Engineering
and Management Summary
Feasibility Study of Potential Corrective Action
Options for
Great, Green, and Bournes Ponds
Introduction: This evaluation
was performed for the Town of Falmouth and the Ashumet Plume Citizens
Committee, as part of the Ashumet Plume Nitrogen Offset Program, with funding
provided by the Air Force Center for Environmental Excellence. The objective of the study was to provide
viable options for corrective actions to reduce nitrogen loading and improve
water quality in Great, Green, and Bournes Ponds.
Excess
nitrogen loading to Great, Green, and Bournes Ponds has been identified as a
significant cause of degradation and impairment of the ponds’ ecosystems. Nitrogen has been identified as the most
significant nutrient controlling the water quality of the ponds. Nitrogen loading leads to over
fertilization, resulting in increased production of macroalgae and
phytoplankton, reduced oxygen levels, and poor habitat for fish and
shellfish. Dissolved oxygen is depleted
during cloudy, low light and nighttime conditions by algal respiration and
biological decay of dead algae.
Increased
sedimentation and reduction in eelgrass habitat associated with excess growth
of algae, along with reduced oxygen levels, have severely reduced shellfish
habitat and resources. During certain
periods, oxygen levels can drop below levels necessary to sustain animal life,
resulting in fish kills. Excess
macroalgae and phytoplankton levels may also generate nuisance conditions, such
as unpleasant odors and impaired visual qualities.
Phase I of the
Ashumet Plume Nitrogen Offset Program was focused on:
·
Assembling critical information on
nutrient loading to summarize existing conditions (Task 1),
·
Establishing and calibrating pond
flushing and water quality models and developing a clear and concise basis for
the assessment of current and future water quality (Tasks 2 and 3),
·
Completing a thorough screening of all
available corrective actions (Task 4), and
·
Developing a comprehensive set of
alternative corrective action options (Tasks 5 and 6).
The
feasibility study (combination of Tasks 4, 5, and 6) was designed to identify and define specific actions that could be
implemented to reduce ecological degradation caused by excess nutrient loading.
Report
Overview:
This summary provides an overview of the completed qualitative and
quantitative assessment of potential water quality improvements based on a
range of feasible corrective actions designed to reduce nitrogen loading to
Bournes, Green, and Great Ponds. The
feasibility study included both an initial screening of potential corrective
action alternatives and a detailed analysis of options to support selection of
corrective action options put forth by the Ashumet Plume Citizens Committee.
Task
4: Initial Screening of Alternatives to
Offset Nitrogen Loading to Great, Green, and Bournes Ponds
The alternatives assessed in the initial screening included
a range of options focused on mitigating the impacts of nutrient overloading on
the ponds. The range of alternatives
was developed based on the experience of the Horsley & Witten, Inc., Applied
Coastal Research and Engineering, and the Center for Marine Science and
Technology, review of the relevant literature, review of actions proposed in
analogous projects, and input from the Ashumet Plume Committee and the citizens
of the Town of Falmouth.
Potential
Corrective Action Alternatives:
Three general categories of potential corrective action alternatives
were assessed including:
1.
nitrogen source control strategies,
2.
methods to limit or manage nitrogen
migration to the ponds, and
3.
in-pond control options designed to
mitigate the impact of current and future nitrogen loading.
Methods
evaluated to accomplish corrective actions included regulatory changes, public
education, and implementation of engineering solutions. Twenty management, regulatory, and
educational options, or tools were initially considered for improving water
quality in the coastal ponds.
Management
Options:
Management options for control of nitrogen migration to the ponds
included controls on runoff, addressing nitrogen deposited on roadways by
atmospheric sources, and removal of nitrogen carried by the freshwater
Coonamessett River, Backus River, and Bournes Brook, draining the upper
watershed, north to Route 28. In-pond
control options were considered to provide long-term management of the symptoms
and harmful effects of high nitrogen levels in the salt ponds. These options included methods to increased
pond tidal flushing such as dredging and re-engineering pond inlets, and
methods to reduce impacts of nutrient overloading such as pond aeration and
harvesting macroalgae.
Regulatory
Options:
Regulatory options were targeted toward controlling nitrogen
sources. However, certain requirements
could be designed to manage and reduce nitrogen migration to the ponds. These regulatory and management tools
included use of the Cape Cod Commission Act, zoning, subdivision control,
health regulations, and wetlands regulations.
Wastewater Treatment Options:
Specific wastewater treatment options were explored to identify improved
wastewater treatment alternatives to significantly reduce the nitrogen load
originating from septic systems, the largest source of nitrogen loading to the
ponds in the watersheds. To be an
effective means of nitrogen source reduction, these systems must include
processes to denitrify the wastewater by converting fertilizing forms of
nitrogen such as nitrate to inert nitrogen gas, reducing nitrogen in discharged
effluent. Wastewater treatment options
considered included centralized wastewater treatment, neighborhood-scale
package wastewater treatment plants, cluster denitrifying systems, and
small-scale, on-site (innovative/alternative) denitrifying septic systems.
