Biotreatment Soil Mix (Bioretention & Bioswale Soil Media)

Q: What is Biotreatment Soil Mix?

Biotreatment Soil Mix is an engineered stormwater soil media used in bioswales, bioretention systems, rain gardens, and LID facilities. It is designed for controlled infiltration, pollutant filtration, and long-term plant health—unlike generic landscape soils that can vary and clog.

Q: Is Biotreatment Soil Mix the same as Bioretention Soil Media or Bioswale Soil Media?

Yes. Biotreatment Soil Mix is commonly referred to as Bioretention Soil Media (BSM) or Bioswale Soil Media. These terms are often used interchangeably by municipalities, engineers, and contractors to describe performance-based engineered stormwater treatment media.

Q: What is Biotreatment Soil Mix made of?

The mix is produced to controlled volumetric ratios using washed sand (typically 60–70%) and stabilized organic compost (typically 30–40%). A common target blend is 60% sand and 40% compost to balance infiltration with biological treatment and plant support.

Q: What infiltration rate does biotreatment soil typically meet?

Biotreatment Soil Mix is commonly designed to meet a minimum infiltration target of 5 inches per hour or greater, depending on project specifications and agency standards. Proper installation and protection from sediment are critical to maintaining infiltration performance.

Q: What projects use Biotreatment Soil Mix?

Typical uses include bioswales and vegetated swales, bioretention planters and curb extensions, rain gardens and green streets, stormwater tree wells, and other Low Impact Development (LID) stormwater treatment applications.

Q: What can cause bioretention soil to fail inspection?

Common causes include excessive fines or dirty sand (leading to surface sealing), over-compaction (loss of pore space and low infiltration), sediment contamination before/after placement, installing when saturated, or allowing construction traffic on the media.

Engineered soil media for bioswales, bioretention systems, and stormwater treatment applications—designed for controlled infiltration, pollutant removal, and long-term plant health.

What Is Biotreatment Soil Mix
It is a purpose-built stormwater treatment media engineered specifically for bioswales, bioretention systems, and LID facilities where predictable infiltration, pollutant capture, and healthy vegetation are critical to performance and regulatory compliance.

This material is not blended like landscape soil. It is produced to controlled volumetric ratios using washed sand and stabilized organic compost, resulting in a uniform media that supports:

Consistent hydraulic behavior
Long-term infiltration without clogging
Nutrient balance for plant establishment
Pollutant attenuation through filtration and biological activity

It is commonly specified on municipal, commercial, and civil projects where stormwater treatment is required by code or permit.

What It’s Used For

Bioswales and vegetated swales
Bioretention planters and curb extensions
Rain gardens and green streets
Stormwater tree wells
Low Impact Development (LID) systems
Flow-through and infiltration-based treatment areas

Bioswale construction Bay Area California

How Biotreatment Soil Mix Works

Biotreatment Soil Mix functions as a living filtration layer within stormwater treatment systems. Its performance is driven by a carefully balanced combination of physical structure, hydraulic conductivity, and biological activity—not by density or compaction.

1. Controlled Infiltration
The sand-dominant structure creates interconnected pore spaces that allow stormwater to move vertically through the media at a controlled rate, preventing surface ponding while avoiding rapid bypass that would reduce treatment effectiveness.

Unlike native soils or amended loams, this engineered blend is designed to:

Drain consistently across the full facility footprint
Resist clogging from fine sediments
Maintain infiltration performance over time

2. Pollutant Removal & Treatment
As stormwater passes through the soil profile, pollutants are removed through multiple mechanisms:

Physical filtration of suspended solids
Adsorption of metals and nutrients onto soil particles
Biological uptake by microbial communities and plant roots
Transformation and breakdown of hydrocarbons and organics

This multi-layered treatment approach is why biotreatment soil is required in many municipal standards.

3. Plant Support Without Compromise
Vegetation plays a critical role in bioswale performance. Biotreatment Soil Mix is engineered to:

Provide sufficient moisture retention for root development
Supply nutrients without excessive fertility
Support microbial activity essential to pollutant processing

The result is vigorous, stable plant growth without sacrificing infiltration capacity.

