Exfiltration Systems: Underground Stormwater Retention BMPs

Topic Overview · Module 16 · Stormwater BMP Design Series

---

1. Introduction to Exfiltration Systems

Topic 1 — Foundational concepts, BMP classification, and detention vs. retention distinctions

Exfiltration is an underground stormwater management approach in which runoff is stored in subsurface structures — trenches or vaults — and gradually released into the surrounding soil. Unlike surface infiltration, water enters and exits entirely below grade, making exfiltration especially valuable where above-ground space is scarce. As a retention-type Best Management Practice (BMP), it reduces runoff volume by returning stored water to the groundwater system rather than simply delaying its discharge.

What Defines Exfiltration

• Water originates from underground storage, not surface ponding — distinguishing it from standard infiltration practices.
• It is classified as a retention system: stored water re-enters the ground rather than being conveyed downstream.
• Particularly suited to dense urban environments where surface land for open ponds or swales is unavailable.

Exfiltration as a BMP

• Exfiltration BMPs fit naturally into urban infill and redevelopment projects where impervious cover is high and open land is limited.
• The two primary physical forms — trenches and vaults — serve the same hydraulic function but differ in geometry and application context.
• Depending on design goals, the same infrastructure can function as either a detention system (managing peak discharge rates) or a retention system (reducing total runoff volume).

---

2. Types of Exfiltration Systems: Trenches and Vaults

Topic 2 — Structural configurations, pollution control media, and cross-sectional design types

Exfiltration systems take two primary structural forms: perforated-pipe trenches that simultaneously store and convey runoff along their length, and underground vaults that store runoff in place. Both can incorporate pollution control media to enhance water quality treatment, and both are sized using cross-sectional configurations tied to site soil conditions.

Exfiltration Trench with Perforated Pipe

• Perforated pipe within the trench increases available storage volume and allows lateral distribution of runoff along the trench length.
• Exfiltration occurs continuously along the full length of the trench, maximizing contact with surrounding soil.
• Rock or pollution control media surrounding the pipe provides additional pollutant removal as water percolates through the trench cross-section.

Stormwater Vaults

• Vaults store runoff underground but do not transport it horizontally — water exfiltrates vertically and laterally through vault walls and floor.
• Common in urban settings where surface footprint is tightly constrained; vaults can be placed under parking lots, plazas, or roadways.
• Available in a wide range of standard manufacturer sizes, simplifying design and permitting for common storage volume requirements.

Pollution Control Media

• Specially selected sorption or filtration media placed within the trench or vault can target specific pollutants such as nitrogen and phosphorus.
• Media selection affects both removal efficiency and system recovery time — denser media slows exfiltration, shifting a fast-recovery system to standard recovery.
• When groundwater protection is a concern, media becomes a critical design component (see Section 6).

Cross-Sectional Configuration Types

---

3. Annual Effectiveness and Recovery Time

Topic 3 — Effectiveness tables, fast-recovery adjustments, and the depth-capture relationship

Determining how much annual runoff pollutant load an exfiltration system will capture requires using standardized effectiveness tables — or their software equivalent — that account for site hydrology, soil permeability, and storage depth. Recovery time is the key variable that separates standard from high-performance exfiltration design.

Standard Recovery Effectiveness Tables

• The AH Volume 1 Appendix O tables provide up to 80 removal efficiency scenarios, covering a range of design treatment depths and site conditions.
• Key input variables include Curve Number (CN) for pervious areas and Directly Connected Impervious Area (DCIA) percentage.
• Interpolating between CN and DCIA values at 5-unit increments can be mathematically tedious — BMPFast automates this process.

Fast-Recovery Adjustment

• Fast recovery is defined as full drawdown of the design treatment volume within 3 hours — typically achievable in limestone or highly permeable media conditions.
• The fast-recovery bonus adds up to 10% additional annual effectiveness, though the benefit diminishes as design storage volume increases.
• BMPFast automatically applies the fast-recovery adjustment when the appropriate cross-section type is selected.

Design Treatment Depth and Capture

---

4. BMPFast Software Application

Topic 4 — Data entry, vault configuration, and perforated pipe trench configuration in BMPFast

BMPFast is the primary computational tool for designing exfiltration systems under Florida’s stormwater regulations. It handles site characterization, BMP sizing, performance standard checking, and — for exfiltration specifically — both vault and perforated pipe trench configurations.

