Prepared in cooperation with the Wisconsin Department of Natural Resources, 

the City of Madison, Cities in the Waukesha Permit Group, Hydro International, 

Earth Tech Incorporated, National Sanitation Foundation International, and 

the U.S. Environmental Protection Agency

Parking Lot Runoff Quality and Treatment Efficiencies 

of a Hydrodynamic-Settling Device in Madison, 

Wisconsin, 2005–6

Scientific Investigations Report 2011–5145

U.S. Department of the Interior

U.S. Geological Survey

Parking Lot Runoff Quality and 

Treatment Efficiencies of a 

Hydrodynamic-Settling Device in 

Madison, Wisconsin, 2005–6

By Judy A. Horwatich and Roger T. Bannerman

Prepared in cooperation with the Wisconsin Department of Natural Resources, 

the City of Madison, Cities in the Waukesha Permit Group, Hydro International, 

Earth Tech Incorporated, National Sanitation Foundation International, and the 

U.S. Environmental Protection Agency

Scientific Investigations Report 2011–5145

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Horwatich, J.A., and Bannerman, R.T., 2012, Parking lot runoff quality and treatment efficiencies of a hydrodynamicsettling device in Madison, Wisconsin, 2005–6: U.S. Geological Survey Scientific Investigations Report 2011–5145, 

35 p. plus 11 app.

Cover: A hydrodynamic-settling device installed in an employee parking lot in downtown Madison, Wisconsin. 

(Photographs from Earth Tech, Inc., September 2005.)
iii

Contents

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

Introduction.....................................................................................................................................................1

Purpose and Scope of Report.............................................................................................................2

Previous Investigations........................................................................................................................3

Site Description..............................................................................................................................................3

Design of Hydrodynamic-Settling Device..................................................................................................5

Methods of Data Collection .........................................................................................................................6

Water-Quality Sampling and Analysis ..............................................................................................6

Particle-Size Analysis .................................................................................................................8

Processing of Water-Quality Samples .....................................................................................8

Quality Control..............................................................................................................................9

Rainfall....................................................................................................................................................9

Flow Monitoring ....................................................................................................................................9

Calibration of Stage ..................................................................................................................10

Calibration of Flow.....................................................................................................................10

Inlet Flows..........................................................................................................................10

System Outlet Flows.........................................................................................................10

Device Outlet Flows..........................................................................................................11

Monitoring Complications .................................................................................................................11

Evaluation of Hydrodynamic-Settling Device .........................................................................................14

Rainfall Data.........................................................................................................................................14

Stormwater Flow Data .......................................................................................................................15

Comparison of Runoff Volumes to Rainfall Volumes............................................................16

Water-Quality Data for Inlet and Outlet..................................................................................17

Water-Quality Data Compared to Other Parking Lots..........................................................18

Particle-Size Distributions........................................................................................................19

Efficiency Calculations ......................................................................................................................21

Efficiency Ratio ..........................................................................................................................21

Sum of Loads ..............................................................................................................................23

Sum of Loads by Particle Size .......................................................................................24

Factors That Affect Variability in Efficiency ................................................................24

Accuracy of Efficiency Calculations ......................................................................................26

Measured Weight of Sediment in Sump................................................................................28

Weight of Sediment in Sump Using Inlet and 

Outlet Suspended-Sediment Loads..................................................................28

Adjustments to Suspended-Sediment Loads ..............................................................29

Particle-Size Distribution of Sediment Retained in Sump............................................................30

Summary........................................................................................................................................................31

References....................................................................................................................................................32
iv

Appendixes 1–11

1. List of References for Previous Investigations......................................................................37

2. Concentrations of selected water-quality constituents in field-equipment blank 

samples collect at inlet, outlet, and bypass of hydrodynamic-settling device in 

Madison, Wisconsin, October 2005 and May 2006 ...............................................................40

 3. Relative percent difference for concentrations of selected water-quality 

constituents in field-replicate samples collected at inlet and outlet of 

hydrodynamic-settling device ..................................................................................................41

4. Precipitation measured by tipping-bucket rain gage at study site during 

water-quality sample collection from hydrodynamic-settling device in 

Madison, Wisconsin, November 2005–September 2006......................................................43

5. Stormwater peak discharge, runoff volumes, and percentage runoff at 

hydrodynamic-settling device in Madison, Wisconsin, November 2005–

September 2006...........................................................................................................................46

6. Concentrations of suspended sediment, adjusted suspended solids, 

volatile solids, and dissolved solids in stormwater samples collected at inlet, 

outlet, and bypass of hydrodynamic-settling device in Madison, Wisconsin, 

Nobember 2005–August 2006 ...................................................................................................49

7. Concentrations of selected water-quality constituents and physical properties 

in stormwater samples collected at inlet, outlet, and bypass of hydrodynamicsettling device in Madison, Wisconsin, November 2005–September 2006 ......................50

8. Mean concentrations of selected polycyclic aromatic hydrocarbons in 

stormwater samples collected at inlet and outlet of hydrodynamic-settling 

device in Madison, Wisconsin, November 2005–August 2006............................................52

9. Particle-size distributions in stormwater samples collected at inlet, outlet, 

and bypass of hydrodynamic-settling device in Madison, Wisconsin, 

November 2005–August 2006....................................................................................................56

10. Loads of suspended solids, adusted suspended solids, volatile solids, and 

dissolved solids in stormwater samples collected at inlet, outlet, and bypass 

of hydrodynamic-settling device in Madison, Wisconsin, November 2005–

August 2006..................................................................................................................................59

11. Loads of selected water-quality constituents in stormwater samples 

collected at inlet, outlet, and bypass at hydrodynamic-settling device in 

Madison, Wisconsin, November 2005–August 2006.............................................................60

Figures

1. Diagram showing location of study area in Madison, Wisconsin, drainage area 

of parking lot, inlets, manholes, hydrodynamic-settling device, and 

storm-sewer pipes........................................................................................................................4

2. Schematic diagram of hydrodynamic-settling device............................................................5

3. Diagram of flow system and location of flow monitoring and water-quality 

sampling equipment for hydrodynamic-settling device.........................................................6

4. Graphs showing comparison of flow measured using dye dilution fluorometer 

and inlet area-velocity meter for two runoff events, May 9 and 16, 2006 .........................11

5. Graph showing stormwater flow volumes at flow system outlet corrected by 

dye dilution in relation to inlet flow volumes, 2006................................................................12
v

7. Photograph showing debris on speed bump overtopping watershed divide with 

stormwater runoff from adjacent City of Madison recycling facility, August 2006 .........13

6. Photograph showing watershed barrier (speed bump) installed to prevent runoff 

from City of Madison recycling facility ...................................................................................13

8. Photograph showing blockage of culvert draining the Madison recycling facility 

adjacent to the Madison Water Utility Administration Building, August 2006 .................13

9. Graph showing cumulative rainfall for study site (2005–6) in relation to cumulative 

frequency for all rainfall greater than 0.05 inch (1949–92) determined on basis of 

National Oceanic and Atmospheric Administration rainfall gage at Dane County 

Regional Airport, Madison, Wisconsin ...................................................................................15

10. Graph showing comparison of percentage of parking lot runoff and rainfall for 

hydrodynamic-settling device, November 2005–August 2006 ............................................17

11. Graph showing particle-size distributions determined at inlet and outlet of 

hydrodynamic-settling device for selected runoff events, November 2005–

August 2006..................................................................................................................................20

12. Graph showing removal efficiency of suspended solids and total solids as a 

function of peak flow for hydrodynamic-settling device system, November 2005–

August 2006..................................................................................................................................26

13. Graph showing variability in suspended-solids removal efficiency relative to 

percentage of particle sizes greater than 125 micrometers for hydrodynamicsettling device, November 2005–August 2006 .......................................................................27

14. Photograph showing slurry tank for drying material vacuumed from bottom of 

hydrodynamic-settling device ..................................................................................................27

Tables

1. Characteristics of drainage area that contribute runoff to hydrodynamic-settling 

device at Madison Water Utility Administration Building, Madison, Wisconsin...............3

2. Limits of detection and analytical methods for inorganic constituents analyzed in 

samples collected at the hydrodynamic-settling device in Madison, Wisconsin.............7

3. Limits of detection and analytical methods for polycyclic aromatic hydrocarbons 

analyzed in samples collected at the hydrodynamic-settling device in 

Madison, Wisconsin.....................................................................................................................8

4. Comparison of monthly rainfall between U.S. Geological Survey rain gage at 

study site and National Oceanic and Atmospheric Administration rainfall gage 

at Dane County Regional Airport, Madison, Wisconsin, November 2005–

September 2006...........................................................................................................................14

5. Summary statistics for selected water-quality constituents in samples collected 

at inlet, outlet, and bypass of hydrodynamic-settling device, Madison, Wisconsin, 

November 2005 through August 2006......................................................................................18

6. Comparison of geometric mean concentrations from this study with geometric 

mean and median concentrations observed in other parking lot studies in 

Wisconsin and Michigan...........................................................................................................19

7. Average particle-size distributions determined at inlet and outlet of 

hydrodynamic-settling device for selected runoff events, Madison, Wisconsin, 

November 2005–August 2006....................................................................................................20

8. Summary of flow-weighted average concentrations, control system efficiency 

ratios, and device efficiency ratios for the hydrodynamic-settling device in 

Madison, Wisconsin, November 2005–August 2006.............................................................22
vi

9. Summary of constituent loads and percentage efficiency for hydrodynamicsettling device in Madison, Wisconsin, November 2005–August 2006 .............................24

10. Particle-size distribution for Maine Department of Transportation road sand ................25

11. Sum of loads and percentage of suspended-sediment load reduction by 

particle size for hydrodynamic-settling device and bypass system in 

Madison, Wisconsin, November 2005–August 2006.............................................................25

12. Comparison of average particle-size distributions between 3 runoff events 

and 17 other runoff events for parking lot in Madison, Wisconsin, 

November 2005–August 2006....................................................................................................29

13. Monitored and estimated loads after concentrations of suspended sediment 

was adjusted for three largest particle sizes at inlet and outlet of hydrodynamicsettling device in Madison, Wisconsin, November 2005 and September 2006................30

14. Particle-size distribution for sediment samples collected in hydrodynamicsettling device in Madison, Wisconsin, September 2006 ....................................................31
vii

Conversion Factors

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

micrometer (μm) 0.00003937 inch (in.)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

liter (L) 0.2642 gallon (gal)

Flow rate

cubic foot per second (ft3/s) 0.028317 cubic meter per second (m3/s)

inch per hour (in/hr) 25.40 millimeter per hour (m/hr)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg)

gram (g) 0.03527 ounce (oz)

Mass per area

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter

Specific gravity

gram per cubic centimeter (g/cm3) 62.43 pound per cubic foot (lb/ft3)

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

°F=(1.8×°C)+32.

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

°C=(°F-32)/1.8.

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Particle sizes of sediment are given in micrometers (μm). A micrometer is one-thousandth of a 

millimeter.

Concentrations of sieved solids as a dried weight are in milligrams per kilogram (mg/kg).
viii

Abbreviations or Acronyms Used in this Report 

DCRA Dane County Regional Airport

DCu dissolved copper

DP dissolved phosphorus

DZn dissolved zinc

EPA Environmental Protection Agency

ETV Environmental Technology Verification 

IJC International Joint Commission

LOQ limit of quantification

MLE Maximum Likelihood Estimation

NOAA National Oceanic and Atmospheric Administration

NPDES National Pollution Discharge Elimination System

NSF International National Sanitation Foundation International

NURP Nationwide Urban Runoff Program

PAH polycyclic aromatic hydrocarbon

PVC polyvinyl chloride

PSD particle-size distribution

R2 correlation coefficient

RPD relative percent difference

SLOH State Laboratory of Hygiene

SM standard methods

SOL summation of loads

SRS standard reference sample

SS suspended sediment

TCu total recoverable copper

TDS total dissolved solids

TP total phosphorus

TSS total suspended solid

TZn total recoverable zinc

USGS U.S. Geological Survey

VSS volatile suspended solids

WDNR Wisconsin Department of Natural Resources
Parking Lot Runoff Quality and Treatment 

Efficiencies of a Hydrodynamic-Settling Device 

in Madison, Wisconsin, 2005–6

By Judy A. Horwatich1 and Roger T. Bannerman2

1

 U.S. Geological Survey.

2

 Wisconsin Department of Natural Resources.

Abstract

A hydrodynamic-settling device was installed in 2004 to 

treat stormwater runoff from a roof and parking lot located at 

the Water Utility Administration Building in Madison, Wis. 

The U.S. Geological Survey, in cooperation with the Wisconsin Department of Natural Resources, the City of Madison, 

cities in the Waukesha Permit Group, Hydro International, 

Earth Tech, Inc., National Sanitation Foundation International, 

and the U.S. Environmental Protection Agency, monitored 

the device from November 2005 through September 2006 

to evaluate it as part of the U.S. Environmental Protection 

Agency’s Environmental Technology Verification Program. 

Twenty-three runoff events monitored for flow volume and 

water quality at the device’s inlet and outlet were used to calculate the percentage of pollutant reduction for the device. The 

geometric mean concentrations of suspended sediment (SS), 

“adjusted” total suspended solids (TSS), total phosphorus 

(TP), dissolved phosphorus (DP), total recoverable zinc (TZn), 

and total recoverable copper (TCu) measured at the inlet were 

107 mg/L (milligrams per liter), 92 mg/L, 0.17 mg/L, 0.05 

mg/L, 38 ug/L (micrograms per liter), and 12 ug/L, respectively, and these concentrations are in the range of values 

observed in stormwater runoff from other parking lots in 

Wisconsin and Michigan.

Efficiency of the settling device was calculated using the 

efficiency ratio and summation of loads (SOL) methods. Using 

the efficiency ratio method, the device reduced concentrations 

of SS, and DP, by 19, and 15, percent, respectively. Using the 

efficiency ratio method, the device increased “adjusted” TSS 

and TZn concentrations by 5 and 19, respectively. Bypass 

occurred for 3 of the 23 runoff events used in this assessment, and the bypass flow and water-quality concentrations 

were used to determine the efficiency of the bypass system. 

Concentrations of SS, “adjusted” TSS, and DP were reduced 

for the system by 18, 5, and 18, respectively; however, TZn 

increased by 5 percent. Some of the TSS concentrations were 

“adjusted” to add the particles that remained on the sieves during sample processing. The loads of SS, “adjusted” TSS, and 

DP were reduced using the SOL method for the settling device 

by 38, 9, and 19 percent, respectively, and TZn increased by 

13 percent. For the bypass system, the loads of SS, “adjusted” 

TSS, and DP had percentage reductions of 39, 12, 22, respectively, however TZn increased by 4 percent. The SOL method 

produced percentage reductions for SS and “adjusted” TSS 

that were twice those for the efficiency ratio method. Removing the two large runoff events on August 23 and 24, 2006, 

from the SOL calculation brought the reduction for SS down 

to 16 and increased “adjusted” TSS by 4 percent. The two 

large runoff events were anomalies in that the runoff volumes 

and dissolved solids concentrations were greatly increased by 

overflow from an adjacent recycling facility.

The SOL method was used to determine the percentage 

of SS load reduction for six different particle sizes for both the 

settling device and bypass system. Essentially no load reduction was observed for particles less than 125 micrometers (μm) 

in diameter, and about a 90-percent reduction occurred for particle sizes greater than 250 μm in diameter. The large removal 

efficiencies for particle sizes greater than 250 μm in diameter 

were further supported by the fact that more than 80 percent 

of the particle sizes trapped in the sump were greater than 250 

μm in diameter. These results support the claim by the manufacturer of achieving a large percentage load reduction for 

particle sizes greater than 250 μm in diameter.

Introduction

Urban runoff can adversely affect aquatic systems by 

altering a stream’s normal flow regime, destroying fish habitat, 

and degrading water quality (Booth and Reinelt, 1993; Horner 

and others, 1994; Masterson and others 1994; Pitt and others, 

1995, Bannerman and others, 1996; Wang and others, 2001; 

Weber and Bannerman, 2004; Richards and others, 2006). To 

help control the effects of urbanization on aquatic systems, 

the Wisconsin Department of Natural Resources (WDNR) has 

promulgated a series of stormwater performance standards 

that attempt to mitigate both water-quantity and water-quality 
effects associated with urban runoff (Wisconsin Administrative Code NR 151, 2004). Water-quality benefits are based on 

reduction of total suspended solids (TSS) loads by 80 percent 

for new development, 40 percent for redevelopment, and 

40 percent for a retrofit reduction. Wisconsin’s municipalities 

will be required to meet these performance standards as part 

fulfillment of their U.S. Environmental Protection Agency 

(EPA) National Pollution Discharge Elimination System 

(NPDES) Phase II permit. Municipalities can select from 

among a number of different types of proprietary and nonproprietary types of stormwater control practices to achieve the 

TSS performance standards, but to reduce the uncertainty in 

the selection process, more information is needed about their 

ability to reduce TSS loads and their total cost.

To help reduce the uncertainty in the selection of proprietary stormwater control practices, a number of products 

have been evaluated in Wisconsin (Waschbusch and others, 

1999, Horwatich and others, 2004, 2010). These evaluations 

included both filtration and hydrodynamic-settling devices, 

which use sedimentation as the principle mechanism for 

removing TSS. These types of devices are usually installed 

underground, which makes them attractive for achieving TSS 

reduction goals at sites with limited above-ground space, such 

as retrofit or redevelopment sites. Results from these evaluations are being used by the WDNR to develop technical standards for both proprietary filters and hydrodynamic-settling 

devices. The technical standards provide criteria for proper 

design and installation. Also, the evaluations help calibrate 

and verify models used to identify the TSS reduction assigned 

to each installation.

As part of the continuing effort to reduce the uncertainty 

in the selection of proprietary stormwater control practices, 

the U.S. Geological Survey (USGS) in cooperation with the 

WDNR, the City of Madison, cities in the Waukesha Permit 

Group, Hydro International (Portland, ME), Earth Tech, Inc. 

(Madison, Wis.), National Sanitation Foundation International 

(NSF International), and the EPA Environmental Technology 

Verification (ETV) Program evaluated the effectiveness of a 

hydrodynamic-settling device, the Downstream Defender®, 

which is manufactured by Hydro International. This single 

chamber device is designed to treat stormwater runoff and 

limit the resuspension of entrained sediment. The newly 

installed device was located at the City of Madison Water 

Utility Administrative Building site. Most of the drainage area 

consists of an employee parking lot and roofs. Monitoring 

of flow was conducted at the inlet, outlet, and bypass to the 

device from November 2005 through September 2006. 

Because the evaluation described in this report is part 

of the ETV program, it followed EPA-approved monitoring 

protocols, and the results are nationally distributed by the EPA 

(U.S. Environmental Protection Agency, 2002a). The ETV 

program sets a national focus on verifying manufacturer’s 

claims for the performance of commercially available stormwater control practices. The EPA cooperates with the NSF 

International as its verification partner, and NSF International 

is in charge of the following tasks: (1) create a national 

protocol to test wet-weather flow technologies, (2) contract 

independent groups to evaluate the effectiveness of the stormwater-treatment practices of interest, (3) review and implement the verification testing plans, and (4) make study results 

available to the general public (U.S. Environmental Protection 

Agency, 2002a). A separate ETV report was prepared with the 

results from the evaluation described herein (U.S. Environmental Protection Agency, 2007).

The purpose of the evaluation described in this report was 

to provide information needed by municipalities to reduce the 

uncertainty they might have in using proprietary practices to 

achieve their pollutant reduction goals. The overall purpose 

of the evaluation can be divided into three specific objectives. 

One specific objective is to determine the efficiency of the 

hydrodynamic-settling device in reducing the contaminant 

loads in runoff from the site. A second specific objective was 

to collect sufficient data to calibrate and verify models estimating TSS reduction by hydrodynamic-settling devices and 

to add to the database characterizing the quality of runoff from 

an office complex. This objective required adding particle-size 

distributions to the constituent list. A third specific objective 

was to verify the manufacture’s claim for TSS reduction.

Purpose and Scope of Report

This report describes the methods and results from the 

evaluation of the efficiency of a hydrodynamic-settling device 

installed at the City of Madison Water Utility Administration 

Building site. The methods include collecting flow volume and 

water-quality data at the inlet, outlet, and bypass locations for 

the device. Methods for collecting rainfall data at the site are 

also described. Results are presented in the form of concentrations and loads of selected constituents at different locations 

in the hydrodynamic-settling device. Separate procedures are 

described for using the concentrations and loads to calculate 

the ability of the device to improve water quality. Results from 

23 runoff events were used for the efficiency calculations. 

Additional calculations are described that test the accuracy of 

the rainfall data, the flow data, and the water-quality data.

Flow volume and water-quality data used for the efficiency calculations were collected from November 2005 

through August 2006. Water-quality samples collected at the 

inlet, outlet, and bypass locations were analyzed for as many 

as 31 constituents. These constituents might include particulate and dissolved forms of solids, trace metals, particlesize distributions, and 18 polycyclic aromatic hydrocarbons 

(PAHs). Some of the constituents had been analyzed by 

previous studies of similar source areas, and these concentrations were compared with the results from this study. Rainfall 

records from the closest weather stations were used to evaluate the accuracy of the rainfall data measured at the study 

site. Sufficient particle-size distribution data were collected 

at the inlet and outlet to represent the efficiency of the device 

2 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
for each particle size. The accuracy of the loads measured at 

the different sampling locations was evaluated by comparing the difference in the inlet and outlet suspended-sediment 

(SS) loads to the amount of sediment trapped in the sump of 

the device. 

Previous Investigations

The USGS has a long history of conducting urban waterquality investigations in Wisconsin. Starting with the EPAfunded International Joint Commission (IJC) study in 1974 

(Bannerman and others, 1979) the USGS has been involved 

continuously with partners in Wisconsin trying to find solutions to urban runoff problems. The IJC study helped characterize the level of pollution contributed from urban areas to 

the Great Lakes. In 1978, the EPA established the Nationwide 

Urban Runoff Program (NURP) to assess the water-quality 

characteristics of urban runoff. When the City of Milwaukee, 

Wis., was chosen by the EPA as a NURP site, a partnership 

between the WDNR and the USGS was developed to evaluate 

urban runoff in Milwaukee. Since the NURP study, the USGS 

and the WDNR have continued their partnership and have 

completed more than 16 studies in at least six cities. Other 

studies have focused on understanding the cause of degradation in urban streams, the level of pollutant control needed to 

improve the urban resources, evaluating the efficiency of different stormwater control practices, and improving modeling 

of urban runoff. Results from all of these efforts have assisted 

Wisconsin and other States in the implementation of more 

cost-effective stormwater management programs. See appendix 1 for a list of references for these previous investigations.

Site Description

The hydrodynamic-settling device was installed in the 

parking lot of the Water Utility Administration Building in 

Madison, Wis., during the fall of 2004. The utility building 

is located at 119 East Olin Avenue in Madison. The latitude 

and longitude coordinates are 43°03’09”N and 89°22’55” W 

(U.S. Environmental Protection Agency, 2007). 

The building grounds cover about 5.5 acres, and 

1.91 acres of the grounds drain to the settling device (fig. 1, 

table 1). The device is sized to treat the runoff from the 

1.91 acres. All the runoff from the 1.91 acres is delivered 

to the device through a storm-sewer collection system. The 

storm-sewer system that collects the runoff reaching the 

device consists of 12- and 15-in. diameter concrete pipes. 

The catch basins do not have sumps, and there are no other 

stormwater control practices in the area. After leaving the 

outlet to flow diversion structure, the treated (and bypassed) 

water enters a wet detention pond also located on the Water 

Utility property.

Water leaving the wet detention pond flows through the 

city storm-sewer system to Wingra Creek, which is a tributary to Lake Monona. Wingra Creek is on the WDNR 303(d) 

impaired waters list (U.S. Environmental Protection Agency, 

1998, 2002b). Wingra Creek’s impairments are aquatic toxicity and contaminated sediment.

Madison, Wis., has a climate typical of interior North 

America, with a large annual temperature range and frequent 

short-period temperature changes. Nearly 60 percent of the 

approximately 33 in. of average annual rainfall falls in the 

months of May through September (National Climate Data 

Center, 2003). The average amount of snowfall is 44 in.

Site Description 3

Table 1. Characteristics of drainage area that contribute 

runoff to hydrodynamic-settling device at Madison Water Utility 

Administration Building, Madison, Wisconsin.

