U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5060

Prepared in cooperation with the Massachusetts Department of Environmental Protection

The Simulated Effects of WastewaterManagement Actions on the Hydrologic 

System and Nitrogen-Loading Rates to Wells 

and Ecological Receptors, Popponesset Bay 

Watershed, Cape Cod, Massachusetts
Front cover photograph. Northward view of the Eel River in East Falmouth, Cape Cod, Massachusetts. The Eel River, which is typical of the 

estuaries along the southern shore of Cape Cod, is a long, narrow saltwater embayment, referred to locally as a coastal pond, that occupies a 

drowned glacial valley in the Mashpee outwash plain. Photography by Denis LeBlanc, June 20, 2013.
The Simulated Effects of WastewaterManagement Actions on the Hydrologic 

System and Nitrogen-Loading Rates 

to Wells and Ecological Receptors, 

Popponesset Bay Watershed, Cape Cod, 

Massachusetts

By Donald A. Walter

Prepared in cooperation with the 

Massachusetts Department of Environmental Protection

Scientific Investigations Report 2013–5060

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Walter, D.A., 2013, The simulated effects of wastewater-management actions on the hydrologic system and 

nitrogen-loading rates to wells and ecological receptors, Popponesset Bay Watershed, Cape Cod, Massachusetts: 

U.S. Geological Survey Scientific Investigations Report 2013–5060, 62 p., at http://pubs.usgs.gov/sir/2013/5060/.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Hydrogeology.........................................................................................................................................4

Geologic Setting...........................................................................................................................5

Hydrologic Setting .......................................................................................................................5

Recharge Areas to Wells and Ecological Receptors.............................................................7

Nitrogen in the Cape Cod Aquifer......................................................................................................9

Sources of Nitrogen ....................................................................................................................9

Transport of Nitrogen................................................................................................................10

Methods of Analysis...........................................................................................................................12

Groundwater-Flow Modeling...................................................................................................12

Regional-Model Design ...................................................................................................12

Modifications to the Regional Model ............................................................................14

Subregional Model Development ..................................................................................14

Simulation of Solute Transport ................................................................................................15

Simulation of Advective Transport.................................................................................17

Simulation of Mass Transport.........................................................................................17

Simulation of Nitrogen Sources and Wastewater-Management Actions ..............18

Effects of Wastewater-Management Actions on the Hydrologic System .........................................23

Water Levels and Streamflow ..........................................................................................................23

Recharge Areas to Receptors ..........................................................................................................29

Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors ..............................34

Nitrogen Loads Estimated by Using Recharge Areas ..................................................................34

Recharge Areas and Traveltimes............................................................................................34

Estimates of Nitrogen Loads for Current and Future Wastewater Conditions................34

The Effect of Dynamic Recharge Areas on Estimates of Nitrogen Loads........................40

Estimates of Time-Varying Nitrogen Loads by Using a Solute-Transport Model.....................42

Estimates of Current Loads ......................................................................................................42

Effect of Surface Waters ..........................................................................................................43

Assessing System Response to Wastewater-Management Actions ...............................45

Limitations of Analysis .......................................................................................................................57

Summary and Conclusions.........................................................................................................................58

References Cited..........................................................................................................................................60
iv

Figures

1. Map showing extent of coastal aquifers in southeastern Massachusetts and 

hydrography of Cape Cod and location of the Popponesset Bay watershed, western 

Cape Cod, Massachusetts ..........................................................................................................3

2. A, Map showing surficial geology, water-table altitudes, general groundwater-flow 

directions, and location of Popponesset Bay on the Mashpee outwash plain and 

B, generalized north-south hydrologic cross section through western Cape Cod, 

Massachusetts..............................................................................................................................6

3. Map showing water-table altitudes, directions of groundwater flow, and recharge 

areas to wells and major ecological receptors in the Popponesset Bay watershed, 

western Cape Cod, Massachusetts...........................................................................................8

4. Generalized section showing different types of receptors, recharge areas, and 

mechanisms of freshwater discharge in coastal watersheds typical of Cape Cod, 

Massachusetts..............................................................................................................................9

5. Map showing A, land use and B, the locations of wastewater-treatment facilities 

and parcels served by septic systems within the Popponesset Bay watershed, 

western Cape Cod, Massachusetts.........................................................................................11

6. A, Map showing extent of the regional model of the Sagamore flow lens with 

hydraulic boundary conditions, locations of production wells, and simulated watertable altitudes, and B, section showing vertical discretization of the original and 

revised models along column 160 transecting Popponesset Bay, western Cape Cod, 

Massachusetts............................................................................................................................13

7. Graphs showing comparison of hydrologic budgets between A, the unconfined 

(original) and confined (modified) regional models, and B, a subzone of the modified 

regional model and the entire subregional model of the Popponesset Bay 

watershed, western Cape Cod, Massachusetts ...................................................................15

8. A, Map showing the subregional model of the Popponesset Bay watershed 

including hydrologic boundaries and simulated water-table altitudes for the regional 

and subregional models, western Cape Cod, Massachusetts............................................16

9. Maps showing A, return-flow rates from MEP vector data and B, the rasterized 

version of the data mapped to the regional and subregional model grids for the 

Mashpee Planning Area, western Cape Cod, Massachusetts ...........................................19

10. Diagrams showing examples of A, vector data of parcel-scale return flow, B, vector 

data mapped to a raster coincident with a model grid, and C, area-weighted mean 

return flow mapped to the model grid.....................................................................................20

11. Maps showing distribution of simulated current recharge rates representing septicsystem and treated-wastewater return flow derived from Massachusetts Estuarine 

Project data for the Mashpee Planning Area, western Cape Cod, Massachusetts .......22

12. Graph showing wastewater return-flow volumes from Mashpee Planning Area 

vector data and rasterized data used as input to both the regional and subregional 

models, Mashpee Planning Area, western Cape Cod, Massachusetts ............................23

13. Map showing distribution of nitrogen-loading rates associated with current 

wastewater return flow in the Mashpee Planning Area, western Cape Cod, 

Massachusetts............................................................................................................................24

14. Maps showing distribution of nitrogen-loading rates associated with wastewater 

return flow for A, the first phase and B, the second phase of partial sewering and 

disposal at wastewater treatment facilities in the Mashpee Planning Area, western 

Cape Cod, Massachusetts ........................................................................................................25
v

15. Graphs showing A, groundwater-discharge to major receptors for current 

wastewater conditions and three simulated wastewater-management actions, 

and B, changes in groundwater discharge from current conditions for the three 

simulated wastewater-management actions, Popponesset Bay watershed, 

western Cape Cod ......................................................................................................................27

16. Maps showing A, water-table altitude for current conditions and potential future 

conditions resulting from three wastewater-management actions, and B, drawdown 

resulting from full sewering within the Mashpee Planning Area, western Cape Cod, 

Massachusetts............................................................................................................................28

17. Maps showing drawdown resulting from A, the first phase and B, the second phase 

of partial sewering and disposal at wastewater treatment facilities within the 

Mashpee Planning Area, western Cape Cod, Massachusetts ...........................................30

18. Map showing recharge areas to wells and ecological receptors for current 

conditions and A, full sewering and B, two sequential phases of wastewater 

management, Popponesset Bay watershed, western Cape Cod, Massachusetts .........31

19. Map showing recharge areas to Popponesset Creek and Ockway Bay for current 

hydrologic conditions and potential future conditions resulting from two phases 

of wastewater management following partial sewering and disposal, and current 

nitrogen-loading rates from parcels within the Mashpee Planning Area, western 

Cape Cod, Massachusetts ........................................................................................................32

20. Map showing recharge areas to ponds in the Popponesset Bay watershed with 

operation of the Upper Cape Cooperative wells (current conditions) and without 

operation of the wells (potential future conditions), western Cape Cod, 

Massachusetts............................................................................................................................33

21. Map showing distribution of recharge areas to wells and individual ecological 

receptors—streams and estuaries—and parcels containing sources of septicsystem return flow, Popponesset Bay watershed, western Cape Cod, 

Massachusetts............................................................................................................................35

22. Map showing traveltimes to wells and ecological receptors within the Popponesset 

Bay watershed for current conditions, Popponesset Bay watershed, western 

Cape Cod, Massachusetts ........................................................................................................36

23. Graph showing distribution of traveltimes from the water table to wells and 

ecological receptors, Popponesset Bay watershed, western Cape Cod, 

Massachusetts............................................................................................................................37

24. Map showing distribution of traveltimes and nitrogen-loading rates within the 

recharge area to the tidal portion of the Mashpee River for current wastewater 

conditions, Popponesset Bay Watershed, western Cape Cod, Massachusetts .............38

25. Graph showing nitrogen-loading rates from recharge areas to individual ecological 

receptors—streams and estuaries—estimated for current (1997–2001) conditions, 

Popponesset Bay watershed, western Cape Cod, Massachusetts...................................39

26. Graph showing nitrogen-loading rates estimated for recharge areas to wells and 

major ecological receptors under current (1997–2001) conditions and conditions 

resulting from full sewering and two phases of partial sewering and disposal at 

wastewater treatment facilities, Popponesset Bay watershed, western Cape Cod, 

Massachusetts............................................................................................................................40

27. Maps showing recharge areas to Popponesset Creek and Ockway Bay for current 

hydrologic conditions and potential future conditions resulting from two phases of 

wastewater management following partial sewering and disposal, and future 

nitrogen-loading rates from parcels within the Mashpee Planning Area, western 

Cape Cod, Massachusetts ........................................................................................................41
vi

28. Maps showing simulated nitrogen concentrations after 30 years of transport from 

current nitrogen sources for altitude ranges A, 0 to -10 feet, B, -90 to -100 feet, and 

C, along vertical section A–A’, near Popponesset Bay watershed, western Cape Cod, 

Massachusetts............................................................................................................................44

29. Graph showing time-varying loads of waste-derived nitrogen to wells and ecological 

receptors over 100 years of transport from current nitrogen sources, Popponesset 

Bay watershed, western Cape Cod, Massachusetts ...........................................................46

