Phase 1C Engineering Content¶

NJ Stormwater BMP Manual — Chapters 4, 5, and 6¶

Pollutant Removal · SWM Standards and Computations · Groundwater Recharge¶

2023 vs. 2026 Edition Comparison Generated March 4, 2026 — OPAL Stormwater Engineering Knowledge System

Section 1 — Stormwater Pollutant Removal Criteria (Chapter 4)¶

1.1 What the Manual Requires¶

Chapter 4 of the NJ Stormwater BMP Manual establishes pollutant removal criteria that BMPs must satisfy when applied to meet water quality standards under N.J.A.C. 7:8. The primary performance metric is Total Suspended Solids (TSS) removal, measured as a percentage removal relative to untreated runoff from the contributing drainage area.

2023 Edition — Core Requirements:

Standard TSS removal target: 80% for sites not using GI BMPs to satisfy the water quality volume (WQV) standard.
The 80% TSS criterion applies to the WQV design storm (1.25 inches over 2 hours for NJ), evaluated as the annual average removal rate across modeled storm events.
Nutrient removal (total nitrogen, total phosphorus) is secondary but required for discharges to Category One (C1) waters or waters with approved Total Maximum Daily Loads (TMDLs) for nutrients. Where applicable, specific nutrient reduction targets are cited in the applicable TMDL.
TSS removal performance values are assigned to individual BMP types in the manual's performance tables (Table 4-1 in both editions), which list low, moderate, and high confidence removal ranges for each BMP based on published research.
BMPs applied in series as part of a treatment train receive combined credit; the method for calculating combined TSS removal is specified (joint probability approach, not simple additive removal).

2026 Edition — Updated Requirements:

The 80% TSS removal standard is retained for Non-GI BMPs.
For GI BMPs, the compliance pathway shifts: a GI BMP that achieves the volumetric reduction standard (complete capture and infiltration/evapotranspiration of the WQV with no direct discharge) is deemed to meet the TSS removal requirement by compliance pathway credit, without separate TSS removal calculation. This is the single most significant procedural change in Chapter 4.
For GI BMPs that do not achieve full volumetric reduction (partial capture), TSS removal must be calculated for the portion of WQV that is not retained, and the 80% standard applies to that fraction.
Nutrient reduction requirements are strengthened for discharges to impaired waters; the 2026 edition explicitly states that for sites within a designated nutrient management area, a treatment train incorporating a bioretention cell or constructed wetland with confirmed nutrient uptake capability is the preferred compliance pathway.

1.2 Where Designers Document Compliance¶

Under both editions, TSS removal compliance is documented in the Stormwater Management Report (SWMR) submitted with permit applications. Required documentation elements include:

Identification of the selected BMP type(s) for water quality compliance
Drainage area(s) directed to each BMP, with impervious area tabulation
WQV calculation for each drainage sub-area
TSS removal efficiency citation (from Chapter 4 performance tables) and supporting reference
For grouped BMPs (treatment trains): combined removal calculation following the joint probability method
2026 addition: if relying on GI BMP volumetric reduction credit for TSS compliance, the SWMR must include the volumetric reduction calculation per Chapter 14 confirming that the full WQV is captured and retained with no surface discharge from the design storm

1.3 Notable Updates Between 2023 and 2026 Editions¶

Item	2023	2026
TSS standard for Non-GI BMPs	80% removal required	Unchanged — 80% retained
TSS compliance for GI BMPs	TSS removal calculated for all BMPs	GI BMPs achieving full volumetric reduction exempt from TSS calculation
Partial volumetric reduction credit	Not explicitly addressed	Partial GI capture: TSS calculated only for undischarged fraction
Treatment train nutrient reduction	Mentioned; TMDL-specific	Expanded; preferred treatment train specified for nutrient management areas
Performance table (Table 4-1)	2023 version; research basis varies	Updated removal values for several practice types reflecting post-2020 studies; bioretention and permeable pavement values revised upward

Section 2 — SWM Standards and Computations (Chapter 5)¶

2.1 What Standards Drive Sizing and Verification¶

Chapter 5 describes the three primary stormwater management standards in N.J.A.C. 7:8 and the computational approach used to size BMPs for each:

Standard 1 — Water Quality (WQ) - Requires capture and treatment (or full volumetric reduction) of the Water Quality Volume (WQV): the runoff generated from 1.25 inches of rainfall over a 2-hour duration for the post-development condition. - Applied to major developments generating one or more acres of new impervious surface or disturbance. - The WQV is calculated using a runoff coefficient derived from Curve Number (CN) methodology for the post-development impervious and pervious area mix.

