Photo: Mill Road vegetated stormwater detention basin

U.S. Department of the Interior WRIR 01-4099

U.S. Geological Survey September 2001

Prepared in cooperation with the Monroe County Department of Health

Effects of a Vegetated Stormwater-Detention 

Basin on Chemical Quality and Temperature 

of Runoff from a Small Residential 

Development in Monroe County, New York

by Donald A. Sherwood

T he vegetated stormwater-detention basin at a small residential development in Monroe 

County, N.Y. has been shown to be effective in reducing loads of certain chemical 

constituents to receiving waters. Loads of suspended solids, nitrogen, and phosphorus have 

been reduced by an average of 14 to 62 percent. The basin has little effect on the temperature of runoff 

between the inflow and the outflow; water temperatures at the outflow during summer storms averaged 

0.5o C higher than those at the inflow.
Base from U.S. Geological Survey

State base map 1:500,000, 1974

0

0

1 2 3 MILES

1 2 3 KILOMETERS

EXPLANATION

BASIN BOUNDARY 

WEBSTER

EAST

ROCHESTER

PITTSFORD

FAIRPORT

ROCHESTER

Genesee River Basin

Mendon Ponds

County Park

VICTOR

MONROE

COUNTY

COUNTY

ONTARIO

MONROE COUNTY

WAYNE COUNTY

Thornell Road

Mill

East

Street

Road

NOT TO SCALE

Lawton

Drive

Housing

development

Mill Road

detention

basin

Mill Creek

Irondequoit

Creek

IRONDEQUOIT

BAY

LAKE ONTARIO

Genesee River

Thomas Creek

(Barge)

Canal

Allen

Creek

Erie

Creek

Iron deq uoit

East Br. Allen

Creek

77° 37' 30" 30'

43°15'

10'

05'

43°

42°

55'

22' 30"

Figure 1. Irondequoit Creek drainage basin and location of Brookside Meadows development and Mill Road artificial wetland, Monro

County, New York

2

Runoff from storm events can cause 

flooding and can transport contaminants 

such as lead, zinc, nutrients, and 

sediment to streams and lakes. Urban 

and suburban development increases the 

amount of impervious area within the 

watershed and thereby increases the 

volume of storm runoff and loads of 

contaminants that discharge to local 

streams. Continued development in the 

Irondequoit Creek watershed (fig. 1) 

often conflicts with the efforts of 

watershed managers concerned with 

water-quality issues. 

Stormwater-detention basins and 

constructed (artificial) wetlands have 

been shown to mitigate the adverse 

effects of storm runoff on stream-water 

quality (Zarriello and Sherwood,1992) 

and have been widely used in varying 

forms throughout Monroe County. 

Stormwater-detention basins are 

INTRODUCTION
Figure 2. Aerial photo of Brookside Meadows residential 

development showing drainage area of the Mill Road 

artificial wetland (Courtesy of Monroe County Environmental 

Management Council, 2000)

Detention basin

Drive

Lawton

Mill Road

3

catchments designed to attenuate peak flows and to promote the settling of suspended sediment 

and associated chemical constituents transported by runoff. Artificial or created wetlands are 

designed to be either permanently or intermittently wet, and can contain either a single 

hydrophilic (strong affinity for water) plant species such as cattails, or a diversity of plants 

including mosses, grasses, cattails, woody shrubs, and trees. Artificial wetlands provide the 

additional benefit of the uptake and removal of runoff-borne contaminants that can adversely 

affect receiving waters. The effectiveness of a detention basin or wetland in decreasing chemical 

and sediment loads depends on several factors, including the retention time during which 

suspended constituents can settle out, and the amount and type of vegetation, which not only 

retards flow through the basin and provides a substrate to which suspended constituents can 

adhere, but also takes up certain constituents directly and provides a setting for additional uptake 

by the associated microbial community. Detention basins are frequently converted to artificial or 

constructed wetlands either by planting vegetation within the detention basin or allowing it to 

grow naturally.

Despite the documented water quality 

improvements provided by detention 

basins and artificial wetlands, the potential 

effect of these impoundments on 

streamwater temperature remains a concern 

because the sudden alteration of water 

temperature downstream from a basin in 

response to stormflows could affect the 

physical and chemical properties of the 

stream as well as the health of the resident 

fish and other aquatic organisms.

