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Groundwater Recharge Rules

Regulatory Summary

N.J.A.C. 7:8-5.4 (referenced as N.J.A.C. 7:8-5.2 in the recharge design context) requires that major development projects maintain pre-development groundwater recharge volumes on an annual basis. The standard applies to all projects meeting the major development thresholds — one acre of disturbance or 0.25 acres of new impervious surface.

The core requirement is straightforward: calculate the annual recharge volume under pre-development conditions, calculate it under post-development conditions, and provide on-site infiltration capacity sufficient to offset the deficit.

Recharge Calculation Framework

The recharge deficit is calculated as:

Rev = Sum[ (Rrate_pre x A_pre) - (Rrate_post x A_post) ]

Where:

  • Rrate = annual recharge rate (inches/year) from BMP Manual Table 6-1, classified by Hydrologic Soil Group (HSG) and land use
  • A = contributing area for each land use category
  • Rev = annual recharge volume deficit that must be provided by infiltration-based BMPs

Impervious surfaces receive a recharge rate of zero regardless of slope or adjacent pervious area.

Recharge Feasibility by Soil Type

HSG Infiltration Rate Recharge Feasibility Notes
A > 0.30 in/hr Excellent Full recharge systems typically feasible
B 0.15 - 0.30 in/hr Good Bioretention and pervious pavement generally effective
C 0.05 - 0.15 in/hr Limited Extended drawdown periods; may need oversized BMPs
D < 0.05 in/hr Poor Technical infeasibility finding typically required
A/D, B/D (dual) Variable Seasonal D-rated parent soil governs; evaluate seasonal saturation

2023 vs. 2026: Key Changes

Topic July 2023 January 2026
Recharge credit eligibility GI practices generally; some ambiguity on ET-only practices GI-only recharge credit — only infiltration-based GI practices qualify; ET-only practices (green roofs, blue roofs) excluded
Green roof recharge credit Ambiguous — some practitioners credited toward recharge Explicitly excluded unless paired with infiltration component
Mounding analysis trigger Soils with infiltration rates > 2.99 in/hr Expanded trigger; structural setback analysis added for sites adjacent to basements or below-grade utilities
Table 6-1 recharge rates 2023 values Updated for several HSG/land-use combinations (turf/lawn on HSG A revised upward)
Soil investigation protocol General guidance; no formal protocol Formal Soil Investigation Protocol with minimum boring counts, depths, and testing methods
SHWT determination General requirement Requires field borings with redoximorphic feature documentation by qualified professional; SSURGO screening only
Net recharge definition Not explicitly defined Clarified as volume reaching the water table, excluding ET losses from practice media or vegetation

GI-Only Recharge Credit (2026)

The 2026 amendment resolves a significant ambiguity from the 2023 rules. Only GI practices that achieve infiltration into native soils qualify for recharge volume credit. Practices that reduce volume solely through evapotranspiration (such as standalone green roofs) do not contribute to the recharge standard, even though they may receive volumetric reduction credit under the water quality standard.

This distinction means that a project relying on green roofs for water quality compliance must still provide separate infiltration-based practices (bioretention, dry wells, infiltration basins, pervious paving) to satisfy the recharge deficit.

Engineering Interpretation

Design Sequence for Recharge Compliance

  1. Characterize soils — Obtain HSG classification from NRCS mapping; confirm with field borings
  2. Calculate pre-development recharge — Apply Table 6-1 rates to existing land use areas
  3. Calculate post-development recharge — Apply Table 6-1 rates to proposed land use areas (impervious = 0)
  4. Determine recharge deficit — Subtract post from pre; this is the design volume
  5. Select infiltration-based GI BMPs — Only practices without impermeable liners that infiltrate into native soil
  6. Verify SHWT separation — Minimum 2 feet from BMP bottom to seasonal high water table for all practice types
  7. Check setbacks — Foundations (10 ft), wells (25 ft), septic (50 ft), property lines (10 ft)
  8. Perform mounding analysis — Required for high-permeability soils (>2.99 in/hr) and sites near basements/below-grade utilities under 2026 rules
  9. Apply K_sat safety factor — Design saturated hydraulic conductivity = geometric mean of tested values x 0.50

Soil Investigation Protocol (2026)

The 2026 edition establishes minimum requirements that did not exist under the 2023 rules:

Requirement 2026 Standard
Borings per BMP Minimum 1 per 5,000 sq ft of BMP footprint
Boring depth Minimum 72 inches below proposed BMP bottom
SHWT confirmation Field redoximorphic features by licensed soil scientist or qualified PE
K_sat testing At least 1 in-situ falling-head or constant-head test per BMP
Design K_sat Geometric mean of tested values with 50% safety factor applied
Acceptable test methods In-situ falling head permeameter or ASTM D5126 (piezometer method)
Prohibited methods Estimated rates from soil texture or pedon descriptions (screening only)

SHWT Separation Requirements

All infiltrating BMPs require a minimum 2 feet of vertical separation between the BMP bottom (bottom of aggregate layer) and the seasonal high water table:

  • Bioretention cells (without underdrain): 2 ft
  • Infiltration basins: 2 ft
  • Dry wells: 2 ft from bottom of gravel reservoir
  • Pervious pavement systems: 2 ft from bottom of gravel reservoir

Technical Infeasibility

Where site conditions preclude infiltration (HSG D soils, high SHWT, contaminated soils, bedrock, stormwater hotspot classification), a technical infeasibility finding may be documented. The engineer must provide supporting soil data and constraint analysis — not a generic statement. Infiltration is prohibited entirely at stormwater hotspot locations (fueling stations, vehicle maintenance yards, hazardous material storage areas).

BMP Implications

Only BMPs that infiltrate into native soil without impermeable liners qualify for recharge credit:

Practice Recharge Credit Key Constraint
Bioretention (no liner, no underdrain) Yes Requires HSG A or B soils; 2 ft SHWT separation
Bioretention (with underdrain) Partial Credit only for volume infiltrated before underdrain activation
Infiltration Basin Yes Large footprint; requires suitable soils
Dry Well Yes Residential scale; requires clean runoff (rooftop)
Pervious Paving Yes Maintenance-dependent; surface clogging risk
Green Roof (standalone) No ET-only; does not reach water table
Cistern/Rain Barrel No Reuse, not infiltration
Wet Pond No Non-GI; no infiltration
Sand Filter w/ Underdrain No Non-GI; discharge via underdrain
MTD No Non-GI; flow-through treatment only

Liner prohibition is absolute — any practice counting toward recharge credit must not use an impermeable liner. Specifications must explicitly state "no impermeable liner" to satisfy reviewer documentation requirements.

Key BMP Pages

Cross References