Prepared in cooperation with the Gwinnett County Department of Water Resources

Total Suspended Solids Concentrations and Yields for 

Water-Quality Monitoring Stations in Gwinnett County, 

Georgia, 1996–2009

Open-File Report 2013–1145

U.S. Department of the Interior

U.S. Geological Survey
Front cover: Yellow River at Georgia Highway 124 near Lithonia, Georgia. Inset photograph by Paul D. Ankcorn, USGS; 

background photograph by Karen W. Stull, USGS.

Back cover: Big Haynes Creek at Lenora Road. Photograph by Stephanie Gillain.
Total Suspended Solids Concentrations and Yields for 

Water-Quality Monitoring Stations in Gwinnett County, 

Georgia, 1996–2009

By Mark N. Landers

Prepared in cooperation with the Gwinnett County Department of Water Resources

Open-File Report 2013 –1145

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Landers, M.N., 2013, Total suspended solids concentrations and yields for water-quality monitoring stations in 

Gwinnett County, Georgia, 1996 –2009: U.S. Geological Survey Open-File Report 2013–1145, 10 p., available online 

at http://pubs.usgs.gov/of/2013/1145/.
iii

Contents

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

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

Total Suspended Solids Concentrations ....................................................................................................4

Quality-Control Samples......................................................................................................................4

Total Suspended Solids and Suspended Sediment Concentration..............................................5

Total Suspended Solids Yields.....................................................................................................................6

Calibration Dataset...............................................................................................................................6

Regression Model Development........................................................................................................7

Accuracy of Estimated Yields .............................................................................................................7

Summary..........................................................................................................................................................9

References....................................................................................................................................................10

Figures

1. Map showing location of study area, watershed boundaries, and monitoring 

stations in Gwinnett County, Georgia........................................................................................2

2. Graph showing total suspended solids concentrations of samples taken 

concurrently at fixed-point sample intake and across the stream cross section

for the 14 study watersheds in Gwinnett County, Georgia.....................................................5

3. Graph showing total suspended solids concentration and suspended 

sediment concentration for 380 storm samples from 14 watersheds in 

Gwinnett County, Georgia, 1996–2009.......................................................................................5

4. Graph showing concentration of total suspended solids and turbidity and 

discharge for samples collected in Suwanee Creek at Suwanee, Georgia, 

1996 –2009.......................................................................................................................................6

5. Graph showing cumulative observed and estimated total suspended solids 

load for 67 storm samples collected for Suwanee Creek at Suwanee, Georgia, 

1996 –2009 ......................................................................................................................................7

Tables

1. Water-quality monitoring stations, Gwinnett County, Georgia .............................................3

2. Statistical summary of total suspended solids concentration for 

water-quality monitoring stations in Gwinnett County, Georgia...........................................4

3. Characteristics of regression models used to estimate concentration 

of total suspended solids for water-quality monitoring stations in 

Gwinnett County, Georgia............................................................................................................8

4. Total suspended solids annual yields, in pounds per acre per year, 

for water-quality monitoring stations in Gwinnett County, Georgia.....................................9
iv

Conversion Factors and Datum

Multiply By To obtain

Length

inch 2.54 centimeter (cm)

inch 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2)

acre 4,047 square meter

Volume

million gallons (Mgal) 3,785 cubic meter (m3

)

Flow/transport rate

cubic foot per second 0.02832 cubic meter per second

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

pound per acre 1.121 kilogram per hectare

Mass

pound 0.4536 kilogram (kg)

Constituent yield

pounds per acre per year 

[(lb/acre)/yr] 1.121 kilograms per hectare per year [(kg/ha)/yr]

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Concentrations of chemical constituents in water are given in milligrams per liter.

Acknowledgments

The author wishes to thank personnel of the Gwinnett County Department of Water Resources 

for their long-term commitment to improved data and understanding of streamwater quality 

in their watersheds. This report is possible because of the field data collection and analysis 

expertise of the U.S. Geological Survey, Gwinnett Project personnel: Paul Ankcorn, Jon Evans, 

John Joiner, Brandon Reyes, Jolene Robichaux, and Karen Stull. 
Total Suspended Solids Concentrations and Yields for 

Water-Quality Monitoring Stations in Gwinnett County, 

Georgia, 1996–2009

By Mark N. Landers

Abstract

The U.S. Geological Survey, in cooperation with the 

Gwinnett County Department of Water Resources, established 

a water-quality monitoring program during late 1996 to 

collect comprehensive, consistent, high-quality data for use by 

watershed managers. As of 2009, continuous streamflow and 

water-quality data as well as discrete water-quality samples 

were being collected for 14 watershed monitoring stations in 

Gwinnett County. 

