Pathogens in Urban Stormwater Systems Prepared by Urban Water Resources Research Council
The single most frequent cause of water quality impairment in the U.S. is elevated fecal indicator bacteria (FIB) (EPA 2014). FIB-related impairments can have significant and costly implications for local governments, businesses, and watershed stakeholders due to beach closures and total maximum daily load (TMDL) compliance and implementation requirements to address these impairments. TMDLs and associated municipal separate storm sewer system (MS4) NPDES permit requirements for FIB load reductions pose unique challenges relative to TMDLs for chemical constituents. FIB are living organisms that occur naturally in the environment and whose sources can move freely throughout watersheds and storm drain systems, even when anthropogenic sources of FIB are controlled. Furthermore, FIB are generally not a direct cause of human health impacts; instead, they are easy-to-measure surrogate parameters that are intended to infer that fecal wastes and associated pathogens may be present. Nonetheless, FIB are currently considered to be the best available practical alternative to monitoring for multiple pathogens associated with human and animal wastes. Although the human health risk associated with exposure to waters impacted by untreated or poorly treated human sewage is well documented, the health risk from recreational exposure to elevated FIB in urban runoff-impacted receiving waters is less well known.
The state of the art and practice in modeling transport and fate of FIB (and pathogens) involves significant uncertainty, more so than traditional water quality constituents. This uncertainty carries forward into evaluation of FIB management strategies, development of appropriate wasteload and load allocations for TMDLs, and regulatory decisions. Nonetheless, MS4 owners/operators are often assigned wasteload allocations in urban FIB TMDLs and may face significant wasteload reduction requirements, which are enforceable through MS4 discharge permits. Although management and correction of human sources of FIB (e.g., leaking sanitary infrastructure, illicit connections, dumpster drainage) to storm sewer systems can reduce FIB loads posing human health risk, many MS4s will need to reduce FIB from other sources as well to meet wasteload reduction targets. Identifying the sources of FIB and their relative contributions can be complex and costly. Load reductions are difficult, especially for the natural, non-human FIB sources, for multiple reasons (e.g., ubiquitous nature of FIB, current limits of technology related to urban stormwater controls, magnitude of reductions targeted). For these and other reasons, there are real questions regarding the attainability of FIB water quality standards in urban watersheds and in MS4 discharges. Depending on the sources of FIB affecting a particular receiving water and the manner in which MS4 permit compliance is assessed, dry weather standards may be attainable in some cases, but consistently attaining standards under wet weather conditions may be infeasible.
To support MS4 permit holders and watershed stakeholders in developing realistic goals and effective strategies for addressing pathogens in urban stormwater systems, this report consolidates information on many facets of FIB impairments, providing information on the following topics:
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