Centralized Treatment Options: Centralized treatment options considered included connecting
homes, schools, and businesses the watersheds to the existing Falmouth
wastewater treatment plant, and construction of new centralized wastewater
treatment facilities in East Falmouth.
Evaluations of centralized treatment options considered collection
systems, treatment works, and effluent discharge systems. The treatment technologies considered for
centralized treatment included three basic types of biological wastewater
treatment systems that are capable of treating wastewater volumes in the
millions of gallons per day (mgd) range, and removing common wastewater
constituents including nitrogen. These
biological treatment systems included suspended growth treatment systems,
attached growth treatment systems, and natural treatment systems.
Neighborhood-Scale Conventional Package Treatment
Plants: In addition to large centralized treatment facilities, smaller
neighborhood-scale conventional package treatment plant systems were
considered. Three types of treatment
systems were identified including suspended growth treatment systems, fixed
media treatment systems, and membrane bioreactor treatment systems. Neighborhood-scale treatment facilities can be placed on
the property of condominium developments, commercial facilities, neighborhood
vacant lots, or adjacent public or neighborhood associations. These systems could be designed to serve
areas of approximately 100 homes, and are appropriate for schools and large
commercial facilities with equivalent wastewater flows.
Innovative On-Site and Cluster
Denitrifying Septic Systems: Alternative or innovative on-site septic
systems that are designed to denitrify wastewater were also considered. Denitrifying systems are available for
individual on-site treatment for one home, or can be scaled-up to multiple-home
cluster systems. These systems
incorporate the use of additional components over conventional septic systems
that include aerobic treatment units, septic tank treatment modules, and
leaching system media/chambers. A
two-step process is necessary to turn fertilizing nitrogen compounds in
wastewater into inert nitrogen gas.
Septic systems receive organic nitrogen compounds such as proteins and
ammonia in wastewater. Most organic
nitrogen compounds are converted to ammonia in the septic tank. Ammonia is converted to nitrate in the
presence of oxygen (aerobic conditions).
This is as far as the treatment process normally goes in a conventional
Title 5 system.
To convert the nitrate to nitrogen gas (denitrification), an innovative
system must include an additional step that includes anaerobic (no oxygen)
conditions. The presence or absence of
oxygen promotes the necessary conditions for the types of bacteria that
accomplish the separate stages of the treatment process.
Stormwater Runoff Controls:
Methods to control stormwater runoff carrying nitrogen from roofs, paved
areas, and lawns within the watershed were considered. Nine techniques to collect and treat
stormwater to remove nitrogen have been evaluated as part of the initial screening
process.
Fresh
Water River Wetland Treatment Systems: Methods to improve nitrogen removal from the
fresh water river systems leading to the salt ponds, including Bournes Brook,
the Backus River, and the Coonamessett River, were evaluated. These methods focused on enhancing natural
processes for nitrogen uptake by plants and, where possible, establishing
conditions that promote denitrification.
Two general categories of wetland systems were considered for wastewater
treatment. These included free surface
wetlands and subsurface flow systems.
Subsurface flow wetland systems primarily rely on water moving through
the root zone of wetland vegetation and may not include open water. Free surface wetlands have open water
surfaces with emergent vegetation, clear open water areas, floating vegetation,
or a combination of all of the above.
Coastal Engineering Alternatives: Coastal
engineering and management alternatives were considered, including
re-engineering the pond inlets and dredging to improve tidal flushing, wetlands construction along the shoreline of
the ponds, in-pond aeration systems,
and biomass removal programs to remove algae.
These systems treat the symptoms, not the cause, of excess nitrogen
loading.
Screening of Alternatives: A screening matrix
of potential corrective actions was developed to qualitatively compare
alternatives and technologies to specific screening criteria. The screening criteria developed for this
evaluation included:
·
Potential for water quality improvement,
·
Ease of Implementation,
·
Technical complexity,
·
Difficulty of the permitting,
·
Public acceptability, and
·
Cost
Screened
alternatives were discussed in detail with the Ashumet Plume Committee leading
to the selection of alternatives and technologies that were carried forward for
detailed analyses.
Tasks 5 and 6: Detailed Analysis of Feasible Corrective
Actions
Regulatory changes and public education alternatives were evaluated by
direct comparison with evaluation criteria and discussion with the Ashumet
Plume Citizens Committee. Engineering
options were also evaluated by developing detailed engineering scenarios and
associated cost estimates. Selected
engineering scenarios were used as examples to predict potential water quality
improvement that may be achieved through implementation. Modeling exercises were completed with water
quality models of the ponds developed specifically for the Nitrogen Offset
Program.
Regulatory, Public Education, and
Management Actions: Planning, zoning, subdivision, health,
wetland regulation, management, and education options evaluated in detail
included:
·
Nominating the
watersheds to Bournes, Green, and Great Ponds, and the ponds themselves, as a
District of Critical Planning Concern (DCPC).
