Introduction to Infiltration Best Management Practices (BMP)

By Vaikko Allen, CPSWQ, LEED-AP; Aimee Connerton LEED-AP; and Cory Carlson, P.E.

Overview

Traditionally, stormwater regulations have dealt primarily with the reduction of flow rates from large infrequent storms and treatment of runoff from smaller, more frequent events. These regulations have been somewhat effective in maintaining and in some cases rehabilitating the integrity of the receiving waters. Flood control and channel protection designs commonly relied on detention systems to attenuate peak flow rates, thereby limiting downstream hydromodification effects, especially erosion and flooding. Treatment controls have also been implemented with varying effectiveness to remove pollutants prior to the release of runoff downstream. While these approaches may effectively address immediate flooding and water quality issues, runoff reduction through infiltration, harvest and use, and evapotranspiration is also needed. Runoff reduction not only helps to restore predevelopment flow patterns in downstream water bodies, but it also reduces pollutant export and can augment local water supply.

The U.S. Environmental Protection Agency (EPA) promotes the role of runoff reduction approaches, referred to as "Green Infrastructure" practices, as being suitable "at a wide range of landscape scales in place of, or in addition to, more traditional stormwater control elements to support the principles of LID" (EPA, 2011). The EPA, along with other notable environmental groups, is signatory to the 2008 Green Infrastructure Action Strategy (EPA, 2008) to promote the benefits of using green infrastructure. Runoff reduction has replaced treatment as the preferred means of treating routine storm events in recent Phase I NPDES stormwater permits and in implementation guidance for Section 438 of the Energy Independence and Security Act (EPA, 2009). Clearly, infiltration will play an increasingly important role in meeting stormwater runoff mitigation regulations.

The benefits of infiltration go beyond limiting the export of pollutants from the site and reducing hydromodification effects. It can also augment groundwater storage, thereby stabilizing stream flows, limiting saltwater intrusion in coastal areas, and providing a source of high-quality extractable water.

Where does the rainwater go?

In natural conditions, rainwater leaves a site by evapotranspiration, overland flow, or deep recharge depending on local soils, rainfall patterns, vegetation, and site topography. Rainwater that is evaporated directly back into the atmosphere or that is transpired by plants is collectively referred to as evapotranspiration. This water may be lost from the surface of the ground or plants. It could also be withdrawn from the shallow root zone by plants.

Rainwater may also leave a site as overland flow if surface depressions fill with water and overflow. Overland flow or surface runoff has traditionally been the focus of civil engineers' site development work. Infiltration is the third possible fate for rainwater. Shallow infiltration or interflow travels laterally through subsurface soils and may emerge to recharge streams, springs, or other surface water features. Deep infiltration describes the downward vertical migration of stormwater into storage aquifers where it remains over the long term or is extracted for use.

Water is often referred to as the universal solvent. As it travels, it tends to transport pollutants it comes in contact with, including hydrocarbons, trash and debris, bacteria, heavy metals, nutrients, and sediment. Given its transport capabilities and the environmental remediation axiom that the farther pollutants travel from their source, the more difficult and expensive they are to recover, it is important to avoid creating infiltration systems that may contaminate soil or water over time.

Infiltration feasibility

A common requirement in new generation stormwater permits is that runoff reduction Best Management Practices (BMPs) must be implemented to the "maximum extent technically feasible." These requirements have forced a more formal definition of feasibility criteria by which stormwater designers and plan reviewers can objectively evaluate the potential for such BMPs on a site. Infiltration system feasibility criteria can be grouped into four categories: soil and groundwater quality protection, structural issues, flow routing, and avoiding disruption of the regional water balance.

Soil and groundwater protection

Migration of water through soil is the primary means of subsurface pollutant transport; however, common pollutants have a wide range of water solubility and soil adsorption potential. At one end of the spectrum are inert pollutants like sediment and plastic trash that are easily strained as water passes through soil and pose no subsurface contamination risk. At the other end of the spectrum are soluble organic contaminants like pyrene, chlorides, nitrates, and some pathogens that can be transported long distances through soil (Clark, Pitt, and Field, 2010). On some sites, the soil or groundwater may already be contaminated and infiltration should be avoided so that pollutant plume migration is not accelerated. Natural soils can be sources of pollutants as well. For example, selenium is abundant throughout the Southwestern United States and can be transported to sensitive surface waters by infiltrated runoff.

Where soluble pollutants are expected to be present in runoff — and especially where they are known to cause downstream water quality impairments — treatment targeting those pollutants prior to infiltration is essential. Where effective treatment cannot be assured, infiltration should be avoided. Infiltration should be avoided at sites with a high risk for groundwater or soil contamination, including fueling stations, car washes, heavy industrial areas, nurseries, and roadways with more than 25,000 Average Daily Traffic (ADT).

