Total Maximum Daily Load for Fecal Coliform Bacteria to Town Branch, North Carolina. Final version submitted to EPA

Transcription

1 Total Maxmum Daly Load for Fecal Colform Bactera to Town Branch, North Carolna Fnal verson submtted to EPA August, 2002 Cape Fear Rver Basn Prepared by: NC Department of Envronment and Natural Resources Dvson of Water Qualty 1617 Mal Servce Center Ralegh, NC (919)

2 TABLE OF CONTENTS 1.0 Introducton Watershed Descrpton Water Qualty Montorng Program Water Qualty Target Source Assessment Pont Source Assessment Nonpont Source Assessment Lvestock Mscellaneous Sources Faled Septc Systems Urban Development/Santary Sewer Overflows Wldlfe Source Assessment Concluson Modelng Approach Model Framework Model Setup Instream Decay Rate Hydrologc Calbraton Water Qualty Calbraton Calbraton Results Predcton Uncertanty Total Maxmum Daly Load Reducton Target Crtcal Condtons Current Condtons Margn of Safety TMDL Allocaton Instantaneous Porton of Standard Seasonal Varaton Summary and Future Consderatons Montorng Implementaton Publc Partcpaton 30 References Cted 31 Appendx I. Observed Data Appendx II. Model Ft Statstcs 39 Appendx III. Model Calbraton Informaton. 41 Appendx IV. Stream Channel Cross Sectons for Subwatersheds 42 Appendx V. Modeled Flow and Fecal Colform 44 1

3 Total Maxmum Daly Load for fecal colform bactera to Town Branch 1.0 INTRODUCTION On the 2000 North Carolna 303(d) lst, the North Carolna Dvson of Water Qualty (DWQ) has dentfed a 3.6-mle segment (16-17) of Town Branch n the Cape Fear Basn as mpared by fecal colform bactera. The mpared segment extends from the stream s source to ts confluence wth the Haw Rver. Ths secton of the stream s located n subbasn Town Branch s desgnated as a class C water. Class C waters are freshwaters that are protected for secondary recreaton, fshng, and propagaton and survval of aquatc lfe. Secton 303(d) of the Clean Water Act (CWA) requres states to develop a lst of waters not meetng water qualty standards or whch have mpared uses. Ths lst, referred to as the 303(d) lst, s submtted bennally to the U.S. Envronmental Protecton Agency (EPA) for revew. The 303(d) process requres that a Total Maxmum Daly Load (TMDL) be developed for each of the waters appearng on Part I of the 303(d) lst. A TMDL s the maxmum amount of a pollutant (e.g., fecal colform) that a waterbody can receve and stll meet water qualty standards, and an allocaton of that load among pont and nonpont sources. The objectve of a TMDL s to estmate allowable pollutant loads and allocate to known sources so that actons may be taken to restore the water to ts ntended uses (USEPA, 1991). Generally, the prmary components of a TMDL, as dentfed by EPA (1991, 2000a) and the Federal Advsory Commttee are as follows: Target dentfcaton or selecton of pollutant(s) and endpont(s) for consderaton. An endpont s an nstream numerc target. The pollutant and endpont are generally assocated wth measurable water qualty related characterstcs that ndcate complance wth water qualty standards. North Carolna ndcates known problem pollutants on the 303(d) lst. Source assessment. Sources that contrbute to the mparment should be dentfed and loads quantfed, to the extent that that s possble. Reducton target. Estmaton or level of pollutant reducton needed to acheve water qualty goal. The level of polluton should be characterzed for the waterbody, hghlghtng how current condtons devate from the target endpont. Generally, ths component s dentfed through water qualty modelng. 2

4 Margn of safety. The margn of safety addresses uncertantes assocated wth pollutant loads, modelng technques, and data collecton. Per EPA (2000a), the margn of safety may be expressed explctly as unallocated assmlatve capacty (porton of TMDL) or mplctly through conservatve assumptons. The margn of safety should be ncluded n the reducton target. Allocaton of pollutant loads. Allocatng avalable pollutant load (TMDL), and hence pollutant control responsblty, to the sources of mparment. The wasteload allocaton porton of the TMDL accounts for the loads assocated wth exstng and future pont sources. The load allocaton porton of the TMDL accounts for the loads assocated wth exstng and future nonpont sources. Any future nonpont source loadng should reman wthn the TMDL that s calculated n ths assessment; n other words, ths TMDL does not leave allocaton for future sources. Seasonal varaton. The TMDL should consder seasonal varaton n the pollutant loads and endpont. Varablty can arse due to streamflows, temperatures, and exceptonal events (e.g., droughts and hurrcanes). Crtcal condtons. Crtcal condtons occur when fecal colform levels exceed the standard by the largest amount. If the modeled load reducton s able to meet the standard durng crtcal condtons, then t should meet the standard at all, or nearly all, tmes. Secton 303(d) of the CWA and the Water Qualty Plannng and Management regulaton (USEPA, 2000a) requre EPA to revew all TMDLs for approval or dsapproval. Once EPA approves a TMDL, then the waterbody may be moved to Part III of the 303(d) lst. Waterbodes reman on Part III of the lst untl complance wth water qualty standards s acheved. Where condtons are not approprate for the development of a TMDL, management strateges may stll result n the restoraton of water qualty. The goal of the TMDL program s to restore desgnated uses to water bodes. Thus, the mplementaton of bactera controls wll be necessary to restore desgnated uses n Town Branch. Although an mplementaton plan s not ncluded as part of ths TMDL, reducton strateges are needed. The nvolvement of local governments and agences wll be crtcal n developng an mplementaton plan and reducton strateges. DWQ wll seek to begn development of the mplementaton plan durng publc revew of the TMDL. 3

