Wood chips and applications of wood chips in stormwater

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This page provides information on wood chips. While providing extensive information on wood chips, there is a section focused specifically on stormwater applications for wood chips.

Overview and description

Wood chips are small- to medium-sized pieces of wood formed by cutting or chipping larger pieces of wood such as trees, branches, logging residues, stumps, roots, and wood waste. They include bark, wood, and often leaves. Wood chips are rich in lignin, suberin, tannins. Common mulch sources include cedar, cypress, straw/hay, pine, and spruce. Organic wood mulch is often a byproduct of the lumber industry (typically shredded bark), wood recycling centers (i.e. pallets) or processed yard waste from public landfills.

In stormwater applications, wood chips are used as a mulch to provide one or more beneficial functions. Potential benefits of wood chips include but are not limited to the following. They

• decompose slowly;
• slowly release nutrients;
• effectively retain and slowly release moisture;
• moderate temperature;
• provide weed control;
• are sustainabile;
• are typically rewlatively cheap to purchase;
• resist compaction;
• create a diverse environment for soil biota; and
• may sequester some pollutants. such as nitrogen.

Physical and chemical properties of wood chips vary depending on the source, method of production, and age. Because of this variability, this page focuses on generic properties of wood chips used as mulch, except where otherwise stated.

Source

• [1]
  • Aged wood mulch is commonly available as bark nuggets, and as both shredded softwood (such as cedar or fir) and hardwood. Wood mulch feedstock may be dictated by region as softwoods are more prevalent on the west coast of the U.S. and hardwoods on the east coast. A double shredded bark 3-inch mulch layer is generally used in biofiltration practices. Triple shredded bark mulch may contain too many fines and single shredded may contain larger mulch more prone to floating. Bark nuggets should also be avoided to prevent floating as they are less dense. When softwoods are used, texture should be evaluated as some softwoods can be somewhat “mouse-nest” in appearance and contribute to floating.

Properties of wood chips

This section includes a discussion of chemical and physical properties of wood chips, and potential contaminants in wood chips,

Chemical-physical properties of wood chips

Chemical and physical properties of wood chips

Potential contaminants in wood chips

Leachate from fresh wood chips is acidic, produces chemical oxygen demand, and releases nutrients. Negative aquatic response to leachate has been observed near wood chipping facilities and may be due to COD, phenols, organic compounds, or resin acids such as isopimaric acid (IA) and dehydroabietic acid (DHAA) (Machrafi et al., 2007; Taylor and Carmichael, 2003; Rex et al., 2016). Toxic effects associated with high oxygen demand from wood stockpiles have been observed in nearby receiving waters (Tao et al., 2005; Kannepalli et al., 2016). Studies indicate leaching of nutrients and organic compounds that contribute to oxygen demand decrease with time (Machrafi et al., 2007).

Wood chips from recycled wood may contain creosote and CCA (chromated copper arsenate). Wood chips from recycled wood is often colored with dyes. Dyes are typically organic- or iron-based and have not been found to be toxic. However, if colored wood chips are used, the wood source should be determined University of Massachusetts, Amherst.

Most studies indicate that diseased mulch cannot transmit pathogens to the roots of healthy trees. Mulch should not be incorporated into soil, under which conditions pathogens may be transmitted to trees ([2]; [3]).

Effects of wood chips on physical and chemical properties of soil and bioretention media

In this section we provide information on effects of wood chips on pollutant attenuation and on physical properties of soil and engineered media.

Effects of wood chips on retention and fate of other pollutants

• general
• nitrogen
• runoff from wood facilities
• [4]
  • Wood mulch is beneficial due to the additional treatment performance it provides to a biofiltration practice. The media stays protected, and infiltration rates maintained while most of the sedimentation occurs on the surface of the mulch within a biofiltration practice. Published research supports greater metals adsorption capacity for hardwood mulch due to its intrinsic properties including humic compounds consisting of carboxyl and hydroxyl functional groups, and greater cation exchange capacity, surface area and pH. Wood mulch is also known to capture oil and grease among other organic compounds. Wood mulch is also a host for microbial and macro-organism activity which supports plant health and pollutant degradation. Wood mulch sustains organic-based removal mechanisms as the mulch is decomposed and replenishes organic material within the media, also preserving water-holding capacity.
• swale/ditch with wood chips
• denitrification
• N removal
• N removal
• N removal

Standards, classification, testing, and distributors

Wood chip standards and specifications

Recommended values for wood chips used in a growth media (Source: see reference list in this section)
pH
Electrical conductivity (ms/cm)
Cation exchange capacity (meq/100g)
Nitrogen (%)
Phosphorus (%)
Potassium (%)
Copper (% minimum)
C:N ratio (minimum)
Lignin (%)
Total organic matter (% minimum)
Moisture (%)
Ash content (%)
Impurities
Fiber content
Expansion	l/kg
Water holding capacity	l/kg

Specific standards do not exist for wood chips, but the Forest Stewardship Council (FSC) certifies wood sources, which guarantees that the wood and bark is responsibly sourced. The FSC Controlled Wood Standard (FSC-STD-040-005 Version 3.0) requires knowledge of where the wood comes from, an evaluation of the risk that a wood source is in violation with unacceptable categories of wood, and mitigate actions to reduce any risk from the wood source.

