Efficiency of a Stormwater Detention Pond in 

Reducing Loads of Chemical and Physical 

Constituents in Urban Streamflow, Pinellas 

County, Florida

By I.H. Kantrowitz anc/W.M. Woodham

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 94-4217

Prepared in cooperation with 

PINELLAS COUNTY, FLORIDA

Tallahassee, Florida 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For additional information write to:

District Chief

U.S. Geological Survey

Suite 3015

227 North Bronough Street

Tallahassee, FL 32301

Copies of this report can be 

purchased from:

U.S. Geological Survey 

Earth Science Information Center 

Open-File Reports Section 

P.O. Box 25286, MS 517 

Denver, CO 80225-0425
CONTENTS

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

Introduction......................................................................................................................................................^ 2

Description of the Detention Pond, Monitoring Sites, and Drainage Basin ..................................................................... 2

Data Collection..........................................................................................................................................^ 4

Constituent Loads ............................................................................................................................................................. 5

Detention Pond Efficiency for Stormwater....................................................................................................................... 7

Measured Pond Efficiency........................................................................................................................................... 8

Adjusted Estimates of Pond Efficiency ....................................................................................................................... 11

Factors Affecting Constituent-Load Changes ............................................................................................................. 11

Errors in Load Adjustment................................................................................................................................ 11

Flushing of Stored Base Flow ........................................................................................................................... 12

Settling and Biochemical Processes .................................................................................................................. 13

Vegetation.......................................................................................................................................................... 13

Restricted Circulation........................................................................................................................................ 14

Load Reduction of Urban-Runoff Contaminants ........................................................................................................ 14

Metals........................................................................................................................_^ 14

Nutrients............................................................................................................................................................ 14

Other Selected Chemical and Physical Constituents......................................................................................... 14

Detention Pond Efficiency For Base Flow........................................................................................................................ 15

Summary and Conclusions................................................................................................................................................ 17

References Cited ............................................................................................................................................................... 18

FIGURES

1. Map showing detention area 3 and the location of the inflow and outflow monitoring sites on

Saint Joe Creek, Pinellas Park, Pinellas County, Fla.................................................................................................. 3

2. Discharge hydrograph for typical storm C and samples selected for chemical

analysis at outflow site (02308935)............................................................................................................................ 6

TABLES

1. Drainage area and land-use characteristics of areas contributing flow into and out of detention area................... 4

2. Hydraulic characteristics of the monitored storms.................................................................................................. 5

3. Computed loads of selected water-quality constituents in stormwater at the inflow site ....................................... 7

4. Computed loads of selected water-quality constituents in stormwater at the outflow site ..................................... 8

5. Daily base-flow loads of selected water-quality constituents at the inflow site...................................................... 9

6. Concentrations of selected water-quality constituents in stormwater and base flow at the inflow site .................. 9

7. Pond efficiencies for selected water-quality constituents in stormwater based on loads measured at the

inflow and outflow sites .......................................................................................................................................... 10

8. Mean-event concentrations of selected water-quality constituents in stormwater at the inflow and

outflow sites ............................................................................................................................................................ 10

9. Estimated pond efficiencies for selected water-quality constituents in stormwater based on adjusted

inflow loads and measured outflow loads ............................................................................................................... 12

10. Daily base-flow loads of selected water-quality constituents at the outflow site before and after

construction of the detention pond.......................................................................................................................... 16

Contents
CONVERSION FACTORS, WATER-QUALITY ABBREVIATIONS, AND 

ACRONYM

Multiply

inch (in.)

foot (ft)

cubic foot (ft3 )

cubic foot per day (ft3/d)

square mile (mi2)

acre

By

25.4

0.3084

0.0283

0.0283

2.590

0.4047

To obtain

millimeter

meter

cubic meter

cubic meter per day

square kilometer

hectare

Temperature in degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) as 

follows:

F=1.8°C+32

WATER-QUALITY ABBREVIATIONS

kg kilograms

mg/L milligrams per liter

ACRONYM

USGS U.S. Geological Survey

IV Contents
Efficiency of a Stormwater-Detention Pond in Reducing 

Loads of Chemical and Physical Constituents in Urban 

Streamflow, Pinellas County, Florida

By I.H. Kantrowitz andW.M. Woodham

Abstract

A multipurpose wet stormwater-detention 

pond in Pinellas Park, Florida, was studied to 

determine its effectiveness in reducing the load of 

selected water-quality constituents commonly 

found in urban streamflow. Water-quality 

samples, and data on streamflow and precipitation 

were collected at the outflow and principal inflow 

of detention area 3 on Saint Joe Creek. To 

compare the constituent loads entering and 

leaving the detention pond, flows and water 

quality were simultaneously monitored at the 

inflow and outflow sites for six storms as well as 

intermittently monitored during periods of base 

flow. Loads of 19 selected chemical and physical 

constituents were determined.

Because all the stormwater entering the 

detention pond was not measured at the inflow site, 

computed stormwater inflow loads were adjusted to 

account for loads from the unmonitored areas. The 

ratio of stormwater volume measured at the outflow 

site to stormwater volume measured at the inflow 

site was used to adjust inflow loads for individual 

storms. Pond efficiencies for selected water-quality 

constituents for each of the storms were estimated 

by dividing the difference in outflow and adjusted 

inflow loads by the adjusted inflow load.

Stormwater loads of the major ions (chloride, 

calcium and bicarbonate) and dissolved solids at the 

outflow site exceeded loads at the inflow site, partly 

as a result of mixing with base flow stored within 

the pond. However, the detention pond was 

effective in reducing the stormwater load of such

urban-runoff contaminants as metals, nutrients, 

suspended solids, and biochemical and chemical 

oxygen demand. Estimated median pond 

efficiencies for reducing constituent loads ranged 

from 25 to more than 60 percent for metals, 2 to 

52 percent for nutrients, 7 to 11 percent for two 

measurements of suspended solids, and 16 to 

49 percent for the oxygen-consuming substances. 

The reductions of constituent loads in stormwater 

are probably a result of dilution with pond water 

(particularly for smaller storms), adsorption, 

chemical precipitation, settling, biologic uptake, 

and oxidation. The establishment of aquatic 

vegetation midway through the study appears to 

have increased the efficiency of the pond in 

reducing loads of urban-runoff contaminants in 

stormwater.

The efficiency of the detention pond in 

reducing base-flow loads was estimated by 

comparing base-flow loads at the outflow site prior 

to and after construction of the pond. Loads of 

major ions and dissolved solids in base flow were 

reduced at median efficiencies ranging from 17 to 

35 percent. Urban-runoff contaminants in base flow 

were generally reduced at higher efficiencies. 

Median efficiencies ranged from 38 to 82 percent 

for metals, 19 to 83 percent for nutrients, 34 to 45 

percent for suspended solids, and 43 to 65 percent 

for the oxygen-consuming substances. The 

reductions in base-flow loads are probably a result 

of adsorption, chemical precipitation, biologic 

uptake, and settling within the pond. These 

processes were more effective in reducing baseflow loads after the establishment of aquatic 

vegetation in the pond.

Abstract
INTRODUCTION

Urban streamflow, particularly stormwater runoff, often contains substantial loads of numerous 

chemical and physical constituents that may adversely 

affect the water quality of receiving waters. These 

constituent loads are caused by the rainfall washoff of 

deposited material such as vehicle emissions, trash, 

pet litter, fertilizers and pesticides applied to lawns, 

and atmospheric deposition. Several constituents 

common in urban streamflow include metals, dust and 

soils, nutrients, pesticides, bacteria, and other natural 

and anthropogenic compounds (Lopez and 

Giovannelli, 1984).

Even relatively small loads of some contaminants can stress the biological integrity of receiving 

waters through various physical and chemical processes. Metals can be toxic to aquatic life and are often 

accumulated and concentrated in the food chain. 

Excess nutrients commonly result in the growth of 

undesirable aquatic plants and contribute to the 

eutrophication process of receiving waters. Suspended 

material in water causes turbidity, which affects the 

penetration of light and inhibits the growth of desirable aquatic plants such as seagrass. The presence of 

oxygen-consuming substances reduces the level of 

oxygen dissolved in water and can kill fish.

Stormwater-detention ponds, commonly used 

to attenuate peak flows and minimize flooding, are 

regarded as one of the most promising management 

practices to reduce contaminant loads (McCuen, 1980) 

and are widely used throughout Florida. Detention 

ponds designed to control flooding and to improve 

water quality by maximizing sedimentation and chemical and biologic processes are referred to as multipurpose stormwater-detention ponds. Ponds designed to 

retain water during nonstorm periods are referred to as 

wet detention ponds.

Urban streamflow in densely populated Pinellas 

County contains concentrations of selected constituents that exceed State standards for treated sewage 

effluent and that also exceed the standards for water 

entering the Gulf of Mexico and Tampa Bay (Lopez 

and Giovannelli, 1984). A multipurpose wet detention 

pond located on Saint Joe Creek in the urban community of Pinellas Park in Pinellas County (fig. 1) was 

selected to study the effectiveness of stormwaterdetention ponds as pollution-control measures. The 

study was conducted from 1984 to 1991 by the U.S. 

