Non-Proprietary BMPs

Soil restoration is an ESD practice applied after construction to deeply till compacted soils and restore their porosity by amending them with compost. These soil amendments can reduce the generation of runoff from compacted urban lawns and may also be used to enhance the runoff reduction performance of downspout disconnections, grass channels, and filter strips (Table 1).

Table 1: Stormwater Functions of Soil Compost Amendments 1
Stormwater Function	HSG Soils A and B		HSG Soils C and D
Annual Runoff Reduction Rate	No CA 2	With CA	No CA	With CA
Simple Rooftop Disconnection	50	NA 3	25	50
Filter Strip	50	NA 3	NA 4	50
Grass Channel	20	NA 3	10	30
Total Phosphorus Removal 4	0		0
Total Nitrogen Removal	0		0
Channel Protection & Flood Mitigation	Partial. Designers can use the RRM spreadsheet to adjust curve number for each design storm for the contributing drainage area, based on annual runoff reduction achieved
1CWP and CSN (2008), CWP, 2007 2 CA= Compost Amended Soils, see Design Specification No. 4 3 Compost amendments are generally not applicable for A and B soils, although it may be advisable to incorporate them on mass-graded B soils to maintain runoff reduction rates. 4 Filter strips in HSG C and D should use composted amended soils to enhance runoff reduction capabilities. See Design Specification No. 2, Sheetflow to Conservation Area or Vegetated Filter Strip.

Compost amended soils are suitable for any pervious area where soils have been or will be compacted by the grading and construction process. They are particularly well suited when existing soils have low infiltration rates (HSG C and D) and when the pervious area will be used to filter runoff (downspout disconnections and grass channels). The area or strip of amended soils should be hydraulically connected to the stormwater conveyance system. Soil restoration is recommended for sites that will experience mass grading of more than a foot of cut and fill across the site:

Existing soils have high infiltration rates (e.g., HSG “A and B” soils), although compost amendments may be needed at mass-graded B soils to maintain runoff reduction rates.
The water table or bedrock is located within 1.5 feet of the soil surface.
Slopes exceed 10%.
Existing soils are saturated or seasonally wet
They would harm roots of existing trees (stay outside the tree drip line)
The downhill slope runs toward an existing or proposed building foundation
The contributing impervious surface area exceeds the surface area of the amended soils

Compost amendments can be applied to the entire pervious area of a development or be targeted in select areas of the site to enhance the performance of runoff reduction practices. Some common design applications include:

As referenced in several of the other specifications, soil compost amendments can be used to enhance the runoff reduction capabilities of allied practices. The specifications for each of these practices contain design criteria for how compost amendments can be incorporated into those designs:

Soil tests are required during two stages of the compost amendment process. The first testing is done to ascertain pre-construction soil properties at proposed amendment areas. The initial testing is used to determine soil properties to a depth 1 foot below the proposed amendment area, with respect to bulk density, pH, salts, and soil nutrients. These tests should be conducted every 5000 square feet, and are used to characterize potential drainage problems, and determine, what if any, further soil amendments are needed.

The second soil analysis is taken at least one week after the compost has been incorporated into the site. This soil analysis should be conducted by a reputable laboratory to determine whether any further nutritional requirements, pH, and organic matter adjustments are necessary for plant growth. This soil analysis should be done in conjunction with the final construction inspection to ensure tilling or subsoiling has achieved design depths.

The runoff reduction volume achieved by soil restoration depends on the site application and the pre-construction hydrologic soil group. When compost amendments are used simply to reduce runoff from compacted lawns, the lower runoff coefficients shown in Table 2 can be used to lower the total treatment volume for the site as a whole. If the soil restoration area accepts runoff from adjacent impervious areas, the higher runoff reduction rates outlined in Table 1 may be used for the indicated practices.

