67 to 09-93 113 159.0 90.0 130.0 150.0 180.0 300.0 42 1.00 .63 .23 F

Turbidity, in NTU 03-70 to 04-78 83 1.1 .0 .5 1.0 1.2 4.5 47 -.02 -2.11 .18 U

Specific conductance,

in μs/cm 05-67 to 09-93 114 365.0 215.0 302.0 352.0 400.0 821.0 41 4.44 1.22 .00 F

Dissolved solids,

in t/d 05-69 to 09-93 93 170.0 .0 30.7 99.1 268 1,030.0 52 .08 .05 .79 F

Dissolved solids,

in t/acre-ft 05-67 to 09-93 101 .3 .2 .2 .3 .3 .7 40 .00 .58 .17 F

Dissolved solids,

in mg/L

05-67 to 09-93 113 223.0 113.0 176.0 212.0 255.0 540.0 42 1.73 .78 .03 F

1976-93 99 218.0 113.0 177.0 211.0 248.0 487.0 55 2.08 .95 .17 F

1987-93 29 220.0 129.0 168.0 210.0 242.0 420.0 27 -3.48 -1.58 .22 F

pH, Dissolved Oxygen, and Suspended Sediment

pH, in standard units 05-67 to 09-93 115 7.5 6.7 7.3 7.5 7.7 8.8 46 -.01 -.15 .15 U

Dissolved oxygen,

percent saturation 05-68 to 09-93 43 35.0 10.0 24.0 30.0 46.0 80.0 28 -.08 -.23 .73 U

Dissolved oxygen,

in mg/L

05-67 to 09-93 105 3.1 .5 2.0 2.7 4.0 14.0 41 -.08 -2.59 .02 U

1976-93 97 2.9 .5 2.0 2.7 3.6 6.3 48 -.02 -.63 .41 U

1987-93 29 2.7 1.1 2.0 2.4 2.9 6.2 27 -.09 -3.50 .16 F

Suspended sediment,

in mg/L

11-75 to 09-93 80 5.9 1.0 2.0 4.0 6.0 44.0 52 .00 .00 .46 U

1976-93 81 5.9 1.0 2.0 4.0 6.0 44.0 44 .24 4.08 .07 U

1987-93 29 5.4 1.0 2.0 4.0 7.0 23.0 28 .00 .00 1.00 U
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 17

Nitrogen, Phosphorus, and Carbon Species

Total ammonia, in mg/L 03-70 to 12-92 112 0.2 <0.01 0.0 0.1 0.1 14.0 65 -0.01 -4.46 0.02 U

Total nitrite, in mg/L2 03-70 to 09-92 83 .01 CD .1 .01 .01 .1 83 .00 .00 .44 U

Total nitrite + nitrate,

in mg/L 05-75 to 11-85 73 .04 CD .1 .02 .04 .3 73 .00 -5.68 .08 U

Total organic nitrogen, in 

mg/L 03-70 to 09-92 123 1.2 .3 .7 .9 1.1 9.9 42 -.01 -.89 .35 U

Total ammonia plus 

organic nitrogen, in mg/L

11-75 to 08-93 114 1.2 .2 .8 .9 1.1 10.0 57 .01 .44 .51 F

Total organic carbon,

in mg/L 05-70 to 11-85 65 16.5 2.0 10.0 13.0 17.0 160 25 -.46 -2.78 .24 U

Total nitrogen, in mg/L

10-73 to 09-93 81 1.8 .4 .9 1.0 1.2 24.0 29 -.02 -1.17 .54 U

1976-93 78 1.2 .4 .8 1.0 1.2 8.5 25 .02 1.34 .36 F

Total phosphorus, 

in mg/L

05-68 to 09-93 128 .1 <.01 .01 .02 .05 1.5 42 .00 1.36 .36 F

1976-93 115 .1 <.01 .01 .02 .05 1.5 57 .00 -.06 .98 F

1987-93 29 .0 <.01 .01 .02 .05 .2 28 .00 .00 .27 U

Trace Metals

Barium, in μg/L 03-78 to 09-93 58 23.4 6.0 13.0 19.5 31.0 53.0 48 -2.00 -8.55 .00 U

Copper, in μg/L 03-70 to 11-90 58 3.6 CD 1.5 3.0 5.0 11.0 58 TS TS .33 U

Iron, in μg/L 05-67 to 09-93 73 57.2 5.0 30.0 50.0 77.0 200.0 42 .00 .00 .90 U

Manganese, in μg/L 05-70 to 05-91 60 11.0 CD 3.0 6.0 9.5 190.0 60 -.11 -.98 .62 U

Nickel, in μg/L2 10-77 to 09-93 52 2.3 CD .7 1.5 2.8 23.0 52 TS TS .80 U

Strontium, in μg/L 05-67 to 05-93 79 26.0 58.0 180.0 230.0 310.0 830.0 33 2.82 1.08 .09 U

Zinc, in μg/L 03-70 to 11-89 54 15.1 CD 6.0 10.0 18.5 60.0 54 TS TS .40 U

Bacteriological and Biological Characteristics

Fecal coliform, in colonies per 100 mL

01-78 to 07-93 65 620.0 2.0 30.8 60.0 125.0 28,000 39 16.54 2.67 .30 U

1987-93 25 80.6 6.0 43.5 60.0 105.0 300.0 17 -4.99 -6.19 .67 U

Fecal streptococcus, in 

colonies per 100 mL

01-78 to 07-93 54 200.0 2.0 47.5 100.0 222.0 1,300 38 3.75 1.88 .76 U

1987-93 25 80.6 6.0 43.5 60.0 105.0 300.0 17 -4.99 -6.19 .67 U

Phytoplankton, in cells 

per mL 11-75 to 11-81 40 8,832 65.0 842.5 1,800 3,050 110,000 17 -1.227 -13.89 .22 U

1Attained significance level. Value less than 0.10 is statistically significant as shown in red.

2At the 25th percentile, constituent is estimated from a log-probability regression procedure.