The following common feasibility criteria related to soil and groundwater protection can be used for preliminary site screening. Wherever there is any uncertainty regarding the presence of subsurface pollution or groundwater migration, a qualified hydrologist, geotechnical engineer, or other professional should be consulted.

• Site is not a gas station, nursery, car wash, or other area where hazardous materials or soluble pollutants are – or will be – handled or stored
• At least 250 feet of separation from known groundwater or soil contamination
• At least 100 feet of horizontal separation from wells, springs, and septic systems
• At least 10 feet from the bottom of the facility to the seasonal high groundwater table
• Site has less than 25,000 ADT
• No infiltration in areas with karst topography such as limestone

Structural issues

Introducing water to soil may cause structural instability. For example, soils with high clay content can expand and contract as moisture levels change and slopes can become unstable when saturated. It is critical that a qualified professional be consulted regarding the placement of infiltration facilities so the stability of building foundations, roads, parking lots, and other site features is not threatened. Common feasibility criteria related to structural integrity of soils and structures include:

• Infiltration facilities should be >50 feet from slopes steeper than 15 percent
• Infiltration facilities should be >8 feet from building foundations or a 1:1 slope from the bottom of the foundation (i.e., 10 feet away from a 10-foot-deep foundation)
• Avoid plastic or expansive soils
• No significant increased risk of geotechnical hazards as determined by a geotechnical professional or an available watershed study

Stormwater and groundwater routing

A surface infiltration system must drain in a reasonable amount of time to avoid creating mosquito habitat and to recover storage volume for subsequent storms. Drain down of water stored on the surface within 72 hours is commonly referenced as a benchmark for West Nile Virus control. Drain down of the design volume within 48 hours is typically required to avoid bypassing a significant amount of rainfall in successive storms. Where longer drain down times are required due to slow-draining soils, the infiltration system storage volume can be increased to maintain a high annual runoff capture volume percentage. The relationship between the storage volume, annual capture percentage, and drain down time depends on local rainfall patterns and is best determined using continuous simulation modeling.

Table 1: Characteristics of hydrologic soil groups

Soil Class	Infiltration	Runoff Rate	Infiltration Rate	Soil Types
A	High	Low	>0.3 in./hr when wet	Sand or sandy loam
B	Moderate	Moderate	0.15 to 0.3 in./ hr when wet	Silt loam or loam
C	Low	Moderate to High	0.05 to 0.15 in./hr when wet	Sandy clay loam
D	Low	High	0.0 to 0.05 in./hr when wet	Clay, silty clay, clay loam

A common rule of thumb for infiltration feasibility is that hydrologic soil groups A and B are suitable, C soils are questionable, and D soils are not good candidates. The hydrologic soil group refers to the infiltration potential of the soil after prolonged wetting.

It is also prudent to remember that infiltrated water does not go "away." A good understanding of subsurface geology can help avoid infiltrating water in one area only to have it reemerge down slope or as a seep in a neighboring property. Routing feasibility criteria are as follows:

• Drain surface storage within 72 hours for vector control.
• Drain subsurface storage within 48 hours or increase storage volume in poor-draining soils.
• Native soil infiltration rate should be at least 0.5 inches per hour.
• Avoid horizontal confining layers, such as ledge, caliche, or clay.
• Avoid infiltrating near utility trenches or other linear deposits of permeable fill that may accumulate and conduct groundwater.

Regional water balance

Most rain falling on land in its natural state is lost to evapotranspiration. For example, in the arid Southwest, less than 10 percent of rainfall makes it to either deep infiltration or runoff (Wilcox, Breshears, and Seyfried, 2003). The rest is intercepted by vegetation or the top few inches of soil. Low impact development (LID) based stormwater management regulations typically focus on maintaining the pre- and post-development runoff rates and volumes. Infiltration of the excess runoff from impervious areas can help meet that goal, but at the same time may cause the natural infiltration rate to be dramatically exceeded. On a local level, this can result in soil instability. On a regional scale, this can also have serious implications.

Water levels and flow rates in most natural water bodies fluctuate with seasonal changes in groundwater levels and direct inflow rates. Infiltrating too much water in an area can prolong the seasonal peak flow or high-water level. This is most dramatic in ephemeral streams, ponds, and wetlands where an unnaturally long wet period can displace endangered or sensitive species.

