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
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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
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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
period)”.
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.
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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.
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“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
require.
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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
Report.
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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
law.
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,
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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
including:
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).
Recommendations:
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.
Sincerely,
Scott Horsley
Water Resources Consultant
References:
“Housing Density and Bacterial Loading In Urban Streams”, Katherine D. Young1 and
Edward L. Thackston, Journal of Environmental Engineering, Vol. 125, No. 12, December,
1999
“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.