Small-Scale Infiltration Basins¶
Infiltration basins are surface depressions designed to temporarily pond stormwater and allow it to percolate into the native soil. They rely entirely on soil infiltration and provide no underdrain — all captured runoff must infiltrate within the required drawdown period.
Source: NJ Stormwater BMP Manual, Chapter 9, Section 9.8 (2026)
Infiltration basins manage stormwater by directing runoff into a shallow, unlined depression where it ponds and gradually infiltrates into the subsoil. Unlike bioretention, infiltration basins have no engineered media layer — treatment depends on natural soil processes, and the design is highly sensitive to soil type and subsurface conditions.
They are surface-based BMPs commonly used on sites with sandy, well-drained soils (HSG A or B) and sufficient setback from buildings and utilities.
Primary stormwater functions:
- Groundwater recharge — primary function; all volume must infiltrate
- Runoff volume reduction — contributes to Water Quality volume compliance
- Moderate water quality treatment — through soil filtration and adsorption
When engineers choose this BMP:
Infiltration basins are selected when the site has confirmed permeable soils (typically ksat ≥ 0.5 in/hr), a water table well below the basin bottom, and the regulatory program requires groundwater recharge credit under N.J.A.C. 7:8-5.4(a). They are not suitable on HSG C/D soils or where the SHWT is within 2 ft of the basin bottom.
Source: NJ Stormwater BMP Manual, Chapter 9, Section 9.8 (2026)
Confirm all values against the current NJ Stormwater BMP Manual, Section 9.8, before use in design. Run Agent 1 with the Chapter 9.8 PDFs to generate a verified 2023/2026 comparison table.
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Contributing drainage area | Typically ≤ 5 acres impervious per basin | Same | Larger areas require Ch. 10 infiltration basin |
| Maximum ponding depth | Sized so WQV÷(Ksat × drawdown time) ≤ available depth | Same | Basin bottom area determines ponding depth |
| Drawdown time | 72 hours maximum | 72 hours maximum | 24–48 hours preferred for water quality |
| Minimum native soil Ksat | ≥ 0.52 in/hr (HSG A/B); field-tested per Ch. 12 | ≥ 0.5 in/hr | Soil survey data not acceptable for design |
| Minimum SHWT separation | 2 ft below basin bottom | 2 ft below basin bottom | LPSS-certified boring required |
| Groundwater mounding check | Required; see Ch. 13 | Required | Mounding must not rise within 2 ft of basin bottom |
| Pretreatment | Required — forebay ≥10% WQV or equivalent filter strip | Required | Protects basin floor from sediment sealing |
| Basin bottom slope | Level or ≤2% grade | Same | Ensures uniform ponding and infiltration |
2026 Update: The 2026 edition adds explicit minimum setback distances from NJDEP-designated protected water supply sources (see Siting Constraints). Additionally, a water balance analysis is required when native Ksat is near the minimum qualifying threshold, confirming 72-hour drawdown under wet antecedent conditions.
Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.8; Ch. 12; Ch. 13
Infiltration basins are among the most site-sensitive BMPs in the manual. The following constraints are critical and must be verified by field investigation:
Seasonal High Water Table (SHWT)
- Minimum 2 ft separation required between the basin bottom and the SHWT
- SHWT must be confirmed by soil borings or monitoring wells — design table values from soil surveys are not sufficient for site design
- See Seasonal High Water Table
Soil Permeability
- Infiltration basin is only viable in HSG A or B soils (ksat ≥ 0.5 in/hr typically)
- Soil permeability must be field-tested per BMP Manual Chapter 12 (double-ring infiltrometer or equivalent); soil survey data alone is not acceptable
- See Soil Permeability Testing
Groundwater Mounding
- Required analysis for all infiltration basin designs
- Mounding must not cause the water table to rise within 2 ft of the basin bottom during or after a design storm
- See Groundwater Mounding
Setbacks and Exclusion Zones
- Minimum setback from structures with basements: 10 ft
- Minimum setback from septic drainfields: 50 ft
- Minimum setback from potable water supply wells: 25 ft
- Minimum setback from NJDEP Protected Water Supply Sources: per 2026 Ch. 10.2 explicit distances (new requirement)
- Not suitable in karst geology — consult geotechnical engineer
- Not suitable on contaminated sites where infiltration could mobilize subsurface pollutants
Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.8; Ch. 12; Ch. 13
Semi-Annual Inspection
- After storm events, confirm that ponded water drains to dry conditions within the 72-hour drawdown period; increasing drawdown time is the primary indicator of surface clogging
- Inspect basin floor for sediment accumulation, surface crust formation, or algal growth that may reduce infiltration rate
- Inspect inlet pipe and inlet protection (rock pad or filter strip) for erosion or blockage
- Inspect pretreatment forebay for sediment fill level
- Inspect emergency overflow spillway for obstruction, erosion, or vegetation encroachment
Sediment Management
- Remove accumulated sediment from pretreatment forebay when sediment depth reaches approximately 50% of the forebay design volume, or annually based on site loading
- Do not allow sediment to reach the basin floor — fine sediment seals the infiltration surface and is the primary long-term failure mechanism
- Remove any sediment deposited on the basin floor promptly; scarify crust if drawdown time is increasing
Vegetation Management
- Maintain native grass or meadow cover on basin floor and side slopes as specified in the planting plan; cover prevents surface erosion and supports biological treatment
- Remove invasive species before they establish; do not allow woody vegetation on basin floor
- Mow side slopes per O&M schedule; remove clippings that could add organic matter to basin floor
Overflow and Emergency Spillway
- Inspect overflow structure or emergency spillway annually and after extreme storm events
- Clear any debris or sediment that obstructs overflow capacity; restore vegetation cover on spillway if eroded
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 9, Section 9.8
Design Errors
- Soil permeability overestimated from soil survey data — leading cause of infiltration basin failure; field-measured Ksat (double-ring infiltrometer or permeameter) required; NRCS soil survey Ksat values are not acceptable for design
- Groundwater mounding analysis omitted or underestimated — basin raises the water table into the 2-ft minimum separation zone during operation; creates saturated conditions that eliminate infiltration capacity and create surface ponding failures
- Contributing drainage area oversized — design ponding depth exceeded during WQV storm; overflow before drawdown completes; basin overtops into undesigned discharge path
- Water balance analysis not performed at marginal Ksat — 2026 requirement: when Ksat is near the minimum qualifying threshold, wet antecedent condition drawdown must be confirmed
Construction Issues
- Basin floor compacted by heavy equipment — the most common construction-phase failure; native soil permeability destroyed before first storm; drawdown fails immediately
- Pretreatment omitted or undersized — fine sediments from tributary drainage area and construction activity reach basin floor during grading and first storm events; surface sealing begins immediately
- Concentrated inflow without energy dissipation — single-point inlet scours the basin floor; channelized flow short-circuits uniform ponding; reduces effective infiltration area
Long-Term Performance Risks
- Progressive surface sealing — fine particles accumulate in upper soil pores over years; drawdown time steadily increases; eventually basin holds water continuously and ceases to infiltrate; scarification of the top 2–6 inches of soil may restore function temporarily
- Septic or groundwater interference — where setback from septic components was inadequate or not checked, infiltration can raise groundwater into drainfield elevation, causing system backup
- Vegetation changes infiltration — establishment of meadow or wetland species on basin floor affects surface infiltration rate unpredictably; organic matter accumulation can reduce permeability over time if maintained vegetation is allowed to decompose in place
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | Infiltration basins qualify as GI; required where feasible |
| N.J.A.C. 7:8-5.4(a) | Groundwater Recharge | Primary regulatory driver for infiltration basin use |
| N.J.A.C. 7:8-5.3(d) | Water Quality Treatment | Volume credit for water quality infiltration |
| N.J.A.C. 7:8-5.4(b) | Stormwater Quantity Control | Peak flow reduction credit if applicable |
BMP Manual Source
- NJ Stormwater BMP Manual, Chapter 9, Section 9.8 (2026)
- NJ Stormwater BMP Manual, Chapter 12 — Soil Testing Criteria
- NJ Stormwater BMP Manual, Chapter 13 — Groundwater Mounding Analysis
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