15.3 Infiltration, Bioretention, and Media Filters

2. Infiltration Basin (12.7.8) and Infiltration Trench (12.7.9)

Infiltration Basin (12.7.8)

An Infiltration Basin is an open, flat-bottomed surface impoundment (stabilized with dense water-tolerant turfgrass or a surface layer of coarse stone/sand) engineered to capture the full Water Quality Volume ($WQ_v$) and percolate $100\%$ of that volume vertically downward into the underlying native soil. Unlike a detention basin, an infiltration basin has no low-flow outlet pipe at the floor; only extreme storms exceeding the design infiltration volume exit over an elevated emergency spillway or high-stage riser.

Infiltration Trench (12.7.9)

An Infiltration Trench (or infiltration gallery) is an excavated linear trench—typically $3\text{ to }8\text{ feet}$ deep and $3\text{ to }10\text{ feet}$ wide—backfilled with clean, double-washed, $1.5\text{- to }3.0\text{-inch}$ coarse open-graded crushed stone ($35\%\text{ to }40\%$ void space) or modular perforated plastic arches/pipes:

• Geotextile & Sand Bottom Lining: The vertical trench walls and top surface (often including a replaceable upper geotextile flap located $2\text{ to }6\text{ inches}$ below a surface pea-gravel filter layer) are lined with non-woven geotextile filter fabric to stop adjacent native soil from migrating into the stone voids. Many designs place a $6\text{-inch}$ layer of clean coarse sand on the trench floor rather than bottom filter fabric, because horizontal bottom fabric tends to blind with biological slime and fine clay.
• Observation Well (Monitoring Standpipe): Every infiltration trench must have a vertical $4\text{- to }6\text{-inch}$ diameter perforated PVC observation well anchored to a baseplate on the trench floor with a lockable surface cap. CESSWI inspectors lower a dipstick or water-level tape into the observation well immediately after a storm and again $24\text{ to }48\text{ hours}$ later to verify that the trench drains within the required $24\text{- to }72\text{-hour}$ drawdown window.
• Mandatory Pretreatment: Both infiltration basins and trenches require dedicated upstream pretreatment—such as a sediment forebay, vegetated filter strip, pea-gravel diaphragm, or hydrodynamic separator—to remove coarse sand, grit, and leaf litter before water enters the infiltration footprint.

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3. Bioretention / Rain Gardens (12.7.10)

Bioretention Cells (and residential/commercial Rain Gardens) are shallow landscaped surface depressions that treat stormwater runoff through physical filtration, chemical adsorption, ion exchange, plant root uptake, and microbial biodegradation.

Five-Layer Engineered Bioretention Cross-Section (Top to Bottom)

1. Surface Ponding Zone ($6\text{ to }12\text{ inches}$ deep):
   • Temporary shallow storage above the mulch surface. Engineered to draw down completely within $24\text{ to }48\text{ hours}$, ensuring standing water disappears long before the $72+\text{ hour}$ threshold required for mosquito eggs and larvae to mature.
2. Shredded Hardwood Bark Mulch Layer ($2\text{ to }3\text{ inches}$ thick):
   • Double- or triple-shredded aged hardwood bark mulch acts as the primary physical pre-filter for fine silt, prevents the underlying soil media from crusting, retains moisture for plants, and chemically adsorbs heavy metals ($\text{Cu, Zn, Pb}$) and petroleum hydrocarbons right at the surface where maintenance crews can easily rake out and replace the mulch every $1\text{ to }2\text{ years}$. (Inspector Alert: Never allow lightweight pine bark nuggets or wood chips, which float during storms and plug the overflow catch basin grate!)
3. Adapted Native Vegetation:
   • Deep-rooted native perennials, sedges, rushes, wet-tolerant shrubs, and trees capable of tolerating both temporary $6\text{- to }12\text{-inch}$ inundation and extended summer drought. Vigorous root growth and decay continually open new vertical macropores, preventing the filter bed from sealing over time.
4. Engineered Bioretention Soil Media ($2.0\text{ to }4.0\text{ feet}$ deep):
   • A laboratory-tested, high-permeability sandy planting mix consisting of $85\%\text{ to }88\%$ medium-to-coarse washed sand (ASTM C33), $8\%\text{ to }12\%$ fines (silt and clay), and $3\%\text{ to }5\%$ organic matter (well-aged leaf compost), achieving a design filtration rate of $1.0\text{ to }5.0\text{ in/hr}$. Never allow contractors to backfill a bioretention cell with ordinary native clay topsoil!
5. Pea-Gravel Choker & Washed Stone Underdrain Bed ($8\text{ to }15\text{ inches}$ total):
   • A $2\text{- to }3\text{-inch}$ pea-gravel choker layer (ASTM #8 stone) bridges the sand media over a $6\text{- to }12\text{-inch}$ washed angular stone bed (ASTM #57 stone) housing a perforated PVC underdrain pipe ($4\text{ to }6\text{ in.}$ diameter) with a vertical cleanout/inspection standpipe, plus an elevated domed overflow catch basin grate/riser set at the maximum $6\text{- to }12\text{-inch}$ ponding elevation to bypass larger storms.

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4. Sand and Organic Filters (12.7.11)

Sand Filters (including Austin surface sand filters, Delaware perimeter trench filters along parking lot edges, and underground vault sand filters) and Organic Media Filters are self-contained, two-chamber filtration systems typically enclosed within an impermeable concrete box or geomembrane liner.

Where Sand and Organic Filters Excel

Because they capture filtered water in an underdrain inside a watertight structure rather than infiltrating it into the underlying ground, Sand and Organic Filters are the premier BMP choice for:

• Stormwater Hotspots: Fueling stations, fleet maintenance yards, industrial plants, and high-turnover commercial parking lots where infiltration into groundwater is prohibited.
• Impervious Clay Soils (HSG D), High Water Tables, or Karst Bedrock: Sites where native soil infiltration is physically impossible or geologically unsafe.

Two-Chamber Operation & Maintenance

1. Chamber 1 — Sedimentation Pretreatment Chamber: A wet or dry settling chamber equipped with a trash screen and submerged oil-skimming baffle wall or weir that settles out coarse sand/grit and traps floating oil, grease, and debris.
2. Chamber 2 — Filter Media Bed: Pre-settled water spreads uniformly over an $18\text{- to }24\text{-inch}$ deep filter bed of either clean washed medium sand (achieving $80\%\text{ to }85\%$ TSS removal) OR organic/sorptive media—such as peat moss, leaf compost, or granular activated carbon (GAC) / zeolite blends—which provide high cation-exchange capacity (CEC) to chemically bind dissolved heavy metals ($\text{Cu, Zn, Pb}$) and dissolved hydrocarbons before discharging through a gravel-encased perforated underdrain.
3. Crust Skimming Maintenance: When fine silt forms a surface crust on the sand bed and the drawdown time exceeds $48\text{ hours}$, maintenance crews must manually rake and skim off the top $2\text{ to }3\text{ inches}$ of crusted, discolored sand and replace it with clean specification sand.

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Summary Comparison: LID Infiltration & Media Filtration BMPs