5 1.1 Watershed Descrpton Town Branch, located n the upper Cape Fear Rver basn, drans nto the Haw Rver about three mles northeast of the Cty of Graham (see Fgure 1). The creek s watershed les entrely wthn Alamance County and s slghtly less than 4 square mles n area. The Cty of Graham (2000 populaton of 12,833) covers more than three-quarters of the watershed. DWQ has an ambent water qualty montorng ste (Storet number B ) near the creek s confluence wth the Haw. Fgure 1. Town Branch Watershed Town Branch watershed # DWQ ambent staton Reach Fle, V3 ( ) Major Roads Graham muncpal boundary Cty of Graham Town Branch Interstates 40 & 85 Haw Rver N # Vcnty Map of Town Branch Watershed Town Branch Mles The land use/land cover characterstcs of the watershed were determned usng 1996 land cover data that were developed from LANDSAT satellte magery. The North Carolna Center for Geographc Informaton and Analyss, n cooperaton wth the NC Department of Transportaton and the Unted States Envronmental Protecton Agency Regon IV Wetlands Dvson, contracted Earth Satellte Corporaton of Rockvlle, Maryland to generate comprehensve land cover data for the entre state of North Carolna. Tabulated land cover/land use data for the 4

6 Town Branch watershed are shown n Table 1. Durng the formaton of ths geographc dataset, developed land was dentfed usng the proporton of synthetc cover present; low densty developed was 50-80% synthetc cover, and hgh densty developed was % synthetc cover (Earth Satellte Corporaton, 1997). Assumng that synthetc cover s mpervous, and that all nondeveloped land cover classes have 1% mpervous cover, the Town Branch watershed s estmated to have 29-42% mpervous surface. Table 1. Land use/land cover n Town Branch watershed. Land Use/Land Cover Town Branch Watershed Acres Hgh Densty Developed 275 (10.8%) Low Densty Developed 991 (38.8%) Cultvated 5 (0.0%) Managed Herbaceous 395 (15.5%) Forest 886 (34.7%) Total 2552 The USGS 14-dgt hydrologc unt code (HUC) for Town Branch s Water Qualty Montorng Program There are two sources of fecal colform data for ths project: 1) ambent montorng data; 2) specal study data collected by the Cty of Graham. More nformaton on each s provded below and a more detaled report s ncluded n Appendx I. Town Branch was lsted as mpared based on data from the ambent montorng staton, whch s located at SR 2109 (Cooper Rd.) or about one-quarter mle upstream from the stream s confluence wth the Haw Rver. Fecal colform samples are collected at ths ste on a monthly bass. The second source of Town Branch fecal colform data s a specal study conducted by the Cty of Graham n July and August of Ten samples were collected at three dfferent stes, ncludng: Town Branch at Cooper Road (ambent ste at SR 2109); Town Branch at the wastewater treatment plant (just upstream of major trbutary, County Home Branch); County Home Branch at Glbreath Street. The purposes of ths study were to evaluate whether the creek was complyng wth the state fecal colform standard, and to provde nformaton on bactera source areas n watershed. 5

7 The ambent montorng data were used n model calbraton. Both sets of montorng data may be seen n Appendx I. 1.3 Water Qualty Target The North Carolna fresh water qualty standard for fecal colform n Class C waters (T15A: 02B.0211) states: Organsms of the colform group: fecal colforms shall not exceed a geometrc mean of 200/100ml (membrane flter count) based upon at least fve consecutve samples examned durng any 30 day perod, nor exceed 400/100 ml n more than 20 percent of the samples examned durng such perod; volatons of the fecal colform standard are expected durng ranfall events and, n some cases, ths volaton s expected to be caused by uncontrollable nonpont source polluton; all colform concentratons are to be analyzed usng the membrane flter technque unless hgh turbdty or other adverse condtons necesstate the tube dluton method; n case of controversy over results, the MPN 5-tube dluton technque wll be used as the reference method. The nstream numerc target, or endpont, s the restoraton objectve expected to be reached by mplementng the specfed load reductons n the TMDL. The target allows for the evaluaton of progress towards the goal of reachng water qualty standards for the mpared stream by comparng the nstream data to the target. For ths TMDL the water qualty target s the geometrc mean concentraton of 200cfu/100ml over a 30-day perod. A geometrc mean s obtaned by calculatng the average of the log values of the ndvdual samples. Bascally, the geometrc mean wll dscount hgher values so that t should be lower than the arthmetc mean (average of measurements, no log taken). Cfu stands for colony-formng unts; t may also be referred to as smply counts n ths assessment. In ths TMDL, DWQ wll consder the entre model perod to address the porton of the standard that lmts the percentage of nstantaneous excursons over 400cfu/100ml to twenty percent. In order to evaluate the fecal colform model, montor water qualty condtons and assess progress of the TMDL, an evaluaton locaton was establshed for the Town Branch watershed. The evaluaton locaton of ths watershed s Town Branch at SR 2109, whch s the locaton of the ambent montorng staton. 6

8 2.0 SOURCE ASSESSMENT A source assessment s used to dentfy and characterze the known and suspected sources of fecal colform bactera n the watershed. DWQ completed a source assessment and used t to develop the water qualty model for the TMDL calculaton. 2.1 Pont Source Assessment General sources of fecal colform bactera are dvded between pont and nonpont sources. Currently, there are no facltes n the watershed that dscharge waste through the Natonal Pollutant Dscharge Elmnaton System (NPDES), whch s consdered to be the regulatory approach for all but the smallest of pont sources. The Cty of Graham has a muncpal treatment faclty (NPDES NC ), however that dscharges nto the Haw Rver. North Carolna also has general wastewater permts for package plants (small sand flter operatons), but there s very lmted nformaton about those; DWQ cannot dentfy any that dscharge to Town Branch. 2.2 Nonpont Source Assessment Nonpont sources of fecal colform bactera nclude those sources that can not be dentfed as enterng the waterbody at a specfc locaton (e.g., a ppe). Nonpont source polluton ncludes urban, agrcultural and background sources. For ths TMDL, background loadng s consdered to be that whch orgnates from wldlfe; ths s prmarly from forestland, but wldlfe are consdered to exst on cropland as well (see Wldlfe below). Fecal colform bactera may orgnate from human and non-human sources. Table 2 lsts the potental human and anmal nonpont sources of fecal colform bactera (Center for Watershed Protecton, 1999). The nonpont sources of fecal colform bactera n Town Branch nclude runoff from urban development (stormwater), sewer lne systems (leaky sewer lnes and sewer system overflows), wldlfe, falng septc systems, and probably llct connectons n unknown locatons Lvestock DWQ conferred wth an Alamance County extenson agent, Paul Walker, to derve estmates for lvestock populatons n the Town Branch watershed. From ths conversaton, DWQ decded that no lvestock resde n the watershed, and that none would be accounted for n the TMDL model. 7

