February 15, 2004 | General

Best Management Practices For Post-Construction Soils

Mark Musick and Howard Stenn
BioCycle February 2004, Vol. 45, No. 2, p. 29
The 1999 listing of Puget Sound Chinook salmon as a threatened species under the Endangered Species Act sent shock waves through western Washington. This was the first time an endangered species was listed in the heart of a major urban area, and salmon recovery efforts have had major repercussions for planning and development processes in and around Seattle.
Recognition that soils are directly related to water quality, and thus indirectly to salmon recovery, prompted the Washington Department of Ecology (DOE) to include a “Post-Construction Soil Quality and Depth” Best Management Practice (BMP) in the recently revised Stormwater Management Manual for Western Washington. Over the next few years, these BMPs will need to be incorporated into local building regulations under terms of new and updated National Pollution Discharge Elimination System (NPDES) permits.
Washington State is the first in the nation to include soil quality and depth BMPs in its storm water management manual. These new requirements will impact both municipal planning departments and the development industry. To facilitate their widespread utilization, public agencies, research scientists and members of the Washington Organic Recycling Council (WORC) have taken the lead in developing specifications and educational programs for implementing the new BMPs.
There are many reasons why wild salmon were added to the Endangered Species list, but one of the most important is habitat destruction. In rapidly urbanizing areas, the construction of roads, play fields, homes and commercial buildings has severely impacted the quantity and quality of water in streams and rivers. The increase in impervious and compacted surfaces exacerbates soil erosion, winter floods and summer droughts that degrade salmon streams and destroy spawning areas. In addition, more pollutants wash into streams from paved areas, compacted soils and lawns.
Snohomish County solid waste planner Sego Jackson and other members of WORC are credited with clarifying the soil/salmon connection. (See “Organics Play Role in Salmon Recovery in Northwest,” BioCycle, April, 2000). During the 1998 “Salmon in the City” conference, they realized that the scientific presentations on stream degradation as a result of development did not acknowledge the critical role of urban soils in water quality, and the opportunity to restore soil functions with organic amendments.
In the spring of 1999, WORC initiated the “Soils for Salmon” campaign in cooperation with public agencies, researchers and the landscape industry. Later that year, WORC hosted two “Soils for Salmon” seminars that brought together specialists including scientists, landscape professionals, and water quality experts to explore the connections between soils and salmon recovery in the Pacific Northwest, and the use of compost amendments to restore soils damaged by development. As Derek Booth, director of the University of Washington’s Center for Water & Watershed Studies, stated, “There is no doubt that the decline of the Pacific Northwest’s salmon populations should catalyze new techniques for land development and construction.”
The WORC team (including Jackson, Josh Marx of King County, Holly Wescott of DOE, Andy Bary of Washington State University, and David McDonald of Seattle Public Utilities) developed a white paper and newsletter that were used to build awareness among policy makers and planners. McDonald also developed a “Soils for Salmon” professional training course, which has been attended by more than 2,000 landscape professionals, designers, engineers and planners over the past five years.
Building awareness and developing practical methods for soil restoration led to the campaign’s most recent success-incorporating soil BMPs into local and state building and storm water codes.
Largely as a result of WORC’s “Soils for Salmon” initiative, the Department of Ecology included a series of development guidelines and maintenance practices to restore the storm water management functions of soils in the 2002 Stormwater Management Manual for Western Washington. The “Post-Construction Soil Quality and Depth” BMP (BMP T5.13), includes provisions to protect native soils where possible and restore natural functions in disturbed soils by adding organic amendments such as compost, mulching exposed soils, and using organics in erosion control (e.g. compost berms and blankets).
The revised storm water manual officially recognizes the ecological functions of naturally occurring, undisturbed soils and vegetation, including water infiltration and storage, nutrient and sediment adsorption, and pollutant biofiltration. According to the DOE manual, “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 storm water 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.”
Top priority is given to preservation of existing soils. According to the storm water manual, “The duff layer and native topsoil should be retained in an undisturbed state to the maximum extent practicable.” Where grading is required, the design guidelines call for on-site stockpiling and reapplication of duff and native topsoil whenever feasible.