In 1994, the U. S. Geological Survey 

(USGS), in cooperation with the Monroe 

County Department of Health and the 

Town of Pittsford, began monitoring a 

diversely vegetated stormwater detention 

basin at a housing development (Brookside 

Meadows) in the Town of Pittsford (figs. 1, 

2) to assess the effect of this basin on water 

quality downstream. The program entailed 

recording water temperature, collecting 

water samples for chemical analysis, and 

collecting flow data for use in calculating 

constituent loads at the inflow and outflow 

of the basin. This report describes the 

effectiveness of the vegetated detention 

basin in decreasing chemical loads entering the 

receiving stream, and the effects of the basin on 

the temperature of the outflow.

.
Figure 3. Photographs showing inflow (B, C) to Mill Road artificial wetland, and outflow pipe (A). 

Artificial Wetland 

Boundary 

INFLOW 

PIPE

A. Outflow pipe and manhole

OUTFLOW 

PIPE

8” Diameter

B. Detail of inflow pipe

INFLOW PIPE

30” Diameter

C. Inflow pipe 

4 BASIN DESCRIPTION AND HISTORY

The Mill Road artificial wetland is a small (0.6-acre) stormwater impoundment that was 

constructed in 1981 as a settling basin to retain stormflows and trap sediment caused by the 

construction of the Brookside Meadows residential development. Sediment was periodically 

removed from the basin during the period of construction (1981-90) but has not been removed 

since then, and sediment from disturbed soils and from impervious (paved) surfaces since 

construction was completed has accumulated to a thickness of about 2 feet above the original 

basin bottom. The growth of vegetation in the basin over the years has, in effect, turned the 

detention basin into an artificial wetland. Part of the basin is permanently wet and sustains 

emergent wetland vegetation (cattails and reeds); the rest of the basin is intermittently wet and 

contains a diversity of natural vegetation such as willows, grasses, and woody shrubs. The basin 

receives storm runoff from Brookside Meadows, a residential development with a steeply sloping 

(8 to 25 percent) drainage area of about 39 acres. Stormflows peak and recede quickly because 

the development is steeply sloped and contains large amounts (13.5 percent) of impervious surface 

area (roofs, streets, etc.). Because of the steep basin slope, runoff rates are highly correlated with 

rainfall intensity. Stormwater is retained in the detention basin because the outflow pipe (8 in. 

dia.) is smaller than the inflow pipe (30 in. dia.) (fig. 3). Runoff from the detention basin flows 

into Mill Creek Tributary, then into Mill Creek, and finally into Irondequoit Creek (fig. 1).
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

0.1

10

0.2

0.3

0.4

0.5

0.7

1

2

3

4

5

7 EXPLANATION

IN FLOW

OUTF LO W

FLOW, IN CUBIC FEET PER SECOND

March 19, 1998

TIME IN HOURS

Inflow

Outflow

March 19, 1998 EXPLANATION

Figure 4. Typical inflow and outflow hydrograph for the Mill Road detention 

basin in response to a moderately high flow

5 DATA COLLECTION

Instrumentation was installed at the inlet and the outlet of the basin in 1995 to collect water 

samples, measure flow, monitor water temperature, and record the amounts of precipitation and 

the water levels in the basin. All data were stored on a data logger, which also governs the 

operation of the water samplers. The data are then used to estimate loads of chemical constituents 

into and out of the basin; these load values in turn are used to assess the effectiveness of the 

basin in reducing chemical loads.

Water samples are collected by flow-activated automatic samplers at the inflow and outflow of 

the basin and analyzed for concentrations of chemical constituents such as nutrients (species of 

nitrogen and phosphorus), suspended and dissolved solids, common ions (chloride and sulfate), and 

lead and zinc. The volume of flow into and out of the basin is measured and used to calculate 

chemical loads. A typical inflow and outflow hydrograph for a moderately high flow event is 

depicted in figure 4. Thermistors recorded water temperatures at the inflow and outflow to assess 

the basin’s effects on the temperature of runoff flowing into the receiving stream.

