Infiltration Basin
An infiltration basin is a stormwater management practice designed as a shallow, excavated impoundment that captures and temporarily stores runoff, allowing it to infiltrate into the underlying native soil. By promoting infiltration, these basins effectively reduce runoff volume, recharge local groundwater aquifers, and filter pollutants from stormwater. They are fundamentally simple structures, typically consisting of an open basin with a flat, vegetated floor over permeable soils. Their effectiveness is highly dependent on site conditions, particularly soil permeability, and they require careful design and consistent maintenance to prevent clogging and ensure long-term performance.
The primary function of an infiltration basin is to mimic natural hydrologic processes by returning stormwater to the ground. This helps maintain baseflow in nearby streams, which is especially beneficial for supporting aquatic ecosystems during dry periods. While they offer high pollutant removal and volume reduction, their application is limited to sites with suitable geology and a low risk of groundwater contamination. Historical performance data indicates a significant rate of failure for improperly sited or maintained basins, making thorough site assessment a critical first step.
Applicability
Infiltration basins are a targeted stormwater solution with strict siting requirements. Their use is constrained by soil characteristics, groundwater proximity, topography, and the nature of the contributing land use. When site conditions are appropriate, they are an excellent choice for groundwater recharge and water quality improvement.
Regional Suitability
Infiltration basins can be adapted for use in most regions of the United States, though modifications are necessary for arid and cold climates. In arid areas, robust pretreatment is essential to manage high sediment loads, and drought-tolerant vegetation should be used. In cold climates, basins may be taken offline during winter to prevent frost heave and avoid infiltrating runoff containing high concentrations of road salt. Their use is strictly prohibited in areas with karst topography due to the high risk of sinkhole formation and rapid groundwater contamination.
Ultra-Urban and Constrained Sites
Application in densely developed ultra-urban environments is rare. Infiltration basins require a relatively large, contiguous, and flat land area, typically consuming 2% to 3% of their drainage area. This space is often unavailable in urban settings. Furthermore, urban soils are frequently compacted or consist of fill material with poor infiltration capacity. There is also a risk that infiltrated water could interfere with nearby building foundations, utilities, and other subsurface infrastructure.
Stormwater Hotspots
Infiltration basins must not receive runoff from land uses classified as stormwater hotspots. These are areas where activities generate runoff with higher-than-typical pollutant concentrations, such as gas stations, vehicle maintenance facilities, or industrial storage yards. Direct infiltration of this highly contaminated runoff poses an unacceptable risk to groundwater quality. If treatment of hotspot runoff is required, it must be fully treated by other means before being routed to any infiltration practice.
Retrofit Applications
Using infiltration basins as a stormwater retrofit in previously developed areas presents challenges. Identifying locations with suitable soils and adequate separation from the water table can be difficult in an established suburban or urban landscape. Because they are best suited for small drainage areas (typically under five acres), retrofitting an entire watershed would require a large number of individual basins, which can be inefficient from a cost and maintenance perspective compared to larger, regional solutions.
Design Criteria
Successful long-term performance of an infiltration basin depends on a design that integrates careful site selection with robust pretreatment, proper sizing, and features that facilitate maintenance.
Feasibility and Siting
A thorough geotechnical investigation is the first step in design. Key feasibility criteria include:
- Soils: The underlying soils must have a field-verified infiltration rate of at least 0.5 inches per hour. Soils should have a clay content below 20% and a combined silt/clay content below 40%. Infiltration basins must not be constructed on fill soils.
- Drainage Area: The maximum contributing drainage area should not exceed five acres. Larger areas increase the risk of rapid sediment accumulation and clogging.
- Separation Distances: The bottom of the infiltration basin must be at least four feet vertically separated from the seasonally high water table and any bedrock layer. A horizontal setback of at least 100 feet from any water supply wells and 25 feet down-gradient from building foundations is required.
- Site Slopes: The basin may not be located on slopes exceeding 6%. The floor of the basin must be graded to be perfectly flat to promote uniform infiltration.
Conveyance
Runoff must be safely conveyed to and from the basin. Infiltration basins should be designed as off-line systems, where a flow splitter diverts only the target water quality volume into the basin. Flows from larger storm events should bypass the facility and be routed to a stabilized downstream conveyance system. All channels leading to the basin must be stabilized with vegetation to prevent erosion and sediment transport into the practice.
Pretreatment
Pretreatment is the most critical design element for preventing premature failure from clogging. A dedicated pretreatment practice, such as a sediment forebay, vegetated filter strip, or grass channel, must be incorporated into the design. This pretreatment cell should have a minimum volume equal to 25% of the total water quality volume being treated. Using multiple pretreatment measures in series is highly recommended to maximize sediment capture before runoff enters the main infiltration area.
