Management of soil and engineered media removed from bioretention basins and similar stormwater treatment devices

Engineered media in a bioretention practice. Image from MPCA's Flickr website.

Stormwater collection and conveyance systems collect and concentrate pollutants to prevent them from reaching lakes, rivers, streams, wetlands, and other waters of the state, where they could have a negative effect on water quality, aquatic animals, or human health. This document provides guidance for management of soil and engineered media that is excavated/removed from bioretention basins and other stormwater collection and treatment systems, which have been designed, constructed, operated, and maintained for the purpose of providing temporary storage or treatment of stormwater.

Background

Lab studies demonstrated that engineered media effectively removes pollutants but eventually breakthrough occurs (Hatt et al., 2011; Morgan, 2011; Gulbaz et al., 2015). The significance of breakthrough is that it indicates a media is no longer able to effectively treat influent and suggests the media is approaching or has reached saturation with a specific pollutant. At this point the media must be replaced. Managing contamination and pollutants in bioretention basins and other devices should be expected and sampling is required prior to disposal or beneficial use (e.g. fill, topsoil, or compost) to determine proper management.

This guidance document will help you think through important steps associated with soil and engineered media removal projects. These may include

• determining who is responsible for managing the material,
• considering the effects of land use within a drainage area,
• sampling soil and engineered media and what laboratory analysis is required,
• calculating BaP (benzo[a]pyrene) equivalents for carcinogenic polycyclic aromatic hydrocarbons (cPAHs), and
• management requirements for contaminated soil/media where contaminated soil would be accepted for disposal.

This document is intended to help those responsible for operation and maintenance of stormwater systems determine what steps to consider during the course of managing a soil/media removal project. This is guidance and is not a comprehensive list of everything you may need to do when managing a sediment removal project. Other considerations may also include

• determining proximity to high value resources or sensitive ecological features,
• landscape variations,
• soil types,
• management of native or invasive species, and
• a wide range of other variables that may be encountered from one municipality to the next or one project to the next.

There are other stormwater collection devices not specifically addressed in this guidance document, and this guidance may be adapted for other stormwater collection devices not specially addressed in this document. A guidance document for management of sediment removed from stormwater ponds can be found at this link. Guidance for materials collected from pretreatment practices can be found at this link.

Sediment or soil removal from lakes, rivers, streams, and wetlands may be subject to additional requirements such as a permit from the Minnesota Department of Natural Resources (DNR) to allow work below the ordinary high water level. Permit determinations are guided by DNR hydrologists based on geographical location. A list of DNR hydrologists by area is available on the DNR website.

Soil and engineered media in bioretention basins and other devices

Bioretention basins and other similar related stormwater treatment devices are constructed with “engineered media”. For many bioretention basins, this engineered media, or bioretention media, will consist of soil and an organic amendment such as compost, peat, wood chips, or coir. However, some bioretention basins or other treatment devices are enhanced with other media, which are often selected for their ability to absorb dissolved pollutants. Several types of media are discussed in more detail at this link. Below is a list and brief description of various media that might be utilized.

• Biochar – a charcoal-like substance that’s made by burning organic material from biomass. Because combustion of organic matter creates PAHs, special attention should be given to measuring PAHs of engineered media containing biochar.
• Pelletized peat – peat is formed by the accumulation of partially decomposed organic matter in certain wetlands. The peat is harvested, dried, and pressed into pellets and sold as a soil amendments or water treatment media.
• Water treatment residuals (WTR), iron and aluminum – the residual material from the treatment of drinking water with coagulation-flocculation. The material consists of the iron and aluminum precipitates from the chemicals used for treatment, as well as sediment and other materials removed from the treated water.
• Water treatment residuals (WTR), calcium – often referred to as “spent lime”, these are the residual materials of treating drinking water with hydrated lime for water softening.
• Iron filings – granular iron that is mixed with sand for a high permeability filter media. The iron oxidizes (i.e. rusts) when exposed to moisture and oxygen, and the oxidized iron surface has a strong affinity to bind dissolved phosphorus and some metals. Due to the nature of the iron filings, including reversal of oxidation if buried underground, engineered media containing iron filings should not be reused as fill material, and is best disposed in a landfill.
• Crushed limestone or calcareous sand – sand to pebble sized calcium carbonate materials that provide physical solids filtration, as well as some removal of dissolved phosphorus.

For a discussion of media mixes link here.