Geological Survey (USGS) in cooperation with the 

Pinellas County Engineering Department.

Streamflow in Saint Joe Creek during, and for a 

period following storms, is referred to in this report as 

"stormwater runoff. Although consisting largely of 

overland flow, most of which is conveyed to the 

stream channel by stormwater sewers, stormwater runoff may also include a small component of flow 

contributed by ground-water discharge as well as 

water released (flushed) from storage from small, 

upstream detention ponds. Streamflow during periods 

of fair weather consists almost entirely of groundwater discharge to the channel and is referred to as 

"base flow". Because of delays in the construction of 

the detention pond, intensive collection of stormwaterrunoff data did not begin until 1989. The delay 

provided the opportunity for more extensive sampling 

of base flow and thus broadened the scope of the study 

to include the full range of urban streamflow.

The purpose of this report is to present a 

comparison of the constituent loads in urban streamflow 

as it enters and leaves a multipurpose wet detention 

pond. The report also presents efficiency data for the 

removal of selected urban-runoff contaminants (metals, 

nutrients, suspended material, and oxygen-consuming 

substances) in both stormwater and base flow within the 

detention pond. Stormwater runoff and base flow were 

measured and sampled upstream and downstream from 

the detention pond. Constituent loads in stormwater 

were computed from intensive sampling of six storms 

during a 2-year period from 1989 to 1991. Constituents 

loads in base flow were computed from 13 grab samples 

collected upstream from the pond during the period 

from 1989 to 1991 and from 16 grab samples collected 

at the downstream site during the period from 1984 to 

1991.

DESCRIPTION OF THE DETENTION 

POND, MONITORING SITES, AND 

DRAINAGE BASIN

The stormwater-detention pond selected for this 

study is located on Saint Joe Creek, a largely 

channelized perennial stream that flows westward and 

drains an urbanized area in central Pinellas County 

(fig. 1). The stream is also referred to locally as "Saint 

Joes Creek," "Joe's Creek" and "Joe Creek." The 

detention pond, known as detention area 3, was 

constructed by the Pinellas County Engineering>~y Department by excavating a 25-acre (0.04 mi ) area to a 

depth of 15 to 20 ft below the general land surface and, 

in two settling basins within the pond, as much as 8 ft

2 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Location of Study Area

within the city of

Pinellas Park in

Pinellas County

Pinellas 

Park

AUTOMATIC 

WATER SAMPLER 

AND RAIN GAGE

AUTOMATIC 

WATER SAMPLER 

AND RAIN GAGE

49th Avenue North

Saint Joe Creek

OUTFLOW 

DISCHARGEGAGING 

STATION 

(02308935)

Detention 

Area

- INFLOW 

DISCHARGE- 

" GAGING 

STATION 

(02308931)

LOW-HEAD 

V-NOTCH WEIR

46th Avenue North

Lealman

EXPLANATION

H SETTLING BASIN 

~3 AREA OF FILL

500 FEET

Figure 1. Detention area 3 and the location of the inflow and outflow monitoring sites on Saint Joe Creek, Pinellas Park, 

Pinellas County, Fla.

below the original streambed elevation. Construction of 

the detention pond was completed in 1989. A 30-in. 

diameter gravity sanitary sewer parallels the channel and 

the southern boundary of the pond but is not known to 

have affected water quality in Saint Joe Creek or 

detention area 3.

Because detention area 3 was excavated below 

the water table and below the level of the pond outlet, it 

contains water over most of its area even under baseflow conditions. During construction, an area of fill was 

placed parallel to the original creek channel in order to 

temporarily isolate the channel from the northern part of 

the pond. After construction, the fill was not completely 

removed, leaving it partially emerged during base-flow 

conditions and overgrown by cattails. Even when submerged during storms, the fill probably restricts circulation between the two halves of the pond (fig.l).

Under base-flow conditions, the entire pond is 

covered by water except for the emergent area of fill. 

With a typical 1 -ft depth of water covering the shallow 

submerged parts of the pond, and with more than 5 ft 

of water in the deeper parts, the pond contains more 

than 1,250,000 ft3 of water. During storms, backwater

conditions are created behind the concrete v-notch 

weir outlet and water is temporarily stored in the pond. 

Approximately 1,000,000 ft3 are stored for each foot 

of water-level rise. Backwater conditions are generally 

dissipated less than 24 hours after cessation of precipitation. Because of the permanent water-storage 

conditions within the pond, however, the residence 

time of stormwater may commonly exceed 24 hours. 

A daily-discharge gaging station and an automatic water sampler were maintained on Saint Joe 

Creek by the USGS from September 1984 through 

September 1991. The gaging station, Saint Joe Creek 

at Pinellas Park (station number 02308935), was 30 ft 

upstream from a box culvert at the intersection of 46th 

Avenue North and 46th Street North and about 900 ft 

downstream from the outlet of the detention pond 

(fig. 1). This station, referred to as the "outflow" site, 

monitored the quantity and chemical quality of flow 

from a 2.55 mi2 drainage basin composed mostly of 

medium-density, single-family residential and 

commercial land uses (table 1). Land-use characteristics of the drainage basin did not change during the 

period of study because the basin experienced little or 

no new development.

Description of the Detention Pond, Monitoring Sites, and Drainage Basin 3
Table 1. Drainage area and land-use characteristics of areas contributing flow into and out of detention area 3 on Saint Joe 

Creek, Pinellas Park, Pinellas County, Fla.

Site

Outflow

Inflow

Intervening area

Drainage area 

(square miles)

2.55

2.00

0.55

Land use, percentage of area

Residential

53

51

60

Commercial

20

22

13

Open and 

recreation

14

13

16

Roads

12

13

9

Institutional

1

1

2

Following construction of detention area 3 in 

1989, a daily-discharge gaging station and an automatic water sampler were installed about 1,100 ft 

upstream from the excavated area. This "inflow" 

gaging station was operated by the USGS from 

October 1989 through September 1991. The inflow 

site, Saint Joe Creek at Lealman (station number 

02308931), was 60 ft upstream from the Seaboard 

Coastline Railroad bridge near the intersection of 49th 

Avenue North and 40th Street North (fig. 1). The 

inflow site monitored the quantity and chemical 

quality of flow from a 2.00 mi2 drainage basin.

A 0.55 mi2 area surrounding the detention pond, 

referred to as the "intervening" area, contributes runoff to the pond that is not monitored at the inflow site

^

(table 1). The intervening area includes the 0.04 mi 

area of the pond. Runoff from the intervening area 

enters the detention pond from three stormwater sewers located on the north, south, and west sides of the 

pond. It was considered impractical to monitor the 

inflow from the sewers; therefore, indirect methods 

were used to estimate loads from the intervening area.

DATA COLLECTION

Water-quality samples and data on streamflow 

and precipitation were collected at the outflow and 

principal inflow of detention area 3 on Saint Joe 

Creek. Samples of stormwater runoff for six storms 

were collected and analyzed at both sites from 

September 1989 through August 1991. One to three 

base-flow samples per year were collected from 1984 

to 1991 at the outflow site. One base-flow sample was 

collected at the inflow site in 1986 and two to five 

samples per year were collected from 1989 to 1991.

The outflow and inflow sites were operated as 

standard USGS gaging stations (Carter and Davidian, 

1968). Continuously monitored water levels in the 

stream were converted to flows using a stagedischarge relation developed for each site by means of 

periodic flowmeter measurements (Buchanan and

Somers, 1969; Kennedy, 1984). Efforts were made to 

take measurements during storms in order to define the 

stage-discharge relation for the higher stages and flows 

of primary interest to this study. Mean daily discharge 

values for both sites have been published in the annual 

data releases of the USGS. Instantaneous discharges 

and discharges for subdivided days were computed 

using the stage-discharge relation and were used to 

compute the constituent loads for base flow and for 

each of the six storms at the outflow and inflow sites.

Sample collection, preservation, shipping, and 

analysis were done according to standard procedures of 

the USGS (Fishman and Friedman, 1989). Manual 

sampling techniques were used to collect water samples 

during base flow. Automatic sampling equipment was 

used at both sites to collect water samples during storms. 

The samplers were stage activated, and the frequency of 

sampling was controlled by a data logger programmed to 

sample at designated time intervals or when the stage 

changed at more than a designated rate. The stage, time 

intervals, and rates of change programmed into the data 

logger were based on the hydrograph characteristics 

observed at the outflow site prior to construction of the 

pond. Generally, 15 to 24 samples were collected at each 

site for each storm. The storage area of the sampling 

equipment was refrigerated and was maintained at a 

temperature of approximately 4 °C.