EFFECT OF DETENTION BASIN ON CONSTITUENT LOADS

Fourteen high flow events between January 1996 and September 1999 were selected as 

representative events at this site. These events were selected because they represent seasonal 

variation, have varying flow conditions, and had adequate sample coverage. Constituent 

loads were estimated from the calculated 

inflow and outflow volumes for each 

storm, multiplied by the mean constituent 

concentrations for the storm. Inflow and 

outflow volumes were calculated from the 

beginning to the end of each 5-minute 

logging interval. 

The inflow volumes of the 14 high flow 

events ranged from 11,600 cubic feet to 

141,000 cubic feet with a mean of 40,400 

cubic feet. Outflow volumes ranged 

from 10,400 cubic feet to 125,300 cubic 

feet, with a mean of 36,900 cubic feet, 

and averaged about 9.5 percent less than 

inflow volumes. The lower flow volumes 

at the outflow are at least partially 

accounted for by infiltration to ground 

water on the southeast side of the basin. 

While amounts of loss to ground water 

have not been quantified, soil cores and an 

injection test indicate that soils there are 

conducive to infiltration.
Constituent or property

Average 

load in

Average 

load out

Overall 

removal 

efficiency1

Volume 40,400 36,900 --

Suspended solids 250 95.5 61.8

Ammonia, as N 0.27 0.20 25.9

Ammonia + organic nitrogen, as N 2.25 1.82 19.1

Nitrite + nitrate, as N 2.66 2.50 6.0

Phosphorus, total as P 0.52 0.32 38.5

Orthophosphate, as P 0.14 0.12 14.3

Total organic carbon 17.9 15.0 16.2

Chloride, dissolved 82.1 71.2 13.3

Sulfate, dissolved 48.5 48.6 -0.21

Zinc, dissolved 10.6 18.0 -69.8

Zinc, total 174 124 28.7

Dissolved solids 167 381 -128

Table 1. Average Inflow volume and outflow volume and constituent 

loads at the inflow and outflow of the Mill Road detention basin for 

fourteen high flow events 1996-99.

[Volume is cubic feet, constituent loads are in pounds. Red type indicates 

difference between inflow and outflow load is statistically significant at α = 

0.05]

1 These removal efficiencies are based on the average inflow and outflow loa

6 REMOVAL EFFICIENCY

Mean loads of most constituents were higher at the inflow than at the outflow (table 

1). Removal efficiencies defined as ((load in - load out) ÷ load in) x 100 generally were 

greatest for those constituents associated with particulate transport—suspended solids (62 

percent), ammonia + organic nitrogen (19.1 percent), and total phosphorus (38 percent). 

Positive removal efficiencies indicate a reduction in load within the basin, either through 

settling or through uptake by plants. Negative efficiencies indicate an increase in load at the 

outflow, generally through resuspension or dissolution of constituents deposited in the wetland 

by previous storms. A statistical test (Wilcoxon signed-rank test) was used to evaluate whether 

differences between the inflow loads and the outflow loads were statistically significant at the 

95-percent confidence level (α = 0.05). If tests do show a statistical significance, that means 

there is a 95-percent chance that an actual reduction in loads occured at the outflow, rather than 

an apparent reduction resulting from a chance arrangement of data.

Removal efficiencies are computed in relation to the inflow loads; therefore, large absolute 

values must be considered in relation to the constituent mass that is entering the basin. Overall 

removal efficiencies (table 1) for each constituent are based on average of the loads for all 

14 events, whereas average removal efficiencies (table 2) are the average of the individual 

event removal efficiencies. Overall removal efficiencies can be misleading; for example, the 

data indicate a positive overall removal efficiency for dissolved chloride, even though only 

two individual events showed a positive removal efficiency, whereas 11 events showed 

a negative removal efficiency and resulted in a negative average removal efficiency. 

This indicates that loads of dissolved chloride at the outflow are likely to be higher 

than those at inflow. Large input 

loads of chloride tended to result 

in positive removal efficiencies 

while small input loads tended 

to produce negative removal 

efficiencies (fig. 5). Other 

constituents whose inflow 

loads appeared to be positively 

correlated with removal 

efficiency were total suspended 

solids, nitrite + nitrate, 

dissolved sulfate, and 

dissolved solids. Constituents 

with positive overall removal 

efficiencies, but negative 

removal efficiencies for 

individual flows, were 

suspended solids, ammonia + 

organic nitrogen, 

orthophosphate, total organic 

carbon, and total zinc (table 2).

