Description:
Exfiltration is a subsurface structure to exfiltrate the stored water into the ground. The “ex” is intended to distinguish between surface (infiltration) and underground movement (exfiltration) of water into the ground. As examples, it can be perforated pipe or vault surrounded by natural rock or pollution control media. A vault typically has no media in the storage area. If the soils and water table allow, stored runoff exfiltrates to the groundwater.
A typical section of a pipe is shown below with the design volume also known as the required treatment volume “RTV”. A recovery time of 72 hours is used for low hydraulic conductivity (such as no limestone) conditions in the left side of the diagram below.
There are procedures for estimating the recoverable volume for a design and operation when the ground layer is limestone (SFWMD, 2016). The design is usually consistent with the section shown on the right in the diagram below with a water table at or below the pipe invert. The time for recovery is typically specified to be one or three hours. All designs must follow applicant handbook calculations.
Hydraulic Conductivity Conditions
Low Hydraulic Conductivity
High Hydraulic Conductivity
Average Annual Capture Effectiveness
The average annual effectiveness in a high hydraulic conductivity rock layer (limestone exfiltration area) was calculated using the original work of Wanielista (1978).
An example of the average annual capture effectiveness for Region 2 expressed as a percentage of total rainfall (Y axis) is shown as a function of design treatment depth (X axis) (Wanielista and Yousef, 1993)
The top curve represents a recovery time of 3 hours, corresponding to the time interval for independent storm events in Florida, assuming no initial abstraction on the catchment (runoff = rainfall).
The capture effectiveness assumes exfiltration of every rainfall event up to the design depth plus at least the design depth for all rainfall events greater than the design depth.
Average annual capture effectiveness for Region 2 as a function of design treatment depth.
Adjustment for Initial Abstraction and Recovery Time
Harper (2007) adjusted the yearly capture to account for initial abstraction on the catchment and for a 72-hour recovery time using long-term simulation of rainfall/runoff.
Annual capture tables are published in the FDEP Applicant’s Handbook, Volume 1 (June 28, 2024) for design depths ranging from 0.25 to 4.0 inches.
For example, increasing recovery from 72 hours to 3 hours at a design depth of 1 inch results in an increased capture efficiency of approximately 8%.
Example Calculation
For a 1.2-acre catchment and a design depth of 1 inch, the design volume is 0.1 acre-foot (1 in × 1.2 acres ÷ 12 in/ft).
BMPTrains is then used to calculate the average annual removal for the catchment conditions and location
Pollution Control Media
Pollution control media may be used for exfiltration in areas where additional removal of nutrients is required before runoff waters enter the groundwater supply. In many applications, exfiltration systems are located beneath traffic areas or parking surfaces. Selected media must provide adequate structural support for vehicles or surface structures.
Input Data
Pipe Trench Configuration
- 100 feet of perforated pipe
- 36-inch diameter pipe
- Trench: 4 feet wide x 6 feet deep
- Surrounded by pollution control media
- Void space: 0.25(fraction, not percent)
- Default condition: natural soils
- Exfiltration time greater than 3 hours
Vault Configuration
- 40 ft x 40 ft x 8 ft deep vault
- Interior volume reduction 0.02 (fraction, not percent)
- Surrounding rock: limestone
- Exfiltration time less than 3 hours