Table 4. Statistical summary and trends of selected water-quality constituents at the Tamiami Canal station (Continued)

[N, total number of observations; n, number of observations used for trend determination. Best trend: F, flow-adjusted concentrations; U, unadjusted concentrations. Other abbreviations: mg/L, milligrams per liter; mL, milliliter; Pt-Co units, platinum-cobalt units; NTU, nephelometric turbidity units; μS/cm, 

microsiemens per centimeter; t/d, tons per day; t/acre-ft, tons per acre feet; °C, degrees Celsius; CD, censored data; TS, trend slopes not reliable due to 

censored data]

Constituent Period of 

record N Mean Minimum

Percentile

Maximum n

Rate of increase or 

decrease

pvalue1

Best 

trend

25 50 75 Units per 

year

Percent 

per year
18 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

Water-quality trends were based on the seasonal 

definition and the resultant flow/concentration pairs or 

concentration values selected within that seasonal definition. The trend slope is presented as change in original units per year and as percent per year for each 

constituent. The trend slope in percent per year is 

expressed as a percentage of the mean concentration by 

dividing the slope by the mean and then multiplying by 

100 (Schertz and others, 1991, p. 19). In tables 3 and 4, 

statistically significant values (marked in red) are represented by p-values less than 0.10. The p-value is the 

attained significance level or probability of rejecting 

the null hypothesis of no trend when there actually is a 

trend. ESTREND allows for the use of high p-values 

(0.10 or higher) when removing flow-related variability (Schertz and others, 1991, p. 24). However, even 

though no flow adjustment was performed for trends at 

the Miami Canal station due to the human influence on 

discharge, a p-value of 0.10 also was used for consistency. Those trends that would not be statistically significant at a p-value of 0.05 are mentioned in the text 

and noted in tables 3 and 4. The best trends also are represented as to whether they are based on unadjusted or 

flow-adjusted concentrations (table 2).

Statistically significant and nonsignificant trends 

are reported at the Miami and Tamiami Canal stations 

for each constituent during its entire period of record. 

For most constituents, the time period is 1966-94 for 

the Miami Canal station or 1967-93 for the Tamiami 

Canal station (tables 3 and 4). However, because trends 

are dependent on the period of record for which 

they originally were determined, trends of selected 

water-quality constituents (dissolved solids, dissolved 

oxygen, suspended sediment, total nitrogen, total phosphorus, fecal coliform, and fecal streptococcus) are 

presented for two or three time periods each at the 

Miami and Tamiami Canal stations. The additional data 

sets for the above constituents help provide further 

understanding of trends at these stations during different time periods (tables 3 and 4). Because of changes in 

program emphasis over time, some constituents span 

shorter time periods; whereas others, such as the major 

inorganic constituents and nutrients, span longer time 

periods. This accounts for the fact that some trends are 

for shorter periods than others. Additionally, for constituents spanning the entire period of record, trends are 

determined for various time periods, but always compared to water-quality data existing at the end of the 

program.

Water-quality trend analysis is defined as estimating the change in water quality over time (Schertz 

and others, 1994, p. 1). The trends presented here for a 

particular constituent over a specific time period represent the overall trend that has occurred during that 

period. However, trend slopes present the rate of 

change in original units per year and as a percentage of 

the mean, but give no indication of how a particular 

trend has occurred. Trends may have occurred as gradual changes over time, abrupt changes, step changes, or 

reversals over the period of record. Therefore, those 

constituents where a statistically significant (p-value 

less than 0.10) trend has occurred, plots of LOWESS 

lines have been generated to illustrate the nonlinear 

variations in the data for the specific period of record. 

Table 5 provides the number of statistically significant 

trends for selected constituents at the Miami and Tamiami Canal stations. Table 6 summarizes temporal 

trends as indicators of improvement or deterioration in 

water quality over time.

Relation of Selected Water-Quality 

Constituents to Discharge

Plots were generated to depict the LOWESS 

lines of concentration as a function of discharge for dissolved solids, suspended sediment, total nitrogen, and 

total phosphorus at the Miami and Tamiami Canal stations. LOWESS minimizes the influence of outliers on 

the smoothed line. A smoothness factor (F) of 0.5 was 

used for the plots shown in figures 7 and 8. The 

smoothness factor is the fraction of observations used 

in fitting the data to the line, and usually ranges from 

0.3 to 0.7 (Schertz and others, 1991, p. 24).

The concentration of dissolved solids as a function of discharge at the Miami Canal station (fig. 7) 

shows no substantial changes with increasing discharge, indicating there is no dilution effect or increase 

in concentration due to washoff. The dissolved-solids 

Table 5. Number of statistically significant trends for 

selected constituents at the Miami and Tamiami Canal 

stations over time 

[MC, Miami Canal station; TC, Tamiami Canal station]

Station Time period Number of 

upward trends

Number of 

downward 

trends

MC 1966-94 13 8

TC 1966-93 8 5
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 19

Table 6. Summary of water-quality indicators showing 

improvement or deterioration at the Miami and Tamiami Canal 

stations over time

[Based on trends determined at a p-value of 0.10]

Water-quality 

constituent

Time

period Trend Effect on water quality

Miami Canal Station

Chloride 1966-94 Upward Deterioration

Magnesium 1966-94 Upward Deterioration

Potassium 1966-94 Upward Deterioration

Silica 1966-94 Upward Deterioration

Sodium 1966-94 Upward Deterioration

Sulfate 1966-94 Upward Deterioration

Turbidity 1970-78 Downward Improvement

Specific conductance 1966-94 Upward Deterioration

Dissolved Solids

1966-94

1976-94

1987-94

Upward

Upward

Upward

Deterioration

Deterioration

Deterioration

pH 1966-94 Downward Deterioration

Suspended sediment 1974-94

1987-94

Upward

Downward

Deterioration

Improvement

Total ammonia 1971-94 Downward Improvement

Total organic carbon 1970-81 Upward Deterioration

Total phosphorus 1987-94 Downward Improvement

Barium 1978-94 Downward Improvement

Iron 1969-94 Downward Improvement

Fecal coliform 1976-94 Downward Improvement

Fecal streptococcus 1987-94 Upward Deterioration

Tamiami Canal Station

Chloride 1967-93 Upward Deterioration

Fluoride 1967-93 Downward Improvement

Magnesium 1967-93 Upward Deterioration

Potassium 1967-93 Upward Deterioration

Sodium 1967-93 Upward Deterioration

Specific conductance 1967-93 Upward Deterioration

Dissolved solids 1967-93 Upward Deterioration

Dissolved oxygen 1967-93 Downward Deterioration

Suspended sediment 1976-93 Upward Deterioration

Total ammonia 1970-92 Downward Improvement

Total nitrite plus 

nitrate 1975-85 Downward Improvement

Barium 1978-93 Downward Improvement

Strontium 1967-93 Upward Deterioration

concentration plot for the Tamiami Canal station (fig. 8) 

illustrates the effect of constituent dilution with increasing discharge. The concentration of suspended sediment 

as a function of discharge at the Miami Canal station 

(fig. 7) indicates a slight increase with increasing discharge. A similar suspended-sediment concentration 

plot for the Tamiami Canal (fig. 8) shows slight dilution 

followed by an increase in sediment with increased discharge. An initial decrease in suspended sediment concentration with increasing discharge, followed by an 

increase in constituent concentration with increasing 

discharge, might reflect initial dilution of a point source 

followed by washoff of nonpoint source contaminants. 

