Phase 3 - Master 2026 Synthesis: Subsurface Engineering, Volumetric Reduction, and Manual Reorganization¶

Source Documents: - 2023: N.J.A.C. 7:8 (July 2023), BMP Chapter 12, BMP Chapter 13, GI Requirement Fact Sheet - 2026: N.J.A.C. 7:8 (January 2026), BMP Chapter 12, BMP Chapter 13, BMP Chapter 14, GI Requirement Fact Sheet

Overview¶

This synthesis covers the part of the NJ Stormwater BMP Manual that governs subsurface feasibility and 2026 volumetric-reduction compliance. Chapter 12 addresses soil testing and seasonal high water table (SHWT) determination. Chapter 13 addresses hydraulic impacts from infiltration BMPs through groundwater mounding analysis. Chapter 14, new in the 2026 manual, consolidates the volumetric-reduction standards that were previously handled through the rules and related guidance rather than through a dedicated BMP Manual chapter.

Read together, these chapters tell a more continuity-heavy story than many early summaries suggested. Chapters 12 and 13 remain the technical foundation for exact-location soil testing, SHWT determination, design permeability, and groundwater-mounding review. The major 2026 shift is the addition of Chapter 14 and its direct alignment with N.J.A.C. 7:8-5.6(d), which gives designers a clearer rule-to-manual path for retention, hydrograph-based alternatives, and offsite volumetric-reduction compliance.

Section 1: Soil Testing and Subsurface Evaluation (Chapter 12)¶

1.1 Role of Soil Testing in BMP Design¶

Chapter 12 identifies five main reasons soil testing is needed in stormwater design: determining Hydrologic Soil Group (HSG), determining soil series for groundwater-recharge calculations, locating the SHWT, determining soil hydraulic conductivity for design and as-built testing, and determining inputs to groundwater-mounding analysis.

For infiltration BMPs, Chapter 12 functions as a feasibility gate. It requires the designer to confirm subsurface conditions at the proposed BMP location, not just rely on generalized planning-level mapping. In both editions reviewed here, the chapter also distinguishes between infiltration BMPs and non-infiltration BMPs that still need SHWT information because they must maintain minimum separation from groundwater.

1.2 Core Testing Framework¶

The core testing framework reviewed in the 2023 and 2026 Chapters 12 is substantially the same:

The NRCS Web Soil Survey is the default starting point for HSG and soil-series information, but field work is required where mapped conditions are uncertain, dual-rated, unavailable, or inconsistent with what is found onsite.
SHWT can be determined from redoximorphic features, direct water-level observation, or NRCS Web Soil Survey depth-to-water-table information, depending on what is observed and the time of year.
Infiltration BMPs must test saturated soil hydraulic conductivity and identify SHWT at the proposed BMP location.
Certain non-infiltration BMPs may still need SHWT information even if they do not rely on infiltration for compliance.

The chapter's general notice also states that soil profile pits, borings, and soil hydraulic conductivity tests, along with associated documentation, must be conducted under the direct supervision of a licensed New Jersey professional engineer.

1.3 SHWT Timing and Location Rules¶

The reviewed Chapter 12 text supports the following SHWT approach in both editions:

Where redoximorphic features are present, SHWT is tied to the observed water table or the elevation of the mottling, depending on what is encountered.
During January through April, direct water-level observation in profile pits or borings may be used.
During May through December, NRCS Web Soil Survey depth-to-water-table information may be used only if the observed soil profile is verified against the detailed NRCS profile description for the soil series present.
If that verification cannot be made during May through December, SHWT remains undetermined and additional testing is required.

In other words, the source language uses January through April for direct measurement and May through December for the alternate NRCS-supported path.

1.4 Quantity and Placement of Soil Explorations¶

The source tables support a structured, table-driven approach in both editions.

For infiltration BMPs, the reviewed requirements include:

A minimum of two soil profile pits within the infiltration area for most proposed infiltration BMPs.
One additional soil profile pit for each additional 10,000 square feet of infiltration area beyond the first 10,000 square feet.
At least one hydraulic-conductivity test for each soil profile pit.
Special rules for small BMPs, multiple BMPs in the same soil mapping unit, and linear BMPs.

