Section 1 — Chapter 4
Stormwater Pollutant Removal Criteria
1.1 Core TSS Removal Requirement
Chapter 4 establishes pollutant removal criteria applicable to stormwater BMPs used to satisfy the water quality standard in N.J.A.C. 7:8. The primary performance metric is Total Suspended Solids (TSS) removal, expressed as an annual average percentage of TSS mass removed relative to untreated runoff entering each BMP. Both the 2023 and 2026 editions hold the fundamental standard at 80% TSS removal for BMPs applied to meet the Water Quality Volume (WQV) standard.
Performance values in Table 4-1 are assigned to each BMP type by confidence tier (low, moderate, high) based on published research. Designers cite the applicable removal efficiency from Table 4-1 as part of the water quality compliance demonstration in the Stormwater Management Report.
1.2 GI BMP Compliance Pathway — 2026 Change
The most significant Chapter 4 update in the 2026 edition is the formal separation of compliance pathways for GI BMPs and Non-GI BMPs:
TSS removal calculated for all BMPs, including GI BMPs, using Table 4-1 performance values. GI practices cited an 80%+ removal efficiency entry from the table to demonstrate compliance.
A GI BMP that achieves full volumetric reduction (VRv ≥ WQV per Chapter 14 method) is deemed to meet the 80% TSS standard by compliance pathway credit — no separate TSS calculation required.
For GI BMPs that achieve only partial volumetric reduction (VRv < WQV), the residual volume (WQV − VRv) must be managed by a separate Non-GI BMP meeting the 80% TSS standard applied to that residual fraction. This partial-credit scenario was not explicitly codified in the 2023 edition.
1.3 Nutrient Reduction Requirements
Nutrient removal (total nitrogen, total phosphorus) is a secondary requirement applicable to discharges to Category One (C1) waters or waters with approved TMDLs specifying nutrient reduction targets. The required removal percentage is site-specific and flows from the applicable TMDL.
1.4 Treatment Train Calculation Method
Where two or more BMPs are used in series toward the TSS standard, combined removal must be calculated using the joint probability method:
Combined TSS Removal = 1 − [ (1 − R₁) × (1 − R₂) × ... × (1 − Rₙ) ] Where R₁, R₂... Rₙ are the fractional TSS removal efficiencies of each BMP in sequence (expressed as decimals, e.g., 0.80 for 80%).
1.5 Chapter 4 Comparison Table
| Item | 2023 | 2026 |
|---|---|---|
| TSS standard — Non-GI BMPs | 80% removal required | Unchanged — 80% retained |
| TSS compliance — GI BMPs (full VR) | TSS removal calculated from Table 4-1 | VRv ≥ WQV → TSS compliance by pathway credit; no Table 4-1 calculation required |
| Partial volumetric reduction | Not explicitly addressed | VRv < WQV → TSS calc required for residual (WQV − VRv) |
| Nutrient reduction — impaired waters | TMDL-specific; general guidance | Treatment train preferred pathway specified for nutrient mgmt areas |
| Table 4-1 performance values | 2023 research basis | Updated for bioretention and permeable pavement (values revised upward per post-2020 studies) |
Section 2 — Chapter 5
SWM Standards and Computations
2.1 The Three NJ Stormwater Standards
Chapter 5 describes the computational basis for meeting the three primary N.J.A.C. 7:8 stormwater management standards applicable to major development:
- Standard 1 — Water Quality (WQ): Capture and treat (or volumetrically reduce) the Water Quality Volume generated from 1.25 inches of rainfall over 2 hours for the post-development condition.
- Standard 2 — Groundwater Recharge: Maintain pre-development average annual groundwater recharge volume. (Design methodology detailed in Chapter 6; see §3 below.)
- Standard 3 — Flood Control: Post-development peak discharge rates for the 2-year and 100-year design storms must not exceed pre-development rates at the point of discharge. Some projects additionally require channel protection analysis tied to MSWMP obligations.