The concentrations of total nitrogen and total phosphorus as a function of discharge at the Miami Canal station 

indicate the effects of dilution and washoff (fig. 7). The 

total nitrogen and total phosphorus plots (fig. 8) for the 

Tamiami Canal station show only initial dilution at 

lower discharges.

Major Inorganic Constituents and Physical 

Characteristics

Major inorganic constituents and physical characteristics were analyzed to determine water-quality 

trends at the Miami and Tamiami Canal stations during 

the period of record (1966-94) or other selected time 

periods. Upward trends of some major dissolved ions 

(chloride, magnesium, potassium, and sodium), specific 

conductance, and dissolved solids were detected at both 

stations; upward trends of silica and sulfate also were 

detected at the Miami Canal station. Downward trends 

of turbidity and fluoride were detected at the Miami and 

Tamiami Canal stations, respectively. Plots were generated to show the LOWESS lines of selected major inorganic constituents and other characteristics as a function 

of time at the Miami and Tamiami Canal stations (figs. 9 

and 10).

Major inorganic constituents, as discussed in this 

report, include those cations (positively charged ions) 

and anions (negatively charged ions) that constitute the 

bulk of the dissolved solids and that commonly occur in 

concentrations exceeding 1.0 mg/L (milligram per liter). 

The major dissolved cations generally are calcium, 

magnesium, sodium, and potassium; the major anions 

are sulfate, chloride, fluoride, and those constituents 

contributing to alkalinity, which primarily are carbonate 

and bicarbonate (Hem, 1985). These charged species in 

solution contribute to the water’s specific conductance, 

defined as the ability to conduct an electric current. Both 

dissolved solids and specific conductance are dependent 

on the degree of mineralization of the water and generally are indicators of inorganic water quality. Silicon, 

which is nonionic, contributes to the dissolved solids 

and is usually reported as silica.
20 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

800

600

400

200

0

100

80

60

40

20

0

0.25

0.20

0.15

0.10

0.05

0

4

3

2

1

0

0 250 500 750 1,000 0 250 500 750 1,000

0 500 1,000 1,500 0 500 1,000 1,500

DISSOLVED SOLIDS, IN

MILLIGRAMS PER LITER

SUSPENDED SEDIMENT, IN

MILLIGRAMS PER LITER

TOTAL PHOSPHORUS, IN

MILLIGRAMS PER LITER

TOTAL NITROGEN, IN

MILLIGRAMS PER LITER

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

Figure 7. LOWESS lines of concentration as a function of discharge for dissolved solids, suspended sediment, total 

nitrogen, and total phosphorus at the Miami Canal station. Smoothness factor is 0.5.
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 21

600

500

400

300

200

100

50

40

30

20

10

0

1.5

1.0

0.5

0

25

20

15

10

5

0

0 1,000 2,000 3,000

0 1,000 2,000 3,000

0 1,000 2,000 3,000

0 1,000 2,000 3,000

DISSOLVED SOLIDS, IN

MILLIGRAMS PER LITER

SUSPENDED SEDIMENT, IN

MILLIGRAMS PER LITER

TOTAL PHOSPHORUS, IN

MILLIGRAMS PER LITER

TOTAL NITROGEN, IN

MILLIGRAMS PER LITER

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

MEAN DAILY DISCHARGE,

IN CUBIC FEET PER SECOND

Figure 8. LOWESS lines of concentration as a function of discharge for dissolved solids, suspended sediment, total 

nitrogen, and total phosphorus at the Tamiami Canal station. Smoothness factor is 0.5.
22 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

6

5

4

3

2

1

0

50

40

30

20

10

0

100

80

60

40

20

0

20

15

10

5

0

9.0

8.5

8.0

7.5

7.0

6.5

800

700

600

500

400

300

200

1960 1970 1980 1990 2000

POTASSIUM, IN

MILLIGRAMS PER LITER

SULFATE, IN MILLIGRAMS

PER LITER

SUSPENDED SEDIMENT, IN

MILLIGRAMS PER LITER

SILICA, IN MILLIGRAMS

PER LITER pH, IN STANDARD UNITS

DISSOLVED SOLIDS, IN

MILLIGRAMS PER LITER

YEAR

YEAR

YEAR

YEAR

YEAR

YEAR

Bias

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000 1960 1970 1980 1990 2000

1960 1970 1980 1990 2000 1960 1970 1980 1990 2000

Figure 9. LOWESS lines of selected major inorganic constituents and other characteristics as a function of time at 

the Miami Canal station. Smoothness factor is 0.5.
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 23

100

80

60

40

20

0

400

300

200

100

0

1,500

1,250

1,000

750

500

250

0

30

20

10

0

30

25

20

15

10

5

0

1.2

1.0

0.8

0.6

0.4

0.2

SODIUM, IN MILLIGRAMS

PER LITER

CHLORIDE, IN

MILLIGRAMS

PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS

PER CENTIMETER

MAGNESIUM, IN

MILLIGRAMS

PER LITER

TURBIDITY, IN

NEPHELOMETRIC

TURBIDITY UNITS

DISSOLVED SOLIDS, IN

TONS PER ACRE FEET

YEAR YEAR

YEAR YEAR

YEAR YEAR

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000

Figure 9. LOWESS lines of selected major inorganic constituents and other characteristics as a function of time at the 

Miami Canal station (continued). Smoothness factor is 0.5.
24 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94 150 125 100755025

