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Module 6: Specified Bioretention Soil & Mulch

Module 6: Specified Bioretention Soil & Mulch. Melanie Mills, MLA Central Coast LIDI. Bioretention Soil Media (BSM). A key element contributing to facility function One of the most challenging design elements. Characteristics of BSM. Key for stormwater objectives and plant health

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Module 6: Specified Bioretention Soil & Mulch

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  1. Module 6: Specified Bioretention Soil & Mulch Melanie Mills, MLA Central Coast LIDI

  2. Bioretention Soil Media (BSM) A key element contributing to facility function One of the most challenging design elements

  3. Characteristics of BSM • Key for stormwater objectives and plant health • high enough infiltration needs • soil composition that supports plant establishment and long-term health • underdrain systems – pollutant removal properties • infiltration rates not so high to preclude pollutant removal

  4. Specifying BSM • BIORETENTION SOIL SPECIFICATION • Soils for bioretention areas shall meet the following criteria. • 1. General Requirements • Be sufficiently permeable to achieve a long-term, in-place infiltration rate of at least 5 inches per hour. Have sufficient moisture retention and nutrient content to support vigorous plant growth. • Bioretention soil shall be a well-blended mixture of compost and mineral aggregate, measured on a volume basis. • 30% - 40% Specified Compost • 60% - 70% Specified Mineral Aggregate. • The mixture shall be well blended to produce a homogeneous mix. • 1.2 Mineral Aggregate for Bioretention Soil • A. General • Mineral Aggregate shall be free of wood, waste, coating such as clay, stone dust, carbonate, etc., or any other deleterious material. All aggregate passing the No. 200 sieve size shall be non-plastic. • B. Mineral Aggregate for Bioretention Soil Texture • Mineral Aggregate for Bioretention Soils shall be analyzed by an accredited lab using #200, #100, #40, #30, #16. #8, #4, and 3/8 inch sieves (ASTM D 422 or as approved by municipality), and meet the following gradation: • Sieve Size Percent Passing (by weight) • Min Max • 3/8 inch 100 100 • No. 4 90 100 • No. 8 70 100 • No. 16 40 95 • No. 30 15 70 • No. 40 5 55 • No. 100 0 15 • No. 200 0 5 • Sieve size gradation to meet Municipality’s specifications. • Composted Material • Compost shall be a well decomposed, stable, weed free organic matter source derived from waste materials including yard debris, wood wastes or other organic materials not including manure or biosolids meeting the standards developed by the US Composting Council (USCC). The product shall be certified through the USCC Seal of Testing Assurance (STA) Program (a compost testing and information disclosure program). • Compost Quality Analysis

  5. Specifying BSM

  6. Specifying BSM • A mix of fine sand and compost (by volume) • 60% to 70% sand • 30% to 40% compost • Specified sand • Specified compost • Installation of bioretention soil • prep, soil mixing/placement, protection of soils from sediments

  7. Alternative Mixes • Soil mix one of the most common failures • BASMAA spec includes methods for evaluating alternative mixes • including “top soil” or “loamy sand” difficult to verify • mixing compost into native soils difficult quality control • performance may start out okay, but fail over time • failures in Bay Area correlate to observation of clay in mix

  8. Aggregate Component • Concrete sand • ASTM C33 • Readily available • Easy to verify

  9. Compost Component • Specified Compost • US Composting Council Certified • participant in “Seal of Testing Assurance Program” • soil lab tested every 120 days

  10. Mixing On-site vs. Premix • Premix available in Bay Area • multiple Contra Costa approved suppliers • meets the spec. • Premix may be more thoroughly mixed • Premix tested as a final mix • will indicate if organic content meets spec. (4%-6%) • will include initial infiltration rate for mix

  11. Soil Installation • Prevent over-compaction of native soils • keep heavy equipment out of bioretention area • fence area • Provide erosion control • keep drainage from entering facility • Scarify native soil • Protect adjacent surfaces from media • pervious surfaces especially

  12. Soil Installation • Do not work in wet weather • Apply in lifts, 6” recommended • Lightly compact lifts • boot compact or landscape roller • light wetting ok, but let dry between lifts • allow to dry overnight before planting • Verify elevations before planting • excavation must allow for design soil mix depth and mulch depth (if used) • Settling

  13. Future Research • Is organic matter lost over time – can it be replenished with top dressing? • Are there special considerations for arid climates? • Do soils need to be replaced at some future point? • Role of fines in water quality treatment • Nutrient leaching • does compost play a role • is media depth a key factor

  14. Regional Considerations Availability of specified compost Availability of premixed local sources

  15. Mulch • Retains moisture • Reduces erosion • Suppresses weed growth • Do not specify bark - floating • 1” – 2” specified compost can be used • Do not apply before rainy season • May reapply as mulch layer breaks down

  16. Floating Mulch TOP 10

  17. www.centralcoastlidi.org

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