Land use Acres Total runoff contribution 

(percent)

Parking lot and 

roadway

1.05 55

Roofs 0.49 26

Lawn 0.29 15

Sidewalks 0.08 4
Y

l In et 10

Downstream Defender Elevation View

Water Utility 

Administration Building

City of Madison

Recycling Facility

Detention Pond

Schematic diagram of hydrodynamic-settling device

Discharge to Wingra Creek

Data from survey on 9/22/2005

Quann-Olin Parkway

Olin Avenue

Outlet

Pond Outlet

Inlet 3

Inlet 5Inlet 4

Inlet 6

Inlet 9 Inlet 11

Madison, WI

Structure

Inlet 11Inlet 10

Inlet 9Inlet 6Inlet 5Inlet 4Inlet 3

Outlet

Pond Outlet

* Invent of outlet pipe from manhole.

Rim Elevation

852.83853.31853.15853.98853.82853.76853.78

nana

Invert Elevation*

850.73850.66850.28850.06849.77849.59849.00848.75

847.84 (low)

849.24 (med)849.49 (high)

Figure 4-2

Site map and drainage area

Madison water utility 

Downstream defender site

Madison, WI

35946 July 2005

InletOutletDownstream defenderRain gageProperty lineStorm sewer

Drainage boundary to downstream defenderDetention pondBuildingLandscapeParking and drivewaysSidewalk

0 50 100

Feet

EXPLANATION

Figure 1. Location of study area in Madison, Wisconsin, drainage area of parking lot, inlets, manholes, hydrodynamic-settling device, and storm-sewer pipes (modified from J.A. Bachhuber, Earth Tech, Inc., written commun., 2005).

4 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
Figure 2. Hydrodynamic-settling device.

Access ports Floatables lid

Outlet pipe

Pipe coupling

Floatables storage

Sediment storage

Concrete manhole

Support frame

Dip plate

Tangential inlet

Center shaft

Center cone

Benching skirt

Design of Hydrodynamic-Settling 

Device

The 6-ft diameter hydrodynamic-settling device installed 

at the Madison Water Utility Administration Building site is 

designed to remove settleable solids, oil, and floatables in 

stormwater runoff. The device has no moving parts and no 

external power requirements. It consists of a cylindrical concrete vessel, with polypropylene internal components, a 304 

stainless-steel support frame, and connecting hardware (fig. 2). 

The concrete vessel is a standard precast cylindrical manhole 

with a tangential inlet pipe that is installed below ground. Two 

ports at ground level provide access for inspection and clean 

out of stored floatables and sediment. The internal components 

consist of two concentric hollow cylinders (the dip plate and 

center shaft), an inverted cone (the center cone), a benching skirt, and a floatables lid (U.S. Environmental Protection 

Agency, 2007).

The device is designed to collect accumulated sediment 

outside the treatment flow path and beneath the benching 

skirt. The internal components help minimize turbulence and 

hydraulic-head losses, which can enhance separation and prevent resuspension of previously stored sediment. The center 

cone is one component that protects resuspension of previously stored sediment by redirecting the main flow upwards 

and inwards under the dip plate into the inner annular space 

(fig. 2).

The installation includes a flow diversion structure that 

is located approximately 13 ft north of the device (fig. 3). 

Stormwater runoff from the drainage area enters the 5-ft diameter flow diversion manhole by a 1.125-ft polyvinyl chloride 

(PVC) pipe (fig. 3, point A). The manhole housed a 0.97-ft 

PVC device inlet pipe (point B), a 1.4-ft PVC device outlet 

pipe (point C), a 1.1-ft system outlet PVC pipe (point D), and 

a 2-ft high weir wall (point E). When flow is bypassed over 

the weir, the hydrodynamic-settling device and the diversion 

structure are acting as a system. Because the settling device 

used for the study is sized to treat flows up to 3 ft3

/s, the 

diversion structure will bypass when flows exceed 3 ft3/s up 

to 8 ft3/s.

The hydrodynamic-settling device installed at the study 

site is designed to remove settleable solids from stormwater runoff. Generally, the removal efficiency of the device 

decreases or possible negative with increasing flow rates, finer 

particles, and cooler water temperatures. For runoff at 15 °C 

and flow rates up to 3 ft3/s, the device will remove over 80 percent of settleable solids when the specific gravity equals 2.65 

with a particle-size distribution (PSD) similar to the Maine 

Department of Transportation’s road sand. Hydro International 

defines “settleable sediment” as particles greater than 62 μm 

in size. Because 80 percent of the particles in the road sand 

PSD are about 290 μm in diameter or greater, the device can 

achieve the performance standard by trapping mostly particles 

in the medium sand-size fraction or greater (U.S. Environmental Protection Agency, 2007).The device has a sediment storage capacity of 56.7 ft3

.

Design of Hydrodynamic-Settling Device 5
Methods of Data Collection

Stormwater runoff was measured and collected at the 

inlet, outlet, and bypass pipes of the hydrodynamic-settling 

device. Each pipe was equipped with automated stormwaterquality samplers and instruments to measure stage height and 

velocity. Rainfall depths were collected by use of a tippingbucket rain gage. Measurement, control, and storage of data 

were done by way of electronic data loggers. Data were automatically retrieved twice daily with telephone modems.

Water-Quality Sampling and Analysis 

Water-quality samples were collected from the inlet, 

outlet, and bypass weir (sampling points B, C, and E, fig. 3) 

of the device over 11 months. Station identification numbers 

6 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

and names for each sampling location are 430310089225401 

Downstream Defender Unit Inlet at Madison, Wis. (point B); 

430310089225402 Downstream Defender System Outlet at 

Madison, Wis. (point C); and 430310089225403 Downstream 

Defender Unit Outlet at Madison, Wis. (point E). The analytical constituents were selected based on the performance information from the manufacturer and the regulated constituents 

that the WDNR might want to control in the future (tables 2 

and 3). Samples were analyzed at the Wisconsin State Laboratory of Hygiene (SLOH), which participates in the USGS 

Standard Reference Sample (SRS) Program (Woodworth and 

Connor, 2003).

Automatic samplers were programmed to collect flowweighted samples from points B, C, and E (fig. 3). To collect 

samples at the inlet (point B), a water-quality sample line 

was installed 2 ft downstream from where flow enters the 

device, and to collect a sample at the outlet, a water-quality 

D

B

C

A

E

Flow-volume monitoring at points B, C, D

Water-quality sampling at points B, C, E

Figure 3. Flow system and location of flow monitoring and water-quality sampling equipment for hydrodynamic-settling device (from 

Hydro International, Portland, Maine, 2004).
sample line was installed 2 ft upstream from where flow exits 

the device (point C). To sample bypassing runoff events, a 

water-quality sample line was installed 1 in. below the top of 

the upstream side weir wall (point E). All sample lines were 

Teflon lined and installed perpendicular to flow and approximately 1 in. from the bottom of the pipe. The automatic samplers were programmed with a pre-sample purge and rinses 

cycle before each sample. Sample lines were not replaced 

between events. The data logger in the monitoring station was 

programmed to initiate a subsample for a predefined volume 

of flow; consequently, more subsamples were collected for 

large-volume runoff events than for small-volume runoff 

events. Flow-weighted sampling allowed for the collection of 

one composite sample per stormwater runoff event, consisting 

of numerous subsamples collected throughout the course of 

the event. This approach resulted in a single flow-weighted or 

“event mean” concentration for each runoff event. The volume 

between subsamples was determined such that a minimum of 

five 1-L subsamples were collected for each event. The maximum sampler capacity was 40 1-L subsamples into 4 glass 

bottles. For events greater than or equal to 0.2 in. of rainfall 

and a minimum of five 1-L subsamples, the subsamples were 

processed for all constituents (tables 2 and 3); otherwise, subsamples were processed for concentrations of SS, TSS, and 

total dissolved solids (TDS). Samples were processed according to the churn-splitting procedure described by Horowitz and 

others (1997) and further described in this report.

Methods of Data Collection 7

Table 2. Limits of detection and analytical methods for inorganic constituents analyzed in samples 

collected at the hydrodynamic-settling device in Madison, Wisconsin.

[mg/L, milligrams per liter; P, phosphorus; μg/L, micrograms per liter; NA, not applicable]

Constituent or characteristic Unit Limit of 

detection

Limit of 

quantification1 Method

Dissolved solids, total mg/L 50 167 SM2540C2

Suspended solids, total mg/L 2 7 EPA 160.23

Volatile solids, total mg/L 2 7 EPA 160.23

Suspended sediment mg/L 2 7 ASTM D3977–972

Phosphorus, dissolved mg/L as P 0.005 0.016 EPA 365.13

Phosphorus, total mg/L as P 0.005 0.016 EPA 365.13

Calcium, total recoverable mg/L 0.02 0.07 EPA 200.72,3

Magnesium, total recoverable mg/L 0.03 0.7 EPA 200.72,3

Zinc, dissolved4 mg/L 1 3 EPA 200.92

Zinc, total recoverable4 mg/L 1 3 EPA 200.92

Copper, dissolved mg/L 1 3 SM3113B2

Copper, total recoverable mg/L 1 3 SM3113B2

Wet-sieve of sediment NA NA NA Burton5

Coulter counter of sediment NA NA NA Burton5

Laser diffraction of sediment NA NA NA Burton5

Microfiltration of sediment NA NA NA Burton5

1

Limit of quantification is the low standard in the calibration curve.

2

American Public Health Association and others (1989); SM, standard methods.

3 U.S. Environmental Protection Agency (EPA) (1986).

4The first 3 events had limit of detection and limit of quantification of 16 and 50, respectively.

5 Burton and Pitt (2002).
Table 3. Limits of detection and analytical methods for polycyclic

aromatic hydrocarbons analyzed in samples collected at the 

hydrodynamic-settling device in Madison, Wisconsin.

[All data in micrograms per liter determined by use of method SW8310 in 

American Public Health Association and others (1989)1]

Constituent Limit of detection Limit of 

quantification2

Acenaphthene 0.064 0.20

Acenaphthylene 0.11 0.34

Anthracene 0.031 0.10

Benzo[a]anthracene 0.093 0.30

Benzo[a]pyrene 0.16 0.52

Benzo[b]fluoranthene 0.13 0.41

Benzo[g,h,i]perylene 0.14 0.44

Benzo[k]fluoranthene 0.12 0.38

Chrysene 0.027 0.09

Dibenzo[a,h]anthracene 0.034 0.11

Fluoranthene 0.11 0.35

Fluorene 0.52 1.7

Indeno[1,2,3-cd]pyrene 0.093 0.30

1-Methylnaphthalene 0.064 0.20

2-Methylnaphthalene 0.049 0.16

Naphthalene 0.042 0.13

Phenanthrene 0.093 0.30

Pyrene 0.11 0.34

1 American Public Health Association and others (1989); SM, standard 

methods.

2

 Limit of quantification is the low standard in the calibration curve.

Particle-Size Analysis

In July 2004, the USGS Wisconsin Water Science Center 

adopted a new method for particle-size analysis. Previous 

methods required a large sample volume to provide enough 

sediment for analysis. Previous methods were not designed 

for the relatively small amounts of sediment observed in 

stormwater samples. The new method requires only about a 

liter of sample and has been incorporated into the evaluation 

described in this report. The new particle-size analysis uses 

a two-step protocol suggested for stormwater by Burton and 

Pitt (2002). The protocol was adopted for use in Wisconsin 

by SLOH.

The first step was to wet sieve the sample for the particle 

sizes of 500, 250, 125, 63, and 32 μm in diameter. All the 

material remaining on the sieves was then dried and weighed 

to determine mass. Particles less than 32 μm in diameter 

remaining in the filtrate were further delimited into four additional particle sizes by either using a laser-based diffraction 

instrument or a Coulter Counter (Beckman Coulter Multisizer 

3 particle-size counter; Graham and Beckman Coulter, Inc., 

2003). These instruments were used to determine the percentage of sediment by mass with diameters less than 14, 8, 5, and 

2 μm in diameter. Laser diffraction was used on the samples 

at the beginning of this evaluation, and the Coulter Counter 

was used on all the remaining samples. Other researchers have 

used a Coulter counter to evaluate particle sizes in stormwater 

(Burton and Pitt, 2002). The Coulter counter was calibrated by 

microfiltering replicate samples with polycarbonate filters.

Processing of Water-Quality Samples

A new procedure was used to improve the accuracy 

and precision of measured quantity of particulate constituents in samples that contained a large amount of sand-sized 

particles (greater than 125 μm in diameter). Previous studies have shown that using a churn to partition samples with 

large quantities of sand had the potential to cause a positive 

bias and to lessen the precision of constituent concentrations 

associated with particulates (Horowitz and others, 1997). 

The use of a wet-sieving process decreased these errors for 

sediment-associated constituent concentrations (Selbig and 

others, 2007). This process consisted of pouring a known 

quantity of sample through sieves of 125, 250, and 500 μm in 

diameter before churning the aqueous portion. Material collected on sieves was sent to the SLOH in individual bottles to 

be dried and weighed. Dried material from each of the sieves 

was then combined and processed for total recoverable metals and phosphorus. This process was used for 10 of 23 runoff 

events, which were determined by stirring the samples and 

observing at least 2 g of material at the bottom of the bottle 

after 1 minute. For samples from the 10 runoff events, large 

amounts of material dropped to the bottom of the glass bottle 

within 1 minute of stirring the sample. The aqueous portion 

of the sample that passed through the sieves was processed 

using typical USGS churning procedures (Horowitz and others, 1997). Concentrations of SS presented in this report may 

include sieved material.

Sample results of the sieved mass were added back to 

the aqueous portion to determine a mean concentration for 

the runoff event by using the following equation (Selbig and 

others, 2007): 

C S I = (( ) m C / / 1000 × s V ) , (1)

where

CI is concentration of sieved solids, in milligrams 

per liter;

Sm is mass of sieved solids after drying, in grams;

Cs is concentration of constituent in sieved solids, 

in milligrams per kilogram; and

V is volume of sieved water, in liters.

8 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
The sieving process eliminates sand-size particles resulting in a negative bias when TSS is analyzed. Conventional 

TSS analysis may include particles as large as 500 μm in 

diameter. To adjust TSS (“adjusted” TSS) concentrations that 

include these larger particles, an independent study was conducted by the USGS. The independent study compared the 

TSS aliquot method to the concentration of SS whole-bottle 

method. Concentration of SS, TSS, and PSD were processed 

on 13 nonsieved samples. From this analysis, particles of 63 

μm in diameter or less were found to have similar concentrations of SS and TSS. However, for particles larger than 63 μm 

in diameter, TSS had lower concentrations than SS. The TSS 

to SS particle-size ratios were 0.29 for 500 μm in diameter, 

0.32 for 125 μm in diameter, and 0.57 for 250 μm in diameter, 

(William Selbig, U.S. Geological Survey, written commun., 

2006). Therefore, SS particle-size concentrations of the sieved 

samples were multiplied by the respective ratios and then 

added to the TSS concentration. The “adjusted” TSS concentrations in this report reflect this process.

Quality Control

The field-equipment blank and replicate samples were 

collected at the inlet and outlet of the settling device and 

analyzed for the same constituents as those for runoff samples (appendix. 2). The equipment blank procedure passed 

deionized water through the sampler and processed the 

water through the churn splitter to validate clean sampling 

procedures (Wilde, 2006). Blank samples were collected at 

the beginning and midpoint of the study to validate cleansampling procedures.

Three equipment blank samples (at points B, D, and E) 

were collected to test clean-sampling procedures. The first 

equipment blank set was collected before sampling began 

(blank set 1) and the second set between runoff events 7 and 

8 (blank set 2). The blank 1 sample contained no detectable 

concentrations at the settling device inlet, device outlet, and 

bypass wall (points B, D, and E, fig. 3). The blank set 2 sample had detectable concentrations of dissolved copper (DCu), 

total recoverable copper (TCu), dissolved zinc (DZn), and 

total recoverable zinc (TZn) at the three sample points, but the 

dissolved concentrations were less than the limit of quantification (LOQ). The TCu concentrations at the three sampling 

points exceeded the LOQ, but TCu concentrations were not 

used to evaluate the device for concentration reductions. Quality-control samples collected directly from the sampler and 

from the bottle of blank water were analyzed for dissolved, but 

not total, concentrations; analyses resulted in detections below 

or at limit of quantification (appendix. 2).

Analytical precision is a measurement of how much an 

individual measurement deviated from a mean of replicate 

measurements. Replicate churn samples were collected during 

several stormwater runoff events to quantify the variability or 

precision in the churn splitting and analytical procedures. The 

relative percent difference (RPD) was calculated to evaluate 

precision in procedures after sample collection.

The relative percent difference equation is:

RPD = − [(X X 1 2 ) / X ] , ×100 (2)

where

X1 is concentration of constituent in a sample;

X2 is concentration of a constituent in a duplicate sample; 

and

X̅ is mean value of X1 and X2.

Replicate samples were collected during runoff events 

5 and 17 to quantify variability in the sampling process. The 

RPD target for TSS was 30 percent or less; for recoverable 

metals, the RDP target was 25 percent or less (appendix 3). 

In replicates for runoff event 17, the RDP target of 25 percent 

was exceeded for TZn. For all other constituents a relatively 

low RPD was reported.

Rainfall

A tipping-bucket rain gage was used for continuous 

rainfall measurement to measure the volume and intensity of 

rainfall for each storm. A data logger recorded the number of 

bucket tips (0.009 in. per tip) every 60 seconds. This gage was 

not designed to record frozen precipitation, so measurements 

during periods of snowfall and freezing rainfall were not 

used. Calibration records showed there was no need to adjust 

rainfall data. All rainfall amounts are listed in appendix 4. To 

accurately record rainfall amounts during varying intensities, 

a microprocessor in the rain gage used a built-in polynomial 

to correct for the intensity, which was based on the tipping 

bucket’s mechanism (Design Analysis Associates, 2001).

The rain gage was 10 ft north of the hydrodynamicsettling device (fig. 1). It was mounted on a 2-in. round pipe 

raised 10 ft above ground. The rain gage was affixed to the 

Water Utility’s security fence. During two rain-gage calibrations, debris was cleaned from inside the gage.

Flow Monitoring

Area-velocity flow meters that use continuous wave Doppler technology were mounted inside the pipes at flow-volume 

monitoring points B, C, and D (fig. 3) to measure stage, velocity, and flow volume. The flow meter at point D measures flow 

from the outlet pipe (point C) and the flow that bypasses the 

weir wall (point E). A stand-alone stage pressure transducer 

and temperature probe were installed on the weir (point E). 

Stage from this probe was used to initiate the bypass sampling 

routine when a given stage threshold was reached. The bypass 

sample tube was located on the upstream side of the weir wall.

Methods of Data Collection 9
Calibration of Stage 

Corrections were applied to stage measurements (October 

4, 2005, and May 10, 2006) that reflect differences between 

water-surface elevations measured manually and those measured with the area-velocity flow meters. Calibration procedures consisted of inflating balls in the pipes near points A and 

B (fig. 3) to seal off a catchment area upstream of the device. 

Flow meters (points B, C, and D) were calibrated simultaneously by moving all three meters into the catchment area. 

Using a garden hose, water stages inside the catchment area 

were increased by increments of 0.1 to 0.15 ft. Ten to 15 readings were taken at each flow meter. These measurements were 

then compared to what was recorded by the respective flow 

meters. Accuracy of the stage records, on average, was estimated to be within ± 2 percent for the three flow meters.

Stage measurements were also checked periodically 

under low flow or standing water conditions. The device never 

completely drained, and offsets were applied directly to the 

flow meters on March 31, 2006, to account for the standing 

water conditions. Before this date, corrections were made to 

the stored stage data. Stage calibration on May 10, 2006, concluded all meters were recording correctly; therefore, no stage 

corrections were necessary.

The stand-alone stage pressure transducer on the weir 

wall was calibrated on May 10, 2006. The design of the probe 

prevented stage from being recorded until water reached a 

height of 0.58 ft. The stage was offset by 0.84 ft.

Calibration of Flow

Stormwater runoff was measured at the inlet, outlet, and 

bypass pipes of the device. A dye dilution system was installed 

to calibrate flow rate at the device inlet (point B). The device 

outlet and system outlet (points C and D) were corrected using 

the calibrated values from the device inlet flows.

Inlet Flows

In April 2006, an automatic dye dilution system was 

installed to calibrate flow. A pump that injected a dye of a 

known concentrations was mounted in pipe at point A (10 ft 

upstream from the weir wall, fig. 3). A fluorometer that samples a mixture of stormwater and dye was mounted alongside 

the device inlet sampling line (a fluorometer measures the 

concentration of dye fluorescence). A separate gage house 

for sampling dye and recording data was located adjacent to 

the sampling gage houses. A dye-dilution event was triggered 

when a given stage threshold was reached at device inlet.

The equation to convert dye concentrations to flow 

follows:

Q q = ×C c , (3)

where

Q is flow being measured, in cubic feet;

q is injection rate, in milliliters per minute;

C is concentration of injected dye, in percent by volume; 

and

c is concentration of measure, in micrograms per liter.

Runoff events from May 9, 11, and 16 produced more 

than 650 sample points of calibration at the inlet meter. Comparison of the inlet area/velocity flow (fig.4, point B) and the 

dye dilution flow yielded a ±8 percent difference per event in 

flows ranging from 0.03 to 0.41 ft3 (fig. 4). Results from these 

events concluded that the device inlet meter was recording 

flow accurately for the majority of flows in the calibration 

range. Fourteen percent of the peak flows were higher than 

the dye-dilution calibration ranges, however accurate stages 

were recorded for the entire pipe depth during the static-stage 

calibration therefore higher flows should be within the + 8 

percent. Flow meters at the settling device and system outlets 

could not be calibrated by dye dilution because it was unclear 

how the dye would mix inside of the device.

System Outlet Flows

Five of the 26 events sampled recorded stormflow 

bypassing the hydrodynamic-settling device and passing 

over the weir wall (point E, fig. 3). To estimate the amount of 

flow bypassing the settling device to the meter at point D, a 

relation correlating flows between the calibrated inlet meter 

(point B) and the system outlet meter (point D) was established. This relation was based on four assumptions: (1) no 

time delay from the inlet and system outlet, (2) only the rises 

in the hydrograph were used (this eliminated inaccuracy with 

meters located backwater from the detention pond), (3) flows 

that recorded bypassing were not included in the relation, and 

(4) runoff events before March 31, 2006, were not used (there 

was a shift in stage before this date; therefore, there was a 

different stage-flow relation). A scatter plot of flows from the 

system outlet (point D) to the device inlet (point B) indicated 

that the system outlet flows were small (fig. 5). A regression 

line of y =1.1557x + 0.0326 for a stage above 0.05 ft was used 

to correct flows at the system outlet. Several other regressions 

fit a higher flow regime but produced larger errors for overall 

flows. For events that did not include bypass flow, the average difference between the volume of flow measured at the 

device inlet and the volume of flow measured at the system 

outlet after the downstream area-velocity data was adjusted 

by –2 percent. For events without bypassing, the average difference between the inlet and the adjusted system outlet flows 

were –2 percent.

10 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
Device Outlet Flows

Flows from the device outlet (point C) flowmeter were 

not used to calculate loads for three reasons: (1) flows from 

device outlet and inlet should be equal, and the inlet meter was 

calibrated with dye dilution; (2) the device outlet flows overestimated inlet flow; and (3) bypassing conditions may affect the 

device outlet.

Monitoring Complications

Backwater from Pond. It was noticed in the fall of 2005 

that stormwater was taking several hours to drain out of the 

device. There was standing water at the system outlet that 

could have affected the device performance. The detention 

pond (downstream from the device) was backing water into 

the manhole. In January 2006, the detention pond control 

structure was widened to reduce pond elevation.

No Flow Data Spikes. Programming changes between 

the data logger and the velocity meter were added that omitted data spikes during no flow. Communications from the 

area-velocity meter to the data logger were managed through 

serial string translation. During periods of no flow, data were 

recorded for the first minute of the hour. When particles were 

not available for the area-velocity meter, the meter could not 

correctly determine the velocity within that minute; therefore, 

the data logger translated the velocity data as an extremely 

high or low data point. To replace the high or low data point 

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

5/16/06 17:00 5/16/06 17:07 5/16/06 17:14 5/16/06 17:21 5/16/06 17:28 5/16/06 17:36 5/16/06 17:43

Area-velocity reading

Fluorometer reading

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

5/9/2006 13:15 5/9/2006 13:36 5/9/2006 13:57 5/9/2006 14:19 5/9/2006 14:40 5/9/2006 15:02 5/9/2006 15:23 5/9/2006 15:45

Flow, in cubic feet per second Flow, in cubic feet per second

Area-velocity reading

Fluorometer reading

Figure 4. Comparison of flow measured using dye dilution fluorometer and inlet area-velocity meter for two runoff events, May 9 

and 16, 2006.

