Assessing Long-Term Pollutant Concentration Trends in Urban Runoff for TMDL Planning and Regulatory Assessmentby L. Donald Duke,
Document Type: Proceeding Paper
Part of: World Water & Environmental Resources Congress 2003
Loads of some pollutants in many urban watersheds include a substantial component originating with non-point sources and stormwater runoff. TMDL planning includes estimating those loads, and available methods provide reasonable accuracy either using land use loading factors or multi-year averages in measured concentrations. However, when TMDLs or NPDES related regulations require pollutant load reduction, available data and methods are much less successful at identifying temporal trends to verify effects of municipal pollutant reduction programs. Data available for many municipal stormwater discharges are limited to NPDES requirements, commonly concentration measured for a few storms per year. Trends are masked by substantial natural variation in concentration caused by varying storm characteristics, source activities, and hydrogeological conditions, and the highly irregular statistical nature of the variation. Statistical identification of trends may require decades of monitoring data to detect even large-scale changes.
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