30. Graphs showing A, time-varying loads of waste-derived nitrogen that have passed 

through ponds prior to discharging at receptors and B, components of nitrogen loads 

to three streams, Popponesset Bay watershed, western Cape Cod, Massachusetts ...47

31. Graph showing A, unattenuated and attenuated loads to streams and ponds 

calculated by using loss terms and B, nitrogen loads to the tidal reach of the 

Mashpee River calculated under different loss assumptions, Popponesset Bay 

watershed, western Cape Cod, Massachusetts ...................................................................48

32. Map showing concentrations of nitrogen at an altitude between 0 and -10 feet 

following 30 years of full sewering in the Mashpee Planning Area, western Cape 

Cod, Massachusetts...................................................................................................................49

33. Graph showing changes in loads of waste-derived nitrogen to wells and ecological 

receptors in the Popponesset Bay watershed following full sewering of the 

Mashpee Planning Area, western Cape Cod, Massachusetts ...........................................51

34. Graph showing changes in loads of waste-derived nitrogen at wells and ecological 

receptors in the Popponesset Bay watershed following two phases of partial 

sewering and disposal of treated wastewater at wastewater-treatment facilities 

within the Mashpee Planning Area, western Cape Cod, Massachusetts ........................52

35. Graphs showing changes in nitrogen loads in wastewater from partial sewering, 

septic-system return flow, and disposal at wastewater-treatment facilities in the 

Mashpee Planning Area for A, all receptors and B, streams and estuaries in the 

Popponesset Bay watershed, western Cape Cod, Massachusetts...................................53

36. Graph showing changes in nitrogen loads from direct groundwater discharge 

resulting from partial sewering and disposal at wastewater-treatment facilities in 

the Mashpee Planning Area for individual receptors in the A, Popponesset Bay 

system and B, the eastern part of the Waquoit Bay system, western Cape Cod, 

Massachusetts............................................................................................................................55

37. Graph showing changes in nitrogen-loading rates from septic-system return flow 

and treated wastewater in response to two sequential phases of partial sewering 

and disposal at wastewater-treatment facilities for different load sources in A, the 

Mashpee River and B, Shoestring Bay, Popponesset Bay watershed, western 

Cape Cod, Massachusetts ........................................................................................................56

Tables

1. Simulated hydrologic budgets and septic-system return flow and disposal at 

wastewater-treatment facilities for current (1997–2001) conditions and potential 

future conditions resulting from hypothetical wastewater-management actions in the 

Popponesset Bay watershed and the Mashpee Planning Area, western Cape Cod, 

Massachusetts............................................................................................................................26

2. Names and corresponding identifiers for freshwater streams, estuaries, and open 

coastal waters that receive nitrogen from terrestrial sources, Popponesset Bay 

watershed and the eastern part of Waquoit Bay watershed, western Cape Cod, 

Massachusetts............................................................................................................................54
vii

Conversion Factors, Datum, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

square mile (mi2) 259.0 hectare (ha)

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Application rate

pounds per acre per year 

[(lb/acre)/yr]

1.121 kilograms per hectare per year 

[(kg/ha)/yr]

Vertical coordinate information is referenced to North American Vertical Datum of 1988 

(NGVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Loads are given in kilograms per day (kg/d) and kilograms per year (kg/yr).

List of Abbreviations

FDM finite-difference method

GIS geographic information system

MassDEP Massachusetts Department of Environmental Protection

MEP Massachusetts Estuaries Project

MMR Massachusetts Military Reservation

MPA Mashpee Planning Area

TMDL Total Maximum Daily Loads

UCC Upper Cape Cooperative

USGS U.S. Geological Survey

WTF wastewater-treatment facility
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The Simulated Effects of Wastewater-Management 

Actions on the Hydrologic System and Nitrogen-Loading 

Rates to Wells and Ecological Receptors, Popponesset 

Bay Watershed, Cape Cod, Massachusetts

By Donald A. Walter

Abstract

The discharge of excess nitrogen into Popponesset 

Bay, an estuarine system on western Cape Cod, has resulted 

in eutrophication and the loss of eel grass habitat within 

the estuaries. Septic-system return flow in residential areas 

within the watershed is the primary source of nitrogen. Total 

Maximum Daily Loads (TMDLs) for nitrogen have been 

assigned to the six estuaries that compose the system, and 

local communities are in the process of implementing the 

TMDLs by the partial sewering, treatment, and disposal 

of treated wastewater at wastewater-treatment facilities 

(WTFs). Loads of waste-derived nitrogen from both current 

(1997–2001) and future sources can be estimated implicitly 

from parcel-scale water-use data and recharge areas delineated 

by a groundwater-flow model. These loads are referred to as 

“instantaneous” loads because it is assumed that the nitrogen 

from surface sources is delivered to receptors instantaneously 

and that there is no traveltime through the aquifer. The use of 

a solute-transport model to explicitly simulate the transport 

of mass through the aquifer from sources to receptors can 

improve implementation of TMDLs by (1) accounting for 

traveltime through the aquifer, (2) avoiding limitations 

associated with the estimation of loads from static recharge 

areas, (3) accounting more accurately for the effect of surface 

waters on nitrogen loads, and (4) determining the response 

of waste-derived nitrogen loads to potential wastewatermanagement actions.

The load of nitrogen to Popponesset Bay on western 

Cape Cod, which was estimated by using current sources as 

input to a solute-transport model based on a steady-state flow 

model, is about 50 percent of the instantaneous load after 

about 7 years of transport (loads to estuary are equal to loads 

discharged from sources); this estimate is consistent with 

simulated advective traveltimes in the aquifer, which have a 

median of 5 years. Model-calculated loads originating from 

recharge areas reach 80 percent of the instantaneous load 

within 30 years; this result indicates that loads estimated from 

recharge areas likely are reasonable for estimating current 

instantaneous loads. However, recharge areas are assumed to 

remain static as stresses and hydrologic conditions change in 

response to wastewater-management actions.

Sewering of the Popponesset Bay watershed would not 

change hydraulic gradients and recharge areas to receptors 

substantially; however, disposal of wastewater from treatment 

facilities can change hydraulic gradients and recharge areas 

to nearby receptors, particularly if the facilities are near the 

boundary of the recharge area. In these cases, nitrogen loads 

implicitly estimated by using current recharge areas that do 

not accurately represent future hydraulic stresses can differ 

significantly from loads estimated with recharge areas that 

do represent those stresses. Nitrogen loads to two estuaries 

in the Popponesset Bay system estimated by using recharge 

areas delineated for future hydrologic conditions and nitrogen 

sources were about 3 and 9 times higher than loads estimated 

by using current recharge areas; for this reason, reliance 

on static recharge areas can present limitations for effective TMDL implementation by means of a hypothetical, but 

realistic, wastewater-management action. A solute-transport 

model explicitly represents nitrogen transport from surface 

sources and does not rely on the use of recharge areas; because 

changes in gradients resulting from wastewater-management 

actions are accounted for in transport simulations, they provide more reliable predictions of future nitrogen loads.

Explicitly representing the mass transport of nitrogen 

can better account for the mechanisms by which nitrogen 

enters the estuary and improve estimates of the attenuation 

of nitrogen concentrations in fresh surface waters. Water 

and associated nitrogen can enter an estuary as either direct 

groundwater discharge or as surface-water inflow. Two 

estuaries in the Popponesset Bay watershed receive surfacewater inflows: Shoestring Bay receives water from the Santuit 

River, and the tidal reach of the Mashpee River receives 

water (and associated nitrogen) from the nontidal reach 

of the Mashpee River. Much of the water discharging into 

these streams passes through ponds prior to discharge. The 

additional attenuation of nitrogen in groundwater that has 
2 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

passed through a pond and discharged into a stream prior to 

entering an estuary is about 3 kilograms per day. 

Advective-transport times in the aquifer generally are 

small—median traveltimes are about 4.5 years—and nitrogen 

loads at receptors respond quickly to wastewater-management 

actions. The simulated decreases in nitrogen loads were 50 

and 80 percent of the total decreases within 5 and 15 years, 

respectively, after full sewering of the watershed and within 

3 and 10 years, for sequential phases of partial sewering and 

disposal at WTFs. The results show that solute-transport 

models can be used to assess the responses of nitrogen loads to 

wastewater-management actions, and that loads at ecological 

receptors (receiving waters—ponds, streams or coastal 

waters—that support ecosystems) will respond within a few 

years to those actions.

The responses vary for individual receptors as functions of hydrologic setting, traveltimes in the aquifer, and the 

unique set of nitrogen sources representing current and future 

wastewater-disposal actions within recharge areas. Changes 

in nitrogen loads from groundwater discharge to individual 

estuaries range from a decrease of 90 percent to an increase 

of 80 percent following sequential phases of hypothetical 

but realistic wastewater-management actions. The ability to 

explicitly represent the transport of mass through the aquifer 

allows for the evaluation of complex responses that include the 

effects of surface waters, traveltimes, and complex changes in 

sources. Most of the simulated decreases in nitrogen loads to 

Shoestring Bay and the tidal portion of the Mashpee River, 79 

and 69 percent, respectively, were caused by decreases in the 

nitrogen loads from surface-water inflow.

Introduction

Estuaries and inshore coastal waters adjacent to 

urbanized and developing areas are at risk for eutrophication 

arising from excess nutrients from onshore sources and the 

resultant degradation of water quality and clarity, harmful 

algal blooms, and loss of critical marine habitat. These inland 

marine waters often have limited exchange with open coastal 

waters and receive freshwater from groundwater discharge 

and surface-water inflows that introduce excess nutrients from 

anthropogenic sources into the estuary. The nutrient generally 

of most concern to marine ecosystems is nitrogen, which can 

be introduced into estuaries and inshore coastal waters from 

a number of onshore sources, including road and roof runoff, 

fertilizers, and wastewater. The economic and recreational 

importance of estuaries and inshore coastal waters, which 

provide habitat for commercial and game fish, makes 

eutrophication and habitat loss an increasing concern to local 

communities as well as the regulatory community.