2.2 Water Quality Volume (WQV) Calculation
WQV is the critical sizing parameter for water quality compliance. It is computed as:
WQV = P × Rv × A P = design storm rainfall depth = 1.25 in Rv = volumetric runoff coefficient for post-development condition A = contributing drainage area (acres or sq ft; units consistent with P) Rv is computed from the area-weighted composite Curve Number (CN): Rv = 1 − (S / (S + P)) where S = (1000/CN) − 10
The WQV determines the minimum volume that the water quality BMP must either store and treat (Non-GI BMP approach) or capture and retain through infiltration/ET (GI BMP volumetric reduction approach).
2.3 Curve Number (CN) Tables — 2026 Update
The 2026 edition updates CN values for several land use / HSG combinations:
| Land Use / Condition | HSG | 2023 CN | 2026 CN | Direction |
|---|---|---|---|---|
| Single-family residential — 1/4 ac lots | B | 69 | 66 | ↓ Lower |
| Conventional turf / lawn | B | 69 | 66 | ↓ Lower |
| Impervious surface (all types) | All | 98 | 98 | Unchanged |
| Commercial / industrial paved | All | 98 | 98 | Unchanged |
| Forest / woods good condition | B | 55 | 55 | Unchanged |
2.4 Flood Control Design Storms
Peak flow calculations use NJ-specific rainfall depths derived from NOAA Atlas 14, applied with the SCS 24-hour Type III storm distribution. Representative values for central NJ (reference only; designers must use project-location-specific Atlas 14 depths):
| Return Period | Approximate NJ Range (in) | Application |
|---|---|---|
| 2-Year, 24-hr | 3.2 – 3.6 | Flood control standard; channel protection |
| 10-Year, 24-hr | 4.8 – 5.5 | Culvert, inlet sizing; some MSWMP requirements |
| 100-Year, 24-hr | 9.0 – 13.0 | Flood control standard; FEMA floodplain analyses |
2.5 Volumetric Reduction Integration — 2026 Workflow
The 2026 edition formalizes a distinct volumetric reduction accounting step within the WQ compliance workflow. The updated design sequence is:
2.6 Chapter 5 Standard Comparison
| Standard | 2023 | 2026 |
|---|---|---|
| WQV design storm | 1.25 in/2 hr; CN from 2023 tables | Unchanged design storm; updated CN for select categories |
| GI BMP WQ compliance | Size BMP to capture WQV volume; TSS from Ch. 4 | VRv ≥ WQV (Ch. 14 calc) → full compliance; residual → Non-GI BMP |
| Flood control rainfall | Atlas 14 NJ-specific depths | Unchanged; explicit citation of current Atlas 14 version added |
| Channel protection | General MSWMP reference | Clarified: direct application when no MSWMP exists |
Section 3 — Chapter 6
Groundwater Recharge
3.1 Recharge Requirement Overview
Chapter 6 establishes requirements for maintaining pre-development groundwater recharge under N.J.A.C. 7:8-5.4. Major developments must not decrease the average annual groundwater recharge volume below pre-development levels beyond what is technically infeasible. Compliance requires infiltration-based BMPs that allow precipitation to percolate into native soils, maintaining hydrologic continuity with the groundwater system.
The recharge volume deficit (pre-development recharge minus post-development recharge) is the design target:
Rev (recharge deficit) = Σ [ (Rrate_pre × A_pre) − (Rrate_post × A_post) ] Rrate = annual recharge rate (in/yr) from Table 6-1, by HSG and land use A = area in each land use/cover category (acres)
The resulting annual volume (acre-ft/yr or cu-ft/yr) is the storage or infiltration volume that on-site BMPs must provide.
3.2 Hydrologic Soil Group (HSG) Feasibility
Excellent infiltration. Full recharge systems (bioretention, pervious paving, infiltration basins) generally feasible.
