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Storm Drain Requirements

BWSC reserves the right to determine whether drainage issues will be addressed during building renovations. Site plan review projects in the City of Boston are required to INFILTRATE A VOLUME OF RUNOFF EQUAL TO 1-INCH OF RAINFALL TIMES THE TOTAL FOOTPRINT OF PROPOSED IMPERVIOUS AREA ON SITE.

General Requirements

  1. Chain Basins - Chain basins are considered to be poor engineering practice. They are not allowed in the public or private way open to public travel. If chain basins are to be built on private property, the last basin before the connection to the BWSC facilities must be a BWSC standard catch basin type with oil trap outlet. For more information see the Standard Details
  2. Drain Lateral Connection - When there is a separate storm drain in the street, the site's storm drain must connect to the storm drain. When there is a combined sewer in the street, the site's sewer and storm drain infrastructure must leave the property in separate pipes and then connect to the combined sewer via a wye connection once in the public right of way. For more information see Standard Details
  3. Connection Detail - Detail of the connection, if it is not be made to an existing wye branch on the drain, must be shown on the plans. Connection to clay, P.V.C., concrete and iron pipes will be made using an approved coring machine and mechanical device or the cutting in of an approved gasket and saddle. For more information see Standard Details
  4. Drain Slope - Minimum pitch is 1/8-inch per foot for building storm drain connections. Building connections shall be bedded in 3/4" minimum crushed stone to at least half the pipe diameter.
  5. Cleanout - A clean out on the portion of the building storm drain on the owner's property at the property line is required where a building is set back from the property line. A cleanout or access structure must also be provided on the downstream end of infiltration systems.
  6. Ten (10) Foot Separation - The storm drain connections shall be laid at least 10 feet apart from any new or existing water services connections.
  7. Cut & Caps - All abandonment of the existing storm drain services must be cut and capped off at the main(s) in the street. All such work must be performed by a Licensed Contractor.
  8. Inspection - BWSC has the right to inspect building sanitary sewers, building storm drains, and other private sewers, grease traps, oil-particle separators, and other appurtenances tributary to the BWSC's wastewater and storm drainage systems. Inspection points must be delineated on the site plan, along with sign off by a BWSC inspector.

Green Infrastructure/Low Impact Development (GI/LID)

For all re/construction projects in the City of Boston it is MANDATORY to retain stormwater on site. A volume of runoff equal to one inch of rainfall times the total impervious area on site must be infiltrated prior to discharge to a storm drain or a combined sewer system for projects less than 100,000 square feet of floor area. For all projects which are at or above 100,000 square feet of floor area, the project must use a volume of runoff equal to 1.25 inches of rainfall times the total impervious area on site. BWSC will accept a 30% void ratio for volume within a stone area. Must provide soil borings and/or test pits conducted by a Registered Professional Engineer or Registered Sanitarian to determine the infiltration rate of the soil on site. A pre- and post- construction Drainage Analysis must be completed in accordance with the Massachusetts Stormwater Handbook (pg. 68). Drainage calculations for the runoff are needed including the storm frequency, time of concentration, peak rate of runoff, and total volume of water for all projects involving over 2,500 square feet of impervious surface. Information, including NOAA Atlas 14 precipitation frequency, can be found on the Precipitation Frequency Data Server website. Infiltration rates will also be used in sizing calculations for Green Infrastructure/Low Impact Development (GI/LID) infiltration practices to treat storm water runoff.

GI/LID practices that utilize infiltration are necessary in order to meet the water quality requirements outlined in the Total Maximum Daily Load (TMDL) for the Charles River and the BWSC Consent Decree. Information about GI/LID infiltration practices can be found in BWSC's Stormwater Best Management Practices: Guidance Document and GI/LID Standard Details can be found in the Massachusetts Stormwater Handbook. Any project with an infiltration system and/or a catch basin addition must also include and Operations and Maintenance (O&M) plan with their site plan material. This Plan must identify; a) the Stormwater Management system's owner(s), b) the party or parties responsible for operations and maintenance, c) a schedule for inspection and maintenance and d) list maintenance tasks required.

Underground Injection Well Code (UIC)

The Massachusetts Department of Environmental Protection (MassDEP) is now tracking all new infiltration systems, currently referred to as underground injection wells, in Massachusetts. Site plan review applicants are required to fill out one of two forms: a form for 2-4 unit residences or a form for all other types of properties (single family homes are exempt and do not require a form). Applications must be submitted to MassDEP for approval. Once applications are approved, MassDEP will assign a UIC number. Applicants must then submit UIC numbers to BWSC. Site plans will not receive BWSC approval prior to receiving a UIC number.

Groundwater Conservation Overlay District (GCOD)

Article 32 of the City of Boston Zoning Code establishes a Groundwater Conservation Overlay District (GCOD) in parts of the City to protect the wood pile foundations of buildings from being damaged by receding groundwater levels. The purpose of the GCOD is to ensure that construction projects do not cause reductions in groundwater levels on the construction site or adjacent lots. GI/LID designed for site plan review projects within the GCOD may not utilize an underdrain or be designed for stormwater capture and reuse. Projects within the GCOD are required to RECHARGE stormwater into the ground to help to raise the level of groundwater to a safe level.

The current GCOD map is maintained by the Boston Planning and Development Agency (BPDA).

