by Ron Alexander
Stormwater management is becoming more difficult—and more important.
Communities are facing increasingly intense storms, expanding areas of pavement and other impervious surfaces, aging infrastructure, and soils that have often been compacted during construction. In regions with dense clay soils, such as the Washington, D.C., metropolitan area, water often moves across the top of the soil instead of into it.
When stormwater cannot infiltrate, it flows across the soil surface, gaining speed and carrying sediment, nutrients, heavy metals, and other pollutants into streams and waterways. It can also contribute to localized flooding, soil erosion, landscape damage, and increased irrigation demands.
Green infrastructure practices, such as bioretention systems and infiltration soils, can help manage stormwater closer to where it falls while also creating functional, attractive landscapes.
Why Stormwater Management Matters
Effective stormwater management addresses three closely related concerns:
- Protecting water quality
- Protecting buildings, roads, landscapes, and other infrastructure
- Protecting natural areas and wildlife habitat
Traditional gray infrastructure, including pipes, drains, and underground storage systems, remains important. But green infrastructure can complement those systems by slowing, storing, infiltrating, and treating stormwater within the landscape.
These systems are designed to imitate natural processes. Instead of moving rainwater away from a site as quickly as possible, they allow water to infiltrate into soil, move through vegetation, evaporate, or be released more gradually.
This can help reduce runoff volume, manage peak flow during storms, limit erosion, and remove some pollutants before water reaches nearby waterways.
What Is Bioretention?
Bioretention is a stormwater management practice that collects runoff, typically in a landscaped area, and allows it to move through specially designed soil media. And doing this while reducing sediment and pollutant content.
Common examples include:
- Bioretention basins
- Rain gardens
- Bioswales
- Landscaped stormwater planters
- Vegetated drainage areas
These features are usually installed in low-lying areas so gravity can direct water into them. A properly designed bioretention system must manage water effectively while also supporting healthy vegetation.
That balance is not always simple. The soil media must allow water to infiltrate quickly enough to manage runoff, but it must also retain enough moisture and nutrients to keep plants alive.
How a Bioretention System Works
Although designs vary, a typical bioretention system contains several layers and components.
Pretreatment Area
Water may enter the system at high velocity, particularly when it is coming from a long road, parking lot, roof drain, or other concentrated source.
A pretreatment area—often made from stone or another durable material—slows the water before it reaches the planting area. This helps reduce erosion and allows some sediment and trash (if contained in the flow) to settle out.
Mulch Layer
A mulch layer protects the surface of the soil media from erosion, helps retain moisture, and can capture certain pollutants.
The best mulch for these systems is generally a heavier, more fibrous material that create a mat that resists floating. Lightweight or highly processed wood (cambium) mulches may wash out of the feature during intense storms.
Bioretention Soil Media
The soil media is the working center of the system.
It must:
- Accept and store stormwater
- Allow water to infiltrate at the required rate
- Support plant growth
- Resist compaction and surface crusting
- Help capture nutrients, metals, and other pollutants
Bioretention soils are usually sand-based. Depending on the specification and intended use, they may contain approximately 60% to 80% sand, along with soil and an organic amendment.
Storage Layer
A layer of gravel or coarse mineral material is often installed beneath the soil media. This layer can temporarily store water while it infiltrates into the underlying soil.
Underdrains and Overflow Structures
Some systems include an underdrain when the native soil does not allow water to infiltrate quickly enough.
Overflow structures may also be needed to safely move excess water out of the system during storms that exceed its designed capacity.
Why Organic Matter is Included in Bioretention Soil
Organic matter plays several important roles in bioretention media.
It helps support the microorganisms that live around plant roots and contribute to nutrient cycling and plant health. Some of these microbes also degrade petroleum contaminants. It also increases the soil’s ability to hold nutrients and certain metals through cation exchange capacity.
Depending on the material, organic amendments may also:
- Improve water-holding capacity
- Reduce bulk density
- Support soil aggregation
- Provide slow-release nutrients
- Help capture pollutants
- Improve plant establishment
A bioretention medium made almost entirely from sand may infiltrate water effectively, but it may not retain enough moisture or nutrients to support healthy vegetation. Organic matter helps create a more functional growing medium. This is key because a plants root system is a benefit to the overall operation of the bioretention system, helping to create pore spaces which allow for water and air movement.