Treatment and Sizing
The basin must be sized to store the entire calculated water quality volume (WQv). The required surface area is determined by the WQv and the infiltration rate of the underlying soils. An accurate calculation can be performed using a dedicated stormwater infiltration sizing calculator. The design must allow the full WQv to dewater, or draw down, through the soil within 48 hours. This timeframe ensures that the soil remains aerobic and storage capacity is available for subsequent storm events. An observation well—a perforated vertical pipe extending to the bottom of the basin—should be installed to monitor drawdown times post-construction.
Landscaping
Vegetation plays a key role in the function and stability of an infiltration basin. The entire contributing drainage area must be fully stabilized with dense vegetative cover before runoff is directed into the basin. The basin floor and side slopes should be planted with water-tolerant turf grasses that can withstand periodic inundation. A healthy turf cover helps maintain soil structure, prevents surface erosion, and aids in water infiltration.
Pollutant Removal
When properly designed and functioning, infiltration basins provide very high pollutant removal rates because stormwater is filtered through the soil matrix, which traps particulates and facilitates chemical and biological processes. As no surface discharge occurs for treated storms, the pollutant load delivered downstream is effectively eliminated. The data below represents estimated removal efficiencies for a basin that successfully infiltrates the full water quality volume. For more detailed performance information, consult the comprehensive pollutant removal database.
| Pollutant | Estimated Removal Efficiency (%) |
|---|---|
| Total Suspended Solids (TSS) | 75 |
| Total Phosphorus (TP) | 60 – 70 |
| Total Nitrogen (TN) | 55 – 60 |
| Metals (Cadmium, Copper, Lead, Zinc) | 85 – 90 |
| Bacteria | 90 |
Source: Adapted from Schueler, 1987. Assumes full infiltration of the design storm volume.
Construction and Cost Considerations
The construction sequence is critical to the long-term success of an infiltration basin. The basin area must be clearly marked and protected from compaction by heavy machinery throughout the construction phase. The basin should never be used as a temporary sediment trap. Excavation of the basin should be one of the final steps in the site construction process, occurring only after the contributing drainage area has been fully stabilized with vegetation. Excavation should be performed with equipment that can reach into the basin from the sides to avoid compacting the basin floor.
Construction costs are moderate compared to other practices, as they primarily involve earthwork. A 1991 study estimated construction costs at approximately $2 per cubic foot of storage provided (SWRPC, 1991). The primary cost concern is the long-term maintenance burden and the potential for expensive rehabilitation if the basin fails due to clogging. Full-scale rehabilitation can involve removing and replacing the top layer of soil, which can be as costly as the initial construction.
Maintenance
Consistent and targeted maintenance is essential to prevent clogging and ensure the continued function of infiltration basins. A legally binding maintenance agreement should specify the responsible party and outline the required activities and schedule. Historically, infiltration basins have one of the highest failure rates of all stormwater practices, almost always due to a lack of maintenance.
| Activity | Schedule |
|---|---|
| Inspect for signs of standing water, erosion, sediment accumulation, and structural damage. Check observation well to confirm drawdown time is under 48 hours. | Semi-Annually and after major storms |
| Mow basin floor and side slopes. Remove litter, debris, and invasive vegetation. Stabilize any eroded areas. | As Needed (typically 2-4 times per year) |
| Core aerate or disc the basin bottom to break up compacted soil surfaces and maintain infiltration capacity. Remove thatch buildup. | Annually |
| Remove accumulated sediment from the pretreatment forebay and basin floor. Restore original grades and re-establish vegetation. | Every 3 to 5 years, or when performance declines |
If an observation well shows that water is ponding for more than 72 hours after a storm, the basin is likely clogged. The first corrective step is typically aggressive aeration or tilling of the surface. If this does not restore function, more intensive rehabilitation, such as removing the top 6-12 inches of soil and sediment and replacing it with a clean sand/compost mix, may be required.
Limitations
Despite their benefits, infiltration basins have significant limitations that restrict their use. The most prominent limitation is their high rate of failure due to surface clogging from sediment accumulation. This requires a commitment to diligent, long-term maintenance that is often underestimated.
Siting constraints are severe. They are unsuitable for sites with impermeable soils (e.g., clays), steep slopes, a high water table, or shallow bedrock. The potential for groundwater contamination makes them inappropriate for treating runoff from stormwater hotspots. If a basin clogs and holds standing water for extended periods, it can become a breeding ground for mosquitoes and develop nuisance odor problems. Aesthetically, a dry, grassy basin can be well-integrated into a landscape, but a clogged, muddy, or poorly vegetated basin can be unattractive.
Frequently Asked Questions
What is the primary purpose of an infiltration basin?