·
Revising Section
305-14 of the Subdivision Rules and Regulations with respect to requirements
governing nutrient loading analyses by prohibiting division of land within the
watersheds to Bournes, Green, and Great Ponds, unless the lots will be served
by a denitrifying wastewater treatment system or be connected to a central
wastewater treatment facility.
·
Revising the
Board of Health regulations to include a new Section FHR 15.2 (6) entitled,
“Septic Systems within the Watershed of Bournes, Green, and Great Ponds.” Include within this section are requirements
that new (upgraded) Title 5 septic systems are to be denitrifying wastewater
treatment systems.
·
As an
improvement to the Town’s Wetlands Protection Bylaw regulations, adopting a
100-foot naturally vegetated buffer strip to sensitive coastal ponds.
·
As an
improvement to the wetland regulations under the Town’s Wetlands Protection
Bylaw, establishing bans on lawns or the use of lawn fertilizers and maximize
natural vegetation within 100 feet of Coastal Resource Areas.
·
Analogous to
the existing regulations for the Black Beach/Great Sippewissett Marsh DCPC,
promulgating lawn limits throughout watersheds to the three ponds.
·
Establishing
criteria for the acquisition of open space to protect water quality as a means
of limiting new sources of nitrogen and maintain vegetative land cover to
mitigate atmospheric nitrogen deposition.
·
Establishing a
public education program to address lawn maintenance and fertilizer use in
Falmouth’s coastal salt pond watersheds.
·
Assessing local
storm drainage systems for prioritization of corrective action to reduce
nitrogen in road runoff or eliminate discharges.
Engineering
Options:
Engineering solutions were assessed by comparison with evaluation
criteria and discussions with the Ashumet Citizen’s Committee. In order to properly address the potential
for water quality improvement, the water quality model developed for this
project, and described in the Task 3 report Water Quality Analysis of Great, Green, and
Bournes Ponds, was used to test engineering scenarios. Results of water quality modeling provided
long-term concentrations of nitrogen at Pondwatcher sampling stations located
throughout the ponds.
The
scenarios that were developed included different types of wastewater treatment
as well as wastewater treatment for different geographic areas. In addition, a scenario was developed to
assess constructed wetland treatment for upper watershed nitrogen loads carried
by freshwater river systems.
Conclusions: Important points identified in the
evaluation of engineering scenarios included:
·
Larger centralized treatment
facilities are the most efficient on a cost per kilogram of nitrogen removed
basis; however, the cost is sensitive to the density of the housing in the area
to be served due to costs associated with the sewer collection systems.
·
Centralized treatment and cluster innovative/alternative
systems are more effective and reliable for removal of nitrogen from wastewater
than smaller onsite innovative/alternative systems. Large cluster systems can be as cost effective as large
conventional wastewater treatment plants.
·
Even though current Title 5
systems do not remove nitrogen from wastewater, a significant cost is borne by
homeowners to maintain and replace these systems due to the limited effective
life of these systems.
The Ashumet Plume Citizen’s Committee has developed the goal of establishing a comprehensive town-wide, long-term plan to
solve nutrient loading impacts to the ponds that includes the following
elements:
The Committee has presented three wastewater
treatment options for the Town of Falmouth to consider:
1.
Use $7.5 million of the money
available from the Air Force to develop a wastewater management district, the
“Nitrogen Management District.”
Available funding would be used for professional services, to purchase
land for treatment and discharge sites, and to fund and permit the treatment systems. The total cost would be $75 million to $100
million, funded by district property owners and/or the Town of Falmouth.
2.
Fund a $7.5 million
sewer/central treatment system for approximately 400 homes at the upper
northern ends of the ponds, to remove about 5 percent of the nitrogen-load at
no cost to those home-owners (except hook-up) or the Town. All other watershed property owners fund
Title 5 systems as needed.
Use $7.5
million to pay for part of a $20 million sewer/central treatment system for more
homes at the upper ends of the ponds to remove 15 percent of the nitrogen load to the ponds. Connecting homeowners would fund the costs
in excess of funds available from the Air Force and all other watershed
property owners would fund individual Title 5 systems as needed.
The
Committee’s consensus has been developed in the Report of the Ashumet Plume
Citizens Committee Water Quality Assessment, Conclusions and Program Options,
October 27, 2000, included in Appendix C of the Task 5 and 6 Report. The Committee’s proposed budget for the
available $8.5 Million is as follows:
Millions
Before
Spring Town Meeting
Present
Consultant Contract $
0.2
Fertilizer
Reduction Program $0.2-0.4
Constructed
Wetlands Demo. $0.2-0.3
Education
& Contingency $0.1-0.3
At
Spring Town Meeting:
Wastewater
Treatment $ 7.5 *
Total
Air Force Funding $
8.5
____
* Plus any additional effort the Town may
decide to undertake