The concentration of major ions (chloride, 

calcium, and bicarbonate), total recoverable metals 

(aluminum, chromium, copper, lead, mercury, and 

zinc), total nutrients (ammonia, nitrite plus nitrate, 

organic nitrogen, phosphorus, and orthophosphate as 

phosphorus), dissolved solids, suspended volatile 

solids, total suspended solids, biochemical oxygen 

demand, and chemical oxygen demand were 

determined at a USGS water-quality laboratory, except 

for bicarbonate, which was computed from the field 

determination of alkalinity. For consistency 

throughout this report, these chemical and physical 

measurements and constituents will be described 

collectively as constituents.

4 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Reported concentrations of metals, particularly 

concentrations less than 0.1 mg/L, could be strongly 

influenced by contamination introduced by sampling 

methods. Because the focus of this study was a 

comparison of loads at the two sites, and because 

sampling methods at both sites were the same, the 

analytical data for metals and the computations based 

on these data were assumed to be valid in a relative 

sense, if not in an absolute sense.

CONSTITUENT LOADS

The load (mass) of a constituent in streamflow is 

the mathematical product of the concentration of the 

constituent (mass per volume) and the total volume of 

water passing a point. Concentration data for 19 constituents and continuous discharge data were available 

at the inflow and outflow sites for each of the six 

storms studied and also for selected periods of base 

flow. The hydraulic characteristics for each of the six 

storms are shown in table 2.

During periods of stormwater runoff, both the 

quantity and chemical quality of the flow may change 

rapidly. Although runoff was monitored continuously, 

there is no practical way to monitor the chemical quality of a variety of constituents in a continuous manner. 

This inability introduces an intuitive but unknown 

error into the computation of load. The method chosen 

to obtain constituent concentrations for load computations in this study is referred to as "discrete sampling" 

and is one of several standard methods commonly 

used, all of which have associated unknown errors. 

Discrete sampling has the advantage of being relatively straightforward, relying on mathematical computation rather than physically blending a composite 

sample. Too, by using data from several chemical 

analyses, discrete sampling minimizes the effect of 

possible sampling error or spurious laboratory data.

To compute storm loads by the discrete method 

of sampling, several samples from among the total 

samples collected during a storm were selected for 

chemical analysis. The samples were chosen so that 

they were representative of discrete intervals on the 

storm hydrograph: two or three samples on the rising 

limb of the hydrograph, one or two at or near the peak, 

and two or three on the falling limb. A hydrograph 

showing the discharge measured at the outflow site 

during a typical storm as well as samples selected for 

chemical analyses are shown in figure 2. The total 

interval representing stormwater flow is bounded by 

the first and last samples collected; within the storm, 

discrete intervals are bounded by the midpoint (based 

on time) between successive samples.

Samples were individually analyzed and 

constituent loads were computed from the product of 

the constituent concentrations and the volume of flow 

occurring during the interval of the storm represented 

by the sample. Each discrete load represents the 

quality of the flow for a part of the storm hydrograph. 

These loads were then summed to determine the total 

constituent load for the storm.

Loads for 19 constituents, along with the 

median value for each, were computed at the inflow 

and outflow sites for the six storms, and are presented 

in tables 3 and 4. Loads were computed from 

discharge data reported to two significant figures and 

concentration data reported to one, two, or three 

significant figures, depending on the constituent and 

the analytical method used. Load computations based 

on concentrations reported to one significant figure, 

are reported to two significant figures in tables 3 and 4. 

In all other cases, loads are reported to three 

significant figures. Some constituents in some storms 

had concentrations below analytical detection limits. 

In these cases, the detection limits were used to 

compute a maximum possible load. These loads are 

indicated by a "less than" symbol in tables 3 and 4. 

The presence of these censored data points made it 

necessary to use median, rather than mean, values to 

summarize the load data (tables 3 and 4).

Table 2. Hydraulic characteristics of the monitored storms on Saint Joe Creek, Pinellas Park, Pinellas County, Fla.

[Rainfall data from 8-inch recording raingage at the inflow site]

Rainfall

Storm

A

B

C

D

E

F

Date

09-25-89

12-08-89

02-23-90

10-11-90

04-24-91

08-19-91

Total 

(inches)

1.44

1.69

2.66

1.54

1.00

1.43

Duration 

(hours)

28.0

15.0

13.5

12.0

4.5

13.0

Intensity 

(inches per 

hour)

0.05

.11

.20

.13

.22

.11

Ten-day 

antecedent

1.96

.01

.18

2.12

.37

.82

Inflow site runoff

(thousands 

of 

cubic feet)

2,510

4,100

4,580

1,060

1,350

3,930

(inches)

0.54

.88

.99

.23

.29

.85

Outflow site runoff

(thousands 

of 

cubic feet)

3,830

5,240

4,620

1,570

1,210

7,050

(inches)

0.65

.88

.78

.27

.20

1.19

Volume ratio

outflow/ 

inflow

1.53

1.28

1.01

1.48

0.90

1.79

Constituent Loads
75

o 

o

LLJ

DC 

LJU 

Q.

hLJJ 

LLJ 

LL

O 

CO

=)

O

LLJ 

O 

DC 

<

I 

O

c/2

Q

50

25 

SAMPLE SELECTED FOR 

CHEMICAL ANALYSIS

2400 0800 1600 

FEB. 22, 1990

2400 0800 1600 

FEB. 23, 1990

2400 0800 1600 

FEB. 24, 1990

2400 0800 1600 

FEB. 25, 1990

2400

24-HOUR TIME

Figure 2. Discharge hydrograph for typical storm C and samples selected for chemical analysis at outflow site (02308935) on 

Saint Joe Creek, Pinellas Park, Pinellas County, Fla.

The greatest constituent loads contained in stormwater at both the inflow and outflow sites are associated 

with naturally occurring dissolved major ions, represented in this study by chloride, calcium, and bicarbonate as well as the dissolved solids residue. Nevertheless, 

relatively small loads of metals, nutrients, and 

suspended solids and the presence of oxygen-consuming substances are often environmentally significant. 

These 15 constituents are collectively referred to in this 

report as "urban-runoff contaminants."

Many constituents in stormwater runoff are also 

in base flow. In addition to the stormwater samples 

collected for this study, 13 base-flow samples were 

collected from December 1986 to July 1991 at the 

inflow site and 16 samples were collected from 

November 1984 to May 1991 at the outflow site. 

Because base-flow quantity and chemical quality are 

relatively constant, daily base-flow load was computed 

as a simple product of the concentrations in a single 

sample and the daily volume of flow for the date of 

sampling. Base-flow volumes were available from the 

record of daily discharge at the gaging stations or, in 

only one case, from an instantaneous measurement of

discharge. The minimum, maximum, and median daily 

constituent loads computed from base-flow samples at 

the inflow site are shown in table 5. Only data from the 

inflow site are presented to avoid the effect of the 

detention pond on the constituent loads. (Data from 

the outflow site are presented in table 10 and are 

discussed elsewhere in this report.)

Constituent concentrations in stormwater and 

base flow at the inflow site are summarized in table 6. 

The data for stormwater are event-mean 

concentrations calculated from load and runoffvolume data (tables 3 and 2) for each of the storms; the 

data for base flow are based on the concentrations of 

individual samples. Median concentrations of urbanrunoff contaminants in stormwater are generally 

greater than in base flow (table 6); furthermore, 

instantaneous concentrations of these constituents in 

stormwater are often significantly greater than the 

mean-event concentrations shown in table 6. The 

relatively high concentrations, coupled with increased 

volumes of flow during storms, result in stormwater 

constituent loads that may be as much as 60 times 

greater than comparable base-flow loads.

6 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Table 3. Computed loads of selected water-quality constituents in stormwater at the inflow site on Saint Joe Creek, Pinellas 

Park, Pinellas County, Fla.