The primary purpose of an infiltration basin is to manage stormwater by capturing runoff and allowing it to soak into the ground. This process recharges local groundwater, reduces the total volume of runoff leaving a site, and filters pollutants. By mimicking the natural water cycle, it helps maintain the health of nearby streams and aquifers. Unlike a detention basin that only slows runoff down, an infiltration basin aims to eliminate it entirely for smaller, more frequent storms.
How is an infiltration basin different from an infiltration trench?
An infiltration basin is a shallow, open impoundment with a vegetated surface that relies on the native soil for infiltration. In contrast, an infiltration trench is a narrow, deep excavation filled with stone aggregate that provides a large subsurface storage volume. Trenches are typically used on sites with limited surface area, such as parking lot islands or linear corridors, while basins require a larger, open footprint. Basins use surface infiltration, while trenches use both bottom and side-wall infiltration.
What happens if an infiltration basin gets clogged?
When an infiltration basin clogs, its surface becomes sealed with fine sediment and organic matter, preventing water from soaking into the soil. The basin will fail to drain within the required 48-hour drawdown period, leading to prolonged standing water. This condition can kill the vegetation, create a mosquito breeding habitat, and cause the basin to function as a simple pond with no water quality or recharge benefits. The only remedy for a clogged basin is physical rehabilitation, such as tilling the surface or removing and replacing the top layer of soil.
Why can’t infiltration basins be used for stormwater hotspots?
Stormwater hotspots are land uses like gas stations or industrial sites that generate runoff with high concentrations of toxic pollutants, such as hydrocarbons or heavy metals. Infiltration basins discharge this runoff directly into the ground. While soil provides some filtering, it cannot reliably remove all dissolved contaminants. Using an infiltration basin at a hotspot creates a direct and unacceptable risk of contaminating groundwater, which may be a source of drinking water for nearby wells or communities. Protecting groundwater quality is a paramount concern.
What kind of soil is required for an infiltration basin?
The ideal soils are sandy loams, loamy sands, or sands, which have high permeability. A site must have a minimum verified infiltration rate of 0.5 inches per hour. Soils with high clay or silt content are not suitable because they drain too slowly, leading to prolonged ponding and system failure. A geotechnical investigation, including soil borings and infiltration tests, is a mandatory step in the design process to confirm that the soils on-site can support the practice.
How does an infiltration basin compare to bioretention?
Both practices use soil to treat stormwater, but they differ in complexity and applicability. An infiltration basin is a simpler system relying on native soils over a large area. A bioretention facility, or rain garden, is typically smaller and uses an engineered soil media, a mulch layer, and diverse plantings. Bioretention is more versatile and can be used on sites with poorer soils by including an underdrain. It also offers higher pollutant removal for certain contaminants due to the enhanced biological activity in the engineered media.
What is the most important maintenance task for an infiltration basin?
The most critical maintenance task is preventing sediment from reaching the basin floor. This involves routine inspection and cleaning of pretreatment measures, such as sediment forebays, and ensuring the contributing drainage area remains well-vegetated. Once sediment clogs the basin floor, infiltration capacity is lost. Regular mowing and periodic aeration of the basin floor are also crucial for preserving the soil’s permeability and preventing surface sealing. Proactive, preventative maintenance is far more effective and less costly than reactive rehabilitation.
Can infiltration basins be used in cold climates?
Yes, but with design modifications. Infiltration can be ineffective when the ground is frozen. Some designs incorporate an underdrain system that can be opened in winter, allowing the basin to function as a dry detention facility until the ground thaws. Another concern is the high chloride concentration from road salt in winter runoff, which can contaminate groundwater. For this reason, basins may be taken offline during the deicing season, with runoff bypassing the facility. Planting salt-tolerant grasses is also recommended.
Are there other common infiltration-based stormwater practices?
Yes, several other practices are designed around the principle of infiltration. Infiltration trenches are stone-filled reservoirs used in space-constrained areas. Dry wells are small, excavated pits filled with stone that manage runoff from rooftops. Perhaps the most widely used alternative is porous pavement, which includes permeable asphalt, pervious concrete, and permeable pavers. These surfaces allow rainfall to pass directly through the pavement into a stone reservoir below, providing treatment and storage while serving as a functional surface for vehicles or pedestrians.
How do I decide if an infiltration basin is right for my site?
The decision depends heavily on site-specific factors. First, conduct a thorough site analysis focusing on soils, depth to water table, and available space. If the soils infiltrate at more than 0.5 inches per hour and there is adequate separation from groundwater, a basin may be feasible. Next, consider the land use to ensure it is not a stormwater hotspot. Finally, weigh the land requirement and long-term maintenance commitment. A comprehensive BMP selector tool can help compare infiltration basins against other practices based on your site conditions and treatment goals.