Maggie Brooks

County Executive

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4301

July 2004

Effects of Jefferson Road Stormwater-Detention Basin on Loads 

and Concentrations of Selected Chemical Constituents in East 

Branch of Allen Creek at Pittsford, Monroe County, N.Y.

Water-quality and streamflow data collected from 1990 through 2000 at East Branch Allen Creek in Monroe 

County, N.Y., indicate that the loads of some nutrients have decreased since the installation of a detention 

basin at Jefferson Road in August 1995. Mean monthly loads of ammonia + organic nitrogen calculated for 

1995-2000 are significantly less than those calculated for 1990-95 while total phosphorus was the only nutrient to show 

no significant differences in mean monthly load between 1990-95 and 1995-2000.

In cooperation with Monroe County Department of Health

by Donald A. Sherwood
Analysis of chemical quality data collected from 

1990 through 2000 at East Branch Allen Creek 

indicated that the detention basin at Jefferson 

Road provides improvement (reduction) in the 

loads of some constituents.

Jefferson Rd.

detention basin

0423204920

STREAMGAGING STATION

EAST BRANCH ALLEN

CREEK NEAR PITTSFORD

(0423204920)

77o37' 30" 30'

43o

15'

10'

05'

43o

42o

55'

22' 30'

Base from U.S. Geological Survey

State base map 1:500,000, 1974

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VICTOR

AREA SHOWN

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ROCHESTER

MONROE

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MONROE COUNTY

WAYNE COUNTY

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FAIRPORT

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1 2 3 MILES

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BASIN BOUNDARY 

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ROCHESTER

Figure 1. Principal geographic features of Irondequoit Creek basin, 

Monroe County, N.Y., and location of Jefferson Road stormwaterdetention basin in Town of Pittsford.

2

Introduction

Stormwater-detention basins are catchments designed 

to reduce downstream flooding and promote the settling of 

suspended sediment. They have been widely used in varying 

forms throughout Monroe County since the 1980’s and have 

been shown to reduce the magnitude of peak flows downstream 

and to decrease the concentrations of some constituents 

transported in storm runoff (Zarriello and Sherwood, 1992; 

Sherwood, 2001). However, the effect these detention basins, 

particularly a large unvegetated basin such as the Jefferson Road 

detention basin, have on chemical constituents is still unclear.

The Jefferson Road stormwater-detention basin was 

constructed on the East Branch of Allen Creek, a tributary to 

Irondequoit Creek (fig. 1), in the summer of 1995 by the Town 

of Pittsford, a suburb of Rochester, to alleviate downstream 

flooding. The detention basin allows water that is impounded 

during high flows to be released slowly to the creek; it also 

supplements flow in the creek, as needed, to provide irrigation 

for downstream country clubs.

Streamflow of East Branch of Allen Creek is continuously 

monitored by the U.S. Geological Survey in cooperation with 

the Monroe County Department of Health at a streamflowgaging station (no. 0423204920) 1.1 miles (mi) downstream 

(north) of the detention basin and 0.2 mi downstream of State 

Highway 31 (fig. 1). The Monroe County Environmental 

Health Laboratory (MCEHL) also collects water samples at 

this site to assess the chemical quality of the stream. The data 

collected at this site since its installation in 1990 provide a 

basis for evaluating the effectiveness of the basin in decreasing 

the stormwater loads of nutrients and other constituents in the 

East Branch of Allen Creek. 
3

OUTLET WEIR BOX

CONTROL

BACK POND

SPILLWAY

�� Back pond

�� Outlet weir box control

Figure 3. View of Jefferson Road detention basin 

in Town of Pittsford, N.Y.: A. View of back pond 

from spillway (also shown on front cover). B. 

Outlet weir box (also shown on back cover). 

Figure 2. Major features of Jefferson Road detention basin area, 

Monroe County, N.Y. 

JEFFERSON ROAD (NYS ROUTE 252)

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NOT TO SCALE

Jefferson Road

detention basin

Back

Pond

Front

Pond

Spillway

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Outlet Weir Box

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RAILROAD

Canal

Siphon

gaging 

station

Description of Detention Basin

The detention basin occupies over 15 acres along the channel 

of East Branch Allen Creek between Jefferson Road (State 

Route 252) and the Consolidated Rail Corporation (CONRAIL) 

railroad tracks (fig. 2). It was placed in full operation during 

the second week of August 1995. The pond is divided into a 

smaller “front” pond and a larger “back” pond by an earthen 

berm with a spillway connecting the two parts (fig. 2). The 

front pond is designed to dissipate the energy of stormflows to 

minimize erosion and promote settling of suspended material. 