[ , no data; <, maximum possible load based on concentration below analytical detection limit;°C, degrees Celsius]

Constituent

Major ions (dissolved) 

Chloride 

Calcium 

Bicarbonate

Storm (loads in kilograms)

A

803 

2,350

B

1,740 

3,950

C

1,300 

3,530

D

398 

953 

2,630

E

543 

1,270 

3,500

F

1,100 

3,520 

10,100

Median

952 

2,940 

3,500

Solids, residue

at 180 °C 9,240 

Urban Runoff Contaminants

Metals (total recoverable): 

Aluminum

Chromium .33 

Copper .32 

Lead .52 

Mercury <.02 

Zinc 3.35

Nutrients (total):

Ammonia (as nitrogen) 10.6 

Nitrite plus nitrate 12.7 

Organic nitrogen 44.9 

Phosphorous 7.08 

Orthophosphate 2.73 

(as phosphorus)

Suspended volatile solids 300

Total suspended solids, residue

at 105 °C 816

Biochemical oxygen demand 308

Chemical oxygen demand 1,910

17,600 13,500 3,740

23.7

.24

.47

1.18

33.9

.56

.81

1.19

4.76

1,180

1,900

4.90

4.85

12.2

82.3

21.8

11.1

780

2,240

381

3,830

2.65 

<.03 

<.03

.08 

<.004

.62

0.56

1.11

22.7

4.88

3.00

120

126

<729

5,230

12.3 

.09 

.16 

.45

<.004 

7.36

4.05

6.73

50.4

7.96

2.64

330

790

227

1,470

13,900

8.27 

.20 

.29

.32

22.0

31.2

6.91

80.8

9.77

6.04

190

710

356

3,060

11,400

12.3

.22

.30

.48

<.01

4.83

4.85

6.91

50.4

7.96

3.00

320

803

308

1,910

Lopez and Giovannelli (1984, p. 68) used a set of 

regression equations to estimate that stormwater loads 

make up 67 to 78 percent of the annual load of selected 

contaminants transported in Saint Joe Creek. The available data from the present study could not be used to 

confirm their estimate because the range in volume of 

the 6 monitored storms was not representative of 20

o

small storms (flow volumes less than 1,000,000 ft that 

occurred during the 2-year study period. Despite the 

inability to estimate loads for small storms, a minimum 

approximation of the annual stormwater loads for the 

urban-runoff contaminants at the inflow site can be 

computed as the product of the median storm loads 

(table 3) and the 19 storms per year that had flow volumes within the range of the monitored storms. Annual 

base-flow loads of the urban-runoff contaminants were 

computed as the product of median daily base-flow load 

(table 5) and 269 days of base flow per year during the 

study period. Based on these approximate annual loads, 

stormwater runoff is estimated to contribute from 

54 percent (chemical oxygen demand) to 82 percent 

(lead) of the total load of urban-runoff contaminants at 

the inflow site. These minimum percentages support the 

estimates made by Lopez and Giovannelli (1984).

DETENTION POND EFFICIENCY FOR 

STORMWATER

A common method used to quantify the 

efficiency of a treatment facility, in this case the 

detention pond, is to divide the change in load by the 

inflow load (Martin and Smoot, 1986); the result is 

expressed as a percent. As an example, if the load of a 

constituent entering the pond were 100 kg and the load 

leaving the pond were 50 kg, the efficiency of the pond 

for that constituent for that particular storm would be 

50 percent. An efficiency of 100 percent is the 

theoretical maximum that could be attained. On the 

other hand, if the load leaving the pond exceeded the 

load entering the pond, the efficiency would be 

expressed as a negative percentage and can exceed 

100 percent. Efficiencies cannot be computed when 

the inflow load (table 3) is a "less-man" value because 

the mathematical expression (Inflow-Outflow)/Inflow 

has no rational solution. However, a minimum 

efficiency can be computed when only the outflow 

load is a "less-than" value.

Detention Pond Efficiency for Stormwater
Table 4. Computed loads of selected water-quality constituents in stormwater at the outflow site on Saint Joe Creek, Pinellas 

Park, Pinellas County, Fla.

[--, no data; <, maximum possible load based on concentration below analytical detection limit; °C, degrees Celsius]

Constituent

Major ions (dissolved): 

Chloride 

Calcium 

Bicarbonate

Storm (loads in kilograms)

A

1,390 

4,110

B

2,810 

6,470

C

1,710 

4,300

D

624 

1,630 

4,570

E

626 

1,460 

4,330

F

2,580 

7,130 

20,600

Median

1,550 

4,200 

4,570

Solids, residue at

180 °C 17,000 28,600 17,100 6,260 6,000 29,100 17,000 

Urban Runoff Contaminants

Metals (total recoverable):

Aluminum - 49.0 45.1 1.56 1.95 9.66 9.66 

Chromium .47 .23 .38 <.04 .04 .33 .28 

Copper .28 .45 .39 <.05 <.03 .23 .26 

Lead .35 .72 .63 <.04 <.03 <.20 <.28 

Mercury <.01 <.15 <.02 <.01 <.01 <.02 <.02 

Zinc 2.73 3.08 3.15 .66 <.34 4.59 2.90

Nutrients (total):

Ammonia (as nitrogen) 9.68 - 4.51 .90 <.44 17.7 4.51 

Nitrite plus nitrate 57.8 - 9.44 1.78 <.69 <3.99 <3.99 

Organic nitrogen 82.4 -- 81.3 21.2 47.8 118 81.3 

Phosphorus 9.43 - 16.0 2.49 4.06 10.5 9.43 

Orthophosphate 4.12 - 8.24 1.09 .43 5.16 4.12 

(as phosphrus)

Suspended volatile solids 730 1,400 810 <44 220 290 510

Total suspended solids, 2,140 3,170 3,110 <44 401 714 1,430 

residue at 105 °C

Biochemical oxygen 

demand

Chemical oxygen demand

188

3,340

429

5,300

88.8

<44

104

653

417

2,960

188

2,960

Measured Pond Efficiency

Individual storm efficiencies based on the loads 

computed from data at the inflow and outflow sites for 

the 19 constituents are listed in table 7. About half the 

efficiencies in table 7 are negative; that is, the load 

leaving the pond was more than the load entering the 

pond. Additional loading to the pond is to be expected 

because of the unmonitored inflows from the intervening area (table 1) and precipitation directly on the 

pond. Stormwater runoff from the 0.51 mi area surrounding the detention pond is collected by stormwater sewers that flow directly into the pond. 

Precipitation falling directly on the 0.04 mi pond also 

contributes to runoff and loads at the outflow site.

The significance of the load contribution from 

the intervening area (including precipitation on the 

pond) can be recognized by comparing the volume of 

flows at the inflow and outflow sites for the six storms 

(table 2). Measured outflow volume exceeded inflow 

volume in all cases except for storm E. Storm E also 

had the fewest observed negative efficiencies of any

monitored storm, doubtless because the intervening 

area contributed little or no flow and associated load. 

For the other storms, the outflow volume was 1.01 to 

1.79 times greater than the inflow volume, with a 

mean ratio for all monitored storms of 1.33. By way of 

comparison, the drainage area at the outflow site is 

1.28 times greater than the drainage area at the inflow 

site (table 1).

Because the measured changes in constituent 

loads (table 7) are based on incomplete inflow data, 

they do not accurately describe the efficiency of the 

detention pond in reducing constituent loads contained 

in stormwater. Mean-event concentrations of urbanrunoff contaminants in stormwater leaving the pond 

(outflow site) are generally less than concentrations at 

the inflow site (table 8), suggesting that these constituents are being removed or diluted within the pond. To 

estimate the true efficiency of the pond, measured 

inflow loads must be adjusted to account for the load 

from the unmonitored intervening area.

8 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Table 5. Daily base-flow loads of selected water-quality constituents at the inflow site on Saint Joe Creek, Pinellas Park, 

Pinellas County, Fla.

[<, maximum possible load based on concentration below analytical detection limit; °C, degrees Celsius]

Constituent

Major ions (dissolved):

Chloride

Calcium

Bicarbonate

Solids, residue at 180 °C

Urban runoff contaminants

Metals (total recoverable):

Aluminum

Chromium

Copper

Lead

Mercury

Zinc

Nutrients (total):

Ammonia (as nitrogen)

Nitrite plus nitrate

Organic nitrogen

Phosphorous

Orthophosphate (as phosphorus)

Suspended volatile solids

Total suspended solids, residue at 105 °C

Biochemical oxygen demand

Chemical oxygen demand

Number of 

samples

12

12

8

13

9

13

13

13

12

13

11

11

11

11

11

12

13

11

12

Minimum

32.3

38.1

250

367

0.080

<.004

.002

<.004

<.001

.002

.040

.060

1.37

.100

.032

2.01

4.01

2.81

<41.6

Maximum

Load in kilograms per day

102

254

535

1040

0.450

.025

.068

.064

.005

.196

.954

1.14

4.517

.587

.308

50.9

95.4

20.0

604

Median

59.5

145

404

616

0.318

.010

.010

.008

<.001

.078

.206

.254

2.70

.343

.159

18.6

29.4

7.49

117

Table 6. Concentrations of selected water-quality constituents in stormwater and base flow at the inflow site on Saint Joe Creek, 

Pinellas Park, Pinellas County, Fla.

