Code of Maine Rules › 06 - DEPARTMENT OF ENVIRONMENTAL PROTECTION › 096 - DEPARTMENT OF ENVIRONMENTAL PROTECTION - GENERAL › Chapter 500 - STORMWATER MANAGEMENT › Appendix 096-500-D - Infiltration basins, dry wells, and subsurface fluid distribution systems

Current through 2024-29, July 17, 2024
Any project proposing infiltration of stormwater must provide
				  adequate pre-treatment of stormwater prior to discharge of stormwater to the
				  infiltration area, or provide for treatment within the infiltration area.
				  Pre-treatment systems provide for removal of a portion of pollutant load before
				  the runoff enters the infiltration area. The infiltration area must minimize
				  discharge of soluble pollutants to groundwater, and must be maintained to
				  assure that its capacity for infiltration and pollutant removal is unimpaired.
				  The Department will determine the adequacy of a pre-treatment system or
				  treatment system based on its assessment of factors including but not limited
				  to: nature of activities with exposure to stormwater within the drainage area
				  of the infiltration system; area to be treated; proximity of drinking water
				  wells; and site conditions, including soils, depth to high groundwater table
				  and depth to bedrock. All drywells and subsurface fluid distribution systems
				  must be registered with and meet all other requirements of the department's
				  Underground Injection Control Program.
Lack of appropriate pollutant removal best management
				  practices (BMPs) may result in violations of the groundwater quality standard
				  established by
				   38
				  M.R.S.§465-C(1). Many
				  pollutants found in stormwater accumulate in the soils in infiltration areas
				  and are released due to chemical changes that occur in the infiltration area
				  over time. Consequently, runoff quality often underestimates the long-term
				  adverse effects on groundwater quality due to these contaminants, and cannot be
				  used as a direct indicator of anticipated adverse effects. Maintenance of the
				  infiltration area may be required to prevent clogging, development of anaerobic
				  conditions, or other conditions that could impair the functioning of the area
				  or increase the risk of pollutant discharge from the infiltration area.
Provided that the standards in this appendix are met, a
				  discharge to groundwater from a stormwater infiltration system will not have a
				  significant effect on the quality or classification of waters of the state and
				  is considered a
de minimis
discharge that is exempt from the
				  waste discharge licensing requirement of the Waste Discharge Licensing Program
				  pursuant to
				   38 M.R.S.
				  §413(1-A). However,
				  nothing in this Chapter may be construed to limit the Department's licensing or
				  enforcement authority under 38 M.R.S. Articles 4-A or 6. The Department may
				  make a site-specific determination that discharges from a site will be
de minimis
in certain cases where the separation from bedrock
				  and seasonal high water table cannot be met and potential discharges of
				  pollutants to groundwater are absent or minimal. All other requirements of this
				  Appendix must be met, and this
de minimis
finding does not
				  constitute a permit shield or other exemption from the requirements of 38
				  M.R.S. Articles 4-A and 6. Regardless of this determination, the Department may
				  require monitoring of water levels or water quality to demonstrate compliance
				  with applicable requirements of 38 M.R.S. Articles 4-A and 6, and other
				  requirements of this Chapter.
NOTE
: Stormwater infiltration systems not
				  meeting the standards described in Appendix D may require a waste discharge
				  permit. An infiltration system serving a development regulated under the
Site Location of Development Act
may be required to meet
				  additional standards. For definitions and provisions associated with the Waste
				  Discharge program, see
				   38 M.R.S.A.
				  §§413
et seq.
, and
				  Department Rules chapters 520
et seq
.
1. Definitions.
As used in this appendix, the
				  following terms have the following meanings.
(a) Drywell. A well or other facility deeper than it is wide,
				  completed above the water table so that its bottom and sides are typically dry
				  except when receiving fluids.
(b) Infiltration basin. A structure wider than it is deep and
				  designed to hold runoff without any means of discharge other than
				  evapotranspiration, infiltration, or emergency bypass.
(c) Non-stormwater discharges. Any discharge to a stormwater
				  management system that is not composed entirely of stormwater, other than
				  discharges directly resulting from fire-fighting at the facility.
				  Non-stormwater discharges can originate from direct connections to industrial,
				  commercial, or residential facilities, or indirectly as surface or subsurface
				  discharges to the stormwater collection system.
(d) Public water supply. Any publicly or privately-owned
				  water-supply system that serves at least 25 people or 15 service connections
				  for at least 60 days per year.