Pollutants in engineered media from stormwater treatment practices

Common pollutants in stormwater include metals, polycyclic aromatic hydrocarbons (PAHs), and various petroleum products from automobiles (e.g. oil, grease, diesel fuel, gasoline).

Research previously conducted by the MPCA on stormwater ponds concluded that polycyclic aromatic hydrocarbons (PAHs) are often responsible for the greatest contamination problems in stormwater pond sediment (Crane et al. 2010). Research conducted on stormwater pond sediments in the Minneapolis-St. Paul, Minnesota metropolitan area showed that PAHs are the primary contaminants of concern affecting disposal decisions of stormwater pond sediment (Polta et al. 2006; Crane 2014; Huang et al.). PAHs persist in the environment and pose a risk to animals, plants, and people at elevated concentrations. These contaminants are formed by the incomplete combustion of organic materials, such as wood, oil, and coal, as well as occurring naturally in crude oil and coal (Crane et al. 2010).

Coal tar-based sealants are a major source of PAHs in urban sediments where these products are used in the surrounding watershed (Mahler et al. 2012). The Minnesota Pollution Control Agency’s (MPCA) research (Crane 2014) determined that coal tar-based sealants were the most important source of PAHs (67.1%), followed by vehicle emissions (cars and trucks) (29.5%) and pine wood combustion (3.4%). The MPCA determined that coal-tar based pavement sealants were the largest source of carcinogenic PAHs to stormwater, and the Minnesota Legislature banned the use of coal-tar based pavement sealants, effective January 1, 2014.

More recently, field studies have focused on assessing pollutant accumulation in engineered media or soil used in infiltration and filtration practices. Some of these studies are summarized below.

• Al-Ameri et al. (2018) studies 78 bioretention practices in Australia and observed that catchment characteristics were significantly correlated with metal accumulation rates. Biofilters in catchments with current or past industrial activities had elevated heavy metal concentrations in the filter media. Zinc concentrations in the surface 0–100 mm exceed both soil quality and ecological guidelines. In contrast, heavy metal concentrations in residential catchments are unlikely to (ever) reach levels that exceed soil quality guidelines for human health, although zinc concentrations approach ecological guideline criteria.
• Furén et al. (2022) conducted a large scale field study of accumulation of organic micro pollutants in bioretention practices. They observed that most PAHs and PCBs were frequently detected, and of 13 phthalates and two alkylphenols, Di(2-ethylhexyl)phthalate (DEHP) and nonylphenol, were quantified regularly. Large inter and intra-site variations were observed, most likely due to differences in pollutant sources, contributing catchment size and/or land uses, with highest levels in filter top layers. However, pollution was detected in all filters regardless age, size and catchment land use.
• Johnson and Hunt (2016) collected soil samples from an 11-year old bioretention cell in Charlotte, NC. Accumulation of zinc, copper, and phosphorus in soil media was characterized. Sampled zinc concentrations were above protective ecological thresholds. Initial phosphorus concentrations increased by a factor of seven.
• Lucke and Nichols (2015) evaluated the pollution removal and hydrologic performance of five, 10-year old street-side bioretention systems. Heavy metal and hydrocarbon testing undertaken on the bioretention systems found that the pollution levels of the filter media were still within acceptable limits after 10 years in operation. Bioretention basin pollution removal performance was highly variable and dependent on a range of factors including inflow pollution concentrations, filter media, construction methods and environmental factors.
• Several studies suggest the concentration of pollutants, particularly metals, is greatest near the surface of the bioretention media and decreases with depth (Linh, 2011; Li and Davis, 2008; Hunt et al., 2012; Hathaway et al., 2011; Furen et al., 2022).

Soil and engineered media removal process

The process for removing soil and engineered media that may be impacted by pollutants includes the following steps. These are discussed in greater detail below.

• Inventory and maintenance needs.
• Evaluating and testing soil/media.
• Engineering, contracting, and work plans.
• Excavating soil/media.
• Site restoration.
• Records and documentation to keep on file.

1. Inventory and maintenance needs. Assessing the need for and planning of soil and engineered media removal projects includes a number of steps ranging from estimating lost capacity of the practice to attenuate pollutants to notifying neighbors about plans to maintain the bioretention basin or other stormwater device. For municipalities who are managing dozens, or sometimes hundreds of stormwater devices, starting with an inventory and a maintenance prioritization process is recommended.