Cape Cod is a rapidly developing area in the coastal 

plain of southeastern Massachusetts (fig. 1). The population of 

Cape Cod more than doubled between 1970 and 2010 with the 

densest development generally in areas near the shore (Cape 

Cod Commission, 2012). As a result, many estuaries on Cape 

Cod have been adversely affected by the discharge of nitrogen 

from sources in the associated watersheds. Eel-grass beds, 

an important habitat for fish and shellfish populations, have 

decreased substantially in most inshore waters since 1950 and 

have been absent from several estuarine systems on Cape Cod 

since the mid-1990s (University of Massachusetts-Dartmouth 

School of Marine Science and Technology, 2012a). In 2001, 

the Massachusetts Department of Environmental Protection 

(MassDEP) initiated the Massachusetts Estuaries Project 

(MEP) in coordination with the University of MassachusettsDartmouth School of Marine Science and Technology to 

assess the ecological health of the Commonwealth’s coastal 

waters and to determine Total Maximum Daily Loads 

(TMDLs) for nitrogen that would establish conditions suitable 

for the growth of eel grass in individual estuarine systems. 

The MEP approach, referred to as the “Linked Watershed-Embayment Management Modeling Approach,” links 

watershed (groundwater-flow) models and embayment 

(hydrodynamic) models in conjunction with the collection 

of water-quality and ecological data to develop the TMDLs. 

Detailed discussions of the application of the approach to a 

number of estuarine systems on Cape Cod are presented in a 

series of reports published between 2003 and 2010 (University 

of Massachusetts-Dartmouth School of Marine Science and 

Technology, 2012). As part of the approach, recharge areas 

(watersheds) delineated by using a groundwater-flow model 

are mapped to detailed spatial data representing current (1997–

2001) nitrogen sources and used to estimate loads from these 

sources to the estuary; these modeled loads are used as inputs 

in a hydrodynamic model of the estuary, which is calibrated to 

represent current water-quality conditions within the estuary 

(Howes and others, 2004). The ecological health of the estuary 

is evaluated, and the rate of nitrogen loading that the system 

can assimilate and still support the growth of eel grass is 

estimated. The same watershed-based method is used to estimate loads resulting from a planned wastewater-management 

action, generally consisting of collection (through sewering), 

treatment, and centralized disposal of wastewater. Finally, the 

same set of hydrodynamic and water-quality models is used to 

determine if the future nitrogen load would result in waterquality conditions that are suitable for the growth of eel grass, 

indicating compliance with the TMDL.

Popponesset Bay is an inland estuary on the south shore 

of western Cape Cod that has been adversely affected by 

development within the estuary’s watershed (fig. 1). The 

estuary has a maximum depth of about 15 feet (ft); daily tidal 

fluctuations in the estuary are between 1 and 1.5 ft (Howes 

and others, 2004). In 1951, there were about 97 acres of eelgrass habitat within the estuarine system, which has a total 

area of about 700 acres; eel grass was absent from the system 

in surveys conducted in 1995 and 2001 (Howes and others, 

2004). As part of the MEP, TMDLs for nitrogen recently 

(2004) have been developed for the individual receptors that 

compose the Popponesset Bay system; a detailed discussion of 
Introduction 3

Figure 1. Extent of coastal aquifers in southeastern Massachusetts and hydrography of Cape Cod and location of the Popponesset 

Bay watershed, western Cape Cod, Massachusetts. Green labels identify flow lenses.

!

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70°40' 70°20' 70°

42°

41°40'

Extent of

Coastal

MASSACHUSETTS Aquifers

Cape Cod

Plymouth-Carver

 Aquifer

Cape Cod Bay

Nantucket Sound

ATLANTIC OCEAN

Buzzards Bay

60

40

20

30

20

10

10

5

10

Popponesset 

 Bay

Sagamore

Monomoy

Nauset

Chequesset

Pamet

Pilgrim

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

Waquoit Bay

Cape Cod Canal

Bass River

SANDWICH

BARNSTABLE

FALMOUTH

MASHPEE

BOURNE

MMR 

WastewaterTreatment

Facility

5

5

MMR

EXPLANATION

Line of equal water-table 

altitude—number is altitude, 

in feet

Popponesset Bay watershed

Massachusetts Military 

Reservation (MMR)

40

0 2.5 5 MILES

0 5 10

10

KILOMETERS
4 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

the application of the MEP approach to the Popponesset Bay 

system is presented in Howes and others (2004).

The response of nitrogen loads to the estuary is of interest 

to local communities that must implement the TMDLs. The 

Town of Mashpee, which had the largest population increase 

on Cape Cod between 2000 and 2003 and encompasses most 

of the Popponesset Bay watershed, is evaluating wastewatermanagement actions to reduce nitrogen loads into the 

Popponesset Bay system in compliance with the TMDLs. 

Terrestrial nitrogen loads estimated by using the MEP 

approach represent what are defined as “instantaneous” loads: 

the load estimated by mapping model-delineated watersheds 

to spatial data on current or future nitrogen sources is assumed 

to be the same as the load discharging to the estuary at that 

time, and, therefore, the approach does not explicitly account 

for the subsurface transport of nitrogen through the aquifer 

from surface sources to the ecological receptor—pond, 

stream, or estuary. Although the use of instantaneous loads 

may be suitable for estimating current nitrogen loads and 

the development of TMDLs, the approach makes it difficult 

for decisionmakers to estimate the time-varying response of 

nitrogen-loading rates to the complex, competing effects of 

sewering and centralized disposal resulting from a wastewatermanagement action. 

The U.S. Geological Survey (USGS), in cooperation with 

the MassDEP, has evaluated calculations by solute-transport 

models of the responses of both the hydrologic system and 

loads of waste-derived nitrogen to wells and ecological receptors within the Popponesset Bay system resulting from a set of 

hypothetical wastewater-management actions similar to those 

being considered by local communities. The purposes of the 

analysis are to (1) develop methods for incorporating detailed 

spatial data on nitrogen sources into a modeling analysis, 

(2) determine how the hypothetical wastewater-management 

actions could affect water levels and streamflows, (3) evaluate how quickly the rate of nitrogen discharge into receptors 

could change in response to wastewater-management actions, 

and (4) demonstrate the applicability of these methods to other 

estuarine systems in southeastern Massachusetts for enhancing implementation of the TMDLs developed by the MEP. 

The analysis has the ancillary benefit of assisting the Town of 

Mashpee in better understanding the dynamics of their coastalaquifer system and protecting local marine ecosystems. The 

methods presented in this report also will have applicability 

to estuarine systems in similar hydrologic environments in 

southeastern Massachusetts and elsewhere.

Purpose and Scope

This report discusses the effects of a set of wastewatermanagement actions—consisting of the collection through 

sewering, treatment, and centralized disposal of wastewater—

on the hydrologic system and ecological receptors within the 

Popponesset Bay watershed. Two sets of effects are discussed: 

(1) the effects on water levels, streamflows, and recharge areas 

and (2) the effects on time-varying rates of nitrogen loading to supply wells and ecological receptors. The report also 

documents the numerical models and methods developed to 

support the analysis and provide information on how similar 

approaches could be applied to other estuarine systems.

The effects of changing anthropogenic stresses—sewering, disposal at wastewater-treatment facilities (WTFs), and 

pumping—on water levels and streamflows are presented 

graphically and discussed. Maps show the effects of such 

stresses on recharge areas to selected ecological receptors, and 

the report includes discussions of how changes in recharge 

areas can affect estimates of nitrogen-loading rates from 

spatial data on nitrogen sources and implications for the use of 

static recharge areas in TMDL implementation.

The methods used to incorporate detailed nitrogen-source 

data into a solute-transport modeling analysis are documented 

in the report, including comparisons of rasterized data used for 

model inputs and original vector data to quantify the accuracy 

of the representation of detailed nitrogen sources in a solutetransport model. The amount of nitrogen loading to wells and 

ecological receptors—ponds, streams, and estuaries—simulated by using a solute-transport model is presented by using 

maps and graphs. The analyses in this report address loads of 

nitrogen only from wastewater sources because (1) wastewater 

is the largest source of nitrogen in coastal watersheds on Cape 

Cod, and (2) management of wastewater is most effective 

for nitrogen-load reduction. The report includes comparisons 

between loads simulated by using a solute-transport model 

and those estimated by using model-delineated recharge areas 

mapped to spatial data on nitrogen sources. The report presents and discusses the responses of nitrogen-loading rates at 

wells and ecological receptors to different wastewatermanagement actions, including (1) no action, (2) complete 

removal of nitrogen through sewering and disposal outside 

of the area, and (3) a two-phase combination of sewering and 

centralized disposal of treated wastewater within and near the 

watershed. Finally, the applicability of the methods to other 

estuarine systems is discussed, as are limitations of the analysis, to allow proper application of the method.

Hydrogeology

Cape Cod is underlain by unconsolidated sediments that 

generally are highly permeable and nearly 1,000 ft thick in 

some areas. The region receives substantial rainfall, and the 

unconsolidated sediments compose the sole source of potable 

water for the region’s communities. The Cape Cod aquifer system consists of six separate flow lenses: Sagamore, Monomoy, 

Nauset, Chequesset, Pamet, and Pilgrim (fig. 1). Each flow 

lens represents a distinct aquifer system that is hydraulically 

separate from adjacent flow lenses. Popponesset Bay and its 

watershed are in the south-central part of the Sagamore flow 

lens—the largest and westernmost of the flow lenses (fig. 1). 

The geologic history and hydrology of Cape Cod have been 

documented in numerous publications, including LeBlanc and 
Introduction 5

others (1986), Masterson and others (1997a), Oldale (1992), 

and Uchupi and others (1996). 

Geologic Setting

The glacial sediments, which consist of gravel, sand, silt, 

and clay and are underlain by crystalline bedrock, were deposited 15,000–16,000 years ago within and near the margins of 

retreating continental ice sheets (Oldale and Barlow, 1986; 

Uchupi and others, 1996). The altitude of the bedrock surface underlying the glacial sediments ranges from about 50 ft 

below North American Vertical Datum of 1988 (NAVD 88) 

near the Cape Cod Canal to more than 900 ft below NAVD 88 

beneath the outer part of Cape Cod; the bedrock surface is as 

deep as 500 ft below NAVD 88 along the shore of Nantucket 

Sound and beneath Popponesset Bay (Fairchild and others, 

2013). The surficial geology of Cape Cod is characterized by 

broad, gently sloping outwash plains and hummocky terrain 

associated with glacial moraines and ice-contact deposits. Outwash sediments, which compose most of the glacial sediments 

underlying Cape Cod, were deposited in fluvial and lacustrine 

depositional environments associated with proglacial lake 

deltas analogous to those seen in present-day fluvial deltas 

(Oldale, 1992). 