This report provides statistical summaries of total 

suspended solids (TSS) concentrations for 730 stormflow and 

710 base-flow water-quality samples collected between 1996 

and 2009 for 14 watershed monitoring stations in Gwinnett 

County. Annual yields of TSS were estimated for each of the 

14 watersheds using methods described in previous studies. 

TSS yield was estimated using linear, ordinary least-squares 

regression of TSS and explanatory variables of discharge, 

turbidity, season, date, and flow condition. The error of 

prediction for estimated yields ranged from 1 to 42 percent 

for the stations in this report; however, the actual overall 

uncertainty of the estimated yields cannot be less than that 

of the observed yields (±15 to 20 percent). These watershed 

yields provide a basis for evaluation of how watershed 

characteristics, climate, and watershed management practices 

affect suspended sediment yield.

Introduction

The U.S. Geological Survey (USGS), in cooperation 

with Gwinnett County Department of Water Resources, 

established a water-quality monitoring program during late 

1996 to collect comprehensive, consistent, high-quality 

data for use by watershed managers. Water-quality sample 

collection began during 1996, and by 2001, 12 watersheds 

were being monitored continuously for streamflow, precipitation, temperature, specific conductance, and turbidity, with 

hydrograph-based sampling of three storm events and three 

base-flow periods every 6 months. Two watersheds were 

added to the study during 2006 –2007 (fig. 1; table 1). This 

report summarizes total suspended solids (TSS) data collected 

at these sites through the 2009 water year (October 1, 2008, to 

September 30, 2009). 

The overall purpose of USGS water-quality monitoring in 

Gwinnett County is to provide a long-term record of comprehensive and consistent hydrologic and water-quality data that can 

be used by county and State watershed managers and engineers to evaluate effectiveness of water-management practices 

on streamwater quality in the county. This water-quality 

monitoring program provides measurements of stream hydrology 

and constituent concentrations and yields. The methods used 

follow USGS protocols and quality-assurance procedures and 

are consistent between watersheds and over time.

Leading causes of stream impairment in the United 

States are sediment-associated constituents, such as bacteria, 

excessive sediment, nutrients, and trace metals (U.S. Environmental Protection Agency, 2002). Sediment concentrations and 

yields change with natural and human-influenced watershed 

characteristics (Landers and others, 2007). Annual yield of 

total suspended sediment is a primary performance criterion in 

Gwinnett County’s Watershed Protection Plan, which is tied to 

its water-use permits (Gwinnett County Department of Water 

Resources, 2000). 

Specific goals of the long-term monitoring program include 

• Monitoring water-quantity and quality status in 

real time;

• Monitoring long-term and seasonal water-quantity 

and quality trends;

• Providing flood-warning data for emergency managers 

and the public;

• Providing data to water managers to evaluate and 

meet regulatory monitoring requirements of permits 

for water-supply withdrawals, wastewater discharges, 

stormwater, source-water watersheds, and total 

maximum daily load studies; and

• Providing data for computation of constituent loads 

and yields.
2 Total Suspended Solids in Gwinnett County, Georgia, 1996–2009

Chattahoochee–

Flint River Basin

Ocmulgee–

Oconee–Altamaha

River Basin

GEORGIA

Ocean Atlantic

Atlanta

Gwinnett County

Eastern Continental Divide

Water-quality monitoring station and name

0 2 4

0 2 4

6 MILES

6 KILOMETERS

EXPLANATION

N

Base from U.S. Geological Survey

1:100,000-scale digital data

Figure 1. Location of study area, watershed boundaries, and monitoring stations in Gwinnett County, Georgia.