The first phase of work identifies need and determines if a soil or engineered media removal project is even necessary. This may include a preliminary survey to gage accumulated sediment volume and reduced performance of the bmp, (e.g. reduced infiltration rate, water quality monitoring); and provide a rough estimate of the number of cubic yards of sediment/soil/media to be removed. Though specific to iron-enhanced sand filters, this section of the manual provides guidance on volume and water quality monitoring and assessment.

2. Evaluating and testing soil/media. Soil/media samples are collected and compared to MPCA’s Remediation Division soil reference values (SRVs) to determine whether excavated soil and engineered media may be beneficially used, or if landfill disposal is more appropriate. This affects work plan development, including contract specifications for bidding projects and is an important part of the management process.

• Guidance for collecting samples and testing soil/media is summarized in Appendix A.
• Guidance for comparing contaminant analytical data (concentrations) to SRVs and calculating B[a]P equivalents is summarized in Appendix B.

There are two sets of SRVs based on the following remediation soil land use categories.

• Residential land includes lawn surrounding single family housing and newly developed single family residences, multi-family housing, condominiums, playgrounds, sports fields, beaches, produce gardens, long-term care facilities, correctional housing, hospitals, campgrounds, child care centers, churches, schools, wildlife areas, local/state/national forests, and public or private erodible trails.
• Industrial land includes lawns, yards, and landscaping that surround hotels, office buildings, retail stores, shopping centers, and restaurants and industrial property, public utility facilities, rail and freight facilities, storage facilities, warehouses, office buildings, and manufacturing facilities.

The analytical results and calculation of B[a]P equivalents are compared to the MPCA’s Remediation Divisions SRV values for these two land use categories to determine management options. Management options include the following.

• Use of excavated soil/media as unregulated fill. Contaminant concentrations from the list of analytes, including cPAHs expressed as B[a]P equivalents and any other site-specific contaminants, are all below the Residential SRVs and Screening Level Soil Leaching Values (SLVs). The excavated soil/media is unregulated fill and does not require any special management. Excavated soil/media may be reused in accordance with the MPCA's BMPs for the Off-Site Use of Unregulated Fill available at: https://www.pca.state.mn.us/sites/default/files/c-rem1-01.pdf.
• Determination of excavated soil as regulated solid waste. One or more of the required list of analytes, including cPAHs expressed as B[a]P equivalents and any other site-specific contaminants, exceed the Residential SRVs but do not exceed the Industrial SRVs. The excavated soil/media requires special management and cannot be used as unregulated fill.
• Excavated soil/media that is not considered unregulated fill is most commonly guided to a solid waste landfill. Depending on the types and concentrations of contaminants, soil/media may need to be disposed of at a Municipal Solid Waste (MSW) landfill that has an industrial solid waste management plan; that do accept contaminated soils. This means contaminated soil/media must go to a MSW landfill that has a liner and a leachate collection system.

MSW landfills in Minnesota that can accept contaminated soil/media are listed at this webpage, or the list can be accessed directly at this link. Some additional landfills that are permitted to accept industrial waste, and which may also accept contaminated stormwater soil/media, include:

• Voyageur Industrial Landfill in Cannon Falls, Minnesota
• Vonco II Landfill in Becker, Minnesota
• Vonco V Landfill in Duluth, Minnesota
• Shamrock Environmental Landfill in Cloquet, Minnesota
• Dem-Con Landfill in Shakopee, Minnesota
• Veolia E S Rolling Hills Landfill in Buffalo, Minnesota
• SKB Rosemount Industrial Waste Facility in Rosemount, Minnesota

Guidance for analytical data comparing contaminants to SRVs and calculating B[a]P equivalents are summarized in Appendix B. This may be an important variable as soil/media removal projects are planned and samples are collected and compared. It is recommended that you consult with contractors and contact disposal or re-use facilities to ensure they will be able to accept your waste and to determine what additional sampling requirements (if any) may be required by the facility.

3. Engineering, contracting, and work plans. Once evaluation of the material is complete, a work plan for removal and management of the material is developed. Work plan development includes a wide range of logistics including, but not limited to the following.