Outwash sediments are broadly divided into three 

depositional units: coarse-grained sand and gravel deposited 

in meltwater streams (topset beds), fine to medium sands 

deposited in nearshore lacustrine environments (foreset 

beds), and fine sand and silt deposited in offshore lacustrine 

environments (bottomset beds) (Masterson and others, 

1997a). Geologic contacts generally are gradational, and 

heterogeneities are common, including lenses of silt and clay 

within the shallow, generally coarse-grained sediments—

possibly representing deposition locally within the fluvial 

outwash plain—and lenses of coarse sediments within deep, 

generally fine-grained sediments, possibly representing 

deposition by gravity flows. Moraines were deposited in 

low-energy environments and generally are finer grained and 

less sorted than are outwash sediments; ice-contact deposits 

were deposited within high-energy fluvial environments 

beneath and inside the ice sheets and generally are coarser 

grained than are the outwash deposits. The Popponesset Bay 

watershed is within the Mashpee outwash plain on western 

Cape Cod (fig. 2A); the outwash plain is the largest on Cape 

Cod and is bounded on the north by the Sandwich moraine and 

on the west by the Buzzards Bay moraine (fig. 2A). Outwash 

sediments underlying the Mashpee outwash plain generally 

become finer grained with depth and to the south with 

increasing distance from the sediment source near the presentday Cape Cod Canal. The western part of Popponesset Bay is 

underlain by ice-contact deposits (fig. 2A).

The water-transmitting properties of the glacial aquifer 

sediments are determined primarily by grain size and degree 

of sorting. The trends in hydraulic conductivity of outwash 

sediments result from the trends in grain size; the hydraulic 

conductivity of sediments generally decreases with depth and 

with increasing distance from sediment sources, or generally 

southward (Masterson and others, 1997b). Previous investigations have identified general relations between sediment grain 

size and hydraulic conductivity as determined from aquifer 

tests (Masterson and others, 1997b; Masterson and Barlow, 

1994). Medium to coarse sand and gravel deposits have 

hydraulic conductivities that range from about 200 to 350 feet 

per day (ft/d). Fine to medium sands have hydraulic conductivities typically ranging from 70 to 200 ft/d. The hydraulic 

conductivity of very fine sand and silt typically ranges from 

30 to 70 ft/d; silt and clay deposits have hydraulic conductivities from about 10 to 30 ft/d. Ice-contact and kame deposits 

consist generally of medium to coarse sand and gravel and 

have hydraulic conductivities similar to those of coarsegrained outwash deposits. Moraine deposits have a variable 

lithology—ranging from gravel and sand to silt and clay—and 

generally have lower average hydraulic conductivities than 

outwash deposits. Most areas, including moraines, have trends 

of decreasing hydraulic conductivity with depth.

Hydrologic Setting

The unconsolidated glacial sediments underlying Cape 

Cod compose an unconfined aquifer system that is bounded 

below by relatively impermeable bedrock, above by the water 

table, and laterally by salt water: Cape Cod Bay to the northeast, Cape Cod Canal to the northwest, Buzzards Bay to the 

west, and Nantucket Sound to the south (fig. 1). The Sagamore 

flow lens on central and western Cape Cod is the largest and 

westernmost of the six separate groundwater-flow lenses that 

underlie Cape Cod; the flow lens is hydraulically separated at 

its northwestern extent from mainland Massachusetts by the 

Cape Cod Canal and from the adjacent flow cell by the Bass 

River at its eastern extent. Recharge from precipitation is the 

sole source of water to the aquifer system. About 45 inches 

(in.) of precipitation falls annually on Cape Cod; slightly more 

than half of the precipitation recharges the aquifer across the 

water table (LeBlanc and others, 1986). The remainder is lost 

to evapotranspiration; surface runoff is negligible owing to the 

sandy soils and low topographic relief of the area.

Groundwater flows outward from regional groundwater 

divides towards natural discharge locations at streams, coastal 

estuaries, and the ocean (fig. 1). On western Cape Cod, the 

groundwater flows outward from the northwestern part of the 

Sagamore flow lens (fig. 2A). Most groundwater flows through 

shallow sediments and discharges to streams and estuaries; 

groundwater recharging the aquifer near groundwater divides 

flows deeper in the aquifer and discharges to the ocean 

(fig. 2B). Most groundwater discharge (about two thirds of 

the total) discharges into salt-water bodies. About 25 percent 

of groundwater is discharged into freshwater streams and 

wetlands, and a small amount (less than 10 percent) is 

removed from the system for water supply (Walter and 

Whealan, 2004). Water-table altitudes on the Sagamore flow 
6 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Figure 2. A, Surficial geology, water-table altitudes, general groundwater-flow directions, and location of Popponesset Bay on the 

Mashpee outwash plain and B, generalized north-south hydrologic cross section through western Cape Cod, Massachusetts.

0 3 6 MILES

0 3 6 KILOMETERS

Saline 

groundwater

Saline 

groundwater

Bedrock

Freshwater-saltwater 

 interface

-100

-200

-300

 200

 100

NORTH SOUTH

Nantucket

 Sound

-400

-500

-600

Fresh groundwater

Cape Cod

 Bay

Aerial recharge

 0

Altitude, in feet

Schematic diagram, horizontal not to scale

VERTICAL SCALE GREATLY EXAGGERATED

B

70 4° 1'20" 70° 09'8"

41 46'46" °

41 30'19" °

30

40

50

15

10

20

0 1.5 3 6 MILES

0 3 6 KILOMETERS

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

20 General direction of 

groundwater flow

EXPLANATION

Line of equal water-table 

 altitude, in feet—contour 

 interval is 5 feet

Mashpee outwash plain Buzzards Bay moraine

Sandwich moraine

Buzzards Bay outwash plain

Harwich outwash plain

Barnstable outwash plain

Cape Cod Bay lake deposits

Nantucket ice-contact deposits

Falmouth ice-contact deposits

Popponesset Bay watershed

Cape Cod Bay

Nantucket Sound

Buzzards Bay

Popponesset Bay

A

Cape Cod Canal

Approximate location of 

generalized hydrologic 

section 

 Top of 

water-table 

 mound 

C

C

C′

C′

65

60

55

45

35

25

5
Introduction 7

lens exceed 65 ft above NAVD 88 (fig. 2A). Water-table 

contours and groundwater-flow patterns are strongly affected 

locally by numerous kettle-hole ponds; these are flow-through 

ponds that focus groundwater flow (Walter and others, 2002). 

Groundwater-flow paths converge in areas upgradient of the 

ponds, where groundwater discharges into the ponds, and 

diverge in downgradient areas, where pond water recharges 

the aquifer. Some ponds have surface-water outlets that 

drain into freshwater streams. Streams generally are areas of 

groundwater discharge (gaining streams) and receive water 

from the aquifer. Some stream reaches may lose water to 

the aquifer (losing streams), however, particularly in areas 

downgradient from pond outflows. 

About 7 percent of the water recharging the Cape Cod 

aquifer system is removed for water supply (Walter and 

Whealan, 2004). Most of this water is returned to the system 

as disposed wastewater, either as dispersed septic-system 

return flow or as point discharges to the aquifer at WTFs. 

Although most of the pumped water is returned to the aquifer, the water is usually disposed in areas away from where 

the water was withdrawn, particularly in areas served by 

public water supply. Large-capacity supply wells decrease 

groundwater levels and can affect natural resources by drying 

vernal pools, drawing down ponds, and decreasing streamflows by changing hydraulic gradients and either intercepting 

groundwater that would have discharged to a stream or by 

direct infiltration of water from the stream. In the vicinity of 

large wastewater-disposal facilities, groundwater can become 

mounded, and streamflows can increase locally as a result.

The Popponesset Bay watershed extends from Nantucket 

Sound to the top of the regional water-table mound on 

the Sagamore lens (fig. 2). Groundwater-flow directions 

in the watershed generally are southward from the top of 

the regional water-table mound to discharge locations at 

streams and Popponesset Bay (fig. 3). The northern part of 

the watershed has several large kettle-hole ponds: MashpeeWakeby Pond, Peters Pond, Snake Pond, and Santuit Pond 

(fig. 3). Freshwater streams—Mashpee River, Quaker Run, 

and Santuit River—are the predominant hydraulic features 

within the central part of the watershed. The Popponesset 

Bay system, to the south, consists of six estuaries defined 

by hydrodynamic, water-quality, and ecological criteria: 

Popponesset Bay, Popponesset Creek, Ockway Bay, Mashpee 

River (tidal), Shoestring Bay, and Pinquickset Cove (fig. 3) 

(Howes and others, 2004). Groundwater-flow patterns in the 

northern part of the watershed are affected by kettle-hole 

ponds, and groundwater-flow directions in the central part of 

the watershed converge towards discharge areas at freshwater 

streams; nearer the coast, groundwater-flow patterns are 

controlled by the geometry of coastal water bodies, which 

receive direct groundwater discharge (fig. 3).

Recharge Areas to Wells and Ecological 

Receptors

The area at the water table that contributes recharge to 

a receptor is referred to as the recharge area to that receptor. 

Natural receptors include coastal waters (estuaries and open 

coastal waters), streams, and ponds; water can also discharge 

into pumped wells. Ponds differ from other natural receptors 

in that water entering ponds can reenter the aquifer to later 

discharge into one of the other types of receptors. Recharge 

areas form a mosaic that is a function of hydraulic gradients 

and the locations and types of receptors. Walter and others 

(2004) present a regional mosaic of recharge areas to major 

receptors for Cape Cod as well as examples of recharge-area 

mosaics for surface-water-dominated and groundwaterdominated estuarine watersheds. 