9 Table 2. Potental sources of fecal colform bactera n urban and rural watersheds (Center for Watershed Protecton, 1999). Source Type Source Human Sources Sewered watershed Combned sewer overflows Santary sewer overflows Illegal santary connectons to storm drans Illegal dsposal to storm drans Non-sewered watershed Falng septc systems Poorly operated package plant Landflls Marnas Non-human Sources Domestc anmals and urban wldlfe Dogs, cats Rats, raccoons Pgeons, gulls, ducks, geese Lvestock and rural wldlfe Cattle, horse, poultry Beaver, muskrats, deer, waterfowl Mscellaneous Sources Illct dscharges (e.g., straght ppes) and any loadng that occurs durng baseflow (no runoff) are called mscellaneous sources n the TMDL allocaton. In the model, mscellaneous sources are treated as a constant, nstream source of bactera. It s necessary to separate these nstream sources from land based ones, because they are defned as one nstream source through modelng. That s, t s dffcult to determne ndvdual estmates for the fecal colform that orgnates from llct dscharges, but t s possble to estmate them cumulatvely n the model durng perods of low streamflow. Durng such perods, contrbutons from nonpont sources of fecal colform, whch ranfall transport, are assumed to be neglgble. What s beng ft to the model, through calbraton, s a constant nstream source of fecal colform Faled Septc Systems Falng septc systems have been cted as a potental source of fecal colform bactera to water bodes (USEPA, 2000). For the most part, household waste from the Cty of Graham and ts surroundngs s treated at the muncpal treatment plant, whch dscharges to the Haw Rver. Based on nvestgaton by the Cty of Graham, t appears that there are at least legacy septc tanks (no longer n actve use) and just a few currently used septc tanks n the watershed (Quck, 2002). DWQ estmates that there are 4 falng legacy septc systems n the Town Branch watershed. Durng the publc comment perod on the draft Town Branch TMDL, the Cty of Graham alerted DWQ to 8

10 addtonal falng septc systems n a neghborhood mmedately north and south of Cheek Lane (Sullvan, 2002). After consultng the map and other nformaton provded by the Cty of Graham, DWQ decded to add 20 more falng septc systems to the TMDL model. Negotatons are n progress to annex ths neghborhood to the Cty of Graham; ths may lead to connectng the neghborhood to Graham s sewage collecton and treatment systems. Addtonally, DWQ assumed that, on average, there are 3 people per system. Assumng the average concentraton of septc waste reachng the stream s 1.0 x 10 4 counts/100 ml and that the septc overcharge flow rate s 70 gallons/day/person (Horsely & Whtten, 1996), the contrbuton from falng septc systems s 4.77 x 10 7 counts/hour. DWQ also assumed that 60% of the septc overcharge reached the stream channel; ths estmate s not scentfcally based and was selected as a seemngly moderate to hgh number for transport from a falng septc system to the stream network. The loadng rate from septc systems usng these assumptons was 1.61 x 10 8 counts/30 days Urban Development/Santary Sewer Overflows Fecal colform bactera can orgnate from varous urban sources. These sources nclude pet waste, runoff through stormwater sewers, llct dscharges/connectons of santary waste, leaky sewer systems and santary sewer overflows. Fecal colform accumulaton rates on urban land cover were derved usng the followng: 1) the proporton of the watershed that s covered by hgh and low densty developed land cover; 2) the types of urban land use that occurs n the watershed (e.g., resdental, and heavy and lght commercal); 3) the fecal colform buld-up (accumulaton) rates for each land-use n 2), as calculated from nstream stormwater samples collected by the Unted States Geologcal Survey (USGS) from December 1993 to September 1997 n Mecklenburg County (Bales et al., 1999). In the USGS study, each of the urban land uses was pared wth a sample ste. The land use descrptons and calculated accumulaton rates for fecal colform may be seen n Table 3. 9

11 Table 3. Rate of accumulaton and maxmum storage of fecal colform by land use (from Bales et al., 1999). Land Use Rate of Accumulaton (count per acre per day) Maxmum Storage (count per acre) Resdental 6.86 x x Heavy Commercal 2.68 x x 10 9 Lght Commercal/Lght Industral 3.20 x x Woods/Brush 5.48 x x A step-by-step descrpton of the approach to determne ntal urban fecal colform accumulaton rates follows. DWQ frst calculated the proporton of low and hgh-densty land cover n the Town Branch watershed. Usng local knowledge of the watershed, the NC GIS land cover data were converted nto the four land use classes referenced by the USGS study (see Table 4). By combnng the proportons of the four land classes wth the accumulaton rates, DWQ assgned a comprehensve urban accumulaton rate of fecal colform. The accumulaton rate s an mportant model parameter that descrbes how much fecal colform s generated on each land use; actual fecal colform loadng (delvery) to the stream network s determned through subsequent modelng. Essentally, the model tracks fecal colform buld-up through the accumulaton rate and smulates fecal colform wash-off as precptaton falls. More descrpton of the model appears later n ths document. Table 4. Estmated converson from NC GIS land cover to land use n USGS study. Note that ths s for urban (developed) land cover only* Watershed Urban Land cover classfcaton Land use classfcaton (from GIS database) (estmated) Town Branch 21.7% hgh densty developed, 78.3% low densty developed 16% lght commercal/ndustral 4% heavy commercal 51% lght resdental 29% woods/brush By combnng the nformaton n Tables 3 and 4, DWQ calculated ntal estmates of urban accumulaton and maxmum storage. The results are shown n Table 5. 10