Products receiving Mulch & Soil Council Certification must pass rigorous screening and are periodically audited to ensure the products meet Council standards. The certification ensures the product label is accurate and all ingredients are listed, and product claims have been verified. A Mulch & Soil Certification also ensures the mulch contains no chromated copper arsenate.

Some general guidelines for material selection are provided below [5].

• Recycled yard waste from public municipalities or recycled wood scraps such as pallets should be avoided due to inconsistencies in source and potential for unwanted trash or chemical additives in the mulch.
• Mulch should be free from waste wood material, harmful chemicals, or inorganic dyes.
• Mulch should not contain any fertilizer, pesticide, or other amendment.
• If mulch is being used in a nutrient sensitive area, a leachate analysis would ensure the mulch does not contribute to nutrient leachate from the practice.
  • Organic wood mulch is often a byproduct of the lumber industry (typically shredded bark), wood recycling centers (i.e. pallets) or processed yard waste from public landfills. Other mulches can include grass clippings, pine needles, straw, sawdust, leaf litter, turf, coir and compost, but these should be avoided for biofiltration applications, particularly high flow media, to prevent hydraulic restriction, flotation, maintenance burdens and nutrient leaching among other issues. Composted or aged wood mulch, which is mulch that has had time to decompose, is a better option over fresh wood mulch for use in most landscape practices, including biofiltration. Composted mulch is typically free of disease, insects and weed seeds. Fresh wood mulch can remove nitrogen from the soil, which may be beneficial to nitrogen removal from stormwater, but can strip nitrogen away from landscape plants. Fresh wood mulch is typically available as wood chips and can be more prone to floating, exposing the media surface to erosion and obstructing overflows. Wood chips typically are not as shredded and decomposed as composted mulch which tends to more easily form a mat after a few storm events due to its more fibrous structure.
  • Qualifying mulch feedstock properties for consistency, texture and size helps ensure the mulch will not restrict flow, but also provide necessary pretreatment performance. Mulch with excessive fines or sediment should be avoided as this can reduce infiltration rates, but most mulch sizes should be less than 1 inch. Figure 2 illustrates a shredded hardwood mulch with ideal consistency and texture. Wood mulch can contain additives such as compost, but this should be avoided as a surface layer for biofiltration practices. Mulch particle size and texture not only affects hydraulic function but also floatability. If floating wood mulch is unavoidable, an open mesh netting can be staked over the surface to prevent mulch from escaping the practice. For a quick float qualification test, place mulch in a clear container filled with water. Stir to ensure even wetting and wait approximately 24 hours to observe if most of the mulch has settled.
  • Wetting wood mulch at installation and ensuring it receives irrigation will reduce the chance of mulch floating from the practice. The wetting and drying process allows caking or matting to occur. Inlet areas, or other areas receiving concentrated flow, should be designed with dissipator stones or other type of erosion control. Denser plant covers can also reduce mulch flotation. Mulch with higher moisture content and therefore greater density can also minimize floatability. Bark nuggets and wood chips should be avoided due to their lighter density and texture that lends itself to increased floatability.
• details

Distributors

• [6]
  • Availability and supply method are also important factors to consider. Sometimes it is more economical to purchase bulk mulch over bagged mulch depending on the size of the biofiltration practice, but bagged mulch can be easier to handle. If you are qualifying mulch you should make sure what you are testing is available year-round so that when it is time for mulch installation you can ensure the qualified product is available. Mulch is a natural product that can change over time, so periodic supplier quality checks are important.

Effects of aging

Prabhu and Thomas (2002) provide an extensive discussion of wood chips decomposition.

Storage, handling, and field application

Keep wood mulch away from the trunks of trees to prevent rot. If you are concerned about termites, use cedar mulch or keep other wood mulches at least 6 inches (15 cm.) from the foundation. Let your mulch age if you aren’t sure of your source. This allows time for any sprays that were used on the tree or diseases it may have had to break down.[7]

Read more at Gardening Know How: Types Of Bark Mulch: Tips For Using Wood Mulch In Gardens https://www.gardeningknowhow.com/garden-how-to/mulch/bark-mulch-in-gardens.htm

Miscellaneous

• [8]
  • Use arborist wood chips. They absorb water and sink; bark repels water and floats and is more likely to clog inflows or overflows
  • Wood chips provide more nutrition than bark, which is essentially sterile
  • Wood chips discourage weed germination by tying up nitrogen at the soil surface; compost provides more nutrition for weeds to get growing

References

• https://s3.wp.wsu.edu/uploads/sites/403/2015/03/wood-chips.pdf
• https://pss.uvm.edu/ppp/articles/woodchips.html
• cedar
• pine
• hemlock
• selecting mulch for biofiltration
• Urban Landscape Management Practices as Tools for Stormwater Mitigation by Trees and Soils