One important benefit of 

artificial or created wetlands 

is the removal of phosphorus 

from stormwater. While 
Table 2. Removal efficiencies for selected constituents for each of the fourteen high-flow 

events at the Mill Road detention basin. 

[all values are percent; diss, dissolved; Location is shown in fig. 1]

Date

Max

storm

disch

Total 

suspended 

solids

Ammonia

Ammonia 

+ organic 

nitrogen

Nitrite + 

nitrate

Total 

phosphorus

OrthophosphateTotal

organic

carbon

Dissolved 

chloride

Dissolved 

sulfate

Dissolved 

zinc

Total 

zinc

Dissolved 

solids

Jan. 18, 1996 2.1 -103 28.6 -15.5 11.4 2.50 22.2 -- 30.1 21.9 -- -- --

Apr. 30, 1996 4.4 11.4 41.7 4.42 -1.87 52.0 37.5 -1.86 -23.6 -24.0 -- -27.0 -30.5

May 11, 1996 2.9 64.1 48.4 34.4 0.94 43.7 22.9 16.7 0.00 0.67 -- 27.6 --

Jul. 3, 1996 4.6 46.6 44.8 -4.92 -28.6 0.00 33.3 -- -135 -46.1 -- -- --

Sep. 13, 1996 4.8 50.9 16.7 -17.1 9.39 10.5 5.88 -4.38 -20.4 -- -- 39.4 -7.69

Dec. 13, 1996 2.8 71.3 44.4 14.1 -1.69 32.9 16.0 18.0 25.9 -10.8 -- 22.6 --

May 19, 1997 (1) 2.2 63.6 15.4 0.00 -41.7 63.6 50.0 16.7 -125 -- 34.7 29.8 -63.8

May 19, 1997 (2) 6.9 80.9 2.27 -5.92 7.51 47.2 12.5 15.8 -19.0 -- -88.0 38.8 -30.3

Aug. 16, 1997 7.8 71.2 25.9 11.1 -55.7 47.8 20.0 -27.0 -215 -- -29.6 42.1 --

Mar. 8, 1998 3.5 60.6 28.6 26.7 4.31 0.00 20.0 10.9 -0.80 -12.2 -101 -47.1 -12.2

Mar. 19, 1998 6.0 72.1 64.3 67.7 8.62 54.6 20.0 41.0 -9.54 -0.63 -230 62.0 -66.5

Aug. 20, 1999 8.4 70.1 47.4 25.9 3.45 32.1 -14.3 4.55 -63.3 -104 -21.8 52.9 8.00

Sep. 7, 1999 1.9 83.9 66.7 29.2 -13.2 50.0 -25.0 47.5 -122 -17.2 -66.7 42.2 -59.1

Sep. 16, 1999 2.0 82.0 56.4 19.5 32.9 27.6 6.25 4.76 -27.0 -10.8 -110 37.4 --

Average 4.3 47.7 38.0 13.5 2.5 33.2 16.2 11.9 -50.3 -20.3 -76.5 26.7 -32.8

DISSOLVED CHLORIDE INFLOW LOAD, IN POUNDS

REMOVAL EFFICIENCY, IN PERCENT

50

25

0

-25

-50

-75

-100

-125

-150

-175

-200

-225

-250

1 2 5 10 20 50 100 200 500 1000

Figure 5. Graph showing relationship between dissolved chloride 

removal efficiency and chloride load at the detention basin inflow

(Location is shown in fig. 1).

7

phosphorus is not the sole cause of 

eutrophication (aging process of water 

bodies), it is frequently the key element 

required by freshwater plants and usually is 

present in the least amount relative to need. 

Therefore, an increase in phosphorus allows 

plants to make use of other nutrients already 

present in the water and thereby promotes 

plant and algal growth. Generally, of all the 

elements required for plant growth in water, 

phophorus is the most easily controlled. 