Chattahoochee River

Buford Lake Sidney Lanier

Dam

Alcovy

River

watershed

Shoal Creek

watershed Pew

Creek

watershed

Apalachee 

 River

 watershed

Level Creek

watershed

Richland Creek

watershed

Crooked Creek

watershed

Sweetwater Creek

watershed

Yellow River

watershed

Big Haynes 

Creek watershed

North Fork

Peachtree Creek

watershed

No

Business

Creek

 watershed

Brushy Fork 

Creek

watershed

Suwanee Creek

watershed

Wheeler Creek

watershed

GWINNETT COUNTY

02335350

02334885

02336030

02205522

02205865

02207385

02208150

02218565

02208130

02217274

02334480

02334578

02207185

02207400

02207120

02218565

Figure 1. Location of study area, watershed boundaries, and monitoring stations in Gwinnett County, Georgia.
Introduction 3

Table 1. Water-quality monitoring stations, Gwinnett County, Georgia.

[USGS, U.S. Geological Survey]

USGS station 

number Station name Yearestablished

Drainage area,

in square miles

02205522 Pew Creek at Patterson Road, near Lawrenceville, GA 2006 7

02207120 Yellow River at GA 124, near Lithonia, GA 1998 162

02207185 No Business Creek at Lee Road, below Snellville, GA 2001 10.1

02207385 Big Haynes Creek at Lenora Road, near Snellville, GA 1998 17.3

02207400 Brushy Fork Creek at Beaver Road, near Loganville, GA 1998 8.15

02208130 Shoal Creek at Paper Mill Road, near Lawrenceville, GA 2006 3.9

02208150 Alcovy River at New Hope Road, near Grayson, GA 1998 30.8

02217274 Wheeler Creek at Bill Cheek Road, near Auburn, GA 2001 1.31

02218565 Apalachee River at Fence Road, near Dacula, GA 2001 5.68

02334480 Richland Creek at Suwanee Dam Road, near Buford, GA 2001 9.34

02334578 Level Creek at Suwanee Dam Road, near Suwanee, GA 2001 5.02

02334885 Suwanee Creek at Suwanee, GA 1998 47

02335350 Crooked Creek near Norcross, GA 1998 8.89

02336030 North Fork Peachtree Creek at Graves Road, near Doraville, GA 2001 1.42

A previous report (Landers and others, 2007) describes 

monitoring methods, physiographic and climatic setting, 

watershed characteristics, analytical methods, water-quality 

concentrations and yields, trends in water quality, and watershed effects on hydrology and water quality for watersheds 

in Gwinnett County. Data and analytical results in Landers 

and others (2007) are limited to the six watersheds that were 

monitored beginning in 1996 through the 2003 water year.

This report provides statistical summaries of TSS sample 

concentrations and annual yields of TSS for each of the 

14 watersheds monitored through the 2009 water year. The 

monitoring methods and analytical approach are the same as 

those described by Landers and others (2007). This report 

updates the quality-assurance summary for TSS and describes 

minor changes to the analytical methods used by Landers and 

others (2007). This report does not include interpretive analysis 

of watershed effects on water quantity and water quality.
4 Total Suspended Solids in Gwinnett County, Georgia, 1996–2009

Total Suspended Solids Concentrations

Water-quality samples were collected during a wide 

range of hydrologic conditions. Methods of data collection 

are described in Landers and others (2007) and follow 

procedures documented in published manuals, techniques, and 

quality-assurance plans (Rantz, 1982a, b; Wilde and others, 

1998; Wagner and others, 2000; Gotvald and Stamey, 2005). 

Quality-control samples also were collected to verify that 

samples were representative and uncontaminated. Concentration data were combined with streamflow and turbidity data 

to compute annual TSS yields. 

Between 1996 and 2009, TSS analyses were conducted 

for about 730 stormflow and 710 base-flow water-quality 

samples collected from the 14 watershed monitoring stations 

listed in table 1. Analytical results for individual samples have 

been provided to the Gwinnett County Department of Water 

Resources and are available at http://nwis.waterdata.usgs.

gov/ga/nwis/qwdata. Users also can access these data through 

the project Web site at http://ga2.er.usgs.gov/urban/gwinnett,

which provides other supporting data. A statistical summary 

of TSS concentrations for each station are listed in table 2. 

These statistical summaries represent stormflow and base-flow 

samples collected from 1996 to 2009. Quality-assurance 

samples are not included in these summaries. 

