TMDL Studies
Pigg River
The TMDL for Old Womans Creek, Snow Creek, Story Creek, and three segments of Pigg River due to water quality violations of the bacteria standard has been approved. The final report is available on the Virginia DEQ website: http://www.deq.virginia.gov/tmdl/apptmdls/roankrvr/piggec.pdf.
The Pigg River, Snow Creek, Story Creek, and Old Womans Creek watersheds are all part of the Roanoke River basin. The hydrologic units composing the watersheds for each stream are: Pigg River, L14-L18; Snow Creek, L17; Story Creek, part of L14; and Old Womans Creek, part of L13. Big Chestnut Creek corresponds to hydrologic unit L15. The Pigg River watershed (to which Snow Creek, Story Creek, and Big Chestnut Creek are tributaries) stretches across Franklin County and into part of Pittsylvania County, covering the northernmost point of Henry County. Old Womans Creek, adjacent to Pigg River, is located entirely inside Pittsylvania County (See figure below).
Location of Pigg River Watershed
The land use distribution in the four watersheds of interest are fairly similar (Table), mainly composed of forest but with a significant portion of agricultural area. Residential areas compose a small portion of all watersheds and are clustered primarily around Rocky Mount and Ferrum, both located in hydrologic unit L14. Pigg River flows east and discharges into Leesville Lake; Old Womans Creek flows north and discharges into Leesville Lake. Leesville Lake discharges to the Roanoke River (USGS Hydrologic Unit Code 03010101), which flows into the Albemarle Sound; the Albemarle Sound discharges to the Atlantic Ocean.
Land use description in TMDL watersheds.|
Watershed |
|
Agriculture |
Residential |
|
|
72% |
26% |
2% |
|
Snow Creek |
71% |
28% |
1% |
|
|
78% |
20% |
2% |
|
Old Womans Creek |
76% |
24% |
<1% |
Landuse in Pigg River and Old Womans Creek
In order to remedy the fecal coliform water quality impairments, Total Maximum Daily Loads (TMDLs) have been developed, taking into account all sources of bacteria and a margin of safety (MOS). The TMDLs were developed for the new water quality standard for bacteria, which states that the calendar-month geometric mean concentration of E. coli shall not exceed 126 cfu/100 mL, and that no single sample can exceed a concentration of 235 cfu/100mL.
There are three point sources permitted to discharge bacteria into the Pigg River basin; one of these is located in the Story Creek No permitted facilities exist in the Old Womans Creek watershed. However, the majority of the bacteria load originates from nonpoint sources.
The Hydrological Simulation Program – FORTRAN (HSPF) (Bicknell et al., 2001) was used to simulate the fate and transport of fecal coliform bacteria in the Pigg River and Old Womans Creek watersheds. As recommended by VADEQ, water quality modeling was conducted with fecal coliform inputs, and then a translator equation was used to convert the output to E. coli for the final TMDL. To identify localized sources of fecal coliform within the watershed, the Pigg River watershed was divided into 23 sub-watersheds (including 2 for Story Creek and 4 for Snow Creek), based on homogeneity of land use, stream network connectivity, and monitoring station locations. The Old Womans Creek watershed was likewise divided into 7 sub-watersheds.
The hydrology component of HSPF was calibrated using data from September 1, 1989 to December 31, 1995; it was validated using data from June 1, 1984 to August 31, 1989. Initial estimates of hydrologic parameters were generated according to the guidance in BASINS Technical Note 6 (USEPA, 2000a). These parameters were refined during calibration. The program HSPEXP (Expert System for the Calibration of HSPF) was used to aid in calibration, and after the successful calibration the default calibration criteria in HSPEXP were met for both the calibration and validation periods.
The water quality component of the HSPF model was calibrated and validated for Pigg River, its tributaries, and Old Womans Creek at 10 monitoring stations. The bacteria model was calibrated to 7 stations (those seven having data during the calibration period) for a rough period of 1994-1998. The bacteria model was validated to all 10 stations for a rough period of 1999-2005. Inputs to the model included fecal coliform loadings on land and in the stream. A comparison of simulated and observed fecal coliform loadings in the stream indicated that the model adequately simulated the fate and transport of fecal coliform bacteria.