• Notification of residents and neighbors
• How to access the site and determining what machinery will be needed to remove soil/media
• Define how soil/media will be removed, measured, and paid for
• Testing or analysis requirements for the destination disposal or treatment facility
• Plans for erosion control
• Tree removal, environmental impact, depth to ground water, and risks associated with the displacement of wildlife or invasive species
• Lack of design and/or construction documentation (no “as-built” records)
• Estimating water management needs and the amount of time and oversight needed to manage water (e.g. drain the stormwater collection system of ponded water)
• What permits (if any) may be required by your local watershed district, county, or the DNR. The MPCA does not require a permit or notification for routine maintenance of stormwater devices. Cities or other stormwater system owners are advised to keep records and documentation of their soil/media removal projects as outlined in this guidance and as required by the Municipal Separate Storm Sewer Systems (MS4) Permit. A permit from the MPCA is required if projects will disturb one or more acres upland. Projects disturbing one or more acres upland are required to have a Construction Stormwater Permit.
• Defining appropriate BMPs for dewatering (e.g., rock riprap, sand bags, plastic sheeting, or other accepted energy dissipation measures), such that the discharge does not adversely affect the receiving water or downstream landowners.
• Ensuring that water from pumping or draw-down activities is discharged in a manner that does not cause nuisance conditions, erosion in receiving channels, or erosion on down-slope properties. This also includes inundation of wetlands causing significant and/or adverse impact. The general rule of thumb is “keep it clear”.
• How soil/media will be transported and a process to track the volume removed.
• Defining logistics, administrative, and engineering requirements, surveys, dewatering processes, site access and easements, rock entrance and off-site tracking needs, coordination with adjacent cities, and/or watershed districts and the Minnesota Department of Transportation.

4. Excavating soil/engineered media. Soil/media excavation projects can take place during the winter or summer. Conducting soil/media removal projects in the winter offers some benefits over summer projects. Winter excavations greatly reduce the risk that rain may cause flooding and erosion during excavation, or turbid runoff conditions. Access with trucks and heavy machinery is easier in the winter when soil surrounding stormwater ponds freezes solid, minimizing soil disturbance or compaction. Adjacent residents and neighbors have windows closed in winter and this means less noise, less dust, less odor, and fewer disturbances overall. Winter excavation projects also have a few drawbacks. They include shorter working days, and possible need for lights after dark to extend the work day. Working in freezing conditions and sub-zero weather could include hard frozen ground; snow and ice cover in work area; and potential frozen water mass increasing cost of landfill disposal.

Summer excavations include the risk of unexpected rainfall events that can complicate a conventional sediment/soil removal project and sometimes delay the project for days and increase the risk to receiving waters down-stream. Small projects may be completed in one day or less and risks associated with unexpected rainfall events can be minimized or avoided altogether. Small projects (less than 1 acres) do not require a construction stormwater permit, but safeguards and best management practices are still required to ensure negative down-stream impacts to receiving waters are prevented. Large projects that will disturb one or more acres upland are required to have a Construction Stormwater Permit to ensure BMPs are implemented as the scale of the project and potential risks to receiving waters increase.

Survey work is usually conducted before and after excavation to estimate the amount of sediment/soil/media to removed and to identify the depths of excavation in order to restore desired capacity. If the removal volume is not defined by surveying, then establishing a standard volume per truck and calculating the volume based on truck loads leaving the site can be used to track the volume in cubic yards.

Excavating or removing sediment from stormwater collection systems requires care to prevent turbid water and pollutants from impacting down-stream waters such as wetlands, streams, rivers, or lakes. This is just as true for winter sediment removal projects as it is for projects conducted during the summer months.

5. Site restoration and erosion control. Site restoration work should be conducted as soon as weather conditions permit and may include:

• Additional cleanup or maintenance of inlet and outlet structures.
• Additional site stabilization work including sediment and erosion control.
• Establishing plants, seed, sod, mulch, or vegetation to prevent erosion (above water line).
• Professional engineer sign-off on project completion.

Erosion control (temporary and permanent) are should be incorporated into plans and specifications for stormwater soil/media removal projects as appropriate. Permanent erosion-control features may include provisions for:

• Vegetative buffer strips around the stormwater device.
• Design of grassed waterways and overflow channels.
• Armoring of spillways or other features needed to prevent erosion for the life cycle of the stormwater collection and conveyance system.

Temporary erosion control features may include provisions such as mulch, tackifiers, or erosion control blankets to prevent erosion until seeding takes root and vegetation becomes established. Erosion of banks, side slopes, spillways, outfalls, channels, and adjacent upland areas disturbed by machinery are all priority areas during site restoration. These areas should be stabilized as quickly as possible to prevent erosion.

Areas susceptible to erosion should be inspected frequently following a sediment/soil/media removal project until vegetation is reestablished or the site is otherwise stabilized. If erosion occurs, the eroded areas should be restored as quickly as possible. If erosion persists, action must be taken immediately to protect downstream receiving waters with permanent erosion control.