Methods of Data Collection 11
0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

System outlet flow, in cubic feet per second

y = 0.0546x2 + 1.1811x

R2 = 0.8984

Inlet flow, in cubic feet per second

Figure 5. Graph showing stormwater flow volumes at flow system outlet corrected by dye dilution in relation to inlet flow 

volumes, 2006.

12 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

with the last valid velocity recorded by the area-velocity 

meter, high and low cutoff thresholds were programmed into 

the data logger. To validate removal of these high or low data 

points, the velocity data recorded by the data logger were 

compared to velocity recorded by the internal memory of the 

area-velocity meter. The area-velocity meter stored 15-second 

data for approximately 2 days then overwrote it with new data. 

Programming changes were made in April 2006.

Power Failures. During the summer of 2006, a few runoff events were missed due to power failures with the samplers. High summer temperatures increased the power needs 

to cool refrigerators. When the sampler tried to take a sample, 

there was not enough power to turn on the pumps, resulting in 

power failures. Adding an extra battery to the system alleviated this problem.

Rodent Damage. Sample tubing was replaced at the 

inlet sampler in June 2006 due to damage from a rodent. This 

caused a runoff event on June 10, 2006, to be missed.

Water Temperature Probe Failure. A temperature probe 

mounted near the bypass wall did not record correctly after 

May 1, 2006. Temperature readings from another monitoring 

site, 3 mi northeast of the settling device, were substituted 

for the remaining events. The substituted probe was installed 

in May 2006 at a downtown Madison, Wis., parking lot 

stormwater monitoring site.

Hydrograph Errors for Two Runoff Events. It was noticed 

that the ends of two flow event hydrographs showed discrepancies in flow; there was more flow at the inlet sampler than 

at the outlet sampler (events for August 24 and 25, 2006). 

To determine which meter was in error, stage versus velocity 

plots were made for several sampled runoff events. From these 

plots, the inlet and system outlet meters produced a regression 

equation with correlation coefficients (R2

) of 0.97 and 0.93, 

respectively, but the system outlet meter had greater scattered. 

By inserting the events in question on the regression plot, 

volumes were visibly low. The tail end of the system outlet 

hydrograph was corrected for those two events by replacing 

the low volume with the recorded inlet volume.

Overflow from Adjacent Recycling Facility. West of the 

Madison Water Utility Administration Building is a City of 

Madison recycling facility with outside storage of yard and 

brush waste. Most of this storage area is impervious. During 

the first inspections of the study site, it was noticed that some 

of the runoff from the recycling facility could potentially enter 

the drainage area being monitored. To reduce the chances of 

this additional runoff from entering the study area, the City of 

Madison constructed a 3-in. high speed bump diversion across 

the driveway connecting the two properties (fig. 6).

Blockage of Culvert Draining Adjacent Recycling Facility. During the last month of the monitoring period, the pipe 

designed to drain a large portion of the adjacent recycling 

facility became clogged with sediment. This pipe (fig. 1) 

is located in a grassy depressed area on the west end of the 

study area parking lot (figs. 7 and 8). Visual observation of 

the debris line around this pipe indicated that the depression 

would fill with runoff and overflow into the drainage area for 

the monitoring site. Based on site inspections, the extent of 

additional drainage could be as much as 4 acres for a total 

drainage area of 5.9 acres. The additional runoff volume 

would vary with rainfall factors such as depth and intensity 

and with extent of blockage at the inlet. The additional flow 

would not decrease the accuracy of the flow meters, but 

the higher flows could increase the chances of some runoff 

bypassing the settling device.
Figure 6. Watershed barrier (speed bump) 

installed to prevent runoff from City of 

Madison recycling facility (fenced area to 

left of photograph).

Figure 7. Debris on speed bump 

overtopping watershed divide with 

stormwater runoff from adjacent City of 

Madison recycling facility, August 2006.

Figure 8. Blockage of culvert draining 

the Madison recycling facility adjacent to 

the Madison Water Utility Administration 

Building, August 2006.

Methods of Data Collection 13
Evaluation of Hydrodynamic-Settling 

Device

Rainfall, flow volume, particle-size, and water-quality 

data were all important in evaluating the effectiveness of the 

hydrodynamic-settling device. A comparison of monitored 

rainfall depths and long-term trends in rainfall depths helped 

determine if the monitoring data were representative of rainfall 

patterns in Madison. The flow data were needed to determine 

the volumes of runoff entering and leaving the settling device 

and bypass system. The ratio of runoff volumes to rainfall 

volumes was used to help test the accuracy of the runoff-flow 

data. Both flow volume and constituent concentrations were 

needed to evaluate the efficiency of the settling device. Particle-size data helped explain the magnitude of the efficiency 

determined for the device.

Rainfall Data

For the study period of November 5, 2005, to September 

12, 2006, the rain gage on the study site recorded 71 rainfall 

events (appendix 4). Because the rain gage could not be used 

to measure frozen rainfall accurately, the data set in appendix 

4 does not include any events in December 2005, January 

2006, and February 2006. In addition to the rainfall depths, 

appendix 4 includes the rainfall volumes, maximum 15- and 

30-minute rainfall intensities, the erosivity index (a measure 

of the erosive force of a specific rainfall), and the number of 

days without rain before each event. Rainfall data collected at 

the site were compared to National Oceanic and Atmospheric 

Administration (NOAA) data collected at the Dane County 

Regional Airport (DCRA), which is about 6 mi from the study 

site (table 4).

The difference between the total from the USGS (onsite) 

rainfall data and the 2005–6 total from the DCRA rainfall data 

was less than 1 percent. This indicates that the rain gage at the 

study site was comparable to the DCRA rainfall data, where 

larger differences generally occurred for individual months, 

but localized convective storms can cause substantial differences over distances as small as 6 mi. The rainfall at the Madison Water Utility Administration Building was 3.5 in. higher 

than the long-term (1971–2000) average at DCRA (National 

Oceanic and Atmospheric Administration, 2007).

Probability distributions for USGS and DCRA rainfall 

data sets were constructed by use of the Weibull plotting 

position (Helsel and Hirsch, 1992). Rainfall amounts for individual rainfall events were computed for both data sets. Rainfall amounts greater than or equal to 0.05 in. (the minimum 

amount recorded during this evaluation) were ranked from 

lowest to highest. A cumulative probability distribution then 

was computed for both data sets by use of the formula:

14 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Table 4. Comparison of monthly rainfall between U.S. Geological Survey rain gage at study site and 

National Oceanic and Atmospheric Administration rainfall gage at Dane County Regional Airport, 

Madison, Wisconsin, November 2005–September 2006.

[Rainfall is presented in inches. USGS, U.S. Geological Survey; DCRA, Dane County Regional Airport;—. no data]

Month and year USGS rain gage DCRA rain gage DCRA long-term average1

November 2005 2.5 3.4 2.3

December 2005 — — —

January 2006 — — —

February 2006 — — —

March 2006 1.7 2.3 2.3

April 2006 5.8 4.2 3.4

May 2006 3.8 4.6 3.2

June 2006 1.9 2.3 4.0

July 2006 5.8 4.2 3.9

August 2006 5.1 5.4 4.3

September 2006 3.4 3.3 3.1

Total 30 30 27

1

Average from 1971–2000 for Dane County Regional Airport, Madison, Wis. (National Oceanic and Atmospheric 

Administration , 2007).
0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

0.0 0.2 0.4 0.6 0.8 1.0

Historical

Downstream defender

Cumulative frequency for all rainfall greater than 0.05 inch

Precipitation, in inches

Figure 9. Cumulative rainfall for study site (2005–6) in relation to cumulative frequency for all rainfall greater than 0.05 inch 

(1949–92) determined on basis of National Oceanic and Atmospheric Administration rainfall gage at Dane County Regional 

Airport, Madison, Wisconsin.

P i n R R = + / , ( ) 1 (4)

where

R is precipitation event;

PR is probability of an event having precipitation less than 

that of event;

i

R is ranking of event R; and

n is total number of events in the data set.

Except for a moderate deviation for rainfall depths 

between 0.65 and 0.9 in., the distribution of the sampled 

events was very similar to the long-term distribution (fig. 9).

Stormwater Flow Data

Stormwater flow data were collected for 47 of the 71 

rainfall events from November 2005 to September 2006 

(appendix 5). All of the flow data were collected during warmweather months. Because the settling device outlet flows 

(point C in fig. 3) were not calibrated, the flow from the device 

outlet was less accurate than the device inlet flows (point B in 

fig. 3). The stormwater flow data from the settling device outlet were excluded from the data analysis. Because the inlet and 

outlet flow volumes are the same, the device inlet flows were 

used to represent the device outlet flows. These flow data were 

also used to determine (1) peak flow rates at the settling device 

inlet and system outlet (point D in fig. 3), (2) inlet volume, 

(3) volume over the flow splitter weir (point E in fig. 3), and 

(4) runoff coefficient for the settling device system. Only the 

flow events on April 16, May 24, July 27, and August 23 and 

24, 2006, had enough bypass volume to significantly affect the 

magnitude of the percentage of runoff. 

Most of the data representing flow had a wide range 

in values. The device inlet peak flows ranged from 0.10 to 

2.9 ft3

/s (appendix 5). The system outlet peak flows ranged 

from 0.04 to 5.8 ft3/s. The maximum peak flows at the system outlet should be higher than the flows at the device inlet 

because the flow splitter is designed to limit the flows to the 

device inlet to 3 ft3. The runoff volumes at the device inlet 

ranged from 138 to 17,200 ft3 with majority of the volumes 

greater than 1,000 ft3. The volume of the sump below the outlet pipe elevation is about 147 ft3

, so the storage volume of the 

device is replaced at least seven times for most flow events.

Water-quality samples were collected for 26 of the 47 

flow events with measured flow (appendix 5). The range of 

rainfall depths for the sampled events was 0.05 to 2.2 in. 

with a mean of 0.59 in. There were 13 runoff events that had 

rainfall depths less than 0.2 in. Only 5 of the 13 events were 

sampled because there was insufficient depth of water in the 

pipe to collect a sample. Twenty-five of the 47 flow events had 

between 0.2 and 1.0 in. of rainfall, and samples were collected 

from 13 of them. One was not sampled due to rodent damage 

on the sample line. There were nine rainfall events greater than 

1 in., and eight were sampled; the event not sampled was due 

Evaluation of Hydrodynamic-Settling Device 15
to overheating of the sampler. Nine of the 25 flow events with 

rainfall greater than 0.2 in. were not sampled because they 

occurred either before the project started or after project sampling was completed but before sediment was removed from 

the device. Overall, the number of stormwater flow events not 

sampled due to equipment problems was minimal for rainfall 

events greater than 0.2 in. Twenty-one of the 25 flow events 

with rainfall greater than 0.2 in. were sampled for a sampling 

efficiency of 84 percent during the sampling period.

The settling device was designed to treat up to 3 ft3/s 

before flow bypassing begins, and bypassing did occur when 

flows exceeded 3 ft3/s (appendix 5). This occurred during 

seven stormwater flow events, and for five of those events, 

water-quality samples were collected at the weir wall. The 

sampled flow events occurred on April 16, May 24, July 27, 

and August 23 and 24, 2006, and the bypass events that were 

not sampled were on July 9 and September 12, 2006. On July 

9, 2006, flow bypassing occurred for 4 minutes of the flow 

event; however, this was too short for the bypass sampler to 

be activated. Stormwater flow events on May 24, July 27, and 

August 24, 2006, had flow bypassing for almost the entire 

event, whereas the events on April 16 and August 23, 2006, 

flow bypassed the settling device for less than 5 percent of the 

flow duration. Of the eight sampled events with rainfall of 1 

in. or greater in depth, five of the stormwater flow events were 

the sampled events with bypass flows (appendix 5). The difference between the bypass flow events with larger rainfall and 

those with no flow bypass is the intensity of the rainfall. The 

peak 15-minute rainfall intensity for the sampled flow events 

with bypass ranged from 2.0 to 5.5 in/hr, whereas the three 

larger events without bypass had peak 15-minute intensities 

ranging from 0.4 to 0.9 in/hr.

Results from 3 of the 26 sampled flow events were not 

used in any of the calculations in the rest of this report. The 

dates for these events were May 15 and 24, and July 27, 

2006. Data from the event on May 15, 2006, were not complete because the sampler at the device outlet did not collect 

enough water to process. Not only did the flow events on May 

24 and July 27, 2006, have continuous bypass, but the pipes 

were surcharging to the point where water was coming out of 

the manhole at the top of the device. The high intensities and 

large rainfall volumes for these two runoff events produced 

pipe-full conditions that lasted more than 15 minutes for each 

event. It was not possible to measure flow accurately under 

these conditions.

Comparison of Runoff Volumes to 

Rainfall Volumes

Another check on the accuracy of the stormwater flow 

and rainfall measurements was used to calculate the runoff 

coefficients for each flow event. By dividing the volume of 

rainfall into the runoff volume, it was possible to determine 

whether the amount of rainfall produced the expected amount 

of runoff. The percentage of directly connected impervious 

area was an important factor in the magnitude of the runoff 

coefficient, and the percentage of connected impervious area 

in the study site was about 84 percent. This value assumed that 

all of the parking lot, roof, and sidewalks were directly connected. For small rainfall depths, almost all of the runoff originated directly from connected areas, but pervious areas and 

disconnected impervious areas contributed during larger flow 

events. Runoff coefficients estimated for areas with different 

amounts of directly connected imperviousness clay soils indicated that the runoff coefficients for the study area probably 

average about 60 percent (Bochis-Micu and Pitt, 2005).

For most sampled runoff events, the runoff coefficients 

ranged between about 75 and 85 percent (appendix 5; fig. 10). 

The average of runoff coefficients for 23 of the sampled events 

was 84 percent. This is higher than the expected value of about 

60 percent. The runoff events on August 23 and 24, 2006, 

significantly increased the average, with runoff coefficients of 

about 133 and 216 percent, respectively. The runoff volume 

for these two events was larger than the recorded rainfall volume. These two runoff events not only occurred during the 

last month of the study when the inlet draining the adjacent 

recycling center was clogged (described in more detail in the 

section on “Methods of Data Collection”), but they also represent relatively large rainfalls. Although the flow measurements 

were probably accurate for these two events, the overflow 

from the adjacent recycling center could have easily increased 

the runoff coefficients above 100 percent. Without these two 

events, the average runoff coefficient decreases to 75 percent.

Although the 75 percent might be close enough to the 

suggested value of 60 percent, it is closer to the average runoff 

coefficient of 70 percent observed for runoff from a hospital 

site in Green Bay, Wis. (Horwatich and others, 2004). The 

hospital drainage area had a connected impervious value of 

90 percent, and the Bochis-Micu curves estimated a runoff 

coefficient of about 70 percent for an area with 90 percent 

connected imperviousness. It might be reasonable to increase 

the study site connected imperviousness by 6 percent to about 

90 percent if the new lawns at the study site are assumed to 

produce more runoff than the calculations done by BochisMicu. Runoff measurements for lawns 1 to 3 years old and 

those older than 10 years showed that the runoff volumes from 

newly developed lawns was significantly greater than runoff 

from older lawns (Legg and others, 1996). The potential for 

extra runoff from the 0.29 acre of new lawn at the Water Utility site could be responsible for the somewhat higher than 

expected average runoff coefficient for the site. By assuming 

a slightly higher percentage of imperviousness for the site, the 

flows and rainfall depths measured in this study appear good 

enough to produce an accurate average runoff coefficient.

16 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
40

80

120

160

200

240

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Precipitation, in inches

Runoff, in percent

Figure 10. Comparison of percentage of parking lot runoff and rainfall for hydrodynamic-settling device, November 

2005–August 2006.

Evaluation of Hydrodynamic-Settling Device 17

Water-Quality Data for Inlet and Outlet

Runoff event mean water-quality concentrations were 

determined for 31 constituents at the inlet and outlet of the 

hydrodynamic-settling device (appendixes 6–8). Eighteen 

of the constituents were individual compounds of polycyclic 

aromatic hydrocarbons (PAHs). Analysis for concentrations of 

SS, TSS, and TDS were done for all 23 sampled runoff events 

at both the inlet and outlet of the device. Depending on the 

amount of water available for analysis, between 17 and 21 of 

the events were processed for other constituents. Testing for 

PAHs was limited to seven of the sampled events because the 

PAH analysis was not performed on samples that were sieved 

before the splitting process. Bypass samples collected at the 

weir wall for three runoff events were analyzed for all the constituents except PAHs. Data from the events sampled on May 

15 and 24, and July 27, 2006, are not included in the appendixes because of previously discussed problems with the flow 

monitoring and sample collection.

When a sample was sieved prior to the sample-splitting 

process, the volatile suspended solids (VSS) and TSS concentrations were not determined for the material left on the 

sieves. An analysis was done, however, on the material left 

on the sieve for TP, TZn, and TCu. Because VSS represents 

the organic material such as leaves and grass clippings in the 

stormwater, a lot of the VSS material would probably be left 

on the sieves. Therefore, VSS concentrations are probably 

underestimated for nine events at the hydrodynamic-settling 

device inlet, four events at device outlet, and three events at 

the weir wall. These events were excluded from the appendixes. This leaves 12 runoff events with VSS concentrations 

for both the inlet and outlet. Total suspended solids concentrations measured in the water were increased to account for 

the amount on the sieves by using a method (described in the 

”Methods of Data Collection” section of the report) that is 

based on the sieve mass and PSD of the material left on the 

sieve.

All of the water-quality constituents except PAHs, TDS, 

and DZn had inlet and outlet concentrations greater than 

detection levels. Dissolved zinc concentrations were less than 

detection limits for only two runoff events, and the dissolved 

solids had four events with concentrations less than detection 

limits. In contrast, the nondetectable compounds composed a 

substantial proportion of the total PAHs results. Nondetectable 

compounds were less than detection limits for samples from 

the device outlet more often than samples from the inlet.

Summary statistics for the individual PAH compounds 

were not computed because of the large number of runoff 

events that had individual concentrations that were less than 

detection limits. To calculate the summary statistics for total 

PAHs, a method was needed to account for the nondetected 

concentrations. Methods included using the limit of detections, 

one-half the limit of detections, and zero. To be consistent 
with other USGS studies, the total PAH concentrations were 

calculated by using zero (Mahler and others, 2005).

Along with the mean and median, the summary statistics for the constituent concentrations at the inlet, outlet, and 

bypass included the geometric mean and the mean (table 5). 

The geometric mean was added because the Shapiro-Wilk 

statistic test for normality indicated that the concentrations 

followed a lognormal distribution (Helsel and Hirsch, 1992). 

Runoff data from many urban sites around the country exhibited similar distributions for event-mean concentrations; these 

concentrations were either lognormal or could be approximated as log normally distributed (Driscoll and others, 1990). 

Data sets that were log normally distributed are best described 

by the geometric mean because it gives less weight to 

extremes than the mean (Helsel and Hirsch, 1992). The mean 

concentrations are also important because they are used to calculate the efficiency of stormwater control practices (Strecker 

and others, 2003).

Water-Quality Data Compared to Other 

Parking Lots

The geometric mean concentrations of SS, “adjusted” 

TSS, TP, dissolved phosphorus (DP), TZn, and TCu were in 

the range of concentrations observed in runoff from other 

parking lots in Wisconsin and Michigan (table 6). The concentrations of TZn are on the lower end of the range observed 

for all the parking lots, and this might reflect the difference 

between an employee parking lot and the more busy commercial parking lots. The similarity between the concentrations 

observed in this study and some of the other parking lots indicate that the efficiencies for the hydrodynamic-settling device 

could be extrapolated to other sites.

18 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Table 5. Summary statistics for selected water-quality constituents in samples collected at inlet, outlet, and bypass of hydrodynamicsettling device, Madison, Wisconsin, November 2005 through August 2006.

[mg/L, milligrams per liter; mg/L, micrograms per liter; {}, bypass sample count; —, no data available; PAH, polycyclic aromatic hydrocarbon]

Constituent Number of 

samples

Geomean Median Average

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

“Adjusted” solids, 

suspended, total, 

mg/L

23 {3} 92 89 368 103 105 314 144 151 381

Sediment, suspended, 

mg/ L

23 {3} 107 90 618 116 104 494 189 154 659

Solids, volatile, 

suspended, mg/ L

14 27 27 — 23 26 — 32 34 —

Solids, dissolved, 

mg/L

23 {3} 99 95 124 88 86 130 112 108 130

Phosphorus, total, 

mg/L

19 {2} 0.17 0.17 0.66 0.16 0.14 0.75 0.25 0.24 0.75

Phosphorus, dissolved, 

mg/ L

19 {2} 0.05 0.05 0.15 0.05 0.04 0.32 0.11 0.09 0.32

Copper, total 

recoverable, μg/L

18 {2} 12 13 25 12 14 28 15 16 28

Copper, dissolved, 

μg/L

17 {2} 4.7 4.7 3.5 4.7 4.9 4.4 5.0 5.1 4.4

Zinc, total recoverable, 

μg/ L 

17 {2} 38 43 130 38 45 130 45 54 130

Zinc, dissolved, 

μg/L

20 {2} 7.6 8.0 10 7.0 8.0 13 8.8 8.9 13

Calculated hardness 

(mg/L), mg/ L

21 {2} 53 54 82 59 59 83 58 62 83

PAHs, total, 

μg/ L 

7 — — — — — — 0.80 0.88 —
Particle-Size Distributions

Particle-size distributions are available for 21 runoff 

events at the inlet and outlet of the hydrodynamic-settling 

device (appendix 9). The nine particle sizes analyzed for this 

study included 500, 250, 125, 63, 31, 16, 8, 4, and 2 μm in 

diameter. The PSD at the inlet and outlet varied for each event. 

For the inlet samples, the portion of sand particles (greater 

than 63 μm in diameter) ranged from 16 to 93 percent, and 

the outlet samples ranged from 36 to 93 percent. Based on 

average particle sizes for all the runoff events, the average 

median particle size for the device inlet was about 50 μm in 

diameter, and the average amount of sand in the samples was 

about 48 percent (fig. 11, table 7). These averages agree very 

well with the averages observed at another parking lot monitored in Madison, Wis. (Horwatich and Bannerman, 2010). 

In the 2010 study, both the average median particle size and 

level of sand in the samples were slightly less at about 50 and 

43 percent, respectively. A wide range in the percentage of 

sand in each sample was also observed in the runoff from the 

other parking lot.

Two groups of runoff events were responsible for the 

extreme values observed in most particle sizes observed at the 

hydrodynamic-settling device inlet. Compared to the average 

median particle size and percentage of sand-sized particles 

for all the runoff events, the events on March 8 and 12, and 

April 2 and 12, 2006, had the smaller percentages. These four 

events had an average median particle size of only about 8 μm 

in diameter, and the average percentage of sand in the samples 

was about 12 percent (fig. 11). At the other extreme, the runoff 

events on August 17, 23, and 24, 2006 (events 24–26), and 

water samples had a relatively large percentage of larger particles. The average percentage of sand-sized particles was 70 

percent for these three runoff events, and the average median 

particle size was more than 500 μm in diameter. These three 

events could have had a relatively large amount of large particles in the runoff because of the overflow from the adjacent 

recycling center. The events occurred during the last month 

of sampling when the inlet draining the adjacent facility was 

clogged. Wood chips transported from the recycling facility were observed deposited on the parking lot (figs. 7 and 8) 

and were observed in the water samples. A few wood chips 

could greatly increase the percentage of larger particles in 

the samples.

Evaluation of Hydrodynamic-Settling Device 19

Table 6. Comparison of geometric mean concentrations from this study with geometric mean and median concentrations observed in 

other parking lot studies in Wisconsin and Michigan.

[mg/L, milligrams per liter; μg/L, micrograms per liter; med, median; —, no data available]

Study

Total 

suspended 

solids

(mg/L)

Suspendedsediment 

concentration 

(mg/L)

Dissolved 

phosphorus 

(mg/L)

Total 

phosphorus 

(mg/L)

Total copper 

(μg/L)

Total zinc

(μg/L)

City of Madison Water 

Utility, Madison (this study, 

(Nov. 2005–August 2006)

92 107 0.05 0.17 12 38

Employee parking lot, Madison, 

Wis. (Horwatich and 

Bannerman, 2010)

15 (med) 19 0.03 0.06 4 20

St. Mary’s Hospital parking lot, 

Green Bay, Wis. (Horwatich 

and others, 2004)

23 (med) 31 0.02 0.06 — 50

City garbage truck parking area, 

Milwaukee, Wis. (Corsi and 

others, 1999)

232 (med) — 0.002 0.26 32 150

Retail parking lot, Madison, Wis. 

(Waschbusch and others, 1999)

50 — 0.02 0.10 — —

Retail parking lot Marquette, 

Mich., (Steuer and others, 1997)

110 — 0.022 0.20 22 178

Retail parking lot, Madison, Wis.

