Subsurface Evaluation,
Groundwater Mounding & Volumetric Reduction

Section 1 — Chapter 12

Soil Testing Criteria and Subsurface Evaluation

1.1  Role of Soil Testing in BMP Design

Chapter 12 provides the site investigation framework for all infiltration-dependent GI BMPs. It functions as a hard feasibility gate — not advisory guidance. The three fundamental questions it must answer are:

1. Is the native soil hydraulically capable of receiving and transmitting the design runoff volume within the required drawdown period?
2. Is the seasonal high water table (SHWT) sufficiently deep to provide the mandatory 2.0-foot separation from the proposed BMP infiltrating surface?
3. What is the measured saturated hydraulic conductivity (Ksat) at the design bottom-of-BMP elevation?

If the answer to any of these is negative, an infiltrating GI BMP cannot be sited at that location. The designer must either modify the BMP design (add an impermeable liner and underdrain to convert to Non-GI configuration) or relocate the facility.

1.2  Core Testing Protocol: 2023 Edition

Soil Textural Classification

Field classification using USDA Soil Texture, supplemented by NRCS Hydrologic Soil Group (HSG) from Web Soil Survey. HSG A and B presumed infiltration-capable pending field verification. HSG C and D generally unsuitable for native infiltration without engineered media replacement throughout the BMP filter zone.

Ksat Testing Method (2023)

Falling-head percolation test (NJDEP or ASTM methods) or field measurement using the Amoozegar compact constant head permeameter or Guelph permeameter at the proposed bottom-of-BMP elevation. Minimum of two test borings per BMP footprint. Ksat results used to confirm HSG classification and calculate the design infiltration rate — applying a safety factor of 0.5× or 0.33× to account for clogging over BMP service life.

SHWT Determination (2023)

SHWT identified via soil morphological indicators — redoximorphic features with Munsell chroma ≤2 with mottles — observed in a soil boring or test pit. Depth recorded relative to the proposed bottom-of-BMP grade. The 2023 edition does not explicitly require that SHWT determination be made by a licensed soil scientist; a PE performing and documenting the boring is generally accepted.

1.3  Key Changes: 2023 vs. 2026 (Chapter 12)

Three additional changes in the 2026 Chapter 12:

Soil Investigation Reference Profile

Section 2 — Chapter 13

Groundwater Mounding Analysis

2.1  Why Mounding Analysis Matters

The static SHWT measured in the Chapter 12 boring represents the pre-BMP groundwater condition. When the BMP begins to operate — recharging the aquifer with storm runoff — the local water table rises. If this mound grows faster than lateral groundwater flow can dissipate it, the mound eventually reaches the BMP bottom. When the separation distance falls below 2.0 feet, infiltration capacity is impaired; when the mound contacts the BMP bottom, the system floods and cannot accept runoff at all.

Chapter 13 exists to predict this mound height under operating conditions and confirm that the 2.0-foot buffer is maintained after accounting for BMP-induced recharge.

2.2  Analysis Triggers

Both editions require mounding analysis when one or more trigger conditions are met. The 2026 edition refines these triggers as follows:

• Tributary impervious area to a single BMP footprint exceeds 10,000 ft²
• [2026 NEW] BMP footprint (bottom-of-BMP area) exceeds 3,000 ft², regardless of tributary area
• SHWT depth is less than 4.0 feet below the proposed BMP bottom-of-BMP elevation
• Site is within a mapped NJDEP Sole Source Aquifer recharge zone
• The approving authority determines analysis is warranted

2.3  Analysis Method: Hantush (1967)

Both editions specify the Hantush (1967) analytical method for estimating water table rise below a rectangular recharging area in an unconfined aquifer. This is a closed-form analytical equation — not a numerical groundwater model — and produces an estimated maximum mound height (Δh) directly below the BMP center. Required inputs:

2.4  2026 Documentation Requirement

The 2026 edition adds a specific documentation obligation not present in 2023: the mounding analysis and all supporting inputs must be compiled as a dedicated subsection within the SWM Report for stormwater permit review. Required contents:

• Hantush analysis worksheet with all parameter values and their sources
• Ksat and aquifer parameter documentation traceable to Chapter 12 field test logs
• BMP geometry (footprint dimensions and bottom-of-BMP elevation from design drawings)
• Calculated Δh and comparison to (h₀ − 2.0 ft) criterion
• Cross-reference to LPSS-certified SHWT depth used as h₀ input

Section 3 — Chapter 14 (New in 2026)

Volumetric Reduction Standards

3.1  2023 Status — No Unified Chapter

The 2023 NJ Stormwater BMP Manual contains no Chapter 14. The volumetric reduction concept — the requirement that a portion of stormwater runoff volume be eliminated through infiltration, evapotranspiration, or beneficial reuse — existed in the 2023 regulatory framework under N.J.A.C. 7:8 but was treated only implicitly in the 2023 manual through GI BMP definitions and WQV sizing methodology in existing chapters. There was no unified chapter synthesizing VRC calculation methods, compliance accounting, or documentation requirements. Engineers assembled volumetric reduction compliance narratives from disparate guidance sources, and reviewing authorities lacked a standardized documentation format to evaluate.

