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Groundwater Mounding

Groundwater mounding is the temporary rise in the groundwater table that can occur beneath an infiltration BMP. In the live source corpus, the governing framework is N.J.A.C. 7:8 together with BMP Manual Chapter 13, not an informal set of numeric screening triggers.

Structured citation evidence for this page lives in the sibling claim manifest.

Concept Explanation

When a BMP infiltrates stormwater into subsoil, the groundwater surface beneath the BMP may rise faster than groundwater can disperse laterally. If that rise is large enough, it can:

  • reduce the effective separation to the seasonal high water table
  • interfere with the BMP's own drain-time performance
  • contribute to surficial ponding, basement impacts, or conflicts with subsurface structures

That is why the regulation requires hydraulic-impact review for infiltration BMPs rather than treating mounding as a narrow specialty check.

Regulatory Trigger

The source-backed regulatory trigger is broad. N.J.A.C. 7:8 requires hydraulic-impact assessment whenever the stormwater management design includes one or more BMPs that infiltrate stormwater into subsoil.

The live citations layer also resolves a common error: the regulation does not codify a universal mounding trigger such as 2.99 in/hr, SHWT within 4 feet, or 5,000 cubic feet per event. Those numbers show up in legacy summaries, not in the governing NJAC trigger text.

Chapter 13 Analysis Framework

BMP Manual Chapter 13 uses the Department's Hantush Spreadsheet, derived from the USGS groundwater-mounding method, as the standard analysis tool for typical cases. The chapter frames the analysis around:

  • recharge rate
  • basin or BMP bottom footprint dimensions
  • initial saturated thickness
  • horizontal hydraulic conductivity
  • specific yield
  • infiltration duration

The spreadsheet is used to estimate the maximum mound height and the lateral extent of the mound so the designer can evaluate whether the proposed configuration creates an adverse hydraulic impact.

Reference datum

The source-backed reference datum is the static seasonal high water table. Chapter 13 states that the spreadsheet sets the SHWT at elevation zero and measures the temporary groundwater rise upward from that datum.

That is the important correction to keep in mind when reviewing older summaries: the current evidence does not support a distinct 2026 "mound apex" reference rule.

Permeability input

Chapter 13 also aligns the mounding analysis with the design-permeability framework used elsewhere in the BMP Manual. The recharge rate used in the spreadsheet is tied to the design permeability rate, which applies the factor of safety of 2.0 to the tested permeability.

The chapter also caps the infiltration duration used in the spreadsheet at 72 hours. If a proposed configuration cannot avoid adverse hydraulic impacts or cannot satisfy the drain-time framework, the design has to be reworked rather than justified with unsupported shortcut thresholds.

Chapter 13 also constrains two other inputs that older summaries often skipped. Specific yield defaults to 0.15; values above 0.15 have to be supported by soil testing from the substratum below the BMP and cannot exceed 0.2 in the spreadsheet. Horizontal hydraulic conductivity is also not a free-form guess: Chapter 13 assumes Kh = 5R in the Coastal Plain and Kh = R outside the Coastal Plain, where R is the recharge-rate input used in the spreadsheet.

Engineering Implications

From a design-review perspective, groundwater mounding is most relevant wherever runoff is intentionally infiltrated into the subsoil, including:

  • infiltration basins
  • dry wells
  • infiltrating bioretention systems
  • infiltrating pervious pavement systems
  • other infiltration BMPs or BMP trains that discharge into subsoil

If the analysis shows that the mound would create adverse hydraulic impacts, the typical response is to redesign the BMP footprint, infiltration depth, infiltration rate assumptions, or BMP selection rather than treating the site as automatically acceptable.

Governing source sections

  • N.J.A.C. 7:8-5.2(h) for the hydraulic-impact requirement
  • BMP Manual Chapter 12 for soil testing inputs
  • BMP Manual Chapter 13 for the groundwater-mounding method and Hantush Spreadsheet use