[data for stormwater are event-mean concentrations; <, concentration below analytical detection limit (baseflow) or maximum possible mean-event 

concentration based on one or more individual concentrations below analytical detection limit (stormwater); X, not applicable; °C, degrees Celsius; 

discharge in cubic feet per second (cfs)]

Stormwater

Constituent or characteristic um e.r ° samples

Major ions (dissolved):

Chloride

Calcium

Bicarbonate

Solids, residue at 180 °C

Urban runoff contaminants

Metals (total recoverable):

Aluminum

Chromium

Copper

Lead

Mercury

Zinc

Nutrients (total):

Ammonia (as nitrogen)

Nitrite plus nitrate

Organic nitrogen

Phosphorous

Orthophosphate

(as phosphorous)

Suspended volatile solids

Total suspended solids,

residue at 105 °C

Biochemical oxygen demand

Chemical oxygen demand

Discharge (cfs)

6

6

3

6

5

6

6

6

6

6

5

5

5

5

5

6

6

5

5

X

f Minimum Maximum Median 1

Concentration in 

milligrams per liter

10

27

81

104

.07

<.001

<.001

.003

<.0001

.02

.02

.04

.63

.09

.04

2

4

2.9

<24

X

15

34

92

152

.30

.005

.006

.012

<.0003

.20

.28

.18

1.3

.21

.10

10

21

5.9

38

X

12

32

91

128

.20

.002

.004

.008

<.0001

.04

.11

.09

.73

.16

.07

5

14

4.2

27

X

Mumber ol 

samples

12

12

8

13

9

13

13

13

12

13

11

11

11

11

11

12

13

12

12

13

F Minimum

Base flow

Maximum Median

Concentration in 

milligrams per liter

14

19

90

125

.04

<.001

<.001

<.001

<.0001

.01

.02

<.02

.55

.05

<.01

1

2

1.4

<10

.82

22

48

126

214

1.0

.008

.012

.010

.0022

.04

.27

.42

1.3

.15

.10

10

39

5.1

190

2.6

16

38

106

154

.08

.003

.002

.002

<.0001

.02

.07

.08

.67

.08

.03

4

8

1.9

42

1.6

Detention Pond Efficiency for Stormwater 9
Table 7. Pond efficiencies for selected water-quality constituents in stormwater based on loads measured at the inflow and 

outflow sites on Saint Joe Creek, Pinellas Park, Pineallas County, Fla.

[individual storm efficiencies are equal to (inflow load-outflow load)/inflow load and are expressed as a percent; >, minimum value because outflow 

concentration was below analytical detection limit; X, efficiency not determined because inflow concentration was below analytical detection limit; --, no 

data; °C, degrees Celsius ]

Constituent

Major ions (dissolved):

Chloride

Calcium

Bicarbonate

Solids, residue at 180°C

Urban runoff contaminants

Metals (total recoverable):

Aluminum

Chromium

Copper

Lead

Mercury

Zinc

Nutrients (total):

Ammonia (as nitrogen)

Nitrite plus nitrate

Organic nitrogen

Phosphorous

Orthophosphate (as phophorous)

Suspended volatile solids

Total suspended solids, residue at 105 °C

Biochemical oxygen demand

Chemical oxygen demand

Storm

A

-73

-75

-

-84

-

-42

12

33

X

18

9

-355

-84

-33

-51

-143

-162

39

-75

B

-61

-64

--

-62

-107

4

4

39

X

35

-

-

-

-

-

-19

-67

-

-

C

-32

-21

-

-27

-33

32

52

47

X

36

7

23

1

27

26

-4

-39

-13

-38

D

-57

-71

-74

-67

43

X

X

>50

X

-6

-61

-60

7

49

64

>51

>63

30

X

E

-15

-15

-24

-15

84

55

>81

>93

X

>95

>89

>90

5

49

84

33

49

54

56

F

-135

-103

-104

-109

-17

-65

21

>38

X

79

43

>42

-46

-7

>15

-53

-1

-17

3

Median 

value

-59

-68

-74

-64

-17

4

21

43

X

36

9

23

1

27

26

-12

-20

-30

-18

Table 8. Mean-event concentrations of selected water-quality constituents in stormwater at the inflow and outflow sites on 

Saint Joe Creek, Pinellas Park, Pineallas County, Fla.

[<, maximum possible mean-event concentration based on one or more individual concentrations below analytical detection limit; °C, degrees Celsius]

Inflow site Outflow site

Constituent

Major ions (dissolved):

Chloride

Calcium

Bicarbonate

Solids, residue at 180°C

Urban runoff contaminants

Metals (total recoverable):

Aluminum

Chromium

Copper

Lead

Mercury

Zinc

Nutrients (total):

Ammonia (as nitrogen)

Nitrite plus nitrate

Organic nitrogen

Phosphorous

Orthophosphate

(as phosphorous)

Suspended volatile solids

Total suspended solids, residue

at 105 °C

Biochemical oxygen demand

Chemical oxygen demand

Number of 

samples

6

6

3

6

5

6

6

6

6

6

5

5

5

5

5

6

6

5

5

Minimum Maximum Median

Concentration in 

milligrams per liter

10

27

81

104

.07

<.001

<.001

.003

<0001

.02

.02

.04

.63

.09

.04

2

4

2.9

<24

15

34

92

152

.30

.005

.006

.012

<.0003

.20

.28

.18

1.3

.21

.10

10

21

5.9

38

12

32

91

128

.20

.002

.004

.008

<0001

.04

.11

.09

.73

.16

.07

5

14

4.2

27

Number of 

samples

6

6

3

6

5

6

6

6

6

6

5

5

5

5

5

6

6

5

5

Minimum Maximum Median

Concentration in 

milligrams per liter

13

33

103

131

.04

<.001

<.001

<.001

<.0001

<01

<02

<.02

.48

.05

.01

<1

<1

1.7

<1

19

44

126

193

.34

.004

.003

.005

<001

.03

.09

.53

1.4

.12

.06

9

24

3.3

41

14

38

103

152

.06

.002

.002

.002

<0002

.02

.04

.04

.62

.09

.03

6

16

2.1

19

10 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Adjusted Estimates of Pond Efficiency Factors Affecting Constituent-Load Changes

In a study of urban streamflow in the Tampa Bay 

area, Lopez and Giovannelli (1984) observed that 

stormwater volume was the most significant variable in 

predicting contaminant loads in stormwater. Their study 

was based on a regression analysis of rainfall, runoff, 

and water-quality data for nine urban drainage basins, 

including Saint Joe Creek. The constituents included as 

independent variables in the regression analysis were 

total recoverable lead, total organic nitrogen, total 

nitrogen (the sum of the nitrogen species analyzed for in 

this study), total phosphorus, biochemical oxygen 

demand, and chemical oxygen demand. Because 

stormwater volume has been shown to be a reasonable 

predictor of stormwater loads, the measured constituent 

loads at the inflow site were volume adjusted to permit a 

comparison with loads at the outflow site. The premise 

behind this adjustment is that, for an individual storm, 

the volume of flow into the pond should be equal to the 

volume of flow out of the pond.

Load data for the inflow site (table 3) were multiplied by the volume ratio for the appropriate storm 

(table 2) to compute an adjusted load that is assumed to 

represent the total constituent loading to the pond 

including the estimated load from the unmonitored 

intervening area. Individual storm efficiencies for the 

19 constituents, based on the adjusted inflow loads and 

the measured outflow loads, are listed in table 9. Because 

there is no way to verify the accuracy of the volumeratio adjustment, and because of the inherent, but 

unknown, errors in the basic computation of loads, pond 

efficiencies in table 9 are considered to be estimates.

The adjusted storm efficiencies listed in table 9 

are more consistent (have a smaller range) among 

storms for each constituent than the unadjusted values 

in table 7 and have fewer negative values. Median 

values are used to summarize the pond efficiency for 

each constituent. The median, rather than the mean, 

was used because of the relatively large number of 

censored data points ("less-than"' values) and because 

it minimizes the effect of outlier data points. Although 

more vigorous methods have been used to mathematically describe constituent-load efficiencies (Martin 

and Smoot, 1986), the relatively small number of data 

sets available for the present study does not justify 

their use. Too, the logic behind the zero-intercept 

regression method used by Martin and Smoot (1986, 

p. 22) is invalidated by the unmonitored intervening 

area and the presence of a base-flow component in the 

stormwater runoff at detention area 3. For this reason, 

median values are believed to adequately describe the 

overall performance of the detention pond.

Data in table 9 indicate that loads of chloride, 

calcium, bicarbonate, and dissolved solids leaving the 

pond consistently exceeded the loads entering the 

pond. These constituents should be little changed by 

the processes operating within the pond. In particular, 

dissolved chloride in water is highly stable and generally does not react or form solute complexes with 

other ions, does not form salts, is not absorbed on 

mineral surfaces, and is not involved in biochemical 

reactions (Hem, 1985, p. 118). Because of the conservative nature of chloride in water, it is reasonable to 

assume that the total loads entering and leaving the 

detention pond should be about the same (ignoring the 

negligible contribution directly from rain on the pond). 

Other studies (for example, Martin and Smoot, 1986, 

p. 27) have determined that detention ponds have little 

or no effect on chloride loads. Therefore, the median 

increase in chloride load of 28 percent (table 9) is of 

interest because of what it may indicate concerning 

factors affecting constituent-load changes in the pond.