(e) Subsurface fluid distribution system. Any system designed
				  to dispose of stormwater beneath the surface of the earth, including, but not
				  limited to, wells, settling tanks, disposal fields, pretreatment filters,
				  pipes, or any other fixture, mechanism, or apparatus used for this
				  purpose.
(f) Zone of contribution or delineated contributing area. The
				  area that contributes water to a water supply well, generally represented as
				  the projection of the three-dimensional volume of water flowing to a
				  discharging well onto a two-dimensional map view.
2. Limitations on the use of infiltration for
				  stormwater treatment
(a) Storage or handling of petroleum products, pesticides,
				  fertilizers, and hazardous substances. Infiltration of runoff from a draining
				  area on which petroleum products, pesticides, fertilizers, hazardous
				  substances, or other materials with the potential to contaminate groundwater
				  are stored or handled, is not allowed unless containment structures are used to
				  isolate these materials from precipitation and provided that they are operated
				  in accordance with a spill prevention, control, and countermeasure plan;
				  operation and maintenance plan; or equivalent document as required by Section
				  7(D)(5)(c)(vi) of
				  this Chapter. This restriction does not apply to storage of heating oil in a
				  tank or tanks with a total volume of 550 gallons or less and serving a single
				  consumptive residential or commercial user.
(b) Storage or handling of road salt or similar materials.
				  Infiltration of runoff from an area on which road salt or similar materials are
				  stored or handled in bulk is not allowed.
(c) Containment structures. Storage and handling areas for
				  petroleum products, road salt, and other potential groundwater contaminants may
				  be isolated within containment structures, buildings, or other enclosures to
				  effectively remove those areas from exposure to stormwater so that infiltration
				  structures may be constructed to serve the remaining drainage area, provided
				  that the facility is operated in accordance with a spill prevention, control,
				  and countermeasures plan; operation and maintenance plan; or equivalent
				  document as required by Section (7)(D)(5)(c)(vi) of this Chapter.
(d) Infiltration of runoff from asphalt or concrete paving or
				  equivalent material. Infiltration of runoff from a total of one acre or more of
				  asphalt or concrete paving or equivalent material at a given project is not
				  allowed except by means of infiltration basins located, designed, operated, and
				  maintained in accordance with this appendix. This limitation does not apply to
				  roads entirely within subdivisions consisting of lots for single-family
				  detached residential housing or to use of pavement alternatives approved by the
				  Department. Use of pavement alternatives is limited by sections
				  2(a) and
				  2(b) of this
				  Appendix, and other requirements of this Chapter.
(e) Infiltration of runoff from lawn areas, vegetated areas,
				  and roofs. Infiltration of runoff from lawn areas and other vegetated areas,
				  playing fields, and roofs of residential and commercial structures where no
				  manufacturing or processing occurs, other than for home-based industries, is
				  allowed, provided that any application of fertilizers, pesticides, and similar
				  turf-management chemicals, is in accordance with a Department-approved
				  management plan and no part of the areas used for infiltration is in the
				  delineated contributing area of a well that is part of a public water supply
				  system. Lawn areas on individual lots that are sold or developed as part of a
				  residential subdivision consisting of lots for single-family detached
				  residential housing are exempt from this requirement.
(f) Non-stormwater discharges, and stormwater from
				  off-sitedrainage systems. Non-stormwater discharges and stormwater from
				  off-sitedrainage systems may not be discharged to an infiltration system,
				  unless the Department determines the additional discharge will not have a
				  negative impact on groundwater quality.
(g) Industrial and vehicle maintenance facilities.
				  Infiltration of runoff from industrial facilities where materials are stored or
				  processed in areas exposed to precipitation and vehicle maintenance facilities
				  where maintenance or storage of equipment occurs in areas exposed to
				  precipitation is not permitted, unless the Department determines the additional
				  discharge will not have a negative impact on groundwater quality.
(h) Certain commercial facilities. Infiltration of runoff
				  from facilities with high use parking lots, drive-up windows, or similar
				  sources of elevated hydrocarbon pollution, is not permitted, unless the
				  Department determines the additional discharge will not have a negative impact
				  on groundwater quality.
(i) Use of vegetated soil filter beds. Discharges to
				  groundwater from infiltrating soil filters as described in Appendix E,
				  subsection
				  4(c)(ii) are
				  considered
de minimis
discharges for the purposes of the Waste
				  Discharge Licensing Program, and do not require a waste discharge
				  license.
