Infiltration Feasibility¶

Infiltration feasibility is the combined assessment that determines whether a site can support stormwater infiltration into native soils. It integrates soil permeability, SHWT depth, groundwater mounding risk, site contamination history, and geologic conditions into a single go/no-go decision that governs BMP selection and groundwater recharge compliance.

Source: NJ Stormwater BMP Manual, Chapters 6, 9, 12, 13; N.J.A.C. 7:8-5.3, 7:8-5.4

Concept Explanation¶

No single soil or site parameter determines whether infiltration is feasible. Instead, the engineer must evaluate multiple constraints simultaneously and arrive at one of three outcomes:

Full infiltration feasible — the site can support infiltration-based GI BMPs without underdrains, qualifying for groundwater recharge credit and GI volumetric reduction compliance
Partial infiltration feasible — some infiltration is possible but constraints require design modifications (e.g., underdrain with elevated outlet creating an internal water storage zone)
Infiltration infeasible — site conditions prohibit infiltration entirely; the designer must document the infeasibility finding and propose Non-GI alternatives

The 2026 amendments to N.J.A.C. 7:8 formalized the requirement that engineers must document GI feasibility analysis before proposing Non-GI alternatives. Generic statements ("GI is infeasible due to site constraints") are no longer acceptable. The analysis must include specific soil investigation data, SHWT determination, and identification of the constraint(s) that preclude infiltration.

Combined Assessment Framework¶

The following decision framework represents the standard evaluation sequence for infiltration feasibility:

Step 1: Soil Permeability
  K_sat >= 0.5 in/hr AND HSG A or B? --> Proceed to Step 2
  K_sat < 0.5 in/hr OR HSG C/D?      --> Infiltration limited; underdrain required
  HSG D with K_sat < 0.05 in/hr?      --> Infiltration generally infeasible

Step 2: Seasonal High Water Table
  SHWT >= 2 ft below BMP bottom?      --> Proceed to Step 3
  SHWT < 2 ft below BMP bottom?       --> Infiltration infeasible at proposed depth
                                           Options: raise BMP, reduce depth, underdrain

Step 3: Groundwater Mounding
  Post-mounding WT still >= 2 ft      --> Proceed to Step 4
  below BMP bottom?
  Mound reduces separation < 2 ft?    --> Reduce infiltration volume, add underdrain,
                                           or split into smaller distributed practices

Step 4: Site Prohibitions
  Contaminated soils?                  --> Infiltration prohibited (NJDEP review)
  Karst geology?                       --> Infiltration prohibited or severely restricted
  Stormwater hotspot?                  --> Infiltration prohibited
  None of the above?                   --> INFILTRATION FEASIBLE

Infiltration vs. Underdrain Decision¶

When full infiltration is not feasible, the designer must choose between:

Design Option	When to Use	Recharge Credit	GI Credit
Full infiltration (no underdrain)	K_sat adequate, SHWT adequate, no prohibitions	Yes — full recharge credit	Yes — volumetric reduction per Ch. 14
Underdrain with elevated outlet (IWZ)	Moderate soils; partial infiltration desired	Partial — credit for infiltrated fraction only	Partial — volumetric reduction for retained volume
Underdrain at invert (no retention)	Poor soils (HSG C/D), shallow SHWT, or prohibitions	None — no recharge credit	Water quality treatment only; does not satisfy GI volumetric standard
Lined system with underdrain	Contaminated soils, hotspots, or karst	None — liner prevents infiltration	Water quality treatment only

The 2026 BMP Manual explicitly states that BMPs providing recharge credit must not use impermeable liners. The liner prohibition is absolute for practices counting toward recharge compliance.

