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Minnesota Stormwater Manual

Wood chips and applications of wood chips in stormwater
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Page summary - wood chips
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. Beneficial properties of wood chips include the following.
  • increase the water holding capacity and water retention of soil and bioretention media;
  • reduce soil temperatures;
  • reduce soil compaction; and
  • can retain nitrogen when designed for this.

Regarding water quality, additional research is needed, but generally:

  • wood chips do not appear to retain phosphorus;
  • wood chips do not leach phosphorus;
  • wood chips are effective at retaining nitrogen when designed for this;
  • wood chips are effective at retaining metals; and
  • wood chips are effective at retaining many organics.

Wood chips may be beneficial for composting.

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

image
image of wood chips
Wood chips. Lignum Group, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

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, and 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 sustainable;
  • are typically relatively 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.

Applications for wood chips in stormwater management

Wood chips have not been extensively used in stormwater applications with the exception of bioreactors designed to promote denitrification. Wood chips have been used as a top mulch in bioretention practices, primarily to control weeds and for erosion protection. Some research is underway to determine other potential benefits of wood chips, such as improvement in soil physical and hydraulic properties and pollutant retention.

Source

As discussed below, the properties of mulch vary with several factors, including the tree species, source of chips (e.g. bark, wood), and initial condition of the chips (e.g. age, moisture content, chip size). Aged wood mulch, which is preferred, 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 (Hills, 2019).

Fresh wood mulch should be avoided. 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. Fresh mulch may also contribute other pollutants, including organic compounds, and increase oxygen demand (Kannepalli et al., 2016).

Shredded wood should be avoided. Shredded wood mulch tends to clog soil and form a mat which can restrict infiltration.

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

Physical and chemical properties of wood chips varies by the source (e.g. species, wood or bark) and the physical dimensions of the wood chips. Chips from wood typically are 70-80 percent cellulose and 20-30 percent lignin, while chips from bark are roughly 50 percent cellulose and 50 percent lignin (Pintor-Ibarra et al., 2017). Other components comprise less than 5 percent of the total dry weight mass of wood chips.

The adjacent table summarizes select physical and chemical properties of wood chips. The table does not differentiate between species or whether the chips are from bark or wood residue. Some general observations regarding these include the following.

  • Bark residues show a high content of inorganic compounds compared to wood residues
  • Bark residues are more acidic than wood residues
  • The major inorganic compounds found in ash are calcium, magnesium, and potassium.
  • Heavy metal concentrations are low
  • Extractives levels in barks are higher than in woods
  • Nitrogen concentrations are lower and C:N (carbon:nitrogen) ratios are higher in coniferous species compared to deciduous species
  • In general, physical properties are more favorable as chip size decreases (e.g. from 12 mm diameter to 4 mm diameter). For example, CEC ( cation exchange capacity) and water holding capacity increase as chip size decreases.

Chemical and physical properties of wood chips. For concentrations of metals, link here.
Link to this table

Property Range found in literature1 Median value from literature
Total phosphorus (mg/kg) 0.027 - 0.24 0.13
Total nitrogen (mg/kg) 0.31 – 1.8 0.38
Total potassium (mg/kg) 100-1600 709
Total carbon (mg/kg) 434-498 47.1
pH 3.34-5.07 3.47
Cation exchange capacity (cmol/kg)1
  • bark: 57-92
  • wood: 30-46
Total calcium (mg/kg) 600-6200 1190
Total magnesium (%) 60-6200 189
Bulk density (g/cm3) 0.138 - 0.422 0.293
Water holding capacity (% by wt) 58.5
Total pore space (%) 60-63 61.5

Primary references for this data:

  • Ima and Mann, 2007
  • Hamid et al., 2016
  • Pintor-Ibarra et al., 2016
  • Kim et al., 2016
  • Chandrasekaran et al., 2012
  • Venner et al., 2011
  • New York State Energy Research and Development Authority, 2013
  • Warner, 1976

1CEC increased as chip size decreased


Potential contaminants in wood chips

The adjacent table summarizes concentrations of heavy metals, selenium, and arsenic in wood chips in comparison with Soil Reference Values (SRV) and Soil Leaching Value (SLV). Concentrations in wood chips were typically well below risk criteria, with the only exception being maximum observed arsenic concentration, which exceeded the residential SRV. Additional data was found in the literature but is not included in the table. Other data in the literature show similar results.

