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4 SECTION TWO BMP T5.13 “POST CONSTRUCTION SOIL QUALITY AND DEPTH” IN THE STORMWATER MANAGEMENT MANUAL FOR WESTERN WASHINGTON Excerpted from the Washington State Department of Ecology’s Stormwater Management Manual for Western Washington, Vol. V: Runoff Treatment BMPs, Chapter 5, pages 5-8 to 5-11 (or pages 105 to 108 in the online PDF file) as revised December 2014. ”BMP” means “Best Management Practice”, a term used for techniques that are recommended or (in this case) required. The Manual can be found online at www.ecy.wa.gov/programs/wq/stormwater/manual.html Purpose and Definition Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions including: water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltration; water interflow storage and transmission; and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal topsoil and sod. Not only are these important stormwater functions lost, but such landscapes themselves become pollution generating pervious surfaces due to increased use of pesticides, fertilizers and other landscaping and household/industrial chemicals, the concentration of pet wastes, and pollutants that accompany roadside litter. Establishing soil quality and depth regains greater stormwater functions in the post development landscape, provides increased treatment of pollutants and sediments that result from development and habitation, and minimizes the need for some landscaping chemicals, thus reducing pollution through prevention. Applications and Limitations Establishing a minimum soil quality and depth is not the same as preservation of naturally occurring soil and vegetation. However, establishing a minimum soil quality and depth will provide improved on-site management of stormwater flow and water quality. Soil organic matter can be attained through numerous materials such as compost, composted woody material, biosolids, and forest product residuals. It is important that the materials used to meet the soil quality and depth BMP be appropriate and beneficial to the plant cover to be established. Likewise, it is important that imported topsoils improve soil conditions and do not have an excessive percent of clay fines. This BMP can be considered infeasible on till soil slopes greater than 33 percent. Design Guidelines Soil retention. Retain, in an undisturbed state, the duff layer and native topsoil to the maximum extent practicable. In any areas requiring grading remove and stockpile the duff layer and topsoil on site in a designated, controlled area, not adjacent to public resources and critical areas, to be reapplied to other portions of the site where feasible. Soil quality. All areas subject to clearing and grading that have not been covered by impervious surface, incorporated into a drainage facility or engineered as structural fill or slope shall, at project completion, demonstrate the following: 1) A topsoil layer with a minimum organic matter content of 10% dry weight in planting beds, and 5% organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the undisturbed soil. The topsoil layer shall have a minimum depth of eight inches except where tree roots limit the depth of incorporation of amendments needed to meet the criteria. Subsoils below the topsoil layer should be scarified at least 4 inches with some incorporation of the upper material to avoid stratified layers, where feasible. 2) Mulch planting beds with 2 inches of organic material. 3) Use compost and other materials that meet these organic content requirements: a) The organic content for “pre-approved” amendment rates can be met only using compost meeting the compost specification for Bioretention (BMP T7.30), with the exception that the compost may have up to 35% biosolids or manure. The compost must also have an organic matter content of 40% to 65%, and a carbon to nitrogen ratio below 25:1. The carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region. b) Calculated amendment rates may be met through use of composted material meeting above; or other organic materials amended to meet the carbon to nitrogen ratio requirements, and not exceeding the contaminant limits identified in Table 220-B, Testing Parameters, in WAC 173- 350-220. The resulting soil should be conducive to the type of vegetation to be established. Editor’s note: “other organic materials” includes other composts not meeting the stringent specification in BMP T5.30, which is designed for bioretention swales. Any mature, stable compost is appropriate for general soil amendment. ---PAGE BREAK--- BMP T5.13 “Post Construction Soil Quality and Depth” – SECTION TWO 5 Implementation Options. The soil quality design guidelines listed above can be met by using one of the methods listed below: 1) Leave undisturbed native vegetation and soil, and protect from compaction during construction. 2) Amend existing site topsoil or subsoil either at default “pre-approved” rates, or at custom calculated rates based on tests of the soil and amendment. 3) Stockpile existing topsoil during grading, and replace it prior to planting. Stockpiled topsoil must also be amended if needed to meet the organic matter or depth requirements, either at a default “pre-approved” rate or at a custom calculated rate. 4) Import topsoil mix of sufficient organic content and depth to meet the requirements. More than one method may be used on different portions of the same site. Soil that already meets the depth and organic matter quality standards, and is not compacted, does not need to be amended. Planning/Permitting/Inspection/Verification Guidelines & Procedures Local governments are encouraged to adopt guidelines and procedures similar to those recommended in Guidelines and Resources For Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual for Western Washington. This document is available at: http://www.soilsforsalmon.org/pdf/Soil_BMP_Manual.pdf Maintenance Establish soil quality and depth toward the end of construction and once established, protect from compaction, such as from large machinery use, and from erosion. Plant vegetation and mulch the amended soil area after installation. Leave plant debris or its equivalent on the soil surface to replenish organic matter. Reduce and adjust, where possible, the use of irrigation, fertilizers, herbicides and pesticides, rather than continuing to implement formerly established practices. Runoff Model Representation Areas meeting the design guidelines may be entered into approved runoff models as “Pasture” rather than “Lawn.” Flow reduction credits can be taken in runoff modeling when BMP T5.13 is used as part of a dispersion design under the conditions described in: BMP T5.10B Downspout Dispersion BMP T5.11 Concentrated Flow Dispersion BMP T5.12 Sheet Flow Dispersion BMP T5.18 Reverse Slope Sidewalks BMP T5.30 Full Dispersion (for public road projects) Figure 5.3.3 – Planting bed Cross-Section (Reprinted from Guidelines and Resources For Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual for Western Washington, 2010, Washington Organic Recycling Council) Related BMP’s in the same volume (Vol. V, Ch. 5) of the Stormwater Management Manual for Western Washington available online at www.ecy.wa.gov/programs/wq/stormwater/manual.html BMP T5.40 Preserving Natural Vegetation BMP T5.41 Better Site Design See also Chapters 7 and 9 in Volume V on Infiltration and Biofiltration/Bioretention BMPs and see Volume III, Appendix C “Low Impact Development Flow Modeling Guidance”