3. Location
(a) Drinking water supply wells. Unless specifically approved
				  by the Department in consultation with the Department of Health and Human
				  Services' Drinking Water Program, infiltration systems must be located no less
				  than 300 feet from any private drinking water supply well, must not be located
				  within the delineated contributing area of a public drinking water supply well,
				  and must be located as far downgradient of any drinking water supply well as
				  practical. Department approval of a reduced setback will be subsequent to
				  Department review and approval of a study by a Maine Certified Geologist
				  demonstrating that discharges from the infiltration system will not result in
				  an unreasonable adverse impact on the quality or quantity of water provided by
				  the well or wells. The Department may require a groundwater monitoring program
				  or place other requirements on the design and operation of the infiltration
				  system or infiltrating soil filter for the protection of the water supply well
				  or wells.
(b) Subsurface wastewater disposal systems. Infiltration
				  systems must be located to meet or exceed the minimum setback distances from
				  existing and proposed subsurface wastewater disposal systems provided in Table
				  7B of the Maine Subsurface Wastewater Disposal Rules, 144A CMR 241. Setback
				  distances may be required by the Department, the local plumbing inspector, or
				  the Department of Health and Human Services' Division of Environmental Health.
				  An infiltration system is considered a major watercourse for the purposes of
				  Table 700.2 determining applicable setback requirements of the Maine Subsurface
				  Wastewater Disposal Rules, 144A CMR 241.
Allowance for lesser setbacks for onsite disposal systems or
				  other disposal systems owned or controlled by the applicant may be requested
				  from the Department, the Department of Health and Human Services, and the local
				  plumbing inspector, but must be approved as part of the Department permit.
				  Infiltration systems must be located as far downgradient of any component of a
				  subsurface wastewater disposal system as practical.
(c) Protected natural resources. Infiltration systems must be
				  located no less than 25 feet from any protected natural resources as defined at
				   38 M.R.S.
				  §480-B, other than fragile mountain
				  areas, and must be located as far upgradient of any such resources as
				  practical.
(d) Surface grade. The pre-construction surface grade must be
				  20% or less at the location of the proposed infiltration system.
(e) Separation from bedrock. Infiltration systems serving one
				  acre or more of impervious area must be located in areas with more than five
				  feet of saturated overburden above the bedrock surface, as measured during the
				  seasonal low water table. This restriction does not apply to runoff from areas
				  of non-asphalt roofing on a building or other facility in which no
				  manufacturing or processing occurs, other than for home-based industries.
				  Separation from bedrock and depth to the water table may be demonstrated by
				  means of test pits, borings, or similar invasive explorations, or by
				  non-invasive geophysical methods such as seismic surveys. Thickness of blast
				  rock or similar disturbed or artificially placed material is not counted toward
				  the required thickness of overburden; blast rock or similar material is not
				  considered as bedrock. Demonstration of a continuous in-situ layer, at least
				  five feet in thickness, of unfractured basal till or marine, estuarine, or
				  lacustrine clay between bedrock and the infiltration structure may substitute
				  for this requirement, at the discretion of the Department.
4. Design and operation
(a) Soil permeability. The permeability of the soil at the
				  depth of the base of the proposed infiltration system must be no greater than
				  2.41 inches per hour, unless a medium with equivalent pollutant removal
				  properties is approved by the Department. Infiltration basins must be lined
				  with a suitable soil filter material, which must be installed over the base and
				  sides of the proposed infiltration system. The soil filter material must be
				  fine enough to filter fine sediments and provide effective absorption of
				  pollutants, but coarse enough to slowly drain the stored stormwater within a 24
				  to 48 hour period. The soil filter material must be well blended and graded and
				  must contain sufficient organic matter to facilitate the removal and treatment
				  of hydrocarbons. Unless otherwise approved by the Department on a case-by-case
				  basis, the soil filter must be at least 18 inches thick, and the surface area
				  of the filter must be no less than 5% of the contributing impervious drainage
				  area. The Department may consider alternative lining materials, such as
				  materials with higher organic content, provided that the applicant can
				  demonstrate that contact time, pollutant removal, and other relevant features
				  of the liner are at least as protective of groundwater quality. The Department
				  may require monitoring of groundwater quality to determine the effectiveness of
				  any treatment.
(b) Vegetation of infiltration basins. All areas of the basin
				  not covered by stone or other non-vegetative covers must be maintained as
				  grass.
(c) Separation from seasonal high water table. The bottom of
				  the infiltration system, including any stone layer or other material below the
				  depth of any manufactured components of the system, must be at least three feet
				  above the elevation of the seasonal high water table.