12 Table 5. Intal (pre-calbraton) estmates of accumulaton and storage. Watershed Rate of Accumulaton Maxmum Storage (count per acre per day) (count per acre) Town Branch 1.03 x x Snce these numbers are based on studes n dfferent, somewhat dstant watersheds (Mecklenburg Co.), they were subject to calbraton n the model. The cty of Graham owns and operates a wastewater treatment plant and sewage collecton system. From , Graham reported seventeen santary sewer overflows (SSOs) of greater than 1000 gallons, ncludng fve SSOs of greater than 50,000 gallons. DWQ dd not explctly account for SSOs n the modelng; rather, DWQ used a relatvely hgh (compared to other land uses), constant value for urban nterflow fecal colform concentraton n the calbraton model, whch, along wth a calbrated urban accumulaton rate may account for leaky sewers and nfrequent SSOs Wldlfe Wldlfe can be a source of fecal colform bactera n forest, wetland, pasture and cropland areas. Wldlfe depost fecal materal n these areas, whch can be transported to a stream n a ran event. Wldlfe n Town Branch watershed s expected to nclude deer, raccoons, squrrels, and brds (ncludng waterfowl). DWQ derved populaton densty estmates for all but squrrels and nonwaterfowl brds; consequently, these anmals were not ncluded n the model. DWQ obtaned estmates for deer populaton of per square mle from the North Carolna Wldlfe Resources Commsson (WRC, 2001). The lower end of the range (15) was appled to cropland and pastureland, and the hgher end of the range (30) was appled to forestland. DWQ assumed that no wldlfe lve n areas wth urban land cover. There was very lttle bass for estmatng populatons of raccoon, duck and geese densty ths s one of many areas of uncertanty n the model. DWQ consdered that there are 5 geese and 10 ducks per square mle of forestland and managed herbaceous land, and that there are 10 raccoons 11

13 per square mle on such land. The numbers for geese, duck and raccoon are not scentfcally based and are ntended as a rough, moderate estmate. 2.3 Source Assessment Concluson All of the aforementoned source assessment data were entered nto a spreadsheet called Fecal Tool, whch calculates accumulaton rates on the dfferent land covers, and loadng from drect sources such as leakng septc systems and mscellaneous nstream sources. TetraTech, Inc. developed Fecal Tool. Output from ths spreadsheet was used n the ntal estmates for the correspondng parameters n the water qualty model. Some of the model parameter estmates calculated n the spreadsheet were later altered through calbraton (e.g., urban colform accumulaton rates). 3.0 MODELING APPROACH An mportant component of the TMDL s to establsh the relatonshp between nstream water qualty and sources of fecal colform. A model that smulates or statstcally characterzes hydrology and water qualty s a helpful tool for ths purpose. Models provde the relatve contrbuton of the sources, as well as the predctons of water qualty resultng from changes n these source contrbutons; these are the basc elements of Total Maxmum Daly Load determnaton. 3.1 Model Framework The model selected for ths TMDL needed to meet several objectves: 1) To smulate watershed loadng and nstream transport of fecal colform bactera, and to capture some of the temporal and spatal varaton that those processes demonstrate. 2) To smulate nstream fecal colform concentratons over several years, so that crtcal condtons (defnton on page 2) may be dentfed. Crtcal condtons wll be the bass for ths TMDL. 3) To evaluate seasonal effects on the producton and fate of fecal colform bactera. EPA s BASINS software ncludes a model, Nonpont Source Model (NPSM), that s suted for TMDL development. NPSM s based on another model, the Hydrologc Smulaton Program FORTRAN (HSPF). Because t meets the objectves stated above, DWQ chose NPSM as the model for ths TMDL. 12

14 NPSM (HSPF) s a dynamc watershed model capable of smulatng nonpont source runoff and assocated pollutant loads. It does ths by trackng water and fecal colform n the watershed. Specfcally, modules named PWATER and IWATER are used to calculate the components of the water budget, and to predct the runoff from pervous and mpervous areas, respectvely (EPA, 1993). The model consders the followng hydrologc processes: precptaton, ntercepton, surface runoff, nterflow, groundwater, evaporaton and evapotranspraton. Fluxes or storages wthn subroutnes of the model smulate these processes. Fecal colform s smulated n the PQUAL and IQUAL modules (from pervous and mpervous land segment) usng smple relatonshps wth water. Fecal colform occurs n both the surface and subsurface outflow, though the former s consdered to be more complex n the model. On the surface, fecal colform can be affected by adheson to the sol, and by lght, wnd, temperature and drect human nfluence. The approach s to smulate fecal colform usng basc accumulaton (buld-up) and depleton rates, n concert wth depleton by wash-off; n other words, fecal colform outflow from the surface s a functon of the water flow and the amount of fecal colform n storage (EPA, 1993). Constant rates are assumed for subsurface loadng from the dfferent land use categores. NPSM (HSPF) performs flow routng and pollutant decay n stream reaches through the RCHRES module. Flow s assumed to be undrectonal and decay s assumed to be frst-order n nature (see secton below). Also, NPSM allows dscrete smulaton of the requred components of the TMDL (e.g., WLA and LA components). 3.2 Model Setup Town Branch was modeled as a sngle watershed (no subwatershed delneaton) based on Reach Fle 3 (RF3) stream coverage and a dgtal elevaton model of the area. The farthest downstream pont of the delneaton was the DWQ ambent water qualty samplng staton, B , or Town Branch at SR 2109 near Graham. Dewberry and Davs Consultants compled hourly meteorologcal data from a weather staton near Greensboro (see Fgure 3 for specfc locaton and ts proxmty to Town Branch watershed). The meteorologcal data begn on 7/1/1996 and end on 8/11/2001. DWQ measured the stream channel cross secton near the watershed outlet (Appendx IV), and ncorporated ths nto the model s f-table (table that descrbes relatonshp between streamflow depth and streamflow volume). 13