The recharge mosaic indicates the mechanisms by which 

anthropogenic compounds can be introduced into an estuarine system (fig. 4). Water entering the aquifer from recharge 

areas to estuaries, along with any associated anthropogenic 

compounds, discharges directly to those estuaries, whereas 

water and associated compounds entering from recharge areas 

to streams discharge to those streams and enter estuaries as 

surface-water inflow. Water entering recharge areas to ponds, 

along with associated compounds, discharges into the ponds 

and can reenter the aquifer as pond-water outflow. The water 

and associated compounds can then either discharge directly 

to a coastal-water body or into a stream, entering the estuaries 

respectively as groundwater discharge or surface-water inflow. 

In some cases, ponds have surface-water outflows, and some 

water and associated compounds discharge directly from the 

ponds into streams, entering the estuaries with surface-water 

inflow (Walter and Masterson, 2011). 

The Popponesset Bay watershed is a surface-waterdominated system owing to the numerous kettle-hole 

ponds and freshwater streams within the watershed (fig. 3). 

Areas contributing recharge directly to ponds, streams, and 

estuaries compose 40, 33, and 27 percent, respectively, of the 

Popponesset Bay watershed (fig. 3). Recharge and associated 

nitrogen discharge from the southern part of the watershed 

directly to an estuary; from the central part of the watershed 

into freshwater streams and then the estuaries as surfacewater inflow; and from the northern part of the watershed 

into kettle-hole ponds, the aquifer, freshwater streams, and 

finally estuaries as surface-water inflow. Some water and 

associated nitrogen discharging into Mashpee-Wakeby Pond 

and Santuit Pond leave the ponds as surface-water outflow 

into the Mashpee River and Santuit River, respectively. 

Some water leaves the watershed through pumped wells. 

These mechanisms control the introduction of nitrogen into 

the Popponesset Bay estuarine system and could affect the 

potential for the attenuation of nitrogen concentrations prior to 

entering the system.
8 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Figure 3. Water-table altitudes, directions of groundwater flow, and recharge areas to wells and major ecological receptors in 

the Popponesset Bay watershed, western Cape Cod, Massachusetts. 

68

60

64

56

48

40

 8

32

24

16

40

70°32' 70°28'

41°40'

41°36'

0 1 2 MILES

0 1 2 KILOMETERS

Wakeby Pond

Peters

Pond

Snake

Pond

Santuit

Pond

Mashpee

Pond

Mashpee River

Quaker Run 

Santuit River

Mashpee River

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

Ockway Bay NANTUCKET 

 SOUND

Shoestring Bay

Pinquickset

 Cove

Popponesset

 Creek

Popponesset

 Bay

FALMOUTH

MASHPEE

SANDWICH

BARNSTABLE

EXPLANATION

Recharge areas to major receptors, 

 Popponesset Bay watershed

Coastal waters

Streams

Ponds

Wells

Location of production well

Line of equal water-table altitude

 —number is altitude, in feet

Top of water-table mound

General direction of groundwater flow
Introduction 9

Nitrogen in the Cape Cod Aquifer

Nitrogen is the element of most concern in the waters 

around and on Cape Cod owing to its role as a limiting nutrient for algal growth in marine and brackish waters and the 

adverse ecological and economic effects of nitrogen-related 

eutrophication in some estuaries. Nitrogen is introduced into 

the groundwater system from both natural and anthropogenic sources; on Cape Cod, the largest source is wastewater. 

Nitrogen generally is transported conservatively from surface 

sources to receptors under the oxic conditions that prevail in 

the aquifer. 

Sources of Nitrogen

Natural sources of nitrogen in coastal watersheds on 

Cape Cod include leaf fall and atmospheric deposition. In the 

16 estuaries for which analyses have been completed by the 

MEP (2012), recharge through natural areas contributed an 

average of 3.2 percent of the total nitrogen load and did not 

exceed 7 percent of the total nitrogen load in any estuary on 

Cape Cod (http://www.oceanscience.net/estuaries/reports.

htm, accessed March 18, 2012, Ocean Science, 2012). 

Howes and others (2004) reported that leaf fall likely is not 

a significant component of the nitrogen load to Popponesset 

Bay. Atmospheric deposition directly onto receiving waters is 

the largest natural source contributing nitrogen to estuaries, 

although atmospheric-deposition rates are affected by the 

anthropogenic release of nitrogen into the atmosphere. 

Atmospheric-deposition rates between 1910 and 1995 on 

Cape Cod ranged from 0.9 to 4.0 kilograms per hectare per 

year (kg/ha/yr), with an increase of about 0.26 kg/ha/yr per 

decade (Bowen and Valiela, 2000). A concentration of about 

1.09 milligrams per liter (mg/L) in precipitation is used by 

the MEP to estimate loads of wet atmospheric deposition 

onto ecological receptors (Howes and others, 2003). The 

median proportion of total nitrogen load resulting from wet 

atmospheric deposition is 12.0 percent for the 16 watersheds 

for which MEP analyses have currently been completed 

(2012). The proportion for individual estuarine systems 

ranged from less than 1 percent to more than 40 percent and 

is a function of the size of a coastal water body relative to the 

size of the watershed (http://www.oceanscience.net/estuaries/

reports.htm, accessed March 18, 2012).

Anthropogenic sources of nitrogen include wastewater, 

fertilizers, and runoff from impervious surfaces. In groundwater in glacial aquifers that underlie agricultural areas, the 

primary source of nitrogen to pumped wells is nitrogen fertilizers (Warner and Arnold, 2010), and fertilizer application 

has resulted in the contamination of glacial aquifers in many 

areas of the United States (DeSimone and others, 2009). In 

urbanized, nonagricultural areas, such as Cape Cod, fertilizer 

application generally is limited to lawns and golf courses. In 

the 16 coastal watersheds for which MEP analyses have been 

completed, the median proportions of total nitrogen loads to 

estuaries resulting from fertilizer application and runoff from 

impervious surfaces were 5.0 and 6.0 percent, respectively 

(http://www.oceanscience.net/estuaries/reports.htm, accessed 

March 18, 2012). 

Wastewater is the largest source of nitrogen to glacial 

aquifers and ecological receptors on Cape Cod; the mean and 

median proportions of total nitrogen loads in the 16 watersheds for which MEP analyses have been completed were 72.1 

and 72.5 percent, respectively (http://www.oceanscience.net/

estuaries/reports.htm, accessed March 18, 2012). Wastewater sources included both untreated return flow from septic 

systems and the discharge of treated wastewater from WTFs. 

Only about 15 percent of residential areas on Cape Cod 

currently are sewered, and the largest source of wastewaterderived nitrogen is septic-system return flow. A mean of about 

92 percent of the total load of wastewater-derived nitrogen 

for the 16 coastal watersheds is currently from septic-system 

return flow (http://www.oceanscience.net/estuaries/reports.

htm, accessed March 18, 2012). 

Figure 4. Different types of receptors, recharge areas, and mechanisms of freshwater discharge in coastal watersheds typical 

of Cape Cod, Massachusetts (modified from Walter and others, 2004).

WEST EAST

Estuary

 (east)

 Local groundwater  divide

Unconfined aquifer

Estuary recharge 

 area (east)

Stream discharge

 to coast

 Stream 

recharge

 area

Land surface

Stream recharge

 area

Well

Well

recharge 

area

Stream Q

Water table

Recharge

Stream recharge area

Pond

Pond outflow

Deep groundwater (underflow)

Estuary

recharge

area (east)

Pond recharge

 area

Regional groundwater divide

Estuary recharge area (west)

Bedrock

NOT TO SCALE

Estuary

(west)
10 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Land uses within the 19-square-mile (mi2) watershed to 

Popponesset Bay include residential commercial, industrial, 

agricultural, and recreational, and open space (fig. 5A). 

Commercial and industrial land uses compose about 6 and 

1 percent of the watershed, respectively; the remaining 

nonresidential land uses, most of which are undeveloped 

or open space, constitute about 46 percent of the watershed 

(fig. 5A). Residences, which occupy about 47 percent of the 

area (fig. 5A), compose the largest single land use in 

the watershed. 

Residential land use provides a potential source of 

nitrogen from septic-system return flow (Persky, 1986) and, 

as part of the MEP process, parcel-scale water-use data were 

compiled in cooperation with local communities to estimate 

the distribution of return flow—and associated nitrogen—

within the watershed. About 33 percent of the watershed 

receives septic-system return flow (fig. 5B). In 2003, about 

32,300 kilograms per year (kg/yr) of nitrogen from wastewater 

sources discharged into the Popponesset Bay system, 

representing about 66 percent of the total nitrogen load to the 

system (Howes and others, 2003). In addition to septic-system 

return flow, wastewater-derived nitrogen is introduced into 

the watershed at four small wastewater-treatment facilities 

(fig. 5B). Anthropogenic nitrogen loads to the Popponesset 

Bay system from fertilizer application and impervious 

surfaces were 5 and 6 percent, respectively; nitrogen loads 

to the Popponesset Bay system from natural sources—wet 

atmospheric deposition and recharge onto natural areas—were 

17 and 5 percent, respectively (Howes and others, 2003).

Transport of Nitrogen

Background, or natural, concentrations of nitrogen 

generally are less than 0.1 mg/L in the Cape Cod aquifer 

(LeBlanc, 1984; Savoie and others, 2012; Barbaro and 

others, 2013). Concentrations of nitrogen in septic-system 

effluent can exceed 30 mg/L as N, and concentrations 

exceeding 10 mg/L have been observed in groundwater near 

residential areas (Weiskel and Howes, 1992). Advective 

transport generally is the dominant component of transport 

for conservative solutes such as nitrate in the aquifer because 

of high recharge rates and the generally high permeability 

of the aquifer sediments (LeBlanc, 1984). Groundwater 

velocities of more than 1.5 ft/d have been observed at 

the USGS Toxics Substances Hydrology Research Site 

near the former wastewater-treatment facility (WTF) at 

Massachusetts Military Reservation (MMR) (fig. 1) (LeBlanc 

and others, 1991). The rate of advective transport in the 

aquifer is related to the location of a source area relative to 

regional groundwater divides and discharge locations; near 

divides, where horizontal gradients are small and downward 

components substantial, groundwater flow is more vertical 

and slower than flow recharged farther from divides, where 

horizontal flow predominates (fig. 2B) (Walter and Masterson, 

2003b; Walter and others, 2004). Traveltimes, defined as the 

total times required for water to move from recharge locations 

at the water table to natural discharge locations, are largest for 

groundwater flow that originates near regional groundwater 

divides and range from essentially zero adjacent to discharge 

boundaries to hundreds of years near groundwater divides 

(Walter and others, 2004).

