Notes
Exhibit 238A (AR 018968–018970): Letter dated September 7, 2000 from Northwest Hydraulic Consultants Inc. (Seattle office) to Ray Hellwig, Regional Director, Washington State Department of Ecology Northwest Regional Office, regarding 401 Certification for Seattle-Tacoma International Airport Master Plan Update (MPU). Reviews the August 2000 Preliminary Comprehensive Stormwater Management Plan (SMP) across four volumes. Identifies discrepancies in HSPF hydrologic model parameters for fill embankment baseflow analysis (Appendix F), finding groundwater plus interflow of ~44% versus 63.6% reported; predicts average groundwater reductions to Miller Creek from subbasins SDW1A and SDW1B of 0.10 cfs (July) and 0.09 cfs (August). Challenges SMP conclusions on water rights acquisition on Miller Creek (Appendix G), domestic withdrawal estimates of 0.01 cfs per user (6,460 gallons/day), irrigation return flows, septic system elimination. Raises concerns about cumulative impacts from future commercial development in acquisition area, inconsistencies between SMP and Natural Resource Mitigation Plan (100-year pre-development discharge at SR-509: 198 cfs vs. 71 cfs; detention volume 42 vs. 100 acre-ft for Miller Creek), preservation of existing basin boundaries, Walker Creek headwaters (Wetlands 44a and 44b, basin SDS7, Des Moines Creek diversion), and Industrial Wastewater System diversions. Filed on behalf of Airport Communities Coalition (ACC) at PCHB Third Runway proceedings.