15 3.2.1 Instream Decay Rate Once fecal colform bactera reach a waterbody, envronmental factors nfluence the extent of ther growth and decay. Physcal factors that nfluence the bactera populatons nclude photo-oxdaton, adsorpton, flocculaton, coagulaton, sedmentaton and temperature (USEPA, 1985). Chemcal toxcty, ph, nutrent levels, algae and the presence of fecal matter may also nfluence the fecal colform populaton. The water qualty model utlzes a frst order decay rate to calculate nstream decay of fecal colform bactera. C t = C o e -kt C t = colform concentraton at tme t (cfu/100ml) C o = ntal colform concentraton (cfu/100ml) k= decay rate constant (day -1 ) t = exposure tme (days) Bacteral de-off has been modeled as a frst-order decay equaton, usng a decay rate (k) between 0.7/day and 1.5/day (Center for Watershed Protecton, 1999). Another study found that the medan decay rate for fecal colform was 1.15/day (Lombardo, 1972); that value was used n the Town Branch model for the exstng condton and allocaton runs. 3.3 Hydrologc Calbraton Because NPSM s drven by precptaton and by the subsequent treatment of the water budget, t s mportant to calbrate the hydrologc parameters pror to calbratng the water qualty parameters. In the hydrologc calbraton, smulated streamflows were compared to the hstorc streamflow data recorded at a contnuous stream gage. There s not a contnuous gage n Town Branch, so nstead DWQ used one at Reedy Fork near Oak Rdge (USGS ), whch s about 30 mles away (see Fgure 2 below). The Reedy Fork watershed s relatvely smlar to the Town Branch watershed n terms of shape, however t has proportonally less urban land cover and s consderably larger n area (about 20 square mles to 4 square mles). To calbrate the model, hydrologc parameters, ncludng nfltraton, upper and lower zone storage, groundwater storage and recesson, nterflow, and evapotranspraton, were adjusted wthn a recommended range untl the smulated and observed hydrographs were as close as possble. DWQ determned the best match by assessng statstcal ft; more descrpton s provded below. 14

16 In te rst at e s 40 & 85 Fnal verson submtted to EPA Fgure 2. Regonal Vew of Important Model Stes $T Haw Rver Reedy Fork stream dscharge gage Town Branch watershed Reach Fle, V1 Major Roads WDM Weather Data Statons Muncpal boundares Weather Staton Greensboro Town Branch Watershed Cty of G raham Haw Rver Town Branch N Mles A four-year perod from 1/1/97 to 12/31/00 was used as the calbraton perod for the hydrologc parameters. Relatve ft of the modeled flow compared to the recorded flow s shown n Fgure 3 below. The hydrologc parameters used to calbrate the model developed at the Reedy Fork gage were assumed to apply to the Town Branch watershed, and were used to develop the water qualty model for Town Branch watershed. 15

17 Fgure 3. Smulated and observed flows recorded at USGS Reedy Fork, Reedy Fork Hydrologc Calbraton Flow (cubc feet per second) /1/97 3/1/97 5/1/97 7/1/97 9/1/97 11/1/97 1/1/98 3/1/98 5/1/98 7/1/98 9/1/98 11/1/98 1/1/99 3/1/99 5/1/99 7/1/99 9/1/99 11/1/99 1/1/00 3/1/00 5/1/00 7/1/00 9/1/00 11/1/00 Observed Flow (cfs) NPSM Flow (cfs) Four conventonal statstcs for assessng model ft are correlaton coeffcent (r), the root mean squared error (RMSE), mean error (ME) and mean absolute error (MAE). DWQ calculated these to calbrate the model. Further descrpton on each of the statstcs s provded below, and the formulas for these statstcs appear n Appendx II; much of ths follows Stow et al. (n revew). A log transformaton was used because, as wth most envronmental data, fecal colform has a log normal dstrbuton. The log transformaton more equally weghts low and hgh values when calculatng ft statstcs (t gves less leverage to the relatvely few hgh values). Correlaton coeffcent, r, s a measure of the varablty n the observed data that s explaned by the model. Ths was chosen nstead of r 2, because r 2 hdes some negatve correlaton. The closer t s to 1, the better. If the correlaton between observed and predcted values s close to 1, the values don t necessarly track each other, they just tend to vary smlarly (Stow et al., n revew). For the Reedy Fork hydrologc calbraton, usng log base 10 values, the four-year correlaton coeffcent s 0.79; ths s a typcal level of predcton for hydrologc models (Stow, 2002). 16

18 The root mean squared error (RMSE) s the standard devaton of the model resduals, whch are the dfference between the model predctons and observed data. A lower RMSE s better (0 s best), though ts value s relatve to what the model attempts to predct; the mean of the observed data s a good measure to compare. In ths applcaton, the RMSE s 0.19 and the observed mean s 1.20 (log base 10 values), whch ndcates decent precson. The average error (AE) and average absolute error (AAE) are measures of aggregate model bas. Agan, closer to 0 s better, though for the mean error, values near zero can be msleadng snce negatve and postve dfferences can offset each other (Stow et al., n revew). For the Reedy Fork hydrologc calbraton, the AE and AAE were 0.01 and 0.13, respectvely. Ths ndcates that the model does not over- or under-predct consstently, and agan, that the precson s good. Table 6. Hydrologc calbraton ft statstcs Statstc Value (usng log base 10) r 0.79 RMSE 0.19 AE 0.01 AAE 0.13 Observatons (n) 1,461 Obs. mean 1.20 DWQ tested the Reedy Fork calbraton on North Buffalo Cr. and found t to be a reasonable ft there as well. North Buffalo Cr. s n between Reedy Fork and Town Branch, and has a proporton of urban land cover that s more smlar to the Town Branch watershed. 3.4 Water Qualty Calbraton Once the hydrologc calbraton s complete the model s calbrated for water qualty by adjustng parameters untl smulated and observed fecal colform concentratons acheve acceptable agreement. To calbrate the model, several parameters were adjusted ncludng the accumulaton rates of fecal colform bactera, wash-off rates, maxmum storage of fecal colform bactera and contrbutons from drect sources. By matchng the trends n smulated and observed 17