Nitrogen in natural waters occurs primarily as nitrate 

(NO3

-

), ammonium (NH4

+

), dissolved nitrogen gas (N2(g)), 

or as organic nitrogen; the predominant species of inorganic 

nitrogen in groundwater is determined by the pH and redox 

conditions of the waters (Hem, 1985). LeBlanc (1984) 

reported that nitrate was the predominant form of nitrogen 

in treated sewage effluent at the MMR wastewater-treatment 

facility, and nitrate emanating from the site has been 

transported, primarily by advection, more than 7 miles (mi) 

downgradient from the source (Barbaro and others, 2013). 

Denitrification occurs within parts of the plume characterized 

by high organic carbon and low dissolved oxygen 

concentrations; nitrogen is primarily in the form of ammonia 

in anoxic parts of the plume (Barbaro and others, 2013). In 

addition, Weiskel and Howes (1992) found that most nitrogen 

emanating from septic systems in a sandy coastal aquifer in 

southeastern Massachusetts was in the form of nitrate.

Denitrification, in which nitrate is reduced to nitrogen 

gas, can remove or attenuate nitrate in an aquifer (Hem, 1985) 

and in most aquifers is coupled with the oxidation of organic 

carbon present in aquifer. The glacial aquifer underlying 

Cape Cod generally is oxic and contains little organic carbon 

(LeBlanc, 1984), and, therefore, nitrate attenuation through 

denitrification is likely to be limited. Most data suggest that 

the oxic, carbon-poor conditions in the Cape Cod aquifer 

generally would be favorable for the persistence of nitrate in 

the subsurface and the conservative transport of nitrate with 

groundwater flow (advection) from surface sources to ecological receptors.

Reducing conditions favorable for denitrification may 

occur in (1) septic-system return flow prior to recharge at 

the water table, (2) large plumes downgradient of WTFs and 

landfills, (3) local-scale reducing environments in the aquifer, 

(4) muddy, organic-rich bottom sediments in estuaries, and 

(5) surface-water bodies. Weiskel and Howes (1992) found 

that a total of about 16 to 36 percent of nitrogen (primarily 

as nitrate) in septic-system effluent was lost during transport over small distances (less than about 2,000 ft), mostly 

within the septic systems or in the underlying unsaturated 

zone prior to recharge at the water table. Local environments 

can be reducing in deep parts of the aquifer, suggesting the 

potential for denitrification and nitrogen attenuation over long 

transport distances and times (Colman and others, 2004). 

Kroeger and Charette (2008) report geochemical conditions 

favorable for and evidence of denitrification within estuarine 

muds in similar hydrogeologic settings. However, the importance of these attenuation processes in the Cape Cod aquifer 

is unknown, and estimates of nitrogen loads to estuaries by 

the MEP are based on the assumption of no attenuation in 

aquifer or estuarine sediments, so that the highest plausible 

load estimates will be used to implement nitrogen TMDLs. 
Introduction 11

Figure 5. A, Land use and B, the locations of wastewater-treatment facilities and parcels served by septic systems within the 

Popponesset Bay watershed, western Cape Cod, Massachusetts.

70°32' 70°28'

41°40'

41°36'

NANTUCKET

SOUND

Popponesset Bay

70°32' 70°28'

Popponesset Bay NANTUCKET

SOUND

Parcel with septic-system return flow

General land use

Residential

Commercial

Industrial

Agricultural

Recreation

Open space

Other (undeveloped)

Wastewater-treatment facility

Golf

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

0 1 2 MILES

0 1 2 KILOMETERS EXPLANATION

A B
12 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Nitrogen in coastal watersheds is attenuated within surface 

waters prior to entering coastal receiving waters; attenuation 

results from geochemical conditions favorable for denitrification and uptake of nitrogen by biota within the surface waters. 

Current nitrogen-load estimates by the MEP are based on the 

assumption that 50 percent of nitrogen is attenuated within 

ponds and 30 percent in streams; the attenuation is accounted 

for by decreasing nitrogen sources within the recharge areas 

to surface waters—50 and 30 percent from recharge areas to 

ponds and streams, respectively (Howes and others, 2003).

Methods of Analysis

Groundwater-flow and single-phase solute-transport 

modeling techniques were used to simulate the effect of 

wastewater-management actions on nitrogen loading to the 

Popponesset Bay estuarine system. First, an existing regional 

groundwater model was modified to facilitate the simulation 

of nitrogen transport (conservatively as nitrate) within the 

Popponesset Bay watershed; second, a subregional model 

of the watershed was developed and linked to the modified 

regional model (fig. 6A); and third, the subregional model was 

linked to a solute-transport code to allow for the simulation 

of nitrogen transport in the aquifer to wells and ecological 

receptors. A Geographic Information System (GIS) was used 

to convert detailed data on nitrogen sources into model inputs.

Groundwater-Flow Modeling

An existing regional model of the Sagamore flow lens 

previously developed to determine the hydrologic budget and 

the effects of current and future well withdrawals on surface 

waters and to delineate recharge areas to wells and ecological receptors was repurposed in support of the MEP (Walter 

and Whealan, 2004; Walter and others, 2004). A detailed 

discussion of the model design and calibration is presented 

in Walter and Whealan (2004). The existing regional model 

was calibrated to current hydrologic conditions and includes 

stresses similar to current stresses. In addition, the existing 

regional model also was the basis for previous work to delineate recharge areas to receptors as part of the MEP process. 

Therefore, it was considered reasonable to modify the regional 

model to facilitate simulations of nitrogen transport discussed 

in this report. 

Regional-Model Design

The regional model of the Sagamore flow lens extends 

from the Cape Cod Canal to the Bass River (fig. 1) and 

uses the finite-difference groundwater-modeling program 

MODFLOW–2005 (Harbaugh, 2005) to simulate groundwater 

flow in the aquifer. The model has 246 rows and 365 columns 

with a uniform horizontal discretization of 400 ft and a total 

active modeled area of 246 mi2 (fig. 6A). The original model 

has 20 layers with thicknesses ranging from 10 ft in the top 

170 ft of saturated thickness to more than 200 ft in the deepest 

layer (fig. 6B).

Surface waters are represented as head-dependent flux 

boundaries, with open coastal waters represented by the GHB 

Package and saltwater wetlands and some freshwater wetlands 

by the DRN Package (Harbaugh and others, 2000) (fig. 6A). 

Freshwater streams and some freshwater wetlands were simulated by the STR package (Prudic, 1989), which allows for an 

accounting of streamflow and pumping-induced streamflow 

depletion. Freshwater ponds are simulated as active parts of 

the aquifer, and simulated pond levels can change in response 

to changing stresses. Ponds are represented as areas of very 

high hydraulic conductivity (100,000 ft/d) within the aquifer; 

the HFB Package (Hsieh and Freckleton, 1993) is used to 

represent pond-bottom sediments implicitly.

A natural recharge rate onto aquifer sediments of 

27.1 inches per year (in/yr) is specified as the sole source 

of water to the aquifer by the RCH Package (Harbaugh 

and others, 2005). This value was obtained from long-term 

precipitation records, was adjusted during model calibration 

(Walter and Whealan, 2004), and is consistent with previous 

investigations on Cape Cod and southeastern New England 

(Barlow and Dickerman, 2000; DeSimone and others, 2001; 

Masterson and others, 1997b; Walter and Masterson, 2003b). 

Recharge onto surface-water bodies was 16 in/yr, representing 

the difference between precipitation and pan evaporation 

(Farnsworth and others, 1982). Recharge was further adjusted 

to account for septic-system return flow in residential 

areas. Parcel-scale water-use data were not available during 

development of the regional model (2002), so the volume of 

generated wastewater was determined from the total volume 

of groundwater withdrawn by each town; a consumptive loss 

of 15 percent was applied, and the remaining volume was 

evenly distributed across model cells representing nonsewered 

residential areas. Recharge also was adjusted in areas of 

existing wastewater-disposal facilities (Walter and Whealan, 

2004). Withdrawal of water from 95 production wells was 

simulated by using the WEL Package (McDonald and 

Harbaugh, 1988) and average pumping rates for 2003 (Walter 

and Whealan, 2004). Pumping at three proposed (2003) 

wells also was represented in analyses presented in Walter 

and Whealan (2004); these wells, which currently (2012) are 

in operation, are referred to as the Upper Cape Cooperative 

wells (fig. 6A) and were installed in the northern part of the 

MMR to offset any possible losses of potable water related to 

contamination from sources on the MMR.

Aquifer properties were estimated from lithologic logs 

and from previously developed depositional models of central 

and western Cape Cod (Masterson and others, 1997b; Byron 

Stone, U.S. Geological Survey, written commun., 2002). 

Hydraulic conductivities varied spatially and with depth and 

ranged from 350 ft/d for coarse sand and gravels to 10 ft/d for 

silt and clay (Walter and Whealan, 2004). The steady-state 

regional models were calibrated by using measured longterm water levels and streamflows; in addition, delineated 

contaminant plumes in and around the MMR to the west of 
Introduction 13

Figure 6. A, Extent of the regional model of the Sagamore flow lens with hydraulic boundary conditions, locations of production 

wells, and simulated water-table altitudes, and B, section showing vertical discretization of the original and revised models along 

column 160 transecting Popponesset Bay, western Cape Cod, Massachusetts.

