Pollutant Removal, SWM Standards, and Groundwater Recharge

Stormwater Pollutant Removal Criteria (Chapter 4)

1.1 What the Manuals Require

Both editions treat Chapter 4 as the stormwater runoff quality chapter for TSS and related pollutant-removal selection logic, but the trigger and framing changed materially between the two eras.

2023 Edition - Core Requirements

• The chapter is keyed to major development that creates at least 0.25 acres of new or additional impervious surface.
• The core performance standard is 80 percent average annual TSS removal from post-construction runoff.
• The runoff quality design storm remains the 1.25-inch / 2-hour water quality design storm.
• Nutrient reduction is still required to the maximum extent feasible, with Chapter 4 using nutrient-removal data as a BMP-selection aid rather than as a fixed statewide numeric nutrient standard.
• The chapter explains treatment trains and the combined-removal calculation for BMPs used in series.

2026 Edition - Core Requirements

• The trigger shifts to increases of one-quarter acre or more of regulated motor vehicle surface, or reconstruction of one-quarter acre or more of motor vehicle surface.
• The chapter adds redevelopment and reconstruction examples that clarify when stormwater runoff quality standards attach to existing motor vehicle surfaces.
• The chapter also adds the "maintain or increase existing treatment" logic where existing water quality treatment is modified or removed.
• The 2026 chapter states that the stormwater management measures used to satisfy runoff-quality reduction must be green infrastructure BMPs unless a waiver or variance from N.J.A.C. 7:8-5.3 is obtained.
• The chapter keeps treatment-train logic and typical nutrient-removal guidance, but updates the framing to the current GI / waiver-or-variance regime and the regulated motor vehicle surface terminology.

1.2 Where Designers Typically Document Compliance

Under both editions, designers typically document Chapter 4 compliance in the stormwater management report through:

• drainage-area tabulations and runoff pathways to each BMP;
• identification of the runoff-quality trigger that applies to the project;
• TSS removal rates for each BMP and any treatment-train calculation;
• nutrient-removal discussion where maximum-extent-feasible nutrient reduction is part of the design narrative; and
• supporting rule/manual references for any non-default or waiver-based compliance path.

For 2026 work, it is also important to separate what is a Chapter 4 runoff-quality demonstration from what is a quantity or volumetric-reduction demonstration elsewhere in the corpus.

1.3 Notable Updates Between 2023 and 2026 Editions

Chapter 4 should therefore remain a runoff-quality chapter in report writing and design review. Volumetric-reduction compliance is real in the 2026 framework, but it is grounded in the quantity standards at N.J.A.C. 7:8-5.6(d) and BMP Manual Chapter 14 rather than in Chapter 4 itself.

SWM Standards and Computations (Chapter 5)

2.1 What Standards Drive Sizing and Verification

Chapter 5 is the computation chapter. In both editions it explains how to calculate runoff rates, volumes, and hydrographs, and how to apply the quantity, quality, and recharge standards through accepted hydrologic and hydraulic methods.

Across both editions, the chapter supports:

• the water quality design storm framework;
• the groundwater-recharge link back to Chapter 6 and the NJGRS workflow;
• the runoff-quantity options tied to hydrologic and hydraulic analysis; and
• the NRCS methodology, including runoff volumes, peak flows, and hydrograph comparisons.

For quantity control, the key framework remains the current and projected 2-, 10-, and 100-year storm events. The chapter supports all three storm events, not a simplified 2-year / 100-year-only reading.

2.2 What Computations Are Commonly Referenced

At a conceptual level, the chapter commonly references:

• runoff-volume and peak-flow calculations under NRCS methods;
• hydrograph comparisons for the quantity-control options;
• time-of-concentration and runoff-routing concepts;
• separate handling of impervious and pervious runoff contributions where the methods require it;
• the prohibition on using misleading composite CN approaches in situations where separate hydrograph development is required; and
• current and projected rainfall-depth handling using NOAA Atlas 14 rainfall data and the rule-based adjustment factors.

The 2026 chapter is clearer and more explicit on the climate-adjusted rainfall side of the quantity analysis. It names current storm events and projected storm events, and ties them back to the rule tables for current precipitation adjustment factors and future precipitation change factors.

2.3 Notable Updates Between 2023 and 2026 Editions

Chapter 5 remains the computational hub for the current and projected storm analysis, but it is not itself the source of the Chapter 14 volumetric-reduction workflow. Where a 2026 engineering narrative addresses volumetric reduction, the governing support comes from N.J.A.C. 7:8-5.6(d) and Chapter 14.

Groundwater Recharge (Chapter 6)

3.1 Recharge Requirement Concepts and Applicability

Chapter 6 remains focused on the recharge standard itself and on the NJGRS-based methodology for meeting it.

