By Tina K. Mudd, PMP, CPESC, REP, QSD/QSP

here are many articles about stormwater compliance that address how to plan for and install best management practices (BMPs) on construction sites, and how and when to notify regulators about changes to plans, but the most successful contractors take an extra step. They create an environment where everyone on the team—environmental engineers or consultants, contractor leadership and onsite crew—assumes responsibility for stormwater management.

Creating a communications loop that provides information to the people involved in oversight, regulation and installation of BMPs leads to a worksite that promotes proper stormwater management as an important part of the construction project. This results in a true team approach to stormwater management that leads to a more successful build that is not hampered by regulatory delays and ultimately this team approach will help the contractor’s bottom line.

The steps to building the ideal communications loop do not happen overnight and require some strategic decisions to change how the entire team views stormwater management.

Build a relationship with agency partners

Don’t wait for a site inspection to meet the regulatory representative. Meet the inspector before you begin building to talk about what BMPs you are implementing and why. Of course, the beauty of a stormwater plan is that you can constantly change it. By building rapport with your regulatory partner you can ensure open communication and a resource to trouble shoot solutions if the initial plan is not working.

In the first call or meeting with inspectors, ask how they want to receive communications: phone call, email or text. People like to communicate in different ways, so ensuring that you are using their preferred method lets them know that you respect their time and how they organize their work responsibilities.

This approach to communications makes the regulatory representative part of the team and eliminates an “us and them” atmosphere. Being open to alternative suggestions creates trust between contractors and inspectors and helps to form a good relationship. In the end, the goal is the same for both parties—to protect our communities’ water resources.

Most importantly, don’t be afraid to reach out to regulators proactively. Some contractors may be hesitant to contact inspectors if there is a problem to resolve because they think it automatically opens an enforcement action. This is not the case if the contractor has been transparent throughout the process. For example, if the dewatering process is not working as expected, communicate to the regulator why it isn’t working and ask if a different tactic can be tried. This type of communication shows that you are committed to proactive permit compliance on the site and are working to find the most effective solution.

Not all construction companies have environmental engineers on staff, so they may rely on consultants to help design and oversee the performance of BMPs as well as work with regulators. No matter how experienced the consultant is, contractor representatives also need to develop relationships with agency representatives. The contractor is ultimately responsible for stormwater management, so there is a risk if a third party is allowed to handle the relationship with no involvement from the contractor.

Communicate with and listen to the construction crew

The more a construction crew knows about stormwater management and why certain measures are taken to control stormwater, erosion, and sediment, the more invested in the process they become. In addition to explaining why specific BMPs are installed at a jobsite and how they work to manage stormwater better, keep the issue top of mind by continually offering information. Include environmental topics along with safety topics in the weekly tailgate talks. All construction crews understand the importance of safety on a jobsite, so the inclusion of environmental topics in these meetings conveys their importance.

Other ways to keep onsite construction crews aware of stormwater management importance include:

• Post a stormwater control map onsite. Place a map where it can be seen during every daily status meeting and encourage forepersons and project managers to use a pen to markup changes and updates. This keeps the plans and the actual onsite BMPs synced so that everyone has the same information.
• Empower employees. All employees should stop work when they see an erosion or water control issue, just as they are empowered to stop work when they notice a safety issue. For example, any employee driving by a silt fence that has fallen should know to report the issue so it can be addressed immediately. The only way this happens is by educating all employees about the importance of that control.
• Educate forepersons, supervisors and project managers. Send forepersons and project managers to courses where they learn about stormwater management, permit requirements and types of controls. They do not have to become experts, but they will benefit from a basic education and the language with which to speak with inspectors. If the inspector shows up unexpectedly, the supervisor or foreperson is not afraid to handle a compliance inspection—in fact, they are proud to do so.
• Listen to the onsite crew. In addition to educating onsite employees through tailgate talks, seminars or online courses, remember that good communication is a two-way street. Ask employees who are installing or working around the BMPs if they have alternate installation ideas. Asking for their input demonstrates the “team” concept of stormwater management.

Lastly, don’t make the most common mistake of assuming the installation, oversight and maintenance of BMPs is someone else’s responsibility. The foreperson and crew are responsible for installing BMPs and numerous other things to build the project. Maintain a realistic perspective while communicating stormwater priorities. Remember that the goal is that everyone—contractor, inspector and onsite crew—works together as a team. 

