55 Walkers Brook Drive, Suite 100, Reading, MA 01867 Tel: 978.532.1900

M E M O R A N D U M

John Aubin, Town Planner TO: Jennifer Miller, Conservation Agent Town of Seekonk,

FROM: M. James Riordan, AICP, LEED AP, Senior Project Manager James I. Pearson, PE, Senior Project Manager Weston & Sampson Engineers, Inc.

DATE: October 28, 2020

SUBJECT: Stormwater Peer Review for Pine Brook Court

Introduction and Purpose Weston & Sampson Engineers, Inc. (Weston & Sampson) was requested by the Town of Seekonk (the Town) to provide a peer review of stormwater management proposed as part of the Pine Brook Court Definitive Plan. Pine Brook Court (also known as “Pine Brook Estates”) is a development proposed in the vicinity of 224 and 250 Arcade Avenue. We understand that stormwater from Pine Brook Court will discharge to the Runnins River, is on the Massachusetts and lists of impaired waters (US Clean Water Act, section 303(d) lists) for multiple pollutants related to stormwater, including pathogens, biodiversity impacts, hypoxia, and metals.

This memorandum summarizes the results of our review. This peer review is prepared pursuant to our October 8, 2020 contract and proposal with the Town. Some of the materials that we reviewed to prepare this letter report were provided to us by the Town and include the following:

• Definitive Subdivision Plan of Pine Brook Court (September 24, 2020) • Stormwater Management Narrative for “Pine Brook Estates” (August 25, 2020) • Order of Resource Area Delineation (SE69-0899) • Decision on “Horton Property” Preliminary Subdivision (June 9, 2020) • Environmental Impact Statement for Pine Brook Estates (October 8, 2020)

Overview of the Proposed Project As described in the Environmental Impact Statement for Pine Brook Estates (EIS), Pine Brook Estates Subdivision is proposed to be located on the east side of Arcade Ave in Seekonk, situated north of the intersection of Ledge Road and east of Marsden Street. The Pine Brook Estates definitive plan proposes nine residential building lots to be accessed by a new roadway of approximately 800 feet in length.

Peer Review This peer review was prepared to address the following general review topics:

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1. Stormwater Design Review 2. Water Quality Related to the Runnins River 3. Additional Comments Related to Stormwater 4. Findings and Recommendations

1.1 Stormwater Design Review

We conducted a review of stormwater management related to construction as well as the stormwater drainage and treatment system that has been proposed as part of the Pine Brook Court Definitive Plan. We offer the following comments:

a. The drainage subcatchment maps that were referenced on page 8 of 26 of the Stormwater Management Narrative were not provided. Flow paths for time of concentration, and other input parameters for the calculations cannot be verified. b. Page 10 of 26 of the Stormwater Management Narrative notes a “constructed stormwater wetland” in the first paragraph. We found no constructed stormwater wetland on the definitive plans. c. Page 12 of 26 of the Stormwater Management Narrative indicates the elevation of the bottom of Basin 1 to be 52.5 feet. This conflicts with the Definitive Plan details, which indicates an elevation of 52.0 feet. This may result in a change to total basin storage volume, which may need to be adjusted throughout the report. d. Page 12 of 26 of the Stormwater Management Narrative indicates an infiltration rate of 8.41 inches per hour. Please provide a reference for this value. 8.41 inches per hour exceeds the highest Rawls table value of 8.27 inches per hour. The 8.41 value should be verified and, if needed, adjusted throughout the narrative. e. Page 13 of 26 of the Stormwater Management Narrative indicates Basin 2 to have a storage volume of 3,375 cubic feet. We would estimate the volume at approximately 4,700 cubic feet. Volume should be verified and adjusted as needed. f. Page 13 of 26 of the Stormwater Management Narrative indicates Basin 3 to have an overflow elevation of 56 - 57 feet. This conflicts slightly with the Definitive Plans, which indicate an elevation of 56.5 feet. The elevation should be verified. Related calculations should be adjusted as needed. g. Page 15 of 26 of the Stormwater Management Narrative indicates total suspended solids (TSS) removal rate of 25%; however, a minimum of 44% is required due to the rapidity of the infiltration rate and proximity to critical areas. This will require redesign of the sediment forebay. This will be an issue for all three of the proposed basins. h. Page 15 of 26 of the Stormwater Management Narrative indicates TSS removal for deep-sump catch basins of 10%. This removal rate can actually be adjusted up to 25%. i. Page 15 of 26 of the Stormwater Management Narrative indicates TSS removal for the sediment forebay of 0%. This can be adjusted up to 25%. However, note that a separate removal rate of 25% is not allowed when a forebay is used as part of an infiltration basin. j. Item 6 on page 17 of 26 of the Stormwater Management Narrative indicates the use of grass swales; however, the Definitive Plans do not clearly show the use of grass swales. Additionally, no credit for grass swales is requested under Standard 4. Please clarify. k. On page 18 of 26, straw “waddles” should be revised to “wattles.” l. Hydrographs in the modeling report are depicted for 20 hours. These should be extended to 24 hours as they are representing 24-hour storms.