Standard 2 — Groundwater Recharge - Requires maintenance of the pre-development groundwater recharge volume on an annual basis. - Calculated as the difference between pre-development recharge and post-development recharge, with recharge rates tabulated by Hydrologic Soil Group (HSG) and land use type (Chapter 6; see Section 3 below).

Standard 3 — Flood Control - Requires that peak discharge rates for the 2-year and 100-year design storms do not exceed pre-development values at the point of discharge. - Channel protection: Some municipalities or MSWMP-governed projects also require a channel protection (Cpv) standard for the 1-year storm or a storm-specific channel erosion threshold, but this is geographically variable. - Flood control analysis uses the NRCS TR-55 methodology (or equivalent accepted hydrologic model) with NJ-specific rainfall depths.

2.2 Referenced Computations (Conceptual Overview)¶

Water Quality Volume (WQV)

WQV is computed from the general rational/runoff equation form applied to the 1.25-inch design storm:

WQV = P × Rv × A

Where: - P = design storm rainfall depth (1.25 in) - Rv = volumetric runoff coefficient for the post-development condition - A = drainage area contributing to the BMP (acres or square feet, consistent with P units)

Rv is derived using Curve Number methodology or composite impervious fraction, depending on BMP sizing method. The WQV determines the minimum storage or infiltration volume required for water quality compliance.

Curve Number (CN) Determination

CN values are assigned by HSG and land use category from the standard NRCS tables, as adapted in the NJ BMP Manual. The post-development CN is calculated as the area-weighted composite of CN values for all cover types in the drainage area. The 2026 edition updates certain CN values (described below in §2.3).

Recharge Volume (Rev)

Rev = (Recharge Rate_predevelopment - Recharge Rate_postdevelopment) × A

Annual recharge rates are tabulated in Chapter 6 (Table 6-1) by HSG and land use category. The deficit is the volume that on-site infiltration BMPs must provide.

Peak Flow Computations (Flood Control)

Stage-discharge relationships for detention basins and routing calculations for culverts and outfalls use standard dam/culvert hydraulics. Hydrologic inputs use NJ-specific Soil Conservation Service (SCS) design storm rainfall depths from the NOAA Atlas 14 tables adopted in the manual (24-hour storm, Type III distribution, applicable for most of NJ):

2-year, 24-hour: approximately 3.2 to 3.6 inches (varies by region/location)
100-year, 24-hour: approximately 9 to 13 inches (significant northeast-southwest variation in NJ)

Note: The NJ BMP Manual specifies using Atlas 14-derived NJ-specific IDF data; the rainfall depths above are representative reference values. Designers must use the applicable depths for the specific project location from the current approved tables.

2.3 Notable Updates Between 2023 and 2026 Editions (Chapter 5)¶

Updated CN Tables (2026)

The 2026 edition updates Curve Number values for several land use categories, particularly: - Single-family residential at various lot sizes (adjusted for observed post-construction conditions) - Conventional lawn areas in HSG B soils (CN value reduced from 69 to 66 based on updated NJ monitoring data) - Commercial/industrial (CN unchanged in most categories; paved surfaces remain CN 98 for all HSGs)

These CN adjustments affect WQV calculations — lower CN values for residential uses reduce computed WQV slightly, potentially reducing required BMP sizing. Engineers reworking existing designs under 2026 standards should recalculate WQV if the applicable CN has changed.

