2122 106TH ST SW 2025-03-12 - Laserfiche WebLink

o Where replacing existing impervious surfaces unless the existing surface is a non -pollution generating surface over an outwash soil with a saturated hydraulic conductivity of four inches per hour or greater. o At sites defined as "high use sites" in Volume I of this manual. o In areas with "industrial activity" as identified in 40 CFR 122.26(b)(14). • Where the risk of concentrated pollutant spills is more likely such as gas stations, truck stops, and industrial chemical storage sites. o Where routine, heavy applications of sand occur in frequent snow zones to maintain traction during weeks of snow and ice accumulation. A local government may designate geographic areas within which permeable pavement, or certain types of permeable pavement, may be designated as infeasible due to year-round, seasonal or periodic high groundwater conditions, or due to inadequate infiltration rates. Designations must be based upon a preponderance of field data, collected within the area of concern, that indicate a high likelihood of failure to achieve the minimum groundwater clearance or infiltration rates identified in the above infeasibility criteria. The local government must develop a technical report, and make it available upon request by the Dept. of Ecology. The technical report must be authored by (a) professional(s) with appropriate expertise (e.g., registered engineer, geologist, hydrogeologist, or certified soil scientist), and document the location and pertinent values/observations of data that were used to recommend the designation and boundaries for the geographic area. The types of pertinent data include, but are not limited to: Standing water heights or evidence of recent saturated conditions in observation wells, test pits, test holes, and well logs. Observations of areal extent and time of surface ponding, including local government or professional observations of high water tables, frequent or long durations of standing water, springs, wetlands, and/or frequent flooding. Results of infiltration tests In addition, a local government can map areas that meet a specific infeasibility criterion listed above provided they have an adequate data basis. Criteria that are most amenable to mapping are: • Where land for bioretention is within an area designated by the local government as an erosion hazard, or landslide hazard • Within 50 feet from the top of slopes that are greater than 20% and over 10 feet vertical relief • Within 100 feet of a closed or active landfill Volume V — Runoff Treatment BMPs — December 2014 5-21