4. Long-Term Stability
Unlike soils with high fines or unstable organic content, this mix is designed to:

Minimize compaction under normal installation practices
Maintain structure during wet/dry cycles
Perform consistently across seasonal conditions

This stability is essential for stormwater facilities that must function reliably for decades.

Material Composition & Blend Design

Biotreatment Soil Mix is engineered using a strict, performance-driven blend design rather than a generic soil recipe. Each component is selected and proportioned to achieve the hydraulic, chemical, and biological characteristics required for stormwater treatment systems.

Blend Composition (By Volume)

Component & Proportion

Washed Sand 60–70%

Stabilized Organic Compost 30–40%

Typical Target Blend:

60% Sand
40% Compost

This ratio aligns with widely adopted bioretention and bioswale specifications and provides a proven balance between infiltration capacity and nutrient availability.

Sand Component (Structural Backbone)

The sand fraction provides the primary hydraulic framework of the mix.

Key characteristics:

Clean, washed, durable aggregate
Angular to sub-angular particles for structural stability
Non-plastic fines only
Low silt and clay content to prevent clogging

The sand gradation is controlled to ensure:

Consistent pore structure
Predictable drainage behavior
Resistance to compaction during installation

This allows stormwater to infiltrate evenly while maintaining sufficient contact time for treatment.

Compost Component (Biological Engine)

The compost fraction supplies the biological and chemical functionality of the soil mix.

Key characteristics:

Fully stabilized and mature organic material
Derived from clean, plant-based feedstocks
Free of biosolids, manure, and contaminated inputs

The compost is selected to:

Support microbial communities responsible for pollutant breakdown
Provide moderate nutrient availability for vegetation
Improve moisture retention without sealing pore spaces

Excessive organic content is avoided to prevent reduced infiltration or long-term settlement.

Uniform Blending Process

All components are homogenized under controlled conditions to minimize segregation and ensure consistent performance from load to load. This uniformity is critical for:

Predictable inspection outcomes
Reliable installation behavior
Long-term system performance

Biotreatment Soil Media Mix aka Bioswale

Hydraulic & Physical Performance Criteria

Biotreatment Soil Mix is engineered to meet the functional performance requirements of modern bioswale and bioretention systems, where both drainage capacity and treatment effectiveness must be balanced. The following criteria represent typical performance targets used by engineers, inspectors, and reviewing agencies.

Hydraulic Performance

Minimum Infiltration - Rate ≥ 5 inches/hour*

Hydraulic Conductivity Test Method - ASTM D2434 (Modified), when required

Drainage Behavior - Uniform vertical percolation

Surface Ponding - Minimized under design storm events

Physical Properties

Soil Texture - Engineered loam

Bulk Density (Dry) - 500–1100 lbs. /yd³

Moisture Content - 30–55% (of dry solids)

Saturation Percentage ~50%

Compaction Sensitivity - Low (under proper installation practices)

Performance Characteristics

Designed to drain efficiently without rapid bypass, allowing sufficient residence time for pollutant treatment
Maintains pore structure under repeated wet–dry cycles
Resists sealing and crusting caused by excessive fines
Supports plant establishment without sacrificing hydraulic performance

Unlike native soils or landscape blends, Biotreatment Soil Mix is formulated so that hydraulic behavior is predictable, repeatable, and compatible with long-term stormwater system operation.

Chemical Properties & Soil Chemistry

The chemical profile of Biotreatment Soil Mix is carefully controlled to support plant health, microbial activity, and pollutant treatment while avoiding conditions that can impair infiltration or cause long-term system failure. Excessive salts, unstable nutrients, or improper pH are common reasons bioswale systems are rejected or underperform—this mix is engineered to avoid those issues.

