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.

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.