TMDL Studies
North Fork Shenandoah and Stony Creek
The TMDL for the North Fork Shenandoah River and Stony Creek for bacteria impairments was approved on September 26, 2006 and is available for download at the DEQ Website.
A part of the Shenandoah River basin, the North Fork Shenandoah watershed (Watershed ID VAV-B45R) is located in Rockingham and Shenandoah Counties, Virginia, and comprises the following watershed IDs B42, B43, B44, B45, B46, B47, B48, B49, and B50. Of those watersheds, three have existing TMDLs: B46-Linville Creek, B47-Smith Creek, and Holmes Creek (part of B45). A section of B42 lies within West Virginia. The North Fork Shenandoah is loosely bounded by the county boundaries to the west and the Massanutten Mountains to the east (See Figure). Harrisonburg lies on the southern boundary and Woodstock is near the northern boundary. The watershed is approximately 454,000 acres in size. North Fork Shenandoah is heavily forested (about 61%) followed by agricultural (about 37%) with the remaining residential (about 2%). The watershed flows north and discharges into the Shenandoah River (USGS Hydrologic Unit Code 02070006), which flows into the Potomac River; the Potomac River discharges into the Chesapeake Bay.
A part of the North Fork Shenandoah watershed, the Stony Creek watershed (Watershed ID VAV-B49R) is located in Shenandoah County, Virginia, loosely bounded by Appalachian Mountains to the west, and Edinburg to the east (See Figure). The Stony Creek watershed is approximately 72,600 acres in size. Stony Creek is mainly a forested watershed (about 70%) within the Appalachian Mountains into the rolling valley. The remaining 30% of the watershed area is primarily agricultural (28%), with a small area devoted to rural developments (2%).
TMDL Studies
Mill Creek
This TMDL was approved on June 5, 2006 and a copy of the report can be downloaded from the Virginia DEQ website.
A part of the North Fork Shenandoah River basin, the Mill Creek watershed (Watershed ID VAV-B48R_MIL01A00) is an upstream portion of state hydrologic unit B48, and is located in Shenandoah County, Virginia as shown in Figure 1.1. The watershed is 29,753 acres in size. The land use in Mill Creek is mainly forest (53%) and pasture (44%), with a small amount of cropland (2%) and urban and residential land uses (1%). Mill Creek flows east and discharges into the North Fork of the Shenandoah River, which flows into the Shenandoah River, which flows into the Potomac River; the Potomac River discharges into the Chesapeake Bay.
Mill Creek was originally listed as impaired on Virginia’s 1998 Section 303(d) Total Maximum Daily Load Priority List and Report due to water quality violations of the general aquatic life standard, cause of impairment unknown. As a result, the Environmental Protection Agency (EPA) added this stream to a 1998 consent order requiring a TMDL by 2010. In 2002, a bacterial impairment on Mill Creek was also added to the Section 303(d) list (VADEQ, 2002a). Land use for the Mill Creek watershed was derived from the National Land Cover Dataset (NLCD). Based on a categorization of the 21 land uses in the NLCD data, the main land use category in Mill Creek is forest, comprising approximately 52% of the total watershed area. Pasture, cropland, and urban/residential acreage accounts for about 43%, 2%, and 1% of the watershed area, respectively, as shown in the Figure below.
The TMDL to address the aquatic life impairment in Mill Creek was developed using sediment as the pollutant. Because Virginia has no numeric in-stream criteria for sediment, a “reference watershed” approach was used to set allowable loading rates in the impaired watershed. The reference watershed approach pairs two watersheds: one whose streams are supportive of their designated uses, and one whose streams are impaired. The non-impaired upper portion of Mill Creek watershed (above station MIL005.67) was selected as the TMDL reference watershed for the Mill Creek watershed. The Generalized Watershed Loading Function (GWLF) model (Haith et al., 1992) was selected for comparative modeling of both the impaired and TMDL reference watersheds in this TMDL study. Channel erosion was modeled explicitly within GWLF using the algorithms included in the AVGWLF adaptation of the GWLF model (Evans et al., 2001) in a modified version of GWLF that corrects for a flow accumulation coding error.
