HomeMy WebLinkAboutSMAST Nantucket Habor Modeling Jan 2012_201401231100090127University of Massachusetts Dartmouth
School of Marine Science and Technology
Coastal Systems Group
706 South Rodney French Blvd.
New Bedford, MA 02744-1221
************************Technical Memorandum ****************************
To:David Fronzuto,Nantucket Marine Department
Richard Ray, Nantucket Health Department
From:Brian Howes, Director Coastal Systems Program
Ed Eichner, Coastal Systems Program
Roland Samimy, Coastal Systems Program
John Ramsey, Applied Coastal Research & Engineering, Inc.
RE:Scenarios 1, 2, 3, 4 of Nantucket Harbor MEP Linked Model
Date:Original: August 23, 2011
Updated:January 4, 2012
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The present Technical Memorandum details the results of four (4) Scenario Runs completed using the
MEP Linked Watershed-Embayment Model developed for the Nantucket Harbor System. Development
of the model and establishment of the nitrogen thresholds for the Nantucket Harbor System are described
in the MassDEP/SMAST MEP Nitrogen Threshold Report for the Nantucket Harbor Estuary1. These
Scenarios are the initial planning runs for this system for the Town of Nantucket. The scenarios focus on
nitrogen management strategies within the watershed (scenario 1 and 2),increasing tidal flushing
(scenario 3 which elevates the jetties) and a combination of increasing tidal flushing and nitrogen
management within the watershed (scenario 4 elevates the jetties and utilizes watershed loading
developed for scenario 2). The effects of the watershed and flushing alterations on nitrogen levels
throughout the waters of the Nantucket Harbor System are compared to the MEP target nitrogen levels
(thresholds) needed to achieve restoration. The scenarios were developed by the Town of Nantucket in
order to provide guidance to municipal officials, private citizens and environmental groups to support
decisions regarding the nitrogen management planning and load allocation related to the stewardship of
this critical coastal system.
At present, historic eelgrass and benthic animal habitat within Nantucket Harbor is showing impairment
in the Head of the Harbor and Polpis Harbor sub-basins, although most of the estuary is generally
supporting high quality habitat. Impairment stems from nitrogen enrichment due to watershed nitrogen
inputs that exceed this estuary's assimilative capacity under its present hydrodynamic regime. As part of
nitrogen management planning, the Town of Nantucket is working with the MEP Technical Team
(through SMAST)in the use of the MEP Linked Model developed for this estuary.
1 Howes, B.L., S.W.Kelley, J.S. Ramsey, R.I. Samimy, D.R. Schlezinger, E. Eichner. 2006. Linked Watershed-
Embayment Modeling Approach to Determine Critical Nitrogen Loading Thresholds for Nantucket Harbor, Town
of Nantucket, MA. Massachusetts Estuaries Project Final Report to Massachusetts Department of Environmental
Protection, Boston MA. 168pp.
Page 1 of 7
The two sewering scenarios assume that wastewater is collected in two areas within the Harbor watershed
and is then treated and discharged at the existing Town Waste Water Treatment Facility.The third
modeling run (scenario 3)focuses on tidal flushing by elevating the main jetties at the Harbor inlet.The
fourth modeling run combines the wastewater loading reduction from scenario 2 with the tidal flushing
alteration in scenario 3. Details of the scenarios are as follows:
Scenario 1, Monomoy:
The MEP Nantucket Harbor linked model was used to evaluate the extent of improvement in
nitrogen related water quality that would be achieved by lowering nitrogen loading to the watershed
from septic systems through the extension of sewer lines into the Monomoy area (Figure 1) and
connecting all dwellings possible. This scenario is based on existing, not buildout, watershed
nitrogen loading and model output was assessed relative to the MassDEP TMDL targets for
Nantucket Harbor.
Scenario 2, Monomoy / Shimmo:
The MEP Nantucket Harbor linked model was used to evaluate the extent of improvement in
nitrogen related water quality that would be achieved by lowering nitrogen loading to the watershed
from septic systems through the extension of the sewer lines into both the Monomoy and Shimmo
areas (see Figure 1) and connecting all dwellings possible. This scenario is based on existing, not
buildout, watershed nitrogen loading and model output was assessed relative to the MassDEP TMDL
targets for Nantucket Harbor.
Scenario 3, Elevate jetties:
The MEP Nantucket Harbor linked model was used to evaluate the extent of improvement in
nitrogen related water quality that would be achieved by elevating the jetties to the main tidal inlet of
the Harbor to at or above high tide (full tide jetty). This scenario is based on existing watershed
nitrogen loading documented in the MEP threshold report and model output was assessed relative to
the MassDEP TMDL targets for Nantucket Harbor.