19 concentratons resultng from a varety of streamflows, the model may be a reasonable predctor of nstream water qualty. Selected values from the water qualty calbraton may be seen n Appendx III. Through model calbraton, DWQ estmated that the constant nstream source (see mscellaneous sources, Secton 2.2.2) s 7.2 x 10 6 counts/30 days. Illct dscharges are assumed to be ncluded n ths estmate. DWQ focused on calbratng urban colform accumulaton and maxmum storage rates n the Town Branch watershed, as that was dentfed as the prmary source through the source assessment. Through calbraton, these values decreased by approxmately 80% from the values shown n Table 5 (estmates from Mecklnburg Co. USGS study). It seems rather evdent that the Charlotte values are much too hgh for the Town Branch watershed. DWQ also calbrated urban nterflow concentraton, whch was consdered to be a constant value n the model. The calbraton perod for the water qualty model spanned October, 1996 nto August, The begnnng tme was lmted by the meteorologcal fle, whch began n July, DWQ allowed three months for the model to stablze before comparng predcted and observed data. The meteorologcal fle also lmted the smulaton end tme Calbraton Results Fecal colform samples collected at B (the ambent montorng staton) between October, 1996 and August, 2001 were compared to smulated concentratons and ranfall collected at the meteorologcal statons. The calbraton objectve was to obtan the best ft to the observed data, as determned by the same statstcs used n the hydrologc calbraton (r, RMSE, AAE, AE; see Secton 3.3). Graphcal ft of the model to observed data s shown n Fgures 4 and 5. These fgures ndcate that the model does a far job at smulatng the response of fecal colform bactera over tme, wth varatons n flow. The model calbraton statstcs for fecal colform are not nearly as good as those for the hydrologc calbraton. Addtonally, the model results ndcate that nstream fecal colform concentratons followng ranfall events frequently exceed the fecal colform standard, whle nstream concentratons durng dry perods typcally do not exceed the standard. Fnally, a plot of modeled flow and fecal colform concentraton appears n Appendx V. One mportant concept that 18

20 ths fgure demonstrates s hgher fecal colform concentratons occur n Town Branch followng ran events. Table 7. Water qualty calbraton ft statstcs Statstc Value (usng log base 10) r 0.47 RMSE 0.79 AE AAE 0.72 Observatons (n) 50 Obs. mean 2.58 Fgure 4. Smulated versus observed fecal colform concentratons from 9/1996 to 3/ FECAL COLIFORM (#/100 ml) /22/96 12/31/96 4/10/97 7/19/97 10/27/97 2/4/98 5/15/98 8/23/98 12/1/98 3/11/99 RAINFALL (n/day) DATE RAINFALL (IN/DAY) MODEL OUTPUT OBSERVED DATA 19

21 Fgure 5. Smulated versus observed fecal colform concentratons from 3/1999 to 8/ FECAL COLIFORM (#/100 ml) RAINFALL (n/day) /31/99 7/9/99 10/17/99 1/25/00 5/4/00 8/12/00 11/20/00 2/28/01 6/8/01 9/16/01 DATE RAINFALL (IN/DAY) MODEL OUTPUT OBSERVED DATA The low correlaton coeffcent ndcates a lack of predctve power. On the other hand, the relatvely small RMSE, AE and AAE suggest that the model may predct the mean farly well, but there may be a lot of ndvdual scatter about the mean. To resolve ths dscrepancy, DWQ examned modelng effcency, whch s calculated usng the followng formula: n = 1 ( O O) n = 1 2 n = 1 ( O O) ( P 2 O) 2 where: O _ = mean of the observatons O = th of n observatons P = th of n predctons 20

22 The modelng effcency measures how well the model predcts relatve to the average of the observatons. In terms of the ts value, the followng gudelnes apply: The closer to 1, the better. A value near 1 ndcates a close match between model predctons and observatons. If > 0, then the model predcts better than the average of the observatons. If < 0, then the mean of the observatons predct better than the model. If = 0, then the model predcts ndvdual observatons no better than the average of the observatons. The modelng effcency for the Town Branch colform model s Ths confrms that the model predctve precson s low Predcton Uncertanty The nablty to accurately smulate specfc observed data ponts can sometmes be attrbuted to more specfc aspects of a model, such as dfferences n ranfall at the meteorologcal gage and n the watershed, or llct pont dscharges. More often though, the lack of agreement between modeled and observed fecal colform s due to the general hgh degree of uncertanty assocated wth predctng any water qualty varable, especally fecal colform. Predcton uncertanty comes from a number of sources, ncludng (from Reckhow and Chapra, 1983 and Reckhow, 1995): Gaps n our scentfc knowledge. Natural varablty spatal and temporal varablty n chemstry, hydrology and ecology s great. Model predctons are on a much coarser scale than what occurs n nature. Measurement error measurement of fecal colform n the feld and laboratory has error. Aggregaton error wth ncreased endpont specfcty (space and tme), the uncertanty assocated wth the predcton ncreases. Model error: Ms-specfcaton model expressons that characterze processes may be wrong. Error n parameters reacton rates may be napproprate. Error n model nputs e.g., loadng terms such as accumulaton rate of bactera have error. 21

23 Unfortunately, many water qualty models employed for TMDL analyss, ncludng NPSM, are not adept at characterzng predcton uncertanty. Wth these models, all we know s that the uncertanty s certan to be large. Emphaszng an adaptve management approach s one way to address ths. Specfcally, the model may gude ntal decson makng, but contnued observaton of the watershed and creek, as fecal colform controls are mplemented (e.g., excluson fencng, leaky santary sewer repar), s expected to be our best approach for determnng the approprate level of management. Ths s especally true, consderng the results of the water qualty model calbraton, for the Town Branch TMDL. 4.0 Total Maxmum Daly Load A Total Maxmum Daly Load s the maxmum amount of a pollutant that a water body can receve and stll meet water qualty standards, and an allocaton of that amount among pont and nonpont sources. A TMDL comprses the sum of wasteload allocatons (WLA) for pont sources, load allocatons (LA) for nonpont sources, and a margn of safety. Ths defnton s expressed by the equaton: TMDL = S WLA + S LA + MOS The objectves of the TMDL are to estmate allowable pollutant loads, and to allocate to the known pollutant sources n the watershed, so the approprate control measures can be mplemented and the water qualty standard can be acheved. The TMDL wll be expressed n unts of counts/30 days, as ths s the perod over whch the water qualty target/standard s evaluated. The two man components of a TMDL, the reducton target, ncludng a margn of safety, and the allocaton strategy wll be presented n the followng sectons. 4.1 Reducton Target To determne the amount of fecal colform load reducton necessary to comply wth the water qualty standard, the perod of crtcal condtons must be establshed. Crtcal condtons, and loadng that represents current condtons and TMDL condtons were determned n the followng manner: 22