The vegetated detention basin at Mill road 

produced a statistically significant reduction 

in total phosphorus load (about 35 percent) 

and in orthophosphate load (about 15 percent) 

in stormflows during 1995-99. These removal 

efficiencies for phosphorus are similar those 

noted under the maximum water-retention 

configuration of another detention basin 

within the Irondequoit Creek Basin (Zarriello 

and Sherwood, 1992).
8 EFFECT OF DETENTION BASIN ON WATER TEMPERATURE

Artifically induced water-temperature changes in streams, caused by the release 

of water from upstream impoundments, may adversely affect downstream aquatic 

ecosystems. Sudden temperature changes in open waters, especially during the summer, 

may affect periphyton, benthic invertebrates, and fish in addition to causing shifts in 

algal predominance.

Nineteen precipitation events over a 5-year period (1995-99) were examined to 

determine the effect of the Mill Road detention basin on the temperature of water leaving 

the basin. Four of the events occured during the fall, three during the winter, four during 

the spring, and eight during the summer. Inflows and outflows are plotted by season 

in figure 6.

Several factors, either individually or in combination, can affect the temperature of 

runoff as it passes through the vegetated detention basin; among these are the temperature 

of the impervious surfaces on which the rain falls, the intensity of precipitation, the air 

temperature during the storm, the water-retention time in the basin, and the amount and 

stage of the foliage development on vegetation within the basin. 

Stormflow entering the basin during cold months generally produces an initial spike 

(increase) in water temperature within the basin, but this decreases rapidly as the flow 

increases. As flow from the event subsides, temperatures return to pre-event levels. 

Outflow temperatures responded similarly, but generally take longer to return to prestorm 

levels. Outflow temperatures from fall though spring were typically 1 to 3  ̊C (2 to 5 

 ̊F) cooler than inflow temperatures.

Summer is the period of greatest concern over the thermal effect of detention basin 

discharges on receiving waters. The duration of the eight summer storms selected for 

analysis ranged from 6 to 40 hours. Typically, inflow temperatures during these storms 

exceeded outflow temperatures until the recession, and thereafter decreased more rapidly 

than outflow temperatures (fig. 7). This is because the temperature of runoff entering 

the basin is initially increased by contact with warm or hot impervious surfaces and, 

thus, caused a sharp initial increase in temperatures at the inflow. When the impervious 

surfaces begin to cool, and precipitation intensity decreased, the water temperature 

at the basin inflow also begins to decrease. The highest temperatures at the inflow 

generally occured shortly after the peak flow. The temperature pattern at the outflow 

was similar—a sharp initial increase as the runoff reached the outflow, followed by a 

temperature peak which occured longer after the outflow peak than it had at the inflow 

peak. Maximum outflow temperatures were usually slightly lower than maximum inflow 

temperatures (table 3), whereas mean outflow temperatures were essentially the same 

as, or slightly higher than mean inflow temperatures. A maximum instantaneous outflow 

temperature of 24.1oC (75 oF) was noted during the July 8, 1998 storm. Maximum inflow 

and outflow temperatures tended to be similar among summer storms, and in five of 

the eight storms, maximum temperatures at the outflow were slightly lower than those 

at the inflow.

In general, mean water temperatures for the eight summer storms were 0.5o C (0.9o F) 

higher at the outflow than at the inflow. Mean outflow temperatures ranged from 17.1 

to 20.9oC (63 to 70o F). The greatest difference between mean water temperature at the 

outflow and that at the inflow was 1.9o C (3.4o F) during the storm of August 20, 1999. 

