Volumetric Reduction Credit (VRC)¶

Regulatory Summary¶

Regulatory Basis¶

The Volumetric Reduction Credit (VRC) is established under N.J.A.C. 7:8-5.5 (2026 edition). It allows stormwater management systems incorporating qualifying Green Infrastructure (GI) BMPs to receive credit toward the water quality volume (WQV) groundwater recharge, and channel protection requirements by demonstrating actual volume reduction through infiltration or evapotranspiration — rather than treating the full design volume through a water quality structure.

How VRC Works¶

Standard stormwater compliance requires treatment of the full Water Quality Volume (WQV) — the runoff generated by the 1.25-inch design storm (80th percentile) from all impervious surfaces.

When a GI BMP infiltrates or evapotranspires a portion of that runoff, the volumetrically reduced portion does not need to pass through a water quality treatment structure. The compliance pathway shifts from treatment to volume elimination.

Compliance Formula¶

WQV = Total WQV from impervious area
VRC = Volume infiltrated/evapotranspired by GI BMP
Remaining WQV requiring treatment = WQV − VRC

If the GI BMP infiltrates 100% of the WQV, no additional water quality treatment structure is required (subject to groundwater recharge and channel protection checks).

Engineering Interpretation¶

GI BMP Eligibility for VRC¶

Only BMPs that function through native soil infiltration or evapotranspiration qualify. The following BMPs earn full or partial VRC:

BMP	VRC Status	Condition
Bioretention	Full VRC	Native infiltration ≥ 0.52 in/hr confirmed by Ch. 12 testing
Infiltration Basin	Full VRC	Ch. 12 required; no liner
Infiltration Trench	Full VRC	Ch. 12 required; no liner; OGS pretreatment for parking
Dry Well	Full VRC	Ch. 12 required; rooftop drainage only
Pervious Pavement	Full VRC	Ch. 12 required; native infiltration below subbase
Green Roof	Partial VRC	ET-based; credit proportional to media depth and ET rate
Cistern	Partial VRC	Credit for pre-storm void only; inter-event depletion analysis required
Grass Swale	Partial VRC	Only where native infiltration occurs along channel bottom
Extended Detention (Ch. 10)	Partial VRC	Only the infiltrated fraction; requires Ch. 12 and no liner

Non-GI BMPs (all Ch. 11 BMPs, and Ch. 9/10 BMPs constructed over liners) earn no VRC regardless of treatment performance.

Conditions Required to Claim VRC¶

To claim VRC for a GI BMP, the following conditions must be met:

Ch. 12 soil characterization — Ksat ≥ 0.52 in/hr measured in the field at the BMP footprint; Ch. 12 testing protocol required.
SHWT separation — Seasonal high water table must be ≥ 2 ft below the bottom of the infiltration surface (or media layer for bioretention/green roof).
No liner — Any impermeable liner eliminates VRC for the lined BMP.
Ch. 13 groundwater mounding analysis — Required for infiltration BMPs > 1 ac drainage area or where SHWT separation is between 2–4 ft.
Design volume documentation — The engineer must calculate and document the infiltrated volume that forms the basis of the credit.

BMP Implications¶

Groundwater Recharge Credit¶

GI BMPs earning VRC simultaneously satisfy the groundwater recharge requirement under N.J.A.C. 7:8-5.4 for the infiltrated fraction. No separate recharge calculation is needed for the volume covered by VRC.

Design Implication¶

VRC is the primary mechanism through which GI BMPs reduce or eliminate the need for conventional water quality treatment structures (sand filters, wet ponds, MTDs). Projects maximizing GI coverage reduce total impervious runoff routed to treatment, which:

Reduces required WQV treatment structure size
Reduces long-term maintenance burden
Supports groundwater recharge compliance

Cross References¶

N.J.A.C. 7:8-5.5 — Volumetric Reduction Credit
N.J.A.C. 7:8-5.4 — Groundwater Recharge
N.J.A.C. 7:8-5.3 — Water Quality Standards
NJ Stormwater BMP Manual, Chapter 12 — Soil Testing for Infiltration
NJ Stormwater BMP Manual, Chapter 13 — Groundwater Mounding Analysis