Water rights are allocated in many areas of the United States based on the historic or presumed availability of groundwater or surface water. Where runoff volume is reduced beyond natural conditions or where it is infiltrated in a location where it can't be extracted, this can effectively deprive a downstream user of water to which they are entitled. Following these simple feasibility criteria can help avoid regional water imbalances:

• Do not violate downstream water rights.
• Coordinate with groundwater management agencies.
• Create no adverse impact on flow rate, duration, and seasonality of downstream waters.

Figure 1: Landscape infiltration BMP featuring storage, treatment and infiltration components	Figure 2: Replacing stone with manufactured structures like this chamber can increase subsurface storage capacity

Infiltration system design

Common infiltration practices include drywells, bioretention, permeable pavement, infiltration trenches, infiltration basins, and subsurface infiltration galleries. (see Figures 1 and 2) Regardless of their form, all infiltration systems have three primary components: storage, treatment, and infiltration.

Storage — Retention volume of a BMP is when runoff accumulates during a storm and is stored prior to infiltration or treatment. In a landscape-based system, storage tends to be the most upstream component in the form of depressional storage. For example, a bioretention cell may store 6 to 12 inches of water in a surface pool that slowly infiltrates at a rate dictated by the native soils. One method of increasing storage in a landscape-based system is to add highly permeable, high-void soil amendments.

In subsurface systems, the storage component is often located between the treatment and infiltration components. For example, runoff may be routed to a hydrodynamic separator or cartridge-based media filter, then to a network of large-diameter perforated pipe from which it percolates into the surrounding native soil. In subsurface systems, storage may be comprised entirely of gravel, but many high-voids options are available to minimize the overall volume of the system. For example, perforated pipes, concrete arches and vaults, and plastic chambers and crates are all commonly used (see Figures 3 and 4). Most of these subsurface storage options are HS20 loading rated and some can be reinforced for greater loads when necessary. This allows the overlying land to be used for nearly any non-building purpose from parks to parking lots.

The storage component volume is typically equal to the volume of runoff produced during the water quality design storm. The maximum allowable effective depth (inches) of water stored in the system can be calculated by multiplying the drain down time (hours) by the design infiltration rate (inches/hour) of the native soils. For example, the maximum infiltration system effective depth onsite with a desired 48-hour drawdown time and native soils with a 1-inch/hour design infiltration rate is 48 inches. The required footprint of the system can be calculated by dividing the storage volume by the effective depth of the system. For more aggressive sizing, the amount of runoff infiltrated during the time it takes to fill the system can be subtracted from the total required storage volume.

Figure 3: Perforated metal pipe	Figure 4: Stormwater infiltration chambers

Treatment — The treatment component should be tailored to expected pollutants onsite, the vulnerability of the downstream waters to those pollutants, and the infiltrating surface's susceptibility to clogging. In a landscape-based system, the treatment component is typically comprised of plants and soil. For example, in a bioretention system, most pollutants are removed and retained in the mulch and upper layer of soil. Landscapebased water quality BMPs such as swales or biofiltration may be used as treatment upstream of subsurface systems, but subsurface treatment is more commonly provided in the form of a hydrodynamic separator or media filter.

Adequate treatment is a crucial component of all infiltration systems. Inadequate treatment will allow pollutants to migrate to the native soils where clogging of interstitial pore space can quickly reduce infiltration rates. In landscape-based BMPs, failure of the mulch and/or top soil layer to infiltrate due to sediment accumulation can lead to frequent bypass or flooding even when the native soils have ample capacity. In all infiltration systems, exhaustion of the infiltration capacity of the native soils will require a complete system rehabilitation which can be very expensive. Incremental initial expenditure on treatment can pay big dividends by extending the life of the entire system.

Ideally, treatment is provided in the form of filtration through engineered soil or filter media that will remove particles in the 5- to 10-micron range and larger. Sometimes, this is not practical due to site constraints or budgetary limitations. A good second option is gravity separation. There are several commercially available hydrodynamic separators that can reliably remove particles as fine as 50 microns without requiring the large footprint and long detention times of traditional plug flow clarifiers or baffle boxes. Where these systems are being considered, independently verified lab and field performance data should be reviewed to ensure that sizing is adequate. The Technology Acceptance and Reciprocity Partnership (Pa. DEP, 2011) and the Technology Assessment Protocol – Ecology (Washington State Department of Ecology, 2011) both provide such verification for proprietary stormwater treatment systems. For those with severe budgetary limitations, catch basin inserts can provide some pretreatment benefits as long as utmost attention to operation and maintenance can be assured.