Thus, the basin appears to have little effect on the temperature of runoff and, thus, should 

have little affect on receiving waters during the summer. 
Figure 7. Graph showing inflow and outflow hydrograph, and inflow and outflow temperature at 

Mill Road vegetated detention basin during a representative stormflow event. (Location is shown 

Figure 6. Average inflow and outflow temperatures at the Mill Road vegetated detention 

basin for selected storms, by season, 1995-99. (Location is shown in fig. 1). WATER TEMPERATURE, IN DEGREES CELSIUS

FALL WINTER SPRING SUMMER

24

10/21/95

10/18/96

11/01/97

10/08/98

1/19/96

2/27/97

1/23/994/13/96

3/27/97

4/19/983/18/99

6/07/96

6/22/96

6/30/967/08/98

8/02/968/20/99

9/13/96

9/16/99

22

20

18

16

14

12

10

8

6

4

2

0

MEAN OUTFLOW TEMPERATURE

MEAN INFLOW TEMPERATURE

EXPLANATION

JUNE 22, 1996

EXPLANATION

TIME IN HOURS

WATER TEMPERATURE,

IN DEGREES CELSIUS

FLOW,IN CUBIC FEET PER SECOND

23

22

21

20

19

18

17

16

15

10

5

2

1

0.5

0.2

0.1

0.05

0.02

Outflow temperature

Outflow

Inflow temperature

Inflow

0.01

1200 1400 1600 1800 2000 2200 2400

9
Table 3. Maximum, minimum, and average inflow and outflow temperatures with event 

duration and flow volume at the Mill Road detention basin for selected events during 

1995-99. [Temperature values are degrees celsius]

Event

Duration

(hours)

Flow

volume

(cubic feet)

Maximum 

inflow 

temperature

Maximum 

outflow 

temperature

Average 

inflow 

temperature

Average 

outflow 

temperature

06/07/96 24 37,000 21.0 20.4 16.2 17.1

06/22/96 13 30,400 21.8 20.8 18.7 19.2

06/30/96 35 62,200 21.5 21.3 20.0 20.2

08/02/96 6 23,300 24.1 22.5 21.0 20.9

09/13/96 12 56,900 19.9 18.7 18.5 17.3

07/08/98 40 259,000 23.0 24.1 19.5 20.3

08/20/99 12 34,600 21.0 23.2 19.9 21.8

09/16/99 17 43,000 19.8 20.5 17.8 19.4

Average 19.9 68,300 21.5 21.4 19.0 19.5

10

SUMMARY AND CONCLUSIONS

This study was undertaken in 1995 to assess the effectiveness of a naturally vegetated 

stormwater-detention basin in decreasing the amounts of contaminants carried by storm runoff 

from a residential development into an adjacent stream.

The basin decreased the average loads of all constituents except dissolved sulfate, dissolved 

zinc, and dissolved solids. The average load of chloride was greater at the inflow (82.1 lbs) 

than at the outflow (71.2 lbs), which indicates a net removal that can be attributed to a single 

high flow during January 1996, in which the inflow load greatly exceeded the outflow load. The 

loads of all other dissolved constituents were greater at the outflow than at the inflow, indicating 

resupension or dissolution of constituents deposited in the basin by previous storms. Overall 

removal efficiencies ranged from +62 percent for suspended solids to -128 percent for dissolved 

solids. The vegetated detention basin was effective in removing total phosphorus (38-percent 

removal efficiency) and orthophosphate (14-percent removal efficiency). These results suggest 

that the basin is effective in decreasing loads of suspended constituents and of constituents that are 

adsorbed to suspended particles, but is not effective in removing dissolved constituents.

Water temperatures at the outflow during fall and winter averaged about 1.4o C (2.5 o F) lower 

than at the inflow, whereas spring water temperatures at the outflow were about the same as at 

the inflow. The thermal effect of the detention basin on receiving waters during summer storms 

was minimal; average maximum inflow and outflow temperatures were essentially equal. The 

average outflow temperature for all eight summer storms exceeded the inflow temperature by 

0.5o C (0.9o F). 

The results of this study indicate that vegetated detention basins can provide an effective means 

of decreasing the loads of certain suspended constituents in suburban runoff without adversely 

affecting the temperature of the receiving water body.
U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225

Copies of this report can be purchased from:

11 REFERENCE CITED

Zarriello, P.J., and Sherwood, D. A., 1992, 

Effects of stormwater detention on the 

chemical quality of runoff from a small 

residential development Monroe County, 

New York: U.S. Geological Survey WaterResources Investigations Report 92-4003, 

57 p.

For more information contact: Subdistrict Chief

U.S. Geological Survey

30 Brown Road

Ithaca, NY 14850
This report and additional earth science information can be found on the World Wide Web at: http://ny.usgs.gov