Chemical Performance Targets (Finished Mix)

pH 6.2 – 8.2

Electrical Conductivity (EC) ≤ 6.0 mmhos/cm

Total Dissolved Salts (TDS) < 2,000 ppm

Organic Matter (LOI) 4 – 8%

Cation Exchange Capacity (CEC) ≥ 10 meq/100g

Sodium Adsorption Ratio (SAR) < 6

Exchangeable Sodium Percent (ESP) < 10

Why These Parameters Matter

pH Control
Maintaining a neutral-to-slightly-alkaline pH range supports:

Broad plant species compatibility
Optimal microbial activity
Stable nutrient availability

Salinity & Electrical Conductivity
Excessive salts are a leading cause of:

Plant stress and failure
Reduced microbial activity
Long-term infiltration decline

The EC and TDS limits ensure the soil remains suitable for sensitive vegetation and stormwater reuse applications.

Organic Matter Balance
Organic matter is intentionally limited to avoid:

Excessive water retention
Pore space collapse
Reduced infiltration rates

The result is a soil that behaves consistently under both wet and dry conditions.

Sodium Control (SAR & ESP)
Low sodium levels help prevent:

Soil dispersion
Loss of structure
Reduced permeability over time

This is especially important in urban stormwater systems exposed to deicing salts, irrigation water, or recycled water sources.

Long-Term Chemical Stability

Biotreatment Soil Mix is formulated to maintain chemical stability over the service life of the facility, reducing the likelihood of:

Nutrient leaching
Soil crusting
Plant decline due to salt buildup

Sand Component Specifications (Gradation & Cleanliness)

The sand fraction is the structural and hydraulic backbone of Biotreatment Soil Mix. Its gradation and cleanliness directly control infiltration performance, resistance to clogging, and long-term stability of the stormwater facility.

Sand Quality Requirements

The sand used in Biotreatment Soil Mix meets the following criteria:

Clean, washed, durable aggregate
Free of clay coatings, organics, wood, roots, or deleterious materials
Non-plastic fines only
No rounded river sand or manufactured fines that reduce pore connectivity

These requirements ensure the sand maintains open pore structure while resisting migration of fines during storm events.

Sand Gradation Requirements

(ASTM D422 or equivalent sieve analysis)

3/8" - 100%

No. 4 90–100%

No. 8 70–100%

No. 16 40–95%

No. 30 15–70%

No. 40/50 5-55%

No. 100 0-15%

No. 200 0-5%

Why This Gradation Matters

This gradation envelope is engineered to:

Promote uniform vertical infiltration
Prevent fine-particle sealing at the surface
Maintain permeability under sediment loading
Reduce long-term maintenance and media replacement

Sand that falls outside this envelope—especially material with excessive fines—often leads to:

Surface crusting
Standing water
Failed inspections
Premature system replacement

Acceptance & Quality Control

Gradation compliance is verified through laboratory testing when required
Sand is inspected for cleanliness prior to blending
Materials not meeting gradation or cleanliness criteria are rejected

Compost Component Specifications (Stability, Maturity & Safety)

Organic OMRI CDFA certified compost bulk-pile

The compost fraction of Biotreatment Soil Mix functions as the biological engine of the system. Its quality directly affects pollutant treatment, plant health, and long-term infiltration performance. For this reason, compost used in this mix is subject to strict maturity, stability, and safety criteria.

Compost Certification & Feedstock Controls

Compost is OMRI & CDFA-certified
Feedstocks are limited to:
- Clean green waste
- Yard trimmings
- Agricultural plant residues

Prohibited materials include:
- Biosolids or sewage sludge
- Manure-based feedstocks
- Post-consumer or post-industrial wood
- Contaminated organic waste

These controls reduce the risk of heavy metals, salts, pathogens, and unstable organic matter entering the stormwater system.

Compost Physical & Chemical Requirements

Organic Matter 35–75% (dry weight)

Carbon-to-Nitrogen Ratio (C:N) 15:1 – 25:1

pH 6.0 – 8.5

Electrical Conductivity (EC) ≤ 6.0 mmhos/cm

Moisture Content 30–55%

Inert Materials < 1% by weight

Stability & Maturity Criteria

Only fully stabilized compost is used to prevent oxygen depletion, odor issues, and infiltration loss after installation.

Compost must meet one or more of the following maturity indicators:

Respiration Test: < 8 mg CO₂-C/g OM/day
Solvita Maturity Index: ≥ 5
Equivalent laboratory-approved stability testing

Compost exhibiting sour odors, excessive heat, or visible undecomposed material is rejected.