Heavy metal concentrations (mg/kg) in wood chips (New York State Energy Research and Development Authority, 2013).
Link to this table

Metal Mean1 Median1 SRV – residential2 SRV – commercial SLV3
Vanadium 0.02 0.57 1.08 16 8
Chromium (III) 0.24 7.36 23160 100000 >100000
Manganese 70.6 272 2104 26000 260.4
Iron 18.1 345 10808 100000 na
Cobalt 0.03 0.14 4.62 69 54.1
Nickel 0.36 1.98 170 2600 352
Copper 1.32 3.41 2192 33000 1404
Zinc 5.93 17 4632 70000 6008
Arsenic 0.05 1.17 0.08 1.2 11.64
Cadmium 0.005 0.079 1.59 23 17.62
Lead 0.25 1.12 300 700 5401
Antimony 0.005 0.397 93 6.2 1.82
Barium 17.4 45.9 41000 3000 3368
Selenium 0.04 0.09 1200 77 5.28

1 13 samples
2SRV=Soil Reference Value (mg/kg)
3SLV=Soil Leaching Value (mg/kg); assumes 3 foot thick media and 3 foot separation from groundwater


Leachate from fresh wood chips is acidic, produces chemical oxygen demand (COD), 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. If mulch is incorporated into soil, pathogens may be transmitted to trees ([1]).

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 phosphorus

Wood chips are not likely to provide significant phosphorus retention in bioretention practices

No specific studies of phosphorus retention by wood chips in bioretention systems were found, though there are studies focused on nitrogen removal that also evaluated phosphorus. Christianson et al. (2017) studied a dual system consisting of a wood chip-based denitrification bioreactor coupled with either a steel slag or acid mine treatment residual system to remove phosphorus. The bioreactor did not remove phosphorus. Kuter et al. (2017) observed that amending soil receiving biosolids with wood chips resulted in no phosphorus retention. Husk et al. (2018) found wood chip bioreactors vary in their effect on phosphorus leaching, with periods where phosphorus was retained and periods when phosphorus was released. Healy et al. (2012, 2015) and Sharrer et al. (2016) found initial leaching of phosphorus from wood chip bioreactors. Parvage et al. (2017) observed phosphorus retention by wood chips in manure-rich paddock soils. Since studies of nitrogen and phosphorus retention generally utilize dual systems, with one designed for nitrogen removal and the other for phosphorus removal, phosphorus retention by wood chips can be considered to be negligible. Similarly, several studies indicate wood chips are not an important source of phosphorus for vegetation, wood chips would not be expected to leach phosphorus in appreciable amounts.

Effects of wood chips on retention and fate of other pollutants

Several studies indicate wood chip bioreactors effectively reduce nitrogen concentrations through denitrification. These studies include specific design considerations for wood chip bioreactors. Many of these studies also indicate effective removal of dissolved organic carbon (Chang et al., 2018; Christianson et al., 2017; Ergas et al., 2010; Lopez-Ponnada et al., 2020; Wan et al., 2018; Kim et al., 2003; Peterson et al., 2015).

Chang et al. (2018) showed copper retention by wood chips.

Hills (2019) conducted a review of wood chips and found that research supports greater metals adsorption capacity for hardwood mulch due to presence of humic compounds consisting of carboxyl and hydroxyl functional groups, greater cation exchange capacity, increased surface area, and favorable 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.

Hopkins et al. (2021) provide a literature review and summary of the effects of wood chips on pollutant retention. Below are direct excerpts from this paper, including references, which can be found in the References section on this page.

  • Woodchips act as the carbon source in the biological degradation of nitrate, sulphate, ammonia, ammonium, and nitrite (Kim et al., 2003; Edwards et al., 2009). Particulate phosphorus is phosphorus adsorbed to suspended sediment, so it is removed by physical filtration along with suspended solids (Choudhury et al., 2016). Orthophosphorus, is removed through sorption, and nitrates can be removed by sorption as well as biological degradation (Xuan et al., 2010).
  • Woodchips have proven effective for heavy metal removal, but some metals, such as arsenic, have not been studied (Bailey et al., 1999; Edwards et al., 2009; Jang et al., 2005; Ray et al., 2006; Seelsaen et al., 2006; Ashoori et al., 2019). Metals are removed through sorption to the woodchips and cation exchange with phenolic hydroxyl groups. The composition of the wood can greatly affect the removal efficiency of the metals (Bailey et al., 1999).
  • Several studies have found woodchips to effectively remove pesticides (Camilo et al., 2013; Ashoori et al., 2019; Bras et al., 1999; Trapp et al., 2001; Huang et al., 2006).
  • Total petroleum hydrocarbons have shown to be effectively removed by wood products in many studies, by sorption to the woodchips as well as physical filtration of sediments to which hydrocarbons are sorbed and biological degradation (Ray et al., 2006; Trapp et al., 2001; Mackay and Gschwend, 2000; Boving and Zhang, 2004; Seo et al., 2009; Melone, 2016).
  • Halocarbons have been found to be removed by wood through sorption (Boving and Zhang, 2004; Seo et al., 2009; Ray et al., 2006; Shenl et al., 2010; Trapp et al., 2001).
  • There is a limited amount of research evaluating the ability of wood to remove water-borne pathogens. Pathogens are removed by sorption to woodchips or physical filtration if they are adsorbed to sediments in the inflow, causing the deactivation of pathogens by natural decay, desiccation, or predation (Soupir et al., 2018).

