Excessive sedimentation is a leading cause of aquatic life use impairments in Virginia. As required by the Clean Water Act, a total maximum daily load (TMDL) must be developed for impaired waters. When developing a TMDL for aquatic life use impairment where sediment has been identified as the primary pollutant, the target sediment load is often determined using a non-impaired reference watershed, i.e., the reference watershed approach (RWA). The RWA has historically been used in Virginia to establish TMDL target sediment loads because there is no numeric ambient water quality criterion for sediment. The difference between the sediment load generated by the reference watershed and the load generated by the impaired watershed is used to determine the sediment load reduction required to meet the TMDL target load in the impaired watershed. Recent quantification of the Chesapeake Bay TMDL based on Phase 5.3 of the Chesapeake Bay Watershed Model (CBWM) offers a simpler and potentially more consistent method of calculating target sediment loads for impaired watersheds within the Chesapeake Bay watershed. Researchers in the Biological Systems Engineering department at Virginia Tech have developed the "disaggregate method" (DM) which uses landuse inputs to, and pollutant load outputs from, the CBWM to determine pollutant load reductions needed in watersheds whose areas are smaller than the smallest modeling segments generally used in the CBWM. The DM uses landuse-specific unit area loads from two CBWM model runs (an existing condition run and TMDL target load run) and a finer-scale, locally assessed landuse inventory to determine sediment loads. The DM is simpler and potential more consistent than the reference watershed approach. This study compared the reference watershed approach and the disaggregate method in terms of required sediment load reduction. Three sediment-impaired watersheds (Long Meadow Run, Taylor Creek and Turley Creek) within the Chesapeake Bay watershed were used for the study. Study results showed that the TMDL development method used to determine sediment loads would have noticeable effects on resulting sediment-load reduction requirements. For Taylor Creek, the RWA required 20.4 times greater reductions in sediment load (tons/yr) when compared to the DM. The RWA also required 9.2 and 10.4 times greater reductions for Turley Creek and Long Meadow Run watersheds, respectively. On a percentage basis, the RWA for reduction Taylor Creek was 7.3 times greater than that called for by the DM. The RWA called for 4.4 and 4.6 times greater percent reductions for Turley Creek and Long Meadow Run watersheds, respectively. An ancillary objective of this research was to compare the sediment load reductions required for the impaired and their respective RWA-reference watersheds, using the DM. This comparison revealed that, both Taylor Creek and Turley Creek watersheds required less sediment load reduction than their respective reference watersheds, while the load reductions required for Long Meadow Run were slightly greater than its reference watershed. There are several issues associated with either the RWA or the DM for developing sediment TMDLs. Those issues are discussed in detail. Recommendations the need for further studies, based in questions raised by the research presented here are also discussed. / Master of Science

This research addresses the Reference Watershed Approach (RWA) in the TMDL process for benthic impairments. In the RWA, do different land use sources (DOQQ and NLCD) or use of alternative water quality models (GWLF and SWAT) result in different stressor loadings? Is there a difference in stressor loadings when different reference watersheds are used? Study results showed that using different land use sources resulted in required stressor reductions that were different by greater than 10%. In one scenario, use of the NLCD-based land use parameters results in 3.5 times greater reductions than use of DOQQ-based land use parameters. With respect to water quality model selection, in two of the three scenarios considered, a difference in stressor reduction requirements of greater than 10% resulted from using different models. Differences in load reduction requirements are also seen when different reference watersheds are used, regardless of the water quality model or the land use source used. Different references result in a difference of as much as 73% in required sediment reductions in the impaired watershed: the required reductions using one reference watershed are 6.2 times as great as when another is used. Possible alternatives to the RWA include water quality standards to set the target level for many of the common stressors on the benthic assemblage, regression equations that relate benthic stressors to the RBP II score, or averaging of stressor reduction requirements obtained from using the Reference Watershed Approach on several different reference watersheds. / Master of Science