Toxicity & Plant Response

To ensure compatibility with sensitive vegetation and microbial communities:

Seed Germination: ≥ 80% of control
Plant Growth Trials: ≥ 80% of control
Ammonium-N: < 500 ppm (dry basis)

These thresholds confirm the compost is non-toxic and biologically stable.

Pathogen Reduction & Environmental Safety

Processed in compliance with Process to Further Reduce Pathogens (PFRP) standards
Salmonella: < 3 MPN per 4 g total solids
Meets EPA 40 CFR Part 503 limits for trace metals

This ensures the material is safe for use in public-facing stormwater infrastructure.

Installation & Handling Guidelines

Proper installation is essential to achieving the intended performance of Biotreatment Soil Mix. Even a correctly engineered media can fail if it is mishandled, over-compacted, or contaminated during construction. The following guidelines reflect best practices commonly required by municipalities and project inspectors.

Pre-Installation Requirements

Subgrade must be properly prepared, uncompacted, and free of standing water
All upstream areas draining to the facility should be stabilized prior to placement
Prevent sediment-laden runoff from entering the biotreatment area before installation
Do not install during or immediately following heavy precipitation events

Material Handling & Placement

Protect material from contamination during transport and staging
Do not stockpile directly on native soil without separation
Avoid mixing with adjacent soils or construction debris
Place material gently to minimize segregation and compaction

Placement & Consolidation

Place soil in loose lifts, typically not exceeding 9 inches
Light consolidation only using:
- Foot traffic
- Hand tamping
- Walk-behind rollers (low pressure)

Do not use mechanical compaction equipment.

Excessive compaction reduces pore space, lowers infiltration rates, and is a common cause of inspection failure.

Moisture Conditions

Do not place when soil is saturated or excessively wet
Avoid placement if free water is visible in the excavation
Light moistening may be used only to improve workability—not compaction

Protection After Placement

Prevent construction traffic within the biotreatment area
Protect installed media from sediment contamination
Delay activation of inlets until surrounding areas are stabilized

Inspection & Acceptance

Final acceptance is typically based on:

Visual uniformity
Proper installation depth
Absence of contamination or over-compaction
Compliance with project specifications

Any material that becomes contaminated or improperly compacted may require removal and replacement at the contractor’s expense.

Quality Control, Submittals & Documentation

Biotreatment Soil Mix is produced and delivered under a quality-control framework intended to support smooth project approvals, successful inspections, and long-term system performance. Documentation is available to meet typical municipal and engineering submittal requirements.

Quality Control Program

Materials are blended to controlled volumetric ratios
Inputs are reviewed for compliance prior to blending
Finished mix is visually inspected for uniformity and contamination
Loads are managed to minimize segregation during transport

Quality control measures are designed to ensure consistent performance from batch to batch, particularly on phased or multi-load projects.

Available Submittals (Upon Request)

The following documentation can be provided for project approval or inspection purposes:

Sand gradation analysis (ASTM D422 or equivalent)
Compost laboratory analysis (STA or equivalent certification)
Mix certification statement confirming compliance with published specifications
Permeability / hydraulic conductivity test results (project-specific, when required)
Batch delivery certification identifying product type and quantity

Project-Specific Testing

Some jurisdictions or engineers may require additional testing based on site conditions or permit requirements. When requested in advance, project-specific testing may include:

Modified ASTM D2434 permeability testing
Supplemental chemical analysis
Confirmation sampling prior to installation

Testing scope and timing should be coordinated before delivery to avoid delays.

Ordering Information, Use Limitations & Legal Disclaimer

Ordering Information

Biotreatment Soil Mix is available for bulk delivery to residential, commercial, and municipal project sites.

Ordering Notes:

Sold by the cubic yard
Minimum quantities may apply depending on delivery location
Advance notice is recommended for large or phased projects
Project specifications should be reviewed prior to ordering

Customers are encouraged to consult with their project engineer or jurisdictional reviewer to confirm suitability before purchase.

Use Limitations

Biotreatment Soil Mix is intended only for stormwater treatment applications and related green infrastructure systems.