Good feasibility. Bioretention and pervious pavement typically work. Verify Ksat and SHWT separation.
Limited. Extended drawdown periods common; infiltration-only BMPs may be undersized. Combination approaches needed.
Poor. Infiltration generally infeasible. Technical infeasibility finding typically required.
Seasonally saturated soils with dual HSG designation (A/D, B/D) are rated as their D-rated parent material for recharge feasibility purposes. Seasonal groundwater fluctuations govern design, not dry-season conditions.
3.3 SHWT Separation Requirements
A minimum 2-foot vertical separation between the bottom of the infiltrating BMP (bottom of aggregate layer) and the Seasonal High Water Table (SHWT) is required for all infiltration-based practices:
- Bioretention cells (no underdrain): 2 ft, measured from bottom of gravel aggregate layer to documented SHWT elevation.
- Infiltration basins: 2 ft below basin floor to SHWT.
- Dry wells: 2 ft from bottom of gravel reservoir to SHWT.
- Pervious pavement systems: 2 ft from bottom of aggregate sub-base to SHWT.
3.4 Setback Requirements
Infiltration BMPs should not be sited directly upslope of gradients exceeding 20%, to prevent lateral groundwater mounding and potential slope instability.
3.5 Stormwater Hotspot Prohibition
Infiltration is prohibited in areas classified as Stormwater Hotspots — sites where runoff contains pollutant concentrations that could contaminate groundwater upon infiltration. Hotspot land uses include fueling stations, vehicle maintenance and washing facilities, hazardous material storage, and high-vehicle-traffic industrial facilities. A written hotspot screening analysis is required for commercial, industrial, and transportation projects before selecting infiltration-based practices.
3.6 Liner and Geotextile Constraints
Both editions explicitly state that recharge-credit BMPs must not use impermeable liners. The prohibition is absolute: any BMP with an impermeable liner may contribute to the water quality standard (TSS removal) but cannot be credited toward the recharge standard. Project specifications must explicitly state "no impermeable liner" for practices counting toward recharge compliance.
Geotextile fabric used to separate filter media from aggregate is permissible provided it does not act as an effective flow barrier (i.e., hydraulic conductivity of the geotextile must not limit system infiltration below the design Ksat).
3.7 2026 Soil Investigation Protocol
The most significant Chapter 6 addition in the 2026 edition is a formal soil investigation protocol specifying minimum field investigation requirements for all recharge-based BMP designs:
3.8 Chapter 6 Comparison Summary
| Item | 2023 | 2026 |
|---|---|---|
| SHWT separation standard | 2 ft — referenced separately by BMP type | 2 ft — consolidated in Table 6-2 for all BMP types |
| Soil investigation protocol | General guidance; no quantitative minimums | Formal protocol: boring count formula, 72-in depth, redox confirmation, Ksat with 50% safety factor |
| SSURGO for SHWT | Acceptable reference without explicit caveats | Screening tool only; borings required for design confirmation |
| Table 6-1 recharge rates | 2023 values | Updated for turf/lawn on HSG A (revised upward); impervious = 0 explicitly retained |
| Liner prohibition | Stated in practice-specific sections | Retained; spec language clarified |
Section 4
Practical Design and Review Implications
4.1 Changes in Design Workflow
The 2026 standards require the following specific workflow adjustments for major development stormwater design:
- GI Feasibility Analysis as a pre-design deliverable. Soil investigation (borings, SHWT, Ksat per 2026 protocol) must be completed before selecting a Non-GI alternative. This is a site investigation task, not a permit-response task, and must be scoped into early project phases.
- Volumetric reduction accounting is a distinct calculation step. WQ compliance now requires: (a) WQV calculation; (b) GI BMP design; (c) VRv calculation per Chapter 14; (d) VRv vs. WQV comparison; and, if residual remains, (e) Non-GI BMP sizing for the residual. Each step must be documented in the SWMR.