About the Expert

Tina K. Mudd, PMP, CPESC, REP, QSD/QSP, is the environmental manager at Granite Construction in Sparks, Nevada. She holds a Bachelor of Science and a Master of Environmental Policy from the University of Nevada, Reno. She is responsible for regional environmental permitting, monitoring and compliance for all aspects of the construction and materials operations. She is a member of the Associated General Contractors (AGC) National Environmental Steering Committee and Chair of the Nevada AGC Environmental Committee.

By Bill Murphy, PE

The city of Des Moines, Iowa, U.S. is investing in its infrastructure to eliminate combined sewer overflows (CSOs). Like many cities, Des Moines has neighborhoods with no existing storm sewer or undersized storm sewer systems that need to be replaced or reconfigured. Removing several blocks of paved streets and relocating an unimaginable number of utility conflicts to install an entirely new storm sewer system
is impractical.

One such example is the Near West Side Sanitary Sewer (NWSSS) project located near Drake University. The city’s consultant engineer for this project, Wes Farrand, PE, Snyder & Associates, incorporated green infrastructure (GI) to intercept stormwater runoff and take multiple gulps of stormwater along the curb to capture, treat and slowly release a reduced flow downstream. A residential neighborhood along the 2800 block of Rutland Avenue in the upper corner of the project’s watershed was selected. This mature neighborhood has no storm sewer inlets along the street. Minor street flooding was common.

The project engineer started by creating depressions between the back of curb and the existing city sidewalk to create shallow basins for ponding stormwater as it slowly drained down through a bioretention system. After first maximizing the areas bounded by street, sidewalks and neighborhood driveways, Farrand reduced the footprint of those proposed areas to account for mature trees, utility poles and any existing underground utility conflicts. What remained were six individual areas where a bioretention system could be installed.

Farrand started his design with the end in mind. Since bioretention systems are typically designed to handle the first flush or a similar design storm event, he accounted for system overflow during major storm events. He designed a beehive inlet structure at each of the six cells. Those structures connected to a collector storm sewer pipe under the street. The underdrain of each bioretention system connected to a corresponding newly installed overflow structure. Since the bioretention cells were built after street construction was completed, it was critical to not damage the newly paved street or residential driveways during construction of the bioretention systems.

Each of these systems receives the first flush from its corresponding drainage area. Any major storm event that fills the bioretention cells and overtops into the overflow structures will continue to drain down the street as it always has, but with much less flooding since each of these cells will be taking a “gulp” and reducing the peak flow. Exact numbers for the design flow calculations is not readily available since we maximized the size of each cell during construction.

Due to late fall construction, proposed plants were planted in the spring. Plants are vital to these systems for long term success, greater pollutant removal and pleasing aesthetics. Plants remove phosphorous and nitrogen from stormwater and their roots provide pathways for water to pass through the media and into the underdrain. Locally available plants that are hardy in regard
to periods of drought and short term inundation with standing water will thrive
in bioretention.

Maintenance was a top concern of the city so Farrand proposed curbside pretreatment devices to capture large debris and most sediment before releasing the runoff into the bioretention cell. Construction of these GI improvements was completed fall 2020. A site visit in March 2021 showed that the pretreatment devices on all six cells worked as designed and captured debris and sediment that would have otherwise settled onto the bioretention systems.

The tools used to clean the debris and sediment from the curbside pretreatment device and the bioretention cell were a shovel, a rake, a pitchfork and buckets. Maintenance was simply a matter of removing the material from the street gutter, through the concrete flume and from above the grate of the pretreatment device. The grate is manufactured as two halves so one person can easily lift and remove the grate. After removing the grate, the sediment is scooped out of the unit and the filter screen is removed for cleaning. Once all cleaning is done, the filter screen and grate are reinstalled.

The pretreatment device at the first bioretention cell on Rutland Avenue yielded 29 lbs (13 kg) of debris and sediment after only five minutes of maintenance cleaning. Following the demonstration of how to properly clean a bioretention cell, the other cells were cleaned by the general contractor or landscape contractor under the city’s direction. No measurements of sediment were taken at the other five.
Every city will need to determine how to best manage green infrastructure after installation. Like any filter, bioretention systems and pretreatment devices must be inspected and cleaned as part of routine maintenance.