The back pond (fig. 3A) is the main storage area. Both portions 

of the pond are unvegetated during the period of data collection. 

Water from East Branch Allen Creek flows uncontrolled into 

the front pond, through the spillway into the back pond, through 

the outlet control structure, and back into the East Branch Allen 

Creek channel. Outflow from the pond is controlled at a “weir 

box” through a system of weirs and valves (fig. 3B). Flow into 

the weir box is through a 30 in. conduit extending 150 feet (ft) 

into the pond with the upstream end 2 ft above the pond bottom. 

When full (466 ft. above NGVD of 1929), the pond contains 

approximately 7,200,000 cubic feet (ft3) of water and has a 

surface area of about 15 acres. The front pond and the back 

pond are connected by a spillway with the crest at 457 ft. above 

NGVD of 1929. 
4

Table 1. Mean monthly loads of selected constituents at East Branch of Allen Creek 

before and after installation of the detention basin at Jefferson Road in the Town of 

Pittsford, N.Y., August 1995.

[Values are in pounds except as noted. Loads are unadjusted, although statistical test was done on 

adjusted loads. Bold type indicates significant difference at α = 0.05. p denotes significance.]

Mean monthly load

Constituent Before After p

Flow (cubic feet per second) 8.71 9.08 0.92

Suspended solids (tons) 108 83.6 .08

Ammonia as N, dissolved 55.6 52.6 .09

Ammonia + organic nitrogen as N, total 1,420 1,280 <.01

Nitrite + nitrate as N, total 2,430 2,190 .10

Total phosphorus as P 247 240 .42

Orthophosphate as P, dissolved 38.2 44.5 .36

Chloride, dissolved (tons) 77.0 107 .42

Sulfate, dissolved (tons) 57.8 57.2 .58

Loads and Concentrations of Selected Constituents

Discharge data and water samples have been collected 

at the East Branch Allen Creek gaging station since May 

1990. The 5 years of data collected before installation of the 

detention basin provide an adequate basis for comparison 

with data collected since then to assess the efficiency of the 

basin in chemical load removal.

Loads

A regression equation that relates constituent concentration 

to flow was used to estimate the monthly mean loads for 

each month before and after basin installation. The monthly 

means were grouped by individual month and averaged; for 

example, the monthly mean loads for each October before 

basin installation were averaged to produce an average monthly 

load for October before basin installation, and this load was 

compared with the average monthly load for October after 

installation. The monthly loads were then divided by the mean 

monthly flow to adjust for differences in flow before and after 

basin installation. 

Data Limitations

Calculating the basin’s effect on chemical loads from 

data collected before and after basin construction rather than 

concurrent data from inflow and outflow sites at the basin 

is complicated by several factors that could introduce bias: 

(1) the preconstruction and postconstruction periods differ in 

frequency and intensity of storms and in their numbers of dry 

(or wet) years; (2) development within the basin drainage area 

has increased; this increase, in turn, has altered the drainage 

patterns and chemical loads carried in storm runoff; (3) during 

the navigation season inflow from the Erie-Barge canal to the 

East Branch of Allen Creek between the basin outflow and the 

gaging station more than 1 mi downstream could affect stream 

chemistry at the gaging station, and (4) large discharges from 

the basin could resuspend material that was recently deposited 

between the basin and the gaging station. Efforts were made 

during the analyses to minimize the effects of these factors; 

nevertheless these factors increase the uncertainty of the 

interpretations. 

Statistical Analysis

A statistical analysis (Wilcoxon rank-sum test) of 

the adjusted mean monthly loads before and after basin 

installation showed some statistically significant (α = 0.05) 

differences in monthly averages (fig. 4). The values for 

total suspended solids, ammonia, nitrite + nitrate, and 

orthophosphate showed statistically significant increases 

for certain months after basin installation, and significant 

decreases for other months. Ammonia + organic nitrogen 

and dissolved sulfate values after basin installation showed 

months with significantly decreased loads but no months 

with increased loads, whereas dissolved chloride showed 

months with significantly increased 

loads, but no months with decreased 

loads. Neither total phosphorus nor 

monthly mean discharge showed 

changes after basin installation that 

were statistically significant.

