INCORPORATING URBAN WET WEATHER SOURCES IN A TMDL: AN IMPROVED APPROACH

Many stakeholders have requested additional guidance on incorporating stormwater and other urban wet weather sources into Total Maximum Daily Loads (TMDLs). Most states reported to the US Government Accounting Office (GAO) that they did not have adequate data on wet weather point sources to adequately estimate wet weather loadings, and others have expressed the belief that urban waterbodies are unable to attain water quality standards during wet weather events.This paper presents a portion of the research results from a Water Environment Research Foundation's (WERF) TMDL Evaluation and Design project. This research project covered multiple issues; the findings presented in this paper focus on the role of urban wet weather sources in TMDLs and recommend an improved process to addressing urban wet weather sources within TMDLs.Using a broad-based review of nearly 200 approved TMDLs, combined with interviews of state TMDL staff, and detailed study of selected case studies, the research team developed several key findings related to wet weather sources in TMDLs. The paper provides recommendations for possible improvements for states, dischargers, and other stakeholders to utilize when confronting a wet weather TMDL. While developed for urban wet weather issues, many of these conclusions will also be appropriate for rural sources.This paper presents a case for the development of separate and distinct event-related wet weather TMDLs independent of dry weather TMDLs. In many cases, these wet weather TMDLs will have different sources, and often, different water quality endpoints. Reasons and advantages for this approach include:Many uses are often impaired and water quality standards are often exceeded only transiently during wet weather eventsThe process of analyzing loads and conditions in wet weather is typically different in wet weather than in dry weather, especially for transient impacts like bacteria and dissolved oxygen.From a program and regulatory standpoint, Combined Sewer Overflow (CSO), Sanitary Sewer Overflow (SSO), and Stormwater programs are handled differently than wastewater National Pollutant Discharge Elimination System (NPDES) discharges, and hence TMDLs should be addressed differently also.It is expected that improvements to a wet weather TMDL approach may also require revised uses and criteria for wet weather TMDLs. Other consequences of this approach will also be detailed.