DERIVING APPROPRIATE NUTRIENT TARGET FOR URBAN STREAMS IN SUPPORT OF TMDL DEVELOPMENT
Abstract:A technically-sound nutrient target to establish a TMDL endpoint is a pressing need, but deriving nutrient target has proven to be challenging. It is even more difficult to set a nutrient target for urban streams, since EPA's recommended eco-region value is often unrealistically low, given the fact that it is derived from both urban and non-urban watersheds in a specific eco-region. For urban watersheds, especially in rapid growth areas, setting nutrient target for an impaired waterbody may become contentious. Dischargers and land development companies generally like to have consistent permit limits. A balance between protection of ambient water quality and designated use and cost of more stringent nutrient controls for WWTPs and urban runoff results from an appropriate nutrient target for an urban stream during TMDL development. However, EPA guidance has not provided an approach to derive a numerical target for urban streams with nutrient impairment. Furthermore, in the existing EPA technical manual, there are no established causal relationships between nutrients and response variables, such as chlorophyll a and periphyton, which makes it difficult to implement nutrient management strategies for waterbodies with nuisance algae problems. Therefore, the objectives of this paper are to introduce practical methodologies for deriving nutrient targets for urban streams. Applicability of each method, namely, percentile method, breakpoint approach and nutrient-periphyton analysis, was compared in setting TMDL endpoints for a nutrient TMDL in impaired stream from a fast-growing urban area. A weight of evidence approach was applied to select the final TMDL endpoint from the range of numerical criteria derived from these methods.
Document Type: Research Article
Publication date: 2005-01-01
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