Brent T. Aulenbach Joshua C. Henley Kristina G. Hopkins 20230404 tabular digital data Reston, VA U.S. Geological Survey https://doi.org/10.5066/P9G8HZTY Brent T. Aulenbach Joshua C. Henley Kristina G. Hopkins 2023 publication USGS Scientific Investigations Report 2023-5035 Reston, Virginia U.S. Geological Survey The dataset contains the sample constituent concentrations and explanatory variable values used for fitting the load regression models that were then used to estimate streamwater constituent loads for 13 watersheds in Gwinnett County, Georgia for water years 2003 to 2020. Load regression models were fit using U.S. Geological Survey (USGS) LOADEST load estimation software. Models were fit for 12 water-quality constituents: total suspended solids, suspended sediment concentration, total nitrogen, total nitrate plus nitrite, total phosphorus, dissolved phosphorus, total organic carbon, total calcium, total magnesium, total lead, total zinc, and total dissolved solids. Explanatory variables include a storm condition indicator variable, season, time and time-squared (trend), and the natural logarithms of streamflow, streamflow during base-flow conditions, base flow, turbidity, and turbidity during stormflow conditions. The purpose of this dataset is to make assessable the sample constituent concentrations and explanatory variable values used for fitting the load regression models that were then used in estimating streamwater constituent loads in support of the USGS Scientific Investigations Report "Hydrology, water-quality, and watershed characteristics in 15 watersheds in Gwinnett County, Georgia, 2002-20". This study is part of a long-term program to monitor and analyze the hydrologic and water-quality conditions of 15 watersheds in Gwinnett County, Georgia by the USGS in cooperation with Gwinnett County Department of Water Resources. 20021010 20200925 publication date Complete None planned -84.2721 -83.8468 34.1514 33.7676 ISO 19115 Topic Category inlandWaters USGS Thesaurus surface water quality streamflow mathematical modeling USGS Metadata Identifier USGS:63503f8fd34e47431c15c5bb Geographic Names Information System Gwinnett County State of Georgia None. Please see 'Distribution Info' for details. None. Users are advised to read the dataset's metadata thoroughly to understand appropriate use and data limitations. Brent T Aulenbach U.S. Geological Survey, Southeast Region Research Hydrologist mailing address

1770 Corporate Drive Suite 500

Norcross GA 30093 US 678-924-6626 678-924-6710 btaulenb@usgs.gov Funded through a cooperative agreement between Gwinnett County Department of Water Resources and the U.S. Geological Survey MacOS Big Sur, version 11.7.1; Microsoft Excel for Mac, version 16.67, 12_GwinnettCoGa_LOADESTModelCalibration.csv 487 KB No formal accuracy assessment was conducted for streamflow and turbidity. See Sauer and Turnipseed (2010) and Turnipseed and Sauer (2010) for typical accuracies in measuring stream stage and discharge, and in the development of stage-discharge relations. Turbidity water-quality monitors were maintained, and sensor records were checked, corrected, or shifted following the quality-assurance and quality-control procedures outlined in Wagner and others (2006). An analysis of the quality assurance and laboratory water-quality-control samples is provided in “Appendix 1. Quality assurance, quality control, and quality assessment summary” in Aulenbach and others (2023). These analyses are based on the following two datasets provided in the data release herein: 09: Streamwater quality assurance sample results for 19 water-quality constituents in 15 watersheds in Gwinnett County, Georgia for years 2000-2020 and 10: Laboratory standard reference samples for the Gwinnett County, Georgia study for years 2014-2020. Aulenbach, B.T., Henley, J.C., and Hopkins, K.G., 2023, Hydrology, water-quality, and watershed characteristics in 15 watersheds in Gwinnett County, Georgia, water years 2002-20: U.S. Geological Survey Scientific Investigations Report 2023-5035. Sauer, V.B., and Turnipseed, D.P., 2010, Stage measurement at gaging stations: U.S. Geological Survey Techniques and Methods, book 3, chap. A7, 45 p., accessed May 24, 2022, at https://doi.org/10.3133/tm3A7. Turnipseed, D.P., and Sauer, V.B., 2010, Discharge measurements at gaging stations: U.S. Geological Survey Techniques and Methods, book 3, chap. A8, 87 p., accessed May 24, 2022, at https://doi.org/10.3133/tm3A8. Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B.A., 2006, Guidelines and standard procedures for continuous water-quality monitors--Station operation, record computation, and data reporting: U.S. Geological Survey Techniques and Methods 1-D3, 51 p., 8 attachments, accessed July 11, 2020 at https://doi.org/10.3133/tm1D3. No formal logical accuracy tests were conducted. Dataset is considered complete for the information presented, as described in the abstract. Users are advised to read the rest of the metadata record carefully for additional details. U.S. Geological Survey 20220405 application/service https://doi.org/10.5066/F7P55KJN Digital and/or Hardcopy 20021001 20200930 publication date NWIS Sample constituent concentration, streamflow, and sonde turbidity for 13 watersheds in Gwinnett County, Georgia Discrete stream water-quality samples were collected at each study site during base-flow and stormflow conditions to encompass the range of constituent concentrations across hydrologic condition and season. For the purposes of sampling, the year was divided into two seasonal grouping--“summer” (May through October) and “winter” (November through April). A minimum of one base-flow sample and three stormflow-composited samples were collected in each season for a minimum of eight samples a year. Base-flow samples were collected with a USGS DH-81 manual sampler using mostly depth-integrated, equal-width-increment (EWI; stream velocities greater than 1.5 feet per second) and multiple vertical (stream velocities less than or equal to 1.5 feet per second) integrating techniques to ensure a representative sample. For storm composite samples, automatic samplers (ISCO refrigerated samplers, models 6712FR and Avalanche, with flow controllers) were programed to pump up to 24 half-liter aliquots of water from a designated point in the stream into a single composite sample, in which an aliquot is collected each time a specified volume of water flows by the site (constant volume, time proportional to flow volume increment protocol; EPA, 1992) such that the sample is representative of the water-quality variations that occurred during the storm. Water-quality sampling methods, analytical methods, and laboratories used are detailed in the associated USGS Scientific Investigations Report (Aulenbach and others, 2023). Stream water-quality data for the 13 watersheds were downloaded from the U.S. Geological Survey (USGS) National Water Information System (NWIS) Web interface at https://waterdata.usgs.gov/nwis on May 24, 2021. Streamflows and turbidities concurrent with the stream water-quality sample were determined at stream gages with field-deployed sonde turbidities. Storm composite sample streamflows and turbidities were computed as averages of their values when sample aliquots were collected from the automatic samplers. Instantaneous 15-minute interval frequency streamflows and turbidities were downloaded from the USGS National Water Information System (NWIS; USGS 2021) AQUARIUS Time-Series application on June 2-3, 2021. Streamflow was monitored using standard USGS protocols, which includes measuring stream stage, making discharge measurements, developing and maintaining a stage-discharge relation (rating curve), and computing discharge (Sauer and Turnipseed, 2010; Turnipseed and Sauer, 2010). Field turbidities were measured using multi-parameter in-situ water-quality meters, which were maintained and their corresponding sensor records checked, using the quality-assurance and quality-control procedures outlined in Wagner and others (2006). Poor quality data were removed from the record. Stream base flows were determined by hydrograph separation of daily average streamflows using the Web-based Hydrograph Analysis Tool (WHAT; https://engineering.purdue.edu/~what/, accessed