(d) Time for drainage. The infiltration system, including the
				  full thickness of the soil filter bed, must be designed to drain completely
				  within 24 to 48 hours following the runoff event. The applicant must provide
				  information on the permeability of the native soils at the depth of the
				  proposed basin, such as in-place well or parameter testing, analyses of soil
				  gradation, or other means acceptable to the Department, indicating that the
				  permeability of the native soils is suitable to allow such drainage, and the
				  Department may require an analysis demonstrating that the level of water table
				  mounding under the system will be below the bottom of the soil filter within 48
				  hours after the end of the storm event.
(e) Impact on depth to groundwater. Infiltration of
				  stormwater may not increase the elevation to the seasonal high water table
				  beneath a surface-irrigation site, land-disposal area for septage or other
				  waste, or other waste management or wastewater management facility, without
				  specific approval by the Department and, if applicable, the Department of
				  Health and Human Services.
(f) Impact on groundwater flow. Stormwater infiltration may
				  not affect the direction of groundwater flows so as to impair any groundwater
				  monitoring programs or cause the migration of existing contaminated groundwater
				  that would result in unreasonable adverse impact on the quality of surface
				  water, groundwater, or drinking water supplies, or to impair any groundwater
				  monitoring programs.
(g) Mounding and seepage. Groundwater mounding due to
				  stormwater infiltration, especially on clay, bedrock, or other low-permeability
				  surfaces, or stormwater discharges to highly permeable materials such as gravel
				  or blast rock, may not cause seepage, high pore-pressures, or other effects
				  that will adversely affect the stability of slopes or conditions of existing
				  infrastructure in the vicinity of the activity. A qualified professional shall
				  assess the potential for seepage and reduction in slope stability, and submit a
				  report of findings, including logs of test borings or other subsurface
				  explorations, modeling, or other means of analysisas determined to be necessary
				  and applicable.
(h) Conveyance of overflow. Infiltration systems must include
				  measures to convey overflow to a stable discharge location.
(i) Control of access. Access to any infiltration area must
				  be controlled during and after construction to prevent compaction of the
				  soil.
(j) Sediment discharge to infiltration structures. Grassed
				  swales, underdrained swales, sediment traps, or similar practices must be
				  incorporated in the design to minimize discharge of sediment to the
				  infiltration system.
(k) Devices to trap petroleum products. Dry wells or
				  subsurface fluid distribution systems receiving runoff from areas of asphalt or
				  concrete paving not prohibited from using infiltration under Section
				  2 must include sump skimmers, sorbent
				  booms, or similar devices to remove petroleum products from runoff. These
				  devices must provide enough sorptive capacity to trap petroleum products for at
				  least six months after construction and after any resurfacing, repaving
				  reconstruction, or similar activity.
5. Maintenance
(a) Snow storage prohibited. Snow removed from any on-site or
				  off-site areas may not be stored over an infiltration area.
(b) Groundwater monitoring. Groundwater quality monitoring
				  may be required by the Department if necessary to demonstrate that the
				  infiltration system will operate in compliance with the Water Classification
				  Program. Groundwater quality monitoring will generally be required for
				  activities infiltrating water from areas of heavy turf-chemical use, such as
				  golf courses and certain athletic fields, and large connected impervious areas,
				  such as parking lots and runways. Groundwater quality monitoring will generally
				  not be required for activities infiltrating water from lawn areas and other
				  vegetated areas, residential developments except for those with large parking
				  areas, playing fields, low-use roads such as residential subdivision roads, and
				  roofs of residential and commercial structures.
(c) Pollution-control devices. Pollution-control devices such
				  as oil-water separators, skimmers, and booms must be inspected regularly to
				  determine if they need to be cleaned or replaced.
(d) Observation wells, measure of sediment accumulation, and
				  points of access for sediment removal. Observation wells to determine the
				  system's performance and access points to allow for the removal of accumulated
				  sediment must be included in the design of subsurface fluid distribution
				  systems. Dry wells and infiltration basins must have staff gauges, marked rods,
				  or similar instrumentation to measure the accumulation of sediment and
				  determine how quickly the system drains after a storm. The maintenance plan for
				  the infiltration system prepared in accordance with Appendix B, Section
				  2 must indicate the expected rate of
				  drainage of the infiltration system and provide for removal of sediment from
				  the infiltration system.
(e) Sediment removal and maintenance of system performance.
				  Sediment must be removed from the system to prevent deterioration of system
				  performance. The system must be rehabilitated or replaced if its performance is
				  degraded to the point that applicable stormwater standards are not met.
6. Additional requirements.
Additional
				  requirements may be applied on a site-specific basis.