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Dry Well

Source: NJ Stormwater BMP Manual, Chapter 9, Section 9.2 (2026)

Dry wells are subsurface infiltration structures — stone-filled trenches or preformed plastic chamber systems — installed below grade that receive concentrated stormwater inflow from individual roof drains or small impervious surfaces and disperse it into the surrounding native soil. They are among the simplest and most compact GI BMPs and are commonly used for residential downspout disconnection and small commercial applications where surface-level BMPs cannot be accommodated.

Unlike bioretention or infiltration basins, dry wells receive concentrated inflow from a single discharge point rather than sheet flow from a broader tributary area. Their function depends entirely on the capacity of the surrounding native soil to accept infiltrated volume at the rate it is applied.

Primary stormwater functions:

  • Groundwater recharge — primary function; all captured volume infiltrates into native soil
  • Volumetric reduction — generates VRC toward GI Requirement when native infiltration confirmed
  • Runoff volume reduction at the source — intercepts rooftop runoff before it enters the conveyance system

When engineers choose this BMP:

Dry wells are selected for residential or small commercial sites with confirmed permeable soils (HSG A or B; Ksat ≥ 0.52 in/hr), a water table well below the dry well bottom, and limited surface area for bioretention or infiltration basins. They serve effectively as distributed rooftop capture elements within a broader treatment train.

Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.2 (2026)

Parameter 2026 Requirement 2023 Requirement Notes
Receiving area Rooftop drainage from residential/small commercial downspouts Same Not designed for impervious pavement runoff
Storage volume Sized to store WQV from tributary roof area Same Based on roof area and 1.25-inch WQV rainfall depth
Minimum Ksat ≥ 0.52 in/hr; field-tested per Chapter 12 ≥ 0.5 in/hr Soil survey data not acceptable for GI credit
Minimum SHWT separation 2 ft below dry well bottom elevation 2 ft LPSS-certified boring required
Drawdown time ≤ 72 hours ≤ 72 hours Confirm under wet antecedent conditions
Setback from foundation walls ≥ 10 ft minimum ≥ 10 ft Prevents saturated soils adjacent to structure
Setback from septic systems ≥ 10 ft minimum Same Check local health code for greater setbacks
Groundwater mounding Required per Chapter 13 if installation pattern or footprint triggers Required Typically triggered on larger multi-unit clusters

2026 Update: The 2026 edition explicitly requires a Chapter 12 Ksat field investigation at the proposed dry well bottom elevation for all GI-qualifying installations. Estimation from soil survey data alone is not acceptable for VRC credit.

Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.2; Ch. 12; Ch. 13 (2026)

Soil Permeability

  • Dry wells require native soil Ksat ≥ 0.52 in/hr — typically HSG A or shallow sandy HSG B soils
  • In HSG C or D soils, dry wells will not draw down within 72 hours and should not be used
  • Field investigation per Chapter 12 (double-ring infiltrometer or permeameter) required
  • See Soil Permeability Testing

Seasonal High Water Table (SHWT)

  • Minimum 2 ft separation between dry well bottom and SHWT required
  • SHWT confirmed by soil boring; certified boring log required for permit submissions
  • See Seasonal High Water Table

Setbacks

  • ≥ 10 ft from any structure with a basement or crawlspace foundation
  • ≥ 10 ft from septic system components (leachfield, tank); check local health code
  • ≥ 25 ft from potable water supply wells (standard; verify with NJDEP)
  • Not suitable in karst terrain — subsurface voids prevent predictable infiltration
  • Not suitable on contaminated fill sites where infiltration would mobilize pollutants

Receiving Area Limitation

  • Dry wells are appropriate only for clean roof runoff; not designed to treat runoff from parking lots, driveways, or high-pollution loading areas
  • First-flush diverter recommended to exclude initial rooftop washoff

Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.2; Ch. 12 (2026)

Annual Inspection

  • Inspect inlet downspout connection and splash pad for erosion or disconnection
  • Confirm that water is not pooling at the ground surface adjacent to the dry well after rainfall events — surface ponding indicates infiltration has slowed below design rate
  • Inspect cleanout pipe (if installed) for debris and access functionality
  • Check that overflow connection (if provided to storm sewer or grassed surface) is clear

Sediment and Debris Management

  • Install and maintain a leaf screen or first-flush diverter on the downspout to prevent leaf litter, organic debris, and roof granules from entering the dry well
  • Replace first-flush diverter cartridge or clean device annually or per manufacturer recommendations
  • If a cleanout standpipe is present, vacuum or flush periodically to prevent organic accumulation in the stone fill chamber

Performance Monitoring

  • If surface ponding or slow drainage is observed, test drawdown by introducing a measured volume of water and timing drainage — compare to design drawdown rate
  • Progressive failure of dry well infiltration capacity is typically not recoverable; excavation and stone replacement may be required at end of service life (10–25 years)

Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 9, Section 9.2 (2026)

Design Errors

  • Soil permeability not field-tested — dry well sized based on soil survey Ksat estimate; actual in-situ permeability lower than assumed; system fails to draw down within 72 hours from first rainfall season
  • SHWT separation not verified — seasonal water table rise into the dry well bottom eliminates infiltration capacity during wet seasons; year-round performance not achievable
  • First-flush diverter not included — roof granules, organic debris, and fine particles enter stone fill and progressively clog void spaces

Construction Issues

  • Stone fill compacted or contaminated during installation — void ratio reduced; storage volume and permeability below design specification
  • Inlet pipe connected at incorrect invert elevation — overflow activated prematurely; full storage volume not utilized
  • No cleanout access installed — maintenance cannot be performed; performance declines without any diagnostic capability

Long-Term Performance Risks

  • Progressive clogging — fine mineral and organic particles accumulate in stone voids over years; infiltration rate declines progressively; surface ponding occurs at later life stage; limited service life (10–25 years) should be anticipated in O&M planning
  • Root intrusion — tree roots seek water in stone chamber; root masses displace stone fill and reduce storage volume; can damage inlet pipe connections

Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.2 (2026)

Governing Regulations

Rule Section Topic Engineering Relevance
N.J.A.C. 7:8-5.3 Green Infrastructure Requirement Dry well qualifies as GI when native infiltration is confirmed per Ch. 12
N.J.A.C. 7:8-5.4(a) Groundwater Recharge Standard Primary regulatory function of dry wells
N.J.A.C. 7:8-5.3(d) Water Quality Treatment Volume credit for WQV infiltrated

BMP Manual Sources

  • NJ Stormwater BMP Manual, Chapter 9, Section 9.2 (2026) — Dry Wells
  • NJ Stormwater BMP Manual, Chapter 12 (2026) — Soil Testing and Ksat Investigation
  • NJ Stormwater BMP Manual, Chapter 13 (2026) — Groundwater Mounding Analysis
  • NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance

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