6. Records and documentation to keep on file. It is important to keep good records about the operation and maintenance of stormwater collection systems. Good records will not only assist with an accurate inventory and triage of stormwater ponds, but they can also provide the basis for sound planning in the future. Important records and documentation for sediment/soil/removal removal projects may include:

• Inspection dates and frequency of inspections (Required by MS4 Permit)
• Description of maintenance and dates performed (Required by MS4 Permit)
• The unique ID# of the stormwater device or feature (Required by MS4 Permit)
• Employee training records (Required by MS4 Permit)
• Volume of sediment/soil/media removed in cubic yards (Required by MS4 Permit)
• Evaluation, testing, and/or laboratory results (Required by MS4 Permit)
• Place of disposition/disposal (Required by MS4 Permit)
• “As Built” prints or plans if they exist
• The name and geographical location of the pond with reference to nearest cross roads
• Contractor information, shipping papers/manifests/contractual agreements
• Any other observations about the sediment/soil/media removal, or work performed, that will help the city/owner operate and maintain the site in the future

 

References

• Al-Ameri, M., B. Hatt, S. Le Coustumer, T. Fletcher, E. Payne, A. Deletic. 2018. Accumulation of heavy metals in stormwater bioretention media: A field study of temporal and spatial variation. Journal of Hydrology. Volume 567, Pages 721-731. https://doi.org/10.1016/j.jhydrol.2018.03.027.
• Furén, R., K. Flanagan, R.J.Winston, A. Tirpak, J.D.Dorsey, M. Viklander, G. Bleckena. 2022. Occurrence, concentration, and distribution of 38 organic micropollutants in the filter material of 12 stormwater bioretention facilities. Science of The Total Environment. Volume 846:10. https://doi.org/10.1016/j.scitotenv.2022.157372.
• Gulbaz, C., M. Kazezyılmaz-Alhan, and N.K. Copty. 2015. Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies. Water, Air, & Soil Pollution, volume 226, Article number: 376. https://doi.org/10.1061/(ASCE)0733-9372(2008)134:6(409).
• Hathaway, J. M., Hunt, W. F., Graves, A. K., and Wright, J. D. 2011. Field evaluation of bioretention indicator bacteria sequestration in Wilmington NC. J. Environ. Eng., 137(12), 1103–1113.
• Hatt, BE; Steinel, A; Deletic, A; Fletcher, TD. 2011. Retention of heavy metals by stormwater filtration systems: breakthrough analysis. Water Science and Technology. 64:9:1913-1919. 10.2166/wst.2011.188.
• Hunt, W.F. and W.G. Lord. 2006. Bioretention Performance, Design, and Construction and Maintenance. North Carolina Cooperative Extension Service.
• Hunt, W., Davis, A., and Traver, R. 2012. Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design. J. Environ. Eng., 138(6), 698–707. doi: 10.1061/(ASCE)`EE.1943-7870.0000504
• Johnson, J.P., W.F. Hunt. 2016. Evaluating the spatial distribution of pollutants and associated maintenance requirements in an 11 year-old bioretention cell in urban Charlotte, NC. Journal of Environmental Management. Volume 184, Part 2, Pages 363-370. https://doi.org/10.1016/j.jenvman.2016.10.009.
• Li, H., and H.P. Davis. 2008. Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies. Journal of Environmental Engineering Volume. 134 Issue 6. https://doi.org/10.1061/(ASCE)0733-9372(2008)134:6(409).
• Linh, N. 2011. What is in bioretention systems? Depth of metal contamination in bioretention media filtering urban stormwater. The University of Aucklund.
• Lucke, T. and P.W.B. Nichols. 2015. The pollution removal and stormwater reduction performance of street-side bioretention basins after ten years in operation. Science of The Total Environment. Volume 536:1 Pages 784-792. https://doi.org/10.1016/j.scitotenv.2015.07.142.
• Morgan, J.G. 2011. Sorption and Release of Dissolved Pollutants Via Bioretention Media. M.S. Thesis, University of Minnesota.
• Polta, R., S. Balogh, and A. Craft-Reardon. 2006. Characterization of stormwater pond sediments. Final project report. EQA Report 06-572. Environmental Quality Assurance Department, Metropolitan Council Environmental Services, St. Paul, MN.