Scenario 4, Monomoy / Shimmo + elevate jetties:
The MEP Nantucket Harbor linked model was used to evaluate the extent of improvement in
nitrogen related water quality that would be achieved by the combined actions of (a)lowering
nitrogen loading to the watershed from septic systems through the extension of sewer lines into
Monomoy and Shimmo areas (Scenario 2) plus (b) increased tidal flushing by elevating the jetties to
the main tidal inlet of the Harbor to at or above high tide (full tide jetty; Scenario 3). This scenario is
based on existing watershed nitrogen loading modification developed in Scenario 2 and model output
was assessed relative to the MassDEP TMDL targets for Nantucket Harbor.
Scenario Results:
As part of the development of the Monomoy and Shimmo sewering scenarios, MEP staff noted the
following watershed/sewer district characteristics:
1. Monomoy sewering scenario (Scenario 1) has 181 new properties to be sewered, all
are in the Harbor subwatershed, some are also within the existing sewer district but not
connected (see figure 1).
Page 2 of 7
2. Monomoy/Shimmo scenario (Scenario 2) has 443 new properties to be sewered
(including the properties in the Monomoy sewering scenario above); 369 are in the
Harbor subwatershed & 74 are in the Quaise subwatershed.
3. In the Monomoy scenario (Scenario 1)there are 1,014 properties that remain
unsewered in the Town subwatershed, 716 of which are developed or developable.
4. Among the remaining 716 developed or developable properties in the Town
subwatershed, 666 are in the existing town sewer district and have either a listing in the
database we received from the town as using a septic system (262 parcels)2 or no listing
of wastewater treatment type (404 parcels).
5. Among the 404 properties within the existing sewer district that have no wastewater
classification type, 106 of them have town assessor-assigned land use codes that would
indicate that they are developed and would need a septic system or sewer connection.
6. Review of the town-supplied databases used in the MEP N loading model indicates
that some of the properties with land use classifications that would not automatically
mean they have a wastewater system have buildings on them. Adjusting the count with
these properties, indicates that the number of potential septic systems inside the sewer
district would be close to 400.
7. Connecting the ~400 properties already inside the existing town sewer district would
have approximately the same watershed nitrogen loading reduction as the proposed
combined sewering of Monomoy/Shimmo.The caveat to this is the need to confirm the
listings in the Town database, which might be a useful next step if the Town opts to move
in this direction at some time in the future.
The effect of extending the sewer lines into the Monomoy area (Scenario 1)and the combined Monomoy
and Shimmo area (Scenario 2)to remove nitrogen loads from existing on-site wastewater septic systems
was to lower the watershed nitrogen load to the Nantucket Harbor Town basin (Scenario 1: Table 1) and
Town and Quaise basins (Scenario 2: Table 2), respectively.No changes in the watershed nitrogen loads
occurred in Scenario 3, only tidal flushing was effected (Table 3).Scenario 4 was the combination of the
septic effluent removal in Monomoy and Shimmo (Scenario 2)plus the tidal flushing enhancement
(Scenario 3).All scenarios resulted in lower nitrogen levels within the receiving waters of the Harbor
(Table 4).
The resulting nitrogen levels within the waters of Nantucket Harbor and its tributary basin, Polpis Harbor,
were lower compared to present conditions in all scenarios, with resulting TN levels in:
Scenario 1 >Scenario 2 >Scenario 3 >Scenario 4.
Scenario 4,the combined effect of extending sewers to Monomoy and Shimmo plus elevating the main
inlet jetties, results in the greatest level of improvement throughout Nantucket Harbor relative to the
TMDL.However, Scenario 3 yields nearly identical results, indicating that almost all of the improvement
in the combined Scenario 4 results were due to the increase in tidal flushing resulting from the
modification of the jetties.Both Scenario 3 and Scenario 4 attain the threshold nitrogen level/TMDL
2 Note that connecting these 262 parcels within the existing sewer district (according to the Town database), would
have a similar effect on lowering watershed wastewater nitrogen as the reductions proposed in Scenario 1.
Page 3 of 7
(0.350 mg/L TN)at the sentinel station within the Head of the Harbor and both are extremely close to the
threshold level/TMDL (0.355 mg/L TN)at the secondary sentinel station within Polpis Harbor.It should
also be noted that all of the scenarios (1,2,3,4)are based upon existing, not build-out, conditions, the
Town must mitigate any new loads from new development to sustain the improvements in any of the
scenarios.