Errors in Load Adjustment

One factor contributing to the greater than 

expected loads of chloride and other major ions at the 

outflow site may be that loads entering the pond from 

the unmonitored intervening area are underestimated 

by the volume-ratio method of load adjustment. An 

underlying assumption of the volume-ratio method is 

that the constituent load generated from the 

intervening area is similar to that generated from the 

area monitored at the inflow site. Although this is a 

reasonable assumption based on the general land-use 

characteristics of the two areas (table 1), the 

assumption was not verified because of the difficulty 

of instrumenting and sampling the sewers that 

discharged stormwater directly into the pond from the 

intervening area. If the inflow loads of the major ions 

were underestimated by the volume-ratio adjustment, 

the inflow loads of the urban-runoff contaminants 

(metals, nutrients, suspended material and oxygenconsuming substances) also might be underestimated. 

In this case, the efficiencies for the urban-runoff 

contaminants shown in table 9 might be too low. There 

are, however, other explanations for the increased 

loads of the major ions (discussed below) so that it is 

not possible to quantify the possible errors or 

uncertainties in table 9 due to underestimation of 

inflow loads.

Detention Pond Efficiency for Stormwater 11
Table 9. Estimated pond efficiencies for selected water-quality constituents in stormwater based on adjusted inflow loads and 

measured outflow loads on Saint Joe Creek, Pinellas Park, Pinellas County, Fla.

[individual storm efficiencies are equal to (inflow-outflow)/inflow and are expressed in percent; efficiencies are calculated using inflow loads adjusted by the 

storm-volume ratio (table 2); >, minimum value because outflow concentration was below analytical detection limit; X, efficiency not determined because 

inflow concentration was below analytical detection limit; , no data; °C, degrees Celsius]

Constituent

Major ions (dissolved):

Chloride

Calcium

Bicarbonate

Solids, residue at 1 80 °C

Urban runoff contaminants

Metals (total recoverable):

Aluminum

Chromium

Copper

Lead

Mercury

Zinc

Nutrients (total):

Ammonia (as nitrogen)

Nitrite plus nitrate

Organic nitrogen

Phosphorous

Orthophosphate (as phosphorous)

Suspended volatile solids

Total suspended solids, residue at 1 05 °C

Biochemical oxygen demand

Chemical oxygen demand

Storm

A

-13

-14

-20

7

435'6

X

47

40

-197

-20

13

1

-59

-71

60

-14

B

-26

-28

-27

-61

25

25

52

X

49

-

-

-

-

-

7

-30

-

--

C

-30

-21

-25

-32

33

52

48

X

36

8

23

2

27

27

-3

-37

-11

-37

D

-6

-16

-17

-13

60

X

X

>66

X

28

-9

-8

37

66

75

>67

>75

52

X

E

-28

-28

-37

-27

82

51

>79

>93

X

>95

>88

>89

-5

43

82

26

44

49

51

F

-31

-13

-14

-17

35

8

56

>65

X

88

68

>68

18

40

52

15

44

35

46

Median 

value

-28

-18

-17

-22

35

25

52

>60

X

48

40

23

2

40

52

11

7

49

16

Flushing of Stored Base Flow

The detention pond contains more than 

1,250,000 ft of water in storage between periods of 

stormflow. Because the average rate of base flow at the

-5

inflow site is about 150,000 ft /d, stormwater remaining in the pond after a storm should be replaced by 

base flow in about 8 days. Chloride and the other 

major ions are more concentrated in base flow than in 

stormwater (table 6). Therefore, when base flow stored 

in the pond is flushed out during a storm, water at the 

outflow site should be more concentrated in the major 

ions than water at the inflow site (which will be largely 

stormwater). As shown in table 8, mean-event concentrations of the major ions at the outflow site exceed 

concentrations at the inflow site. In fact, the median 

values in tables 6 and 8 show that the concentrations 

of chloride, calcium, bicarbonate and dissolved solids 

in stormwater at the outflow site (14, 38, 103, and 

152 mg/L respectively) are more similar to the 

concentrations in base flow at the inflow site (16, 38, 

106, and 154 mg/L) than they are to stormwater at the 

inflow site (12, 32, 91, and 128 mg/L). This similarity 

exists despite the indication, discussed in a later

section, that these constituents may be somewhat 

reduced in the pond under base-flow conditions.

The effect of the flushing of stored base flow on 

concentrations and loads at the outflow site is mitigated by conditions which often prevent the complete 

replacement of stormwater by base flow in the interval 

between successive storms. Base-flow conditions 

preceding some storms existed for too short a period to 

completely replace stormwater stored in the pond. 

Storms A and D had the most significant antecedent 

precipitation of any of the monitored storms (table 2) 

and also had the smallest chloride-load increases 

(table 9). Also, the effect of stored base flow is less 

pronounced for storms with flow volumes that exceed 

the normal volume of base flow stored in the pond 

(1,250,000 ft3 ). For these larger storms, the concentrations of the major ions in stored base flow would be 

diluted by stormwater. Nevertheless, the storage and 

subsequent flushing of base flow from the pond might 

largely explain the negative efficiency of the pond 

with regard to chloride and other major ions.

To the extent that stored base flow might be 

responsible for the increase in concentrations and 

loads of the major ions and dissolved solids at the

12 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
outflow site, it might also be partly responsible for the 

decrease in stormwater concentrations and loads of 

metals, nutrients, suspended solids, and biochemical 

and chemical oxygen demand (tables 8 and 9). These 

urban-runoff contaminants are less concentrated in 

base flow than they are in stormwater (table 6), so that 

base flow flushed from the pond during storms might 

decrease the load at the outflow site relative to the load 

at the inflow site. Another result of the flushing of base 

flow during storms is to moderate the peak concentration of urban-runoff constituents downstream from the 

detention pond. Tables 6 and 8 show that concentrations of most constituents in water released from the 

pond (outflow site) are closer to the concentrations 

found in base flow than they are to concentrations in 

stormwater (inflow site).

The effect of stored base flow on pond 

efficiency is more pronounced for smaller storms in 

which base flow is a more significant component of 

flow at the outflow site. The two smallest storms 

monitored, storms D and E, generally showed the 

greatest removal (positive efficiency) of urban-runoff 

contaminants (table 9). Of the 14 urban-runoff 

contaminants for which efficiencies were calculated, 9 

had the greatest load reduction in storm E and 4 in 

storm D. Three of the four major ions had the greatest 

load increase in storm D, further supporting the 

significant role of stored and subsequently flushed 

base flow in affecting loads at the inflow site, 

particularly for smaller storms.

Dilution of stormwater by stored base flow 

reduces peak concentrations and loads in Saint Joe 

Creek downstream from the detention pond; however, 

it does not affect the long-term mass balance of 

constituents entering and leaving the pond. Dissolved 

constituents are not removed by this process but rather 

are simply redistributed in time. Any stormwater (with 

higher concentrations of urban-runoff contaminants) 

that is stored within the pond after storms would 

subsequently be flushed from the pond by base flow. 

There is, however, some evidence to suggest that other 

processes active in the detention pond may have a 

more permanent effect on constituent loads.

Settling and Biochemical Processes

As described in a later section, the relatively 

greater loads of urban-runoff contaminants anticipated 

to occur in base flow at the outflow site following 

storms were not observed and, in fact, constituent 

loads within the pond under base-flow conditions

appear to have been appreciably reduced. The 

processes assumed to be responsible for these 

reductions in base-flow loads are adsorption, chemical 

precipitation, settling, biologic uptake, and oxidation. 

These processes, referred to collectively in this report 

as settling and biochemical processes, are also 

assumed to be active in the pond during and 

immediately following storms. Settling and 

biochemical processes are probably least significant in 

reducing constituent loads for small storms (when 

dilution plays a major role) and are most significant 

during larger storms, such as storms A, B, C and F 

with flow volumes 3 to more than 5 times greater than 

the volume of water stored in the pond. Settling and 

biochemical processes result in permanent load 

reductions only to the extent that urban-runoff 

contaminants in biota and sediments are not recycled 

by decay of vegetation, resuspension of sediments, 

and mobilization of adsorbed ions.

Vegetation

Excluding the low-volume storms D and E 

during which dilution was probably a major factor, 12 

of the 14 urban-runoff contaminants for which 

efficiencies were calculated had the greatest load 

reductions during storm F (table 9). Storm F had the 

highest volume of flow at the outflow site of any of the 

monitored storms so that load reduction due to dilution 

was relatively low and settling and biochemical 

processes were largely responsible for the significant 

load reductions. What further distinguishes storm F 

from the other high-volume storms (A, B, and C) and 

what may explain the higher rates of pond efficiency is 

that vegetation in the pond had become well established 

at the time of this storm whereas it was more sparse 

during the earlier high-volume storms. The presence of 

an extensive plant community in the pond obviously 

promotes biologic uptake of nutrients and possibly 

metals. Plants also oxygenate the water which helps 

reduce biochemical and chemical oxygen demand. 

Additionally, plants decrease flow velocities in the pond 

which promotes the settling of suspended material and 

also increases residence time within the pond. Longer 

residence time, in turn, provides more opportunity for 

settling and biochemical processes to reduce loads.