For sites that must be cleared and graded, the DOE guidelines require that all disturbed and compacted soils “shall be amended to mitigate for lost moisture infiltration and moisture holding capacity.” The goal is to restore the landscape to the capacity of “the original undisturbed soil native to the site.”
To reestablish the water holding capacity of disturbed soils, the BMP calls for a minimum 8-inches of topsoil over subsoil scarified to a depth of 4-inches, “with some incorporation of the upper material to avoid stratification, where feasible.” In addition, the topsoil layer should have “a minimum organic matter content of ten percent dry weight and a pH from 6.0 to 8.0 or matching the pH of the original undisturbed soil,” and the final product should be “conducive to the type of vegetation to be established.” Maintenance practices to extend the benefits, including the use of mulches, are recommended to protect exposed soils.
Four options are provided for meeting the new post-construction requirements: 1) Leave native soil and vegetation undisturbed; 2) Amend existing soil in place with compost or other suitable organic matter; 3) Stockpile existing topsoil prior to grading for reapplication and amend if necessary; or 4) Import topsoil with required organic matter content.
To assist both planning departments and the construction industry with implementing the Soil Quality and Depth BMP, in 2001 the Snohomish County Solid Waste Division contracted with Stenn Design, a consulting firm, to coordinate a team of landscape, soils and compost industry professionals to develop tools, specifications and teaching materials. The team included project coordinator Howard Stenn, soil scientist Craig Cogger of Washington State University, landscape architect Jeff Girvin of The Berger Partnership, consultant Jeff Gage of Compost Design Services, plus technical reviewers including other scientists, building inspectors, regulators, and landscape professionals.
Specific tasks included an extensive review of the scientific literature on soil amendment application rates and methods, soil specifications, and field verification methods. Based on this information, the team developed specifications in standard industry formats (American Public Works Association), model permitting procedures, verification methods and payment measures. The proposed specifications and procedures were refined following extensive review by landscape industry professionals, municipal code inspectors, soil scientists, and regulators.
The tools and specifications developed by Snohomish County’s “Soil Amendment Project” are incorporated in a comprehensive 40-page training manual entitled “Guidelines & Resources for Implementing Soil Depth & Quality BMP T5.13.” The manual includes information on the role of soil quality in storm water management, a copy of the Soil Quality and Depth BMP from the Stormwater Management Manual, specifications, model permitting forms, an amendment rate calculator (Excel spreadsheet tool), and a waterproof field guide for inspection and verification procedures.
Based on their review of the literature and consultation with landscape professionals, the project team’s proposed specifications modified the BMP by reducing the organic matter requirement for turf areas from ten percent to five percent. The specifications allow the requirements to be met through a choice of soil amendment options. Project planners can choose a “default” preapproved amendment rate, or they can calculate a custom rate based on specified soil and amendment tests. The specifications describe preapproved and calculated rates for both planting beds and turf areas in scenarios where the native soil is to be amended in place or stockpiled and reapplied, or where topsoils amended with compost are to be imported. The training manual includes a detailed guide to assist professional specifiers with preparing Soil Management Plans to meet the BMP. The primary steps include reviewing site landscape and grading plans, visiting the site to determine preconstruction soil conditions, selecting amendment options, and identifying compost, topsoils and mulch materials. Detailed sampling and testing instructions are provided for calculating custom amendment rates. The manual includes a simple form for a Soil Management Plan that incorporates all the information required to submit for planning department approval.
In addition to planning and permitting, the training manual also covers verification procedures for building inspectors. The inspectors’ “field guide” includes an outline of the primary conditions to be confirmed, a list of site inspection supplies, and a five-step process for verifying compliance with the BMPs. In addition, the manual provides inspectors with a one-page Field Verification Form.
One challenge the project team wrestled with was developing an effective method for inspectors to verify the organic content of soils after amendment. Beyond concerns of cost, time and accuracy of the various tests available, there was the issue of what to do when calculated amendment rates do not produce the desired organic content upon inspection. Whether the deficit is due to variations in soil conditions or amendment quality, uneven incorporation, or imprecise calculations, resolving even minor discrepancies could be costly, time consuming and contentious – possibly resulting in litigation.