0

40302010

0

10

86420

0.80.60.40.2

0

120100

80604020

0

600500400300200100

25201510

50

1,000

800600400200

5040302010

0

1960 1970 1980 1990 2000

CHLORIDE, IN MILLIGRAMS

PER LITER

POTASSIUM, IN

MILLIGRAMS PER LITER

DISSOLVED OXYGEN, IN

MILLIGRAMS PER LITER

FLUORIDE, IN

MILLIGRAMSPER LITER

SODIUM, IN MILLIGRAMS

PER LITER

DISSOLVED SOLIDS, IN

MILLIGRAMS PER LITER

MAGNESIUM, IN

MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER

CENTIMETER

SUSPENDED SEDIMENT,

IN MILLIGRAMS PER LITER

YEARYEARYEAR

YEARYEARYEAR

1960 1970 1980 1990 20001960 1970 1980 1990 2000

1960 1970 1960 1970 

1960 1970 1980 1990 2000

1960 1970 1980 1990 20001960 1970 1980 1990 2000

1970 1975 

Figure 10. LOWESS lines of selected major inorganic constituents and other characteristics as a function of time at the Tamiami C
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 25

The highest rate of increase for the cations was 

determined for potassium (3.35 percent per year) at the 

Miami Canal station during 1966-94. The LOWESS 

line of potassium shows an overall increase during the 

period of record, with a rather abrupt rate of increase 

beginning in about 1980 (fig. 9). The rate of increase 

for potassium was 1.34 percent per year at the Tamiami 

Canal station (table 4). The rates of increase for sodium 

were 1.54 percent per year at the Miami Canal station 

(1966-94) and 1.45 percent per year at the Tamiami 

Canal station (1967-93); the rates for magnesium were 

1.20 percent per year at the Miami Canal station (1966-

94) and 1.48 percent per year at the Tamiami Canal station (1967-93). However, the LOWESS lines for 

sodium and magnesium at the Miami Canal station 

shows decreasing trends for the last several years of 

record (fig. 9). The cations potassium, sodium, and 

magnesium occur in fertilizers, and upward trends of 

these constituents may be related to fertilizer usage. 

Median concentrations of potassium, sodium, magnesium, and other cations were determined at the Miami 

and Tamiami Canal stations. Median concentrations of 

all these cations were higher at the Miami Canal station 

than at the Tamiami Canal station (tables 3 and 4).

The highest rate of increase for anions was determined for sulfate (4.74 percent per year) at the Miami 

Canal station in 1966-94 (table 3). Results also indicate 

rates of increase for chloride at the Miami Canal station 

(1.23 percent per year) and the Tamiami Canal station 

(1.07 percent per year) as presented in tables 3 and 4, 

respectively. However, trends for chloride at both stations are relatively flat for the last few years of record. 

An upward trend in chloride at the Miami Canal station 

(fig. 9) might be due to increased urban or agricultural 

activities, whereas the upward trend for this anion at 

the Tamiami Canal station might be due to discharge of 

more highly mineralized water from Water Conservation Areas 3A and 3B through structures S-343A, 

S-343B, and S-344 (fig. 1). An upward trend in sulfate 

at the Miami Canal station might be due to increased 

urban or agricultural activities, as well as sulfur-emitting discharges in atmospheric deposition. Smith and 

others (1987, p. 8), however, concluded that sources of 

sulfate in NASQAN stations across the Nation were 

more likely to be terrestial (rather than atmospheric) in 

origin because of the higher frequency of trends in 

basins where the statistical association between the 

ratio of atmospheric deposition to aquatic yield of 

sulfur is low. The LOWESS lines for sulfate and chloride at the Miami Canal station and for chloride at the 

Tamiami Canal station are shown in figures 9 and 10. 

The fluoride trend for the Tamiami Canal station shows 

an overall downward trend; however, there seems to be 

no trend for about the last 15 years of record (fig. 10).

Median concentrations of chloride, sulfate, and 

other anions were determined at the Miami and Tamiami Canal stations. Median chloride concentrations 

were higher at the Miami Canal station (70 mg/L) than 

at the Tamiami Canal station (17 mg/L). During periods 

of backpumping in the northern Everglades, highly 

mineralized connate ground water is drawn into the 

canals, which might account for the higher chloride 

concentrations (Parker and others, 1955, p. 733).

Median concentrations of sulfate were 5.8 mg/L 

at the Miami Canal station and 1.0 mg/L at the Tamiami 

Canal station. Caution is in order in the interpretation 

of the sulfate trend at the Miami Canal station (fig. 9); 

a known positive bias in sulfate concentrations was 

discovered in 1989 based on a turbidimetric method in 

laboratory use since 1982 (Schertz and others, 1994, 

p. 35). This bias was generally evident in water samples where sulfate concentrations were less than 

75 mg/L and median concentrations were less than 

20 mg/L, which correlated with the data from the 

Miami and Tamiami Canal stations (tables 3 and 4). 

Additionally, color values greater than 20 Pt-Co units 

(platinum-cobalt units) may also have contributed to 

the positive bias (tables 3 and 4). The LOWESS line for 

sulfate indicates an overall upward trend in sulfate over 

time (fig. 9), which coincides with the upward trends 

for other ions. What effect this bias may have had on 

the overall trend remains unknown.

Results indicate upward trends for silica and 

dissolved solids at the Miami Canal station (fig. 9), 

and a general upward trend for dissolved solids at the 

Tamiami Canal station (fig. 10). The rate of increase for 

silica was 0.82 percent per year at the Miami Canal station during 1966-94. The rates of increase for dissolved 

solids were 0.41 percent per year at the Miami Canal station during 1966-94 (table 3), and 0.78 percent per year 

at the Tamiami Canal station during 1967-93 (table 4). A 

slightly downward trend for dissolved solids occurred at 

the Tamiami Canal station during the last few years of 

record (fig. 10). There also were upward trends at the 

Miami Canal station during 1976-94 (0.38 percent per 

year) and 1987-94 (0.83 percent per year). The trend for 

dissolved solids in 1987-94 would not have been statistically significant at an alpha level of 0.05 as noted in 

table 3. Median concentrations of silica and dissolved 

solids were higher at the Miami Canal station than at the 
26 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

Tamiami Canal station (tables 3 and 4), indicating 

greater mineralization. Upward trends in silica and dissolved solids tend to indicate an overall deterioration in 

water quality. High concentrations also can be detrimental to aquatic organisms.

