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Review from Water Resources Consultant

Scott W. Horsley

Water Resources Consultant

65 Little River Road • Cotuit, MA 02635 • 508-364-7818

Dear Mr. Casavant and Members of the Zoning Board of Appeals:

I have been retained by abutters to the project to review the proposed River Marsh

Village Development project located at Water Street in Pembroke, MA. I have reviewed

the application materials, documents posted on the town website, and other relevant

data and resources published by the Massachusetts Department of Environmental

Protection (MADEP), Massachusetts Geographic Information System (MAGIS), United

States Geological Survey (USGS), USDA Natural Resources Conservation Service (NRCS)

and United States Environmental Protection Agency (USEPA). This comment letter

updates and supplements my previous letter dated 3 May 2021.

Qualifications: My consulting practice is dedicated to projects where I can assist in the

preservation and/or the restoration of water resources. I have over 30 years of

professional experience in the field of water resources management and on a broad

range of water contamination and restoration projects. I have served as a consultant to

federal, state, and local government agencies, non-governmental organizations (NGOs),

and private industry throughout the United States, Central America, the Caribbean, the

Pacific Islands, Bulgaria, and China. I have assisted in the development and presentation

of a nationwide series of U.S. Environmental Protection Agency (USEPA) workshops on

drinking water protection, wetlands management, and watershed management. I have

also served on numerous advisory boards to the USEPA, the National Academy of Public

Administration, Massachusetts Department of Environmental Protection (DEP),

Massachusetts Executive Office of Energy and Environmental Affairs (EEA), and the

National Groundwater Association. I have received national (USEPA) and local awards

for my work in the water resources management fields. I currently serve as Adjunct

Faculty at Harvard University Extension School and Tufts University, where I teach

courses in water resources policy, wetlands management, green infrastructure, and low


impact development (LID). These courses focus on the critical role of local governments

who have the primary responsibility and authority of regulating land uses in critical

water resource protection areas.

General Comments: The proposed project includes a wastewater treatment plant and a

stormwater infiltration system in very close proximity to each other, the North River,

and extensive wetland systems. The site is low-lying with freshwater and coastal

wetlands. Portions of the project are proposed within the North River Scenic River

Corridor. Shallow groundwater conditions that exist on the site will be exacerbated by

groundwater mounding (water level rises) associated with the proposed wastewater

and stormwater facilities. The proximity of the project to wetlands and coastal waters

raises serious questions regarding water quality impacts and the implications of flooding

and sea level rise.

The project proposes to discharge both wastewater and stormwater to the ground.

According to the applicant’s submittals the volumes and rates of infiltration into the

ground will significantly exceed existing (predevelopment) rates. This will raise

groundwater levels further compromising critical depth to water table issues. State

regulations require that minimum vertical separation to the “seasonal

high water table” is maintained. According to the site topography and elevations the

water table is shallow throughout much of the site and it is questionable whether these

minimum separation distances can be met in post-development conditions when

groundwater mounding is considered. Furthermore, the project will impair the quality

of the North River and the associated wetlands.

My specific comments are as follows:

1. Groundwater Mounding: Groundwater mounding occurs when stormwater or

wastewater are infiltrated into the subsurface. This results in an increased elevation of

the water table and changes groundwater flow directions and velocities.

Figure 1 - Groundwater mounding resulting from infiltration of stormwater or wastewater


The applicant has revised the stormwater design to avoid conducting a groundwater

mounding analysis on the basis that the stormwater infiltration system is now 4.0 feet

above the estimated seasonal high groundwater (ESHGW). They are making this claim

based upon the proposed bottom of the infiltration system at elevation 16.60 and the

ESHGW at test pit TPB-1-3 at 12.54 (a difference of 4.06 feet at that test pit location).