June 2021; Lim and others, 2005). The simple local minimum method was used, which required no parameters to be fit. Hydrograph separations were performed on June 4, 2021 and June 10, 2021. Outlier constituent concentrations that had high influence and leverage on the fit of their concentration relation were removed from this model calibration dataset to prevent inordinate effects on load model predictions (Helsel and others, 2020). These outliers were not necessarily in error but may represent some condition that occurs infrequently. Outliers were identified as concentrations that greatly deviated from any observed relations with the explanatory variables used in the models (logarithm of streamflow, logarithm of turbidity, and day-of-year) and versus concentrations of other constituents where there is a relation. Outliers were also identified during the model fitting process using the USGS LOADEST load estimation software (Runkel and others, 2004) from output plots of model residual concentrations versus explanatory variables, which reflect model deviations while accounting for the effects of all model explanatory variables at once. Outliers were also identified in LOADEST using the Grubbs and Beck outlier test criteria (Grubbs and Beck, 1972). Separate calibration models were fit for water years 2003-2010 and 2010-2020 to improve model predictions. In a few cases, outliers identified in water year 2010 were removed from only one of these two calibration periods and these are noted in the comments column. Model calibration datasets were finalized on January 18, 2022. Aulenbach, B.T., Henley, J.C., and Hopkins, K.G., 2023, Hydrology, water-quality, and watershed characteristics in 15 watersheds in Gwinnett County, Georgia, water years 2002-20: U.S. Geological Survey Scientific Investigations Report 2023-5035. Grubbs, F.E., and Beck, G., 1972, Extension of sample sizes and percentage points for significance tests of outlying observations: Technometrics, v. 14, no. 4, p. 847-854, accessed May 10, 2022 at https://doi.org/10.2307/1267134. Helsel, D.R., Hirsch, R.M., Ryberg, K.R., Archfield, S.A., and Gilroy, E.J., 2020, Statistical methods in water resources: U.S. Geological Survey Techniques and Methods, book 4, chap. A3, 458 p., accessed March 5, 2021, at https://doi.org/10.3133/tm4a3. [Supersedes USGS Techniques of Water-Resources Investigations, book 4, chapter A3, version 1.1.] Lim, K.J., Engel, B.A., Tang, Zhenxu, Choi, Joongdae, Kim, K.-S., Muthukrishnan, Suresh, and Tripathy, Dibyajyoti, 2005, Automated web GIS based hydrograph analysis tool, WHAT: Journal of the American Water Resources Association, v. 41, no. 6, p. 1407-1416, accessed July 11, 2020, at https://doi.org/10.1111/j.1752-1688.2005.tb03808.x. Runkel, R.L., Crawford, C.G., and Cohn, T.A., 2004, Load estimator (LOADEST)--A FORTRAN program for estimating constituent loads in streams and rivers: U.S. Geological Survey Techniques and Methods, book 4, chap. A5, 69 p., accessed July 11, 2020, at https://doi.org/10.3133/tm4A5. Sauer, V.B., and Turnipseed, D.P., 2010, Stage measurement at gaging stations: U.S. Geological Survey Techniques and Methods, book 3, chap. A7, 45 p., accessed May 24, 2022, at https://doi.org/10.3133/tm3A7. Turnipseed, D.P., and Sauer, V.B., 2010, Discharge measurements at gaging stations: U.S. Geological Survey Techniques and Methods, book 3, chap. A8, 87 p., accessed May 24, 2022, at https://doi.org/10.3133/tm3A8. U.S. Environmental Protection Agency [EPA], 1992, NPDES storm water sampling guidance document: U.S. Environmental Protection Agency, EPA 833-8-92-001, 123 p., accessed November 21, 2019, at https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1002YY0.txt. Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B.A., 2006, Guidelines and standard procedures for continuous water-quality monitors--Station operation, record computation, and data reporting: U.S. Geological Survey Techniques and Methods 1-D3, 51 p., 8 attachments, accessed July 11, 2020 at https://doi.org/10.3133/tm1D3. NWIS 20220118 12_GwinnettCoGa_LOADESTModelCalibration.csv Comma Separated Value (CSV) file containing sample constituent concentrations and explanatory variable values used for fitting the load regression models Producer Defined Sample_num Sample number Producer Defined 1 2507 Assinged sequential integer 1 Site_nu U.S. Geological Survey site number Producer Defined "02205865" Sweetwater Creek at Club Drive near Lilburn, Ga. Producer defined "02207120" Yellow River at Ga 124, near Lithonia, Ga. Producer defined "02207185" No Business Creek at Lee Road, below Snellville, Ga. Producer defined "02207385" Big Haynes Creek at Lenora Road, near Snellville, Ga. Producer defined "02207400" Brushy Fork Creek at Beaver Road, nr Loganville, Ga. Producer defined "02208150" Alcovy River at New Hope Road, near Grayson, Ga. Producer defined "02217274" Wheeler Creek at Bill Cheek Road, near Auburn, Ga. Producer defined "02218565" Apalachee River at Fence Road, near Dacula, Ga. Producer defined "02334480" Richland Creek at Suwanee Dam Road, near Buford, Ga. Producer defined "02334578" Level Creek at Suwanee Dam Road, near Suwanee, Ga. Producer defined "02334885" Suwanee Creek at Suwanee, Ga. Producer defined "02335350" Crooked Creek near Norcross, Ga. Producer defined "02336030" North Fork Peachtree Creek at Graves Rd, near Doraville, Ga. Producer defined Watershed Watershed names used in the associated U.S. Geological Survey Scientific Investigations Report. Watershed drainage area defined by site specified in value definition shown in parentheses Producer Defined Sweetwater Creek Sweetwater Creek at Club Drive near Lilburn, Ga. (USGS site number 02205865) Producer defined Yellow River near Lithonia Yellow River at Ga 124, near Lithonia, Ga.Yellow River at Ga 124, near Lithonia, Ga. (USGS site number 02207120) Producer defined No Business Creek No Business Creek at Lee Road, below Snellville, Ga. (USGS site number 02207185) Producer defined Big Haynes Creek Big Haynes Creek at Lenora Road, near Snellville, Ga. (USGS site number 02207385) Producer defined Brushy Fork Creek Brushy Fork Creek at Beaver Road, nr Loganville, Ga. (USGS site number 02207400) Producer defined Alcovy River Alcovy River at New Hope Road, near Grayson, Ga. (USGS site number 02208150) Producer defined Wheeler Creek Wheeler Creek at Bill Cheek Road, near Auburn, Ga. (USGS site number 02217274) Producer defined Apalachee River Apalachee River at Fence Road, near Dacula, Ga. (USGS site number 02218565) Producer defined Richland Creek Richland Creek at Suwanee Dam Road, near Buford, Ga. (USGS site number 02334480) Producer defined Level Creek Level Creek at Suwanee Dam Road, near Suwanee, Ga. (USGS site number 02334578) Producer defined Suwanee Creek Suwanee Creek at Suwanee, Ga. (USGS site number 02334885) Producer defined Crooked Creek Crooked Creek near Norcross, Ga. (USGS site number 02335350) Producer defined North Fork Peachtree Creek North Fork Peachtree Creek at Graves Rd, near Doraville, Ga. (USGS site number 02336030) Producer defined Strt_SDate Sample collection start date and time - this is the collection date of discrete samples and the start date of composite samples Producer Defined 10/10/2002 8:30 9/25/2020 7:30 m/d/yyyy h:mm End_SDate Sample collection end date and time Producer Defined NA Not applicable. No end date for discrete samples Producer defined 12/5/2002 10:15 9/17/2018 6:20 m/d/yyyy h:mm Storm Storm indicator variable Producer Defined 1 Stormflow conditions Producer defined 0 Base-flow conditions Producer defined ln_Flow Natural logarithm of streamflow. Streamflow is instantaneous for discrete samples and an average for storm composite samples. (Corresponds to model streamflow parameter lnQ in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined -3.912 8.523 natural logarithm of cubic feet per second 0.001 lnQ_BaseQ Natural logarithm of streamflow during base-flow conditions as indicated by the "Storm" indicator variable. Calculated as product of the "ln_Flow" variable and the difference of one minus the "Storm" indicator variable. Equal to 0 during stormflow conditions. (Corresponds to model base-flow parameter (1-S)lnQ in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined -3.912 6.273 natural logarithm of cubic feet per second 0.001 ln_BaseQ Natural logarithm of daily average base flow for sample collection start date. Base flow determined from hydrograph separation. (Corresponds to model base-flow parameter lnQb in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined -4.605 7.031 natural logarithm of cubic feet per second 0.001 ln_Turb Natural logarithm of turbidity. Turbidity is instantaneous for discrete samples and an average for storm composite samples. (Corresponds to model turbidity parameter lnT in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined . No Data Producer defined -1.139 8.438 natural logarithm of Formazin Nephelometric Units 0.001 lnTrb_Strm Natural logarithm of turbidity during storm conditions as indicated by the "Storm" indicator variable. Calculated as the product of the "ln_Turb" and "Storm" variables. Equal to 0 during base-flow conditions. (Corresponds to model turbidity parameter SlnT in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined . No Data Producer defined 0.0 8.438 natural logarithm of Formazin Nephelometric Units 0.001 Sin_DOY Sine of sample collection start date day-of-year - used to fit seasonal variations in constituent concentrations. (Corresponds to model seasonal parameter sin-theta in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined -1.0 1.0 0.001 Cos_DOY Cosine of sample collection start date day-of-year - used to fit seasonal variations in constituent concentrations. (Corresponds to model seasonal parameter cos-theta in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined -1.0 1.0 0.001 DTime Sample collection start date, in decimal years. (Corresponds to model time-trend parameter time in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined 2002.773 2020.7453 decimal years 0.0001 DTime.Sq Sample collection start date squared, in decimal years. (Corresponds to model time-trend parameter time^2 in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined 4011099.9 4083411.5 decimal years squared 0.1 Flow Instantaneous (discrete samples) or average (storm composite samples) streamflow. (Corresponds to model streamflow explanatory variable Q in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined 0.02 5030.0 cubic feet per second 0.01 BaseQ Daily average base flow for sample collection start date - determined from hydrograph separation. (Corresponds to model base-flow explanatory variable Qb in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined 0.01 1131.0 cubic feet per second 0.01 Turbidity Instantaneous (discrete samples) or average (storm composite samples) turbidity. (Corresponds to model turbidity explanatory variable T in equation 1 of associated USGS Scientific Investigations Report Aulenbach and others [2023]) Producer Defined . No Data Producer defined 0.3 4620.0 Formazin Nephelometric Units 0.1 TSS.rmk Total suspended solids remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TSS Total suspended solids (USGS parameter code 00530) Producer Defined . No Data Producer defined 1.0 5700.0 milligrams per liter 1 SSC_rmk Suspended sediment concentration remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined SSC Suspended sediment concentration (USGS parameter code 80154) Producer Defined . No Data Producer defined 1 9300 milligrams per liter 1 TN_rmk Total nitrogen remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TN Total nitrogen (USGS parameter code 00600) Producer Defined . No Data Producer defined 0.2 6.4 milligrams per liter as nitrogen 0.01 NO3NO2_rmk Total nitrate plus nitrite remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined NO3NO2 Total nitrate plus nitrite (USGS parameter code 00630) Producer Defined . No Data Producer defined 0.082 6.5 milligrams per liter as nitrogen 0.001 TP_rmk Total phosphorus remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TP Total phosphorus (USGS parameter code 00665) Producer Defined . No Data Producer defined 0.0035 1.8 milligrams per liter as phosphorus 0.0001 DP_rmk Dissolved phosphorus remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined DP Dissolved phosphorus (USGS parameter