41°30'17"

70°9'8" 41°46'56"

-100

 0

-240

Layer 27

ALTITUDE

IN FEET

Layer 1

Bedrock

-497 Column 160

New layers

Existing layers

Confined layers (1-7) Popponesset 

 Bay

B

NORTH SOUTH

MODEL SCHEMATIC ONLY, NOT TO SCALE

35

10

5

0 2.5

0 2.5

5 MILES

5 KILOMETERS

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

70°41'41"

60

65

50

55

40

15

20

25

30

Row 97

No flow

Nantucket Sound

A

Popponesset 

 Bay

Column 160

Row 1, Column 1

 Row 246, 

Column 365

C

C′

C C′

45

Production well—Upper Cape

 Cooperative wells

Mashpee Planning Area

Coastal boundary 

Estuary boundary 

Saltwater boundary

Pond

Stream 

Freshwater boundary

EXPLANATION

Production well—town

Simulated water-table altitude, 

 in feet

40

Popponesset Bay subregional 

 model
14 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

the Popponesset Bay watershed were used as indicators of 

advective flow paths to adjust hydraulic parameters to better 

match regional hydraulic gradients (Walter and Whealan, 

2004). Initial estimates of recharge and intrinsic aquifer 

properties were adjusted during the calibration process to 

achieve an acceptable match between observed and simulated 

hydrologic conditions at the calibration points (Walter and 

Whealan, 2004). Although recharge and pumping stresses 

vary through time, and versions of the regional models 

that incorporate these time-varying stresses are available, 

steady-state models likely are adequate for the simulation 

of advective flow because the time scale of the variability of 

these stresses is shorter than the time scale of transport in the 

aquifer (Walter and Masterson, 2003).

Modifications to the Regional Model

Two sets of modifications were made to the regional 

model to facilitate better simulation of nitrogen transport: (1) 

conversion of the model to a linear (confined) configuration 

and (2) addition of model layers to increase the vertical 

discretization at depth. The existing model is an unconfined 

(non-linear) model that has seven layers with uniform 

thicknesses of 10 ft above NAVD 88—these layers can go 

dry as the simulated head falls below the bottom of a layer. 

Although this model configuration is sufficient for the 

simulation of groundwater flow, the resultant nonlinearity can 

be problematic for solute-transport simulations. The model 

was converted to a confined configuration—or linearized—by 

changing layer tops and bottom to conform to the simulated 

water table. For a given set of stresses, the water-table altitude 

was first simulated by using the unconfined configuration. 

In each model cell, the bottoms of the 7 layers above sea 

level were spaced equally between the simulated water-table 

altitude and 10 ft below sea level (NAVD 88) (fig. 6B), and the 

layers were represented as confined. Model boundaries, which 

were assigned to different layers on the basis of altitude, were 

all assigned to the top layer by using the same altitude as in 

the original model.

The original regional model of the Sagamore flow lens 

has a uniform discretization of 10 ft for the top 17 layers, 

which were above -100 ft (Walter and Whealan, 2004). In the 

modified regional model, layer bottoms were adjusted for the 

top 7 layers as part of the linearization process, so that the 

thicknesses of the 8 layers between the water table and an altitude of -10 ft (as described above) would be variable but less 

than 10 ft. The next 10 layers down to -100 ft have a uniform 

thickness of 10 ft in both the original and linearized models 

(fig. 6B). The bottom 3 layers in the original model, which 

can be truncated by the bedrock surface, were subdivided into 

10 layers in the modified regional model to decrease layer 

thicknesses and thus the amount of numerical dispersion in 

the solute-transport simulations. The maximum thicknesses of 

the bottom layers in the original and modified regional models 

are 257 and 107 ft, respectively, in the deepest part of the 

simulated aquifer along regional model column 160 underlying Popponesset Bay (fig. 6B).

Although the bottom altitudes of the layers in the two 

model versions differ for the top seven layers, the resulting 

combined transmissivity of the layers is the same for both 

model configurations; likewise, subdivision of the bottom 

4 layers into 11 layers does not change the total transmissivity 

in that part of the aquifer. As a result, the unconfined and 

confined model solutions are essentially identical. The 

simulated mean head difference between the confined and 

unconfined models at 301 head-observation locations (Walter 

and Whealan, 2004) was about 0.02 ft; the simulated fluxes 

at hydrologic boundaries were within 1 percent for each 

boundary type (estuary, coast, and streams) (fig. 7A). Both 

confined and unconfined models were run and compared for 

all scenarios to ensure consistency between the models.

Subregional Model Development

A subregional groundwater model was developed that 

encompasses about 5 mi2 on western Cape Cod. The model 

domain includes the entire Popponesset Bay watershed and 

extends from near the top of the water-table mound (the 

regional divide for the Sagamore lens) at its northern extent 

to Popponesset Bay and Nantucket Sound at its southern 

extent (fig. 6A). The subregional model has 592 rows and 

264 columns that coincide with the regional-model grid, but 

with a uniform horizontal discretization of 100 ft. The model 

has 27 layers with the same vertical discretization as the 

modified regional model (fig. 6B). The hydrologic boundaries 

in the subregional model coincide with those of the regional 

model (fig. 8). The finer horizontal discretization could 

have facilitated a more detailed representation of hydraulic 

boundaries; however, coincident boundaries were chosen to 

allow for consistency between the analyses presented here 

and analyses done as part of a previous modeling effort in 

support of the MEP (Walter and others, 2004). The only 

change in hydrologic boundaries was for freshwater streams, 

which were represented as streams by using the STR Package 

(Prudic, 1989) in the regional model and as drains in the 

subregional model by using the DRN Package. The change 

in the stream-simulation method minimized overly complex 

input data files arising from the subregional discretization; 

because streams on Cape Cod generally are gaining features 

that receive groundwater discharge, the representation of 

streams as drains was appropriate. The subregional model was 

linked hydraulically to the regional model with a specifiedhead boundary along its external boundary by using the CHD 

Package (Harbaugh and others, 2005); the specified heads 

along the external boundary are the same as the heads in the 

regional model at coincident cells.

Hydraulic stresses (recharge and pumping) and intrinsic 

properties—hydraulic conductivity and boundary leakances—

are the same in the subregional model and coincident parts 

of the regional model. Hydraulic conductivities of the 

shallowest outwash and ice-contact deposits that underlie 
Introduction 15

the Popponesset Bay watershed and comprise coarse fluvial 

sand and gravel range from about 300 ft/d in the northern 

part of the watershed to about 180 ft/d near Nantucket Sound. 

Hydraulic conductivities in the deepest sediments that underlie 

the watershed and comprise fine sand, silt, and clay deposited 

in low-energy lacustrine environments are about 10 ft/d. The 

Figure 7. Comparison of hydrologic budgets between A, the 

unconfined (original) and confined (modified) regional models, 

and B, a subzone of the modified regional model and the entire 

subregional model of the Popponesset Bay watershed, western 

Cape Cod, Massachusetts.

0.0

10.0

20.0

30.0

40.0

Flow rate, in millions of cubic feet per day

Estuaries

Open coastal 

 waters

Streams

Wells

Recharge

Total

Wells

Recharge

 Net exchange

 across regional 

 model boundaries

Hydrologic-budget component

0.0

2.0

4.0

6.0

8.0

Flow rate, in million s of cubic feet per day

Estuaries

Open coastal 

 waters

Unconfined model

Confined model

EXPLANATION

Subregional model

EXPLANATION

A

B

Coincident area of 

 the regional model

 Dispersion + Diffusion –

Advection – Sorption + Reactions

regional and subregional models yield essentially identical 

solutions because the subregional model is consistent with 

the coincident part of the regional model and linked to it 

by a consistent specified-head boundary. Simulated watertable altitudes and gradients in the subregional model and 

the coincident part of the regional model also are essentially 

identical (fig. 8). Similarly, recharge and fluxes across 

hydrologic boundaries and subregional model boundaries 

agree within 1 percent (fig. 7B).

Simulation of Solute Transport

Sources of anthropogenic contaminants on Cape Cod 

include point sources—such as landfills, WTFs, and industrial 

sites—and nonpoint sources, such as septic systems. Contaminants are transported as dissolved solutes with groundwater 

flow from the water table underlying surficial sources to wells 

and ecological receptors. As an example, contaminant plumes 

around the MMR on western Cape Cod (fig. 1) have migrated 

several miles downgradient since the mid-1950s from a 

number of sources on the facility (U.S. Air Force Center for 

Environmental Excellence (AFCEE), 2000; AFCEE, written 

commun., 2008). The one-dimensional subsurface transport of 

a solute can be expressed as 

δC/δt = DL (δ2C/δx2) + Dd (δ2C/δx2) – 

νx (δC/δx) – (Bd /n) (δC(s)/δt) + (δC/δt)rxn, (1)

where

C = concentration of dissolved solute,

t = time,

DL = longitudinal dispersivity,

Dd = diffusivity,

νx linear groundwater velocity,

Bd = bulk density of aquifer sediments,

n = porosity,

C(s) = concentration of sorbed solute, and

rxn = subscript indicating chemical or biological 

reactions (modified from Fetter, 1992).

In the case of nitrate, which is conservative and 

nonreactive in oxic, carbon-poor aquifers, the last two terms 

of the equation (sorption and reactions) can be ignored, and 

diffusion generally is insignificant in advection-dominated 

systems. The result is a transport equation in which the 

change in concentration with time is a function of advection 

and dispersion. Advection refers to transport of a solute with 

groundwater flow; dispersion, which occurs in all natural 

aquifers, refers to the spreading of solute mass caused by 

aquifer heterogeneity and preferential flow paths relative to 

average groundwater flow.
16 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Figure 8. A, The subregional model of the Popponesset Bay watershed including hydrologic boundaries and simulated watertable altitudes for the regional and subregional models, western Cape Cod, Massachusetts.