However, when you examine the three test pits within the footprint of the proposed

facility this indicates a significant water table gradient (slope) of 10% in an easterly

direction ranging from elevation 9.98 (TP-B-1-1), to 12.54 (TP-B-1-3). This is an increase

of 2.56 feet over a distance of 25 feet. I have prepared a water table map showing

ESHGW elevations (see figure below). According to this gradient the ESHGW in the

eastern corner of the proposed infiltration system (where there is no test pit provided)

would be approximately 14 feet providing only 2.6 feet of vertical separation and thus

requiring a groundwater mounding analysis.

Figure 2 - Groundwater Elevations at Stormwater Infiltration System

Additionally, the site plan includes stormwater and wastewater infiltration areas that

are near to each other and adjacent to steep slopes and wetlands. Each of these two

facilities will cause a rise (mound) in the underlying groundwater and these effects will

overlap and be cumulative. A groundwater mounding analysis is relatively simple and

would provide an important evaluation of these impacts.

Furthermore, the MADEP Stormwater Handbook requires that the groundwater

mounding analysis will show that the infiltration will not break out on adjacent steep

slopes or within nearby wetlands. MADEP Stormwater Handbook, Volume 3, Chapter 1,

page 28 provides criteria by which to evaluate groundwater mounding impacts on

wetlands. It states, “The mounding analysis must also show that the groundwater

mound that forms under the recharge system will not break out above the land or water

surface of a wetland (e.g., it doesn’t increase the water sheet elevation in a Bordering

Vegetated Wetland, Salt Marsh, or Land Under Water within the 72-hour evaluation


Figure 3 - Proximity of Stormwater and Wastewater Infiltration Areas

2. The Project Does Not Comply with MADEP Standard 6 (Critical Areas): The

applicant’s Stormwater Report states, “The proposed project does not discharge to any

critical areas. Not Applicable” and provides no further information.

However, MADEP Stormwater Handbook (Volume 1, Chapter 2) states, “If a proponent

is proposing a project that is in the watershed of a water body with a TMDL1, and if the

project is subject to wetlands jurisdiction, the proponent must select structural BMPs

that are consistent with the TMDL”. The North River has been designated by MADEP as

“impaired by pathogens” and is incorporated into the 2014 Final Pathogen TMDL for the

South Coastal Watershed by the MADEP as part of their compliance with the Clean

Water Act.

The project site is within the designated TMDL and is immediately upstream of shellfish

growing areas (see attached figure). Shellfish growing areas are considered “critical

areas” and the MADEP Stormwater Handbook states, “Stormwater discharges within the

1 A TMDL is a Total Maximum Daily Load report which indicates the maximum amount of a pollutant that

can be discharged to a water body and identifies pollutant sources. TMDL reports are required when the

state identifies waters are “impaired” (not meeting state water quality standards). The TMDL report for

the North River identifies pet wastes as a primary source of pathogens.


Zone II or Interim Wellhead Protection Area of a public water supply and stormwater

discharges near or to any other critical area require the use of the specific source

control and pollution prevention measures and the specific structural stormwater best

management practices determined by the Department to be suitable for managing

discharges to such areas, as provided in the Massachusetts Stormwater Handbook. A

discharge is near a critical area if there is a strong likelihood of a significant impact

occurring to said area, taking into account site-specific factors”.

Figure 4 - Location of Project Relative to Shellfish Growing Areas

Given the size and density of the proposed project and the high probability of the

project to generate significant quantities and loads of pet wastes and the fact that is

immediately upstream of shellfish growing areas this project meets this definition as

“near a critical area”. Pet waste have been identified as a significant pathogen pollutant

source in high density developed areas2. Hence, the project site design must

incorporate features to prevent pathogen pollution. These features should include

limiting the density of the project (thereby limiting the number and density of pets) and

naturally-vegetated buffers and setbacks (50 - 100 feet) from wetlands.

The proposed stormwater infiltration and detention practices are insufficient to meet

this standard for three reasons – 1) they do not treat an adequate volume (1 inch)

associated with critical areas, 2) they provide inadequate buffers to adjacent wetlands,

and 3) the pet waste source areas include both pervious and impervious areas and the

proposed stormwater facilities are designed only for impervious areas.