However, more is not always better. Excessive organic matter may increase nutrient movement or cause the media to settle over time. The appropriate amount depends on the materials, plants, pollutant concerns, and performance requirements for the project.
Choosing Organic Amendments for Bioretention Media
Compost has long been used as the organic component of bioretention soils, but it is not the only option.
Other stabilized organic amendments may also provide the necessary carbon, water-holding capacity, nutrient retention, and biological activity.
Bloom Woody Blend, for example, is a mixture of Bloom soil conditioner and woody material. It is used as a compost alternative in approved regional bioretention specifications.
Bloom is produced by DC Water through anaerobic digestion and thermal hydrolysis, a high-temperature, high-pressure treatment process. The resulting material contains stabilized organic matter, slow-release nutrients, and iron. Iron is key to bonding certain heavy metals.
When Bloom is blended with sawdust, the woody component increases the carbon content and helps moderate nutrient (nitrogen) availability. The product also possesses a very low percentage of water-extractable phosphorus, an important consideration when selecting materials for stormwater applications.
Bloom Woody Blend has been used in hundreds (?) of bioretention features in the Washington, D.C., region, including projects where plant establishment and continued infiltration were both important design goals.
Learn more about Bloom for bioretention.
Nutrient Movement Requires a Balanced Approach
Nutrient movement is a legitimate concern when organic materials are used in stormwater systems. However, nutrient content alone does not tell the whole story.
It is important to understand:
- How much nitrogen is readily available
- How much nitrogen is in slow-release organic forms
- How much phosphorus is water-extractable
- How the material interacts with the other ingredients
- How quickly vegetation will become established
Only a portion of the total phosphorus in many composts is available during the first year. Some biosolids-based products have even lower percentages of readily available phosphorus.
The goal should not be to create a medium completely devoid of nutrients. Plants require nutrients to establish and remain healthy. Without strong vegetation, the system may become more vulnerable to erosion, compaction, weed growth, and declining performance.
Using native or well-adapted plants, establishing vegetation promptly, and avoiding unnecessary amounts of organic matter can help reduce nutrient loss.
Plants are part of the stormwater system, they are not simply decorative additions to a bioretention feature.
Their roots help maintain pore space and infiltration pathways within the soil. Plants also take up water and nutrients, support soil biology, protect the surface from erosion, as well as improve the appearance of the system.
Appropriate plants should generally:
- Tolerate both wet and dry conditions
- Develop deep or extensive root systems
- Require limited supplemental fertilizer
- Be suited to the local climate
- Provide seasonal visual interest
- Withstand the expected maintenance conditions
Native plants are frequently used because they are adapted to regional conditions and may require fewer inputs once established.
A bioretention feature that does not maintain healthy vegetation may gradually lose both its appearance and its stormwater-management function.
What Are Infiltration Soils?
Bioretention features collect water in defined landscape areas. Infiltration soils apply a related principle across larger portions of a site.
Instead of directing all runoff into one basin or feature, designers can improve the soil in lawns, planting beds, slopes, and other landscaped areas so those surfaces absorb more rainfall before it reaches the primary stormwater feature.
This can reduce runoff volume and peak flow while easing pressure on downstream infrastructure.
In some cases, infiltration soils are manufactured from sand, soil, and organic matter. In others, organic amendments are incorporated into dense native soils to improve porosity and reduce compaction.
Because sand is heavy and expensive to transport, adding stabilized organic matter to suitable existing soil may be a more practical option on many projects.
Organic Matter Can Improve Infiltration
Adding organic matter to fine-textured, dense soil can:
- Lower bulk density
- Increase large pore space
- Improve water-holding capacity
- Increase hydraulic conductivity (percolation)
- Support microbial aggregation
- Reduce susceptibility to compaction
Research has shown that adding organic amendments to subsoils and sandy soils can improve infiltration and water retention.
Bloom and Bloom Woody Blend have demonstrated similar effects in soil-blending research, including increasing water-holding capacity, and lowering bulk density and improved hydraulic conductivity with local clay/silt-based soils.
The best results occur when the material is incorporated into the soil profile rather than left on the surface.