(Bannerman and others, 1993)

58 — 0.05 0.19 15 178

Retail parking lot, Milwaukee, 

Wis. (Bannerman and 

others, 1983)

48 — 0.02 0.10 — 133
0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350 400 450 500

All inlet runoff events

All outlet events

Inlet events 1−5

Inlet events 24−26

Particle size, in micrometers

Percentage of sediment less than a given particle size by weight

Figure 11. Particle-size distributions determined at inlet and outlet of hydrodynamic-settling device for selected runoff events, 

November 2005–August 2006.

20 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Table 7. Average particle-size distributions determined at inlet and outlet of hydrodynamic-settling device for selected runoff events, 

Madison, Wisconsin, November 2005–August 2006.

[All values are in percent less than by mass; μm, micrometers in diameter]

Particle size (μm) 500 250 125 63 31 16 8 4 2

Device inlet—all runoff events 88 78 67 52 42 34 28 23 10

Device outlet—all runoff events 97 91 77 59 47 38 31 25 12

First four device inlet runoff events 98 95 89 83 76 59 51 39 15

Last three device inlet runoff events 44 39 35 29 24 22 18 14 9
Water samples collected at the outlet of the hydrodynamic-settling device had a larger percentage of fine particles 

than inlet samples (fig. 11, table 7). From the inlet to the outlet, the average median particle size decreased from 52 to 44 

μm in diameter. The average percentage of particles less than 

63 μm in diameter increased from 52 to 59 percent. Some 

of the largest changes in the percentage of particles sizes 

occurred in the 16 and 250 μm in diameter. Average percentages of particles less than 16 μm in diameter increased from 

about 34 to 38 percent, and the average percentage of particles 

greater than 250 μm in diameter decreased from about 22 to 

9 percent (fig. 11). This increase in finer particle sizes coupled 

with the decrease in larger particle sizes indicates that the settling device was preferentially trapping the larger particles.

In previous studies of stormwater control practices, the 

PSD was shown to have some effect on the efficiency of the 

device (Waschbusch, 1999; Horwatich and others, 2004). 

Based on average PSD at the inlet, a device would need to 

control all the particles greater than about 250 μm in diameter 

to achieve a 20-percent reduction in TSS. To achieve the performance standards in NR151, particles greater than 90 μm 

in diameter and about 3 μm in diameter would need to be 

controlled to achieve the 40-percent and 80-percent reduction 

in TSS reduction, respectively. With all the variability in the 

PSD, the efficiency of the settling device would be expected to 

vary somewhat among runoff events.

Efficiency Calculations

Two methods typically used by investigators to determine 

the removal efficiency of constituents by a stormwater control practice are the efficiency ratio and summation of loads 

(SOL) (National Cooperative Highway Research Program, 

2006). The efficiency ratio uses runoff event mean concentration of contaminants detected in samples collected during the 

study. The SOL is used to evaluate the treatment efficiency on 

a percentage basis by comparing the sum of the influent and 

effluent loads (the product of multiplying the constituent concentration by the runoff volume) for all monitored events. 

Each method uses data from the inlet and outlet to produce a single number that is designed to represent removal 

efficiency of the device. Unfortunately, these methods are not 

designed to evaluate the statistical differences in the data, so 

there is insufficient information generated by the methods to 

determine whether the differences in water-quality measurements for samples from the inlet and outlet are significant. 

These efficiency calculations can be supplemented with a 

statistical test that indicates whether the medians for nonparametric concentrations are statistically significant (Helsel and 

Hirsch, 1992).

A paired statistical test was used to determine whether the 

constituent concentrations at the inlet were greater than those 

at the outlet. A paired statistical test was considered valid for 

this data set because concentrations at the inlet and outlet were 

paired for each runoff event. Most of the constituents were log 

normally distributed; therefore, the nonparametric one-sided 

Wilcoxon signed-rank test was applied (Helsel and Hirsch, 

1992). A test for significance and efficiency ratios calculations 

was not done for calcium and magnesium because the concentrations of these two constituents were only used in the calculation of hardness. In addition, a test for PAHs was not done 

because only seven samples were collected, and results from 

these samples were mostly nondetections.

Inlet and outlet concentrations were significantly different at the 95-confidence level for 4 of the 28 constituents 

analyzed during this study. Concentrations in runoff at the 

settling device inlet were larger than concentrations at the 

device outlet for SS and DP, but the concentrations for VSS 

and TZn were significantly larger at the device outlet than at 

the inlet. At the 84-percent confidence level, the “adjusted” 

TSS inlet concentrations were larger than the outlet concentrations. Because VSS was not analyzed for the particulate matter 

trapped on the sieves during sample processing, the number of 

runoff events available for the efficiency calculation was relatively small. For this reason, VSS efficiencies are not reported 

for this study even though there is a significant difference 

between the inlet and outlet concentrations. Concentrations of 

TDS, TP, TCu, DCu, and DZn were not significantly different 

for the inlet and outlet samples, so efficiencies were not determined for these constituents.

Efficiency Ratio

The efficiency ratio comparison evaluates treatment 

efficiencies on a percentage basis by dividing the constituent concentration at the outflow of the hydrodynamic-settling 

device by the concentration at the inflow of the device and 

multiplying the quotient by 100. This method of calculating 

efficiencies of a device weights all runoff events equally. For 

example, a large volume of flow with large constituent concentrations has the same weight as a small volume of flow 

with small constituent concentrations. The device outlet concentration could be affected by the water stored in the device 

between events; however, sufficient runoff volume exchange 

occurred for the runoff events to minimize the effect of this 

stored volume. As discussed previously, the volume exchanges 

at least 8 times for most events, and the volume is replaced 

at least 16 times for more than one-half of the runoff events. 

Efficiency ratio calculations were done for both the device and 

Evaluation of Hydrodynamic-Settling Device 21
the stormwater control system. The efficiency ratio calculation 

for the settling device is represented by the following equation 

(U.S. Environmental Protection Agency, 1999): 

Device efficiency ratio average concentration at sampli = −1 ng po C

average concentration at sampling po B

 

 

int

int

( )

( )





 



 * , 100

where

sampling point B is the device inlet, and

sampling point C is the device outlet.

Because three of the sampled runoff events had flow over 

the bypass weir, the efficiency ratio was also calculated for 

the bypass system. An efficiency ratio for the bypass system 

of the device accounted for the constituent concentrations 

sampled at the weir wall. This usually results in a smaller efficiency ratio than determined for the settling device, but it is a 

(5)

more realistic assessment of how the device protects receiving 

waters. A flow-weighted average concentration was used to 

calculate the system efficiency because the bypass concentrations only represented three runoff events. Flow-weighted 

average concentrations were determined for device inlet, 

device outlet, and system outlet by summing the total loads 

then dividing by their respective total volumes. The calculation for the stormwater control system efficiency ratio is represented by the following equation:

System efficiency ratio weighted average concentration a = t s ampling po B int i − weighted average concentration at po nt

 int

C

weighted average concentration at sampling po B

( )

( + weighted average concentration a t po E int )

















* , 100 (6)

where

point B is the device inlet;

point C is the device outlet; and

point E is the bypassing over the weir wall.

Both settling device and control system efficiency ratios 

were calculated for concentrations of SS, “adjusted” TSS, DP, 

and TZn (table 8). The efficiency ratio for SS was reduced by 

19 percent and the “adjusted” TSS increase by 5 percent. An 

increase of “adjusted” TSS might be due to the average percentage of sand in the samples at about 50 percent (table 7), 

the difference in the SS efficiency ratio and “adjusted” TSS 

efficiency ratio could be that the TSS analysis does not applying very well to water samples containing a significant amount 

of larger particles with a high specific gravity (Gray and others, 2000; de Ridder and others, 2002; Clark and Siu, 2008; 

Roesner and others, 2007; Selbig and Bannerman, 2007; Clark 

and others, 2008; Kayhanian and others, 2008). By taking an 

aliquot from a sample, the TSS analysis can bias against larger 

particles that settle rapidly before the aliquot is taken. In contrast, the analytical method for concentration of SS uses the 

entire sample volume; therefore, the true concentration of SS 

is more representative by this method.

A positive efficiency ratio of 23 percent was observed for 

TZn at another hydrodynamic-settling monitored in Milwaukee, Wis. (Horwatich and others, 2010). Part of the reason that 

the efficiency ratio was increased at the outlet in this study 

might be the average concentration at the device inlet was 

about five times smaller than observed at the Milwaukee site. 

The settling device efficiency ratio for TDS was similar to the 

28 percent observed for the site in Milwaukee. Because the 

control system efficiency ratios were similar to the device efficiency ratios for all the constituents, the three bypass events 

appear to have little effect on the benefits of the device. It 

was not clear why the concentration of DZn increased at the 

outlet, but anoxic conditions from the stagnant water at the 

bottom of the device could have caused oxidation reduction of 

DZn-complexes.

22 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Table 8. Summary of flow-weighted average concentrations, control system efficiency ratios, and device efficiency ratios for the 

hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006.

[mg/L, milligrams; μg/L, micrograms per liter]

 Constituent

Number of 

samples 

device/ control

Flow-weighted 

average inlet 

concentration

Flow-weighted 

average outlet 

concentration

Flow-weighted 

average bypass 

concentration

Device 

efficiency 

ratio, in 

percent

Control system 

efficiency 

ratio, in 

percent 

Suspended-sediment 

concentration (mg/L)

23/3 189 154 659 19 18

Suspended solids, total 

“adjusted” (mg/L)

23/3 144 151 381 –5 5

Phosphorus, dissolved (mg/L) 19/2 0.106 0.090 0.319 15 18

Zinc, total recoverable (μg/L) 19/2 45.1 53.6 127 –19 –5
Sum of Loads

The SOL is used to evaluate the treatment efficiency 

on a percentage basis by comparing the sum of the influent 

and effluent loads (the product of multiplying the constituent concentration by the runoff volume for an event) for 

sampled events. The SOL method of calculating efficiencies is 

weighted by the runoff volume of each event. This means that 

a small number of events with larger runoff volumes can have 

more effect on the SOL than a large number of events with 

small runoff volumes. The settling device inlet runoff volumes 

were used to calculate both the inlet and outlet loads because 

the inlet volumes were calibrated and considered more accurate than the outlet volumes. As discussed for the efficiency 

ratio calculations, the water stored in the device between runoff events was considered too small to have a significant effect 

on the SOL.

The equation calculating the device percentage of load 

reduction is:

Summation of loads sum of outlet loads

sum of inlet loa 

 = −1

d





 



 



 



. (7)

The SOL for the settling device does not account for the 

load that bypassed the device without being treated. By including the load that bypassed in the efficiency calculation, a more 

realistic assessment of the device’s benefits to the receiving 

waters is possible. Calculating the SOL for the whole system 

will usually lower the percentage of load reduction values 

(Waschbusch, 1999). 

The equation calculating the control system percentage of 

load reduction using the sum of the loads is:

Summation of loads for the system sum of the devic = 100 * e inlet load sum of the device outlet load

sum of the de

 

 

( ) −

( ) vice inlet load + sum o f the bypass load

















,

where

sum of inlet load is the sum of (runoff volume at point 

B*concentration at point B);

sum of outlet load is the sum of (runoff volume at point 

C*concentration at point C); and

(8)

sum o f bypass l oad = the sum of ( ) volume at point o D r − unoff volume a t point B concentration at p o E int  

 *

where

point B is the device inlet;

point C is the device outlet; and

point E is the bypassing over the weir.

As for the efficiency ratios, the SOLs were only calculated for constituents with significantly different inlet and 

outlet runoff event mean concentrations. The SOLs for the 

settling device and the bypass system were calculated for 

“adjusted” TSS, SS, DP, and TZn (table 9, appendix 10 and 

11). Similar to the results for the efficiency ratio, the percentage of load reduction for SS is larger than the “adjusted” TSS. 

As discussed before, the smaller “adjusted” TSS load is probably due to the loss of some of the larger particles during the 

TSS analysis. As expected, the percentages of load reductions 

for the bypass system were slightly less than the reduction 

for the settling device. The similarity between the reductions 

for the settling device and bypass system indicates that the 

amount of bypass flow was insufficient to reduce the benefit of 

the device. The bypass loads for both “adjusted” TSS and SS 

were less than 15 percent of the total inlet load, and the bypass 

load for DP was only 4 percent of the inlet load.

The percentage of bypass system load reduction for 

“adjusted “ TSS of 9 percent is lower than the reduction 

observed in evaluations of other hydrodynamic-settling 

devices. Monitoring of settling devices in a Madison urban 

stormwater study (Waschbusch, 1999) and in a Milwaukee 

highway runoff study (Horwatich and others, 2010) indicated 

TSS load reductions of 21 and 25 percent, respectively. The 

TZn load reduction for these two other studies was about 

17 percent compared to an increase in load observed for the 

study described in this report. The only obvious difference in 

the runoff characteristics between the sites in this study and 

the other two studies was that the TZn runoff event mean concentrations were much higher at the device inlet for the other 

two studies. The bypass system load reductions for DP measured at the two Madison sites were very similar at 17 percent.

If the two runoff events on August 23 and 24, 2006, had 

not been sampled, the percentages of load reduction for SS 

and “adjusted” TSS would have been significantly less. The 

SS load at the inlet without these two event is 469 lb and at the 

outlet is 396 lb and the for “adjusted” TSS load inlet is 368 lb 

and outlet is 383 lb. Removing these two events from the SOL 

Evaluation of Hydrodynamic-Settling Device 23
24 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Table 9. Summary of constituent loads and percentage efficiency for hydrodynamic-settling device in Madison, 

Wisconsin, November 2005–August 2006.

[SOL, summation of loads; lb, pounds; %, percent]

Constituent Inlet SOL (lb) Outlet SOL (lb) Bypass SOL (lb) Device SOL (%) System SOL (%)

Suspended sediment 1,590 979 228 38 39

Suspended solids, total “adjusted” 1,090 993 134 9 12

Phosphorus, dissolved 1.15 0.930 0.049 19 22

Zinc, total recoverable 0.211 0.239 0.0193 –13 –4

calculations lessens the settling-device percentages of load 

reduction to 16 percent for SS and to –4 percent for “adjusted” 

TSS (appendix 10), which is a notable change in load reduction when comparing it to using all 23 runoff events in the 

SOL calculations. Similar reductions would be expected for 

DP, but it was not measured for the runoff event on August 24.

The August 23 and 24 runoff events have a large effect 

on SOL because they not only represented about 70 percent 

of the total inlet load but they also experienced relatively 

large reductions in SS and “adjusted” TSS. For SS the device 

reductions on August 23 and 24 were 38 and 54 percent, 

respectively, and the “adjusted” TSS reductions were 0 and 

26 percent, respectively (appendix 10). Because these two 

runoff events had additional runoff from an adjacent city recycling facility, an unusually large amount of runoff and particulate matter was delivered to the settling device. Although these 

two events should be included in the efficiency calculations, 

the effect of these runoff events does indicate that the performance of the device might be somewhat less for similar sites 

not affected by additional sources of particulate matter, such as 

recycling facilities and soil erosion.

The effect of the August 23 and 24 runoff events might 

also explain the large differences between the efficiency ratios 

and percentage of load reductions for SS and “adjusted” TSS 

(tables 8 and 9). When the two events are removed from the 

SOL calculation, the resulting percentage of load reductions 

for SS and “adjusted” TSS are very similar to the efficiency 

ratios. The percentage reduction in load is 16 percent for SS, 

which is very close to the efficiency ratio of 19 percent. Differences in the two types of efficiency calculations are relatively small for DP because the August 24 runoff event was 

not included in either calculation. The effect of the two August 

runoff events demonstrates potential differences between the 

two methods of determining efficiencies of a settling device 

but also puts emphasis on evaluating the uniqueness of any 

runoff event when calculating the SOL. 

Sum of Loads by Particle Size 

Calculating the SOLs by particle size determines how the 

PSD might affect the efficiency of the hydrodynamic-settling 

device. The manufacturer’s claim for the performance of the 

device is based on a given PSD (U.S. Environmental Protection Agency, 2007). For runoff at 15 o

C, the manufacturer 

claims the device will remove more than 80 percent of the 

settleable solids with a PSD similar to the Maine Department 

of Transportation road sand and a specific gravity of 2.65. 

Almost all the particles in Maine Department of Transportation road sand are in the sand-size fraction or greater than 

63 μm in diameter (table 10). The median particle size for the 

Maine Department of Transportation is 700 μm in diameter. To 

achieve an 80-percent removal with the Maine Department of 

Transportation PSD, the device would have to be only efficient 

at removing particles greater than 250 μm in diameter.

The settling device inlet and outlet SOL by particle size 

was determined by multiplying the concentration of SS for 

each particle size times the inlet volume. Because the particlesize analysis captures all the particles at each size, the analysis 

is a representation of SS and not TSS. The percentage of SS 

load reduction was determined for each particle size for both 

the settling device and bypass system (table 11). Most of the 

reduction in SS occurred for particles greater than 250 μm 

in diameter. Because about 45 percent of the particles were 

greater than 250 μm in diameter, an estimate of the percentage of load reduction for SS could be made by multiplying 

the 90-percent reduction times 45 percent. The result of this 

calculation is 40 percent, and this is very similar to the SS load 

reduction determined for the settling device (table 8). These 

results support the manufacturer’s claim of achieving a high 

percentage reduction for the larger particles.

Factors That Affect Variability in Efficiency 

Suspended-sediment settling-device efficiencies for 

individual runoff events (fig. 12) varied from as low as –47 

percent to as high as 70 percent. Two potential sources of variability between runoff events are the factors affecting the settling velocity of the particulate matter and any bias caused by 

the sampling techniques. Variability due to the settling velocity of the particles is more easily evaluated with the available 

data, but literature is available to support some speculation 

about the importance of sampling bias. Because sedimentation 

is the primary mechanism for the settling device to remove 

suspended particulate matter from the water column, the 
Evaluation of Hydrodynamic-Settling Device 25

Table 10. Particle-size distribution for Maine Department of Transportation road sand (U.S. Environmental Protection Agency, 2007).

[>, greater than]

Particle size 

(micrometers) >10,000 2,000 1,000 500 320 250 125 63

Percent less than 98 78 58 43 30 15 8 5

[SOL, summation of loads; lb, pounds; %, percent; >, greater than]

Table 11. Sum of loads and percentage of suspended-sediment load reduction by particle size for hydrodynamic-settling device and 

bypass system in Madison, Wisconsin, November 2005–August 2006.

Particle size 

(micrometers)

Device inlet SOL 

(lb)

Device outlet SOL 

(lb)

Bypass SOL

 (lb)

Inlet load 

percentage of 

total load

Device SOL 

(%)

Bypass system 

SOL

(%)

>500 242 15 32 17 94 83

500–250 406 41 22 28 90 85

250–125 129 121 20 9 6 5

125–63 114 133 22 8 –17 –14

63–32 110 112 18 7 –2 –1

32–14 448 473 76 31 –6 –5

most commonly used equations for estimating the degree of 

sedimentation depend on knowing the settling velocity of the 

particles (Minton, 2005; Strecker and others, 2005; Field and 

others, 2006). Settling velocity is affected by particle size and 

shape, specific gravity of the particles, and water temperature. 

Peak-flow through rate and the degree of turbulence during a 

runoff event are two other factors frequently found in equations used to design sedimentation devices.

Using the ratio of VSS to “adjusted” TSS as an indicator 

of specific gravity of the particles, one source of variability is 

certainly the wide range in percentage of VSS. The range in 

percentage of VSS for the inlet runoff events was 17 to 67 percent with an average of 41 percent for 13 events (appendix 6). 

The average concentrations for VSS of 32 mg/L and TSS 

“adjusted” 98 mg/L for a ratio of 33 percent for 13 events 

(appendix 6). These particles probably have their origins in 

dead vegetative material, and many of them would be captured 

in the sand-sized fractions. The specific gravities for the particulate matter in most stormwater samples can range from 1.5 

to 2.5 (Burton and Pitt, 2002). Variability in the specific gravities may be affected by the organic content of the particles 

(Kayhanian and others, 2008). Specific gravities of organic 

detritus found in stormwater samples have been reported to 

range from 1.1 to 1.8 g/cm3 (Christina and others, 2002; Kayhanian and others, 2008). Because the particles associated 

with the VSS concentration would be less likely to settle to 

the bottom of the device, the removal efficiency of the device 

could be affected by the percentage of VSS in the runoff.

Results from this study show a high variability in 

removal efficiencies with peak flows that are less than the 

design flow (fig. 12). The efficiencies tended to be all positive 

at the lowest peak flows. Four runoff events with peak flows 

between about 1.8 and 2.7 ft3

/s had negative efficiencies. Testing of two other hydrodynamic-settling devices in Wisconsin 

also showed a high variability in concentrations of SS and 

TSS removal efficiencies for peak flows that were less than the 

design flow (Waschbusch, 1999; Horwatich and other, 2010). 

Three of the events (April 16 and August 23 and 24, 2006) 

in this study had bypass system peak flows that were greater 

than the design flow, but the flow splitter prevented the flow 

from exceeding the design flow at the device inlet. Contrary to 

results from the other four runoff events with high peak flows, 

these three events had positive efficiencies. Their efficiencies 

were enhanced by the high percentage of particles greater 

than 125 μm in diameter (appendix 9). For the two other 

hydrodynamic-settling devices monitored in Wisconsin, the 

removal efficiencies tended to be low or negative when their 

peak flows exceeded the design flows. Changes in peak flows 

by themselves do not seem to be good indicators of changes 

in efficiencies except a when a negative efficiency might be 
-80

-60

-40

-20

0

20

40

60

80

0.0 1.0 2.0 3.0 4.0 5.0 6.0

System suspended-sediment concentration removal, in percent

Peak discharge, in cubic feet per second

Figure 12. Removal efficiency of suspended solids and total solids as a function of peak flow for hydrodynamic-settling device 

system, November 2005–August 2006.

26 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

expected as peak flows approach or exceed the design flow at 

the inlet.

Although the variations in the removal efficiency of a wet 

pond (House and others, 1993) was shown to be related to all 

particle sizes (Greb and Bannerman, 1997), the results from 

the study described herein indicate that the only relationship 

between influent particle size and removal efficiency might 

be for particle sizes greater than 125 μm in diameter. The SS 

efficiencies tend to be greater if the percentage of 125 μm 

in diameter particles is more than about 45 percent (fig. 13). 

Above 45 percent, there is a trend of increasing SS efficiency 

with increasing percentage of 125 μm in diameter particles. 

Influent particle size does not seem to significantly affect the 

variability in the SS efficiency for particles less than 125 μm 

in diameter. Particle size appears to have a somewhat stronger 

effect on SS efficiencies than peak flow, but neither one by 

themselves have a strong enough effect to explain all the variability in the SS efficiencies.

The other source of variability to consider is potential 

bias due to the way the samples were collected. A number 

of authors have reported potential problems with trying to 

collect a representative sample with an automatic sampler, 

especially if there are large amounts of sand-sized particles 

in the water column (Clark and others, 2008; DeGroot and 

others, 2009; Fowler and others, 2009). When the sampler 

is not able to intake the proper proportion of large particles, 

the concentration of SS can be underestimated (Clarke and 

others, 2008). Conversely, the concentration of SS might be 

overestimated if the larger particles are stratified in the pipe, 

and the sampling tube intake is located near the bottom of 

the pipe (DeGroot and others, 2009; Selbig and Bannerman, 

2010), as in this study. One possible indicator of stratification 

is a relatively large percentage of larger particles in a sample. 

Several events had greater than 50 percent of the particles in 

the sand size fraction for the inlet samples (appendix 9). These 

events are also characterized by relatively large flows and SS 

concentrations. If the inlet SS concentrations for these events 

were positively biased due to stratification, the load reductions 

would be overestimated for the events. Since these events also 

represent a large percentage of the total inlet SS load for the 

study period, the SOL for the SS could also be an overestimate. The SOL for other particulate constituents, such as TSS 

and TP, would be less affected, because the laboratory procedures for retrieving an aliquot from the sample bottle tends to 

reduce the percentage of large particles in the sub-sample to be 

analyzed (Selbig and Bannerman, 2011).

Accuracy of Efficiency Calculations

One way of checking the accuracy of the measured SS 

loads at the inlet and outlet of a settling device is to weigh 
the sediment that is retained in the sediment-storage sump. 