United States Environmental Protection Agency () has prepared a guidance manual to

address pathogen pollution issues in Massachusetts TMDL areas. This document,

2 According to MADEP each gram of dog feces contains 23 billion fecal coliform organisms. The water

quality standard for shellfishing areas is 14 organisms/100 milliliters. The track record for pet owners

cleaning up after their pets is not good. Therefore, we must rely on other means to control this pollution

source – lower density and vegetated buffers.


“Mitigation Measures to Address Pathogen Pollution in Surface Waters: A TMDL

Implementation Guidance Manual for Massachusetts” identifies pet waste as a

significant pathogen pollution source and references another document, “Guidance

Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters”.

USEPA, Office of Water 1993.

That report states, “EPA recommends that no habitat-disturbing activities should occur

within tidal or nontidal wetlands. In addition, a buffer area should be established that is

adequate to protect the identified wetland values. Minimum widths for buffers should

be 50 feet for low-order headwater streams with expansion to as much as 200 feet or

more for larger streams”.

The Town of Pembroke’s Wetlands Bylaw recognizes the importance protective buffers

and states, “Lands within 100 feet of the specified resource areas, and with 200 feet of

rivers, streams, and creeks, are presumed important to the protection of these

resources because activities undertaken in close proximity to resource areas have a high

likelihood of adverse effect upon them either immediately, as a consequence of

construction, or over time, as a consequence of daily operation or existence of the

activities. These adverse impacts from construction and use can include, but not be

limited to, erosion, siltation, loss of groundwater recharge, poor water quality, and loss

of wildlife habitat. The Commission therefore may require that the applicant maintain a

strip of continuous, undisturbed vegetative cover within the aforementioned 100 foot

or 200 foot area, unless the applicant convinces the Commission that the area or part of

it may be disturbed without harm to the values protected by this chapter”.

The figure below shows a 100-foot undisturbed buffer (North River) as recommended by

USEPA on the proposed project plan and indicates that the proposed project as

currently designed significantly encroaches within this area. Given the significant size of

the project, its proximity to extensive wetlands, the North River, and its location

immediately upstream of shell growing areas, and the MADEP TMDL status, minimum

setback/buffers of 50 feet to wetlands associated with the intermittent/headwater

stream and 100 feet to the wetlands associated with the North River would be a

reasonable and defensible protective measure that the town of Pembroke could



Figure 5 - Proposed Undisturbed Buffer Area

3. The Project Application Provides Insufficient Information Regarding Wastewater

Treatment and Disposal. The site plans show a wastewater treatment plant but has

provided no details regarding water table conditions or pollutant loading. There is no

design, evaluation, proposed grading, groundwater modeling, or wastewater treatment

technology description provided. The applicant may hope to defer discussion of the

wastewater discharge to MADEP and its permitting under the Groundwater Discharge

Permit Regulations. However, the hydrologic and water quality impacts of the

wastewater discharge are relevant to the town’s review of the project and its

relationship to the adjacent stormwater practices, impacts on wetlands, and impacts on

the North River. As indicated earlier in this letter there are also likely to be overlapping

groundwater mounding impacts between the wastewater and stormwater systems.

Hence there should be a comprehensive evaluation of the overall, cumulative impacts of

the project.

4. The Proposed Dry Detention Basin May Flood: The proposed dry detention basin is

designed at elevation 15.50 with an estimated seasonal high groundwater (ESHGW)

elevation of 15.00. (see figure below). This provides only 0.5 feet of separation to

groundwater (under existing conditions).

The bottom of the basin is to be constructed of permeable materials including loamy

sand and gravel that will allow infiltration of stormwater into the subsurface on a

regular basis (every rain event). The frequent discharge of stormwater into this area will

create long-term groundwater mounding and will result in a higher seasonal

groundwater elevation. As designed the “dry” basin will likely be saturated or

inundated and may not provide the storage volume that is modeled in the Stormwater



In their recent peer review letter of July 25, 2021 Merrill Engineering stated that, “the

regulations specify that if the water table is within 2 feet of the bottom of the basin

problems with standing water may occur. We recommend that this be addressed by the

design engineer”.