Trends were determined for physical characteristics that include color, turbidity, and specific conductance. No upward or downward trends were determined 

for color; however, median values were 50 Pt-Co units 

at the Miami Canal station (table 3) and 30 Pt-Co units 

at the Tamiami Canal station (table 4), indicating 

higher concentrations of organic material in the Miami 

Canal water. This might be due to the organic peat soils 

found in the water-conservation and agricultural areas 

through which water in the Miami Canal flows as compared to the marls or sands prevalent in the Big Cypress 

National Preserve. A statistically significant downward 

trend for turbidity was detected at the Miami Canal station (fig. 9), with a rate of decrease of -2.37 percent per 

year during 1970-78 (table 3). This trend would not 

have been statistically significant at an alpha level of 

0.05 as noted in table 3. The median turbidity value of 

1.0 NTU (nephelometric turbidity unit) was the same at 

both stations (tables 3 and 4) and was well below the 

State freshwater standard (Florida Department of Environmental Protection, 1993) of less than or equal to 

29 NTU above natural background conditions. The 

maximum turbidity values for the Miami and Tamiami 

Canal stations were 11 and 4.5 NTU, respectively. As 

for specific conductance, an upward trend was detected 

at both the Miami and Tamiami Canal stations during 

the period of record, with rates of increase of 0.88 and 

1.22 percent per year, respectively, (tables 3 and 4). A 

downward trend for specific conductance was apparent 

at the Tamiami Canal station, but only during the last 

few years of record (fig. 10). Whereas median specific 

conductance values of 640 μS/cm (microsiemens per 

centimeter) at the Miami Canal station and 352 μS/cm 

at the Tamiami Canal station were below the State 

freshwater standard of 1,275 μS/cm (Florida Department of Environmental Protection, 1993), a maximum 

specific conductance of 1,350 μS/cm detected at the 

Miami Canal station did exceed the State standard. The 

upward trend in specific conductance indicates that an 

increase in dissolved solids occurred in both canals 

over time, which might be a result of natural phenomena or anthropogenic activities.

Water hardness is reported as an equivalent 

amount of calcium carbonate (CaCO3) and is dependent primarily on calcium and magnesium. No Federal 

or State standards exist for hardness, but waters are 

classified as soft from 0-60 mg/L, moderately hard 

from 61-120 mg/L, hard from 121-180 mg/L, and very 

hard at greater than 180 mg/L (Hem, 1985, p. 159). 

Based on median concentrations, water is very hard 

(230 mg/L) at the Miami Canal station and is hard 

(150 mg/L) at the Tamiami Canal station (tables 3 and 

4). Limestone present in the surficial aquifer system 

and Big Cypress National Preserve, where the stations 

are situated, is a natural source of calcium and magnesium, which affect hardness.

pH and Dissolved Oxygen

Trend determinations were made for the timedependent constituents pH and dissolved oxygen. The 

characteristics of pH and dissolved oxygen are related 

through the process of photosynthesis, which results in 

the uptake of carbon dioxide and the manufacture of 

dissolved oxygen by aquatic plants. Both pH and dissolved oxygen may exhibit diel variations, and together 

may be positively correlated to be higher during daylight hours because of photosynthetic activity and 

lower during night-time hours because of plant respiration. For this study, dissolved-oxygen concentrations 

were determined only in the daytime, and thus, represent concentrations that occurred during photosynthetic activity. However, the downward trends in pH at 

the Miami Canal station (fig. 9) and dissolved oxygen 

at the Tamiami Canal station (fig. 10) did not correlate 

with an accompanying statistically significant trend in 

either dissolved oxygen at the Miami Canal station 

(table 3) nor pH at the Tamiami Canal station (table 4), 

which would be expected if the trends were dependent 

on photosynthetic activity alone.

Results indicate downward trends in pH 

(-0.17 percent per year) at the Miami Canal station 

during 1966-94 (fig. 9 and table 3) and in dissolved 

oxygen (-2.59 percent per year) at the Tamiami Canal 

station during 1967-93 (figs. 9 and 10; tables 4 and 5). 

There were, however, no statistically significant trends 

in dissolved oxygen at the Tamiami Canal station for 

the 1976-93 and 1987-93 periods. A downward trend in 

dissolved oxygen usually results from an increase in 

oxygen-demanding materials (most likely organic) in 

water, and generally is indicative of increased anthropogenic activities and deteriorating water-quality conditions. Median pH values were 7.6 at the Miami Canal 

station during 1966-94 and 7.5 at the Tamiami Canal 

station during 1967-93, which were well within the 
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 27

range (6.0-8.5) established by the Florida Department 

of Environmental Protection (1993) for freshwater 

standards. The median concentrations of dissolved 

oxygen were 3.3 mg/L (1987-94) and 2.7 mg/L (1967-

93) at the Miami and Tamiami Canal stations, respectively, which were below the FDEP standard.

Suspended Sediment

Suspended sediment is material that is maintained in suspension in a water column due to the 

upward components of turbulent currents or material 

that exists in suspension as a colloid. This solid material may result from the disintegration of rocks and may 

include chemical and biochemical precipitates, as well 

as decomposed organic material. The concentration of 

suspended sediment in a stream is closely related to 

environmental and land-use factors (both urban and 

agricultural), intensity and volume of precipitation, 

geology and soil types, and physical characteristics of 

the stream. Suspended-sediment concentrations in 

southern Florida streams tend to be lower than those in 

other areas across the Nation, especially the western 

areas, which probably is due to the abundant vegetative 

cover that limits soil bank erosion. High suspendedsediment concentrations may adversely affect stream 

water quality by increasing turbidity, and thus, inhibiting photosynthetic activity. Suspended material, highly 

organic in nature, may increase the water column oxygen demand, resulting in hypoxic conditions. Additionally, high concentrations of suspended sediment may 

adversely affect aquatic life. Smith and others (1987, 

p. 13) found a close association between increasing 

trends in suspended sediment and basins in which land 

use results in high rates of soil erosion.

Results indicate a statistically significant upward 

trend (3.56 percent per year) in suspended sediment at 

the Miami Canal station during 1974-94, a statistically 

significant upward trend (4.08 percent per year) at the 

Tamiami Canal station during 1976-93 (but no detectable trend during 1987-93), and a statistically significant downward trend (-14.44 percent per year) at the 

Miami Canal station during 1987-94 (tables 3 and 4). 