During the first summer after construction of the 

pond, between storms C and D (table 2), vegetation 

became well established along the banks of the pond, 

on the fill material across the center of the pond, and in 

areas of shallow water throughout the pond. The

Detention Pond Efficiency for Stormwater 13
presence of a vegetative community during the final 

three monitored storms (D, E, and F) appears to have 

greatly improved the efficiency of the pond in removing urban-runoff contaminants (table 9).

Restricted Circulation

The northern half of the detention pond is nearly 

isolated from the through-flowing Saint Joe Creek during base-flow conditions (fig. 1). Removal of water by 

evaporation and plant transpiration in this part of the 

pond might result in the accumulation of water with 

somewhat higher concentrations of all dissolved constituents. This relatively enriched water would be 

flushed out of the pond during higher stages of flow 

associated with storms (when circulation between the 

two halves of the pond occurs) thus adding to the load 

measured at the outflow site. To the extent that this 

occurred, the apparent efficiency of the pond in reducing stormwater loads of urban-runoff contaminants 

could be increased by enhancing the circulation of 

water within the pond during base-flow conditions, 

thus minimizing the concentration of constituents by 

evaporation and transpiration. This would, by itself, 

have little or no effect on the total load transported by 

Saint Joe Creek, but it would help further reduce loads 

during and immediately after storms.

Load Reduction of Urban-Runoff 

Contaminants

In the following discussion, median values 

based on all six storms are used to describe the efficiency of the pond in reducing the loads of urban-runoff contaminants. Long-term efficiencies might likely 

be higher under conditions of mature vegetation and 

enhanced circulation. Also, no attempt is made to 

quantify the various physical, biologic and chemical 

processes assumed to be responsible for the observed 

reductions in stormwater load.

Metals

The pond removed metals at an estimated 

median efficiency ranging from 25 percent for chromium to greater than 60 percent for lead (table 9). Aluminum and zinc, the metals with by far the largest 

inflow loads (table 3), had estimated efficiencies of 35 

and 48 percent, respectively. Aluminum is the only 

metal with negative efficiencies for individual storms. 

With the lack of additional geochemical data, particularly pH values, the failure of the pond to consistently 

remove aluminum cannot be fully explained. However, for three of the five storms for which aluminum 

was monitored, a minimum of 35 percent of the aluminum load entering the pond was estimated to have 

been removed (table 9). Mercury was not present in 

sufficient concentration to compute loads. Loads of all 

other metals (chromium, copper, lead, and zinc) were 

consistently reduced in the pond, most likely as a 

result of dilution, adsorption, chemical precipitation, 

and the settling of the suspended phase.

Nutrients

Nutrients were removed at an estimated median 

efficiency ranging from 2 percent for organic nitrogen 

to 52 percent for orthophosphate. Pond efficiencies for 

the removal of nitrogen species in stormwater are 

highly variable, both from storm to storm for individual species, and species to species for individual 

storms (table 9). This likely reflects the complex 

chemistry of nitrogen and its occurrence in various 

oxidation states. The negative efficiencies for nitrite 

plus nitrate (-197 percent) and organic nitrogen 

(-20 percent), and the reduced positive efficiencies for 

phosphorus and orthophosphate for storm A (table 9) 

could be the result of suspected sources of contamination adjacent to the pond. In general, however, the 

loads of all nitrogen species were reduced in the pond, 

probably as a result of dilution and biologic uptake. 

Phosphorus and orthophosphate loads were reduced in 

all storms, probably due to chemical precipitation as 

well as dilution and biologic uptake.

Other Selected Chemical and Physical 

Constituents

The pond seems to function erratically as a 

settling basin. Loads of suspended volatile solids 

increased (negative efficiency) for two of the six 

storms, and loads of suspended total solids increased 

for three of the storms. Overall, suspended volatile 

solids and suspended total solids were removed at estimated median efficiencies of only 11 and 7 percent, 

respectively. However, efficiencies for these constituents were consistently positive for the later storms (D, 

E, and F), possibly reflecting more settling of 

suspended material, less resuspension of bottom sediments, and less erosion of bank and fill material coincident with the establishment of natural vegetation in 

and adjacent to the pond.

14 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Biochemical oxygen demand and chemical 

oxygen demand, commonly used as measures of total 

oxygen consuming substances in water and good relative indicators of environmental stress from organic 

material, were reduced at estimated median efficiencies of 49 and 16 percent, respectively. The significant 

reductions of the loads of these indicators in the pond 

for most storms (table 9) indicate biologic oxidation of 

organic material within the pond.

DETENTION POND EFFICIENCY FOR 

BASE FLOW

Although stormwater runoff can often "shock" 

receiving waters because of the delivery of large contaminant loads within relatively short periods of time, 

base-flow loads are, nevertheless, also of environmental significance. As discussed in this report and Lopez 

and Giovannelli (1984), base-flow loads of contaminants make up at least 20 percent of the estimated 

annual contaminant load transported in Saint Joe 

Creek.

Detention area 3 functions as a modified 

wetland during base-flow conditions because of the 

relative shallowness of the water and the presence of 

cattails and other emergent vegetation. The wetland 

characteristics of the detention pond were best developed after the first summer of operation (between 

storms C and D). Data developed by Martin and 

Smoot (1986) indicate that a small, natural wetland in 

central Florida was more effective in reducing constituent loads than a relatively deep detention pond. 

Therefore, constituent loads in base flow entering 

detention area 3 are expected to be reduced by the 

physical, chemical, and biologic processes active in 

the shallow water, wetlandlike environment of the 

pond.

Base-flow loads at the inflow and outflow sites 

cannot be compared directly in time because the

-5

volume of storage in the pond (1,250,000 ft ) far

o

exceeds the daily inflow rate (150,000 ft /d). Typical 

base-flow detention time of the pond is about 8 days, 

although it may be as little as 4 days if circulation to 

the northern half of the pond is restricted. The 

method chosen to estimate the efficiency of the pond 

in reducing base-flow loads was to compare loads at 

the outflow site before and after construction of the 

pond.

Nine base-flow samples were collected at the 

outflow site between November 1984 and April 1988

prior to construction, and seven samples were 

collected between September 1989 and May 1991 

after construction was completed. The minimum, 

maximum, and median daily constituent loads for the 

pre- and post-construction periods are shown in table 

10. The median values were used to compute apparent 

base-flow efficiencies by dividing the difference in 

median load (preconstruction minus postconstruction) 

by the preconstruction median load. The results, 

expressed as a percentage, also are presented in 

table 10.

Based on the data in table 10, constituent loads 

in base flow appear to be significantly reduced in the 

detention pond. The method of comparison presented 

in table 10 assumes that pre- and post-construction 

conditions were similar except for the presence of the 

pond. Changing conditions that could appreciably 

affect constituent loads generated within the drainage 

basin, and therefore invalidate the comparison made in 

table 10, are land use, streamflow rates, and chemical 

concentrations in base flow. Streamflow rates and 

chemical concentrations are, at least in part, controlled 

by land use. Because the basin experienced little or no 

new building development during the study period, 

reductions in base-flow load are not likely to be due to 

changes in basin characteristics.

Loads are a function of chemical concentration 

and rates of streamflow discharge. Although median 

discharges associated with the pre- and post-construction samples are similar, the range of flow is less in the 

postconstruction period (table 10). This might explain, 

at least in part, the apparent reduction in load. Constituent chemical concentrations, however, are also lower 

in the postconstruction period, suggesting that the positive efficiencies represent an actual reduction of load 

within the pond.

Unlike stormwater, base-flow loads of the major 

ions (chloride, calcium, bicarbonate and dissolved 

solids) apparently are reduced within the pond. Some 

of this reduction may be the result of mixing with 

residual stormwater in the pond. However, data 

presented by Martin and Smoot (1986, p. 49 and p. 51) 

show significant removal (positive median 

efficiencies) of major ions in the wetland treatment 

part of the detention pond-wetland system they 

studied. At least some of the load reduction of the 

major ions in detention area 3 may, therefore, be due 

to processes active in the wetlandlike environment that 

exists in the pond during base-flow conditions.

Detention Pond Efficiency for Base Flow 15
Table 10. Daily baseflow loads of selected water-quality constituents at the outflow site before and after construction of the 

detention pond on Saint Joe Creek, Pinellas Park, Pineallas County, Fla.