Quality-Control Samples

The USGS develops quality-assurance and qualitycontrol procedures to ensure that water-quality data meet 

standards and accurately represent stream conditions. These 

procedures are documented in published manuals, techniques, 

and quality-assurance plans (Rantz, 1982a, b; Wilde and 

others, 1998; Wagner and others, 2000; Gotvald and Stamey, 

2005). Because fixed-point samplers collect samples from a 

single point in the stream cross section, concurrent replicate 

samples must be collected to ensure that the point sample 

is representative of the entire cross section. Two concurrent 

samples are taken—one from the stream cross section using 

depth integrated, equal-width-increment (EWI) methods as 

described in Wilde and others (1998), and another from a point 

in the stream cross section using a fixed-point sampler. Both 

samples are independently processed and analyzed. 

Forty-four pairs of concurrent samples were collected 

from 1996 to 2009 in all of the monitored watersheds 

and were analyzed for concentrations of TSS (fig. 2). An 

equal-value line has been added to figure 2 to show where 

concentrations from the concurrent samples would be equal. 

Of the 44 pairs of concurrent TSS samples, 73 percent of the 

fixed-point sampler concentrations were within 10 percent 

of the concurrent EWI concentrations, and 90 percent were 

within 25 percent. The four samples with differences greater 

than 25 percent are from four different sites. 

Equipment-blank quality-control samples are used to 

check for the presence of constituents from sampling equipment 

that could contaminate an environmental sample. Equipment 

contamination may be caused by inefficient cleaning of manual 

or fixed-point samplers. Of the 52 blank samples collected, 

12 percent had constituent concentrations greater than laboratory analytical detection limits. None of these detected concentrations, however, was greater than 4 milligrams per liter, 

which is well below any stormflow concentrations detected 

during this study. These quality-control sample results are 

similar to those from Landers and others (2007).

Table 2. Statistical summary of total suspended solids concentration for water-quality monitoring stations in Gwinnett County, Georgia.

[USGS, U.S. Geological Survey; mg/L, milligrams per liter; %, quartile percentage of samples that are less than listed concentrations for each station]

USGS station 

number

Number of samples Concentration, in mg/L Period of

Base flow Stormflow Mean Maximum Minimum 25% 50% 75% sampling

02205522 8 25 203 1,100 1 12 98 230 2006–2009

02207120 64 66 131 1,300 1 4 23 180 1996–2009

02207185 47 48 113 1,300 1 4 23 124 2001–2009

02207385 84 64 119 990 1 5 10 174 1996–2009

02207400 84 61 126 1,400 1 8 18 160 1996–2009

02208130 6 18 138 720 2 14 77 165 2006–2009

02208150 60 61 206 1,600 1 5 17 313 1996–2009

02217274 47 48 275 5,400 1 4 46 233 2001–2009

02218565 39 38 237 1,800 1 4 13 320 2001–2009

02334480 44 51 614 6,900 1 10 150 745 2001–2009

02334578 45 50 222 1,630 1 4 60 273 2001–2009

02334885 63 67 142 870 1 7 36 226 1996–2009

02335350 64 71 279 3,120 1 4 65 380 1996–2009

02336030 47 46 112 870 1 4 17 140 2001–2009
Total Suspended Solids Concentrations 5

0 100 200 300

Equal-value line

400 500 600

Cross-section sample total suspended solids concentration,

in milligrams per liter

Point sample total suspended solids concentration,

in milligrams per liter

600

500

400

300

200

100

0

Figure 2. Total suspended solids concentrations of 

samples taken concurrently at fixed-point sample 

intake and across the stream cross section for the 

14 study watersheds in Gwinnett County, Georgia.

Figure 2. Total suspended solids concentrations 

of samples taken concurrently at fixed-point sample 

intake and across the stream cross section for the 

14 study watersheds in Gwinnett County, Georgia.

Total Suspended Solids and Suspended 

Sediment Concentration

Analyses from previous investigations show that the 

amount of suspended sediment in streams is greater than 

indicated by the TSS analysis (Gray and others, 2000; Landers 

and others, 2007). The amount of suspended material in water 

is usually determined using laboratory analytical methods 

for TSS and (or) suspended sediment concentration (SSC). 