The upward trends indicate some deterioration of water 

quality. At the Miami Canal station, the downward 

trend is an indication of improvement in water quality 

over the last few years of the period of record. Median 

concentrations of suspended sediment at the Miami and 

Tamiami Canal stations were 3.0 and 4.0 mg/L, respectively. The suspended sediment trends at the Miami 

Canal station during 1974-94 and 1987-94 were not 

statistically significant at an alpha level of 0.05 as 

noted in table 3.

Nitrogen, Phosphorus, and Carbon Species

Nitrogen, phosphorus, and carbon species are 

needed for the growth and maintenance of all organisms, especially plants. Water bodies that receive 

increased concentrations of nitrogen and phosphorus 

tend to have dense plant growth or algal blooms and 

usually become eutrophic (Hem, 1985, p. 128). Elevated concentrations of nitrogen and phosphorus can 

be attributed to municipal wastewater, industrial wastewater, or agricultural and urban runoff. Nitrogen occurs 

in natural waters in the form of organic nitrogen, 

ammonia, nitrite, and nitrate. Phosphorus also may 

exist in both organic and inorganic states. Organic carbon can be contributed to a water body through plant 

and animal waste. Total organic carbon is a measure of 

the dissolved and suspended organic carbon in a water 

sample.

Trends were determined for selected nitrogen, 

phosphorus, and carbon species at the Miami and Tamiami Canal stations. Statistically significant downward 

trends in total ammonia were detected at both stations 

(figs. 11 and 12). The rates of decrease for ammonia 

were -5.63 percent per year at the Miami Canal station 

during 1971-94, and -4.46 percent per year at the Tamiami Canal station during 1970-92 (tables 4 and 5). The 

median concentration of total ammonia was 0.1 mg/L 

at the Tamiami Canal station and 0.2 mg/L at the Miami 

Canal station, both exceeding the FDEP freshwater 

standard of 0.02 mg/L (Florida Department of Environmental Protection (1993).

According to Schertz and others (1994, p. 38), a 

known positive analytical bias occurred between 1980 

and 1986 in the analysis of total ammonia and total 

ammonia plus organic nitrogen (Kjeldahl nitrogen). 

This bias was introduced by the mercuric chloride tablets used for field preservation of nutrient samples, and 

was caused by an additional positive matrix effect 

introduced by the sodium chloride carrier in the tablets. 

In 1986, the preservation of nutrient samples using the 

tablets was discontinued, and mercuric chloride 

ampoules were substituted. This bias, however, did not 

seem to affect the overall trends of total ammonia at the 

Miami and Tamiami Canal stations (figs. 11 and 12).

Total nitrite plus nitrate and total phosphorus 

were the only other species that demonstrated 
28 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

statistically significant downward trends. The rates 

of decrease were -12.94 percent per year for total 

phosphorus at the Miami Canal station during 

1987-94 (fig. 11 and table 3) and -5.68 percent per 

year for total nitrite plus nitrate at the Tamiami 

Canal station during 1975-85 (fig. 12 and table 4). 

The statistically significant downward trend for 

total phosphorus (-12.94 percent per year) at the 

Miami Canal station coincides with the statistically 

significant downward trend for suspended sediment 

(-14.44 percent per year) over the same time period 

(table 3). However, the total phosphorus trend was 

not statistically significant at an alpha level of 0.05 

as noted in table 3. A strong statistical association 

(p-value less than 0.001) exists between trends in 

total phosphorus and suspended sediment at 

NASQAN stations across the Nation (Smith and 

others, 1987, p. 13). The coexisting downward 

trends in total phosphorus and suspended sediment 

may also suggest that the source of total phosphorus 

was primarily nonpoint in origin over this time 

period. These decreasing trends indicate a general 

improvement in water quality, perhaps due to a 

reduction in agricultural or urban runoff. Median 

concentrations of the nitrogen species were higher 

at the Miami Canal station, which may be attributed 

to flow through the highly urbanized and agricultural areas. Median concentrations of total phosphorus (0.02 mg/L) were the same at both the 

Miami and Tamiami Canal stations during all time 

periods.

The only statistically significant upward 

trend (2.83 percent per year) was reported for total 

organic carbon at the Miami Canal station during 

1970-81 (fig. 11 and table 3). Median concentrations of total organic carbon were 20 mg/L at the 

Miami Canal station (table 3) and 13 mg/L at the 

Tamiami Canal station (table 4). The higher concentration at the Miami Canal station probably 

reflects the influence of the highly organic soils 

prevalent in the agricultural and water-conservation 

areas.

Trace Metals

Trace metals commonly occur in concentrations less than 1.0 mg/L (Hem, 1985, p. 129), and in 

small amounts, some may be toxic to plants, animals, and humans. Many trace metals occur naturally in the environment, but some may be present 

1.5

1.0

0.5

0

1970 1975 1980 1985 1990 1995

TOTAL AMMONIA, IN

MILLIGRAMS PER LITER

YEAR

100

80

60

40

20

0

1965 1970 1980 1990 1995

TOTAL ORGANIC CARBON,

IN MILLIGRAMS PER LITER

YEAR

Bias

0.08

0.06

0.04

0.02

0

1984 1986 1988 1990 1992 1994 1996

TOTAL PHOSPHORUS, IN

MILLIGRAMS PER LITER

YEAR

Figure 11. LOWESS lines of total ammonia, total organic 

carbon, and total phosphorus as a function of time at the 

Miami Canal station. Smoothness factor is 0.5.
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 29

as a result of urban, agricultural, or atmospheric sources. 

Trace metals can adsorb to the bottom sediments and reenter the water column due to changes in the oxidation/reduction potential. Trend determinations were 

made for dissolved barium, copper, iron, manganese, 

nickel, strontium, and zinc. Most samples were analyzed 

using the Seasonal Kendall censored procedure, which 

does not provide for removal of flow-related variability. 

Where censored data consist of greater than 5 percent of 

the total data values for a specific trace metal, the trend 

slopes might not be reliable, thus, the results were not 

recorded. Data sets for many of the water samples were 

too highly censored to make definitive trend analyses 

for the trace metals at the Miami and Tamiami Canal 

stations.

