Bacteria TMDLs for Lakes: A Proposed Methodology to a Challenging Endeavor

Iowa, like many states, has seen its 303(d) list grow rapidly in recent years with increased monitoring and assessment data. The number of lakes for which primary contact recreation is impaired due to high levels of bacteria accounts for a substantial portion of the recent additions to Iowa's 303(d) list. The development of bacteria TMDLs for lakes requires answering at least four challenging questions: (1) What is the critical volume and/or location in the lake for water quality compliance, (2) what critical conditions, hydrological or otherwise, must be considered when developing the TMDL, (3) how will bacteria source loads and in-lake bacteria dynamics be modeled, and (4) what are the implications of high variability and high overall bacteria levels for TMDL implementation? This paper and presentation examine these and other challenges to developing bacteria TMDLs for lakes. An approach utilized by the Iowa Department of Natural Resources (IDNR) for Lake Geode, a 174-acre lake located in Henry and Des Moines Counties in southeast Iowa, serves as the case study.