To resolve this issue, the project team decided to verify amendment rates by requiring submittal of original delivery tickets for all soil and mulch products. Inspectors will match the delivery tickets to the Soil Management Plans to confirm product types and sources, delivery locations, and total quantities. If materials other than those listed in the Soil Management Plan were delivered, lab test results must be provided to determine if they contain equivalent amounts of organic matter as the products approved in the Soil Management Plan. Incorporation of amendments and scarification of subsoils are checked by digging pits for simple visual examination at specified repetitions per acre and use of a metal rod to check for compaction at numerous locations per site. When an inspector feels that a soil does not meet the approved plan, an independent certified soil scientist or similar professional will be called upon to conduct sampling for organic matter testing at a soil laboratory.
Additional resources provided in the manual include a directory of permitted composting facilities in the Puget Sound region, a list of soil and compost analytical laboratories serving western Washington, a spreadsheet for calculating amendment rates for different soils, and references to current literature on the relationship between compost, soils, and habitat protection.
With the knowledge base established and soils BMPs included for the fist time in the revised Stormwater Management Manual, WORC undertook a year-long campaign to educate public planners and landscape professionals about the new regulations. The Puget Sound Water Quality Action Team provided $44,000 in funding, and WORC conducted seven workshops between April 2002 and May 2003 to encourage the incorporation of the new soil BMPs into project planning, budgeting and implementation.
The workshop series, entitled “Stormwater Strategies: Soil Protection and Restoration,” was developed in recognition of the fact that the new soil management strategies will succeed only if both regulators and development professionals understand how to implement the BMPs. The training sessions reviewed the relationship between storm water and soils, introduced the new post- construction soil quality and depth BMPs, and provided information on writing, implementing and evaluating Soil Management Plans. Participants were given “hands on” opportunities to study compost and soil profile samples, and “walked through” examples of how to calculate the amount of organic materials needed to attain minimum organic content percentages.
The one-day workshops attracted a total of 226 participants, including public works project managers, developers, landscape architects and landscape contractors, surface water engineers, grading and building inspectors, specifiers, and compost manufacturers. Howard Stenn, Michael Broili and Peter Moon comprised the core trainer team. Ed O’Brien of DOE’s Water Quality Program attended all seven workshops to introduce the reasoning behind the inclusion of the soils BMPs in the storm water manual. He also came to answer questions and listen to industry concerns about the need for offset incentives for implementation. In response, a committee is working to determine how to credit soil enhancement by reducing requirements (e.g. expenses) for on-site storm water detention and conveyance infrastructure.
For developers, the most cost-effective way to offset the costs of the soil quality requirements will be to stop stripping existing topsoil and vegetation off of construction sites. Eventually, this should encourage greater clustering of development, the preservation of native vegetation and soils, and the reduction of impervious surfaces. By encouraging on-site infiltration, the increased costs will also be offset by savings on reduced storm water infrastructure, including smaller pipes and detention ponds.
Increased soil quality and depth will also contribute to more vigorous, healthier plants, which will result in fewer callbacks for landscape contractors. Homeowners will benefit from reduced costs for irrigation, fertilizers and pesticides. And of course salmon (and all other species – including humans) will benefit from increased water quality.
With the success of its initial workshop series, WORC plans to adapt its presentation materials for varying audiences. In the future, it hopes to provide a wide range of educational programs for municipalities, industry groups and professional associations. Protecting and enhancing soils with recycled organic materials is gaining recognition as an important low impact development strategy as a result of WORC’s work.
Howard Stenn is with Stenn Design in Vashon, Washington. Mark Musick is a Contributing Editor to BioCycle.

Documents referenced in this article are available via the following sites: Stormwater Management Manual for Western Washington, Washington State Department of Ecology Water Quality Program, August 2001, Volume V – Runoff Treatment BMPs,
9915.pdf, pages 5-12, 5-13, or pages 101-102 of that PDF file. Guidelines & Resources for Implementing Soil Depth & Quality BMP T5.13,

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