The weight of the sediment retained in the device should be 

reasonably close to the calculated reduction in SS loads. To 

remove as much as possible from the sediment trapped in 

the storage sump, the sediment was removed from the sump 

on three different dates. On September 15, 2006, the City of 

Madison filled a vacuum truck to capacity with water and 

sediment from the settling device. The material was transported to the USGS Water Science Center in Madison where 

the particulates in the slurry were allowed to settle while still 

in the truck. The truck drained the water from the slurry into 

a city storm sewer. The remaining slurry was dumped into a 

-60

-40

-20

0

20

40

60

80

0 10 20 30 40 50 60 70 80 90

Percentage of particle sizes greater than 125 micrometers

Overall total suspended solids and suspended-sediment 

concentration removal, in percent

Figure 13. Variability in suspended-solids removal efficiency relative to percentage of particle sizes greater than 125 micrometers for 

hydrodynamic-settling device, November 2005–August 2006.

Figure 14. Slurry tank for drying 

material vacuumed from bottom of 

hydrodynamic-settling device.

Evaluation of Hydrodynamic-Settling Device 27
tank (tank 1) consisting of a plastic tarp held up by a wooden 

frame (fig. 14).

More sediment and water were removed from the treatment chamber on September 20 and 21, 2006, using a 2-in. 

trash pump and a 3-in. trash pump, respectively. The slurry 

was stored in a second tank (tank 2) made with a plastic 

tarp. After the sediment suspended in tanks 1 and 2 had been 

allowed to settle, the fairly clear supernatant was pumped out 

of the tanks. Several 1-L samples of the supernatant water 

were composited to determine the concentrations of SS and 

“adjusted” TSS. The sediment remaining in the bottom of the 

two tanks was placed in a hot house and dried for several days. 

The temperature of the hot house was approximately 105 °F. 

The dried sediment was weighed, and subsamples were collected for PSD and VSS analysis.

Measured Weight of Sediment in Sump

The dry weight of the sediment removed from the sediment-storage sump was 415 lb. This weight is low because it 

was not possible to remove all the sediment from the sump. 

The circular shape of the control structure in the settling 

device prevented the suction hoses from reaching the outer 

edge of the sump. Water remaining in the sump made it impossible to use a camera to assess how much sediment was left 

in the sump. Infiltration of groundwater quickly replaced the 

water pumped from the sump. Another method was selected to 

estimate the amount sediment remaining in the sump.

The dry weight of sediment remaining in the sump was 

estimated by multiplying the measured dry device weight 

of the sediment removed from the sump times a conservative estimate of the wet volume of sediment remaining in 

the sump. A dry device weight of 53 lb/ ft3 was calculated by 

dividing the volume of the wet sediment removed from the 

sump into the dry weight of the sediment. To estimate the wet 

volume of sediment left in the sump, an estimate was made 

of the width and depth of the sediment remaining around the 

edge of the circular sump.

The sediment deposited in the sump formed a shallow 

cone (Robert Andoh, Hydro International, oral commun., 

2007), so the depth of the sediment around the edge of the 

sump was less than in the middle. The average depth of sediment measured in the center of the sump was 0.35 ft. It was 

assumed that the depth around the edge was about one-half 

of the depth in the middle or about 0.18 ft. The width of the 

deposit was assumed to be 4 in. around the edge of the sump, 

which is 6 ft in diameter. The wet volume of the sediment 

remaining in the sump was determined to be about 1.1 ft3

. 

By multiplying the device dry weight times the assumed wet 

volume of sediment left in the sump, the amount of sediment 

left in the sump was estimated to be 58 lb. This brings the 

total sediment trapped in the device during the study period to 

approximately 473 lb.

Weight of Sediment in Sump Using Inlet and Outlet 

Suspended-Sediment Loads

The difference between the inlet and outlet SS loads for 

the sampled runoff events was 609 lb (table 9). Although this 

number is about 1.3 times higher than that in the sump, it still 

compares somewhat favorably with the estimate of 473 lb 

trapped in the device. However, the 611 lb of sediment does 

not include the SS inlet and outlet loads for all the unsampled 

runoff events. Unsampled events during the monitoring 

period total 48. Rainfall less than 0.2 in. in depth represented 

31 events. Most low volume runoff events did not produce 

a sufficient water depth in the pipes to collect samples. Low 

runoff volumes from these events probably do contribute a 

small amount to the total SS load. The SS load would be more 

affected by the 17 unsampled events that had rainfall depths 

between 0.2 and 1.0 in. Only one event with more than a 1-in. 

rainfall depth was not sampled. A method to estimate inlet and 

outlet SS loads for unsampled events was developed.

The approach to finding a method for estimating constituent loads for unsampled runoff events started with trying to 

estimate SS loads for sampled events. Multiple models were 

developed by using a statistical method of stepwise regression. Rainfall and runoff characteristics were used as the independent variables, and SS load was the dependent variable. 

Logs of rainfall duration and peak flow were the independent 

variables that most closely estimated the sampled inlet and 

outlet SS load. Using these two independent variables in a 

regression relationship produced reasonably robust correlation 

coefficient (R2

) values for the inlet and outlet of 76 and 78 percent, respectively. Estimates of the SS load were determined 

by retransforming the data set back to original units using the 

Maximum Likelihood Estimation (MLE) method that is commonly used in environmental disciplines (Helsel and Hirsch, 

1992).

To test the regression models for the inlet and outlet SS 

loads, the measured SS loads at the inlet and outlet were estimated using the MLE concentrations and runoff volumes for 

the sampled runoff events. The total modeled SS load for the 

sampled events was 1,320 lb at the inlet and 985 lb at the outlet. The measured inlet and outlet SS loads for sampled events 

were 1,590 and 979 lb, respectively. These values are very 

close to the modeled ones with only a 17-percent difference at 

the inlet and a –5 percent difference at the outlet. The regression models should provide good estimates of SS loads for the 

unsampled events.

The estimated event loads for unsampled events were 

1,451 lb at the inlet and 1,118 lb at the outlet. The unsampled 

runoff events were estimated to contribute 333 lb of sediment 

to the sump. By adding the measured and estimated SS loads, 

the total inlet SS load for the runoff events was 3,040 lb, 

and the total SS load at the outlet was 2,100 lb. The difference between these two numbers indicated that the amount of 

28 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
sediment that should have been retained at the bottom of the 

tank was 942 lb. 

A combination of measured and estimated inlet and outlet 

SS loads overestimated the amount of sediment retained in the 

tank by a factor of two. This result certainly indicates that the 

efficiency calculation might overestimate for SS, and the same 

might be true for some of the other constituents containing a 

lot of particulate matter, such as TZn. Because the removal 

efficiency results presented in tables 8 and 11 might be overestimates, it was important to evaluate sources of error in the SS 

loads and to select a method for estimating the potential error 

in the SS efficiencies.

Adjustments to Suspended-Sediment Loads 

It is unlikely that the process of retrieving and weighing 

the amount of sediment trapped in the sump was off by a factor of two. Although there is some error in weighing the sediment in the sump, the difference between the calculated and 

measured weight of the sediment in the sump is more likely 

due to a positive bias in the SS loads. Some of that bias could 

be from the loads estimated for the unsampled runoff events, 

but the regression equations seem to produce reasonably good 

numbers. If estimated SS loads for the unsampled events 

were high by as much as 50 percent, the total estimate for the 

weight of sediment in the sump would still be about 1.6 times 

higher than the measured load. A more likely cause of positive 

bias in the SS loads is the stratification of particles in the inlet 

and outlet pipes.

Recent efforts to evaluate the accuracy of sampling for 

concentrations of SS and TSS at the bottom of stormwater 

pipes have shown that the concentration of larger particles is 

higher near the bottom of a pipe (Smith, 2002; DeGroot and 

others, 2009; Smith and Granato, 2010). Samples collected 

from throughout the water column at the 12-in. diameter 

inlet of an oil-grit settling device in Boston, Massachusetts, 

showed that the concentration of particles greater than 62 μm 

in diameter tended to be higher at the bottom of the pipe 

(Smith, 2002; Smith and Granato, 2010). At a flow rate of 

0.45 ft3, the samples collected at 1 in. from the bottom of the 

inlet pipe had concentrations of SS that were twice as high as 

the samples collected 2.5 in. from the bottom. Testing at the 

St. Anthony Falls Laboratory in Minnesota concluded that 

automatic samplers were not accurately monitoring particles 

larger than coarse silt when a single intake is located at the 

bottom of the pipe (DeGroot and others, 2009). Compared to 

the concentration fed into a 1.5-ft diameter pipe, the measured 

concentrations of coarse silt (44 to 88 μm in diameter) and fine 

sand (125 to 180 μm in diameter) were about 1.5 and 5 times 

higher, respectively, than they should be. For coarser sand 

(250 to 355 μm in diameter), the measured concentrations 

were about 10 to 13 times higher. Without sufficient vertical 

mixing, the stratification of larger particles in a pipe appears to 

impose a positive bias on the measured concentrations of SS 

and TSS.

Smith (2002) applied an adjustment factor to inlet loads 

in the oil-grit settling device because the inlet loads would 

have been overestimated without the adjustment. The outlet 

loads were not adjusted because the outlet samples contained 

very few particles greater than 62 μm in diameter. A similar 

adjustment factor cannot be developed with the data from the 

study described in this report, but the median percentage of 

particles greater than 63 μm in diameter was more than twice 

that observed in the Smith (2002) study. For seventeen runoff 

events, the average percentages of particles less than 63 μm in 

diameter were 55 percent for the device inlet and 57 percent 

for the outlet (table 12). Three of the largest runoff events had 

median percentages of particles greater than 63 μm in diameter 

of 62 percent for the device inlet and 33 percent for the outlet. 

Stratification of the particles was even more likely for the largest events because more than 32 percent of the particles at the 

inlet were greater than 500 μm in diameter.

The approach to reducing the bias in the SS loads 

involved reducing the concentrations of each particle-size 

range greater than 63 μm in diameter until the difference 

between the inlet and outlet SS loads matched the measured 

weight of sediment in the sump. The concentrations were 

Evaluation of Hydrodynamic-Settling Device 29

Table 12. Comparison of average particle-size distributions between 3 runoff events and 17 other runoff events for parking lot in 

Madison, Wisconsin, November 2005–August 2006.

[All data are in percent by mass; <, less than; μm, micrometer in diameter]

Event group

Particle size (μm)

500 250 125 63 32 14 8 5 <2

Inlet pipe

Three events 68 52 45 38 31 26 21 17 8

Other 17 events 92 82 71 55 44 35 30 24 11

Outlet pipe

Three events 99 95 83 67 54 48 40 29 15

Other 17 events 97 90 76 57 45 37 31 24 11
reduced for the particle-size ranges of 63 to 125, 125 to 250, 

and greater than 250 μm in diameter for both the inlet and the 

outlet. Based on work done at the St. Anthony Falls Laboratory, a different percentage reduction was selected for each 

particle-size range. The percentage reductions selected for the 

ranges of 63 to 125, 125 to 250, and greater than 250 μm in 

diameter were 3, 15, and 30 percent, respectively. The 15- and 

30-percent reductions were determined by multiplying the 

3-percent value by 5 and 10, respectively. These two multiples 

approximate the relative errors observed at the St. Anthony 

Falls Laboratory for fine and coarser sand.

Although the testing by DeGroot and others (2009) supports using much larger reductions than 3, 15, and 30 percent, 

these values were based on the Degroot’s relative reductions 

for each particle-size range and produced a value close to the 

measured weight of sediment in the sump. The much smaller 

reductions are justifiable when the differences between the 

laboratory and field testing are considered. Because the 

particles in both the inlet and the outlet water only traveled a 

short distance before the sampler intake tube, they had much 

less time to settle to the bottom than the particles injected into 

the pipe at the St. Anthony Falls Laboratory. After being well 

mixed in the flow splitter, the inlet water only had 2 ft to travel 

before the intake tube. At the St. Anthony Falls Laboratory, the 

particles traveled about 35 ft before reaching the intake tube. 

In addition, the settling rate for many of particles in the runoff 

water was probably less than the silica sand used for testing 

by DeGroot and others (2009). With inlet VSS concentrations 

that averaged 32 percent of the average “adjusted” TSS concentrations many of the particles would have a relatively low 

specific gravity and be much less likely to settle to the bottom 

of the pipe.

The difference between the “adjusted” SS loads for the 

inlet and outlet of the hydrodynamic-settling device is 505 lb 

(table 13). This value is very close to the weight of 473 lb 

measured for the sediment removed from the sump. Although 

there is a large degree of uncertainty in applying the laboratory findings from the St. Anthony Falls Laboratory to data 

collected from the environment, the process of matching the 

measured weight of sediment in the sump gives some validity 

to the magnitude of the adjustments. Because the relatively 

small adjustments were only needed for the sand-sized particles, the stratification of the sand-sized particles in the inlet 

and outlet pipes was probably responsible for overestimating 

the weight of sediment in the sump. However, the uncertainty in using results from the St. Anthony Falls Laboratory to adjust the SS loads still makes it inappropriate to use 

the “adjusted” SS loads to calculate new SOL values for SS 

(table 9).

The appropriate use of the “adjusted” SS loads is to 

determine if the error caused by the stratification increases or 

decreases the SOL value. The SOL for SS using the adjusted 

sampled SS loads is 30 percent. Because this is less than the 

38 percent calculated for the sampled events (table 9), the 

stratification of the sand particles increases the SOL values. 

This positive bias is probably true for all of the particulate 

constituents. The SOLs (table 9) are probably overestimated 

except the ones for DP. Future studies need to address the 

stratification issue by modifying the sampling methods or by 

developing site-specific adjustment equations, such as developed by Smith (2002), and Smith and Granato (2010).

Particle-Size Distribution of Sediment Retained 

in Sump

Five subsamples were randomly collected from the sump 

sediment, and each subsample was sieved to determine the 

PSD. Volatile solids were also determined for each of the five 

subsamples. Average PSD and VSS values were determined 

for the sediment in the sump by averaging the results of the 

five subsamples. At 8 percent in the sump, the average VSS 

was much smaller than the average of 32 percent observed for 

the inlet water samples . With a lower specific gravity, volatile 

solids such as bits of vegetation are less likely to be trapped 

by the settling device. The majority of the particles trapped in 

the device were in the sand or larger sized particles (table 14). 

More than 80 percent of the particles in the sump were 250 μm 

30 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Table 13. Monitored and estimated loads after concentrations of suspended 

sediment were adjusted for three largest particle sizes at inlet and outlet of 

hydrodynamic-settling device in Madison, Wisconsin, November 2005 and 

September 2006.

 [All values in pounds; SS, suspended sediment]

Site Adjusted monitored 

SS load1

Adjusted estimated 

SS load1

Total adjusted 

SS load

Inlet 1,220 1,115 2,335

Outlet 855 975 1,830

1

 Percentages of 3, 15, and 30 were used on each monitored runoff event to reduce SS concentration for particle sizes 63 to 125, 125 to 250, and greater than 250 micrometers in diameter, 

respectively.
in diameter or larger. This finding supports the observed high 

removal efficiencies for particles greater than 250 μm in diameter but the very low removal efficiencies for particles smaller 

than 250 μm in diameter (table 9)

Similar PSDs were observed for the sediment removed 

from the sumps of two other hydrodynamic-settling devices 

(Waschbusch and others, 1999; Horwatich and others, 2010). 

The majority of the particles trapped in their sumps were 

in the sand-sized fraction. About 80 percent of the particles 

were greater than 250 μm in diameter for the Madison study 

(Waschbusch and others, 1999), and the Milwaukee study had 

about 60 percent of the particles greater than 250 μm in diameter (Horwatich and others, 2010).

Summary 31

Table 14. Particle-size distribution for sediment samples collected in hydrodynamic-settling device in Madison, Wisconsin, 

September 2006.

[<. less than]

Particle size , 

in micrometers <16,000 <8,000 <4,000 <2,000 <1,000 <500 <250 <125 <63

Percent less than 97 95 91 82 69 45 19 7 3

Summary

The U.S. Environmental Protection Agency (EPA) created the Environmental Technology Verification Program to 

verify the performance claims made by manufacturers of proprietary stormwater control practices. The National Sanitation 

Foundation International was funded by the EPA to distribute 

funds for the program and to ensure the quality of the data 

collection. The Wisconsin Department of Natural Resources 

(WDNR) has used the results of the verification program to 

develop technical standards for proprietary stormwater control 

practices. To support these efforts of assigning appropriate 

pollution reduction efficiencies to proprietary stormwater control practices, the U.S. Geological Survey in cooperation with 

WDNR, the City of Madison, cities in the Waukesha Permit 

Group, Hydro International, Earth Tech, Inc., and National 

Sanitation Foundation International and the EPA monitored the 

performance of a hydrodynamic-settling device installed at the 

stormwater discharge point of a city water utility administration building site in Madison, Wis. An employee parking lot 

and roofs composed most of the drainage area.

Sedimentation is the principle process for removing pollutants in the settling device’s single treatment chamber. The 

6-ft diameter device installed in 2004 for this study was sized 

to treat flows up to 3 ft3

/s, and it bypassed any flows in excess 

of 3 ft3/s. Only three stormwater runoff events from November 

2005 through September 2006 had significant bypass volumes 

ranging from 10 to 20 percent of the total runoff volume.

Twenty-three of the runoff events monitored for flow 

and water quality at the device’s inlet and outlet were used 

to calculate the percentage pollutant reduction for the device. 

Bypass flow and water-quality data were included in the calculations to determine the efficiency of the system for the three 

runoff events with bypass. Of the 28 water-quality constituents 

analyzed in the water samples, only suspended sediment (SS), 

“adjusted” total suspended solids (TSS), volatile suspended 

solids (VSS), dissolved phosphorus (DP)S, and total recoverable zinc (TZn) showed the difference between the inlet and 

outlet concentrations to be statistically different at the 84-percent confidence level or greater. Some of the TSS concentrations were “adjusted” to add the particles that remained on the 

sieves during sample processing. Device outlet concentrations 

for TZn and VSS were significantly higher than the inlet concentrations. Efficiency calculations were only done for constituents with a significant difference between the device inlet 

and outlet concentrations. Because nine runoff events did not 

have inlet VSS concentrations, they were not included in the 

efficiency calculations.

Removal efficiency of the hydrodynamic-settling device 

was calculated using the efficiency ratio and summation of 

loads (SOL) methods. Using the efficiency ratio method, 

the device reduced concentrations of SS and DP by 19 and 

15 percent, respectively. Using the efficiency ratio method, the 

device increased “adjusted” TSS and TZn concentrations by 

5 and 19, respectively. Bypass occurred for 3 of the 23 runoff 

events used in this assessment, and the bypass flow and waterquality concentrations were used to determine the efficiency 

of the bypass system. Concentrations of SS, “adjusted” TSS, 

and DP were reduced for the system by 18, 5, and 18, respectively, however TZn increased by 5 percent. The loads of SS, 

“adjusted” TSS, and DP were reduced using the SOL method 

for the settling device by 38, 9, and 19 percent, respectively, 

and TZn increased by 13 percent. For the bypass system, the 

loads of SS, “adjusted” TSS, and DP had percentage reductions of 39, 12, and 22, respectively, and a 4-percent increase 

for TZn. Although the SOL method calculated percentage 

reductions for SS and “adjusted” TSS that were twice those 

for the efficiency ratio method, just removing data for the 

two large runoff events on August 23 and 24, 2006, from the 

SOL calculation brought the reduction for SS down to 16 and 

increased the load for the “adjusted” TSS by 4 percent.

The amounts of runoff and solids concentrations were 

unexpectedly high for runoff events on August 23 and 24, 

2006, because a clogged stormwater inlet caused overflow 

from an adjacent city recycling facility. These two events were 
so large they represented 70 percent of the inlet load for the 

23 sampled events. The effect of the two large events not only 

demonstrates potential differences between the two methods 

of determining efficiencies of a device but also shows the 

importance of evaluating the uniqueness of any runoff events 

when calculating the SOL.

Suspended-sediment removal efficiencies for individual 

runoff events varied from as low as –47 percent to as high 

as 70 percent. Using the ratio of VSS to “adjusted” TSS as 

an indicator of specific gravity of the particles, one source 

of variability was the wide range in the percentage of VSS. 

The range in percentage of VSS for runoff events was from 

16 to 67 percent with an average of 41 percent for 14 events. 

Changes in peak flows by themselves do not seem to be good 

indicators of changes in removal efficiencies except when a 

negative efficiency might be expected as peak flows approach 

the design flow at the inlet. Some of the variability was probably due to the large variability in the percentage of sand in 

the samples and the large average amount of sand in the samples of about 48 percent. The strongest relationship observed 

between removal efficiency and the PSDs was the increasing SS efficiency when the amount of particles greater than 

125 μm in diameter exceeded 45 percent.

The SOL method was used to determine percentage 

SS load reduction for six different particle sizes for both the 

settling device and bypass system. Essentially no reduction 

was observed for particles less than 125 μm in diameter, and 

about a 90-percent reduction occurred for particles greater 

than 250 μm in diameter. The high removal efficiencies for 

particles greater than 250 μm in diameter was further supported by the fact that more than 80 percent of the particles 

trapped in the sump were greater than 250 μm in diameter. 

These results support the manufacture’s claim of achieving a 

high percentage reduction for particles greater than 250 μm in 

diameter.

Stratification of larger particles in the settling device inlet 

and outlet pipes appeared to result in a positive bias in the 

SOL for SS. When the difference between the inlet and outlet 

loads overestimated the weight of the sediment retained in 

the sump by a factor of two, the cause was assumed to be the 

positioning of the sampling intakes at the bottom of the pipes 

where the concentration of larger particles can be higher. The 

approach to reducing the bias in the SS loads involved lowering the concentrations of each particle-size range greater than 

63 μm in diameter until the difference between the inlet and 

outlet SS loads matched the weight of sediment in the sump. 

The SOL for SS using the modified inlet and outlet SS loads 

was 30 percent. Future studies need to address the stratification issue by modifying the sampling methods or by developing site-specific adjustment equations.

References

American Public Health Association and others, 1989, Standard methods for the examination of water and wastewater 

(17th ed.): Washington, D.C., American Public Health Association [variously paged].

Bannerman, R.T., Baun, K., Bohn, M., Hughes, P.E., and 

Graczyk, D.J., 1983, Evaluation of urban nonpoint source 

pollution management in Milwaukee County, Wisconsin—volume 1 for U.S. Environmental Protection Agency, 

Region V: Wisconsin Department of Natural Resources 

Publication PB 84–114164 [variously paged].

Bannerman, R.T., Konrad, J., Becker, D., Simsiman, G., 

Chesters, G., Goodrich-Mahoney, J., and Abrams, B., 

1979, Surface water quality monitoring data—volume 3, 

The IJC Menomonee River Watershed Study: Chicago, 

U.S. Environmental Protection Agency, EPA–905/4–79–

029–C [variously paged].

Bannerman, R.T., Owens, D.W., Dodds, R.B., and Hornewer, 

N.J., 1993, Sources of pollutants in Wisconsin stormwater: 

Water Science Technology, v. 28, no. 3–5, p. 241–259.

Bannerman, R.T., Legg, A.D., and Greb, S.R., 1996, Quality 

of Wisconsin stormwater 1989–94: U.S. Geological Survey 

Open-File Report 96–458, 26 p.

Bochis-Micu, C., and Pitt, R., 2005, Impervious surfaces in 

urban watersheds, in Proceedings of WEFTEC.05, the 78th 

Annual Water Environment Federation Annual Technical 

Exhibition and Conference (CD-ROM), Washington, D.C., 

Oct. 29–Nov. 2, 2005: Water Environment Federation, Alexandria, Virginia, p. 3124–3141.

Booth, D.B., and Reinelt, L.E., 1993, Consequences of urbanization on aquatic systems—measured effects, degradation 

thresholds, and corrective strategies, in Watersheds’ 93, 

Alexandria, Virginia, 1993, Proceedings: U.S. Environmental Protection Agency, p. 545–550. 

Burton, G.A., Jr., and Pitt, R.E., 2002, Stormwater effects 

handbook—a toolbox for watershed managers, scientists, 

and engineers: Boca Raton, Florida, Lewis Publishers, 

929 p.

California Department of Transportation, 2004, BMP Retrofit 

Pilot Program Final Report: Sacramento, California, Division of Environmental Analysis, Report ID CTSW–RT–01–

050 [variously paged]. 

32 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
Clark, S.E., and Siu, C.S., 2008, Measuring solids concentration in stormwater runoff—comparison of analytical methods: Environmental Science and Technology, v. 42, no. 2, 

p. 511–516.

Clark, S.E., Siu, C.Y.S., Pitt, R., Roenning, C.D., and 

Treese, D.P., 2008, Peristaltic pump autosamplers for solids 

measurement in stormwater runoff: Water Environment 

Research, v. 80, 9 p.

Corsi, S.R., Greb, S.R., Bannerman, R.T., and Pitt, R.E., 1999, 

Evaluation of the multi-chambered treatment train, a retrofit 

water-quality management practice: U.S. Geological Survey 

Open-File Report 99-270, 24 p.