The TSS and SSC analytical methods are slightly different 

and typically produce different results. The SSC analytical 

method measures the dry weight of the sediment in an entire 

sample of a known volume. Laboratory methods for TSS 

differ among laboratories, but usually involve measuring the 

dry weight of sediment in a subsample of the available sample 

volume, rather than in the entire sample. Gray and others 

(2000) evaluated 3,235 paired SSC and TSS sample analyses 

and found that this subsampling procedure can undermeasure 

the total sediment in a sample, particularly when the amount 

of sand (particles larger than 0.062 millimeter [mm] and less 

than 2 mm) in a sample exceeds about one-quarter of the total 

sediment mass.

Landers and others (2007) evaluated paired TSS and 

SSC sample analyses for 160 samples collected between 

2001 and 2004. As part of the current study, 380 stormflow 

samples collected during 2001–2009 in the 14 study 

watersheds were similarly analyzed. The concentration of 

TSS typically was less than SSC, as shown in figure 3, in 

which most of the data plot on the SSC side of the line of 

equal value. The relation of TSS concentrations and SSC for 

the 150 samples with less than 25 percent sand has limited 

scatter; however, the relation for the 230 samples with greater 

than 25 percent sand content has significant scatter, and 

TSS concentrations tend to be much less than SSC for these 

samples, in agreement with the finding of Gray and others 

(2000). The best fit between TSS and SSC by linear, ordinary 

least squares regression of all 380 samples (log-transformed) 

is obtained where SSC is equal to about 1.6 times the TSS 

with an R2

 of 0.76 and a p-value less than 0.0001. Thus, 

actual SSC in these streams typically are about 60 percent 

greater than indicated by TSS values, which is the same result 

obtained by Landers and others (2007).

10,000

1,000

100

10

Suspended sediment concentration, in milligrams per liter

Total suspended solids concentration, in milligrams per liter

10 100 1,000 10,000

Sand greater than 25 percent

Sand less than 25 percent

EXPLANATION

Figure 3. Total suspended solids (TSS) concentration 

and suspended sediment concentration (SSC) for 380 

storm samples from 14 watersheds in Gwinnett County, 

Georgia, 1996–2009.

Figure 3. Total suspended solids (TSS) concentration 

and suspended sediment concentration (SSC) for 

380 storm samples from 14 watersheds in Gwinnett 

County, Georgia, 1996 –2009.
6 Total Suspended Solids in Gwinnett County, Georgia, 1996–2009

Total Suspended Solids Yields

The load of a constituent is the mass transport per unit 

time (for example, tons per year). The yield is simply load 

per unit area (for example, pounds per acre per year). Yield is 

obtained by dividing load by watershed area. Yields are loads, 

normalized for the watershed size. Thus, yields often are used 

to compare loads between watersheds. 

For a sample collected during a storm event, the average 

concentration and discharge are known, and the observed load 

is simply the product of concentration (mass per unit volume), 

discharge (volume per unit time), and the duration of the 

sample. Although discharge data are available continuously for 

the monitoring stations of this study, values of concentration 

are available only for periods when a sample was collected. 

For unsampled periods, TSS concentration can be estimated 

using relations of concentration to discharge and (or) turbidity, 

as illustrated in figure 4 for Suwanee Creek. Estimated load 

is then taken as the product of estimated TSS concentration, 

discharge, and duration. The methods used herein follow 

USGS guidelines (Rasmussen and others, 2009). 

Annual TSS yields for the period 1998–2003 and discussion of how land use and climate affect yields are included in 

Landers and others (2007) for the six watersheds included in 

that report. The approach used herein differs from that used in 

Landers and others (2007) in that turbidity was included as an 

explanatory variable. Turbidity was not used as an explanatory 

variable in Landers and others (2007) because continuous, 

consistent turbidity data from in-stream turbidimeters were not 

available at most of the sites until the 2003 water year. 

The turbidimeters used in this study have a maximum 

reporting level of 1,000 Formazin nephelometric units 

(FNU), and were calibrated to at least that amount for the 

study period. Turbidity readings higher than this range may 

be censored and are unreliable. Turbidity values exceeded 

1,000 FNU for an average of 11 hours per year at the study 

sites, and these values were removed from the continuous 

record for this analysis as recommended in Rasmussen and 

others (2009).

