Infiltration Trench¶
Source: NJ Stormwater BMP Manual, Chapter 10, Section 10.2 (2026)
Infiltration trenches are narrow, rock-filled linear structures excavated into native soil that receive stormwater runoff, temporarily store it in the stone void space, and disperse it into the surrounding native soil under gravity. They function as distributed subsurface infiltration systems, accepting both concentrated inflow from surface inlets and sheet flow across a perforated manifold, depending on the configuration.
Unlike dry wells (which receive point-source rooftop drainage), infiltration trenches serve larger contributing drainage areas and are common elements of parking lot drainage systems, roadside management, and treatment train designs. The stone reservoir distributes the stored volume laterally across the trench length, allowing a larger soil contact area for infiltration than a single-point system.
GI Classification (2026): Infiltration trenches qualify as Green Infrastructure when native soil infiltration capacity is confirmed per Chapter 12 and no impermeable liner is installed. They generate VRC credit for the volume infiltrated into native groundwater.
Primary stormwater functions:
- Groundwater recharge — primary function; all stored volume infiltrates into native soil
- Volumetric reduction — VRC credit for WQV infiltrated; meets GI Requirement (GI config)
- Peak flow reduction — storage in stone void space attenuates peak discharge
- Limited water quality — coarse TSS removal through stone filtration; minimal nutrient removal
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.2 (2026)
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Trench width | 2–6 ft typical; maximum determined by excavation stability | Same | Wider trench provides greater infiltration contact area |
| Stone specification | Clean NJDEP-specified open-graded aggregate; no fines | Same | Fine aggregate eliminates void storage capacity |
| Geotextile | Filter fabric on sides and top (not bottom) to prevent soil intrusion from sides/surface | Same | No geotextile below stone — allows infiltration to native soil |
| Minimum Ksat | ≥ 0.52 in/hr at trench base elevation; field investigated per Chapter 12 | ≥ 0.5 in/hr | 2026: explicit Ch. 12 requirement for GI qualification |
| SHWT separation | ≥ 2 ft from trench base to SHWT | ≥ 2 ft | LPSS-certified boring required |
| Drawdown time | ≤ 72 hours from full storage volume | ≤ 72 hours | Confirm under wet antecedent conditions |
| Setbacks | ≥ 10 ft from structures; ≥ 50 ft from potable water wells; ≥ 100 ft from septic | Same | Verify local health code for greater setbacks |
| Groundwater mounding | Chapter 13 analysis required when trench length > 100 ft or SHWT < 4 ft below base | Chapter 13 reference | 2026: explicit triggers defined |
| Pre-treatment | Oil/grit separator or sump catch basin required upstream for parking lot drainage | Same | Protects trench from petroleum-based clogging |
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.2; Ch. 12; Ch. 13 (2026)
Soil Permeability
- Native soil Ksat ≥ 0.52 in/hr required; typically HSG A and upper-B soils
- Not suitable in HSG D soils, urban fill with low permeability, or karst terrain
- Chapter 12 investigation required at proposed trench base elevation
- See Soil Permeability Testing
Seasonal High Water Table
- ≥ 2 ft separation below trench base required; certified soil boring log required
- High fluctuating water tables can submerge trench base during wet seasons
- See Seasonal High Water Table
Setbacks
- ≥ 10 ft from any structure with a foundation
- ≥ 50 ft from potable drinking water wells
- ≥ 100 ft from septic system leachfield
- Not suitable on contaminated fill or brownfield sites (infiltration mobilizes contaminants)
Groundwater Mounding
- Long trenches (> 100 ft) generate lateral groundwater mounding in marginal soils; Chapter 13 mounding analysis required
- See Groundwater Mounding
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.2 (2026)
Annual Inspection
- Inspect surface inlet (catch basin, inlet grate) for sediment and debris accumulation; clean inlet sump annually or when 50% full
- Observe trench surface area for ponding after design storms; ponding that persists
72 hours indicates stone or native soil clogging has reduced infiltration capacity
- Inspect observation well or cleanout (if installed) for water level confirmation
Catch Basin and Pre-treatment
- If an oil/grit separator or pre-treatment sump is upstream of the trench, clean per manufacturer specification; pre-treatment device failure accelerates trench clogging
- Vacuum-clean pretreatment sump at least annually; more frequently for parking lot drainage
Geotextile Monitoring
- Inspect exposed geotextile at trench top or surface inlets for progressive clogging
- Geotextile at trench sides should not be replaced without full trench assessment; consult engineer if clogging is suspected in stone void space (not just surface)
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 10, Section 10.2 (2026)
Design Errors
- Chapter 12 investigation not performed — VRC credit based on assumed or estimated Ksat; actual in-situ permeability below design value; system fails to drain within 72 hours
- Geotextile placed below stone on trench bottom — prevents infiltration into native soil; system functions as detention only; no groundwater recharge; VRC not generated
- No pre-treatment provided for parking lot drainage — petroleum hydrocarbons, oil, and fine sediment enter stone fill directly; biofilm formation clogs stone pores and geotextile; severe premature failure
Construction Issues
- Stone fill contaminated with fines — contractor substitutes aggregate not meeting clean stone specification; void ratio insufficient; storage volume below design
- Trench base not cleaned before stone placement — disturbed soil at trench bottom reduces infiltration rate at the critical infiltration contact zone
Long-Term Performance Risks
- Progressive clogging — fine particles enter stone void space over years; void ratio decreases; drawdown time increases; eventual failure of infiltration function; expected service life 15–25 years depending on sediment loading and drainage area land use
- Petroleum clogging — parking lot drainage without adequate pre-treatment introduces hydrocarbon films to stone media; biological clogging accelerates significantly
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.2 (2026)
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | Qualifies as GI (no liner; Ch. 12 Ksat confirmed) |
| N.J.A.C. 7:8-5.4(a) | Groundwater Recharge | Primary regulatory function; all infiltrated volume generates recharge credit |
| N.J.A.C. 7:8-5.3(d) | Water Quality Treatment | Limited TSS removal through stone filtration and native soil |
BMP Manual Sources
- NJ Stormwater BMP Manual, Chapter 10, Section 10.2 (2026) — Infiltration Trench
- NJ Stormwater BMP Manual, Chapter 12 (2026) — Soil Investigation and Ksat Testing
- NJ Stormwater BMP Manual, Chapter 13 (2026) — Groundwater Mounding Analysis
- NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance
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