Table 2: Runoff Coefficients for Use for Different Pervious Areas
Hydrologic Soil Group	Undisturbed Soils 1	Disturbed Soils 2	Restored and Reforested 3
A	0.02	0.15	0.02
B	0.03	0.20	0.03
C	0.04	0.22	0.04
D	0.05	0.25	0.05
Notes: 1 Portions of a new development site, outside of limits of disturbance, which are not graded and do not receive construction traffic. 2 Previously developed sites, and any site area inside the limits of disturbance as shown on the ESC plan 3 Areas with restored soils that are also reforested to achieve a minimum 75% forest canopy

The depth of compost amendment is based on the relationship between the surface area of the soil amendment to the contributing area of impervious cover that it receives. Table 3 presents some general guidance derived from soil modeling by Holman-Dodds (2004) that evaluates the required depth to which compost must be incorporated. Some adjustments to the incorporation depth were made to reflect recommendations of Roa Espinosa (2006), Balousek (2003), Chollak and Rosenfeld (1998) and others.

Table 3: Short-cut Method to Determine Compost and Incorporation Depths
	Contributing Impervious Cover to Soil Amendment Area Ratio 1
	IC/SA = 0 2	IC/SA = 0.5	IC/SA = 0.75	IC/SA = 1.0 3
Compost (in) 4	2 to 4 5	3 to 6	4 to 8	6 to 10
Incorporation Depth (in)	6 to 10 5	8 to 12	15 to 18	18 to 24
Incorporation Method	Rototiller	Tiller	Subsoiler	Subsoiler
Notes: 1 IC is the contributing impervious cover (square feet) and SA is the surface area of compost amendment (square feet) 2 For amendment of compacted lawns that do not receive off-site runoff 3 In general, IC/SA ratios greater than 1 should be avoided 4 Average depth of compost added 5 lower end for B soils, higher end for C/D soils

Once the area and depth of compost are known, the designer can estimate the total amount of compost needed, using an estimator developed by TCC, (1997):

C = compost needed in cubic yards
A = square feet amended
D = depth of compost added (inches)

Compost shall be derived from plant material and provided by a member of the US Composting Seal of Testing Assurance (STA) program. See www.compostingcouncil.org for a list of local providers.

The compost shall be the result of the biological degradation and transformation of plant derived materials under conditions that promote anaerobic decomposition. The material shall be well composted, free of viable weed seeds, and stable with regard to oxygen consumption and carbon dioxide generation. The compost shall have a moisture content that has no visible free water or dust produced when handling the material. It shall meet the following criteria, as reported by the U.S. Composting Council STA Compost Technical Data Sheet provided by the vendor:

100% of the material must pass through a half inch screen
The pH of the material shall be between 6 and 8
Manufactured inert material (plastic, concrete, ceramics, metal, etc.) shall be less than 1.0 % by weight
The organic matter content shall be between 35 and 65%
Soluble salt content shall be less than 6.0 mmhos/cm
Maturity should be greater than 80%
Stability shall be 7 or less
Carbon/nitrogen ration shall be less than 25:1
Trace metal test result = “pass”
The compost should have a dry bulk density ranging from 40 to 50 lbs/ft3.

No special adaptations are needed in karst terrain, but the designer should take soil tests to ensure that soil pH is adjusted to conform to pre-existing soil conditions found in limestone dominated areas.

Designers should evaluate drainage and water table elevations to ensure the entire depth of soil amendment will not become saturated (i.e., a minimum separation depth of two feet from groundwater). Compost amendments are most cost effective when used to boost the runoff reduction capability of grass filter strips, grass channels and rooftop disconnection.

Compost amendments are ineffective when longitudinal slopes exceed 5%, so some terracing may be needed on steeper slopes.

Soil restoration is not recommended for areas that will be used for snow storage.

Soil amendments can improve the runoff reduction of drainage swales in open section right of way and medians.

The construction sequence for compost amendments differs depending whether the practice will be applied to a large area or a narrow filter strip such as in a rooftop disconnection or grass channel. For larger areas, a typical construction sequence is as follows.

Prior to building, the proposed area should be deep tilled to a depth of 2 to 3 feet using a tractor and sub-soiler with two deep shanks (curved metal bars) to create rips perpendicular to the direction of flow.
A second deep tilling to a depth of 12 to 18 inches is needed after final building lots have been graded.
It is important to have dry conditions at the site prior to incorporating compost
An acceptable compost mix is then incorporated into the soil using a roto-tiller or similar equipment at the volumetric rate of one part compost to two parts soils
The site should be leveled and seeds or sod used to establish a vigorous grass cover. Lime or irrigation may initially be needed to help the grass grow quickly.
Areas of compost amendments exceeding 2500 square feet should employ simple erosion control measures, such as silt fence, to reduce the potential for erosion

The first step is usually omitted when compost is used for narrower filter strips.

