Seasonal High Water Table (SHWT)¶

The Seasonal High Water Table is the highest elevation that groundwater reaches during the wet season (typically late winter through early spring in New Jersey). SHWT depth directly controls whether infiltration-based BMPs can be used at a site and how they must be designed.

Source: NJ Stormwater BMP Manual, Chapters 6, 9, 12; N.J.A.C. 7:8-5.4

Concept Explanation¶

Groundwater levels fluctuate seasonally in response to precipitation, evapotranspiration, and regional aquifer conditions. The SHWT represents the worst-case (shallowest) depth to groundwater that a site experiences on a recurring annual basis. It is the controlling elevation for infiltration BMP design because:

A BMP that functions well when the water table is deep may fail when the water table rises seasonally to within inches of the practice bottom
Saturated conditions beneath a BMP eliminate the unsaturated zone needed for pollutant attenuation through filtration and adsorption
Standing water in the root zone kills vegetation, destroying the biological treatment capacity of planted systems like bioretention

The 2-Foot Separation Requirement¶

Both the 2023 and 2026 editions of the NJ BMP Manual require a minimum 2-foot vertical separation between the bottom of any infiltrating BMP and the SHWT. This applies uniformly to:

BMP Type	Separation Measured From
Bioretention (without underdrain)	Bottom of aggregate/storage layer to SHWT
Infiltration basins	Basin floor to SHWT
Dry wells	Bottom of gravel reservoir to SHWT
Pervious pavement	Bottom of gravel subbase reservoir to SHWT

The 2026 edition consolidated these requirements into a single reference table (Table 6-2), replacing scattered references across multiple chapters in the 2023 edition. The consolidation eliminates prior inconsistencies in how reviewers and designers interpreted the separation standard for different BMP types.

Why 2 feet? The unsaturated zone between the BMP bottom and the water table serves three functions: (1) it provides additional filtration and pollutant attenuation as water percolates downward, (2) it acts as a buffer against groundwater mounding during storm events, and (3) it prevents the BMP from becoming a groundwater discharge point (where the water table rises into the practice, reversing flow).

Engineering Evaluation¶

Field Investigation Methods¶

SHWT determination requires site-specific field investigation. The 2026 BMP Manual specifies two primary methods and explicitly limits the role of desktop data:

Method 1: Soil Borings with Redoximorphic Feature Identification

This is the standard and most commonly used method. A licensed soil scientist or qualified professional examines the soil profile in test pits or borings for redoximorphic features — color patterns (mottles, gleying, oxidized root channels) that indicate the historic extent of seasonal saturation.

Investigation Element	2026 Requirement
Who performs the work	Licensed soil scientist or PE with demonstrated geotechnical training
Minimum boring depth	72 inches below proposed BMP bottom
Documentation	Boring logs with soil descriptions, Munsell color notation, redox feature depths
Number of borings	Minimum 1 per BMP footprint up to 5,000 sq ft; +1 per additional 5,000 sq ft

Redoximorphic features are the definitive indicator of SHWT because they reflect long-term (decades to centuries) patterns of saturation, not a single season's conditions.

Method 2: Monitoring Wells

Monitoring wells provide direct, real-time measurement of groundwater elevation but must be observed over a sufficient period (ideally one full wet season) to capture the seasonal peak. Monitoring wells are most useful for:

Sites where redoximorphic features are ambiguous (e.g., fill soils, disturbed profiles)
Projects where the designer needs to confirm that SHWT is lower than redox features suggest (redox features can persist long after drainage improvements have lowered the water table)
Detailed hydrogeologic investigations for large infiltration systems

Desktop Screening (SSURGO / Web Soil Survey)

The NRCS SSURGO database and Web Soil Survey provide estimated SHWT depths by soil map unit. The 2026 BMP Manual explicitly permits SSURGO data for screening purposes only. Design confirmation requires field borings. Submittals that rely solely on SSURGO estimates for SHWT depth are non-compliant under the 2026 standards.

Evaluating Results¶

Once SHWT is established, the engineer compares it to the proposed BMP bottom elevation:

Available separation = Ground surface elevation
                     - BMP excavation depth (ponding + media + aggregate)
                     - SHWT elevation

If available separation >= 2 ft --> Infiltration feasible (subject to other constraints)
If available separation < 2 ft  --> Infiltration infeasible at this depth
                                    Options: raise BMP, use underdrain, select alternative BMP

Affected BMPs¶

SHWT is a go/no-go constraint for all infiltration-based practices:

BMP	SHWT Impact	Design Response	Link
Small-scale bioretention	2 ft separation required; if not met, must add underdrain (loses recharge credit)	Raise practice or switch to underdrain design	Bioretention
Large-scale bioretention	Same 2 ft requirement; larger excavation depths make SHWT conflicts more common	Underdrain with IWZ may partially preserve recharge	BMP Library
Infiltration basins	Deep practices most vulnerable to SHWT conflicts; require favorable conditions	Relocate or abandon infiltration approach	BMP Library
Dry wells	Point-source infiltration; 2 ft separation from bottom of gravel reservoir	Reduce depth or relocate	BMP Library
Pervious pavement	Subbase reservoir depth adds to total excavation; SHWT can limit reservoir volume	Reduce reservoir depth and add underdrain	BMP Library

Non-infiltrating BMPs (extended detention basins, wet ponds, manufactured treatment devices) are not directly constrained by SHWT because they do not rely on percolation into native soils. However, shallow SHWT can affect construction dewatering requirements and long-term structural stability of embankments.

N.J.A.C. 7:8 Rule Citations¶

Rule Section	Topic	Relevance to SHWT
N.J.A.C. 7:8-5.4(a)	Groundwater Recharge	Recharge credit requires infiltration; SHWT < 2 ft from BMP bottom prohibits infiltration
N.J.A.C. 7:8-5.3	Green Infrastructure Requirement	Shallow SHWT is a documented site constraint that supports a GI infeasibility finding
N.J.A.C. 7:8-5.2	Soil Testing (2026)	SHWT determination procedures specified; qualified professional standard required

BMP Manual Chapter Citations¶

Chapter 12 — Soil Testing Criteria: Defines SHWT determination methods, boring requirements, qualified professional standards, and the prohibition on SSURGO-only documentation for design
Chapter 6 — Groundwater Recharge: Establishes the 2-foot separation requirement as a precondition for recharge credit; Table 6-2 (2026) consolidates separation standards across all BMP types
Chapter 13 — Groundwater Mounding: SHWT is the starting water table elevation for mounding analysis; mounding that reduces effective separation below 2 ft is a failure condition
Chapter 9, Section 9.7: References SHWT as a siting constraint for small-scale bioretention