10200 19TH AVE SE COSTCO FUEL STATION 2016-01-01 MF Import

of time that sto�:nwzter spends in contact with vegetation - and hence, will reduce the removal efFicie�cy of the facilities. Ttie overall result of the proposed site modifications will be an i�crease in phosphorus �oading leaving the stormwater facilities. This is not acceptable. �i he storm��vater mcnitoring rPport shows that the amount of phosphorus leaving the site is already significanely yreater than what v,�as predicted during the EIS process. Requirements for modifications to stormN,ater facilities: The original stermwater facilities were designed and constructed to result in a stated loading of phosphorus leaving the stormwater facilities. Modified site conditions (including phosphorus loading and modified stormwater facilities) must result in the same or reduced predicted loading of phosphcirus dcwnstream of the starmwater facilities. If phosphar�s loading will increase as the result of the proposed parking Ict modifications and gas station, then the phosphorus rerr�oval capucities of the stormwater treatment facilities must also be increased. If the hydraulic residance time of stormwater in vc�getated portions of the stormwater facilities must be c!ecreased to meet the site plar goals, then an alternate treatment strategy that will increase phosphorus removal through some ether mechanism will be required instead. Steps to take: 1) Provide evidence showirg that the samples taken from the sand filter sampling II portal were indeed contaminated with high phosohorus containing sediments or include these sample concentra��ons in the pFusph�rus loading analysis for the site: a. Provide the total suspended solids concentrations of the samples that were deemed "contaminated" b. Sample sediment in the sand filter portal to determine its phosphorus ccmtent 2) Determine the source of the add�tional phosphorus found in the sand filter sampling portal (idea: conduct leach studies of soils found in fill bank along north side of swale). 3) Calculate phosphorus loading bcsed or� modified site conditions. Take into accourt any sources of phosphorus �iscovered in step 1. 4) Design source controls and stormwater treatment facilities to reduce the phosphorus loading leaving the stormwater facilities tc the range given in the EIS (from 1.41 to 3.35 pounds, with an average of 2.2 pounds). 5 I