Setting of Total Maximum Daily Loads for an Urban Water Body
The State of Oregon has established water quality standards for meeting the goals and requirements of the United States Clean Water Act. The normal model for standards is followed in Oregon where the standard is composed of designated beneficial uses and water quality criteria or action levels which are set to maintain these beneficial uses. Section 303 of the Clean Water Act requires that water bodies that fail to meet water quality, usually through non attainment of beneficial uses and measured through chronic violation of water quality criteria such as temperature, chlorophyll a (as an indicator for excessive algae growth) and dissolved oxygen, be identified as “water quality limited.” When this water quality limited designation is in place for a water body, the state must establish a total maximum daily load (TMDL). The Columbia Slough in Portland, Oregon, been designated as water quality limited.
The Columbia Slough is an urban water body (see Figure 21.1) that is significant to the metropolitan Portland community, because of large undeveloped reaches and potential as a valuable fish and wildlife habitat within an urbanized area. The area also contains significant commercial and industrial facilities such as the Portland International Airport. Like most water bodies, the Columbia Slough is usually thought of as a single entity and usually studied with catch-all solutions for the whole water body. The Slough, however, is more appropriately classified into several distinct reaches with unique hydraulic regimes, pollutant sources, and water quality conditions. This classification of the Slough into reaches for TMDL-setting facilitated the analysis of water quality problems and solutions. The classification into reaches did not, however, deter the consideration of watershed effects and in fact improved the chances of implementing more comprehensive watershed-wide plans.
The City of Portland and other affected municipalities and the Oregon Department of Environmental Quality have worked cooperatively to determine appropriate TMDLs for a number of pollutants that are degrading water quality. This group formed the “Core Team” that effectively came together and produced a rational plan to improve water quality of the Columbia Slough within the real and regulatory constraints imposed.
The many processes followed by the Core Team to determine appropriate TMDLs are discussed below. This includes the development of the overall TMDL process and designation and acceptance of the possible forms of TMDLS. The studies (CH2M HILL, 1995) performed included development of databases for water quality and other data management and analysis, estimation of stormwater and other pollutant loadings using SWMM and other techniques, and instream water quality modelling using CE-QUAL-W2 (Wells et al., 1995).
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