24 1) The calbrated model was rerun for the entre, nearly 5-year perod. 2) Smulated fecal colform concentratons for the nearly 5-year perod were plotted as rollng 30- day geometrc mean concentratons. A 30-day geometrc mean s determned by calculatng the geometrc mean of an ndvdual day s fecal colform predcton and the daly predctons for the 29 days that precede t. The rollng aspect s acheved by movng to the next day and performng the same calculaton (the earlest date from the prevous 30-day geometrc mean calculaton would be dropped). 3) DWQ appled load reductons n the model untl all of the 30-day geometrc means were below the standard. The date on whch the last 30-day geometrc mean peak falls below the crteron determnes when crtcal condtons occur. The 30-days pror to and ncludng ths date form the crtcal condtons perod, durng whch the TMDL s determned. 4) For the 30-day crtcal perod, DWQ calculated current loadng from the calbrated model, before any load reductons were taken. 5) Next, agan usng the 30-day crtcal perod, DWQ calculated loadng durng TMDL condtons (after load reductons were taken), when the predcted nstream fecal colform concentratons ndcated no exceedances of the water qualty standard. Wth no predcted exceedances of the standard, the model fulflls the TMDL crteron of allowng the maxmum load whle stll achevng water qualty standards. 6) For both the current condton case and the TMDL condton case, the smulated daly fecal colform loads from sources such as runoff from all lands, leakng septc systems and mscellaneous sources were summed for the 30-day crtcal perod. These sx steps wll be further explaned n the followng sectons Crtcal Condtons The Town Branch fecal colform montorng data ndcate that elevated fecal colform levels occur throughout the year, prmarly durng wet weather condtons. To determne when crtcal condtons occur, DWQ appled load reductons n the model untl all of the 30-day geometrc means were below the standard. The date on whch the last 30-day geometrc mean peak falls below the crteron determnes when crtcal condtons occur. By defnton, the 30-days pror to and 23

25 ncludng ths date form the crtcal condtons perod. See Fgure 6 below. For the Town Branch fecal colform model, ths occurred on February 5, The crtcal condtons occur between January 7, 1998 and February 5, Fgure 6. Reducton of rollng 30-day geometrc mean of fecal colform from exstng loadng to TMDL allocaton. 400 FECAL COLIFORM (#/100 ml) 200 crtcal condtons 2/5/ /12/96 4/30/97 11/16/97 6/4/98 12/21/98 7/9/99 1/25/00 8/12/00 2/28/01 9/16/01 DATE PREDICTED (EXISTING) TMDL ALLOCATION WATER QUALITY CRITERION Ran was recorded n Greensboro on 17 days durng the 30-day crtcal perod; at least 0.10 nch fell on 11 of those days. To wt, November and December, 1997 had 2.19 and 2.66 nches of precptaton, respectvely, whle the 30-day crtcal perod had 7.62 nches of precptaton. Include the day before the crtcal perod (January 6) and the total ncreases to 8.35 nches. Relatvely hgh fecal colform accumulaton rates resulted from the model calbraton (see Secton 3.4), and consequently, rany perods such as the crtcal condtons perod produce hgh nstream 24

26 fecal colform concentratons, as even small (>0.10 nch precptaton) events wash recently accumulated bactera nto the stream network Current Condtons Current loadng condtons were calculated by summng the loadng from the crtcal 30-day perod before any reductons were taken (calbrated model). DWQ separated the prncpal colform source categores, as used n NPSM, n Table 8; these nclude runoff from all lands, leakng septc systems and mscellaneous sources. Runoff from all lands ncludes estmated fecal colform load from deposts by wldlfe, as well as an estmate of loadng from urban areas. Leakng septc systems estmates loadng related to septc systems. Mscellaneous sources s an estmate of loadng from unknown, or llct, nstream sources. Accordng to the model, storm-drven runoff from all land provdes the largest load of fecal colform bactera to the stream. Loads from mscellaneous sources are constant loads that are appled drectly to the stream; these sources wll have the greatest mpact on nstream water qualty durng perods of low flow. Table 8. Summary of predcted exstng colform loads n the Town Branch watershed. Runoff from all lands 1 Leakng septc systems Mscellaneous sources 2 Instream conc. 3 (counts/30 days) (counts/30 days) (counts/30 days) (counts/100 ml) 1.03 x x x Includes urban runoff and wldlfe. 2 Includes llct dscharges. 3 Maxmum smulated concentraton durng the crtcal perod (geometrc mean) Margn of Safety TMDL condtons were calculated by summng the loadng from the crtcal 30-day perod after reductons were taken to brng all of the 30-day geometrc means below the crteron. Before ths s done, however, a margn of safety must be ncluded. A TMDL requrement s that a margn of safety must be ncluded to provde further nsurance that the mpared waterbody wll meet ts desgnated uses once load reductons are realzed. The margn of safety may be accounted for mplctly, through conservatve (more protectve of water qualty) model assumptons, or explctly, by reservng a porton of the allocated load. The Town Branch 25

27 TMDL ncludes explct and mplct margns of safety; more explanaton on the margn of safety follows below. In Fgure 7 below, observe that the target for the rollng 30-day geometrc mean of fecal colform s 170 counts/100 ml, nstead of the standard of 200 counts/100 ml. By usng ths lower target, DWQ provdes an explct margn of safety for the Town Branch TMDL. Ths explct margn of safety may be nterpreted to account for 15% greater assurance of achevng the nstream water qualty target. [( )/200]*100 = 15% Also, an mplct margn of safety s ncluded because the model assumes that bactera delvered from the land surface do not decay as t travels from ts source to the stream network; n realty, some decay would occur, though t s dffcult to estmate or model. Fgure 7. Reducton of rollng 30-day geometrc mean of fecal colform to nclude explct margn of safety. 400 FECAL COLIFORM (#/100 ml) /12/96 4/30/97 11/16/97 6/4/98 12/21/98 7/9/99 1/25/00 8/12/00 2/28/01 9/16/01 DATE PREDICTED (EXISTING) TMDL ALLOCATION WATER QUALITY CRITERION 26

