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
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.
