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DEVELOPMENT OF BACTERIA AND BENTHIC TMDLS: A CASE STUDY, LINVILLE CREEK, VIRGINIA

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Two Total Maximum Daily Load Plans (TMDLs) were developed for Linville Creek, Rockingham County, Virginia. The first addressed the benthic impairment. The benthic impairment TMDL was developed using the reference watershed approach. This approach identifies a comparable watershed within the same physiographic region (an area defined primarily by similar geology and topography) that has a healthy benthic community. The Upper Opequon Creek was selected as the TMDL reference watershed from a list of watersheds around the state with benthic monitoring data and a non-impaired status. A stressor analysis performed on existing monitoring data identified sediment as the major stressor for Linville Creek. Modeling was performed with a modified version of the GWLF (Generalized Watershed Loading Function) model. Model inputs were developed for both watersheds considering surface runoff, stream bank and channel erosion, and point sources of sediment. Model simulations were performed over a 10-year period to generate average annual sediment loads from all sources within each watershed. The TMDL load (t/yr) is calculated as the modeled unitarea load of the identified pollutant from the benthically unimpaired TMDL reference watershed (t/ha-yr) times the area of the impaired watershed (ha), and becomes the target for pollutant reductions from the impaired watershed. Allocation scenarios were developed using reductions from agriculture and from channel erosion sources resulting primarily from unrestricted livestock stream access. The sediment TMDL for Linville Creek is 34,549 t/yr and requires an overall reduction of 12.3% from existing loads.

The second TMDL addressed the bacteria impairment. Virginia has recently moved from fecal coliform standard to an Escherichia Coli (E. Coli) standard for bacteria impairments. Under the new E. coli standard, both the magnitude of the numerical criterion and the permissible violation rate of the criterion have decreased. As a result, the E. coli standard is significantly more restrictive than the previous fecal coliform standard. For the bacteria impairment TMDL, the Hydrologic Simulation Program – FORTRAN (HSPF) was used to simulate the fate and transport of fecal coliform bacteria in the Linville Creek watershed. A mathematical relationship supplied by the Virginia Department of Environmental Quality was used to translate fecal coliform loads to in-stream E. coli loads. The bacteria TMDL requires an overall fecal coliform load reduction of 96% compared to the existing load.
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Document Type: Research Article

Publication date: 2003-01-01

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