The example table in both editions shows the same overall logic. For example:

a 9,000 square foot infiltration basin requires two soil profile pits
a 25,450 square foot infiltration basin requires four total soil profile pits
a 35,000 square foot porous paving system requires five total soil profile pits
ten 400 square foot rain gardens require one shared soil profile pit and one soil boring per BMP

1.5 Practical Takeaway from Chapter 12¶

The strongest Chapter 12 takeaway is continuity. The 2023 and 2026 text supports exact-location testing, table-driven pit and boring counts, January-April direct SHWT measurement, May-December use of NRCS data when verified, and direct linkage to later Chapter 13 groundwater-mounding inputs.

Section 2: Groundwater Mounding Analysis (Chapter 13)¶

2.1 Purpose of Mounding Analysis in Infiltration Design¶

Chapter 13 explains why a static SHWT reading from Chapter 12 is not enough for infiltration design. When runoff is infiltrated into subsoil, groundwater can temporarily mound beneath the BMP. If that mound reaches the bottom of the BMP, infiltration slows or stops and nearby structures may be affected.

The chapter anchors this requirement directly to N.J.A.C. 7:8-5.2(h), which requires the design engineer to assess hydraulic impacts whenever stormwater management includes one or more BMPs that infiltrate stormwater into subsoil. The rule language specifically identifies adverse impacts such as surficial ponding, flooding of basements, interference with subsurface sewage disposal systems or other subsurface structures, and interference with the functioning of the BMP itself.

2.2 Method and Inputs¶

In both reviewed editions, Chapter 13 relies on the Hantush Spreadsheet developed by USGS in cooperation with the Department. The chapter presents the analysis as an analytical spreadsheet-based method, not a fully custom groundwater model.

The reviewed text supports the following inputs and defaults:

Recharge rate is based on the tested vertical hydraulic conductivity of the most restrictive soil layer below the infiltration BMP, using the design permeability rate after applying a factor of safety of 2.
The highest recharge rate allowed in the initial model is 10 inches per hour.
The maximum duration of infiltration used in the spreadsheet is 72 hours.
Specific yield defaults to 0.15 unless tested values justify something higher, with a cap of 0.2.
Horizontal hydraulic conductivity defaults to 5R in the Coastal Plain and R outside the Coastal Plain.

The chapter also explains that the initial model is only the first step. If the calculated mound reaches the bottom of the BMP, the recharge rate must be reduced, the duration adjusted proportionally, and the design iterated until the BMP either works or must be redesigned.

2.3 Continuity Between 2023 and 2026¶

The reviewed 2023 and 2026 Chapter 13 text is again continuity-heavy. The core method, spreadsheet workflow, recharge-rate logic, 72-hour maximum duration, specific-yield defaults, and horizontal-conductivity defaults appear materially consistent across the two editions.

Chapter 13 itself is best read as a method chapter rather than as a list of broad statewide trigger thresholds. The governing emphasis is on the analytical workflow, not on a separate trigger framework based on:

10,000 square feet tributary area
3,000 square feet BMP footprint
SHWT less than 4 feet below the BMP
Sole Source Aquifer mapping
an approving-authority override

Those factors may appear in design heuristics or other materials, but they are not presented as general Chapter 13 rules in the reviewed text.

2.4 Broader-Corpus Support from BMP-Specific Chapters¶

While Chapter 13 itself is method-focused, the broader BMP corpus reinforces how the requirement operates in practice.

BMP-specific chapters in both editions, including bioretention and infiltration-basin chapters, repeatedly direct designers back to Chapter 12 for soil testing and to Chapter 13 for groundwater-mounding assessment. Those chapters also state that where mounding analysis identifies adverse impacts, the BMP must be redesigned or relocated, and that the analysis must provide details and supporting documentation on the methods used and assumptions made.

That broader-corpus support is sufficient for a practical conclusion: infiltration BMPs are not supposed to rely on Chapter 13 as a background reference only. The manuals treat groundwater-mounding analysis as a real design check that can force redesign when hydraulic impacts are identified.

2.5 Practical Takeaway from Chapter 13¶

The result is a clear and still-important Chapter 13 story. The manuals support a specific Hantush-based method, a defined set of spreadsheet inputs and defaults, a 72-hour maximum infiltration duration, and a redesign-if-adverse workflow tied to N.J.A.C. 7:8-5.2(h).