The Wilcoxon rank-sum test 

was also used to identify significant 

differences in mean monthly 

constituent loads for all months 

before, and all months after, basin 

installation. The only constituent 

to show a statistically significant 

difference was ammonia + organic 

nitrogen, which showed a smaller 

median load after basin installation 

(table 1). The slight increase in 

mean monthly flows, from 8.71 ft3/s 

(cubic feet per second) before basin 

construction to 9.08 ft3/s after could 

result from the peak of record flow in 

July 1998 and/or an increased amount 

of inflow from the canal. 
5

0

30

10

20

200

1,000

400

600

800

0.4

0.1

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10

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800

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Before basin installation

After basin installation

FLOW SUSPENDED

SOLIDS

AMMONIA

AMMONIA +

ORGANIC NITROGEN

NITRITE

 +

NITRATE

TOTAL

PHOSPHORUS

ORTHOPHOSPHATE DISSOLVED

CHLORIDE

DISSOLVED

SULFATE

Statistically significant 

differences in median

MEAN MONTHLY DISCHARGE,

IN CUBIC FEET PER SECOND ADJUSTED MEAN MONTHLY LOAD, IN POUNDS

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Junejuly

Aug

Sept

ADJUSTED MEAN MONTHLY LOAD,

IN POUNDS

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Junejuly

Aug

Sept

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Junejuly

Aug

Sept

Figure 4. Mean monthly constituent load (in pounds) for each month before and after the 1995 installation of Jefferson Road 

detention basin Monroe County, N.Y. Loads have been adjusted for before and after differences in flow by dividing the mean 

monthly load by the mean flow for that month.
6

Table 3. Results of seasonal Kendall trend test on 

concentrations of selected constituents at East Branch 

of Allen Creek before and after installation of the detention 

basin at Jefferson Road in the Town of Pittsford, N.Y., 

August 1995. 

[Trend test was considered significant at α = 0.05]

Trend

Constituent 1990-95 1996-2000

Suspended solids None None

Ammonia None None

Ammonia + organic nitrogen None None

Nitrite + nitrate None None

Total phosphorus None Downward

Orthophosphate None None

Chloride, dissolved None None

Sulfate, dissolved None None

Table 2. Median concentrations of selected constituents at East Branch 

of Allen Creek before and after installation of the detention basin at 

Jefferson Road in the Town of Pittsford, N.Y., August 1995.

[Bold type indicates a significant difference at α = 0.05. Values are in milligrams 

per liter except as noted. p denotes significance.]

Median concentration 

Constituent Before After p 

Suspended solids 119 102 0.18

Suspended solids 119 102 0.18

Ammonia, as N dissolved .02 .02 .27

Ammonia + organic nitrogen, as N, total .78 .60 <.01

Nitrite + nitrate, as N, total 1.5 .96 <.01

Total phosphorus, as P .10 .10 .45

Orthophosphate, as P, dissolved .02 .02 .14

Chloride, dissolved 96 109 .04

Sulfate, dissolved 86 79 .13

Concentrations 

Concentration of a given constituent 

may be a more reliable measure of the 

basin’s effectiveness than the load because 

concentration is measured as mass per 

unit volume (mg/L), whereas load (mass) 

is a function of concentration, volume of 

flow, and some unit of time. It is possible 

for lower concentrations to produce 

large loads as a result of high flow and 

for high concentrations to produce large 

loads at lower flows; therefore, the better 

measure of improvement in contamination 

is reflected in the level of concentration 

which is independent of flow. 

Constituent concentrations before and 

after the installation of the detention basin 

were compared through boxplots (fig. 5) 

and the Wilcoxon rank-sum test; results 

indicate significant decreases in median 

concentrations of ammonia + organic 

nitrogen (from 0.78 mg/L to 0.60 mg/L) 

and nitrite + nitrate (from 1.5 mg/L to 

0.96 mg/L) after basin installation. Both 

of these nutrients are represented as 

total values that include the particulate 

component, which tends to settle out 

during detention. The concentrations of 

these nutrients fluctuate seasonally and are 

related to flow. Concentrations of chloride 

generally are higher during the winter 

when road salt application is the greatest 

and lower at other times. The increased 

median concentration of dissolved chloride 

(from 96 mg/L to 109 mg/L) after basin 

installation (table 2) is probably a result of 

increased road salt usage. 