- Updated CN values must be verified. Projects using 2023 CN tables being revised under 2026 standards must confirm whether affected CN values have changed and recalculate WQV if so.
- Soil borings required for all recharge BMPs. Desktop assessment via SSURGO alone is no longer sufficient. Project schedules must accommodate on-site soil investigation before BMP sizing can be finalized.
4.2 Documentation Expected in 2026 Submittals
| Document Element | 2023 Expectation | 2026 Expectation |
|---|---|---|
| GI Feasibility Analysis | Required when Non-GI proposed; format informal | Required before Non-GI; must document soil investigation per 2026 protocol (borings, SHWT, Ksat, setback analysis) |
| WQV Calculation | CN-based; single step | CN-based with 2026 updated values; annotate where CN differs from 2023 |
| GI BMP Compliance Path | TSS removal calculation from Table 4-1 | VRv calculation per Chapter 14; VRv vs. WQV comparison documented; TSS calc not required if VRv ≥ WQV |
| Residual Volume Compliance | Not codified as separate step | Explicit residual volume calculation; Non-GI BMP sized for residual; 80% TSS per Table 4-1 for residual portion |
| Soil Investigation Report | Referenced soil mapping; no formal protocol | Formal report: boring logs, redox documentation, Ksat test results, SHWT depth, design Ksat with 50% safety factor applied |
| Recharge Volume Calculation | Table 6-1 rates; light documentation | Updated Table 6-1 rates; annual recharge deficit clearly calculated; BMP infiltration volume confirmed to match deficit |
4.3 Typical Plan Review Observations
The following issues are consistent with documented requirements in Chapters 4–6 and reflect common incomplete elements in stormwater management submittals:
Where a GI BMP is proposed but does not capture the full WQV, reviewers under the 2026 framework will check for a separate residual volume calculation and a compliant Non-GI BMP sized accordingly. Submittals that treat partial GI volumetric reduction as full compliance are deficient.
The 2026 protocol requires design Ksat = geometric mean of tested values × 50% safety factor. Submittals that use raw measured Ksat values without applying the safety factor are noncompliant with the 2026 soil investigation protocol.
The 2026 edition explicitly designates SSURGO as a screening tool only. Submittals relying on SSURGO-estimated SHWT depths without field borings documenting redoximorphic features will not satisfy the 2026 standard. Reviewers will request boring logs showing mottled horizon data.
BMPs claiming recharge compliance credit must explicitly state "no impermeable liner" in project specifications. Specifications that are silent on liner use leave the reviewer unable to confirm compliance with the recharge standard's liner prohibition.
Commercial, industrial, and transportation project submittals that do not include a hotspot screening analysis are commonly flagged. The 2026 edition emphasizes hotspot screening as a pre-design step; its absence suggests the infiltration feasibility analysis is incomplete.
Submittals using simple additive TSS removal percentages for series BMPs (e.g., 80% + 75% = 155%) are noncompliant. The joint probability method is required: Combined = 1 − [(1 − R₁)(1 − R₂)]. This yields 95% for the example above — still exceeds the 80% standard, but the method must be shown.
The 2026 NJ Stormwater BMP Manual updates to Chapters 4–6 reflect a maturation of the regulatory framework from prescriptive BMP selection to performance-based, quantitative compliance documentation. The introduction of volumetric reduction equivalence for GI BMPs in Chapter 4, the structured WQ compliance workflow in Chapter 5, and the formalized soil investigation protocol in Chapter 6 collectively raise the floor of required engineering rigor for stormwater design in New Jersey.
Practitioners should treat the 2026 framework as one that rewards early-phase site investigation and GI feasibility analysis with streamlined compliance pathways, while imposing higher documentation obligations on projects that rely on Non-GI alternatives or that propose partial volumetric reduction. The reviewer "gotchas" identified in §4.3 represent the most consistent points of friction between submittal quality and regulatory expectations under the updated standard.