Section 3: Volumetric Reduction Standards (Chapter 14 - New in 2026)¶

3.1 Status in 2023¶

The 2023 BMP Manual had no Chapter 14. That does not mean volumetric reduction did not exist in 2023; it means the topic was handled through the rules and planning guidance rather than a dedicated BMP Manual chapter.

The 2023 GI fact sheet focused on the March 2021 green-infrastructure requirement. Its emphasis was on distributed, small-scale, close-to-source BMPs and on contributory drainage-area limits for GI BMPs used to meet groundwater-recharge and water-quality standards. The 2026 fact sheet remains materially similar in that respect.

3.2 What Chapter 14 Adds in 2026¶

What is genuinely new in 2026 is a dedicated BMP Manual chapter that consolidates the volumetric-reduction standards already rooted in N.J.A.C. 7:8-5.6(d).

The reviewed Chapter 14 and N.J.A.C. 7:8 text support three main compliance pathways:

Onsite retention of the water quality design storm by incorporating green-infrastructure BMPs.
Where onsite retention is technically impracticable or where infiltration is prohibited for the runoff type at issue, compliance through hydrograph-based performance: the post-construction water-quality-design-storm peak flow rate must be lower than that of equivalent woods on HSG D soil, and the runoff-hydrograph duration must be longer.
In the alternative, all or part of the standard may be addressed through removal of existing impervious surface and/or offsite retention of equivalent or greater water-quality-design-storm runoff volume, subject to the same-HUC-14 and related rule conditions.

3.3 What Chapter 14 Says About BMP Types¶

Chapter 14 presents a more nuanced picture than a strict GI-only volumetric-credit system.

Chapter 14 explicitly states that onsite retention can be achieved through infiltration BMPs such as dry wells, bioretention systems, infiltration basins, pervious paving systems, and sand filters. It also states that green roofs, cisterns, and GI wet ponds can satisfy the standard through evapotranspiration and beneficial reuse.

Most importantly, the reviewed 2026 Chapter 14 text also states that where onsite retention cannot be met, the volumetric-reduction standards can instead be met by BMPs that attenuate the water-quality-design-storm peak flow rate and extend runoff-hydrograph duration. It gives examples including:

grass swales
underdrained bioretention systems
underdrained pervious paving systems
underdrained sand filters
vegetative filter strips
standard constructed wetlands

That makes Chapter 14 broader than a strict binary framing in which only GI-classified infiltration BMPs contribute to volumetric-reduction compliance.

3.4 Offsite and Alternative Compliance Paths¶

The reviewed Chapter 14 and N.J.A.C. 7:8-5.6(d) text also support:

offsite impervious-surface removal as an alternative way to address all or part of the standard
offsite retention of equivalent or greater water-quality-design-storm runoff volume
same-HUC-14 location rules, with a Watershed Management Area fallback for public transportation entities where same-HUC-14 compliance is technically impracticable
a requirement that offsite portions of the project be fully reviewable and constructed prior to, or concurrent with, the major development
a public-transportation exception where volumetric-reduction standards do not apply if the project meets major-development status solely because it increases the capacity of an existing stormwater conveyance system

3.5 Practical Takeaway from Chapter 14¶

The important 2026 change is the addition of a dedicated chapter that consolidates the rule pathways for onsite retention, hydrograph-based alternatives where infiltration cannot be used, and offsite compliance options. That makes the 2026 manual substantially easier to use for volumetric-reduction analysis.

Section 4: Integration of New and Reorganized Content in the 2026 Manual¶

4.1 The Real 2026 Structural Change¶

The strongest supportable structural change is straightforward: 2026 adds Chapter 14, while 2023 did not have a dedicated volumetric-reduction chapter.

That matters because Chapters 12 and 13 already supplied the subsurface-feasibility and groundwater-impact pieces in both editions. The addition of Chapter 14 in 2026 creates a clearer sequence for designers working on infiltration-capable or retention-driven stormwater design:

Use Chapter 12 to establish subsurface conditions and design inputs.
Use Chapter 13 to test whether infiltration causes unacceptable groundwater impacts.
Use Chapter 14 and N.J.A.C. 7:8-5.6(d) to determine whether the site's overall volumetric-reduction obligation is met through retention, hydrograph-based alternatives, or approved offsite measures.

4.2 Where the Broader BMP Corpus Fits¶

The broader BMP-specific chapters reinforce this sequence in practical design use.