Appendix A: Sampling Plan

This technical guidance should be shared with staff or environmental consultants responsible for sampling sediments and interpreting the analytical results for the owner or responsible party. It is the responsibility of the owner or responsible party to either train their staff or select consultants who can perform these tasks.

Bioretention Soil/Media sampling

The U.S. Environmental Protection Agency’s (EPAs) report on “Methods for Collection, Storage and Manipulation of Sediments for Chemical and Toxicological Analyses: Technical Manual” (EPA 2001) provides guidance on sediment monitoring plans, collection of whole sediments, field sample processing, transport and storage of sediments, sediment manipulations, and quality assurance/quality control (QA/QC) issues. This report should be used as a resource by owners or responsible parties, and their consultants, for sampling and processing stormwater pond sediments. In particular, this user friendly document provides pictures of sediment sampling equipment, flowcharts for making decisions, checklists, and boxes of important bulleted items.

Soil/media characterization

Bioretention basins and other stormwater devices can have a wide variety of sizes and designs. The concentration of pollutants in the stormwater they are receiving also varies, depending on land use and pollutant sources in the watershed contributing runoff. The soil and other media used in construction can also vary – while most rainwater gardens are simple construction with soil amended with compost and mulch, some stormwater devices may amend the soil with other media for enhanced pollutant removal. Possible media types include biochar, water treatment residuals (WTRs), granular iron, and crushed limestone. These media are typically used for their ability to absorb dissolved pollutants (e.g. dissolved phosphorus, dissolved metals), as well as physically filter particulates.

Polycyclic aromatic hydrocarbons (PAHs) are the most common pollutant to exceed MN SRVs in stormwater pond sediment. Based on the MPCA’s 2009 stormwater pond study (Crane 2014), coal tar-based sealant sources comprised 67.1% of total PAHs in surface sediments of ponds located primarily in residential, commercial, and industrial land use areas. Higher concentrations of PAHs will occur in stormwater pond sediments in watersheds where coal tar-based sealants are used on driveways and parking lots than in watersheds where either asphalt-based sealants (which have much lower concentrations of PAHs), no sealant, or other material such as concrete, permeable pavers, or gravel are used for driveways and parking lots. Even though a statewide ban on coal tar-based sealants went into effect January 1, 2014 in Minnesota, abraded coal tar-based sealant particles from existing driveways and parking lots will continue to wash off into stormwater collection and conveyance systems for years to come. As these parking lots and driveways are sealed with asphalt-based sealants in the future, and with the elimination of new applications of coal tar-based sealants, concentrations of PAHs in sediment deposits are expected to decrease over time. The MPCA requires owners or responsible parties of rainwater gardens and other stormwater bmp media to sample soil/media prior to potential offsite reuse or disposal to determine concentrations of pollutants. At a minimum, the following baseline parameters should be analyzed for all soil/media:

• PAHs: 17 cPAHs, 10 noncarcinogenic PAHs
• Metals: the 8 RCRA metals (arsenic, barium, cadmium, chromium, lead, mercury, selenium, silver), and copper.
• Diesel range organics (DRO)
• Gasoline range organics (GRO)

The specific cPAHs and noncarcinogenic PAHs can be found in MPCA’s “Summary of Stormwater Pond Sediment Testing Results” spreadsheet available on MPCA’s website at: https://www.pca.state.mn.us/sites/default/files/wq-strm4-79.xlsx. There are not human health based soil criteria for DRO and GRO. However, the MPCA’s guidance for unregulated fill https://www.pca.state.mn.us/sites/default/files/c-rem1-01.pdf states that soils should have less than 100 mg/kg total petroleum hydrocarbons (TPH), which can be determined by summing the results of DRO and GRO. Additionally, some landfills request DRO and GRO testing for characterization of the soil. Lighter weight petroleum components of gasoline and diesel fuel are not likely to persist in bioretention basins, due to evaporation or microbial degradation in the soil. However, heavier weight petroleum products, such as motor oil, that are also measured by DRO may persist. It is important to request that the laboratory perform a “silica gel cleanup” prior to performing the DRO test, otherwise natural organic matter in the soil will be included in the test results.

Additional considerations

It is the responsibility of the owner or responsible party to evaluate the drainage area of each stormwater collection system to determine whether spills, improper disposal, or the potential for a release from commercial or industrial operations indicate that sampling for other contaminants is needed. For example, if sediment is being removed from a pond in an industrial park and there has been a release of contaminants known to accumulate in sediments (example, nickel from a metal plating facility), the owner or responsible party should include those contaminants on the list for sampling.