Entering Site and Catchment Data

• Performance standards (OFW, impaired water body designations) must be selected at the outset — they govern the minimum annual removal percentage required.
• CN values for non-DCIA pervious areas and total DCIA acreage are entered separately to reflect the hydrologic contribution of each land cover type.
• Pollutant Event Mean Concentration (EMC) values for the site’s land use category are available within the BMPFast database and auto-populate by default.

Configuring the Vault Option

• Required inputs include vault length, width, depth, and void space percentage — all typically available from manufacturer specifications.
• BMPFast checks the design against both the specified performance standard and the net improvement standard simultaneously.
• A storage volume equivalent to more than 4 inches of design treatment depth over the contributing drainage area typically yields very high annual removal effectiveness.

Configuring the Perforated Pipe Trench Option

• Primary inputs are pipe diameter and total pipe length; the software calculates effective storage volume from these parameters and the trench cross-section type.
• Typical layouts use parallel trench runs connected by perpendicular segments to distribute flow across the available site footprint.
• When properly sized, both vault and perforated pipe trench configurations can achieve equivalent annual removal percentages for the same contributing catchment.

---

5. Example Project: Medical Plaza in Fort Lauderdale

Topic 5 — Real-world permit application demonstrating end-to-end exfiltration design with BMPFast

A medical plaza development in Fort Lauderdale converting agricultural land to high-intensity commercial use illustrates the complete exfiltration design process. The site’s limestone geology, regulatory context, and constrained footprint made underground exfiltration the preferred BMP solution — and the project demonstrates that both vault and pipe trench alternatives can satisfy the same stringent performance standard.

Site Characteristics and Performance Requirements

• Land use is classified as high-intensity commercial due to frequent traffic generation — a category with elevated pollutant EMC values.
• The site is subject to both OFW and impaired water designations, requiring the most stringent criterion: 95% annual pollutant removal.
• A geotechnical report confirming limestone permeability is required documentation to justify the 3-hour fast-recovery design assumption.

The example demonstrates a key design principle: when a site has both fast-recovery soils and a large required storage volume relative to its drainage area, multiple physical configurations (vault or trench) can satisfy the same performance standard. The choice between them is primarily driven by constructability, cost, and parcel geometry rather than hydraulic performance differences.

---

6. Groundwater Protection and Sorption Media

Topic 6 — Sorption media design, recharge volume analysis, and service life estimation

Because exfiltration systems return water directly to the subsurface, they can affect groundwater quality — particularly near springs, estuaries, or impaired water bodies. When groundwater protection requirements apply, sorption media is added to the vault or trench to reduce pollutant concentrations before recharge occurs. BMPFast supports this analysis through a dedicated groundwater discharge module.

When Groundwater Protection Is Required

• Groundwater recharge analysis is triggered by site proximity to springs, estuaries, or impaired water bodies where subsurface discharge quality is regulated.
• Regulators may limit both the volume of recharge and the pollutant concentration of recharged water.
• In BMPFast, the groundwater analysis is activated by selecting YES on the groundwater discharge configuration screen.

Selecting and Adding Sorption Media

• Sorption media reduces the system’s exfiltration rate to approximately 5 inches per hour, slowing drawdown and extending contact time with the treatment material.
• Adding media shifts the system classification from fast-recovery to standard recovery — an important consequence that reduces annual effectiveness slightly but improves recharge water quality significantly.
• The media type and dimensions must be entered in BMPFast before recalculating the summary output to reflect the updated performance.

Recharge Volume and Concentration Results

Without sorption media, the average recharge pollutant concentration defaults to the site’s Event Mean Concentration (EMC) — meaning untreated runoff quality is effectively transferred directly to the groundwater. TP removal results are also reported on the BMPFast summary page alongside TN.

Media Service Life Estimation

• Service life is calculated in BMPFast via the Tools menu within the exfiltration module — inputs include vault floor area and media depth.
• A media depth of 2 feet over an 8,000 square foot vault base yielded a 50-year service life in the example project — well above typical design thresholds.
• A service life exceeding 30 years is generally desirable to avoid the high cost and disruption of excavating the vault for media replacement during the asset’s operational life.

---

Quick Reference: Exfiltration System Comparison

---