[load in kilograms per day; discharge in cubic feet per second (cfs)] <, maximum possible load based on concentration below analytical detection limit; 

no data; X, not determined; °C, degrees Celsius; apparent efficiency is equal to (median preconstruction load minus postconstruction load)/preconstruction 

load]

Constituent or ( 

characteristic

£

Major ions (dissolved):

Chloride

Calcium

Bicarbonate

Solids, residue at

180°C

Urban runoff contaminants

Metals (total recoverable):

Aluminum

Chromium

Copper

Lead

Mercury

Zinc

Nutrients (total):

Ammonia (as nitrogen)

Nitrite plus nitrate

Organic nitrogen

Phosphorous

Orthophosphate

(as phosphorous)

Suspended volatile solids

Total suspended solids,

residue at 105 °C

Biochemical oxygen

demand

Chemical oxygen demand

Discharge (cfs)

Preconstruction

Slumber 

of 

samples

3

4

5

9

8

8

8

8

8

9

9

9

9

9

3

9

9

8

9

Minimum

67.5

138

367

528

<.003

.004

.003

<.001

.049

.078

.039

1.97

.117

.059

17.6

5.35

4.14

4.40

1.1

Maximum

153

288

813

1376

--

.037

.024

.099

.002

.621

1.90

1.57

4.23

.621

.352

61.2

141

17.6

290

2.5

Median

108

218

553

830

--

.018

.013

.028

.001

.084

.391

.134

2.64

.274

.132

24.5

58.7

8.22

152

1.6

Number 

of 

samples

7

7

4

7

5

7

7

7

6

7

6

6

6

6

6

7

7

5

6

7

Postconstruction

Minimum

33.4

23.9

171

267

.064

<.003

<.002

.003

<.001

.016

.016

.048

.922

.042

.016

3.18

4.16

1.43

26.9

.65

Maximum

99.4

218

454

905

.440

<.023

.026

.019

.002

.139

.264

.165

3.35

.296

.176

46.4

204

12.3

207

1.9

Median '

78.8

163

359

686

.166

.009

.008

.005

<.001

.042

.065

.086

2.15

.150

.065

16.2

32.3

2.90

87

1.7

Apparent 

efficiency

27

25

35

17

50

38

82

X

50

83

36

19

45

51

34

45

65

43

--

All the urban-runoff contaminants, except for 

organic nitrogen and suspended volatile solids, had 

load-reduction efficiencies that exceeded the efficiencies of the major ions (table 10). Because 3 of the 7 

postconstruction base-flow samples were collected 

within 4 days of the preceding storm and another sample was collected within 8 days of the preceding storm, 

mixing of base flow with stormwater remaining in the 

pond might have been expected to increase, rather than 

decrease the load at the outflow site, at least for these 

four samples. This is because stormwater contains 

higher concentrations of urban-runoff contaminants 

than does base flow (table 6) and because the minimum time required to completely flush stormwater 

from the pond under base-flow conditions is 4 days. 

Nevertheless, there was no correlation between postconstruction loads at the outflow site and the duration

of base-flow conditions prior to sampling; the maximum load of 6 of the 15 urban-runoff contaminants 

occurred in the 3 base-flow samples collected within 

4 days of the preceding storm as compared to 7 of the 

15 maximum loads that occurred in 3 samples 

collected between 14 and 45 days after the preceding 

storm. Additionally, 14 of the 15 minimum constituent 

loads in postconstruction base flow at the outflow site 

occurred in samples collected after the summer of 

1990 when vegetation had become well established in 

the pond. Therefore, the relatively high efficiencies 

computed for metals, nutrients, suspended solids, biochemical oxygen demand and chemical oxygen 

demand, could be related to settling and biochemical 

processes (adsorption, chemical precipitation, biologic 

uptake and oxidation) which appear to be most active 

under wetlandlike conditions.

16 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida
Apparent efficiencies for the removal of constituent loads of urban-runoff contaminants in base flow 

(table 10) range from 38 to 82 percent for the metals, 

19 to 83 percent for the nutrients, 34 to 45 percent for 

suspended solids, 65 percent for biochemical oxygen 

demand, and 43 percent for chemical oxygen demand. 

Except for copper and orthophosphate, the base-flow 

efficiencies in table 10 exceeded the comparable 

stormwater efficiencies (table 9) for every constituent 

with available data. Therefore, the slow, shallow flow 

conditions and relatively long residence times associated with base flow in the detention pond are more 

effective in reducing constituent loads than the rapid, 

sometimes turbulent, deeper flow conditions and 

shorter residence times occurring during storms.

SUMMARY AND CONCLUSIONS

Streamflow and water-quality data were 

collected at the inflow and outflow of a multipurpose, 

wet detention pond on Saint Joe Creek, Pinellas 

County, Florida, in order to determine the 

effectiveness of the pond in reducing the load of 

urban-runoff contaminants. Loads for 19 constituents 

(4 major ions and 15 urban-runoff contaminants) in 

stormwater were computed for six storms at the inflow 

and outflow sites. The greatest loads contained in 

stormwater are associated with naturally occurring 

major ions, as indicated by the loads for chloride, 

calcium, bicarbonate, and dissolved solids. However, 

the relatively small loads of metals, nutrients, and 

suspended solids, and the presence of oxygenconsuming substances in stormwater runoff can be of 

environmental concern.

Because all the stormwater inflow to the detention pond was not measured at the inflow site, loads 

computed for the inflow site were adjusted to account 

for the estimated load from the unrnonitored area. 

Stormwater load data at the inflow site were adjusted 

by the ratio of the stormwater volume measured at the 

outflow and inflow sites. On the basis of the adjusted 

inflow data, it is estimated that the pond removes metals at median efficiencies ranging from 25 percent 

(chromium) to more than 60 percent (lead). Estimated 

median efficiencies of removal for nutrients in stormwater ranged from 2 percent (organic nitrogen) to 52 

percent (orthophosphate). Suspended solids were 

removed at estimated median efficiencies of 7 percent

(for volatiles) to 11 percent (for total), and biochemical and chemical oxygen demands had estimated 

median efficiencies of 49 and 16 percent, respectively. 

Efficiencies for the suspended solids were notably 

greater for the last three storms sampled, possibly 

because of the establishment of vegetation within the 

pond and on the exposed slopes.

Pond efficiencies for the reduction of base-flow 

constituent loads could not be determined from 

comparison of synchronous data at the inflow and 

outflow sites because the daily volume of base flow 

entering the pond only represents about 12 percent of 

the fair-weather volume of water stored in the pond. 

However, the apparent efficiency of the pond in 

reducing base-flow loads was estimated based on a 

comparison of base-flow loads at the outflow site prior 

to and after construction of the pond. The major ions, 

including dissolved solids, were reduced at apparent 

median efficiencies ranging from 17 percent 

(dissolved solids) to 35 percent (bicarbonate). 

Generally, the urban-runoff contaminants in base flow 

were reduced at greater apparent efficiencies, ranging 

from 38 percent (copper) to 82 percent (lead) for 

metals, 19 percent (organic nitrogen) to 83 percent 

(ammonia) for nutrients, 34 percent (volatiles) to 45 

percent (totals) for suspended solids, and 43 to 65 

percent for biochemical and chemical oxygen demand.

Despite the difficulty of making direct 

comparisons at the detention pond, constituent loads 

of both the stormwater and base-flow components of 

urban streamflow are clearly reduced within the pond. 

Dilution (mixing with base flow stored in the pond) is 

significant in reducing the stormwater loads of urbanrunoff contaminants in relatively small storms. 

Adsorption, chemical precipitation, settling, biologic 

uptake, and oxidation within the pond are important 

processes in the reduction of constituent loads during 

larger storms and during base-flow conditions. Settling 

and biochemical processes are typical of wetland 

environments; the establishment of aquatic vegetation 

appears to have increased the efficiency of the pond in 

reducing stormwater and base-flow constituent loads. 

Further reductions in the loads of urban-runoff 

contaminants leaving the pond during storms could 

possibly be attained by improving the circulation of 

base flow within the pond between storms.

Summary and Conclusions 17
REFERENCES CITED

Buchanan, T.J., and Somers, W.P., 1969, Discharge 

measurements at gaging stations: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 3, chap. A8, 65 p.

Carter, R.W., and Davidian, J., 1968, General procedure for 

gaging streams: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 3, chap. A6, 

13 p.

Fishman, M.J. and Friedman, L.C., 1989, Methods for 

determination of inorganic substances in water and 

fluvial sediments: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 5, chap. Al, 

545 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. 

Geological Survey Water-Supply Paper 2254, 264 p.

Kennedy, E.J., 1984, Discharge ratings at gaging stations: 

U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. A10, 59 p.

Lopez, M.A., and Giovannelli, R.F., 1984, Water-quality 

characteristics of urban runoff and estimates of annual 

loads in the Tampa Bay area, Florida, 1975-80: U.S. 

Geological Survey Water-Resources Investigations 

Report 83-4181,76 p.

Martin, E.H., and Smoot, J.L., 1986, Constituent-load 

changes in urban stormwater runoff routed through a 

detention pond-wetlands system in central Florida: 

U.S. Geological Survey Water-Resources 

Investigations Report 85-4310, 74 p.

McCuen, R.H., 1980, Water quality trap efficiency of storm 

water management basins: American Water Resources 

Association, Water Resources Bulletin, v. 16, p. 15-21.

18 Efficiency of a Stormwater-Detention Pond in Reducing Chemical Loads in Urban Streamflow, Pinellas County, Florida