Hopkins et al. (2021) provide a brief discussion of the importance of wood chip shape and source on retention of pollutants. They concluded that high-lignin sources show a greater capacity to retain some pollutants, such as metals, compared to sources with a lower lignin concentration.

Effects of wood chips on soil physical and hydraulic properties

Wood chips increases the water holding capacity and water retention of soil and bioretention , though these increases are less than compost and other mulches that have greater surface area (Davis and Whiting, 2013; van Donk et al., 2011; Perry). Shaw et al. (2005) observed an increase in water holding capacity of 0.81 in/ft in a soil in San Diego, California. Salzmon et al. (1958) observed water holding capacities of 99-138 percent of dry weight for wood chips from four species (red and jack pine, aspen, birch), with the lowest adsorption being for birch.

Wood chips reduce soil temperatures by retaining soil water and blocking direct solar radiation, and reduce diurnal temperature fluctuations. Temperature effects are typically limited to the upper 10 cm of the soil or media (Abdul Bari Awan, 1964; Kotze, 2012; van Donk et al., 2011, Perry).

Research shows mixed results for effects of wood chips on soil bulk density. Wood chips appear to have beneficial effects on reducing soil compaction, particularly when incorporated, but minimal impact in uncompacted soils (Choi et al., 2005; Venner et al., 2011; Tahboub et al, 2008; Qu et al., 2019; Scharenbroch and Watson, 2014; Antieau, 2017).

Effects of wood chips on soil fertility, plant growth, and microbial function

Effects of wood chips on soil fertility, plant growth, and microbial function are somewhat affected by source and physical properties (e.g. size, shape). Wood chips have few negative effects on fertility, plant growth and microbial function and has the potential to improve soil physical conditions.

There are limited scientific studies of effects of wood chips on plant growth. Van Donk et al. (2011) and Horschat (2007) observed decreased weed growth with mulch and increasing mulch thickness. Free (1971) conducted a 15-year study consisting of a 5-year vegetable rotation of sweet corn, beans, tomatoes, cabbage, and peas and comparing 14 different treatments, including several in which 10 tons per acre moist weight (7 tons dry weight) of wood chips were added each year. Some conclusions from the study include the following.

  • Yields of most crops were improved with the addition of wood chips and best when the chips were topdressed on the soil surface after the crops were planted instead of being plowed under.
  • Soil organic matter (SOM) and nitrogen increased in the chip-amended plots, while they dropped in the chipless plots without cover crops.

There is considerable information on use of wood chips for gardening. In general, wood chips improve soil hydrology, reduce soil temperatures, may tie up nitrogen, decrease soil pH, lead to a fungi-dominated soil biota ([2], [3], [4], [5], [6]). The effect therefore is dependent on the vegetation. Deep-rooted plants, particularly shrubs and trees, may benefit from use of wood chips, while other vegetation may be adversely impacted.

Standards, classification, testing, and distributors

Wood chip standards and specifications

Specifications exist for wood chips and pellets used for energy. Specific standards do not exist for wood chips used for other practices, 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 (Hills, 2019).

  • 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.
  • Mulches that include grass clippings, pine needles, straw, sawdust, leaf litter, turf, coir and compost should be avoided for biofiltration applications, particularly high flow media, to prevent hydraulic restriction, flotation, maintenance burdens and nutrient leaching.
  • 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 be more prone to floating, exposing the media surface to erosion and obstructing overflows.
  • Mulch sizes should be less than 1 inch, but excessive fines or sediment should be avoided as this can reduce infiltration rates. Additives such as compost should be avoided as a surface layer for biofiltration practices.

Distributors

Caution: The Minnesota Pollution Control Agency does not endorse specific distributors of wood chips or wood chip products

Availability and supply method are 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 [7]. This website provides some information to frequently asked questions when selecting a mulch. In selecting a distributor, it is recommended the mulch meet Mulch & Soil Council Certification.