Volumetric Reduction Integration (2026)

The most operationally significant Chapter 5 update is the formal integration of volumetric reduction accounting into the WQ standard compliance workflow: - Where a GI BMP is proposed, the designer first calculates the Volumetric Reduction Volume (VRv) that the practice provides under the WQV design storm, using the Chapter 14 sizing methodology. - If VRv ≥ WQV: full compliance with the water quality standard is achieved; no TSS removal calculation required. - If VRv < WQV: the residual volume (WQV − VRv) must be managed by a compliant BMP with documented TSS removal performance (Chapter 4 standard applies to residual volume).

Chapter 5 Standard Comparison Table

Standard	2023 Design Basis	2026 Design Basis
WQV design storm	1.25 in / 2 hr; Rv from CN tables	Unchanged; updated CN values for some categories
GI BMP compliance	GI BMPs shown to meet WQV storage requirement	Volumetric reduction accounting per Ch. 14; VRv ≥ WQV required
Residual/partial GI compliance	Not explicitly codified	WQV − VRv residual treated by Non-GI BMP to 80% TSS
Flood Control rainfall	Atlas 14 NJ-specific depths	Unchanged; explicit citation of current Atlas 14 version added
Channel protection	References general MSWMP guidance	Explicit that channel protection requirements flow from MSWMP; direct application when no MSWMP exists

Section 3 — Groundwater Recharge (Chapter 6)¶

3.1 Recharge Requirement Concepts and Applicability¶

Chapter 6 establishes requirements for maintaining pre-development groundwater recharge rates and quantities. The regulatory basis is N.J.A.C. 7:8-5.4, which requires that major development does not decrease the pre-development average annual groundwater recharge volume by more than what is technically infeasible.

Applicability: The recharge standard applies to major developments — those with 1 acre or more of impervious surface or land disturbance. Smaller projects and redevelopment projects on existing impervious area may receive modified or exempted recharge requirements depending on MSWMP authorization.

Core requirement: Maintain pre-development recharge rate and volume. This is typically achieved by: 1. Infiltration-based GI BMPs: Bioretention cells with underdrain removed or disabled, infiltration basins, dry wells, or pervious pavement that allows water to percolate into native soils. 2. Disaggregated recharge credit: Where full recharge is not feasible, the recharge standard may be met via a combination of on-site practices that collectively provide the annual recharge deficit volume. 3. Technical infeasibility finding: If site conditions (high SHWT, HSG D soils, contaminated soils, bedrock, stormwater hotspot classification) preclude infiltration, a technical infeasibility determination may be granted and recharge provided via alternative means (e.g., groundwater injection in rare cases) or waived with documentation per the applicable stormwater approval pathway.

3.2 Recharge Design — Key Parameters¶

Hydrologic Soil Group Classification

The recharge standard's achievability is fundamentally tied to the soil's hydraulic conductivity and HSG designation:

HSG	Infiltration Rate	Recharge Feasibility
A	> 0.30 in/hr	Excellent; full recharge systems typically feasible
B	0.15 – 0.30 in/hr	Good; bioretention and pervious pavement generally work
C	0.05 – 0.15 in/hr	Limited; infiltration-only BMPs may be undersized; extended drawdown periods
D	< 0.05 in/hr	Poor; infiltration generally infeasible; technical infeasibility finding typically required
A/D, B/D (dual)	Variable (seasonally saturated)	D-rated parent soil governs; consider seasonal ground saturations

SHWT Separation Requirements

Both manual editions require minimum vertical separation between the bottom of an infiltrating BMP and the Seasonal High Water Table (SHWT): - Bioretention cells (without underdrain): Minimum 2 feet of separation between the basin floor (bottom of aggregate layer) and the measured SHWT. - Infiltration basins: Minimum 2 feet (same standard). - Dry wells: 2 feet from bottom of gravel reservoir to SHWT. - Pervious pavement systems: Minimum 2 feet from bottom of gravel reservoir to SHWT.

The SHWT is established through on-site soil borings or test pits, typically conducted by a licensed soil scientist. The 2026 edition explicitly references the NRCS SSURGO database as a permitted first-screening tool for SHWT estimation but specifies that borings are required for design confirmation.

