Posted on

EXH AR024642: Memo Alternative Dilution Modeling Procedures Storm Water Outfalls

Document Library
Sea-Tac Airport
Third Runway
PCHB
Box 10

A 1999 internal technical memo from Parametrix, Inc. evaluates three alternative water-quality dilution modeling methods for assessing whether proposed stormwater outfalls at the Port of Seattle could exceed Washington State water quality standards. The methods range from a static single-event approach based on Ecology's Permit Writer's Manual to a dynamic continuous simulation using HSPF hydrologic models and a probabilistic EPA-funded model developed by researcher Dominic Di Toro. The memo recommends continuing with the continuous simulation approach while considering budget, Ecology's involvement, and the nature of ambient and effluent pollutant concentration data.

Open full document

Notes

Memo dated 8/10/99 from Parametrix, Inc. to Linda Logan (cc: Brian Pippen, Ken Ludwa, Paul Fendt) reviewing alternative dilution modeling procedures for evaluating reasonable potential for exceedance of State water quality standards (WQS) at proposed Port Of Seattle stormwater outfalls. Identifies three methods: (1) static single-event method based on Ecology Permit Writer’s Manual (July 1998) using 95th percentile effluent pollutant concentration and HSPF/ANNIE probability-exceedence relationships; (2) dynamic continuous simulation using HSPF hourly hydrologic results for one-hour acute and 96-hour chronic dilution factor (DF) boundaries per WAC volumetric limitations; (3) EPA-funded probabilistic model per Di Toro (ASCE Journal of Environmental Engineering, Vol. 110, June 1984) implemented by Versar Environmental Risk Management. Discusses NPDES permit compliance, WAC, ambient background concentration, joint probability distributions, and considerations including Ecology involvement, budget, and client expectations. AR 024642–AR 024644.

Sidebar On/Off
V V