Analysis of soil/media samples for particle size and total organic carbon (TOC) is optional, but this information may be useful for some beneficial reuse scenarios of the excavated sediment. The analytical laboratory will provide guidance on the mass of sediment needed for each analysis. Field sampling should be conducted early in the process to provide timely assessments of management options. Sediment sampling for required analytical parameters must be conducted regardless of the volume of soil/media to be excavated from the rainwater garden or other bmp. General guidance for characterizing soil/media is as follows:

• Sampling depth: Sampling should be to the planned depth of excavation or greater. It is the responsibility of the owner or responsible party to collect soil/media samples that will cover the depth to be excavated. The important issue is to submit a sample to the analytical laboratory that is representative of the entire depth interval to be excavated. It is not acceptable to only skim the surface of the soil/media since doing so may miss out on the historical record of sediments (and contaminants) deposited in different depth intervals, or concentrated in soil amendments below the surface.
• Sampling equipment: Most bioretention basins soil/media can be most easily sampled with a hand auger. The hand auger should be of stainless steel construction. A shovel may help remove surface soils or vegetation to access deeper soils, but best practice is to avoid sampling soils that have come into contact with other tools not designated for sampling. Material such as crushed limestone or sand-iron media may require a shovel to facilitate sampling, as a hand auger is unlikely to be effective in this material.
• Sampling location data: Geopositional (GPS) coordinates need to be collected at the location of each sample site.
• Sample number and design: The number of samples to be collected depends on the volume of soil/media to be removed and reused or disposed. A sample should be collected for every 2,000 cubic yards of soil/media that is removed. Note that landfills may have different requirements for number of samples per soil volume.
• The goal is to collect soil/media samples that are representative of the material that will be removed to maintain/restore the functionality of the bioretention basin or other device.
• Multiple samples may to be collected for larger volume removals (>2,000 cubic yards).
• Excavation/removal area. Sample sites may be selected randomly or in a transect from the main inlet to the outlet of the basin/device. When sediment/soil/media removal is targeted only for a certain location(s) within the basin (e.g., a sediment delta near an inlet), sample sites should be selected in the same manner except that the candidate areas for site selection should be defined by boundaries of the targeted area rather than the entire basin.
• Excavation areas greater than X acres or Y cubic yards: For planned soil/media removal within stormwater basins or portions thereof that are larger than X acres, divide the area into roughly equal size subsections so that no subsection is larger than X acres. Select three or more sites to subsample within each subsection to create a representative composite of the subsection. Soil/media from each subsample needs to homogenized (mixed well) in a precleaned container (e.g., large 4 L Pyrex mixing cups work well; larger volumes can use precleaned plastic buckets). For a given subsection, an equal aliquot of soil/media from each associated subsample is then composited together to form the sediment sample for that quadrant that is submitted to the analytical laboratory. When soil/media removal is targeted only for a certain location(s) within a stormwater basin/device (e.g., a sediment delta near an inlet), sample sites should be selected in the same manner except that the candidate areas for site selection should be defined by boundaries of the targeted area rather than the entire stormwater device.
• Field replicate samples: To provide a measure of field precision, it is recommended to collect one field replicate sample for every 10 samples or less collected for analysis (i.e., 10% of samples should be collected in replicate). The goal of a replicate is to be as similar in space and time as one of your “primary” samples. Select the sample(s) to be replicated. One can generate a field replicate with surplus sediment from the homogenized sample already collected for that sample. Adherence to the same procedures and timeline will enhance your analytical precision and results.

Sample collection, handling, and processing (prior to submittal to laboratory) practices.

• Remove any rocks, pebbles, trash, or large pieces of detritus from each subsample and composite sample.
• Overlying water, if present, needs to be decanted from the subsamples.
• Composite sediment samples in the field prior to splitting into the sample jars.
• Sample homogenization and splitting: Sediment samples should be homogenized (mixed well) before splitting the sample into pre-cleaned jars for analyses. Precleaned large stainless steel spoons can be used for mixing.
• Sample labeling and laboratory bottles: The laboratory will provide pre-cleaned sample jars and labels for clients, including separate containers for PAHs, metals, and in some cases percent moisture analysis. Use a permanent marker to fill out the sample label. It is often helpful to pre-label your bottles (before adding sample) both to avoid confusion and the difficulty of attaching labels to wet surfaces. It is also helpful to wrap clear packing tape around the label to secure it on the jar because labels may easily come loose while on ice in coolers during transport.
• Sample percent moisture analysis: Laboratories measure the percent moisture in the samples to convert the results to dry weight measurements. In some cases, the laboratory will provide a separate sampling container for percent moisture analysis.
• Sample transport, storage, and tracking: Store the samples on ice in a cooler during field sampling. Sample tracking forms or chain-of-custody forms must be used during field sampling to record observations about the samples and to provide field sampling information (e.g., sample station, date, time, sampling equipment, analyses to be done). Most analytical laboratories will provide their clients with chain-of-custody forms.

