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Stream Stormwater TMDL Development in Maine using Stressor Identification and Impervious Cover Analysis

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How do you restore an urban stream impaired by nonpoint sources using the TMDL framework? During the 1990's Maine DEP began placing small urban streams on the 303(d) list that did not meet Maine's tiered aquatic life standards. These impairments were primarily due to 'urban runoff'. TMDL development requires a detailed description of the impairments and that means clarifying the source of the impairment. In order to determine the best method to achieve water quality standards, Maine selected 4 urban streams for intense data collection and subsequent analysis using EPA's Stressor Identification Process (SI). Ideally, the SI process would narrow down causal agents and identify a probable TMDL model to solve water quality problems. The SI process confirmed that the observed impairments resulted from many stressors, with stormwater as the common denominator. Multiple stressors do not fit well into the traditional TMDL loading analysis. Concurrent with Maine's efforts, EPA Region 1 engaged in a group process with the states to identify innovative TMDL methods to step up regional production of TMDLs. Stormwater emerged as a priority cause of impairment and therefore resulted in the development of the 'Impervious Cover Method'(IC), by ENSR, which defined a relatively simple TMDL analysis using impervious cover as a surrogate for stormwater impacts. Maine choose the IC approach for TMDL development since it offered a logical segue from the conclusions of the Stressor Identification Process. To apply this approach, Maine developed a specific policy that connected impervious targets to Maine's tiered water quality standards based on macroinvertebrate sampling results. The resulting TMDL defines a %IC targets and identifies the pollutant load as the existing %IC in the watershed. The wasteload and load allocations are defined as the %IC reductions needed to achieve the target, but attainment of water quality standards are the ultimate goal. The %IC reductions also serve as a restoration concept for the adaptive implementation of Best Management Practices (BMPs) within the watershed, to reduce stormwater impacts. The reaction from the Municipal Separate Storm Sewer System (MS4) communities was mixed; some acknowledged the approach as reasonable, while others took the reduction in impervious cover literally and considered the goals unreasonable. EPA, Region 1 encouraged Maine to pursue %IC TMDL approach, but EPA's technical and legal review has resulted in multiple revisions and delays. Final approval is still pending, but EPA is committed to resolving technical difficulties and approval is likely. Will this convoluted TMDL process answer the questions on restoration and attainment of water quality standards? Current community efforts developing watershed management plans are promising, but aquatic biological outcomes are difficult to predict.


Document Type: Research Article


Publication date: 2007-10-01

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