Once the curbside pretreatment device is cleaned, the bioretention cell must be cleaned. Thanks to the pretreatment device, there is no sediment in the cell. Most of the debris in this cell is leaf litter due to the mature trees in this residential neighborhood. Similar bioretention systems in retail or industrial areas typically collect a large percentage of trash rather than vegetation. Since this is a new project, this cell had not yet received its proposed plantings at the first site visit, which made cleaning easy and relatively quick. After 20 minutes of removing debris—mostly by hand—87 lbs (39 kg) of material was collected. That brought the total amount of sediment and debris collected from this first system to 116 lbs (53 kg). With a drainage area of 115,000 ft2 (10,684 m2) for this one cell, that is a lot of pollution captured locally and not allowed to flow downstream through the city of Des Moines and eventually into the Des Moines River.

Filters collect debris and sediment after plantings installed to make cleaning and maintenance of the bioretention cell easier.

Plantings were added in the spring to create the “bio” component of the cell. A site visit several months later showed that sediment and debris inside the pretreatment units proved that the “filter” was working as intended and kept the debris out of the bioretention cells. It will be much easier to remove the sediment and debris from the pretreatment units than it would be to carefully try to remove it from the bioretention cells without damaging the plants.

There will still be some maintenance required in each bioretention cell but it will be much simpler and quicker than it would have been without the pretreatment units. Any filter that is actually working should be dirty and need to be cleaned or replaced. The same concept applies to your furnace filter at home and the air filter in your car.

Since this Rutland Avenue project is part of a very large public improvement project there is a long transition period after the GI construction is complete but before the entire project is finished. During that time the city, contractor and neighbors are not sure who is responsible for maintenance. That is part of the education that is necessary following installation of bioretention cells that engineers must communicate to cities.

This one city block of GI makes a significant difference in reducing localized flooding in this residential neighborhood. However, the bioretention systems only make a minor difference in the flooding and water quality in the NWSSS watershed because it is only one block of GI, but it is one small step in the right direction. In order to make a significant impact on a watershed scale these types of practices, along with permeable paving, soil cells for trees, inline stormwater filters, underground detention and more would have to be installed and maintained throughout the watershed. The natural progression is to implement GI strategies throughout the entire city. That will require long term strategies from all stakeholders.
Think of it like the starfish story that makes the rounds on social media. A guy walking along the beach is picking up dying starfish and tossing them back into the ocean to save their lives. Another man walking towards him says “There are countless dying starfish. Do you actually think you are making a difference?” The first guy picked up another starfish, tossed it into the water and said with a smile, “It made a difference for that one.” 

About the Expert

Bill Murphy, PE, civil engineer, Quick Supply Co., ASP Enterprises, Bowman Construction Supply, Cascade Geosynthetics. He spent the first 15 years of his career as civil engineering consultant before becoming a stormwater expert in the construction supply industry more than 10 years ago.

By Billur Kazaz, MS, CPESC-IT; Michael A. Perez, Ph.D., CPESC; Wesley N. Donald, Ph.D., CPESC

Flocculants are an effective solution to improve the sediment capture performance of construction stormwater management practices. With the appropriate product selection, dosage and application, construction stormwater can be rapidly treated to remove fine-sized soil particles from suspension.¹ This study investigates the use of flocculants for construction stormwater applications in the United States by presenting results from a state-of-the-practice survey. The survey was distributed to state departments of transportation (DOTs) to investigate current flocculant implementations on construction sites. The study highlights the perspective of state DOTs on using flocculants for construction stormwater treatment.

Use of Flocculants

Flocculants are water-soluble polymers that have been used by many different industry applications for solid-liquid separation purposes including water treatment, mining and construction stormwater management. Flocculation occurs as a result of a chemical process that binds small soil particles with a bridging mechanism and forms larger flakes that settle out of suspension.² Construction stormwater management has shifted its focus on these chemical agents in a positive way for reducing erosion and treating sediment-laden runoff. Research studies show that these chemicals are highly effective in construction stormwater treatment with adequate application techniques and dosage recommendations.^3-7 On the other hand, these chemicals have the potential to create risks for polluting water bodies and damaging aquatic life in case of an overdose and improper implementation.

