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

Steqs to take: 1) Delermine the dead storage, live storage, hydraulic residence time and vegetative cover/characteristics of the portion of the wetiand to be filled. 2) Design a new mitigation wetland that provides comparable increases to all N�ater quality function characteristics in addition to the quantified characteristics of the originally designed mitigation �vetland. Storm Drainane Facilities: Existing monitoring/ana{yses: As part of the EIS proc�ss, the amount of phosphorus that wauld leave the stormwater facilities during a yearlong period was determined. The amount of phosphorus that leaves the stormwater facilities is a function of the quantity of phosphorus entering the stormwater facilities and the phosphorus r2moval efficiency of the stormwater facilities. The Costco Storm Drainage Monitoring Final Report, dated December 1997, showed that, with a confidence level of 95%, 5.40 +/- 1.7 pounds of phosphorus left the stormwater facilities during the monitoring year. The analysis done during the EIS process predicted that a range of 1.41 to 3.35 pounds of phosphorus would leave the stormwater facilities, with an average of 2.2 pounds. The report primarily attributes the larger quantities of phosphorus leaving the stormwater facilities to above-normal rainfall during the monitoring period. The report states "rainfall has a reiatively high concentration of phosphorus compared to treated storm�vater draining from the site (Costco Storm Drainage Monitoring Final Report, „ December 1997). And it is true that atmospheric deposition is a source of phosphorus. However, the tota/amount of phcsohorus that would be deposited due to atmospheric conditions wouid not increase dur tn increased rainfall. Rainfall itself is not the source. The source is particles suspended iri the atmosphere. These particles are either entrained in raindrops as they fall, or they are washed off surfaces where they have been deposited durin� dry periods. Increased rainfall would result in an increased volume of more dilute concentrations of phosphorus — not an increase in the total quantity of phosphorus. In addition, some samples with relatively high concentrations of phosphorus were not included in the loading analysis. Flad these samples been included, the calculated amount of phosphorus leaving the site would have been much greater. The samples in question were taken from the sampling portal within the sand filter. The report says that these samples were contaminated by sediments, and were therefore not indicative of the actual phosphorus concentrations in the water. However, total suspended solids 3