28 4.1.4 TMDL Allocaton To calculate the TMDL, DWQ summed the loads from the crtcal 30-day perod after reductons were taken to brng all of the 30-day geometrc means below the crteron, plus the explct margn of safety (170 counts/100 ml). The allocaton strategy for the Town Branch fecal colform TMDL s lmted to nonpont sources, as there are no permtted pont sources n the watershed. An allocaton scenaro that predcts complance wth the nstream water qualty crteron and the requred reductons from the ndvdual categores may be seen n Table 9. A nonlnear relatonshp between fecal colform nputs and nstream fecal colform concentraton produces an nequtable reducton n load and concentraton n percent terms (51% reducton n concentraton requres 60 or more % reducton n load). Table 9. Allocaton strategy by major nonpont sources for TMDL condtons Runoff from all lands (counts/30 days) Leakng septc systems (counts/30 days) Mscellaneous Sources 2 (counts/30 days) Instream f.c. concentraton 1 (counts/100 ml) 3.25 x x x % reducton 60% reducton 60% reducton 51% reducton 1 Maxmum smulated nstream concentraton durng crtcal perod. Percent reducton represents the dfference n smulated nstream concentraton between the exstng loads (Table 8., 350 counts/100 ml) and TMDL allocaton scenaro (ths table, 170 counts/100 ml). 2 Should not nclude llct dscharges. The nonpont sources are summed to produce a load allocaton (LA), whch s dsplayed n Tables 10 and 11 below. Tables 10 and 11. Allocaton strategy by TMDL component for Town Branch. In terms of load: Wasteload allocaton (WLA) (counts/30 days) Load allocaton (LA) (counts/30 days) Explct Margn of safety (MOS) 1 TMDL (counts/30 days) (counts/30 days) x x x Explct margn of safety s equal to 22% snce the load allocaton s reduced by that much from the TMDL allocaton (e.g., [(4.15 x x )/4.15 x ] * 100 = 22%). 27

29 In terms of concentraton: Wasteload allocaton (WLA) (counts/100 ml) Load allocaton (LA) (counts/100 ml) Explct Margn of safety (MOS) 1 TMDL (counts/100 ml) (counts/100 ml) Explct margn of safety s equal to 15.0% snce the nstream water qualty target s reduced to 170 counts/100 ml from 200 counts/100 ml (e.g., [( )/200] = 15%). The explct margn of safety dffers dependng on whether t s vewed n terms of concentraton or load (15% vs. 22%, respectvely). Ths s because of a nonlnear relatonshp between load and concentraton. The mplct margn of safety, from the assumpton that fecal colform bactera do not decay as they are transported from the land surface to the stream network, s not quantfed nor ncluded n the tables above. Bascally though, by not ncludng ths decay, the lsted load allocaton s hgher than f the decay were ncluded. Consequently, the actual (expected) load allocaton wll be lower than what s shown n Tables 10 and 11. Because less fecal colform should actually be delvered to the streams, theren les the mplct margn of safety. 4.2 Instantaneous porton of standard To assess the nstantaneous porton of the fecal colform standard, DWQ consdered observed data and predctons from the full modelng perod. The observed data from 1995 to 2001 show that 44% of the samples were over 400 counts/100 ml. The calbrated model has 28% of the daly predctons over 400 counts/100 ml. The TMDL model that meets the geometrc mean part of the fecal colform standard (overall about a 70% reducton) drops the percent of daly predctons above 400 counts/100 ml to less than 19% over the nearly 5 year model perod. 4.3 Seasonal varaton DWQ used a nearly 5-year smulaton perod to assess the TMDL. Ths longer perod allows for consderaton of seasonal varaton. 28

30 5.0 SUMMARY AND FUTURE CONSIDERATIONS The sources of fecal colform n the Town Branch watershed nclude urban sources n the Graham area, and wldlfe n the forested areas of the watershed. The Nonpont Source Model n EPA s Basns software was used to smulate nstream fecal colform concentratons and to allocate the fecal colform loads to the varous sources. In order for the water qualty target to be met, the fnal allocaton of the fecal colform requres the major sources (not wldlfe as that s consdered part of background) to reduce loadng by approxmately 70%. DWQ consders the major source to be runoff from urban areas, possbly ncludng leakng sewer lnes. In fact, based on the specal study montorng whch s shown n Appendx I, and based on the model results, t appears that f Town Branch s to meet the fecal colform standard, the Cty of Graham must consderably reduce loadng from runoff of urban areas, as well as address SSOs, possbly through nfrastructure repar. As evdenced by Appendx V, the fecal colform problem n Town Branch occurs followng ran events. Addtonal montorng s requred to dentfy specfc sources of fecal colform, n terms of both area and anmal. Fnally, t would be benefcal to the fecal colform mparment of Town Branch f the Cty of Graham would annex, and connect to the sewer system, the neghborhood n the southern porton of the watershed where septc system falure rate s hgh (see Secton Falng Septc Systems). 5.1 Montorng Fecal colform montorng wll contnue on a monthly nterval at the ambent montorng ste (SR2109). The contnued montorng of fecal colform concentratons wll allow for the evaluaton of progress towards the goal of reachng water qualty standards. In addton to ths data collecton, further fecal colform montorng may be consdered. Addtonal montorng could focus on fecal colform source assessment n the watershed; ths would further ad n the evaluaton of the progress towards meetng the water qualty standard. Also, a synoptc survey of nstream fecal colform durng storm events may mprove model calbraton durng those mportant loadng events. To comply wth EPA gudance, North Carolna may adopt new bactera standards n the near future usng Eschercha col (E. col) and enterococc. Thus, future montorng efforts to measure complance wth ths TMDL should nclude E. col and enterococc. Per EPA recommendatons (EPA, 2000b), f future montorng for E. col/enterococc ndcates the standard has not been 29