Both editions support the same high-level structure:

• major development must address groundwater recharge unless an exemption or waiver applies under the rules;
• the recharge standard can be demonstrated through Requirement 1 (maintain 100 percent of average annual pre-construction groundwater recharge volume) or Requirement 2 (infiltrate the increase in projected 2-year runoff volume);
• Chapter 6 primarily develops the data and procedures for Requirement 1;
• Chapter 5 carries the companion computational material for Requirement 2; and
• the NJGRS remains the core spreadsheet-based method for annual recharge analysis and recharge-BMP sizing.

3.2 What the Chapter Actually Emphasizes

The chapter's real emphasis is:

• annual recharge accounting;
• pre- and post-development recharge comparisons;
• NJGRS assumptions, inputs, and limits;
• recharge-deficit sizing logic for BMPs;
• the relationship between contributory impervious area and recharge-BMP sizing; and
• the fact that recharge design depends on soil and site conditions that are evaluated through supporting soil data.

The chapter does not itself function as a detailed soil-investigation protocol chapter. It points to other material for that level of design and testing detail.

3.3 Notable Updates Between 2023 and 2026 Editions

The 2026 chapter reads more like a current user guide for the NJGRS workbook and related recharge-BMP inputs, but its core recharge logic remains continuous with the 2023 edition.

The main takeaways are:

• Requirement 1 / Requirement 2 logic is still central.
• NJGRS remains the primary annual-recharge analysis tool.
• The 2026 text is more explicit in walking users through spreadsheet inputs, checks, and BMP-calculation worksheets.
• The chapter continues to rely on actual soil and site conditions, but the detailed field-testing, boring-count, SHWT-separation, and mounding-analysis procedures are developed in later design chapters rather than in Chapter 6 alone.

For practice-level detail, Chapter 12 provides the soil-testing requirements, Chapter 13 provides the groundwater-mounding inputs and design-rate treatment, and BMP-specific chapters in Chapters 9 and 10 provide many of the SHWT, underdrain, drain-time, and liner details that engineers use in final design.

Practical Design and Review Implications

4.1 What Changes in Design Workflow

The engineering workflow implications that emerge from Chapters 4 to 6 are straightforward:

1. Runoff-quality review is more redevelopment-aware in 2026. The Chapter 4 trigger now turns on regulated motor vehicle surface and reconstruction logic, so engineers must classify the project trigger carefully rather than defaulting to the older impervious-surface shorthand.
2. Quantity analysis must address all three design storms. Chapters 5 and the companion rule text support current and projected 2-, 10-, and 100-year analysis, not a reduced storm set.
3. Climate-adjusted rainfall handling is more explicit. The 2026 Chapter 5 presentation makes current and projected rainfall adjustments a more visible part of the quantity workflow.
4. Recharge remains a two-path framework. Chapter 6 still centers Requirement 1 versus Requirement 2, with NJGRS-based annual recharge work remaining a core design path.

Where the workflow reaches beyond Chapters 4 to 6, the broader manual makes the connection clear: volumetric reduction is grounded in N.J.A.C. 7:8-5.6(d) and Chapter 14, and the detailed infiltration soil testing and groundwater-mounding analysis are developed in Chapters 12 and 13.

4.2 What Changes in Documentation Expected in a Submittal

4.3 Typical Reviewer Issues That Are Actually Supported

The following review themes remain central:

• Wrong runoff-quality trigger: In 2026, engineers can no longer rely on the older "new impervious only" shorthand where regulated motor vehicle surface or reconstruction controls.
• Quantity analysis missing one or more required storm events: The 2-, 10-, and 100-year set remains required where those quantity-control options are used.
• Improper hydrograph or CN handling: Chapter 5 continues to warn against oversimplified runoff-computation shortcuts, especially where separate hydrographs or non-composite handling are required.
• Recharge narrative detached from the chosen requirement: Recharge reports should say whether they are using Requirement 1 or Requirement 2 and show the corresponding analysis path.
• Broader design material misattributed to Phase 1C chapters: If volumetric reduction, soil-testing protocol details, or groundwater-mounding inputs are discussed, they should be tied to the chapters that actually develop them.

4.4 Bottom Line

The Phase 1C report still supports a meaningful 2023-to-2026 engineering comparison, but the story is more disciplined than many shorthand summaries suggest:

• Chapter 4 changes the runoff-quality trigger and redevelopment logic more than it changes the underlying TSS-treatment math.
• Chapter 5 keeps the three-storm quantity framework and makes climate-adjusted rainfall handling more explicit.
• Chapter 6 remains the recharge-methodology chapter, not the place to source every later-design detail about soil testing, SHWT separation, or mounding.

Where 2026 engineering practice truly does pull in volumetric reduction, detailed soil-testing rules, or groundwater-mounding conventions, those points are better sourced from the broader corpus and should be labeled that way.

End of Phase 1C Engineering Content Primary source scope for this report: NJ Stormwater BMP Manual Chapters 4-6, 2023 and 2026 editions. Limited fallback references above are explicitly identified as broader-corpus support.