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1.2 Water Quality Related to the Runnins River

The Runnins River has its headwaters in Seekonk, Massachusetts in the wetlands east of Prospect Street, near Walker Street. From there, the Runnins flows in a southwesterly direction towards East Providence, Rhode Island, where it then forms the boundary between Massachusetts and Rhode Island. It continues flowing along the state line to its end at Mobil Dam. Below Mobil Dam (the boundary between fresh and salt water), the river broadens out and empties into Hundred Acre Cove and becomes the Barrington River.

The Runnins River is impaired for several water quality parameters. An impaired water is a water listed pursuant the US Clean Water Act, section 303(d) (CWA s. 303(d)), because it does not meet its water quality goals and is, therefore, considered not to be “fishable/swimmable.” CWA s. 303(d) listed waters must be scheduled for a total maximum daily load (TMDL) study, which is watershed-based water quality analysis aimed at determining pollutant sources and the pollutant load reduction required to bring a waterbody back into compliance with state and federal water quality standards.

Massachusetts lists the Runnins River as impaired for the following pollutants.

• Escharichia Coli (e coli) • Benthic Macroinvertebrates • Dissolved Oxygen • Fecal Coliform • Mercury in Fish Tissue • Nutrient/Eutrophication Biological Indications

From a stormwater management perspective, the primary pollutant concerns are bacteria (i.e., e. coli and fecal coliform) and nutrients/eutrophication (assumedly phosphorus since this is a freshwater system). Mercury in fish tissue is likely the result of mercury deposition in sediment from years past and will not be significantly improved through stormwater management. Nutrient reduction may indirectly help to improve dissolved oxygen and benthic macroinvertebrates.

The applicant proposes to use infiltration basins to manage water quality related to stormwater. Infiltration is a good choice for management of both bacteria and phosphorus. Both pollutants will be captured in the soil matrix of the infiltration system and the soils of the surrounding landscape. The Massachusetts Stormwater Handbook indicates of up to 60% removal of phosphorus from stormwater through the application of infiltration basins; however, to be effective the proposed basins will need to be designed in accordance with state design standards.

Revisions should be made pursuant to section 1.1 of our review. Also note that the Runnins River was misspelled as the “Runnings” River on page 3 of 26 of the Stormwater Narrative.

1.3 Additional Comments Related to Stormwater

The following comments are provided related to specific requirements of Seekonk’s enforceable development policy:

a. We reviewed the “Environmental Impact Statement for Pine Brook Estates” (EIS) for compliance with section 5.6 of the Rules and Regulations Governing the Subdivision of Land in Seekonk,

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Massachusetts (hereinafter the “Subdivision Regulations”). In part, section 5.6 requires development of a nutrient loading rate, which would provide for improved understanding of potential eutrophication of the Runnins River. We did not find this calculation in the EIS. We believe that nutrient loading should be addressed as the Runnins River is impaired for “nutrient/eutrophication.” b. According to section 8.4.4 of the Subdivision Regulations backfilling must consist of suitable materials uniformly distributed and compacted. We did not find backfilling discussed in the application materials that we reviewed. The applicant should clarify any proposed backfilling to be conducted as part of the Pine Brook Estates stormwater best management practice construction. c. Page 19 of the Stormwater Narrative discusses operation and maintenance and makes note of an operation and maintenance agreement in Appendix D. The operation and maintenance agreement is actually in Appendix C. This should be revised.

1.4 Findings and Recommendations In summary, Weston & Sampson requests that the applicant address the comments provided in sections 1.1, 1.2, and 1.3 of this peer review memorandum.

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