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.