A New Approach to Sediment TMDL Watersheds in the Southern Piedmont

Non-Technical Summary: Many streams in the southern United States turn a muddy brown during storms due to high levels of suspended sediment. Where this sediment comes from is not clear. Traditionally, it has been assumed that the sediment comes from eroding farm fields, construction sites, forest sites where timber is being cut, and/or unpaved roads. However, the real source may be erosion from stream banks where sediment was deposited for more than a century of cotton farming that dates back to the Civil War era. This historic or legacy sediment eroded from old cotton fields at tremendous rates due to the poor farming practices of the period and the nature of the soil that makes it susceptible to erosion. This project will use the North Fork Broad River, a sediment-impaired stream in Georgia, as a case study for the new approach. The study will use a new technique called sediment fingerprinting to determine the source of the stream sediment. The chemical compound, cesium-137, was dispersed in the atmosphere during the nuclear bomb tests of the 1950s and 1960s and subsequently deposited as fallout on the earth's surface. This compound adsorbed tightly to clay particles and didn't move deeper in the soil. Since there are no natural sources of cesium-137 in the environment, it serves as a unique tracer for erosion of surface soils. If the suspended sediment in streams is low in cesium-137, then it indicates that the major source is sediment that has not been at the surface in the last 50 years or the buried legacy sediments eroded from stream banks. <P> Approach: Many streams in the southern Piedmont region are impaired because of high concentrations of suspended sediment and scheduled for development of Total Maximum Daily Load (TMDL) implementation plans. For these waters, it is not clear if the source is upland erosion from agricultural sources or bank erosion of historic sediment deposited in the flood plains during the 19th and 20th century when cotton farming was extensive. If the TMDL implementation plan addresses the wrong source, stakeholders will become disillusioned with the TMDL process. A new approach is needed to determine if bank erosion of legacy sediment is a significant source and how to implement sediment TMDLs in streams that are in an unstable stage of channel evolution. We propose using the North Fork Broad River, a sediment-impaired watershed in Georgia, as a case study for the new approach. Our hypothesis is that the North Fork Broad River is in an unstable stage and that bank erosion of legacy sediment is a major source of the current, high sediment load. We will use sediment fingerprinting (Cs-137 and Be-7, heavy metals, base cations, C, N, S and P) and mixing models to identify the primary sources of erosion. Our preliminary results indicate that Cs-137 can be used to distinguish upland sources of erosion from bank erosion of legacy sediment and that bank erosion is the dominant source of the sediment in the North Fork Broad River. We will use geomorphic assessment (visual ratings at fixed channel intervals) and computer modeling (AGNPS combined with CONCEPT) of channel evolution to determine if the stream is stable or unstable and the best approach to reduce sediment load. Our outreach and education component will promote this approach to stakeholders within the watershed and TMDL specialists, educators, and students throughout the southern region. The priority question we address is what are the hydrologic and geomorphic conditions needed to restore aquatic ecosystems impacted by sediment pollution.