Calibration Dataset

The dataset used to calibrate the regression model 

for computation of annual TSS loads consists of measured 

streamflow discharge, TSS concentration, and turbidity (in 

FNU). The date and flow condition (stormflow or base flow) 

of the sample was also included in the calibration dataset. The 

sampling design and data collection methods are described in 

Landers and others (2007). Calibration datasets for each site 

were evaluated graphically and statistically for correlation, 

multicollinearity, and outliers. Outliers were removed only 

if they could be discredited with a high degree of confidence 

(for example due to noted equipment problems). The number 

of samples in the calibration dataset ranged from 77 to 148 for 

12 of the stations and from 24 to 33 for the two stations that 

began operation in 2006 (table 2).

The design for this study includes collection of samples 

in both base-flow and stormflow conditions. These two 

flow conditions tend to form two populations in the TSS-todischarge relation, as discussed in Landers and others (2007). 

1

1,000

100

10

1

1,000

100

10

1

10

Turbidity, in Formazin nephelometric units

Total suspended solids concentration, in milligrams per liter

Sample discharge, in cubic feet per second

100 1,000 1 10

Figure 4. Concentration of total suspended solids and (A) turbidity and (B) discharge for samples collected in 

Suwanee Creek at Suwanee, Georgia, 1996–2009.

A B

Estimated TSS

EXPLANATION

Stormflow

Stormflow

Base flow Base flow

Observed TSS

Figure 4. Concentration of total suspended solids (TSS) and (A) turbidity and (B) discharge for samples 

collected in Suwanee Creek at Suwanee, Georgia, 1996–2009.
Total Suspended Solids Yields 7

The flow-condition explanatory variable is a binary variable 

that accounts for whether the sample was collected in a 

base-flow or stormflow condition. Each time step was classified as stormflow if the turbidity was greater than 20 FNU; 

otherwise it was classified as base flow, based on evaluation 

of the threshold turbidity for sampled events. For periods with 

missing turbidity data, a time step was classified as base flow 

if the streamflow was less than the flow observed in previous 

streamflow records between 50 to 75 percent of the time for 

that day of the year.

Regression Model Development

Linear, ordinary least-squares regression was used to 

estimate TSS concentration for unsampled periods following 

methods described in Rasmussen and others (2009). This 

method was used to select the most accurate estimator of 

concentration from explanatory variables of discharge, turbidity, 

season, date, and flow condition for each monitoring station. 

An explanatory variable was included in the regression model 

if it met the following criteria: the coefficient was reasonable 

in sign and magnitude, model accuracy was improved, it was 

statistically significant (at a p-value less than or equal to 0.05 

for season and time and 0.01 for all other variables), residual 

analyses indicated adherence to the assumptions of regression, 

and multicollinearity was not significant. The relations were 

developed for logarithms of measured TSS and the explanatory variables, as is typical for load estimation. Estimated 

logarithmic TSS was untransformed (back to linear space) and 

multiplied by an untransformation bias-correction factor (Duan, 

1983; Landers and others, 2007; Rasmussen and others, 2009).

Two regression models were developed for each 

station—one including and one excluding turbidity as an 

explanatory variable. This was done so that TSS concentrations could be estimated for those time steps with missing 

or unreliable turbidity data. Including turbidity as an explanatory variable for estimated TSS concentration improved the 

regression model accuracy for every station. Multicollinearity 

between turbidity and discharge, where both were included 

as explanatory variables, was evaluated using the variance 

inflation factor and was found to not be a problem for these 

analyses (Helsel and Hirsch, 1992; Rasmussen and others, 

2009). The regression models are summarized in table 3. The 

R2 (coefficient of determination) for the regression models 

ranged from 0.87 to 0.95, and the regression standard error 

ranged from 43 to 93 percent. All were statistically significant 

at a p-value of less than 0.001. 

The time step for each station, listed in table 3, is based 

on the typical duration (mean and median) of sampled storm 

events, as in Landers and others (2007). The explanatory 

variables were recorded at 15-minute intervals and averaged 

over the time-step interval for each station. The TSS load 

for each time step was then computed from the product of 

the estimated TSS concentration, streamflow discharge, and 

time-step duration. The load was summed for each water year 

and divided by watershed area to obtain annual yield in units 

of pounds per acre per year for the 14 watersheds of this study 

(table 4). 

