Estimating TMDL Background Loading From Existing Data
Case Western Reserve University
Basin Program Funds:
In Ohio, 3,563 miles of stream are impaired by sediment, according to the U.S. Environmental Protection Agency (U.S. EPA). This is the eighth-highest reported total of the 50 states. For reaches impaired by sediment, the state of Ohio and other states in the basin are developing TMDLs for this parameter. The estimation of the background or natural loading of sediment to waterways in TMDL process is critical. Yet there are not universally accepted approaches for such estimations.
The TMDL for sediment is the maximum quantity of suspended sediment that can enter the waterway without affecting the beneficial uses of the waterway. It is calculated as the sum of all allotments of point source suspended sediment, all allotments of nonpoint sources, background or natural sources of sediment, and a margin of safety. There are several methods available to calculate the background levels, but most present difficulties. Determining the loading from a pristine site could be used but there are few large basins suitable for such a comparison. Scaling up from smaller basins to large basins is possible, but the delivery ratio varies from near 1.0 for certain small basins to 0.1 for large basins. Such field studies also can be prohibitively expensive and time-intensive. Calculations of expected background loading of sediment can be based upon the Universal Soil Loss Equation, assuming an undisturbed basin, but this model is used for determining erosion on the land surface, not determining fluxes of sediment through waterways.
One approach that has merit is to use extant data to determine suspended sediment yields from basins in Ohio and other Great Lakes states and data on land use in these basins to derive a relationship for sediment loading as a function of the level of disturbance and basin size.
This project developed suspended sediment rating curves for 104 drainage areas with U.S. Geological Survey gauging stations; calculated the amount of forest cover in each of the 104 watersheds; developed the relationships between sediment loading, drainage area and percent forest cover; and developed the relationship between forest cover percent and sediment loads. A model was completed.
Background loadings of suspended sediment can be estimated from existing data on sediment yield as a function of watershed area. The approach is broadly applicable to streams in Ohio and the Great Lakes region and transferable elsewhere. The approach is efficient since it relies on already collected and evaluated data. Furthermore, the calculation of sediment load in development of the method is useful in the TMDL process in and of itself because determination of present loading relative to TMDL’s is part of the process in deciding allotments for point and nonpoint source pollutants. Finally, the overall approach can be generalized for the determination of background loading for other parameters as well. More details of the requirements of a TMDL are described in 40 CFR 130.2 and 130.7 and Section 303(d) of the Clean Water Act (CWA), as well as U.S. EPA Pollution Prevention Act (1991).
Contact: Dr. Peter J. Whiting, 216-368-3989