The U.S. Environmental Protection Agency emphasizes the significance of proper dosage guidance and adequate application techniques in flocculant usage for minimizing pollution in downstream water bodies.⁸ Therefore, state agencies are cautious when integrating flocculants into their specifications for construction stormwater treatment. This research was performed to understand the current perspective of DOTs on flocculant selection, dosage and application guidance.

Survey Questions and Distribution

The survey was developed and delivered using an online survey platform with multiple-choice questions that primarily focused on identifying flocculant users and non-users among the DOT’s. Survey participants received questions based on their flocculant usage. Three questions were asked of the non-using agencies and up to ten questions were asked of flocculant users. Depending on their responses to questions, the survey would deviate into differing paths to ensure appropriate questions were asked. DOTs that integrate flocculants into their construction stormwater management specifications received detailed questions on usage purpose, approved flocculant types by their state, dosage and application guidance, and residual monitoring.

The survey was distributed to lead construction stormwater professionals of 51 DOTs, in 50 states and Washington D.C. The email invitation provided an anonymous link to the survey. The questionnaire was kept active for a month, and three distribution cycles were planned for reminders. Some of the participants preferred to complete the survey via phone interviews. A total of 37 agencies responded to the survey invitations and completed the survey questions by phone or online. Specifications and design manuals of non-participating agencies were reviewed to provide complete knowledge on the flocculant usage for construction stormwater treatment in the U.S.

Survey Findings

The state-of-the-practice survey data provided useful data for evaluating current flocculant usage in the U.S. Results indicated that 31 DOTs (61%) are hesitant on using flocculants on construction sites, with only 20 DOTs (39%) using flocculants. Figure 1 displays the map of flocculant usage in the U.S. based on the survey data and the specifications of non-participating agencies. Solid colors represent the survey data, while dashed pattern symbology represents information gathered from non-participating agency manuals and specifications. Orange-colored fill represents state highway agencies that do not allow flocculants and blue represents states that use flocculants for construction stormwater treatment.

The use of flocculants are most common in southeastern states and along the west coast of the U.S. Alaska, Connecticut, District of Columbia, Hawaii, Illinois, Kentucky, Massachusetts, Michigan, Montana, New Jersey, New York, Pennsylvania, Rhode Island and West Virginia are the states that did not respond to the survey invitation. Based on the erosion and sediment control manuals of these states only Alaska, Connecticut, District of Columbia, Illinois, New York, Rhode Island and West Virginia allow the use of flocculants.

The reasons behind the hesitation for using flocculants on job sites were investigated by the questionnaire. Fifty percent of the state agencies surveyed consider their current stormwater management practices as sufficient for treating construction stormwater. State agencies are primarily concerned about polluting downstream water bodies due to inadequate dosage and application rates.
DOTs that responded that they allow flocculant use received additional questions in the questionnaire for gathering detailed information on their perspective. The purpose of flocculant usage on job sites was asked in the questionnaire. The results showed that 12 (92% of) “flocculant allowing” agencies use these chemical agents for sediment control on construction sites and four (31%) of these agencies use flocculants as an erosion control together with sediment control to reduce soil erosion on slopes.
Survey results also indicated that the most common flocculant types preferred by state agencies are anionic polyacrylamide (PAM) (62%), chitosan (38%) and polyaluminum chloride (PAC) (23%), respectively.

Improperly applied, flocculants can be highly toxic to the downstream aquatic environment.⁹ Therefore, the survey also focused on understanding the current dosage and application guidance that has been adapted by the DOTs. Figure 2 (page 16) illustrates the results for the dosage and application guidance question in the survey. The results show that most of the DOTs (55%) are relying on manufacturer guidance, and 23% of responding agencies do not have any developed guidance.

The survey results identified the demand for residual monitoring on construction sites for protecting downstream water bodies from the toxic impacts of these chemical agents. Based on the survey responses, residual monitoring is required by the state agency or regulatory in only three states: California, Florida and South Dakota.