Accuracy of Estimated Yields

Overall accuracy refers to the actual in-stream load or 

yield compared to estimated load or yield. The accuracy of 

observed load from measured discharge and TSS concentration for a sampled event depends on errors associated with 

field sampling and processing and with laboratory analysis. 

As discussed in Landers and others (2007), the measurement accuracy of loads from measured discharge and TSS 

concentration is generally on the order of ±15 to 20 percent. 

The accuracy of estimated load or yield from estimated TSS 

concentration and discharge depends on the accuracy and 

number of samples in the calibration dataset, the range of 

measured and observed conditions, uncertainty in the regression estimator (model standard error), and time duration of 

the estimate. For example, estimated annual yield typically is 

more accurate than estimated daily yield. 

The accuracy of estimated TSS yields was evaluated 

as the cumulative difference between the observed and 

estimated yield for the sampled storm events for each station. 

The cumulative observed and estimated yields are shown in 

figure 5 for Suwanee Creek for the 67 storm events for which 

samples were obtained at this site from 1996–2009. Note 

that figure 5 shows the cumulative load for sampled storms 

only and does not show the cumulative load for all storms 

occurring over the period of record. The error of prediction for 

each site is the cumulative difference between observed and 

estimated yield as a percentage of the total observed yield. The 

error of prediction by this method ranged from 1 to 75 percent 

for the 14 stations in this study; however, the actual overall 

uncertainty of the estimated yields cannot be less than the 

measurement uncertainty (±15 to 20 percent). The standard 

error of the regression models also is shown in table 3.

1996

800

700

600

500

400

300

200

100

0

19971998199920002001200220032004200520062007200820092010

Total suspended solids,

in pounds per acre

Cumulative storm-

 event yield

Observed

Estimated

EXPLANATION

Figure 5. Cumulative observed and estimated total 

suspended solids load for 67 storm samples collected 

for Suwanee Creek at Suwanee, Georgia, 1996–2009. 

Figure 5. Cumulative observed and estimated total 

suspended solids load for 67 storm samples collected 

for Suwanee Creek at Suwanee, Georgia, 1996–2009. 
8 Total Suspended Solids in Gwinnett County, Georgia, 1996–2009

Table 3. Characteristics of regression models used to estimate concentration of total suspended solids for water-quality monitoring 

stations in Gwinnett County, Georgia.

[USGS, U.S. Geological Survey]

USGS 

station 

number

Number of samples

Regression

R2

Regression 

model

standard

error,

in percent

1

Error of 

prediction,

in percent

2

Explanatory variables

Base 

flow

Stormflow Discharge Turbidity Flow

condition Season Date

Model 

time step, 

in hours

02205522 8 25 0.91 62 –7 — • • — — 6

8 25 0.89 72 • — • • — 6

02207120 61 64 0.94 54 –8 • • • • — 24

64 66 0.93 55 • — • • — 24

02207185 47 48 0.92 62 1 • • • • • 6

47 48 0.89 71 • — • • • 6

02207385 80 62 0.92 60 –23 • • • • — 12

84 64 0.89 74 • — • • — 12

02207400 81 61 0.90 57 16 • • • • • 12

84 61 0.88 66 • — • • • 12

02208130 6 18 0.95 44 –4 — • — — — 6

6 18 0.89 68 • — • • — 6

02208150 58 60 0.95 54 38 • • • • — 8

60 61 0.94 58 • — • • — 8

02217274 47 48 0.93 63 –26 • • • • • 8

47 48 0.88 93 • — • • • 8

02218565 39 38 0.95 56 –19 — • — • • 4

45 39 0.90 85 • — • • — 4

02334480 42 50 0.90 87 75 • • • — — 6

44 51 0.90 92 • — • • — 6

02334578 43 50 0.95 56 –4 — • • • • 6

45 50 0.90 84 • — • • — 6

02334885 60 66 0.95 43 9 • • • • • 12

63 67 0.92 55 • — • • • 12

02335350 60 67 0.95 61 –2 • • • • — 6

64 71 0.93 71 • — • • — 6

02336030 43 46 0.89 72 –5 • • • • — 4

47 46 0.87 81 • — • • — 4

1

 The error of prediction for each site is the cumulative difference between observed and estimated yield using both equations as a percentage of the total 

observed yield. The typical measurement error is less than ± 20 percent.