Infiltration — The native soil infiltration rate often drives decisions about the size and form of the infiltration system. The lower infiltration rates are, the shallower and broader the system must become. There are a range of methods for determining infiltration rates, from local soil maps to detailed soil testing. The actual infiltration rate is usually converted to a design infiltration rate by applying a safety factor that accounts for uncertainty in the known rate, assumed reduction in performance over time, treatment effectiveness, and other factors. Safety factors commonly range from 2-10. With a factor of safety of two, the allowable system height would be halved. The design infiltration rate is then used to establish system dimensions and size.

Great care must be taken to ensure that the interface between the storage or treatment component and the native soils is not fouled either by materials passing through the treatment system. In systems where gravel or high-void subsurface structures such as perforated pipes or chambers are used, it is also critical that native soils are prevented from migrating into the storage component. If this happens, subsidence of soils at the surface may occur. The barrier between subsurface storage gravel or galleries and native soils is most often a nonwoven geotextile. During construction, compaction of native soils must be avoided to ensure that optimal infiltration rates are preserved.

Table 2: Draw down times, infiltration rates, and allowable system heights.

Design native soil infiltration rates (in/hr) required to draw down subsurface storage reservoirs of varying depth
Subsurface storage Depth (in)		24	36	48	60	72	120
Drawdown Period (hr)	24	1	1.5	2	2.5	3.0	5.0
	48	0.5	0.8	1.0	1.3	1.5	2.5
	72	0.33	0.5	0.7	0.8	1.0	1.7
	96	0.25	0.4	0.5	0.6	0.8	1.3

Maintenance

Regular inspection and maintenance is critical to ensure the ongoing effectiveness of any BMP. Ideally, the treatment component of the infiltration system is effective enough to prevent the need for restoration of the native soil infiltration capacity. The treatment component must also be accessible to facilitate inspection and maintenance. In bioretention or permeable pavement systems, preventing occlusion of the pavement or BMP surface by sediment is the primary objective of maintenance activity. Inspection should occur at least semiannually with removal of sediment, trash, and debris as needed. Accumulation of these materials on the surface of bioretention systems can also reduce their storage capacity. Mulch should be replaced at least annually and, if needed, the surface of the BMP can be raked to regenerate infiltration capacity.

A similar inspection and maintenance schedule should be kept for subsurface treatment systems such as hydrodynamic separators and media filters. Depending on the sizing of these systems and the pollutant loads received, maintenance intervals can range from six months to several years with annual or semi-annual maintenance being most common. With adequate treatment, the available volume of subsurface storage systems such as perforated pipes or chambers should remain very close to the design volume over the long term. To ensure proper flow routing, inspection of any internal flow control structures should be part of the inspection protocol.

When treatment fails due to improper design, installation, or lack of maintenance, system failure is imminent. In landscape-based systems, failure is often experienced as extended ponding or excessive bypass as gross solids smother the surface of the BMP and reduce surface infiltration capacity. Complete system rehabilitation may or may not be required depending on whether solids have migrated into the structure of the native soil. In the best case scenario, the surface of the BMP can be cleaned and replaced with increased future source control practices on site.

Underground infiltration systems can be more difficult to maintain because they are often under a paved or landscaped area and include plastic, concrete, or metal structures. Extended drain down time is the most common indicator that maintenance is needed. Low-profile systems or those with extensive internal structural supports such as most crate-type systems can be difficult to inspect, impossible to enter, and very challenging to rehabilitate. Other systems such as large-diameter perforated pipe or concrete galleries can easily be inspected and entered if necessary due to their larger, open internal structure. Maintenance typically includes clearing of debris or accumulated sediment from the bottom of the storage system.

Some subsurface infiltration designs call for a fabric layer surrounding the storage structure. For example, perforated pipe is commonly wrapped with a nonwoven geotextile with another geotextile layer between the surrounding gravel and the native soils. This first fabric layer prevents sediment from passing through to the second fabric barrier and into the native soils. If the first barrier clogs, the pipe can be cleaned using jetting equipment. Similar strategies are used for chamber and crate systems with varying effectiveness based on the openness of the internal storage system structure and adequacy of access. Regardless of the system type, complete excavation and reconstruction may be required if significant solids have migrated into the native soil.

Conclusion

As regulatory pressure and increased appreciation of the value of our water resources drives the use of green infrastructure technologies, we will learn important lessons relating infiltration system design decisions to the long-term effectiveness. At this point, it is clear that technical feasibility screening and proper siting are critical first steps. Careful attention to the three components of all infiltration systems — storage, treatment, and native soil infiltration — can produce a system with treatment that effectively protects the infiltration capacity of native soils, and a storage volume and shape that will capture the design storm volume and infiltrate it within the desired drain down period. Given the cost and difficulty of rehabilitating infiltration systems where native soil infiltration capacity has been compromised, a conservative approach to design is advised. Conservative approaches include: increasing design infiltration factors of safety, providing robust treatment controls, and providing a storage volume equal to the design water quality volume.

