TMDL Studies
Mill Creek Watershed, Page County
The TMDL final report is available for download at the DEQ website.
Located in Page County, Virginia, the Mill Creek (VAV-B38R, 8,221 acres) watershed is located southwest of Luray. Mill Creek is a tributary of the South Fork of the Shenandoah River (USGS Hydrologic Unit 02070005) (See Figure 1).Water quality samples collected in Mill Creek over a period of 11½ years(December 1991 – June 2003) indicated that 51% of the samples violated the instantaneous water quality standard for fecal coliform. The instantaneous freshwater water quality standard for fecal coliform under which the Mill Creek impairment was listed specified that fecal coliform concentration in the stream water shall not exceed 1,000 colony forming units (cfu) per 100 mL. Due to the frequency of water quality violations, Mill Creek was placed on Virginia’s 1998 303(d) list of impaired water bodies for fecal coliform.
Figure 1: Location of the Mill Creek Watershed.
Contributions from various sources within the Mill Creek watershed were represented in HSPF to simulate bacteria loadings for existing conditions. Average annual loads were calculated using meteorological inputs for the same 1990-2002 period used in neighboring Hawksbill Creek. Forty-one percent of the fecal coliform in the mean daily fecal coliform concentration comes from cattle directly depositing in the stream, 52% from upland areas due to runoff, 5% from wildlife directly depositing in the stream, and approximately 2% from straight pipes, sanitary sewer overflows, runoff from impervious areas, and contributions from interflow and groundwater.
For existing conditions in Mill Creek, simulated bacteria concentrations exceeded the calendar-month geometric mean water quality standard 77% of the time, and by larger amounts during low flow periods and the summer. During the summer when stream flow is lower, cattle tend to spend more time in streams, increasing direct fecal coliform deposition to streams when water for dilution is least available. Based on amounts of bacteria produced in different locations, monthly bacteria loadings to different land use categories were calculated for each subwatershed for input into the respective models. For the TMDL allocation scenarios, a target of zero violations of both the instantaneous and geometric mean water quality standards was used. Various allocation scenarios were developed to meet the standard as shown in the Table below.
Scenarios 4 and 5 represent alternative Stage 1 reduction scenarios with single standard criterion violation rates less than 10%. Scenario 6 shows that violations of the single sample criterion cannot be eliminated even with 100% reductions from everything except background wildlife sources. The last scenario – Scenario 7 – was developed to eliminate all violations of both the calendar-month geometric mean and the single sample criteria and requires, in addition to all the previous reductions, a 40% reduction in the direct deposits in land deposited waste from wildlife, whose reductions are exempted under current state guidelines.
TMDL Studies
North River Watershed
The approved TMDL final report is available for download at the DEQ website.
The North River watershed (VAV-B16R, VAV-B17R, VAV-B23R, 523,298 acres) is located in Rockingham and Augusta Counties, Virginia, encompassing the cities of Harrisonburg and Staunton (See figure below). North River flows east, merges with South River, and discharges into the South Fork of the Shenandoah River (USGS Hydrologic Unit Code 02070005). The South Fork of the Shenandoah River joins with the North Fork of the Shenandoah River to form the Shenandoah River; the Shenandoah River flows into the Potomac River; the Potomac River discharges into the Chesapeake Bay. The North River watershed includes the Beaver Creek watershed. Water quality samples collected at three stations in the North River watershed during the 2004 assessment period violated the instantaneous water quality standard for bacteria 29, 33, and 37% of the time.
Location of the North River Watershed.
There are sixteen small (1,000 gpd) and two larger point sources permitted to discharge bacteria in the North River watershed; however, the majority of the bacteria load originates from nonpoint sources. The nonpoint sources of bacteria are mainly agricultural and include land-applied animal waste and manure deposited on pastures by livestock. A significant bacteria load comes from cattle and wildlife directly depositing feces in streams. Forested area is the largest land use in the watershed at 49% of the total watershed area, followed by pasture at 43%.
The HSPF model was used to model hydrology and water quality. The hydrology component of the model was calibrated to one flow gage located midway through the model; the water quality portion of the model was calibrated to three monitoring stations located throughout the watershed. The HSPF model was then used to generate allocation scenarios for North River.
Contributions from various sources in the North River watershed were represented in HSPF to establish the existing conditions for a representative 5-year period that included both low and high-flow conditions. Meteorological data from 1988-1992 were paired with bacterial loading and land use data for existing conditions to establish this baseline scenario. Results of the calibrated HSPF model predict that an estimated 91% of the E. coli in the mean daily E. coli concentration at the watershed outlet currently comes from areas covered by a previously developed TMDL. Of the remaining 9% of the mean daily E. coli concentration, 51% comes from upland contributions of cattle, wildlife, humans, and pets; 23% from wildlife directly depositing in the streams; 19% from cattle directly depositing in the stream; 5% from interflow and groundwater; and 3% from straight pipes directly discharging in the stream. Simulated bacteria concentrations exceeded the calendar-month geometric mean water quality standard 63% of the time at the watershed outlet.
A stressor analysis was also completed for North River to change its impairment category. Analysis of the available data showed that, in similar fashion to the bacteria TMDL, most of the benthic stressors were coming from watersheds for which benthic TMDLs had already been completed. We put together a document that supported the reclassification of North River from category 5A, impaired and requiring a TMDL, to category 4A, impaired but not needing a TMDL because a TMDL to address the pollution is already in place.
TMDL Studies
Beaver Creek Watershed
The TMDL final report is available for download at the DEQ Website.
The Beaver Creek watershed (VAV-B18R, 10,205 acres) is located in Rockingham County, Virginia, to the west of Harrisonburg. Beaver Creek is a tributary of the North River (USGS Hydrologic Unit Code 02070005), which in turn, is a tributary of the South Fork of the Shenandoah River. The Shenandoah River flows into the Potomac River. The Potomac River discharges into the Chesapeake Bay. A major contributor to flow in Beaver Creek is a spring located about 4.4 stream miles upstream of the watershed outlet.
Water quality samples collected in Beaver Creek over a period of 10 years (1994 – 2003) indicated that 52% of the samples violated the new instantaneous water quality standard for fecal coliform (400 colony forming units (cfu) / 100 ml). The majority of the bacteria come from nonpoint sources, agriculture being the largest of these sources. The HSPF model was used to determine allocation reductions. Since there was no stream gage within the watershed, only the water quality portion of the model could be calibrated. Calibrated hydrologic parameters were taken from a model file for a nearby watershed. A large spring whose flow intercepted Beaver Creek about halfway down the watershed necessitated the development of a TMDL at two locations in the watershed: upstream of the confluence with the spring, and at the watershed outlet. The low flow above the spring confluence caused high reductions to be called for in all source categories in the upper portion of the watershed. Two possible reduction scenarios were developed for the lower portion of the watershed, each with high reductions in bacteria from pasture areas and straight pipes and varied reductions from other sources.
Land use within the Beaver Creek Watershed
Beaver Creek was also listed for a benthic impairment. A stressor analysis performed by Center personnel was completed for Beaver Creek to delist the benthic impairment for two reasons. A historic polluter shown to be the likely source of organic matter (the primary historic stressor the stream) had closed down in 2002. Additionally, three of four recent benthic assessments show no impairment on the creek; the remaining assessment shows only a slight impairment (benthic impairments are listed based on assessment of severe or moderate impairment, not slight impairment).