Setback Requirements

Setbacks for infiltration BMPs from site features that must both editions recognize: - Foundations/footings: 10 feet minimum horizontal distance - Private wells: 25 feet minimum (varies; check local approval authority) - Septic systems (leach fields): 50 feet minimum - Property lines: 10 feet minimum - Slopes greater than 20%: Not to be used directly upslope

Stormwater Hotspots (Prohibition Zone)

Infiltration is prohibited in Stormwater Hotspot areas — defined locations where runoff carries pollutants at concentrations that could contaminate groundwater if infiltrated (e.g., fueling stations, vehicle maintenance yards, hazardous material storage, high-vehicle-traffic industrial facilities). Water quality pretreatment prior to infiltration is required in transitional or lower-risk situations; full hotspot classification prohibits infiltration entirely.

Recharge Volume Calculation

Annual recharge volumes are calculated using Table 6-1 recharge rates (in/year) by HSG and land use:

Rev = Σ [ (Rrate_pre × A_pre) - (Rrate_post × A_post) ]

Where Rrate values are annual recharge rates (in/year) from Table 6-1 for each land use category before and after development. The resulting deficit volume (acre-feet/year or cubic feet/year) must be provided by the infiltration BMP, expressed as a design volume for sizing purposes.

3.3 Common Design Constraints¶

Liner Prohibition

Both editions explicitly state that BMPs providing recharge credit must not use impermeable liners. The liner prohibition is absolute for practices counting toward recharge compliance. Practices with liners may still be used for water quality compliance (i.e., surface filtration bioretention with raised underdrain meeting TSS standards) but do not count toward the recharge standard.

Geotextile Wrapping

While geotextile fabric may be used to encapsulate filter media (preventing migration of fines into aggregate), it must not act as an impermeable barrier. Geotextile specifications must confirm compatibility with native soil system infiltration rates.

Clay Soils and Fill

Where native soils are poor infiltrators but underlying materials have higher permeability, the designer must evaluate the vertical soil profile to the design depth plus 2 feet below BMP bottom. Fill soils introduced during site grading may alter natural infiltration rates and must not be credited as native soil without independent testing.

3.4 Notable Updates Between 2023 and 2026 Editions (Chapter 6)¶

Soil Investigation Protocol (2026)

The 2026 edition introduces a formal Soil Investigation Protocol for recharge-based BMP design, specifying minimum requirements: - Number of soil borings or test pits: minimum one per BMP footprint up to 5,000 sq ft; one additional for every additional 5,000 sq ft - Boring/pit depth: Minimum 72 inches below proposed BMP bottom - SHWT determination: Documented field redoximorphic features confirming SHWT depth; SSURGO data allowed for screening only - Saturated hydraulic conductivity testing: At least one in-situ falling-head or constant-head permeability test per BMP; design K_sat used at geometric mean of tested values with 50% safety factor applied

The 2023 edition had no equivalent standardized soil investigation requirement; it referenced general soil testing guidance but did not set procedural minimums.

Updated Recharge Rates (Table 6-1, 2026)

Recharge rates in Table 6-1 were updated for several land use/HSG combinations: - Turf/lawn on HSG A soils: revised upward (reflecting improved understanding of maintained lawn infiltration) - Forested HSG B: unchanged - Impervious surface: recharge rate = 0 (unchanged; explicitly noted that no recharge credit is given to any impervious cover regardless of slope or adjacent pervious area)

Separation Requirement Clarification (2026)

The 2026 edition explicitly standardizes the 2-foot SHWT separation requirement across all infiltrating BMP types in a consolidated table (Table 6-2), replacing the dispersed references in Chapter 6 that were present in the 2023 edition. The 2023 edition had equivalent requirements but referenced them at different sections for different BMP types, creating inconsistency in reviewer and designer interpretations.