The reviewed bioretention, infiltration-basin, pervious-paving, and sand-filter chapters point designers back to Chapter 12 for soil testing and to Chapter 13 for mounding analysis. They also explain that soil tests must be performed at the exact proposed BMP location, that design permeability should use the slowest tested permeability rate with a factor of safety of 2, and that post-construction testing is required in several BMP chapters.

Those cross-references support the view that Chapters 12 and 13 are operational design chapters used throughout the BMP Manual. The integration is practical and recurring, even where the manuals do not present it as a single formalized 2026 design contract.

4.3 What Did Not Change as Much as the Earlier Draft Claimed¶

The reviewed fact sheets and BMP chapters point instead to a more focused interpretation:

the GI fact sheets in both eras remain focused on distributed, close-to-source, drainage-area-limited green infrastructure
the BMP-specific chapters in both eras already tie infiltrating practices back to Chapters 12 and 13
the major new organizational move in 2026 is the standalone volumetric-reduction chapter, not a wholesale rewrite of the soil-testing or mounding-analysis regime

Section 5: Implications for Stormwater Engineering Practice¶

5.1 Exact-Location Soil Testing Remains Foundational¶

The manuals continue to require that designers confirm subsurface conditions at the proposed BMP location. The practical implication is that early design must still coordinate grading, BMP layout, soil mapping, SHWT determination, and hydraulic-conductivity testing so that Chapter 13 and BMP-specific design checks are based on actual site conditions rather than only planning-level maps.

5.2 Factor-of-Safety-Based Design Permeability Still Drives Design¶

The reviewed Chapter 13 text and BMP-specific chapters support continued use of the tested most-restrictive soil horizon and a factor of safety of 2 to determine the design permeability rate. That affects drawdown calculations, mounding-analysis inputs, and whether infiltration-based design remains feasible.

5.3 Groundwater Mounding Is a Real Redesign Trigger¶

Across the rule text and BMP-specific chapters, the manuals consistently treat groundwater mounding as a real engineering constraint. If analysis shows adverse hydraulic impacts, the BMP must be redesigned, relocated, or changed in type. That remains one of the clearest practice implications of the subsurface chapters in both editions.

5.4 2026 Makes Volumetric-Reduction Compliance Easier to Assemble¶

The important 2026 practice change is that volumetric-reduction compliance is easier to explain and defend because Chapter 14 consolidates the rule pathways in one place. Designers now have a dedicated manual chapter discussing:

onsite retention by infiltration
retention through evapotranspiration or reuse
hydrograph-based alternatives where infiltration cannot be used
offsite compliance options and their rule conditions

That is a meaningful improvement in workflow.

5.5 Summary of Practice Impacts¶

Practice Area	Takeaway
Soil testing	Chapters 12 in both editions require exact-location subsurface investigation, SHWT determination, and conductivity testing tied to proposed BMP conditions.
Boring and pit counts	The manuals use table-driven exploration requirements, not a flat two-boring rule for all BMPs.
Groundwater mounding	Chapters 13 in both editions use the Hantush Spreadsheet method, defined defaults, and a redesign-if-adverse workflow tied to N.J.A.C. 7:8-5.2(h).
BMP chapter integration	BMP-specific chapters in both eras repeatedly point designers back to Chapters 12 and 13 and require supporting documentation where hydraulic impacts are identified.
2026 volumetric reduction	The real 2026 change is the dedicated Chapter 14 consolidation of retention, hydrograph-based alternatives, and offsite compliance options under N.J.A.C. 7:8-5.6(d).

Synthesis¶

The source picture is more continuity-heavy and more useful than the sharper break sometimes implied in early summaries. The 2023 and 2026 Chapters 12 and 13 provide the same core soil-testing and groundwater-mounding framework, with the same emphasis on exact-location testing, design permeability, and avoidance of adverse hydraulic impacts. The genuine 2026 shift is the addition of Chapter 14, which consolidates the volumetric-reduction standards and their alternatives into a dedicated BMP Manual chapter aligned with N.J.A.C. 7:8-5.6(d).

For the OPAL system, the Phase 3 master content is therefore best framed as a story of consolidation. Chapters 12 and 13 remain the technical foundation for infiltration feasibility. Chapter 14 adds a clearer rule-to-manual bridge for volumetric reduction, including onsite retention, hydrograph-based alternatives where infiltration is impracticable or prohibited, and offsite compliance options where the rules allow them.