code 00666) Producer Defined . No Data Producer defined 0.0018 0.13 milligrams per liter as phosphorus 0.0001 TOC_rmk Total organic carbon remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TOC Total organic carbon (USGS parameter code 00680) Producer Defined . No Data Producer defined 0.46 16.0 milligrams per liter as carbon 0.01 Ca_rmk Total calcium remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined Ca Total calcium (USGS parameter code 00916) Producer Defined . No Data Producer defined 1.8 28.0 milligrams per liter 0.1 Mg_rmk Total magnesium remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined Mg Total magnesium (USGS parameter code 00927) Producer Defined . No Data Producer defined 0.48 28.0 milligrams per liter 0.01 TPb_rmk Total lead remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TPb Total lead (USGS parameter code 01051) Producer Defined . No Data Producer defined 0.1 66.0 micrograms per liter 0.01 TZn_rmk Total zinc remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TZn Total zinc (USGS parameter code 01092) Producer Defined . No Data Producer defined 1.3 360.0 micrograms per liter 0.1 TDS_rmk Total dissolved solids remark code indicating whether the concentration is below its laboratory reporting limit (censored) Producer Defined << empty cell >> Concentration is not censored Producer defined < Concentration is below its laboratory reporting level (censored) as indicated by its concentration Producer defined TDS Total dissolved solids (USGS parameter code 70300) Producer Defined . No Data Producer defined 23 230 milligrams per liter 1 Comments Comments regarding cases where outliers from water year 2010 were removed from only one of the two calibration periods, water years 2003-2010 and 2010-2020 Producer Defined << empty cell >> No Data Producer defined Indicates cases where outliers in water year 2010 were removed for just one of the calibration periods, water years 2003-2010 and 2010-2020. Comment indicates constituent and calibration period. Constituent outlier values in this situation are retained in the dataset This dataset contains the sample constituent concentrations and explanatory variable values used for fitting the load regression models. These models were used to estimate streamwater constituent loads for 13 watersheds in Gwinnett County, Georgia for water years 2003 to 2020. Models were fit for 12 chemical constituents: total suspended solids (TSS), suspended sediment concentration (SSC), total nitrogen (TN), total nitrate plus nitrite (NO3NO2), total phosphorus (TP), dissolved phosphorus (DP), total organic carbon (TOC), total calcium (Ca), total magnesium (Mg), total lead (TPb), total zinc (TZn), and total dissolved solids (TDS). Model calibration samples included both discrete base flow and storm-composited samples and were collected between September 10, 2002 through September 25, 2020. Regression models were fit to the model calibration dataset using an 11-parameter equation, which included the following explanatory variables: storm condition indicator, season, time and time-squared (trends), and the natural logarithms of streamflow, streamflow during base-flow conditions, base flow, turbidity, and turbidity during stormflow conditions. Streamflows and turbidities for storm composite samples were determined as the average of the instantaneous streamflows and turbidities at the time of each sample aliquot. Daily average base flow was determined by hydrograph separation using the Web-based Hydrograph Analysis Tool (WHAT; Lim and others, 2005) using the simple local minimum method on the daily average streamflows. The flow condition indicator variable indicates whether the sample represented storm conditions (=1) or base-flow conditions (=0). Outlier concentrations that had high influence and leverage on the fit on their concentration relation were removed from this model calibration dataset. Models were fit for each constituent-watershed combination and separate models were fit for water years 2003-2010 and 2010-2020 to improve model predictions. For models that included turbidity as an explanatory variable, a