Results indicated statistically significant downward trends in barium and iron at the Miami Canal station (fig. 13 and table 3) and in barium at the Tamiami 

1.0

0.8

0.6

0.4

0.2

0

1970 1975 1980 1985 1990 1995

TOTAL AMMONIA, IN

MILLIGRAMS PER LITER

YEAR

Bias

0.30

0.25

0.20

0.15

0.10

0.05

0

1970 1975 1980 1985 1990

TOTAL NITRITE PLUS

NITRATE, IN MILLIGRAMS

PER LITER

YEAR

Figure 12. LOWESS lines of total ammonia and total nitrite plus 

nitrate as a function of time at the Tamiami Canal station. 

Smoothness factor is 0.5.

Canal station (fig. 14 and table 4). The downward trend 

in iron at the Miami Canal station was not statistically 

significant at an alpha level of 0.05 as noted in table 3. 

A weak trend also was detected for strontium at the 

Tamiami Canal station (fig. 14 and table 4), with a 

median concentration of 230 μg/L (micrograms per 

liter). The rates of decrease for barium were 

-3.86 percent per year at the Miami Canal station and 

-8.55 percent per year at the Tamiami Canal station. The 

concentration of barium in natural water is controlled by 

barium sulfate solubility, which usually limits the concentration of barium to a narrow range. Median concentrations of barium were 31 and 19.5 μg/L at the Miami 

and Tamiami Canal stations, respectively; these concentrations are not considered to be toxic to aquatic life.

The rate of decrease for iron was -2.43 percent per 

year at the Miami Canal station (table 3). The median 

iron concentration was 100 μg/L, which is well below 
30 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

the FDEP freshwater standard of 1,000 μg/L (Florida 

Department of Environmental Protection, 1993). The 

solubility of iron in water depends on several factors 

including the oxidation/reduction potential and the pH 

of the natural system (Hem, 1985, p. 76).

Bacteriological and Biological Characteristics

Bacteriological determinations were made for 

two types of indicator bacteria, fecal coliform and fecal 

streptococcus, at the Miami and Tamiami Canal stations. The presence of these bacteria might be a signal 

that pathogenic disease-producing bacteria or viruses 

exist in the specific water body. There is a close association between fecal coliform density and Salmonella 

contamination, with a sharp increase in Salmonella 

detection when fecal coliform colonies exceed 200 colonies per 100 mL (milliliters) according to the U.S. 

Environmental Protection Agency (1976). Contamination with both types of bacteria may come from industrial/municipal and urban/agricultural runoff.

At the Miami Canal station, results indicate a statistically significant downward trend in fecal coliform 

(-3.84 percent per year) during 1976-94, but no detectable trend during 1987-94. A statistically significant 

upward trend in fecal streptococcus (45.86 percent per 

year) also was found at the Miami Canal station during 

1987-94 (table 3). The downward trend in fecal 

coliform may indicate an improvement in water quality 

over time (1976-94), whereas the upward trend in fecal 

streptococcus may indicate a deterioration in water 

quality during the last few years of the period of record 

(fig. 15). Additionally, the trends for fecal coliform 

during 1976-94 and for fecal streptococcus during 

1987-94 are not statistically significant at an alpha 

level of 0.05 as noted in table 3. The deterioration in 

80

60

40

20

0

400

300

200

100

0

1975 1980 1985 1990 1995

1960 1970 1980 1990 2000

BARIUM, IN MICROGRAMS

PER LITER

IRON, IN MICROGRAMS

PER LITER

YEAR

YEAR

Figure 13. LOWESS lines of barium and iron as a function of time 

at the Miami Canal station. Smoothness factor is 0.5.
Analysis of Water-Quality Trends at the Miami and Tamiami Canal Stations 31

water quality may be due to inadequately treated sewage or runoff from urban or agricultural areas. There 

were no statistically significant upward or downward 

trends in bacteriological characteristics at the Tamiami 

Canal station (table 4). Median concentrations of fecal 

coliform and fecal streptococcus were higher at the 

Miami Canal station than at the Tamiami Canal station 

(tables 3 and 4). This probably is because the Miami 

Canal traverses areas that are influenced more by urban 

and agricultural activities.

Biological determinations were made for phytoplankton at the Miami and Tamiami Canal stations. 

Phytoplankton consist of microscopic free-floating 

plants that are mostly blue-green algae, diatoms, or 

green algae. Their movements are dependent on the 

prevailing current within the water column, and their 

growth is dependent on solar radiation and nutrient 

concentrations within the water body. Heavy nutrient 

loads (especially phosphorus) to a stream, lake, or estuary may cause the water body to become eutrophic, 

with an increase in phytoplankton growth or algal 

blooms and the development of hypoxic or anoxic conditions, resulting in fish kills. Heavy phytoplankton 

growth might also result in undesirable aesthetic 

effects, manifested in unpleasant color and odor problems. Conversely, low phytoplankton growth might be 

an indication that a water body is oligotrophic or poorly 

nourished with respect to nutrients. Results indicate no 

statistically significant upward or downward trends in 

phytoplankton at the Miami and Tamiami Canal stations for the period of record (tables 3 and 4). Median 

concentrations of phytoplankton were 840 and 

1,800 cells/mL (cells per milliliter) at the Miami and 

Tamiami Canal stations, respectively.

60

50

40

30

20

10

0

1,000

800

600

400

200

0

1975 1980 1985 1990 1995

1960 1970 1980 1990 2000

BARIUM, IN MICROGRAMS

PER LITER

STRONTIUM, IN MICROGRAMS

PER LITER

YEAR

YEAR

Missing data record

Figure 14. LOWESS lines of barium and strontium as a 

function of the Tamiami Canal station. Smoothness factor is 

0.5.
32 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

SUMMARY

A study was conducted by the USGS to analyze 

water-quality trends in two areas of southern Florida. 

Selected major inorganic constituents and physical 

characteristics; pH and dissolved oxygen; suspended 

sediment; nitrogen, phosphorus, and carbon species; 

trace metals; and bacteriological and biological characteristics were determined at two USGS daily discharge 

stations over a long-term period (1966-94). The stations used in this study are the Miami Canal station 

west of Biscayne Bay in Miami and the Tamiami Canal 

station along U.S. Highway 41 in the Big Cypress 

National Preserve in Collier County.