Figure 6 - Cross Section of Dry Detention Basin

5. The Project is Inconsistent with the Scenic and Recreational River Protective Order

for the North River. I have read Attorney Warren Baker’s letter and interpretation of

the applicability of this law in favor of the proposed project. However, my reading of

the Protective Order and my understanding of wetlands science as referenced in the law

leads me to a different conclusion – one that provides an appropriate level of protection

for the North River wetland resources and is consistent with the stated intent of the


Wetlands are dynamic and migrate in response to ecological, geological, hydrologic, and

climatic changes and shifts. This is most obvious in a river system where the meander of

the river and associated wetlands change over time. Recognizing their dynamic nature,

wetland delineations performed under the Massachusetts Wetlands Regulations are

only valid up to three years.

The Scenic and Recreational River Protective Order for the North River was enacted by

the Massachusetts Legislature with the intent of protecting these valuable wetland

resources. It regulates all development projects within 300 feet of the North River

Corridor. The Applicant has provided their interpretation of this delineation on their site

plans based upon a 1978 map that was included with the original legislation and are

suggesting that they can avoid the performance standards provided in the law. No one

would expect a wetland boundary to remain static over a 43-year period (since 1978).

The Protective Order defines its jurisdiction over the “Corridor” as including all lands

within 300 feet of the “natural bank”. The “natural bank” is defined in the Protective

Order as, “In wetland areas, it means the landward edge of the salt or freshwater marsh,


as defined in G.L. c. 131, s. 40”. In the case of this project area there is a freshwater

marsh from which this setback should be measured as stated in the law.

According to this law the Corridor extends 300 feet landward of the bordering vegetated

wetland (BVW). Specifically, the Protective Order states:

“In wetland areas, it means the landward edge of the salt or freshwater marsh, as

defined in G.L. c. 131, s. 40 (Massachusetts Wetlands Protection Act)”

This clearly indicates that the project is within the jurisdiction of this statute and is

subject to review by the North River Commission. The law contains several prohibitions


a) “degradation of water quality”

b) harmful alteration of wetlands”.

c) “placement or construction of structures, subsurface sanitary facilities, roadways and

driveways on any wetland (as defined in G. L. c. 131, s. 40). This includes a prohibition

of these structures within the 100-year floodplain (bordering land subject of flooding).


1. Require the applicant to conduct a groundwater mounding analyses to evaluate the

cumulative impacts of the proposed stormwater infiltration, dry detention, and

wastewater discharges. This analysis should provide the required assessment of

potential breakout of water on the proposed steeply graded (20%) slopes and within the

adjacent wetlands.

2. Revise the project plans to reduce the project density and provide for a minimum 50-

foot undisturbed (and naturally vegetated) buffer to wetlands adjacent to the

intermittent/headwater stream and a minimum 100-foot undisturbed (and naturally

vegetated) buffer to wetlands adjacent to the North River.

3. Revise the project plans to incorporate North River Corridor 300-foot setback from

current wetland boundaries consistent with the Scenic and Recreational River Protective

Order for the North River.

Thank you for the opportunity to submit these comments. Please contact me directly

with any questions that you might have.


Scott Horsley

Water Resources Consultant


“Housing Density and Bacterial Loading In Urban Streams”, Katherine D. Young1 and

Edward L. Thackston, Journal of Environmental Engineering, Vol. 125, No. 12, December,


“Guidance Specifying Management Measures for Sources of Nonpoint Pollution in

Coastal Waters”, U.S. EPA, Office of Water 1993. Washington, DC

“Mitigation Measures to Address Pathogen Pollution in Surface Waters: A TMDL

Implementation Guidance Manual for Massachusetts”. U.S. EPA.

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