This product is not intended for:

Structural fill or load-bearing applications
Building pads or foundations
Turf-only installations
Areas subject to heavy vehicular loading
Applications requiring clay-rich or fine-textured soils

Performance depends on proper design, installation, and maintenance. Use outside the intended scope may result in reduced effectiveness or system failure.

Material Variability Notice

Biotreatment Soil Mix is composed of naturally occurring materials. As a result:

Minor variation in color, texture, or moisture content may occur
Some separation may happen during transport
Material is guaranteed at the point of production, not at the jobsite

These variations do not affect compliance with published specifications.

Legal Disclaimer & Limitation of Liability

All materials are provided as is, with no warranties—express or implied—including, but not limited to, warranties of merchantability or fitness for a particular purpose. North Bay Materials makes no guarantees that this product will meet the requirements of any specific project, permit, or jurisdiction. Final approval is the responsibility of the buyer, engineer, or governing authority. To the fullest extent permitted by law, North Bay Materials and its suppliers shall not be liable for any direct, indirect, incidental, special, or consequential damages arising from the use or installation of this material, including but not limited to loss of profits, project delays, or rework costs. By purchasing or using this product, the buyer acknowledges and accepts these limitations.

FAQs - Biotreatment Soil Mix

1) What is Biotreatment Soil Mix?

Biotreatment Soil Mix is an engineered stormwater soil media used in bioswales, bioretention planters, rain gardens, and LID systems. It’s designed for controlled infiltration, pollutant filtration, and long-term vegetation support—unlike landscape soil blends, which can vary and clog.

2) Is Biotreatment Soil Mix the same as Bioretention Soil Media or Bioswale Soil Media?

Yes. These terms are often used interchangeably by engineers, municipalities, and contractors:

Biotreatment Soil Mix
Bioretention Soil Media (BSM)
Bioswale Soil Media / Bioswale Soil Mix

The key is that it’s a performance-based engineered media, not standard topsoil.

3) What is Biotreatment Soil Mix made of?

This mix is produced to controlled volumetric ratios using:

Washed sand (typically 60–70%)
Stabilized organic compost (typically 30–40%)

A common target blend is 60% sand / 40% compost, balancing infiltration performance with plant-supporting biology.

4) What infiltration rate does biotreatment soil typically meet?

Biotreatment Soil Mix is commonly designed to meet a minimum infiltration target of 5 inches per hour or greater, depending on project requirements and agency specs. Performance depends on proper installation, protection from sediment, and avoiding over-compaction.

5) What projects use Biotreatment Soil Mix?

Typical applications include:

Bioswales and vegetated swales
Bioretention planters, curb extensions, and green streets
Rain gardens and infiltration-based treatment areas
Stormwater tree wells
Low Impact Development (LID) systems
Municipal / commercial stormwater compliance projects

6) What can cause bioretention soil to fail inspection?

The most common causes are:

Too many fines / dirty sand (surface crusting, ponding)
Over-compaction (reduced pore space, low infiltration)
Sediment contamination before/after placement
Installing when saturated/wet, or allowing construction traffic on the media

7) Do you provide submittals or documentation for approval?

Yes—documentation can be provided upon request depending on project needs, such as:

Sand gradation analysis (ASTM D422 or equivalent)
Compost lab analysis (STA / certification-type documentation)
Mix certification statement
Permeability / hydraulic conductivity test results (when required and coordinated)

8) Is this product suitable for structural fill or building pads?

No. Biotreatment Soil Mix is intended for stormwater treatment and green infrastructure only. It is not intended for:

Structural fill or load-bearing applications
Building pads/foundations
Areas with heavy vehicular loading
Turf-only installations
Applications requiring clay-rich or fine-textured soils

9) Can the mix look different between loads?

Yes. Because it’s made from naturally occurring materials, minor variation in color, texture, and moisture can occur. Some separation can happen during transport, but these variations do not necessarily indicate a performance issue when handled and installed correctly.

10) Do you deliver Biotreatment Soil Mix outside the Bay Area?

Yes. Delivery is available from the Bay Area through Sacramento-area counties. Delivery fees vary based on distance and quantity, and full transfer loads may qualify for free delivery to many areas.