Submit samples to analytical laboratories At the end of each field sampling day, either transfer the samples directly to the analytical laboratory, which is preferred, or store them in an interim refrigerator or freezer (depending on the specifications of the laboratory) prior to submittal. Some laboratories may provide a courier pick-up service. When out of-town laboratories are used, ship the samples on ice in sturdy coolers using an overnight courier; also use packing peanuts and consider wrapping each jar in bubble wrap. The analytical laboratories will provide guidance on the holding times for samples based on the analytical parameter. To increase the success of the analytical work, follow these steps prior to submitting the sediment samples:

• Matrix Spike/Matrix Spike Duplicate (MS/MSD) analysis: To assess analyte recovery and precision, request/confirm that the laboratory will spike and analyze one Matrix Spike and one Matrix Spike Duplicate per 20 samples or less, as is usual standard practice.
• Sample tracking/chain of custody: Provide a copy of the sample tracking or chain of custody form to the analytical laboratory when the samples are submitted or shipped to them.

 

Appendix B. Carcinogenic PAHs and determination of benzo[a]pyrene equivalents

Appendix B provides guidance for comparing contaminant concentrations from stormwater pond sediment to the MPCA’s Remediation Division Soil Reference Values (SRVs) and instructions for calculating benzo[a]pyrene (B[a]P) equivalents for carcinogenic polycyclic aromatic hydrocarbons (cPAHs).

Soil Reference Values (SRVs): SRVs are risk-based values derived to assess potential human health exposures from soil at a Remediation cleanup site using a reasonable maximum exposure (RME) scenario. RME scenarios are intended to protect an entire population without being overly conservative by using reasonable upper bound estimates for the most sensitive exposure parameters and central tendency estimates for less sensitive exposure parameters. They are intended to evaluate both potential non-cancer and cancer risks associated with a contaminant present in soil. Two separate SRVs are calculated for each contaminant, one for non-cancer risk and one for cancer risk. The final SRV reported as the Residential or Industrial SRV is the lower of the two. In other words, it is the smallest concentration of the contaminant that could potentially pose either a non-cancer or cancer risk. For example, for contaminant “X”, if the non-cancer SRV is 10 mg/kg and the cancer SRV is 5 mg/kg, then the final SRV is reported as 5 mg/kg.

The method for calculating the BaP equivalents values for samples with non-detect results can be found in Appendix C of the MPCA’s “Soil Reference Value Technical Support Document” (April 2022) https://www.pca.state.mn.us/sites/default/files/c-r1-05.pdf. The document describes the Kaplan Meier statistical method for calculating the BaP equivalents.   Table B-1. List of PAHs to be analyzed in stormwater soil/media/sediment
Noncarcinogenic PAHs

• Acenaphthene
• Acenaphthylene
• Anthracene
• Benzo[g,h,i]perylene
• Fluoranthene
• Fluorene
• 2-Methylnaphthalene
• Naphthalene
• Phenanthrene
• Pyrene

Carcinogenic PAHs

• Benzo[a]anthracene
• Benzo[b]fluoranthene
• Benzo[j]fluoranthene
• Benzo[k]fluoranthene
• Benzo[a]pyrene
• Chrysene
• Dibenz[a,h]acridine
• Dibenz[a,h]anthracene
• 7H-Dibenzo[c,g]carbazole
• Dibenzo[a,e]pyrene
• Dibenzo[a,h]pyrene
• Dibenzo[a,i]pyrene
• Dibenzo[a,l]pyrene
• 7,12-Dimethylbenz[a]anthracene
• Indeno[1,2,3-cd]pyrene
• 3-Methylcholanthrene
• 5-Methylchrysene

Note: A combination of benzo[b]fluoranthene, benzo[j]fluoranthene, and/or benzo[k]fluoranthene frequently coelute together when sediments are analyzed