Section 4 — Practical Design and Review Implications¶

4.1 Changes in Design Workflow¶

The transition from 2023 to 2026 standards requires several specific adjustments in how stormwater design is sequenced:

GI Feasibility Analysis moves earlier. The 2026 framework requires documented GI feasibility (soil investigation, SHWT mapping, setback analysis, hotspot screening) before proposing any Non-GI BMP alternative for the water quality standard. This is a pre-design task, not a permit-response task. Engineering scope of work for major projects should include a GI feasibility phase scoped to the new soil investigation protocol.
Volumetric reduction accounting is now a distinct calculation step. Under the 2026 standard, the design sequence for water quality compliance is: (a) calculate WQV; (b) design proposed GI BMP; (c) calculate VRv per Chapter 14; (d) compare VRv to WQV; (e) if residual exists, size a compliant Non-GI BMP for the residual. This is a more structured and multi-step workflow than the 2023 approach of sizing a single BMP to the WQV.
Soil borings are required for all recharge-based BMPs. Under the 2026 soil investigation protocol, desktop assessment is insufficient for design confirmation. Site investigation must be scheduled into the project timeline before final BMP sizing.
CN table updates require verification for existing designs. Projects designed under 2023 standards that are being updated or revised must confirm whether WQV and recharge calculations use the 2026 CN and Table 6-1 values, as updated values may alter BMP sizing even if the site design is otherwise unchanged.

4.2 Changes in Documentation Expected in a Submittal¶

Document Element	2023 Expectation	2026 Expectation
GI feasibility analysis	Required when Non-GI proposed; format was informal	Required before Non-GI; must follow soil investigation protocol and include documented SHWT, K_sat, setback analysis
WQV calculation	CN-based; single step	CN-based with updated values; annotate if 2026 CN differs from 2023
GI BMP compliance pathway	TSS removal calculation from Ch. 4 tables	Volumetric reduction calculation per Ch. 14; VRv vs. WQV comparison documented
Residual WQV compliance	Not explicitly codified	Residual volume calculation; TSS removal for residual per Ch. 4
Soil investigation report	Referenced soil mapping; no formal protocol	Formal soil investigation report: boring logs, redox feature documentation, K_sat test results, SHWT depth, design K_sat with safety factor
Recharge volume calculation	Table 6-1 rates; minimal documentation standard	Table 6-1 updated rates; annual recharge deficit with confirmation that infiltrating BMP volume matches deficit

4.3 Typical Reviewer Observations (Supported by Manual Language)¶

The following observations reflect documented requirements in Chapters 4–6 that are frequently incomplete in submittals:

Partial volumetric reduction not addressed (2026): Where a GI BMP is proposed but the WQV is not fully met, reviewers will check that the residual volume is separately identified and that a compliant Non-GI BMP is sized for the residual. The 2023 framework did not require this residual analysis as a separate step.
K_sat safety factor not applied: The 2026 soil investigation protocol requires the design K_sat to be the geometric mean of tested values reduced by a 50% safety factor. Submittals using raw measured K_sat values without the safety factor applied are noncompliant with the 2026 protocol.
SHWT documentation from SSURGO alone: Reviewers will flag submittals where SHWT depth is supported only by SSURGO database estimates. The 2026 edition explicitly states that borings with redoximorphic feature documentation are required for design confirmation; SSURGO is screening only.
Liner language absent from bioretention specifications: Submittals providing recharge credit through bioretention must explicitly state "no impermeable liner" in the project specifications. Specifications that are silent on liner use may not satisfy the reviewer's documentation requirement.
Hotspot screening not included: Submittals for commercial developments (fueling, vehicle service, industrial) that do not include a hotspot screening analysis or hotspot classification determination are commonly deficient. The 2026 edition does not change the hotspot prohibition but emphasizes the screening obligation as a pre-design step.
Combined TSS removal method not shown for treatment trains: Where two or more BMPs are used in series toward the 80% TSS standard, the calculation must use the joint probability method, not simple additive removal. Submittals that add individual removal percentages (e.g., Bioretention at 80% + Sand Filter at 85% ≠ 165% combined) are noncompliant.

End of Phase 1C Engineering Content Source Reference: NJ Stormwater BMP Manual, Chapters 4–6; 2023 and 2026 Editions. Regulatory basis: N.J.A.C. 7:8.