The principal tool used in this study was the 

computer program ESTREND, which utilizes the Seasonal Kendall trend test − a nonparametric, distribution-free test for monotonic change in water quality 

with time. The null hypothesis for this test assumes that 

the probability distribution of the random variable is 

independent of time. It is also assumed that the data 

values come from the same statistical distribution. This 

test allows for removal of extraneous variation in 

water-quality data caused by both seasonality and discharge. The test is a modification of the Mann-Kendall 

test and compares subsequent data values from the 

same seasons. The p-value, or probability of rejecting 

the null hypothesis of no trend, when there is in fact a 

statistically significant trend (p-value less than 0.10), is 

determined from a standard normal distribution. The 

test allows for the removal of flow-related variability 

for a water-quality record by applying the Seasonal 

Kendall trend test on a time series of residuals developed from concentration/discharge regression models 

or on unadjusted concentrations. The Seasonal Kendall 

slope estimator, which presents the rate of change in 

50,000

40,000

30,000

20,000

10,000

0

1,500

1,000

500

0

1960 1970 1980 1990 2000

1986 1988 1990 1992 1994 1996

FECAL COLIFORM

BACTERIA, IN COLONIES

PER 100 MILLILITERS

FECAL STREPTOCOCCUS

BACTERIA, IN COLONIES

PER 100 MILLILITERS

YEAR

YEAR

Figure 15. LOWESS lines of fecal coliform and fecal streptococcus 

bacteria as a function of time at the Miami Canal station. Smoothness 

factor is 0.5.
Summary 33

original units per year and percent per year is the 

median of all the pairwise comparisons from all the 

seasons. The ESTREND program also employs a 

Seasonal Kendall trend test for censored data and a 

maximum likelihood estimator for multiple censored 

data values.

Upward trends in the cations sodium, potassium, 

and magnesium were detected at the Miami and Tamiami Canal stations. Additionally, the anions chloride 

and sulfate showed upward trends at the Miami Canal 

station, whereas only chloride showed an upward trend 

at the Tamiami Canal station. Fluoride exhibited a 

downward trend at the Tamiami Canal station. The rate 

of increase for sulfate (4.74 percent per year) at the 

Miami Canal station was the highest rate of increase for 

any cation or anion demonstrating statistically significant trends. However, a known bias may have affected 

the analytical results for sulfate based on a turbidimetric method in laboratory use at the time. No trends in 

hardness were detected at either station.

A statistically significant downward trend for 

turbidity was detected at the Miami Canal station; however, the record spans only the 1970-78 period. No 

statistically significant trend for turbidity was detected 

at the Tamiami Canal station, nor for color at either station. Upward trends in specific conductance were 

detected at the Miami Canal station (1966-94) and the 

Tamiami Canal station (1967-93), which correlated 

with upward trends in dissolved solids at both stations 

during the same time periods. The Miami Canal station 

also showed upward trends in dissolved solids for the 

1976-94 and 1987-94 periods. Median values for specific conductance at the Miami and Tamiami Canal stations were 640 and 352 μS/cm, respectively, which are 

below the FDEP standard of 1,275 μS/cm.

Results indicate a downward trend in pH at the 

Miami Canal during 1966-94. A similar downward 

trend in dissolved oxygen occurred at the Tamiami 

Canal station during 1967-93. Median pH values from 

the Miami Canal and Tamiami Canal stations were 7.6 

and 7.5, respectively, both within the FDEP freshwater 

Class III standard range of between 6.0 and 8.5 for 

freshwater. The median concentrations of dissolved 

oxygen were 3.3 and 2.7 at the Miami and Tamiami 

Canal stations, respectively, which are below the FDEP 

standard.

Temporal trends in suspended sediment indicate 

a statistically significant upward trend at the Miami 

Canal station during 1974-94, a statistically significant 

downward trend at the same station during 1987-94, 

and a statistically significant upward trend at the Tamiami Canal station during 1976-93 period. Median concentrations of suspended sediment at the Miami and 

Tamiami Canal stations were 3.0 and 4.0 mg/L, respectively.

Nitrogen, phosphorus, and carbon species results 

indicate downward trends in total ammonia at both stations; however, a known bias occurred in ammonia 

analysis from the early to mid-1980’s as a result of mercuric chloride tablets used for nutrient preservation. 

This bias, however, did not seem to have affected the 

overall trends. Median and maximum concentrations of 

total ammonia at the Miami and Tamiami Canal stations exceeded the FDEP standard of 0.02 mg/L. The 

only other statistically significant downward trend was 

for total phosphorus at the Miami Canal station during 

1987-94. A statistically significant upward trend in 

total organic carbon was indicated at the Miami Canal 

station during 1970-81.

For the analysis of trace metals, statistically significant downward trends were indicated for iron and 

barium at the Miami Canal station during 1969-94 and 

1978-94, respectively, and for barium (1978-93) at the 

Tamiami Canal station. There was a weak upward trend 

for strontium during 1967-93 at the Tamiami Canal station. The median concentration of iron at the Miami 

Canal station was 100 μg/L, which is below the FDEP 

standard of 1,000 μg/L.

Bacteriological analyses indicate a statistically 

significant downward trend for fecal coliform and a 

statistically significant upward trend in fecal streptococcus at the Miami Canal station. There were no statistically significant trends in either bacteria type at the 

Tamiami Canal station. Biological characteristics at 

both stations consisted of phytoplankton analyses, with 

no statistically significant trends in phytoplankton 

found at either station. Median concentrations of phytoplankton at the Miami and Tamiami Canal stations 

were 840 and 1,800 cells/mL, respectively.

A summary of temporal trends was done at the 

Miami and Tamiami Canal stations as indicators of 

either improvement or deterioration in water quality. 

At the Miami Canal station, improvement in water 

quality was indicated by 7 downward trends, and deterioration was reflected by 13 upward trends and 

1 downward trend; at the Tamiami Canal station, 

improvement in water quality was reflected by 4 downward trends and deterioration in water quality was 

reflected by 8 upward trends and 1 downward trend. 

Most downward trends indicate improvement in water 
34 Analysis of Water-Quality Trends at Two Discharge Stations—One within the Big Cypress National Preserve and One near 

Biscayne Bay—Southern Florida, 1966-94

quality over time. However, the downward trend in pH 

indicates deterioration in water quality over time and 

the potential for harmful effects on aquatic life. Trends 

at the Miami Canal station were flow adjusted, however, a p-value of 0.10 was still used. Trends for turbidity, dissolved solids, suspended sediment, total 

phosphorus, iron, fecal coliform, and fecal streptococcus were determined not to be statistically significant at 

an alpha level of 0.05 at the Miami Canal station.

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