Cristina, C., Tramonte, J., and Sansalone, J.J., 2002, 

A granulometry-based selection methodology for separation 

of traffic-generated particles in urban highway snowmelt 

runoff: Water, Air and Soil Pollution, v. 136, p. 33–53.

DeGroot, G.P., Gulliver, J.S., and Mohseni, O., 2009, Accurate 

sampling of suspended solids, in World Environmental and 

Water Resources Congress 2009 and Great Rivers 2009 

ASCE Conference, Proceedings: Kansas City, Missouri, 

American Society of Civil Engineers, p. 807–813.

de Ridder, S.A., Darcy, S.I., Calvert, P.P., and Lenhart, J.H., 

2002, Influence of analytical method, data summarization 

method and particle size on total suspended solids removal 

efficiency, in Strecker, E.W., and Huber, W.C., eds., Global 

solutions for urban drainage, proceedings of the 9th International Conference on Urban Drainage, Portland, Oregon, 

2002: American Society of Civil Engineers report no. 

PD–02–006.1

Design Analysis Associates, 2001, “Smart” SDI–12 tipping 

bucket rain gauge—model H–340SDI, WATERLOG series 

owner’s manual, ver. 1.1: Logan, Utah, 26 p.

Driscoll, E.D., Shelley, P.E., and Strecker, E.W., 1990, Pollutant loadings and impacts from highway stormwater runoff, 

volume I—design procedure: Federal Highway Administration Final Report FHWA–RD–88–006, 61 p.

Field, R., Tafuri, A.N., Muthukrishnan, S., Acquisto, B.A., and 

Selvakumar, S., 2006, The use of best management practices (BMPs) in urban watersheds: Lancaster, Pennsylvania, 

DEStech Publications,. ISBN 1–932078–46–0 [variously 

paged].

Fowler, G.D., Roseen, R.M., Ballestero T.P., Guo, Q., and 

Houle, J., 2009, Sediment monitoring bias by autosampler 

in comparison with whole volume sampling for parking lot 

runoff, in World Environmental and Water Resources Congress 2009, Kansas City, Missouri, Proceedings: Reston, 

Virginia, American Society of Civil Engineers, p. 1–9.

Graham, M.D., and Beckman Coulter, Inc., 2003, The Coulter 

Principle—foundation of an industry: Journal of the Association for Laboratory Automation, v. 8, no. 6, p. 72–81.

Gray, J.R., Glysson, G.D., Torcios, L.M., and Schwartz, G., 

2000, Comparability of suspended-sediment concentration 

and total suspended solids data: U.S. Geological Survey 

Water-Resources Investigation Report 00–4191, 12 p.

Greb, S.R., and Bannerman, R.T., 1997, Influence of particle size on wet pond effectiveness: Water Environment 

Research, v. 69, no. 6, p. 1134–1138.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: New York, Elsevier, 522 p.

Horner, R.R., Skupien, J.J., Livingston, E.H., and 

Shaver, E.H., 1994, Fundamentals of urban runoff management—technical and institutional issues: Washington, 

D.C., Terrene Institute and U.S. Environmental Protection 

Agency, p. 43–56.

Horowitz, A.J., Hayes, T.S., Gray, J.R., and Capel, P.D., 1997, 

Selected laboratory tests of the whole-water sample splitting 

capabilities of the 14-liter churn and the Teflon cone splitters: U.S. Geological Survey Office of Water Quality Technical Memorandum 97.06, 28 p.

Horwatich, J.A., and Bannerman, R.T., 2010, Parking lot runoff quality and treatment efficiency of a stormwater-filtration device, Madison, Wisconsin, 2005–07: U.S. Geological 

Survey Scientific Investigations Report 2009–5196, 50 p.

Horwatich, J.A., Bannerman, R.T., and Pearson, Robert, 2010, 

Highway-runoff quality and treatment efficiency of a hydrodynamic-settling device and a stormwater-filtration device 

in Milwaukee, Wisconsin: U.S. Geological Survey OpenFile Report 2010– 5160, 50 p.

Horwatich, J.A., Corsi, R.S., and Bannerman, R.T., 2004, 

Effectiveness of a pressurized stormwater filtration system 

in Green Bay, Wisconsin—a study for the Environmental 

Technology Verification Program of the U.S. Environmental Protection Agency: U.S. Geological Survey Scientific 

Investigations Report 2004–5222, 19 p.

House, L.B., Waschbusch, R.J., and Hughes, P.E., 1993, Water 

quality of an urban wet detention pond in Madison Wisconsin, 1987–88: U.S. Geological Survey Open-File Report 

93–172, 57 p.

Kayhanian, M., Rasa, E., Vichare, A., and Leatherbarrow, J.E., 

2008, Utility of suspended solid measurements for stormwater runoff treatment: Journal of Environmental Engineering, v. 134, no. 9, p. 712–721.

Legg, A.D., Bannerman, R.T., and Panuska, J., 1996, Variation 

in the relation of rainfall to runoff from residential lawns in 

Madison, Wisconsin, July and August 1995: U.S. Geological Survey Water-Resources Report 96–4196, 11 p.

References 33
Mahler, B.J., Van Metre, P.C., Bashara, T.J., Wilson, J.T., 

and Johns, D.A., 2005, Parking lot sealcoat—an unrecognized source of urban polycyclic aromatic hydrocarbons: 

Environmental Science and Technology, v. 39, no. 15, 

p. 5560–5566.

Masterson, J.P., and Bannerman, R.T., 1994. Impacts of stormwater runoff on urban streams in Milwaukee County, Wisconsin, in National Symposium on Water Quality, Proceedings: American Water Resource Association, p. 123–133.

Minton, G.R., 2005, Stormwater treatment biological, chemical, and engineering principals: Seattle, Washington, Sheridan Books, Inc., 472 p.

National Climatic Data Center, 2003, Local climatological 

data—Madison, Wisconsin—annual summary with comparative data: Asheville, North Carolina, National Climatic 

Data Center, ISSN 0198–5728, 8 p.

National Cooperative Highway Research Program, 2006, 

Evaluation of best management practices for highway runoff control: Washington, D.C., Transportation Research 

Board, NCHRP Report 565, 111 p., 2 app.

National Oceanic and Atmospheric Administration, 2007, 

Dane County Regional Airport Madison precipitation 

records, 1949–2007: Asheville, North Carolina, National 

Climatic Data Center, 42 p.

Pitt, R., Field, R., Lalor, M., and Brown, M., 1995 Urban 

stormwater toxic pollutants—assessment, sources and 

treatability: Water Environment Research , v. 67, no. 3, 

p. 260–275.

Richards, K.D., Scudder, B.C., Fitzpatrick, F.A., Steuer, J.J., 

Bell, A.H., Peppler, M.C., Stewart, J.S., and Harris, M.A., 

2006, Effects of urbanization on stream ecosystems along 

an agriculture-to-urban land-use gradient, Milwaukee to 

Green Bay, Wisconsin, 2003–2004: U.S. Geological Survey 

Scientific Investigations Report 2006–5101–E, 210 p

Roesner, L.A., Pruden, A., and Kidner, E.M., 2007, Improved 

protocol for classification and analysis of stormwater-borne 

solids: Water Environment Research Federation, 04–SW–4, 

124 p.

Selbig, W.R., and Bannerman, R.T., 2010, Development of a 

depth-integrated sample arm to reduce solids stratification 

bias in stormwater sampling: Water Environment Research 

Federation, v. 83, no. 4, p. 347–357

Selbig, W.R., and Bannerman, R.T., 2007, Evaluation of street 

sweeping as a stormwater-quality management tool in three 

residential basins in Madison, Wisconsin: U.S. Geological 

Survey Scientific Investigations Report 2007–5156, 103 p.

Selbig, W.R., Bannerman, Roger, and Bowman, George, 2007, 

Improving the accuracy of sediment-associated constituent 

concentrations in whole-stormwater samples by wet sieving: 

Journal of Environmental Quality, v. 36, no. 1, p. 226–232. 

Smith, K.P., 2002, Effectiveness of three best management 

practices for highway-runoff quality along the southeast 

expressway, Boston, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 02–4059, 62 p.

Smith, K.P., and Granato, G.E., 2010, Quality of stormwater 

runoff discharged from Massachusetts highways, 2005–07: 

U.S. Geological Survey Scientific Investigations Report 

2009–5269, 198 p.

Steuer, J.J., Selbig, W.R., Hornewer, N.J., and Prey, J., 1997, 

Sources of contamination in an urban basin in Marquette, 

Michigan, and an analysis of concentrations, loads, and data 

quality: U.S. Geological Survey Water-Resources Investigations Report 97–4242, 25 p.

Strecker, E.W., Huber, W.C., Heaney, J.P., Bodine, D., Sansalone, J.J., Quigley, M.M., Pankani, D., Leisenring, M., and 

Thayumanavan, P., 2005, Critical assessment of stormwater 

treatment and control selection issues: Water Environment 

Research Federation, Report no. 02–SW–1, ISBN 1–84339–

741–2, 290 p.

Strecker, E.W., Quigley, M.M., and Urbonas, Ben, 2003, 

A reassessment of the expanded EPA/ASCE National BMP 

Database, in National Conference on Urban Storm Water—

Enhancing Programs at the Local Level, Chicago, Illinois, 

2003, Proceedings: U.S. Environmental Protection Agency, 

Office of Research and Development, EPA/625/R–03/003, 

p. 555–574.

U.S. Environmental Protection Agency, 1986, Test methods for 

evaluation of solid waste (3d ed.): Washington D.C., Office 

of Solid Waste and Emergency Response [variously paged]. 

U.S. Environmental Protection Agency, 1998, National water 

quality inventory—1998 report: U.S. Environmental Protection Agency Report EPA–841–F–00–006 [variously paged].

U.S. Environmental Protection Agency, 1999, Preliminary 

data summary of urban storm water best management 

practices: U.S. Environmental Protection Agency, EPA–

821–R–99–012 [variously paged].

U.S. Environmental Protection Agency, 2002a, ETV verification protocol stormwater source area treatment technologies: EPA/NSF wet-weather flow technologies pilot, v.4.1 

[variously paged].

34 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
U.S. Environmental Protection Agency, 2002b, Memorandum on 2002 Integrated Section 305(b) Reports and 

303(d) Lists and the impact of the 305(b) Reports on 

Annual S106 Grant Funding Targets, signed March 1, 

2002, by Michael B. Cook, Director, EPA Office of 

Wastewater Management.

U.S. Environmental Protection Agency, 2007, Environmental Technology Verification Report—stormwater source 

area treatment device—Hydro International Downstream 

Defender®: U.S. Environmental Protection Agency, 07/31/

WQPC–WWF, EPA/600/R–07/121, 60 p., accessed September 2007 at http://www.nsf.org/business/water_quality_protection_center/pdf/Hydro_Verification_Report.pdf.

Wang, L., Lyons, J., Kanehl, P., and Bannerman, R., 2001, 

Impacts of urbanization on stream habitat and fish across 

multiple spatial scales: Environmental Management, v. 28, 

p. 255–266.

Waschbusch, R.J., 1999, Evaluation of the effectiveness of 

urban stormwater treatment unit in Madison, Wisconsin, 

1996–97: U.S. Geological Survey Water-Resources Investigations Report 99–4195, 49 p.

Waschbusch, R.J., Selbig, W.R., and Bannerman, R.T., 1999, 

Sources of phosphorus in stormwater and street dirt from 

two urban residential basins in Madison, Wisconsin, 1994–

95: U.S. Geological Survey Water-Resources Investigations 

Report 99–4021, 47 p.

Weber, D.N., and Bannerman, R., 2004, Relationships between 

impervious surfaces within a watershed and measures of 

reproduction in fathead minnows (Pimephales promelas): 

Hydrobiologia, v. 525, p. 215–228.

Wilde, F.D., ed., 2006, Collection of water samples, 

in National field manual for the collection of waterquality data: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 9, chap. A4 

[variously paged].

Wisconsin Administrative Code, 2004, Wisconsin Department 

of Natural Resources—runoff management—chap. NR151: 

State of Wisconsin [variously paged].

Woodworth, M.T., and Connor, B.F., 2003, Results of the 

U.S. Geological Survey’s analytical evaluation program 

for standard reference samples distributed in March 2003: 

U.S. Geological Survey Open-File Report 03–261, 109 p.

References 35

Appendix 1 37

Appendix 1. List of References for 

Previous Investigations

Bachhuber, J.A., Horwatich, J.A., Corsi, S.R., and Bannerman, R.T., 2002, Environmental Technology Verification 

Program—development of a protocol for testing commercial stormwater treatment devices and two case examples in 

Wisconsin, in StormCon 2002, Marco Island, Florida, 2002, 

Proceedings: Santa Barbara, California, Forester Communications, 12 p.

Bannerman, R.T., Baun, K., Bohn, M., Hughes, P.E., and 

Graczyk, D.J., 1983, Evaluation of urban nonpoint source 

pollution management in Milwaukee County, Wisconsin, volume 1 for U.S. Environmental Protection Agency, 

Region V: Wisconsin Department of Natural Resources 

Publication PB 84–114164 [variously paged].

Bannerman, R.T., Dodds, R.B., Owens, D.W., and Hughes, 

P.E., 1992, Source of pollutants in Wisconsin stormwater, volume 1 for U.S. Environmental Protection Agency, 

Region V: Wisconsin Department of Natural Resources 

Grant number C9995007–01 [variously paged].

Bannerman, R.T., Legg, A.D., and Greb, S.R., 1996, Quality 

of Wisconsin stormwater 1989—94: U.S. Geological Survey Open-File Report 96–458, 26 p.

Bannerman, R.T., Owens, D.W., Dodds, R.B., and Hornewer, 

N.J., 1993, Sources of pollutants in Wisconsin stormwater: 

Water Science Technology, v. 28, no. 3–5, p. 241–259.

Corsi, S.R., Graczyk, D.J., Owens, D.W., and Bannerman, 

R.T., 1997, Unit-area loads of suspended sediment, suspended solids, and total phosphorus from small watersheds 

in Wisconsin: U.S. Geological Survey Fact Sheet 195–97, 

4 p.

Corsi, S.R., Greb, S.R., Bannerman, R.T., and Pitt, R.E., 1999, 

Evaluation of the multi-chambered treatment train, a retrofit 

water-quality management practice: U.S. Geological Survey 

Open-File Report 99–270, 24 p.

Corsi, S.R., Walker, J.F., Wang, L., Horwatich, J.A., and Bannerman, R.T., 2005, Effects of best-management practices 

in Otter Creek in the Sheboygan River priority watershed, 

Wisconsin, 1990–2002: U.S. Geological Survey Scientific 

Investigations Report 2005–5009, 26 p.

Graczyk, D.G., Hunt, R.J., Greb, S.R., Buchwald, C.A., and 

Krohelski, J.T., 2003, Hydrology, nutrient concentrations, 

and nutrient yields in nearshore areas of four lakes in northern Wisconsin, 1999–2001: U.S. Geological Survey WaterResources Investigations Report 03–4144, 64 p.

Graczyk, D.J., Walker, J.F., Horwatich, J.A., and Bannerman, 

R.T., 2003, Effects of best-management practices in the 

Black Earth Creek priority watershed, Wisconsin, 1984–98: 

U.S. Geological Survey Water-Resources Investigations 

Report 03–4163, 24 p.

Greb, S.R., Bannerman, R.T., Corsi, S.R., and Pitt, R.E., 2000, 

Evaluation of the multichambered treatment train, a retrofit 

water-quality management practice: Water Environmental 

Research, v. 72, no. 2, p. 207–216. 

Horwatich, J.A., and Bannerman, R.T., 2010, Parking lot runoff quality and treatment efficiency of a stormwater-filtration device, Madison, Wisconsin, 2005–07: U.S. Geological 

Survey Scientific Investigations Report 2009–5196, 50 p.

Horwatich, J.A., Corsi, R.S., and Bannerman, R.T., 2004, 

Effectiveness of a pressurized stormwater filtration system 

in Green Bay, Wisconsin—a study for the Environmental 

Technology Verification Program of the U.S. Environmental Protection Agency: U.S. Geological Survey Scientific 

Investigations Report 2004–5222, 19 p.

House, L.B., Waschbusch, R.J., and Hughes, P.E., 1993, Water 

quality of an urban wet detention pond in Madison, Wisconsin, 1987–88: U.S. Geological Survey Open-File Report 

93–172, 57 p.

Legg, A.D., Bannerman, R.T., and Panuska, J., 1996, Variation 

in the relation of rainfall to runoff from residential lawns in 

Madison, Wisconsin, July and August 1995: U.S. Geological Survey Water-Resources Report 96–4196, 11 p.

Martinelli, T.J., Waschbusch, R.J., Bannerman, R.T., and 

Wisner, A., 2002, Pollutant loading to stormwater runoff 

from highways—impact of a freeway sweeping program: 

Wisconsin Department of Transportation Final Report 

WI–11–01, 94 p.

Owens, D.O., Jopke, P., Hall, D.W., Balousek, J., and Roa, A., 

2000, Soil erosion from two small construction sites, Dane 

County, Wisconsin: U.S. Geological Survey Fact Sheet 

FS–109–00, 4 p.

Robertson, D.M., Graczyk, D.J., Garrison, P.J., Wang, L., LaLiberte, G., and Bannerman, R.T., 2006, Nutrient concentrations and their relations to the biotic integrity of wadeable 

streams in Wisconsin: U.S. Geological Survey Professional 

Paper 1722, 156 p.

Selbig, W.R., and Bannerman, R.T., 2007, Evaluation of street 

sweeping as a stormwater-quality management tool in three 

residential basins in Madison, Wisconsin: U.S. Geological 

Survey Scientific Investigations Report 2007–5156, 103 p.
38 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Selbig, W.R., and Bannerman, R.T., 2008, A comparison of 

runoff quantity and quality from two small basins undergoing implementation of conventional and low-impactdevelopment (LID) strategies: Cross Plains, Wisconsin, 

water years 1999–2005: U.S. Geological Survey Scientific 

Investigations Report 2008–5008, 66 p.

Selbig, W.R., Bannerman, R.T., and Bowman, G., 2006, Use 

of wet sieving to improve the accuracy of sediment and 

sediment-associated constituent concentrations in wholewater samples, in 8th Federal Interagency Sedimentation 

and Hydrologic Modeling Conference, Reno, Nevada, 2006, 

Proceedings of the Federal Interagency Sedimentation Conferences, 1947 to 2006.: Subcommittee on Sedimentation, 

p. 878–885.

Selbig, W.R., Bannerman, R.T., and Bowman, G., 2007, 

Improving the accuracy of sediment-associated constituent 

concentrations in whole storm water samples by wet sieving: Journal of Environmental Quality, v. 36, no. 1, 7 p.

Steuer, J.J., Selbig, W.R., Hornewer, N.J., and Prey, J., 1997, 

Sources of contamination in an urban basin in Marquette, 

Michigan, and an analysis of concentrations, loads, and data 

quality: U.S. Geological Survey Water-Resources Investigations Report 97–4242, 25 p.

Steuer, J.J., Selbig, W.R., and Hornewer, N.J., 1996, Contaminant concentration in stormwater from eight Lake Superior 

basin cities, 1993–94: U.S. Geological Survey Open-File 

Report 96–122, 16 p.

Stewart, J.S., Wang, L., Lyons, J., Wierl, J.A., and Bannerman, 

R.T., 2001, Influences of watershed, riparian corridors, and 

reach-scale characteristics on aquatic biota in agricultural 

watersheds: Journal of the American Water Resources Association, v. 37, p. 1475–1487.

Stuntebeck, T.D., 1995, Evaluating barnyard best management 

practices in Wisconsin using upstream-downstream monitoring: U.S. Geological Survey Fact Sheet 221–95, 4 p.

Stuntebeck, T.D., and Bannerman, R.T., 1998, Effectiveness of barnyard best management practices in Wisconsin: 

U.S. Geological Survey Fact Sheet 051–98, 4 p.

Stuntebeck, T.D., Komiskey, M.J., Owens, D.W., and Hall, 

D.W., 2008, Methods of data collection, sample processing, 

and data analysis for edge-of-field, streamgaging, subsurface-tile, and meteorological stations at Discovery Farms 

and Pioneer Farm in Wisconsin, 2001–07: U.S. Geological 

Survey Open-File Report 2008–1015, 60 p.

U.S. Environmental Protection Agency, 1982, Results of 

the Nationwide Urban Runoff Program—volume II, 

appendices: U.S. Environmental Protection Agency 

[variously paged]. 

U.S. Environmental Protection Agency, 1983, Results 

of the Nationwide Urban Runoff Program— volume I, final report: Washington, D.C., Water Planning 

Division[variously paged]. 

U.S. Environmental Protection Agency, 2004a, Environmental Technology Verification Report—stormwater source 

area treatment device—Arkal Pressurized Stormwater 

Filtration System: U.S. Environmental Protection Agency, 

EPA/600/R–04/084 [variously paged].

U.S. Environmental Protection Agency, July 2004b, Environmental Technology Verification Report—stormwater source 

area treatment device—The stormwater management StormFilter using ZPG filter media: U.S. Environmental Protection Agency, 04/17/WQPC–WWF, EPA/600/R–04/125, 

65 p.

U.S. Environmental Protection Agency, 2005a, Environmental 

Technology Verification Report— stormwater management 

StormFilter using perlite filter media: U.S. Environmental 

Protection Agency, 05/23/WQPC–WWF, EPA 600/R–

05/137, 56 p.

U.S. Environmental Protection Agency, 2005b, Environmental Technology Verification Report—stormwater source 

area treatment device—Vortechnics, Inc., Vortechs system, 

model 1000: U.S. Environmental Protection Agency, 05/24/

WQPC–WWF, EPA 600/R–05/140, 66 p.

Walker, J.F., Graczyk, D.J., Corsi, S.R., Owens, D.W., and 

Wierl, [Horwatich] J.A., 1995, Evaluation of nonpointsource contamination, Wisconsin—land-use and best-management-practices inventory, selected streamwater-quality 

data, urban-watershed quality assurance and quality control, 

constituent loads in rural streams, and snowmelt-runoff 

analysis, water year 1994: U.S. Geological Survey OpenFile Report 95–320, 21 p. 

Waschbusch, R.J., 1995, Stormwater-runoff data in Madison, 

Wisconsin, 1993–94: U.S. Geological Survey Open-File 

Report 95–733, 33 p. 

Waschbusch, R.J., 1999, Evaluation of the effectiveness of 

urban stormwater treatment unit in Madison, Wisconsin, 

1996–97: U.S. Geological Survey Water-Resources Investigations Report 99–4195, 49 p.

Waschbusch, R.J., 2003, Data and methods of a 1999–2000 

street sweeping study on an urban freeway in Milwaukee 

County, Wisconsin: U.S. Geological Survey Open-File 

Report 03–93, 41 p. 

Waschbusch, R.J., Bannerman, R.T., and Greb, S.R., 1997, 

Yields of selected constituents in street runoff in Madison, 

Wisconsin, 1994–95: U.S. Geological Survey, written commun. to the Wisconsin Department of Natural Resources, 

26 p.
Appendix 1 39

Waschbusch, R.J., Selbig, W.R., and Bannerman, R.T., 1999, 

Sources of phosphorus in stormwater and street dirt from 

two urban residential basins in Madison, Wisconsin, 1994–

95: U.S. Geological Survey Water-Resources Investigations 

Report 99–4021, 47 p.

Weigel, B.M., Emmons, E., Stewart, J.S., and Bannerman, R., 

2005, Buffer width and continuity for preserving stream 

health in agricultural landscapes: Madison, Wisconsin, 

Wisconsin Department of Natural Resources Research/

Management Findings, Wisconsin Department of Natural 

Resources, PUB–SS–756 2005, 4 p.

Wierl [Horwatich], J.A., Giddings, E.M.P., and Bannerman, 

R.T., 1998, Evaluation of a method for comparing phosphorus loads from barnyards and croplands in Otter Creek 

Watershed, Wisconsin: U.S. Geological Survey Fact Sheet 

FS 168–98, 4 p.

Wierl, [Horwatich], J.A., Rappold, K.F., and Amerson, F.U., 

1996, Summary of the land-use inventory for the nonpointsource evaluation monitoring watersheds in Wisconsin: 

U.S. Geological Survey Open-File Report 96–123, 23 p.
40 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 2. Concentrations of selected water-quality constituents in field-equipment blank samples collect at inlet, outlet, and bypass 

of hydrodynamic-settling device in Madison, Wisconsin, October 2005 and May 2006.