Conclusion

The survey findings show that proper guidance for the selection and application of flocculants is needed for state highway agencies to overcome hesitations and adopt their use. The results of this study will provide valuable knowledge for further studies on flocculants to identify the research needs by presenting the perception of the state agencies for flocculant usage. Research studies can highly benefit from the state-of-the-practice survey results to understand the potential needs of the practitioners and develop effective guidance on flocculant usage.

Results show that residual monitoring, dosage and application guidance are the factors that hold DOTs back from adapting flocculants and need further investigation. The fact that most flocculants tend to be soil dependent changes their performance based on soil characteristics, therefore, manufacturer guidance might not be reliable with changing site conditions and soil types. Based upon DOT feedback, further studies focusing on developing dosage and application guidelines by conducting lab-scale and large-scale performance testing would be beneficial for the construction stormwater industry. 

References

1) Mclaughlin, R. A., and A. Zimmerman. Best Management Practices for Chemical Treatment Systems for Construction Stormwater and Dewatering. Report No. FHWA-WFL/TD-09-001. Federal Highway Administration, Vancouver, WA, 2008, p.12.

2) Dao, V. H., N. R. Cameron, and K. Saito. Synthesis, Properties, and Performance of Organic Polymers Employed in Flocculation Applications. Polymer Chemistry, Vol. 7, No. 1, 2016, pp. 11–25. https://doi.org/10.1039/c5py01572c.

3) Przepiora, A., D. Hesterberg, J. E. Parsons, J. W. Gilliam, D. K. Cassel, and W. Faircloth. Field Evaluation of Calcium Sulfate as a Chemical Flocculant for Sedimentation Basins. Journal of Environmental Quality, Vol. 27, No. 3, 1998, pp. 669–678. https://doi.org/10.2134/jeq1998.00472425002700030026x.

4) Harper, H. H., J. L. Herr, and E. H. Livingston. Alum Treatment of Stormwater: The First Ten Years. Journal of Water Management Modeling, 1999. https://doi.org/10.14796/JWMM.R204-09.

5) A. K. Bhardwaj, and R. A. McLaughlin. Simple Polyacrylamide Dosing Systems for Turbidity Reduction in Stilling Basins. Transactions of the ASABE, Vol. 51, No. 5, 2008, pp. 1653–1662. https://doi.org/10.13031/2013.25324.

6) Rounce, D., B. Eck, D. Lawler, and M. Barrett. Reducing Turbidity of Construction Site Runoff via Coagulation with Polyacrylamide and Chitosan. Transportation Research Record: Journal of the Transportation Research Board, Vol. 2309, No. December 2012, pp. 171–177.

7) Kang, J., and R. A. McLaughlin. Simple Systems for Treating Pumped, Turbid Water with Flocculants and a Geotextile Dewatering Bag. Journal of Environmental Management, Vol. 182, 2016, pp. 208–213. https://doi.org/10.1016/j.jenvman.2016.07.071.

8) The United States Environmental Protection Agency. Construction General Permit (CGP). Environmental Protection Agency, Washington D.C., 2017.

9) The United States Environmental Protection Agency. Stormwater Best Management Practice Polymer Flocculation. October 2013.

About the Experts

Billur Kazaz, MS, CPESC-IT, is a graduate research assistant pursuing a Ph.D. in civil engineering at Auburn University. Her research focuses on UAS-based aerial stormwater inspections and the use of flocculants in construction stormwater treatment. She earned her master’s degree at Iowa State University under the supervision of Dr. Michael A. Perez.

Michael A. Perez, Ph.D., CPESC, is an assistant professor in the Department of Civil and Environmental Engineering at Auburn University. His specialization includes construction and post-construction stormwater practices, methods and technologies.

Wesley N. Donald, Ph.D., CPESC, is a research associate in the Department of Civil and Environmental Engineering at Auburn University. He specializes in construction stormwater management applications and conducts research on construction stormwater practices.

By Ted Hartsig, CPSS; Steven Polk, PE

Collection and treatment of stormwater is an exercise of managing imbalanced resources. Urban stormwater runoff focuses increased volumes of water into limited areas where it is expected to be collected and treated. The typical design response to the increased amount of water is to try and balance this system by installing engineered soils and biological systems that nature may or may not be part of a balanced a landscape. As a result, there may be unintended drainage or stormwater treatment problems that require more energy and capital to maintain over time.