3.2  The GI Requirement: Water Quality Volume (WQV)

Chapter 14 centers on the GI Requirement — the N.J.A.C. 7:8 obligation that major development must demonstrate volumetric reduction equal to or exceeding the Water Quality Volume (WQV) for the project's net impervious surfaces. Only GI BMPs generate Volumetric Reduction Credit (VRC) toward this requirement.

3.3  Volumetric Reduction Credit (VRC) by BMP Type

Infiltrating BMPs

Bioretention without impermeable liner (with or without underdrain depending on configuration), infiltration basins, dry wells, and permeable pavement meeting infiltration performance criteria. VRC is computed as the portion of the WQV that infiltrates into native soil during and within the drawdown period. Credit is capped at 100% of the tributary WQV — an oversized infiltrating BMP does not generate surplus credit beyond its drainage area's WQV.

Evapotranspirative BMPs

Green roofs and bioretention systems where ET is the primary volume reduction mechanism. VRC = volume of stormwater removed by ET during and after the design storm event. Chapter 14 provides default ET credit values by BMP type; design engineers may use site-specific ET calculations supported by regional meteorological data and crop coefficient methods.

Rainwater Harvesting Systems

Cisterns, rain barrels, and graywater reuse systems. VRC = volume of stormwater beneficially reused annually. Chapter 14 requires demand-side calculations demonstrating consistent beneficial reuse — systems that are rarely emptied before the next storm event do not receive full VRC credit. The designer must document expected demand schedules (irrigation, toilet flushing, etc.).

3.4  VRC Compliance Table: Required Submission Format

Chapter 14 requires that all major development stormwater permit submissions include a formal Volumetric Reduction Compliance Table. The required fields are:

Example only — illustrative BMP IDs. The 2026 standard requires cumulative VRC ≥ total project WQV. In this example, the project is deficient and must either add GI BMP capacity or convert D-1 to a GI-compliant configuration via the Chapter 8 retrofit pathway.

Section 4

Integration of New and Reorganized Content in the 2026 Manual

4.1  The Chapter 12–13–14 Sequential Design Workflow

The 2026 manual's most important structural improvement is the formalization of a three-chapter sequential workflow for subsurface GI BMP design. Each chapter answers a distinct question at a distinct project phase:

In the 2023 manual, steps 1 and 2 existed but step 3 had no dedicated chapter — meaning engineers assembled volumetric compliance accounting from disparate regulatory and manual sources. The 2026 structure creates a complete evidence chain from site feasibility through permit compliance.

4.2  GI / Non-GI Classification System Codification

The 2026 manual's Chapter 14 codifies the GI BMP classification list with formal 2026 terminology aligned to N.J.A.C. 7:8 definitions. The 2023 edition treated GI vs. Non-GI classification as largely implicit. 2026 GI BMP list (generates VRC credit):

• Bioretention — without impermeable liner; VRC method depends on underdrain configuration
• Infiltration basins (Chapter 10.2)
• Dry wells (Chapter 10.x)
• Cisterns and rainwater reuse systems
• Green roofs (ET credit method)
• Grass swales (when meeting 2026 infiltration performance criteria)
• Permeable pavement (when meeting 2026 infiltration performance criteria)

Non-GI BMPs — extended detention basins, sand filters with impermeable liners, constructed wetlands over impermeable subgrade, blue roofs functioning as detention only — satisfy the Water Quality Standard and Quantity Standard but generate zero VRC toward the GI Requirement.

4.3  Cross-References Established in 2026 Chapter 6

The 2026 Chapter 6 (Stormwater Management Standards — Engineering Design) now formally cross-references Chapters 12, 13, and 14 as required design inputs for all GI BMP designs. The 2023 Chapter 6 cross-referenced Chapters 12 and 13 but had no Chapter 14 equivalent to cite. This tightens the documentation chain from the design standards chapter to the feasibility investigation and volumetric compliance demonstration.

Section 5

Implications for Stormwater Engineering Practice

5.1  Licensed Soil Scientist Now Effectively Required

The 2026 Chapter 12 LPSS requirement is the most structurally significant practice change. Project planning schedules must now account for:

• Engaging an LPSS early in design — during preliminary design or pre-application phase
• Scheduling wet-season borings (November–April) aligned to the proposed BMP layout at design grade
• Coordinating boring locations with civil grading design to ensure tests are at correct future grade elevations, not existing surface
• Incorporating the LPSS report and certification into the stormwater permit submission

This adds cost and schedule to early design phases but reduces the risk of permit denial or post-construction performance failures rooted in inaccurate SHWT assumptions made without field verification.

5.2  Mounding Analysis Becomes a Documented Submission Item

The 2026 Chapter 13 documentation requirement elevates groundwater mounding analysis from an internal design check to a reviewable permit submission item. Design engineers must retain the Hantush analysis worksheet with all inputs traceable to field test data. Reviewing authorities can now specifically request and evaluate the mounding analysis as a named SWM Report subsection — creating accountability that did not exist under the 2023 standard.

5.3  Volumetric Reduction Must Be Tabular and Explicit

The Chapter 14 VRC compliance table requirement changes the submission standard for all major stormwater projects. A narrative description of GI BMP types or a general statement that the GI Requirement is met is no longer sufficient. The table format requires BMP-by-BMP accounting that survives line-item review — matching the existing standard for TSS removal compliance documentation.

5.4  Summary Comparison: 2023 vs. 2026 Practice Workflow