Test methods

Most test methods for wood chips relate to use of chips as a fuel source. Typical parameters of interest include moisture content, ash content, particle size, bulk density, nitrogen content, chlorine content, sulphur content, and trace metals. Products certified by the Mulch and Soil Council have undergone testing prior to certification. Certification by the Mulch and Soil Council only applies to packaged materials. The organization offers an Elite Bulk Program which applies to bulk distributors of mulch. Though Elite Bulk Program distributors are not certified, they comply with specific requirements described here. Several private companies offer testing, typically for use of wood chips as a fuel source. These can readily be found on the internet.

The City of Vancouver provides bioretention details, including recommended properties and tests for wood chip mulches. Link here for a summary of this information.

Effects of aging

Typical C:N ratios for wood chips are 400-800:1, resulting in slow decomposition of wood chips. Studies of wood chip decomposition focus on stockpiles. The rate of decomposition varies with several factors, including temperature, pH, moisture, source, and presence of decomposers (e.g. fungi, bacteria, insects). Slaven et al. (2011) reported degradation rates, on a mass basis, of 3-19 percent over a 180 day period, with increased rates for wood chips containing bark. They also report greater degradation rates as surface area of the wood chips increases and for chip shapes that promote increased porosity. Heinek et al. (2013) observed monthly biomass loss of 1% up to 4% depending on biomass quality, water content and temperature. Standard woodchips showed lowest biomass losses and a low microbiological growth. Bark woodchips and forest residues tend to be more susceptible for fungal growth and highest biomass loss could be proved for bark woodchips. Other studies provide information about processes involved in degradation, differences between wood chip sources, and effects of different factors such as temperature and moisture ([8], [9], [10], [11]).

Fresh wood mulch should be avoided. 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. Fresh mulch may also contribute other pollutants, including organic compounds, and increase oxygen demand (Kannepalli et al., 2016).

Storage, handling, and field application

Packaged wood chips should present no specific storage concerns provided the material is kept containerized. Stored bulk material should be managed to prevent excessive drying, decomposition, splitting and cracking of chips, and excessive thermal heating due to decomposition. Store in cool, sheltered areas.

Handling of wood chips presents no specific concerns other than dust, which may cause respiratory, skin, or allergic reactions. Material Safety Data Sheet.

City of Vancouver recommended wood chip mulch properties
Property Test method Minimum value
Mass per unit area ASTM D65661 ≥ 9.9 OZ/YD2 (340 G/M2)
Water holding capacity ASTM D7367 ≥ 1,200%
Material color Observed Green
Vegetation establishment ASTM D73221 ≥ 250 %
Functional longevity 2 ASTM D5338 <= 3 months
Ecotoxicity EPA 2021.0 48-hour LC50 > 100%
Biodegradability ASTM D5338 Yes

The City of Vancouver developed several details that include wood chip mulch. The designs can be found [here. The following information can be extracted from these details, including information in the adjacent table.

  • Establish a 3 inch arborist wood chip mulch with the following specifications.
    • Furnish wood chip mulch that is chipped from cleared site vegetation or from an approved offsite source. Ensure that chipped material is free of any noxious weeds or invasive vegetation.
    • Mulch may contain wood, wood fiber, bark, branches, and leaves, but may not contain weeds, weed seeds, trash, or visible amounts of soil.
    • Provide wood chip mulch in the following size range: 3-inch to 1/4-inch chipped wood material and vegetative debris. Minimize fine material within blend.
  • Virgin wood fibers derived from clean, whole wood chips. Erosion control products shall be hydraulically applied, phytosanitized and upon application form an intimate bond with the soil surface to create continuous, porous, absorbent and flexible erosion resistant blankets that allow for healthy plant growth. Hydraulic mulch wood with tack is an acceptable product for this application.
  • Apply at the following rates
    • Slopes <= 4H:1V, 2000 lb/acre (2240 kg/ha)
    • Slopes > 4H:1V, 2500 lb/acre (2800 kg/ha)

Most information on application of wood chips, primarily as a mulch, are associated with gardening ([12]; [13]; [14]; [15]; [16]). Some recommendations are provided below.

  • 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.
  • Pull the mulch back from the base of the plants so there is a 2-inch space between the stems and mulch.
  • Weed the garden thoroughly before applying mulch. Pull up all visible weeds, removing as much of the roots as possible. Hoe the soil between plants to cut off any germinating weeds just beneath the soil surface.
  • Water the bed thoroughly if needed. Moisten the soil the amount necessary for the type of plants growing there, or to at least a 6 inch depth.
  • 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.
  • Replenish mulch in spring or fall as needed.

There are also several videos on wood chip mulch application ([17]; [18]; [19]).

Sustainability

Using locally produced wood chips is a sustainable activity and keeps a useful product out of landfills.

References

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