Soil is the resource that most directly affects the stormwater management imbalance. Soil is often considered to be a relatively innate mechanical system meant to accept, move and manage large amounts of stormwater and releasing it slowly, removing pollutants along the way. Soil, however, must be recognized for what it is: A living, breathing, dynamic system that can treat stormwater via infiltration into the ground. But it is also a balance of physical structure, dynamic biology and chemistry. This balance is necessary for stormwater management structures and the landscapes surrounding them. Getting this balance correct is what makes stormwater management successful.

Early infiltration design guidance in most regions of the United States required sand to be used as the predominant soil type for stormwater infiltration. The reason was based primarily on “book values” that logically show sand, especially coarse sand, as having high infiltration rates. However, design guidance for stormwater best management practices (BMPs) is changing as more regional policies allow for higher silt and clay (“fines”) content in biofiltration soil mixes (BSM).¹ While some regions minimize the allowance for fines in BSM materials based on the potential that fine particles will migrate and clog infiltration filters, other regions are realizing that silts and clays actually help stabilize BSM materials and help maintain consistent infiltration and percolation in stormwater BMPs for longer periods of time.² Much of this is because of increased biological activity in bioretention stormwater management systems.³

Why Stormwater BMPs Fail

When soil is disturbed, whether by tilling the soil for agriculture, grading the soil for construction or even in blending soils for specialized BSM, the soil structure that holds sand, silt and clay particles together is often broken.⁴ The resulting separation of silt and clay causes these fine particles to move easily and migrate between sand particles to form clay pans. Similarly, these same fine particles are easily dislodged and eroded from nearby soils that are captured in stormwater BMPs. The result is clogging of the BSM caused by the formation of sediment layers or clay pans. Because clay tends to have more cohesive properties that bind small particles together, silt is often more easily transported to form the blocking layers. Discussion with Steven Polk, PE, with Stormwater STL LLC in St. Louis, Missouri a company that inspects and maintains hundreds of stormwater BMPs quarterly, revealed that poor soil mixtures and clogging near the surface are the main causes of bioretention basin failure. Subsequent repair of these systems can run into tens of thousands of dollars. Often, the repair is not permanent and will need to be repaired again in the future.

In contrast to sand-based BSM, soil with strong aggregated peds that are often a congregation of sand, silt and clay bound together by organic “glues,” chemical bonding, and physical adhesion are effective for stormwater management. Well-aggregated soils provide effective macroporosity through which water readily flows while also retaining micropores that hold water for plant growth and chemical exchange sites, thereby effectively removing pollutants from the water. These types of soils are maintained through active plant growth and the activities of soil microflora (bacteria and fungi) and fauna (earthworms, nematodes, insects, and more). In fact, it is the dynamic actions of plant root growth, microbial transformations and turbidation of soil by worms and other small animals that contribute to healthy soil and the development of macropores that facilitate infiltration and percolation of water. This healthy soil retains an effective stormwater management performance for several years after BMP construction.^3,5,6
Maximizing the “Bio” in Bioretention

As described above, an essential factor for successful BSM in stormwater BMPs is healthy soil biology. This is not limited to plants, but also the microbial communities of the soil itself. Sand-based BSM provides the primary function of moving water into the soil but it relies on the addition of compost and possibly other performance enhancing devices to filter pollutants from incoming stormwater.⁷ However, studies conducted in the states of Washington and California have shown that the inclusion of too much compost in BSM often results in the release of phosphorus, nitrogen and copper into the stormwater effluent passing through the BSM.⁸ The growth and development of natural microorganisms, particularly fungi, will produce natural organic matter, and their activity will serve to reduce the concentrations of both organic and metal contaminants in stormwater runoff.⁹

Plants and soil microorganisms depend on each other for coexistence. Healthy vegetation releases polysaccharides into the soil that stimulate microbial growth and activity. Abundant microbes, in turn, provide plants with the nutrients they need to grow and be healthy. Soil bacteria and fungi also release compounds that build soil structure and promote water movement into and through the soil while also retaining important nutrients. Plants and microbes will break down organic contaminants while often assimilating and immobilizing inorganic metals, binding them long-term to soil organic matter. To make all this work, clays and silts help create the environment where soil microbes can flourish.