[LOD, limit of detection; LOQ, limit of quantification; mg/L, milligrams per liter; <, less than; —, no sample processed; μg/L, micrograms per liter]

Blank1 Blank2 Blank2.1

10/4/2005 5/8/2006

Constituent Unit Inlet Outlet Bypass Inlet Outlet Bypass Outlet LOD LOQ

Suspended-sediment 

concentration

mg/L <2 <2 <2 <2 <2 <2 2 7

Suspended solids, 

total

mg/L <2 <2 <2 <2 <2 <2 2 7

Volatile solids, total mg/L <2 <2 <2 <2 <2 <2 2 7

Dissolved solids mg/L <50 <50 <50 <50 <50 <50 50 167

Phosphorus, total mg/L <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 0.005 0.016

Phosphorus, dissolvedmg/L <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 0.005 0.016

Copper, total recoverableμg/L — — — 27 8 18 1 3

Copper, dissolved μg/L — — — <1 2.3 1.2 <1 1 3

Zinc, total recoverableμg/L <16 <16 <16 21 3 11 16 50

Zinc, dissolved μg/L <16 <16 <16 2 3 2 3 16 50

Calcium, total 

recoverable

mg/L <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 0.02 0.07

Magnesium, total 

recoverable

mg/L <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 0.03 0.07
Appendix 3 41

Appendix 3. Relative percent difference for concentrations of selected water-quality constituents in field-replicate samples 

collected at inlet and outlet of hydrodynamic-settling device in Madison, Wisconsin, April and June 2006.—Continued

[Target, minimum criteria for acceptance of quality-control sample data without qualification; %, percent; Rep, replicate; RPD, relative percent difference; 

mg/L, milligrams per liter; μg/L, micrograms per liter; —, no sample processed] 

Event 5 Event 17

4/12/2006 RPD

(%)

6/25/2006 RPD

Constituent Target Site Rep 1a Rep 1b Rep 2a Rep 2b (%)

Suspended sediment 

(mg/L)

30 Inlet 120 120 0.0 84.1 105 –24.9

Outlet 95 94 1.1 89.1 102 –14.5

Bypass — — — — — —

Suspended solids, total 

(mg/L)

30 Inlet 121 119 1.7 106 105 0.9

Outlet 97 98 –1.0 105 105 0.0

Bypass — — — — — —

Solids, volatile (mg/L) 30 Inlet 20 20 0.0 52 52 0.0

Outlet 19 19 0.0 49 51 –4.1

Bypass — — — — — —

Solids, dissolved (mg/L) 30 Inlet 74 76 –2.7 60 56 6.7

Outlet 86 84 2.3 52 56 –7.7

Bypass — — — — — —

Phosphorus, total (mg/L) 30 Inlet 0.135 0.136 –0.7 0.148 0.153 –3.4

Outlet 0.118 0.114 3.4 0.157 0.153 2.5

Bypass — — — — — —

Phosphorus, dissolved 

(mg/L)

30 Inlet 0.03 0.032 –6.7 0.055 0.055 0.0

Outlet 0.034 0.033 2.9 0.041 0.042 –2.4

Bypass — — — — — —

Copper, total recoverable 

(μg/L)

25 Inlet 15 13 13.3 10 9 10.0

Outlet 13 12 7.7 8 9 –12.5

Bypass — — — — — —

Copper, dissolved (μg/L) 25 Inlet 5.5 5.2 5.5 3.5 3.5 0.0

Outlet 4.9 4.6 6.1 2.7 2.9 –7.4

Bypass — — — — — —
Appendix 3. Relative percent difference for concentrations of selected water-quality constituents in field-replicate samples 

collected at inlet and outlet of hydrodynamic-settling device in Madison, Wisconsin, April and June 2006.—Continued

[Target, minimum criteria for acceptance of quality-control sample data without qualification; %, percent; Rep, replicate; RPD, relative percent difference; 

mg/L, milligrams per liter; μg/L, micrograms per liter; —, no sample processed] 

Event 5 Event 17

4/12/2006 RPD 6/25/2006 RPD

Constituent Target Site Rep 1a Rep 1b (%) Rep 2a Rep 2b (%)

Zinc, total recoverable 

(μg/L)

Zinc, dissolved (μg/L)

Calcium, total recoverable 

(mg/L)

Magnesium, total recoverable (mg/L)

Calculated hardness 

(mg/L)

25

25

25

25

25

Inlet

Outlet

Bypass

Inlet

Outlet

Bypass

Inlet

Outlet

Bypass

Inlet

Outlet

Bypass

Inlet

Outlet

Bypass

36

34

—

6

5

—

16

16

—

7

7

—

69

68

—

38

33

—

6

6

—

16.7

15.7

—

7.2

6.4

—

71.4

65.5

—

–5.6

2.9

—

0.0

–20.0

—

–4.4

3.7

—

–4.3

4.5

—

–4.1

3.7

—

47

48

—

7

5

—

9.6

9.8

—

3.5

3.7

—

38.3

39.7

—

49

46

—

11

6

—

9.7

9.4

—

3.6

3.5

—

39.2

38

—

–4.3

4.2

—

–57.1

–20.0

—

–1.0

4.1

—

–2.9

5.4

—

–2.3

4.3

—

42 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
Appendix 4 43

Appendix 4. Precipitation measured by tipping-bucket rain gage at study site during water-quality sample collection from hydrodynamic-settling device in Madison, Wisconsin, November 2005–September 2006.—Continued[mm, month; dd, day; yyyy, year; hh, hour; mm or min, minute; in., inch; in/h, inch per hour; ft-lb/acre, foot-pound per acre; ft3, cubic foot; —, no data available]

Sampling 

event 

number

Start date and time 

(mm/dd/yyyy hh:mm)

End date and time 

(mm/dd/yyyy hh:mm)

Precipitation 

duration 

(hh:mm)

Total 

precipitation 

(in.)

Maximum 15-min 

intensity 

(in/h)

Maximum 30-min 

intensity

 (in/h)

Erosivity index 

(hundreds of ft-lb/

acre/in/hr)

Precipitation 

volume 

(ft3)

Antecedent dry 

times 

(dd hh:mm)

11/05/2005 15:20 11/06/2005 07:20 00:00 1 0.25 0.22 1.8 6,930 2008:2411/12/2005 16:09 11/12/2005 22:05 22:23 0.16 0.22 0.14 0.2 1,120 3119:3611/14/2005 18:44 11/14/2005 21:19 13:59 0.07 0.07 0.05 0.03 499 1315:3611/15/2005 13:04 11/16/2005 01:35 12:24 0.65 0.25 0.23 1.3 4,490 1518:0011/26/2005 12:10 11/26/2005 14:25 02:15 0.05 0.04 0.04 0.02 374 2618:11

1 11/27/2005 07:05 11/27/2005 11:05 04:00 0.11 0.07 0.05 0.05 749 0016:401 11/28/2005 03:07 11/28/2005 07:24 04:17 0.44 0.43 0.34 1.3 3,060 0016:02

12/23/2005 04:16 12/23/2005 10:28 — — — — — — —01/01/2006 19:30 01/02/2006 13:36 — — — — — — —01/24/2006 09:34 01/24/2006 10:23 — — — — — — —

01/28/2006 05:11 01/29/2006 08:41 — — — — — — —

02/03/2006 05:13 02/03/2006 08:32 — — — — — — —03/06/2006 14:50 03/06/2006 19:03 04:13 0.15 0.11 0.11 0.14 1,060 3106:18

2 03/08/2006 17:50 03/08/2006 22:26 04:36 0.71 0.29 0.29 1.7 4,930 0122:473 03/12/2006 18:04 03/13/2006 08:49 14:45 0.44 0.22 0.18 0.66 3,060 0319:38

03/17/2006 12:23 03/17/2006 13:06 00:43 0.05 0.14 0.09 0.04 374 0403:3403/23/2006 15:50 03/23/2006 17:06 01:16 0.06 0.11 0.09 0.05 437 0602:4403/30/2006 22:51 03/31/2006 01:55 03:04 0.14 0.11 0.07 0.08 936 0705:4504/02/2006 10:54 04/02/2006 12:53 01:59 0.21 0.22 0.16 0.28 1,440 0208:59

4 04/02/2006 19:53 04/03/2006 05:54 10:01 1 0.36 0.23 2 6,990 0007:00

04/06/2006 22:03 04/07/2006 08:48 10:45 1 0.65 0.45 4 7,240 0316:09

5 04/12/2006 04:50 04/12/2006 07:46 02:56 0.37 0.32 0.25 0.77 2,560 0420:026 04/13/2006 21:36 04/13/2006 22:04 00:28 0.09 0.32 — — 624 0113:50
Appendix 4. Precipitation measured by tipping-bucket rain gage at study site during water-quality sample collection from hydrodynamic-settling device in Madison, 

Wisconsin, November 2005–September 2006.—Continued

[mm, month; dd, day; yyyy, year; hh, hour; mm or min, minute; in., inch; in/h, inch per hour; ft-lb/acre, foot-pound per acre; ft3, cubic foot; —, no data available]

Sampling Precipitation Total Maximum 15-min Maximum 30-min Erosivity index Precipitation Antecedent dry Start date and time End date and time event duration precipitation intensity intensity (hundreds of ft-lb/ volume times (mm/dd/yyyy hh:mm) (mm/dd/yyyy hh:mm) number (hh:mm) (in.) (in/h) (in/h) acre/in/hr) (ft3

) (dd hh:mm)

7 04/16/2006 03:39 04/16/2006 04:29 00:50 0.09 0.18 0.14 0.11 624 0205:35

7 04/16/2006 09:16 04/16/2006 10:09 00:53 0.07 0.22 0.13 0.08 499 0004:47

7 04/16/2006 13:47 04/16/2006 16:23 02:36 0.97 2 1.3 12 6,740 0003:38

04/19/2006 04:24 04/19/2006 05:40 01:16 0.14 0.4 0.2 0.25 936 0212:01

04/22/2006 01:24 04/22/2006 02:06 00:42 0.05 0.18 0.09 0.04 374 0219:44

8 04/29/2006 14:33 04/30/2006 16:11 01:38 1.7 0.44 0.36 5.1 11,600 0712:27

05/01/2006 14:41 05/01/2006 14:43 00:02 0.16

—

—

— 1,120 0022:30

9 05/01/2006 21:08 05/01/2006 21:34 00:26 0.24 0.8 0.49 1.2 1,690 0006:25

10 05/09/2006 10:31 05/09/2006 17:21 06:50 0.37 0.22 0.18 0.55 2,560 0712:57

11 05/11/2006 06:17 05/11/2006 08:55 02:38 0.08 0.11 0.07 0.05 562 0112:5611 05/11/2006 11:57 05/12/2006 06:23 18:26 0.78 0.14 0.11 0.7 5,430 0003:02

12 05/15/2006 15:40 05/15/2006 17:33 01:53 0.05 0.11 0.07 0.03 374 0309:17

13 05/16/2006 15:23 05/16/2006 18:06 02:43 0.11 0.22 0.11 0.1 749 0021:50

14 05/17/2006 15:26 05/17/2006 16:15 00:49 0.22 0.43 0.27 0.52 1,560 0021:20

15 05/24/2006 18:39 05/24/2006 18:59 00:20 1.4 5.5

—

— 9,980 0702:24

05/30/2006 10:53 05/30/2006 11:05 00:12 0.09

—

—

— 624 0515:54

05/30/2006 13:06 05/30/2006 13:29 00:23 0.11 0.4

—

— 749 0002:01

06/06/2006 05:52 06/06/2006 08:21 02:29 0.11 0.11 0.07 0.06 749 0616:23

06/10/2006 00:32 06/10/2006 06:35 06:03 0.63 0.29 0.25 1.3 4,370 0316:11

06/14/2006 11:28 06/14/2006 12:27 00:59 0.05 0.14 0.09 0.04 374 0404:53

16 06/18/2006 05:05 06/18/2006 09:28 04:23 0.09 0.07 0.07 0.05 624 0316:38

16 06/18/2006 12:15 06/18/2006 13:02 00:47 0.09 0.22 0.14 0.11 624 0002:47

06/21/2006 05:50 06/21/2006 06:44 00:54 0.06 0.18 0.09 0.05 437 0216:48

06/24/2006 23:29 06/24/2006 23:57 00:28 0.04 0.11 —

— 250 0316:45

44 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6
Appendix 4 45

Appendix 4. Precipitation measured by tipping-bucket rain gage at study site during water-quality sample collection from hydrodynamic-settling device in Madison, 

Wisconsin, November 2005–September 2006.—Continued

[mm, month; dd, day; yyyy, year; hh, hour; mm or min, minute; in., inch; in/h, inch per hour; ft-lb/acre, foot-pound per acre; ft3, cubic foot; —, no data available]

Sampling Precipitation Total Maximum 15-min Maximum 30-min Erosivity index Precipitation Antecedent dry Start date and time End date and time event duration precipitation intensity intensity (hundreds of ft-lb/ volume times (mm/dd/yyyy hh:mm) (mm/dd/yyyy hh:mm) number (hh:mm) (in.) (in/h) (in/h) acre/in/hr) (ft3) (dd hh:mm)

17 06/25/2006 15:11 06/25/2006 21:24 06:13 0.47 0.5 0.45 1.8 3,240 0015:14

17 06/26/2006 03:07 06/26/2006 08:12 05:05 0.32 0.18 0.16 0.43 2,250 0005:43

18 07/09/2006 19:39 07/09/2006 19:47 00:08 0.36 — — — 2,510 1311:27

19 07/11/2006 07:17 07/11/2006 16:08 08:51 1.9 0.86 0.76 12 13,000 0111:30

20 07/19/2006 21:15 07/20/2006 06:59 09:44 0.96 1.4 0.88 7.7 6,660 0805:07

21 07/22/2006 16:44 07/22/2006 20:07 03:23 0.37 1.4 0.72 2.6 2,570 0209:45

07/25/2006 23:50 07/26/2006 01:04 01:14 0.06 0.16 0.08 0.04 416 0303:43

22 07/27/2006 12:06 07/27/2006 15:34 03:28 2.2 4.6 3.7 56 15,000 0111:02

23 08/06/2006 05:52 08/06/2006 11:00 05:08 0.71 0.36 0.28 1.7 4,920 0914:18

08/09/2006 18:44 08/09/2006 18:47 00:03 0.06 — — — 416 0307:44

27 08/17/2006 15:57 08/17/2006 17:42 01:45 0.29 0.72 0.48 1.2 2,010 0721:10

08/18/2006 13:30 08/18/2006 17:50 04:20 0.06 0.08 0.04 0.02 416 0019:48

25 08/23/2006 22:42 08/24/2006 06:59 08:17 1.6 2.1 1.2 19 11,100 0504:5226 08/24/2006 12:59 08/24/2006 15:12 02:13 1.4 2.6 1.5 21 9,360 0006:00

08/25/2006 05:17 08/25/2006 13:39 08:22 0.95 1.7 0.88 8 6,590 0014:05

09/03/2006 18:26 09/03/2006 21:08 02:42 0.28 0.32 0.18 0.42 1,940 0904:47

09/04/2006 05:28 09/04/2006 07:56 02:28 0.35 0.36 0.28 0.83 2,430 0008:20

09/10/2006 14:48 09/10/2006 23:38 08:50 0.35 0.2 0.14 0.41 2,430 0606:52

09/11/2006 10:23 09/11/2006 11:54 01:31 0.06 0.12 0.08 0.04 416 0010:4509/11/2006 15:10 09/11/2006 17:10 02:00 0.34 0.6 0.42 1.2 2,360 0003:16

09/11/2006 18:50 09/11/2006 19:54 01:04 0.14 0.28 0.2 0.24 971 0001:40

09/12/2006 02:13 09/12/2006 06:02 03:49 0.56 0.4 0.28 1.3 3,880 0006:19

09/12/2006 08:53 09/12/2006 11:29 02:36 0.17 0.2 0.14 0.2 1,180 0002:51

09/12/2006 14:19 09/12/2006 18:01 03:42 0.6 0.6 0.42 2.1 4,160 0002:50
46 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6 Appendix 5. Stormwater peak discharge, runoff volumes, and percentage runoff at hydrodynamic-settling device in Madison, Wisconsin, November 2005– September 2006.—Continued [mm, month; dd, day; yyyy, year; hh, hour; mm, minute; in., inch; ft3, cubic foot; ft3/s, cubic foot per second. Shading indicates data that have been excluded from efficiency calculations] Sampling event number Start date and time (mm/dd/yyyy hh:mm) End date and time (mm/dd/yyyy hh:mm) Total precipitation (in.) Peak discharge at unit inlet (ft3/s) Inlet volume (ft3

)

Peak discharge at 

system outlet 

(ft3/s)

Bypassing volume 

over weir wall 

(ft3

)

Percentage 

runoff 

for system

11/5/2005 15:59 11/6/2005 1:28 0.79 0.22 2,690 0.86 49.1

11/6/2005 1:39 11/6/2005 5:47 0.19 0.1 829 0.32 62.9

11/15/2005 13:14 11/16/2005 3:34 0.62 0.21 3,010 0.61 70

1 11/27/2005 06:05 11/28/2005 11:09 0.55 0.25 3,010 0.91 78.92 03/08/2006 17:57 03/09/2006 01:27 0.71 0.19 1,880 1 38.23 03/12/2006 18:31 03/13/2006 05:08 0.43 0.18 1,370 0.42 46

3/30/2006 22:53 3/31/2006 2:49 0.14 0.16 838 0.07 86.3

4/2/2006 11:12 4/2/2006 14:01 0.21 0.26 1,110 0.29 76.2

4 04/02/2006 20:39 04/03/2006 06:50 1 0.54 5,910 0.38 85.2

4/6/2006 22:18 4/7/2006 8:34 1 1.5 6,500 1.6 93.8

5 04/12/2006 05:01 04/12/2006 08:32 0.37 0.54 1,980 0.63 77.26 04/13/2006 21:38 04/13/2006 22:55 0.09 0.86 458 0.59 73.47 04/16/2006 04:04 04/16/2006 17:30 1.1 2.6 6,230 5.8 1,220 97.7

4/19/2006 4:26 4/19/2006 4:56 0.14 0.87 441 0.7 45.4

8 04/29/2006 16:43 04/30/2006 10:50 1.6 0.69 8,480 0.66 76.49 05/01/2006 21:14 05/01/2006 22:33 0.25 2.4 1,570 2 90.6

10 05/09/2006 12:01 05/09/2006 18:02 0.37 0.38 2,090 0.35 81.5

11 05/11/2006 06:48 05/12/2006 06:12 0.86 0.19 5,040 0.18 84.5

12 05/15/2006 17:31 05/15/2006 18:53 0.05 0.19 242 0.04 69.813 5/16/2006 15:39 5/16/2006 18:59 0.11 0.28 596 0.2 78.214 05/17/2006 15:32 05/17/2006 17:28 0.22 0.74 1,310 0.85 85.9
Appendix 5 47

Appendix 5. Stormwater peak discharge, runoff volumes, and percentage runoff at hydrodynamic-settling device in Madison, Wisconsin, November 2005–September 2006.—Continued[mm, month; dd, day; yyyy, year; hh, hour; mm, minute; in., inch; ft3, cubic foot; ft3/s, cubic foot per second. Shading indicates data that have been excluded from efficiency calculations]

Sampling Total Peak discharge at Peak discharge at Bypassing volume Percentage Start date and time End date and time Inlet volume event precipitation unit inlet system outlet over weir wall runoff (mm/dd/yyyy hh:mm) (mm/dd/yyyy hh:mm) (ft3) number (in.) (ft3/s) (ft3/s) (ft3) for system

15 05/24/2006 18:45 05/24/2006 20:56 1.4 — — — — —

05/30/2006 10:59 05/30/2006 11:30 0.09 0.37 328 0.27 52.6

5/30/2006 13:28 5/30/2006 13:53 0.11 0.75 372 0.71 48.8

6/6/2006 6:00 6/6/2006 8:49 0.11 0.1 475 0.04 62.3

6/10/2006 0:49 6/10/2006 6:55 0.63 0.43 3,190 0.54 73

16 6/18/2006 8:03 6/18/2006 13:59 0.17 0.46 899 0.23 76.3

6/21/2006 5:54 6/21/2006 6:18 0.06 0.21 138 0.06 33.2

17 06/25/2006 17:29 06/26/2006 09:02 0.79 0.89 4,250 0.67 77.6

18 7/9/2006 19:41 7/9/2006 20:58 0.36 2.3 1,430 2.6 40 58.9

19 07/11/2006 08:35 07/11/2006 16:09 1.9 1.8 11,000 1.5 83.5

20 07/19/2006 21:15 07/20/2006 08:19 0.96 2.4 4,680 2.5 70.3

21 07/22/2006 16:48 07/22/2006 18:07 0.36 2.2 1,870 1.9 74.9

07/26/2006 01:04 07/26/2006 01:34 0.06 0.32 199 0.07 47.8

22 07/27/2006 12:24 07/27/2006 14:53 2.2 — — — — —23 8/6/2006 6:42 8/6/2006 12:13 0.71 0.57 3,660 0.5 74.4

24 08/17/2006 16:21 08/17/2006 17:39 0.29 1.5 1,300 1.3 64.7

25 8/23/2006 22:53 8/24/2006 7:43 1.6 2.7 13,500 4.4 1,220 133
48 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6 Appendix 5. Stormwater peak discharge, runoff volumes, and percentage runoff at hydrodynamic-settling device in Madison, Wisconsin, November 2005– September 2006.—Continued [mm, month; dd, day; yyyy, year; hh, hour; mm, minute; in., inch; ft3, cubic foot; ft3/s, cubic foot per second. Shading indicates data that have been excluded from efficiency calculations] Sampling Total Peak discharge at Peak discharge at Bypassing volume Percentage Start date and time End date and time Inlet volume event precipitation unit inlet system outlet over weir wall runoff (mm/dd/yyyy hh:mm) (mm/dd/yyyy hh:mm) (ft3

) number (in.) (ft3/s) (ft3/s) (ft3) for system

26 08/24/2006 13:25 08/24/2006 17:49 1.4 2.9 17,200 4.6 3,720 216

08/25/2006 05:43 08/25/2006 17:00 0.95 2.8 14,300 4.5 345 222

09/03/2006 18:32 09/03/2006 21:49 0.28 0.32 1,370 0.23 70.6

09/04/2006 05:49 09/04/2006 08:00 0.35 0.59 1,990 0.54 82

9/10/2006 15:08 9/10/2006 23:45 0.35 0.31 2,310 0.63 95.2

9/11/2006 11:30 9/11/2006 12:46 0.06 0.22 372 0.45 89.4

9/11/2006 15:13 9/11/2006 21:19 0.48 1 4,280 0.87 129

9/12/2006 2:29 9/12/2006 12:44 0.73 1.6 14,500 0.55 286

9/12/2006 14:35 9/12/2006 20:25 0.6 2.1 14,000 1.9 337

Count 47 45 45 45 5 45

Minimum 0.05 0.1 138 0.04 40 32

Maximum 2.2 2.9 17,200 5.8 3720 340

Average 0.59 0.94 3,850 0 0 70
Appendix 6 49

Appendix 6. Concentrations of suspended sediment, adjusted suspended solids, volatile solids, and dissolved solids in stormwater 

samples collected at inlet, outlet, and bypass of hydrodynamic-settling device in Madison, Wisconsin, Nobember 2005–August 2006.