---

Quiz Questions

1. A critical component of infiltration is the rate that rainfall will infiltrate into the soil. What soil groups are good for infiltration?
   1. Soil groups A and B are suitable
   2. All groups (A-D) because water will eventually drain
   3. Soils with a clay mixture
   4. Soils with high runoff rate will also have high infiltration rate
2. Infiltration systems have these primary components:
   1. Storage, treatment, and infiltration
   2. Infiltration only
   3. Storage and infiltration, there is no treatment involved
   4. Treatment and infiltration
3. Storage component volume is typically equal to the volume of runoff produced during the water quality design storm. Maximum allowable effective depth (inches) of water stored in the system is calculated by:
   1. Drain down time (hours) multiplied by the Design Infiltration Rate (inches/hour)
   2. Q = ciA
   3. E=mc²
   4. F=ma
4. What system may be needed when low infiltration rates are present?
   1. Increase the footprint and decrease the depth of the storage system
   2. Does not change the infiltration system
   3. Increase infiltration rate factor of safety
   4. Mix clay into the soil
5. What is the primary reason for maintaining an infiltration system?
   1. Preventing occlusion of the pavement or BMP surface by sediment
   2. Remove the captured pollutants
   3. Regenerate infiltration capacity
   4. All of the above
6. What are the different fates of rainfall??
   1. Evapotranspiration, overland flow, infiltration
   2. Infiltration only
   3. Overland flow and infiltration
   4. Evapotranspiration and overland flow
7. Water drain down is critical:
   1. To avoid creating mosquito habitat
   2. To recover storage volume for subsequent storms
   3. As one measurement to know if a system needs to be maintained
   4. All of the above
8. Migration of water through soil is the primary means of subsurface pollutant transport.
   1. True
   2. False
9. What are the types of infiltration systems?
   1. Perforated CMP
   2. Concrete vaults placed on stone
   3. Subsurface chambers or crates
   4. All of the above
10. In the arid Southwest, what percentage of rainfall makes it to either deep infiltration or runoff?
    1. Less than 10 percent
    2. 20-30 percent
    3. 40-50 percent
    4. 85 percent

---

Vaikko Allen, CPSWQ, LEED-AP, is a Regional Regulatory Manager for CONTECH and has 14 years of stormwater management experience. He holds a B.S. degree in Environmental Science and Policy from the University of Southern Maine with a concentration in Water Resources and also holds patents for several stormwater BMPs.

Aimee Connerton, LEED-AP, is a CONTECH Project Consultant covering the states of Maryland and Delaware. She earned her B.S. degree in Chemistry from the University of Maryland, College Park, with a minor in Economics.

Cory Carlson, P.E., is the Product Manager for Detention and Infiltration at CONTECH. He has a B.S. degree in Civil Engineering from the University of Missouri, earned an MBA from Rockhurst University, and is a registered Professional Engineer in the state of Kansas.

---

REFERENCES

• EPA, 2011, Low Impact Development (LID), U.S. Environmental Protection Agency, www.epa.gov/owow/NPS/lid
• EPA, 2008, Managing Wet Weather with Green Infrastructure Action Strategy, U.S. Environmental Protection Agency, www.epa.gov/npdes/pubs/gi_action_strategy. pdf.
• EPA, 2009, Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act, U.S. Environmental Protection Agency, www.epa.gov/oaintrnt/documents/epa_swm_guidance.pdf.
• Clark, Shirley, Pitt, B., and Field, R., 2010, Groundwater contamination potential from infiltration of urban stormwater runoff, Published in Effects of urbanization on groundwater: an engineering case-based approach for sustainable development, Reston, Va., American Society of Civil Engineers.
• Wilcox, B.P., Breshears, D.D., and Seyfried, M.S., 2003, Water balance on rangelands, Encyclopedia of Water Science, Marcel Dekker, New York, pages 791-794.
• Pa. DEP, 2011, A State Tool to Promote Scientifically Sound, Cost-effective Environmental Decision-making, Pennsylvania Department of Environmental Protection, www.dep.state.pa.us/dep/deputate/pollprev/techservices/tarp.
• Washington State Department of Ecology, 2011, Evaluation of Emerging Stormwater Treatment Technologies, www.ecy.wa.gov/programs/wq/Stormwater/ newtech/index.html