Chemistry of a Healthy BSM

The chemical nature of the healthy BSM and a successful stormwater BMP is sometimes the most difficult to understand and manage. Healthy BSM will have slightly acidic to near-neutral pH and low salt content. The BSM should be relatively low in macronutrients (primarily nitrogen and phosphorus) because most nutrients needed for sustaining plants in the stormwater BMP are often inherent in the soil or in stormwater influent. An important part of the soil chemistry and success of the BMP is the ability of the soil to absorb nutrients as well as contaminants. This is often reflected in a measure of the soil’s cation exchange capacity (CEC). Fine-textured soils have higher CEC than do coarse, sandier soils. To make up for the low CEC of sandy soils, compost, peat or, recently, coconut coir fiber, is added to improve the CEC of sand-based BSM. These organic materials will decompose with time and must be replenished as part of the BMP maintenance program.

Balanced Soil and Successful BMPs

The “right” soil for stormwater BMPs will vary by location, as every site is different. Every stormwater BMP has different functional and environmental needs requiring the appropriate soil to support effective stormwater management. The typical engineered, sand-based soil recommended or required by many stormwater management programs is essentially a “one-size-fits-all” approach that often fails.
Every stormwater BMP has different functional and environmental needs requiring the appropriate soil that is necessary to support effective stormwater management. The use of more natural soils, especially if soils don’t have to be blended, screened or pulverized works very well. If there is a “happy medium” for stormwater bioretention, soils with sandy loam to loam texture—typically about 45% to 70% sand, 10% to 25% clay, and 15% to 35% silt—work very well. The sandy loam to loam texture has sufficient clay and silt to provide soil chemical and biological stability, yet enough sand to allow adequate drainage.
Designing BSM that has physical, biological and chemical balance is appropriate for the local environment and the proper function of each stormwater management system. This will require many designers to rethink the design of stormwater BMPs to include more natural, native soils, accept finer-textured soils and consider plants and soil biology more. 

References

1) Hodgins B., Seipp B. 2018. Bioretention System Design Specifications & “Performance Enhancing Devices.” Center for Watershed Protection. Washington, D.C.

2) Shanstrom N. 2016. How Sandy Does Bioretention Soil Need to Be? Deeproot Blog Entry. April 16, 2016.

3) Ayers E. 2009. Pedogenesis in Rain Gardens: The Role of Earthworms and Other Organisms in Long Term Soil Development. Ph.D. Dissertation. University of Maryland.

4) USDA-NRCS (U.S. Department of Agriculture – Natural Resources Conservation Service). 2010. Soil Glue. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051280.pdf.

5) Skorobogatov A. (2014). Hydrological Functionality of Plants and Its Application to Stormwater Management (Unpublished master’s thesis). University of Calgary, Calgary, AB.

6) Mehring A., Levin L. 2015. Potential Roles of Soil Fauna in Improving the Efficiency of Rain Gardens Used as Natural Stormwater Treatment Systems. Journal of Applied Ecology 52:1445-1454.

7) Hodgins B., Seipp B. 2018.

8) Herrera Environmental Consultants. 2020. Bioretention Media Blends to Improve Stormwater Treatment: Final Phase of Study to Develop New Specifications. Final Report. King County, Washington Department of Natural Resources and Parks. January 2020.

9) McIntyre J., Davis J., Kappenberger T. 2020. Plant and Fungi Amendments to Bioretention for Pollutant Reduction over Time. Final Report to Washington State Department of Ecology Stormwater Action Monitoring. September 25, 2020.

Special thanks to Lillian Stroeker (Olsson) for assistance with research of stormwater BMP soils

About the Experts

Ted Hartsig, CPSS, is a senior soil scientist with Olsson, Inc. in Overland Park, Kansas. His 37 years of experience have been focused on soil design and restoration for environmental and stormwater management in urban and rural locations throughout the U.S.
Steven Polk, PE, is a professional engineer and owner of Stormwater STL, a firm specializing in stormwater BMP management and compliance, offering inspection, maintenance, native plant stewardship, testing, construction and consulting expertise for stormwater quality systems in St. Louis and throughout the Midwest U.S.