That stated, ror established turf, core aeration followed by topdressing can help move organic matter into the root zone, break up surface crusting, and improve the soil’s ability to accept water.
Compaction Can Undermine Performance
Compaction is one of the greatest threats to both plant health and stormwater infiltration.
When soil is compacted, the pore spaces that normally hold air and water are compressed. The soil becomes denser, roots have more difficulty growing, and rainfall is more likely to run across the surface.
Even modest increases in bulk density can begin to restrict root growth.
Construction activities can quickly damage soil structure, particularly when heavy machinery operates on wet soils. For that reason, stormwater infiltration soils should be protected during installation and not used as staging or equipment-access areas.
Long-term maintenance may include:
Core aeration
Deep-tine aeration
Replenishing mulch
Repairing bare areas
Removing accumulated sediment
Replacing clogged or contaminated media where necessary
Maintaining dense, healthy vegetation
Bioretention Performance Changes Over Time
The infiltration rate measured when a soil blend is first produced will not remain unchanged.
Media settles after installation, roots grow through it, organic matter decomposes, sediment accumulates, and maintenance practices affect the soil structure.
This does not mean the system has failed. It means bioretention is a living system that must be monitored and managed (maintained).
One continuing challenge in the industry is the limited amount of long-term monitoring data available for many bioretention installations. Designers and owners are improving their understanding of maintenance practices, but more research is still needed to determine how different systems change over decades.
Pollutant Capture Should Inform the Design
Bioretention soils can help capture nutrients, sediment, and certain metals, but no single media blend is ideal for every pollutant.
Organic matter can bind many metals. Iron-rich materials may be particularly useful for capturing metals such as copper, zinc, and cadmium.
Roadside projects in Washington State, for example, have used compost-amended soils to increase infiltration while reducing concentrations of dissolved heavy metals collected in stormwater systems.
The most effective design begins by identifying the pollutants of greatest concern and selecting media components that have documented value for treating them.
Designers should also recognize that sand, gravel, and other mineral ingredients may release pollutants of their own. All major components should be evaluated—not only the organic amendment.
Simpler Specifications May Be More Successful
Bioretention specifications can become extremely complicated.
A specification may include separate requirements for the sand, soil, organic amendment, additives, and the final blended media. Each additional requirement may increase cost and make it more difficult for local suppliers to produce a compliant blend.
Special ingredients such as biochar and humic substances may have promising uses, but they should not be added without a clear purpose and supporting data.
A successful specification should be rigorous enough to achieve the necessary performance without becoming so complicated that contractors cannot source, blend, or install the material reliably.
Whenever possible, specifications should be based on measurable performance and locally available materials.
Green Infrastructure Works Best as a System
Bioretention systems should not be viewed as isolated landscape features.
Their performance can be improved by combining them with:
- Infiltration soils
- Vegetated swales
- Green roofs
- Permeable pavement
- Soil restoration
- Tree planting
- Cisterns and rainwater storage
- Erosion-control practices
Improving soils in the landscaped areas surrounding a bioretention basin can allow rainfall to infiltrate before it ever reaches the basin. This expands the effective stormwater capacity of the entire site.
The landscape itself becomes part of the infrastructure.
Better Soil Is a Long-Term Investment
Green infrastructure practices can manage stormwater volume, reduce peak flow, improve water quality, and support attractive public and private landscapes.
They can also provide a form of insurance.
A modest investment in better soils and thoughtful stormwater design can help reduce future erosion, flooding, landscape failure, property damage, and irrigation requirements.
The key is to treat the ‘soil’ as essential piece of site infrastructure rather than as a material added at the end of construction.
As storms become less predictable and more intense, and developed areas continue to expand, communities will need landscapes that do more than just look attractive. Bioretention and infiltration soils allow those landscapes to actively store, clean, and manage water while supporting healthy plant growth.
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*Ron Alexander is a horticulturist, author, educator, and one of North America’s leading experts on compost, recycled organics, and manufactured soils. With more than 40 years of experience consulting with landscape architects, municipalities, manufacturers, and the green industry, he has helped advance the science and practical application of compost and organic soil amendments. Ron has contributed to numerous books and authored hundreds of technical publications and is a frequent speaker on soil health, sustainable landscapes, compost utilization, and soil amendment technologies.