2

 Two regression models were developed for each station‚ one including and one excluding turbidity as an explanatory variable: •, explanatory variable 

included in regession model; —, explanatory variable not included in regession model.
Summary 9

Table 4. Total suspended solids annual yields, in pounds per acre per year, for water-quality monitoring stations in Gwinnett County, Georgia.

[USGS, U.S. Geological Survey; —, data insufficient to estimate value]

USGS station 

number 2002 2003 2004 2005 2006 2007 2008 2009

02205522 — — — — 644 371 265 937

022071201 — — 1,040 1,950 430 230 223 4,770

02207185 249 1,860 460 1,140 268 191 75 642

022073851 — — 701 1,050 248 150 121 412

022074001 — — 636 1,380 208 176 95 386

02208130 — — — — — 238 235 1,260

022081501 — — 1,650 2,370 491 371 281 1,870

02217274 1,700 3,680 2,980 841 635 295 119 424

02218565 674 3,490 2,220 1,730 554 332 203 971

02334480 2,170 6,690 4,690 14,100 2,330 1,270 947 2,410

02334578 519 2,270 2,030 1,690 432 287 234 599

023348851 — — 897 1,490 381 235 273 854

023353501 — — 2,180 3,300 1,030 834 685 3,350

02336030 721 1,190 2,460 1,260 554 419 325 2,330

1

 Yields for 1998–2003 reported in Landers and others, 2007.

Summary

The U.S. Geological Survey water-quality monitoring 

program in Gwinnett County provides a long-term record of 

comprehensive and consistent hydrologic and water-quality 

data that can be used by watershed managers and engineers 

to protect and enhance the streams in the county. This report 

provides statistical summaries of total suspended solids 

(TSS) concentrations for 730 stormflow and 710 base-flow 

water-quality samples collected between 1996 and 2009 

for 14 watershed monitoring stations in Gwinnett County. 

Regression models were developed to estimate annual yields 

of TSS for each of the 14 watersheds through the 2009 water 

year. The monitoring methods and analytical approach are the 

same as those described in a previous study. This report does 

not include interpretive analysis of watershed effects on water 

quantity and water quality.

Quality-assurance analysis indicated that sample 

concentrations from the fixed-location pumping samplers are 

representative of the average cross-section concentrations for 

all 14 of the study watersheds. Analysis of equipment-blank 

quality-assurance samples indicated that equipment contamination was not a problem. Concentrations obtained using TSS 

analytical methods typically underestimate the suspended 

sediment concentration (SSC) in a water-quality sample. 

Comparison of 380 paired TSS and SSC samples indicate that 

SSC is roughly equal to 1.6 times TSS for these watersheds, in 

agreement with a similar evaluation in a previous study.

TSS loads and yields for each station were estimated 

using linear, ordinary least-squares regression. The method 

was used to select the most accurate estimator of TSS from 

explanatory variables of discharge, turbidity, season, date, 

and flow condition for each monitoring station. The error of 

prediction for the estimated yields ranged from 1 to 42 percent 

for the stations in this report; however, the actual overall 

uncertainty of the estimated yields cannot be less than that 

of the observed yields (±15 to 20 percent). These watershed 

yields provide a basis for evaluation of how watershed 

characteristics, climate, and watershed management practices 

affect suspended sediment yield.
10 Total Suspended Solids in Gwinnett County, Georgia, 1996–2009

References

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Manuscript approved on June 24, 2013

Edited by Kimberly A. Waltenbaugh

Layout by Caryl J. Wipperfurth

Science Publishing Network, Raleigh PSC

For more information about this publication, contact:

USGS Georgia Water Science Center

1770 Corporate Drive, Suite 500

Norcross, GA 30093

telephone: 678-924-6700

http://ga.water.usgs.gov/
Landers—Total Suspended Solids in Gwinnett County, Georgia, 1996–2009—Open-File Report 2013–1145