Developing Bacteria TMDLs Using a Modified Load Duration Curve Approach

In developing E. coli TMDLs for four tributaries to the West Fork White River in Indiana, a modified Load Duration Curve (LDC) approach was applied. This approach typically involves the generation of a flow duration curve for each particular sampling site or location and creation of a LDC by taking the product of the flow duration curve and the water quality standard for the parameter of interest. Sampled loads are also plotted as points, with those that fall above the LDC denoted as violations of the water quality standard. Required load reductions are then determined as the difference between the observational data and the LDC.The LDC methodology is a simple and well-tested TMDL approach, and is especially pertinent for nonpoint source dominated watersheds. Agricultural row crop land uses are predominant in all four of the watersheds requiring E. coli TMDLs. While a few scattered point source discharges and some combined sewer overflows also contribute to the E. coli loadings in these watersheds, nonpoint sources are the most significant source categories, providing further justification for the selection of the LDC methodology as the approach for determination of the E. coli TMDLs.Due to its ease of application and affordability, the LDC approach is rapidly becoming a popular method for coliform bacteria TMDLs across the nation. Where deterministic water quality models have achieved limited success in the accurate and consistent quantification of bacteria loadings, the LDC method provides a straightforward, simple approach for assessing these highly unpredictable parameters. Other specific advantages of the method include:The ability to establish loading capacity for the full spectrum of flows in the water body, rather than for a characteristic critical low flow;No requirement to establish a critical storm flow or statistical flow for assessment of wet weather conditions;Quick correlation of loading with flow profiles, which provides insight into the major contributing source categories within a watershed; and,Comprehensibility of results, to stakeholders, regulators, source contributors, and other interested parties.Typical LDC methodologies are applied at a single location within a watershed and assess standards violations over a range of flows at that location. Here, an incremental loading estimation method was used for multiple locations within each watershed. This method incorporates existing monitoring data and assesses land use, soils, and source contribution differences between observation points. The approach utilized gauged flow data where available. In ungauged watersheds, flow duration curve estimations were made using flow information from neighboring gauges, drainage area ratios, and relative composite SCS curve number adjustments.