These results indicate that sewer extension to Monomoy and Shimmo areas, alone, is insufficient to meet
the threshold targets for restoring Nantucket Harbor’s impaired resources.The elevation of the jetties
provides more improvement in nitrogen levels, but also is insufficient on its own to meet the threshold
nitrogen level/TMDL at both sentinel stations.Both sewer extension and alteration of hydrodynamics
had positive effects on nitrogen levels.It should also be noted that the parcel analysis conducted within
the existing Town sewered area indicated that a large number of parcels may not presently be connected
to the existing sewer system presenting an opportunity for additional removal of septic nitrogen (if the
town database can be confirmed).
Page 4 of 7
Figure 1. Nantucket Harbor Existing and Proposed Wastewater Collection Areas. The area colored yellow indicates the existing sewer collection
area, while the parcels outlined in red are in the Scenario 1/Monomoy collection area and the parcels outlined in purple are in the Shimmo
collection area. Scenario 2 combines the Monomoy and Shimmo collection areas. Subwatershed boundaries for Nantucket Harbor are shown in
green.Scenario 3 uses existing nitrogen loads and elevates the inlet jetties, increasing tidal flushing.Scenario 4 combines the results of Scenario
2 with elevating the main jetties to the Harbor.
Existing Sewer Area
Monomoy Sewer Area
KEY
Shimmo Sewer Area
Nantucket Harbor Watershed
Page 5 of 7
Table 1.Nitrogen loads used for Scenario 1. The difference between the
present watershed load and Scenario 1 stems from the removal of septic
system wastewater nitrogen load from the Monomoy area (Figure 1).
Subwatershed Area
Present
Watershed
(kg/day)
Scenario 1
Watershed
(kg/day)
Atmospheric
(kg/day)
Benthic
(kg/day)
Head of the Harbor 1.858 1.858 22.239 -17.082
Polpis Harbor 3.529 3.529 2.190 27.370
Quaise 2.123 2.123 20.126 43.643
Town 15.901 15.208 13.888 -2.775
Table 2.Nitrogen loads used for Scenarios 2 and 4. The difference between
the present watershed load and Scenario 1 stems from the removal of septic
system wastewater nitrogen load from the Monomoy and Shimmo areas (Figure
1).
Subwatershed Area
Present
Watershed
(kg/day)
Scenarios 2 &4
Watershed
(kg/day)
Atmospheric
(kg/day)
Benthic
(kg/day)
Head of the Harbor 1.858 1.858 22.239 -16.953
Polpis Harbor 3.529 3.529 2.190 27.335
Quaise 2.123 1.962 20.126 43.517
Town 15.901 14.784 13.888 -2.775
Table 3.Nitrogen loads used for Scenarios 3.Since this scenario only involves
the elevation of the jetties, the present watershed nitrogen load, atmospheric
load, and benthic flux is the same as used in the MEP Threshold Report
assessment of existing conditions.
Subwatershed Area
Present
Watershed
(kg/day)
Scenarios 3
Watershed
(kg/day)
Atmospheric
(kg/day)
Benthic
(kg/day)
Head of the Harbor 1.858 1.858 22.239 -17.211
Polpis Harbor 3.529 3.529 2.190 27.441
Quaise 2.123 2.123 20.126 43.896
Town 15.901 15.901 13.888 -2.793
Page 6 of 7
Table 4.Results of watershed loading reductions and elevation of jetties detailed as Scenarios 1-4 above.
Total nitrogen concentrations at each water quality station under present conditions and for each
scenario are presented in mg/L. Sentinel stations (stations 2.1 and 4) are shown in bold. TN
threshold levels at the respective sentinel stations are 0.350 and 0.355 mg/L.
Station
Station
ID
Present TN
mg/L
Scenario 1
mg/L
Scenario 2
mg/L
Scenario 3
mg/L
Scenario 4
mg/L
Head of the Harbor-Upper 2 0.397 0.396 0.396 0.395 0.394
Head of the Harbor-Mid 2.2 0.390 0.389 0.388 0.387 0.387
Head of the Harbor-Lower 2.1 0.353 0.352 0.352 0.350 0.349
Pocomo Head 3 0.340 0.339 0.339 0.336 0.336
Quaise Basin 3.1 0.325 0.325 0.324 0.321 0.321
East Polpis 4 0.361 0.361 0.360 0.357 0.357
West Polpis 4.1 0.371 0.370 0.370 0.367 0.367
Abrams Point 5 0.296 0.296 0.296 0.292 0.292
Monomoy 6 0.291 0.290 0.290 0.287 0.286
Mooring Area 7 0.285 0.284 0.284 0.281 0.281
Page 7 of 7