The seasonal Kendall trend test was 

used to examine concentration data for 

the presence of statistically significant 

(α = 0.05) trends before and after basin 

installation to ensure that there were no 

already existing upward or downward 

trends that might obscure the effect of 

the detention basin. The only constituent 

for which a trend was indicated was total 

phosphorus, which showed a downward 

trend after basin installation (table 3). This 

trend could be a result of settling within the 

detention basin, but this cannot be verified.
7

Figure 5. Ranges in concentration of selected constituents in water samples from East Branch Allen Creek before 

and after 1995 installation of the Jefferson Road detention basin in Town of Pittsford, N.Y. 

0

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25th percentile

Median

75th percentile

Data value less than or equal to 1.5 times the

interquartile range outside the quartile

Outlier data value less than or equal to 3

and more than 1.5 times the

interquartile range outside the quartile

Outlier data value more than 3 times the

interquartile range outside the quartile

FLOW SUSPENDED 

SOLIDS

AMMONIA + 

ORGANIC NITROGEN

NITRITE

+

NITRATE

TOTAL

PHOSPHORUS

ORTHOPHOSPHATE

DISSOLVED

CHLORIDE

DISSOLVED 

SULFATE

BEFORE AFTER BEFORE AFTER BEFORE AFTER

DISCHARGE, IN CUBIC FEET PER SECOND CONCENTRATION, IN MILLIGRAMS PER LITER

CONCENTRATION, IN MILLIGRAMS PER LITER

AMMONIA
8

FOR MORE INFORMATION: 

Subdistrict Chief

U.S. Geological Survey

30 Brown Rd.

Ithaca, NY 14850

Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

This report and additional earth science information 

can be found on the World Wide Web at 

http://ny.usgs.gov

Conclusions

Discharge and water-quality data collection at East Branch 

Allen Creek from 1990 through 2000 provide a basis for 

estimating the effect of the Jefferson Road detention basin on 

loads and concentrations of chemical constituents downstream 

from the basin. Mean monthly flow for the 5 years prior to 

construction of the detention basin (8.71 ft3/s) was slightly 

lower than after (9.08 ft3/s). The slightly higher mean monthly 

flow after basin construction may have been influenced by the 

peak flow for the period of record that occurred in July 1998 

or variations in flow diverted from the canal. No statistically 

significant difference in average monthly mean flow before and 

after basin installation was indicated.

Total phosphorus was the only constituent to show no 

months with significant differences in load after basin 

construction. Several constituents showed months with 

significantly smaller loads after basin construction than before, 

whereas some constituents showed certain months with smaller 

and some months with greater loads, after basin construction. 

Statistical analysis of the “mean monthly load” for all 

months before and all months after construction of the detention 

basin showed only one constituent (ammonia + organic 

nitrogen) with a significantly lower load after construction and 

none with higher loads. 

Median concentrations of ammonia + organic nitrogen 

showed a statistically significant decrease (from 0.78 mg/L to 

0.60 mg/L) after basin installation, as did nitrite + nitrate (from 

1.50 mg/L to 0.96 mg/L); in contrast, the median concentration 

of dissolved chloride increased from 95.5 mg/L before 

basin installation to 109 mg/L thereafter. A trend analysis of 

constituent concentrations before and after installation of the 

detention basin showed that total phosphorus had a downward 

trend after installation.

Analysis of the data collected at East Branch Allen Creek 

indicates that the Jefferson Road detention basin, in some 

cases, provides an improvement (reduction) in loads of some 

constituents. These results are uncertain, however, because 

hydrologic conditions before basin installation differed from 

those in the 5 years that followed, and because inflow from the 

Erie-Barge canal may alter the water quality in the 1-mi reach 

between the basin outflow and the gaging station.

Selected References

Sherwood, D.A., 2001, Effects of a vegetated stormwaterdetention basin on chemical quality and temperature of 

runoff from a small residential development in Monroe 

County, New York: U.S. Geological Survey WaterResources Investigations Report 01-4099, 8 p.

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 Water-Resources 

Investigations Report 92-4003, 57 p. 

Printed on recycled paper