[All data are in milligrams per liter; —, no sample processed; <, less than]

Sampling date 

(month/day/year)

Sampling 

event 

number

Suspended sediment Suspended solids, 

total adjusted Solids, volatile Solids, dissolved Samples sieved

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass In/Out/Bypass

11/27/2005 1 8 8 — 7 8 — — — — 114 128 — Yes/No

03/08/2006 2 127 166 — 130 147 — — — — 260 238 — Yes/No

03/12/2006 3 64 52 — 61 52 — — — — 192 86 — Yes/No

04/02/2006 4 58 36 — 60 36 — — — — 96 84 — Yes/No

04/12/2006 5 120 95 — 124 97 — — — — 74 86 — Yes/No

04/13/2006 6 419 538 — 398 545 — 66 79 — 182 216 — No/No

04/16/2006 7 270 190 1,006 191 188 527 — — — 62 94 <50 Yes/No/Yes

04/29/2006 8 35 30 — 35 30 — 22 18 — 54 56 — No/No

05/01/2006 9 165 202 — 144 217 — 47 69 — <50 64 — No/No

05/09/2006 10 22 21 — 23 24 — 12 11 — 74 52 — No/No

05/11/2006 11 25 22 — 24 19 — 16 13 — 88 60 — No/No

05/15/2006 12 — — — — — — — — — — — —

05/16/2006 13 58 61 — 60 65 — 29 28 — 122 — — No/No

05/17/2006 14 156 143 — 159 153 — 70 61 — 58 66 — No/No

05/24/2006 15 — — — — — — — — — — — —

06/18/2006 16 57 45 — 61 46 — 24 15 — 124 180 — No/No

06/25/2006 17 104 104 — 106 105 — 52 49 — 60 52 — No/No

07/09/2006 18 450 662 — 371 584 — — — — 80 72 — Yes/Yes

07/11/2006 19 57 37 — 35 38 — 9 10 — <50 <50 — No/No

07/19/2006 20 128 137 — 81 120 — 20 30 — 52 137 — No/No

07/22/2006 21 116 127 — 103 119 — 21 24 — <50 <50 — No/No

07/27/2006 22 — — — — — — — — — — — —

08/06/2006 23 59 37 — 51 37 — — — — <50 58 — No/No

08/17/2006 24 705 211 — 332 207 — — — — 78 98 — Yes/No

08/23/2006 25 527 327 476 349 349 314 — — — 136 112 90 Yes/Yes/Yes

08/24/2006 26 624 285 494 395 292 301 — — — 226 222 170 Yes/No/Yes

Count 23 23 3 23 23 3 12 12 — 23 22 3

Geomean 107 90 618 92 89 368 27 27 — 99 95 124

Median 116 104 494 103 105 314 23 26 — 88 86 130

Average 189 154 659 144 151 381 32 34 — 112 108 130

Maximum 705 662 1,006 398 584 527 70 79 0 260 238 0

Minimum 8 8 476 7 8 301 9 10 0 <50 <50 <50
50 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 7. Concentrations of selected water-quality constituents and physical properties in stormwater samples collected at inlet, 

outlet, and bypass of hydrodynamic-settling device in Madison, Wisconsin, November 2005–September 2006.

[mg/L, milligrams per liter; μg/L, micrograms per liter; >, greater than; —, no sample processed. Shading indicates data that have been excluded from 

efficiency calculations]

Sampling 

event 

number

Phosphorus, total 

(mg/L)

Phosphorus, dissolved 

(mg/L)

Copper, total recoverable 

(μg/L)

Copper, dissolved 

(μg/L)

Zinc, total recoverable 

(μg/L)

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

1 0.070 0.13 — 0.05 0.07 — — — — — — — — — —

2 0.18 0.23 — 0.04 0.03 — 16 25 — 4.7 5.1 — 46 66 —

3 0.10 0.09 — 0.03 0.03 — 10 10 — 4.7 3.1 — 21 24 —

4 0.09 0.08 — 0.03 0.03 — 7 6 — — — — 22 17 —

5 0.14 0.12 — 0.03 0.03 — 15 13 — 5.5 4.9 — 36 34 —

6 — — — — — — — — — — — — — — —

7 0.22 0.23 0.40 0.05 0.04 0.04 22 21 16 3.6 3.7 1.8 45 48 136

8 0.25 0.09 — 0.03 0.05 — 7 6 — 2.8 3.7 — 19 21 —

9 0.16 0.19 — 0.03 0.03 — 16 20 — 2.1 1.6 — 45 62 —

10 0.11 0.10 — 0.05 0.04 — 9 15 — 6.9 9.9 — 23 30 —

11 0.17 0.17 — 0.10 0.10 — 6 6 — 4.8 4.4 — 18 18 —

12 — — — — — — — — — — — — — — —

13 — — — — — — — — — — — — — — —

14 0.23 0.23 — 0.03 0.03 — 15 14 — 4.2 3.5 — 64 55 —

15 — — — — — — — — — — — — — — —

16 — — — — — — — — — — — — — — —

17 0.15 0.31 — 0.06 0.08 — 10 17 — 3.5 5.6 — 47 94 —

18 0.29 0.36 — 0.07 0.06 — 29 36 — 7.0 6.2 — 100 129 —

19 0.09 0.10 — 0.06 0.05 — 6 5 — 3.9 4.0 — 17 18 —

20 0.12 0.14 — 0.04 0.04 — 9 10 — 4.6 7.5 — 40 50 —

21 0.12 0.12 — 0.04 0.01 — 14 12 — 4.0 5.9 — 36 41 —

22 — — — — — — — — — — — — — — —

23 0.16 0.14 — 0.07 0.06 — 8 9 — 5.0 4.0 — 36 31 —

24 0.34 0.30 — 0.09 0.06 — 25 28 — 8.0 6.0 — 102 128 —

25 1.7 1.4 1.1 1.13 .87 0.60 37 34 40 9.0 8.0 7.0 95 99 118

26 — — — — — — — — — — — — — — —

Count 19 19 2 19 19 2 18 18 2 17 17 2 18 18 2

Geomean 0.17 0.17 0.66 0.05 0.05 0.15 12 13 25 4.7 4.7 3.5 38 43 127

Median 0.16 0.14 0.75 0.05 0.04 0.32 12 14 28 4.7 4.9 4.4 38 45 127

Average 0.25 0.24 0.75 0.11 0.09 0.32 15 16 28 5.0 5.1 4.4 45 54 127

Maximum 1.7 1.4 1.1 1.1 0.87 0.60 37 36 40 9.0 9.9 7.0 102 129 136

Minimum .07 0.08 0.40 0.03 0.01 .04 6 5 16 2.1 1.6 1.8 17 17 118
Appendix 7 51

Appendix 7. Concentrations of selected water-quality constituents and physical properties in stormwater 

samples collected at inlet, outlet, and bypass of hydrodynamic-settling device in Madison, Wisconsin, November 

2005–September 2006.—Continued

[mg/L, milligrams per liter; μg/L, micrograms per liter; >, greater than; —, no sample processed. Shading indicates data that have 

been excluded from efficiency calculations]

Sampling 

event 

number

Zinc, dissolved 

(μg/L)

Calcium, total recoverable 

(mg/L)

Magnesium, total recoverable 

(mg/L)

Calculated hardness 

(mg/L)

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

1 — — — 11 16 — 4 5 — 43 60 —

2 >16 >16 — 22 22 — 11 11 — 101 99 —

3 >16 >16 — 18 11 — 9 5 — 80 46 —

4 — — — 12 9 — 5 4 — 50 38 —

5 6 5 — 16 16 — 7 7 — 69 68 —

6 — — — — — — — — — — — —

7 4 4 5 17 20 16 8 10 6 77 89 67

8 11 8 — 8 8 — 3 2 — 30 29 —

9 5 5 — 14 18 — 6 8 — 61 78 —

10 11 10 — 9 7 — 3 2 — 34 27 —

11 10 10 — 9 8 — 3 3 — 35 30 —

12 — — — — — — — — — — — —

13 — — — — — — — — — — — —

14 8 8 — 14 15 — 6 7 — 60 63 —

15 — — — — — — — — — — — —

16 — — — — — — — — — — — —

17 7 11 — 10 19 — 4 7 — 38 40 —

18 7 9 — 26 35 — 13 18 — 117 161 —

19 5 5 — 5 5 — 2 2 — 21 22 —

20 5 6 — 14 15 — 6 6 — 59 59 —

21 3 5 — 11 13 — 5 6 — 48 55 —

22 — — — — — — — — — — — —

23 12 11 — 8 8 — 3 3 — 30 29 —

24 14 15 — 15 20 — 6 8 — 64 82 —

25 24 21 20 19 22 23 9 10 10 84 97 99

26 — — — — — — — — — — — —

Count 15 15 2 19 19 2 19 19 2 19 19 2

Geomean 8 8 10 13 13 19 5 5 8 53 54 82

Median 7 8 13 14 15 19 6 6 8 59 59 83

Average 9 9 13 14 15 19 6 6 8 58 62 83

Maximum 24 21 20 26 35 23 13 18 10 117 161 99

Minimum 3 4 5 5.0 5.2 16 2 2 6 21 22 67
52 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 8. Mean concentrations of selected polycyclic aromatic hydrocarbons in stormwater samples collected at inlet 

and outlet of hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006. 

[All concentrations in micrograms per liter; PAH, polycyclic aromatic hydrocarbons; <, less than; —, no sample processed]

Sampling 

event 

number

2-Methylnaphthalene1-Methylnaphthalene

AcenaphthyleneAcenaphthene Anthracene

Benzo[b] 

fluoranthene

Benzo[k]

fluoranthene

Benzo[a]

pyrene Chrysene

Inlet

1 — — — — — — — — —

2 — — — — — — — — —

3 — — — — — — — — —

4 — — — — — — — — —

5 — — — — — — — — —

6 — — — — — — — — —

7 — — — — — — — — —

8 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.059

9 <0.049 <0.064 <0.11 <0.064 <0.031 0.23 <0.12 0.16 0.2

10 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.036

11 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 <0.027

12 — — — — — — — — —

13 — — — — — — — — —

14 <0.049 <0.064 <0.11 <0.064 0.048 0.39 0.17 0.24 0.35

15 — — — — — — — — —

16 — — — — — — — — —

17 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.11

18 — — — — — — — — —

19 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.04

20 — — — — — — — — —

21 — — — — — — — — —

22 — — — — — — — — —

23 — — — — — — — — —

24 — — — — — — — — —

25 — — — — — — — — —

26 — — — — — — — — —

Outlet

1 — — — — — — — — —

2 — — — — — — — — —

3 — — — — — — — — —

4 — — — — — — — — —

5 — — — — — — — — —

6 — — — — — — — — —
Appendix 8 53

Appendix 8. Mean concentrations of selected polycyclic aromatic hydrocarbons in stormwater samples collected at inlet 

and outlet of hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006. —Continued

[All concentrations in micrograms per liter; PAH, polycyclic aromatic hydrocarbons; <, less than; —, no sample processed]

Sampling 

event 

number

Fluoranthene9HFluorene

Indeno 

[1,2,3-cd] 

pyrene

Phenanthrene Pyrene

Benzo 

[g,h,i] 

perylene

Benzo [a] 

anthracene

Dibenzo 

[a,h] 

anthracene

Naphthalene Total PAH

Inlet

1 — — — — — — — — — —

2 — — — — — — — — — —

3 — — — — — — — — — —

4 — — — — — — — — — —

5 — — — — — — — — — —

6 — — — — — — — — — —

7 — — — — — — — — — —

8 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.06

9 0.38 <0.52 0.16 0.2 0.3 0.22 <0.093 <0.042 <0.042 1.85

10 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.04

11 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.00

12 — — — — — — — — — —

13 — — — — — — — — — —

14 0.59 <0.52 0.25 0.31 0.5 0.33 <0.093 <0.053 <0.042 3.18

15 — — — — — — — — — —

16 — — — — — — — — — —

17 0.17 <0.52 <0.093 <0.093 0.15 <0.14 <0.093 <0.10 <0.042 0.43

18 — — — — — — — — — —

19 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.04

20 — — — — — — — — — —

21 — — — — — — — — — —

22 — — — — — — — — — —

23 — — — — — — — — — —

24 — — — — — — — — — —

25 — — — — — — — — — —

26 — — — — — — — — — —

Outlet

1 — — — — — — — — — —

2 — — — — — — — — — —

3 — — — — — — — — — —

4 — — — — — — — — — —

5 — — — — — — — — — —

6 — — — — — — — — — —
54 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 8. Mean concentrations of selected polycyclic aromatic hydrocarbons in stormwater samples collected at inlet 

and outlet of hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006.—Continued

[All concentrations in micrograms per liter; PAH, polycyclic aromatic hydrocarbons; <, less than; —, no sample processed]

Sampling 

event 

number

2-Methylnaphthalene1-Methylnaphthalene

AcenaphthyleneAcenaphthene Anthracene

Benzo[b] 

fluoranthene

Benzo[k]

fluoranthene

Benzo[a]

pyrene Chrysene

Outlet

7 — — — — — — — — —

8 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.046

9 <0.049 <0.064 <0.11 <0.064 <0.031 0.32 0.15 <0.32 0.29

10 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.043

11 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 <0.027

12 — — — — — — — — —

13 — — — — — — — — —

14 <0.049 <0.064 <0.11 <0.064 0.1 0.38 0.18 0.26 0.36

15 — — — — — — — — —

16 — — — — — — — — —

17 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 0.12

18 — — — — — — — — —

19 <0.049 <0.064 <0.11 <0.064 <0.031 <0.13 <0.12 <0.16 <0.050

20 — — — — — — — — —

21 — — — — — — — — —

22 — — — — — — — — —

23 — — — — — — — — —

24 — — — — — — — — —

25 — — — — — — — — —

26 — — — — — — — — —
Appendix 8 55

Appendix 8. Mean concentrations of selected polycyclic aromatic hydrocarbons in stormwater samples collected at inlet 

and outlet of hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006. —Continued

[All concentrations in micrograms per liter; PAH, polycyclic aromatic hydrocarbons; <, less than; —, no sample processed]

Sampling 

event 

number

Fluoranthene9HFluorene

Indeno 

[1,2,3-cd] 

pyrene

Phenanthrene Pyrene

Benzo 

[g,h,i] 

perylene

Benzo [a] 

anthracene

Dibenzo 

[a,h] 

anthracene

Naphthalene Total PAH

Outlet

7 — — — — — — — — — —

8 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.05

9 0.55 <0.52 <0.21 0.25 0.43 0.28 0.11 <0.048 <0.042 2.38

10 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.04

11 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.034 <0.042 0.00

12 — — — — — — — — — —

13 — — — — — — — — — —

14 0.59 <0.52 0.26 0.31 0.49 0.35 <0.093 <0.056 <0.042 3.28

15 — — — — — — — — — —

16 — — — — — — — — — —

17 0.16 <0.52 <0.093 <0.093 0.14 <0.14 <0.093 <0.10 <0.042 0.42

18 — — — — — — — — — —

19 <0.11 <0.52 <0.093 <0.093 <0.11 <0.14 <0.093 <0.10 <0.042 0.00

20 — — — — — — — — — —

21 — — — — — — — — — —

22 — — — — — — — — — —

23 — — — — — — — — — —

24 — — — — — — — — — —

25 — — — — — — — — — —

26 — — — — — — — — — —
56 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 9. Particle-size distributions in stormwater samples collected at inlet, outlet, and bypass of 

hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006.

[All data are in percent by mass; <, less than; μm, micrometer; —, no sample processed]

Sampling date 

(month/day/year)

Sampling 

event 

number

Particle size

<500 μm <250 μm <125 μm

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

11/27/2005

03/08/2006

03/12/2006

04/02/2006

04/12/2006

04/13/2006

04/16/2006

04/29/2006

05/01/2006

05/09/2006

05/11/2006

05/15/2006

05/16/2006

05/17/2006

05/24/2006

06/18/2006

06/25/2006

07/09/2006

07/11/2006

07/19/2006

07/22/2006

07/27/2006

08/06/2006

08/17/2006

08/23/2006

08/24/2006

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

Minimum

Maximum

96

100

99

99

100

—

100

98

100

97

93

—

—

87

—

96

93

100

93

100

89

—

85

27

55

50

27

100

92

100

99

99

99

—

100

98

98

97

98

—

—

94

—

99

97

100

95

94

89

—

98

100

98

100

89

100

—

—

—

—

—

—

100

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

86

74

74

100

84

99

98

98

97

—

64

84

94

95

81

—

—

74

—

91

80

76

89

56

86

—

73

24

51

42

24

99

85

97

98

98

98

—

94

87

91

92

77

—

—

89

—

92

85

88

91

90

83

—

91

92

95

96

77

98

—

—

—

—

—

—

81

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

80

64

64

81

73

95

91

96

94

—

53

66

79

77

66

—

—

61

—

78

61

65

81

44

79

—

53

20

45

39

20

96

70

96

96

95

94

—

83

66

71

70

62

—

—

68

—

77

67

58

84

80

62

—

74

73

80

86

58

96

—

—

—

—

—

—

64

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

68

57

57

68
Appendix 9 57

Appendix 9. Particle-size distributions in stormwater samples collected at inlet, outlet, and bypass of 

hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006.—Continued

[All data are in percent by mass; <, less than; μm, micrometer; —, no sample processed]

Sampling date 

(month/day/year)

Sampling 

event 

number

Particle size

<63 μm <31 μm <16 μm

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

11/27/2005

03/08/2006

03/12/2006

04/02/2006

04/12/2006

04/13/2006

04/16/2006

04/29/2006

05/01/2006

05/09/2006

05/11/2006

05/15/2006

05/16/2006

05/17/2006

05/24/2006

06/18/2006

06/25/2006

07/09/2006

07/11/2006

07/19/2006

07/22/2006

07/27/2006

08/06/2006

08/17/2006

08/23/2006

08/24/2006

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

Minimum

Maximum

64

93

90

88

79

—

42

41

50

55

55

—

—

43

—

54

39

44

56

28

54

—

39

16

38

34

16

93

50

93

92

92

83

—

57

39

52

48

47

—

—

49

—

49

36

40

61

59

40

—

52

54

69

74

36

93

—

—

—

—

—

—

43

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

55

51

43

55

55

88

81

79

70

—

34

28

36

44

46

—

—

32

—

39

29

27

37

18

36

—

30

13

29

29

13

88

35

88

86

86

71

—

44

28

34

37

36

—

—

37

—

36

27

24

40

39

29

—

38

44

56

63

24

88

—

—

—

—

—

—

31

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

38

45

31

45

48

74

59

57

50

—

24

21

23

33

40

—

—

22

—

34

25

25

29

14

35

—

28

12

27

27

12

74

24

70

76

66

50

—

29

20

23

31

31

—

—

25

—

35

24

19

37

33

29

—

34

36

53

61

19

76

—

—

—

—

—

—

20

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

36

42

20

42
58 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 9. Particle-size distributions in stormwater samples collected at inlet, outlet, and bypass of 

hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006.—Continued

[All data are in percent by mass; <, less than; μm, micrometer; —, no sample processed]

Sampling date 

(month/day/year)

Sampling 

event 

number

Particle size

<8 μm <4 μm <2 μm

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

11/27/2005

03/08/2006

03/12/2006

04/02/2006

04/12/2006

04/13/2006

04/16/2006

04/29/2006

05/01/2006

05/09/2006

05/11/2006

05/15/2006

05/16/2006

05/17/2006

05/24/2006

06/18/2006

06/25/2006

07/09/2006

07/11/2006

07/19/2006

07/22/2006

07/27/2006

08/06/2006

08/17/2006

08/23/2006

08/24/2006

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

Minimum

Maximum

44

65

49

47

39

—

19

19

17

29

37

—

—

18

—

29

21

16

22

12

32

—

27

10

23

22

10

65

21

59

63

56

40

—

24

18

19

27

29

—

—

20

—

34

19

11

33

24

26

—

31

27

45

51

11

63

—

—

—

—

—

—

15

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

32

32

15

32

42

48

35

32

28

—

15

16

12

24

33

—

—

13

—

25

17

12

16

11

26

—

26

8

21

14

8

48

18

42

46

40

29

—

17

15

13

22

24

—

—

14

—

33

15

7

28

18

22

—

29

21

37

35

7

46

—

—

—

—

—

—

10

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

29

20

10

29

23

17

11

9

7

—

4

6

4

8

15

—

—

4

—

17

8

6

9

8

13

—

24

5

16

5

4

24

15

9

12

12

7

—

5

8

3

10

9

—

—

3

—

31

7

3

19

9

9

—

23

14

26

13

3

31

—

—

—

—

—

—

2

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

26

7

2

26
Appendix 10 59

Appendix 10. Loads of suspended solids, adusted suspended solids, volatile solids, and dissolved solids in stormwater samples 

collected at inlet, outlet, and bypass of hydrodynamic-settling device in Madison, Wisconsin, November 2005–August 2006. 

[All data in pounds; —, no sample processed]

Sampling date 

(month/day/

year)

Sampling 

event 

number

Suspended sediment Adjusted suspended 

solids, total Solids, volatile Solids, dissolved

Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass Inlet Outlet Bypass

11/27/2005 1 0.402 0.387 — .356 .387 — 0.14 0.34 — 30.5 27.9 —

03/08/2006 2 15 19.6 — 15.4 17.4 — 3.05 4.46 — 30.5 27.9 —

03/12/2006 3 5.51 4.48 — 5.26 4.48 — 1.11 1.28 — 16.4 7.36 —

04/02/2006 4 21.5 13.4 — 22.3 13.4 — — — — 35.4 31. —

04/12/2006 5 14.9 11.8 — 15.4 12.1 — 2.47 2.35 — 9.15 10.6 —

04/13/2006 6 12.1 15.6 — 11.5 15.8 — 1.9 2.27 — 5.23 6.2 —

04/16/2006 7 106 74.4 76.6 74.8 73.6 40.1 10.9 17.5 4.95 24.1 36.6 1.9 

04/29/2006 8 18.7 16 — 18.7 16. — 11.6 9.53 — 28.6 29.6 —

05/01/2006 9 16.3 19.9 — 14.2 21.4 — 4.61 6.76 — 2.45 6.27 —

05/09/2006 10 2.89 2.76 — 3.02 3.15 — 1.57 1.44 — 9.66 6.78 —

05/11/2006 11 7.92 6.97 — 7.6 6.02 — 5.03 4.09 — 27.7 18.9 —

05/15/2006 12 — — — — — — — — — — — —

05/16/2006 13 2.19 2.3 — 2.26 2.45 — 1.09 1.05 — — — —

05/17/2006 14 12.8 11.8 — 13.1 12.6 — 5.72 4.99 — 4.74 5.4 —

05/24/2006 15 — — — — — — — — — — — —

06/18/2006 16 3.22 2.55 — 3.45 2.6 — 1.35 .84 — 6.97 10.1 —

06/25/2006 17 27.8 27.8 — 28.3 28.1 — 13.8 13. — 15.9 13.8 —

07/09/2006 18 40.4 59.5 — 33.4 52.5 — 4.37 6.43 — 7.14 6.43 —

07/11/2006 19 39.4 25.6 — 24.2 26.3 — 6.17 6.86 — 17.2 17.2 —

07/19/2006 20 37.6 40.3 — 23.7 35.3 — 5.84 8.76 — 15.2 40. —

07/22/2006 21 13.6 14.8 — 12.0 13.9 — 2.44 2.79 — 2.9 2.9 —

07/27/2006 22 — — — — — — — — — — — —

08/06/2006 23 13.5 8.44 — 11.6 8.44 — 5.89 4.08 — 5.67 13.1 —

08/17/2006 24 57.6 17.2 — 27.2 16.9 — 3.73 5.92 — 6.33 7.95 —

08/23/2006 25 446 276 36.2 295 295 23.9 48.7 51.2 5.03 114 94 6.85

08/24/2006 26 674 308 115 427 315 69.9 81.5 81.5 15.6 242 238 39.5 
60 Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6

Appendix 11. Loads of selected water-quality constituents 

in stormwater samples collected at inlet, outlet, and bypass at 

hydrodynamic-settling device in Madison, Wisconsin, November 2005–

August 2006.

[All loads in pounds; —, no sample processed]

Sampling 

event 

number

Phosphorus,

dissolved

Zinc,

total recoverable

Inlet Outlet Bypass Inlet Outlet Bypass

1 0.0025 0.0034 — — — —

2 0.0042 0.0038 — 0.0054 0.0077 —

3 0.0026 0.0024 — 0.0018 0.0021 —

4 0.0114 0.01 — 0.0081 0.0063 —

5 0.0037 0.0042 — 0.0044 0.0042 —

6 — — — — — —

7 0.0187 0.0148 0.0027 0.0174 0.0187 0.0104

8 0.0159 0.0238 — 0.0101 0.0111 —

9 0.0028 0.0026 — 0.0044 0.0061 —

10 0.0059 0.0047 — 0.003 0.0039 —

11 0.0311 0.0315 — 0.0057 0.0057 —

12 — — — — — —

13 — — — — — —

14 0.0021 0.0025 — 0.0052 0.0045 —

15 — — — — — —

16 — — — — — —

17 0.0146 0.022 — 0.0125 0.0249 —

18 0.0061 0.0057 — 0.0089 0.0115 —

19 0.0384 0.0357 — 0.0117 0.0123 —

20 0.0117 0.0105 — 0.0117 0.0146 —

21 0.0049 0.0014 — 0.0042 0.0048 —

22 — — — — — —

23 0.0168 0.0129 — 0.0082 0.007 —

24 0.0071 0.0046 — 0.0083 0.0104 —

25 0.9488 0.7339 0.0458 0.0801 0.083 0.009

26 — — — — — —
Publishing support provided by the U.S. Geological Survey Science 

Publishing Network, Columbus Publishing Service Center

For more information concerning the research in this report, contact the:

Director, Wisconsin Water Science Center

U.S. Geological Survey

8505 Research Way

Middleton, Wisconsin 53562

http://wi.water.usgs.gov/
Horwatich and Bannerman—Parking Lot Runoff Quality and Treatment Efficiencies of a Hydrodynamic-Settling Device, Madison, Wisconsin, 2005–6—SIR 2011–5145