Engineering Evaluation¶

Required Investigations¶

The 2026 Soil Investigation Protocol (Chapter 12) requires the following site data to support the infiltration feasibility determination:

Investigation	Purpose	Minimum Standard (2026)
Soil borings / test pits	HSG classification, soil profile, SHWT	1 per BMP footprint (up to 5,000 sq ft); +1 per additional 5,000 sq ft; 72 in below BMP bottom
In-situ permeability testing	Design K_sat	Falling-head permeameter or ASTM D5126; geometric mean with 50% safety factor
SHWT determination	Separation verification	Redoximorphic feature identification by qualified professional
Mounding analysis	Groundwater rise evaluation	Hantush method or equivalent when triggers are met (high K_sat, shallow WT, large volumes)
Environmental site assessment	Contamination screening	Phase I ESA minimum for commercial/industrial sites; Phase II if contamination suspected
Geologic mapping review	Karst / bedrock identification	NJGS geologic maps; borings to confirm depth to bedrock

Documenting the Feasibility Finding¶

Under the 2026 rules, the Stormwater Management Report must include a GI Feasibility Analysis section containing:

Site plan showing proposed BMP locations and soil investigation points
Boring logs with soil descriptions, Munsell color notation, and redox feature depths
K_sat test results with safety factor calculation
SHWT depth determination and available separation calculation
Mounding analysis results (if triggered)
Hotspot screening determination
Contamination screening results
Conclusion: feasible, partially feasible, or infeasible — with specific constraint(s) identified

Prohibitions: When Infiltration Is Not Permitted¶

Certain site conditions create absolute prohibitions on stormwater infiltration:

Karst Geology

Sites underlain by carbonate bedrock (limestone, dolomite) are susceptible to sinkhole formation when concentrated infiltration dissolves subsurface rock. The NJ BMP Manual prohibits or severely restricts infiltration in mapped karst areas. Where karst is identified, the designer must use lined or underdrained systems that prevent percolation to bedrock.

Contaminated Soils

Infiltrating stormwater through contaminated soils can mobilize pollutants and carry them to groundwater. Sites with known or suspected contamination (brownfields, former industrial uses, underground storage tank sites) require NJDEP review before any infiltration practice is approved. In most cases, infiltration is prohibited until remediation is complete.

Stormwater Hotspots

Locations where runoff carries pollutants at concentrations that could contaminate groundwater — fueling stations, vehicle maintenance yards, hazardous material storage areas, high-traffic industrial facilities. Full hotspot classification prohibits infiltration entirely. Lower-risk transitional areas may allow infiltration with pretreatment, but this requires case-by-case NJDEP determination.

High Water Table (< 2 ft separation)

While not an absolute prohibition in all cases (the BMP can sometimes be raised or redesigned), a SHWT that cannot be separated from the BMP bottom by 2 feet effectively prohibits infiltration at that location.

Affected BMPs¶

The infiltration feasibility assessment applies to every practice that relies on percolation into native soils:

BMP	Feasibility Outcome	Design Path	Link
Bioretention (no underdrain)	Requires full feasibility	Full infiltration + recharge credit	Bioretention
Bioretention (with underdrain)	Used when partial or no infiltration	Water quality treatment; partial or no recharge	Bioretention
Infiltration basins	Requires full feasibility; most demanding	Full infiltration in HSG A/B soils only	BMP Library
Dry wells	Requires full feasibility	Point-source infiltration for rooftop runoff	BMP Library
Pervious pavement	Requires feasibility for unlined subbase	Underdrain alternative for poor soils	BMP Library
Extended detention basins	Not dependent on infiltration	Non-GI; detention and treatment focus	BMP Library

N.J.A.C. 7:8 Rule Citations¶

Rule Section	Topic	Relevance to Infiltration Feasibility
N.J.A.C. 7:8-5.3	Green Infrastructure Requirement	Documented feasibility analysis required before proposing Non-GI; hierarchical GI application
N.J.A.C. 7:8-5.4(a)	Groundwater Recharge	Recharge compliance depends on infiltration feasibility; technical infeasibility finding available when conditions preclude infiltration
N.J.A.C. 7:8-5.2	Soil Testing (2026)	Defines investigation requirements that feed the feasibility determination

BMP Manual Chapter Citations¶

Chapter 12 — Soil Testing Criteria: Soil investigation protocol, K_sat testing methodology, SHWT determination, boring requirements
Chapter 13 — Groundwater Mounding Analysis: Mounding assessment methods and acceptance criteria
Chapter 6 — Groundwater Recharge: Recharge standard, HSG-based recharge rates, separation requirements, hotspot prohibition, liner prohibition
Chapter 9 — Small-Scale GI BMPs: Siting constraints for bioretention and other small-scale practices that depend on infiltration feasibility
Chapter 14 — Volumetric Reduction Standards (2026): Volumetric reduction credit is available only when infiltration feasibility is confirmed