Construction inspection involves digging a test pit to verify the depth of mulch, amended soil and scarification. A rod penetrometer should be used to establish the depth of uncompacted soil at one location per 10,000 square feet

When soil compost amendments are applied on private residential lots, homeowners will need to be educated on their routine maintenance needs, understand the long-term maintenance plan, and be subject to a deed restriction or other mechanism enforceable by the qualifying local program to ensure that infiltrating areas are not converted or disturbed. The mechanism should, if possible, grant authority for local agencies to access the property for inspection or corrective action. In addition, the GPS coordinates for all amended areas shall be provided upon facility acceptance to ensure long term tracking.

A simple maintenance agreement should be provided if soil restoration is associated with more than 10,000 square feet of reforestation. A conservation easement or deed restriction may be required to make sure the newly developing forest cannot be cleared, which also identifies a responsible party to ensure that routine forest improvements are made (i.e., thinning, invasive plant removal, etc.). Soil compost amendments within a filter strip or grass channel should be located in public right-of-way, or within a dedicated stormwater or drainage easement.

Successful soil compost amendments require certain tasks be undertaken in the first year.

Initial inspections: For the first six months following amendments, the site should be inspected at least once after each storm event that exceeds a half- inch.

Spot Reseeding: Inspectors should look for bare or eroding areas in the contributing drainage area or around the soil restoration area, and make sure they are immediately stabilized with grass cover.

Fertilization: Depending on the amended soils test, a one-time, spot fertilization may be needed in the fall after the first growing season to increase plant vigor.

Watering: Water once every three days for first month, and then weekly during first year (Apr – Oct), depending on rainfall.

There are no major ongoing maintenance needs associated with soil compost amendments, although the owners may want to de-thatch the turf every few years to increase permeability. The owner should also be aware that there are maintenance tasks needed for filter strips, grass channels, and reforestation areas.

The following references and resources were used to develop this master specification:

Balusek. 2003. Quantifying decreases in stormwater runoff from deep-tilling, chisel-planting and compost amendments. Dane County Land Conservation Department. Madison, Wisconsin.

City of Portland. 2008. Soil Specification for Vegetated Stormwater Facilities. Portland Stormwater Management Manual. Portland, Oregon

Composting Council (TCC). 1997. Development of a landscape architect specification for compost utilization. Alexandria, VA.
http://www.cwc.org/organics/org972rpt.pdf

Holman-Dodds, L. 2004. Chapter 6. Assessing infiltration-based stormwater practices. PhD Dissertation. Department of Hydroscience and Engineering. University of Iowa. Iowa City, IA.

Lenhart, J. 2007. Compost as a soil amendment for water quality treatment facilities. Proceedings 2007 LID Conference. Wilmington, NC

Roa-Espinosa. 2006. An introduction to soil compaction and the subsoiling practice. technical note. Dane County Land Conservation Department. Madison,Wisconsin.

APPENDIX A:
INITIAL MINIMUM DESIGN CRITERIA FOR REFORESTATION, DISCONNECTION, FILTER STRIPS AND GRASS CHANNELS

A. SITE REFORESTATION.

Several design criteria apply when compost amendments are used to enhance the performance of reforested areas. Site reforestation involves planting trees on existing turf or barren ground at a development site with the explicit goal of establishing a mature forest canopy that will intercept rainfall, increase evapo-transpiration rates and enhance soil infiltration rates. Reforestation areas at larger development sites and for individual trees for smaller development sites are eligible under certain qualifying conditions

The minimum contiguous area of reforestation must be greater than 5000 square feet
A long term vegetation management plan must be prepared and filed with the local review authority to maintain the reforestation area in a natural forest condition

The reforestation area must be protected by a perpetual stormwater easement or deed restriction that indicates that no future development or disturbance can occur within the area
Reforestation methods must achieve 75% forest canopy within ten years
The planting plan must be approved by the appropriate local forestry or conservation authority, including any special site preparation needs
The construction contract should contain a care and replacement warranty extending at least 3 growing seasons to ensure adequate growth and survival of the plant community
The reforestation area shall be shown on all construction drawings and ESC plans during construction

SOIL COMPOST AMENDMENT