second model without turbidity terms was fit to prevent gaps in load estimates when turbidity data was not available. The optimal set of model terms was then determined from the dataset for each station and constituent combination using a forward-stepwise regression with the corrected Akaike Information Criterion (AICc; Akaike, 1974; Burnham and Anderson, 2002). Model-term selection was done in JMP(R) statistical data analysis software version 16.1.0 (SAS Institute Inc., 1989-2021) using an ordinary least squares (OLS) fitting approach. Final model fits and loads were estimated using the USGS LOAD ESTimator software (LOADEST; Runkel and others, 2004) using the adjusted maximum likelihood estimates (AMLE) algorithm (Cohn, 1988; Cohn and others, 1989, 1992). Further details on model development are available in Aulenbach and others (2023). For constituent-watershed combination where concentration models were weak (R-square less than 0.2, based on the forward stepwise regression concentration-model fit), loads were alternatively estimated using the Beale ratio estimator (BRE; Beale, 1962) using the same water-quality model calibration dataset. The BRE is a commonly used ratio estimator approach in which an average sample-estimated load is scaled by its ratio of the annual average and sample average streamflows. Akaike, H., 1974, A new look at the statistical model identification: IEEE Transactions on Automatic Control, v. 19, no. 6, p. 716-723, accessed July 11, 2020, at https://doi.org/10.1109/TAC.1974.1100705. Aulenbach, B.T., Henley, J.C., and Hopkins, K.G., 2023, Hydrology, water-quality, and watershed characteristics in 15 watersheds in Gwinnett County, Georgia, water years 2002-20: U.S. Geological Survey Scientific Investigations Report 2023-5035. Beale, E.M.L., 1962, Some uses of computers in operational research: Industrielle Organisation, v. 31, p. 51-52. Burnham, K.P., and Anderson, D.R., 2002, Model selection and multimodel inference--A practical information-theoretic approach (Second Edition). Springer-Verlag, New York City, New York, ISBN: 13: 978-0387953649. Cohn, T.A., 1988, Adjusted maximum likelihood estimation of the moments of lognormal populations from type I censored samples: U.S. Geological Survey Open-File Report 88-350, 34 p., accessed July 11, 2020, at https://doi.org/10.3133/ofr88350. Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and Summers, R.M., 1992, The validity of a simple statistical model for estimating fluvial constituent loads: An empirical study involving nutrient loads entering Chesapeake Bay: Water Resources Research, v. 28, no. 9, p. 2353-2363, accessed July 11, 2020, at https://doi.org/10.1029/92WR01008. Cohn, T.A., DeLong, L.L., Gilroy, E.J., Hirsch, R.M., and Wells, D.K., 1989, Estimating constituent loads: Water Resources Research, v. 25, no. 5, p. 937-942, accessed July 11, 2020, at https://doi.org/10.1029/WR025i005p00937. Lim, K.J., Engel, B.A., Tang, Zhenxu, Choi, Joongdae, Kim, K.-S., Muthukrishnan, Suresh, and Tripathy, Dibyajyoti, 2005, Automated web GIS based hydrograph analysis tool, WHAT: Journal of the American Water Resources Association, v. 41, no. 6, p. 1407-1416, accessed July 11, 2020, at https://doi.org/10.1111/j.1752-1688.2005.tb03808.x. Runkel, R.L., Crawford, C.G., and Cohn, T.A., 2004, Load estimator (LOADEST)--A FORTRAN program for estimating constituent loads in streams and rivers: U.S. Geological Survey Techniques and Methods, book 4, chap. A5, 69 p., accessed July 11, 2020, at https://doi.org/10.3133/tm4A5. SAS Institute Inc., 1989-2021, JMP, ver. 16.1.0: Cary, North Carolina. U.S. Geological Survey GS ScienceBase mailing address

Denver Federal Center, Building 810, Mail Stop 302

Denver CO 80225 United States 1-888-275-8747 sciencebase@usgs.gov Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. Digital Data https://doi.org/10.5066/P9G8HZTY None 20230404 Brent T Aulenbach U.S. Geological Survey, Southeast Region Research Hydrologist mailing address

1770 Corporate Drive Suite 500

Norcross GA 30093 US 678-924-6626 678-924-6710 btaulenb@usgs.gov FGDC Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998