Infiltration & Inflow Reduction Letter of Agreement & Understanding (LOAU)

The Department of Environmental Protection (DEP), in cooperation with the Massachusetts Water Resources Authority and its member communities, are implementing a coordinated approach to flow control in the MWRA regional wastewater system, particularly the removal of extraneous clean water (e.g., infiltration/ inflow (I/I)) in the system. In April of 2014, the Massachusetts DEP promulgated new regulations regarding wastewater. The Commission has a National Pollutant Discharge Elimination System (NPDES) Permit for its combined sewer overflows and is subject to these new regulations 314 CMR 12.00, section 12.04(2)(d). This section requires all new sewer connections with design flows exceeding 15,000 gallons per day (gpd) to mitigate the impacts of the development by removing four gallons of infiltration and inflow (I/I) for each new gallon gpd of wastewater flow. In this regard, any new connection or expansion of an existing connection that exceeds 15,000 gpd of wastewater shall assist in the I/I reduction effort to ensure that the additional wastewater flows are offset by the removal of I/I. Currently, a minimum ratio of 4:1 for I/I removal to new wastewater flow added. The Commission supports the policy, and will require proponent to develop a consistent inflow reduction plan. The 4:1 requirement should be finalized at least 90 days prior to activation of water service and will be based on the estimated sewage generation provided on the project site plan.

New sewer connections that add estimated sewer flows GREATER THAN OR EQUAL TO 15,000 GPD are required to remove a corresponding amount of Infiltration/Inflow or, pay a fee and submit a 4:1 Infiltration & Inflow Reduction Compliance Letter of Agreement & Understanding (LOAU) stating the property owner's intention to pay the Fee

Particle Separators

When outdoor parking and paved areas GREATER THAN OR EQUAL TO 7,500 SQUARE FEET are proposed, particle separators must be installed for drain lines. The type of separator used will depend on the DRAINAGE AREA SERVED. For storm drain separators, discharge must be directed to the stormwater/combined system, or wetland areas if approved for it. The guidelines and Standard Details are provided by BWSC.

National Pollutant Discharge Elimination System Construction General Permit (NPDES CGP)

IF construction site operations result in the DISTURBANCE OF ONE ACRE OR MORE of land, or are part of a planned disturbance of one acre or more, an Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) Construction General Permit (CGP) must be obtained. To obtain this permit developers or owners need to prepare a Stormwater Pollution Prevention Plan (SWPPP) and submit a Notice of Intent (NOI) to the EPA. The NOI can be filed electronically with the EPA.

Filing the NOI certifies that a SWPPP has been prepared and will be implemented prior to starting construction activities. SWPPPs must address the phosphorus TMDL for the Charles River. Information on preparing a SWPPP can be found on the EPA's website.

Site plans will not receive BWSC approval prior to receiving EPA NOI tracking numbers and SWPPP plans (in PDF format). Erosion and Sediment control plans that comply with the Massachusetts Stormwater Policy must also be submitted for review by BWSC before site plans will be approved.

Upon approval of site plans, developers are REQUIRED to notify BWSC of the start date of construction activities. All NPDES CGP sites will be inspected by BWSC within 8 weeks of the start of construction activities and periodically until the end of the job under the Stormwater Construction Inspection and Enforcement Program. During construction SWPPP operators will comply with BWSC's Construction Site Inspection and Enforcement Program in maintaining their SWPPPs and conducting their own inspections and keeping inspection logs, per requirement of the CGP.

Weekly inspection reports are REQUIRED to be sent to BWSC electronically on a monthly basis. Inspection reports can be emailed to

Sites found not in compliance with their NPDES permits by the BWSC may be subject to BWSC and ISD enforcement measures including stop work orders and fines from BWSC's Regulations Regarding the use of Sanitary and Combined Sewers and Storm Drains. Additionally, failure to obtain the NPDES CGP can result in EPA fines up $37,500 per day for applicable construction sites.

Dewatering Discharge Permit

A Dewatering Discharge Permit application must be filed for certain discharges to BWSC's wastewater or storm drainage systems.

The following are considered "allowable" discharges to the BWSC combined and storm drainage system:

  • Diverted tide, river or stream flows;
  • Uncontaminated springs;
  • Flows from riparian habitats and wetlands;
  • Rising groundwater;
  • Uncontaminated groundwater infiltration to the drains;
  • Water main, hydrant flushing and other discharges from potable water sources associated with routine maintenance of the water distribution system;
  • Irrigation water;
  • Non-commercial car washing;
  • Swimming pool discharges which have been dechlorinated;
  • Street and pavement wash waters;
  • Discharges or flows from emergency firefighting activities; and
  • Building washdown water which does not contain detergents

Any collected water discharged into a storm drain leading to a combined sewer system or into a combined sewer directly requires an MWRA Sewer Use Permit.

With the exception of the aforementioned "allowable" discharges, all other discharges to the combined or storm drainage system must obtain a Dewatering Discharge Permit. Discharges to storm drains that require a Drainage Discharge Permit include, but are not limited to:

  • Dewatering drainage;
  • Subsurface drainage;
  • Non-contact cooling water;
  • Non-contact industrial process waters;
  • Uncontaminated cooling water;
  • Uncontaminated industrial process water or waters associated with the excavation of a foundation or trench;
  • Hydrological testing;
  • Groundwater treatment/remediation;
  • Removal or installation of an underground storage tank;
  • Foundation drains, crawl space pumps or footing drains.

Either a NPDES General Permit or NPDES Exclusion for the discharge must also be obtained and provided to BWSC. Drainage Discharge Permits may be revoked, suspended or reissued if the BWSC determines that the discharge is or may cause or contribute to a water quality problem or a violation of the BWSC's NPDES Permit. In the case of construction site dewatering, the duration of the permit shall not exceed the time period necessary to keep a site dewatered during construction. For more information please see the Dewatering Permit section of the BWSC website.