130

83

67 68

666

777

80

79

78

99

41

65

76

81

66

129

179

64

142

144

178

82

143

63

7

6

12

8

10

9

11

70°32' 70°28'

EXPLANATION

Individual receptors

Estuaries and open coastal waters—

 number is identifier in table 2

Freshwater streams—number is 

 identifier in table 2

Freshwater ponds

Water table from subregional 

 model—number is altitude, in feet, 

 interval is 2 feet

Location of production well

Water table from regional model—

 number is altitude, in feet, interval 

 is 2 feet

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

Constant head boundary

Extent of Popponesset Bay model

80

6

41°36'

41°40'

Popponesset Bay watershed

60

50

30

40

20

10

68

20

20
Introduction 17

Simulation of Advective Transport

Groundwater-flow models are mathematical representations of advective transport based on the assumption that mass 

is conserved along flow lines in an aquifer. Advection generally is the dominant transport mechanism in highly permeable, well-sorted aquifers, such as that which underlies Cape 

Cod, and, therefore, groundwater-flow models can be used to 

approximate transport. The use of particle tracking to represent the transport of nitrate in the Cape Cod aquifer implicitly 

is presented in Walter (2006). The advective transport of 

contaminants in the aquifer was simulated by using the USGS 

particle-tracking software algorithm MODPATH (Pollock, 

1994), which uses cell-by-cell flows from the groundwaterflow model to track the movement of a particle or a field 

of particles through the simulated aquifer. Particle-tracking 

results were geoprocessed and displayed graphically, in part, 

by using the USGS postprocessing software MODTOOLS 

(Orzol, 1997).

Particle tracking was used to delineate recharge areas to 

wells and ecological receptors by seeding a field of particles 

at the water table and tracking them forward to a receptor. 

Particles entering a receptor of interest were identified; the 

starting locations of those particles were processed into georeferenced polygons by using a GIS. Traveltimes of individual 

particles from the water table to the receptor were processed 

into separate polygons. The porosity of aquifer sediments, to 

which velocities and traveltimes are inversely proportional, 

was specified as 0.3, typical of sandy glacial sediments.

Simulation of Mass Transport

Solute-transport codes explicitly represent the transport 

of a solute through the aquifer by solving independently for 

concentrations and, with additional terms, can account for 

dispersion, diffusion, sorption, and simple one-component 

reactions during transport (eq. 1). Nitrate is assumed to be the 

predominant nitrogen species given the geochemical conditions in the aquifer; sorption and reactions are assumed not to 

be important components of transport. Furthermore, diffusion 

is assumed to be negligible compared to advection and dispersion. The assumption of conservative transport also maximizes 

plausible estimates of mass-loading rates, an assumption that 

may be useful for adequate resource protection. 

The solute-transport capabilities of particular interest in 

this analysis are (1) the explicit simulation of the transport 

of mass through the aquifer and surface waters, (2) the direct 

estimation of mass-loading rates at receptors, and (3) accounting for changing hydraulic gradients and dynamic recharge 

areas. The subsurface transport of nitrate was simulated by 

using the three-dimensional transport code MT3DMS (Zheng, 

1999). MT3DMS simulates several components of transport, including advection, dispersion, and diffusion, and can 

simulate some single-species chemical reactions, including 

contaminant decay and various types of linear and nonlinear 

sorption. MT3DMS is linked to MODFLOW by the LMT6 

Package (Zheng and others, 2001) and uses cell-by-cell flows 

and velocities from the flow model to simulate the movement 

of nitrate through the aquifer. The model calculates dissolved 

and sorbed concentrations of the solute of interest at cells 

within the active model domain at specified transport time 

steps. This information can be used to develop maps and cross 

sections of solute concentrations, breakthrough curves, and 

estimates of time-varying mass-loading rates at receptors. 

Mass-loading rates of nitrate at wells and ecological receptors 

were determined by using the Transport Observation (TOB) 

Package (Zheng, 2010).

The most important components of transport are 

advection and, to a lesser extent, dispersion for conservative 

solutes like nitrate. Dispersion, like heterogeneity, is scale 

dependent and generally increases with increasing transport 

distance. Dispersivity is near zero in a perfectly homogenous 

system and increases as the degree of heterogeneity increases. 

The spreading of mass in the aquifer affects concentrations 

and mass-loading rates at receptors. Longitudinal dispersion 

refers to a spreading of mass in the direction of groundwater 

flow and is the largest dispersive component; transverse 

and vertical dispersion refer to spreading of mass along the 

two axes orthogonal to the groundwater-flow direction and 

are much smaller than the longitudinal component. Based 

on an empirical relationship derived from literature values, 

longitudinal dispersivity ranges from 42 to 55 ft for transport 

distances of 7,500 and 13,500 ft, respectively (Luckner 

and Schestakow, 1991). Spitz and Moreno (1996) suggest 

longitudinal dispersivities of about 90 ft for a plume-scale 

transport distance of about 13,000 ft. AFCEE (2000) found 

that longitudinal, transverse, and vertical dispersivities of 

35, 3.5, and 0.35 ft best matched plume geometries at a 

similar transport distance at the MMR. Transport distances 

from sources to receptors in the Popponesset Bay watershed 

typically are estimated to be less than 10,000 ft from recharge 

areas to major receptors (fig. 3); thus, dispersivity between 35 

and 90 ft may be applicable at the watershed scale on the basis 

of literature values.

The transport equation can be solved by using either a 

finite-difference method (FDM) or particle-based methods 

such as the methods of characteristics. The FDM was used 

for these simulations because the method allows for accurate 

conservation of mass and is computationally efficient. In 

advection-dominated systems, particle-based methods generally produce less numerical dispersion; however, conservation 

of mass can be inaccurate. The use of a mass-balance method, 

such as FDM, in advection-dominated systems can result in 

numerical dispersion; however, the problem can be minimized 

by using numerical models with a suitably small discretization. Numerical dispersion is a function of model discretization, velocity, and time-step length and can be approximated 

from the equation

α X V T

2 2 , (2)
18 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

where

α is numerical dispersion,

∆X is cell length,

V is velocity, and

∆T is the time-step length (Fletcher, 1991)

With a cell length of 100 ft, a velocity of 1 ft/d, and a 

time step of 4 days, the approximate numerical dispersion is 

52 ft. The Peclet number, which is defined as the ratio of cell 

length to dispersivity, is about 2. For advection-dominated 

systems, a Peclet number of less than about 4 indicates that 

the FDM is appropriate for use in solving the solute-transport 

equation. The criteria used to determine model discretization 

include the need to (1) minimize numerical dispersion 

(through minimal spatial and temporal discretization) and 

(2) limit the size of the model to make the transport simulation 

tractable. The finite-difference discretization of 100 ft used 

in the subregional model produces a reasonable numerical 

dispersion according to literature values, and still allows 

for a tractable model size for transport simulations—about 

4.2 million cells. Numerical dispersion, as an artifact of 

a finite-difference solution, is a behavior equivalent to 

dispersion within an aquifer (Bear, 1979, p. 262). The 

approximate numerical dispersion in the solute-transport 

model—52 ft estimated from equation 1—is within the 

range of literature values between 35 and 90 ft for transport 

distances typical for the Popponesset Bay watershed. This 

estimate indicates that numerical dispersion alone may 

reasonably represent the general scale of real dispersion over 

watershed-scale transport distances in the aquifer. As with 

particle-tracking analyses, a porosity of 0.3 was specified 

for the solute-transport simulations. The effect of simulated 

dispersivities on time-varying nitrate concentrations and 

loads to receptors in the western Cape Cod aquifer system is 

presented in Walter (2006). It should be noted that dispersion 

affects the rate of change of concentrations and loads at a 

receptor, but not the steady-state loads, which are attained 

after sufficiently long transport times.

Hydrodynamic mixing of solutes within ponds cannot 

be explicitly represented in the transport model but can be 

implicitly represented by enhanced dispersion through the 

ponds. Ponds are represented as areas of high hydraulic conductivity and, as a result, flow is focused through the ponds. 

Open surface waters have an effective porosity of 1; however, 

the porosity of the ponds was specified to be the same as the 

aquifer (0.3) to increase velocities. The larger velocities result 

in more dispersion within the pond, approximating mixing.

Simulation of Nitrogen Sources and WastewaterManagement Actions

A number of potential wastewater-management actions 

are being evaluated to implement TMDLs for nitrogen in 

the Popponesset Bay watershed and the eastern part of the 

Waquoit Bay watershed; these two areas compose a 39-mi2

area referred to as the Mashpee Planning Area (MPA) (figs. 6, 

9A). Potential wastewater-management actions involve the 

collection (through sewering), treatment, and disposal of the 

treated wastewater at WTFs. The simulation of wastewaterderived nitrogen transport from sources to receptors and of 

the response of the system to wastewater-management actions 

requires a detailed understanding of point (WTF) and nonpoint 

(septic-system) sources within the watershed and the possible 

effects of changes in these sources on nitrogen-loading rates to 

the estuarine system. 

As part of the MEP process, parcel-scale water-use 

data were compiled for the Popponesset Bay watershed 

in cooperation with local communities to estimate the 

distribution of return flow and associated nitrogen within 

the watershed. The database includes water-use data from 

the three towns in the watershed (Barnstable, Mashpee, and 

Sandwich) for different periods for each town between 1997 

and 2001; the data were linked to digital parcel and taxassessor data from 2000 and 2001 to quantify the amount 

and location of septic-system return flow (Howes and others, 

2003). Parcel-scale water-use data were obtained from the 

Town of Mashpee (Jeff Gregg, GHD, Inc., written commun., 

August 15, 2011) and are consistent with data developed by 

the MEP. Septic-system return flow was estimated from the 

parcel-scale water-use data by applying a consumptive loss of 

10 percent to be consistent with the MEP analysis (Howes and 

others, 2003) and was normalized to the areas of the parcels 

to convert return-flow volumes into rates (in feet/day). About 

33 percent of the watershed currently (1997–2001) receives 

return flow; rates per parcel within the MPA range from 

0.0002 to 0.8 ft/d (fig. 9A).

A GIS was used to convert the spatial distribution of 

return-flow rates, referred to as vector data (fig. 9A), into 

a rasterized form suitable for input into the groundwater 

model. The process consisted of three steps: (1) mapping the 

vector data to a data raster coincident with the model grid, 

(3) calculating the area-weighted mean return flow within each 

raster cell, and (2) mapping the rasterized data to the model 

grid (figs. 9B, 10). A data raster, as used in this report, refers to 

the spatial representation of data as values in a continuous grid 

of square cells. The result is a raster of area-weighted mean 

return-flow rates that is coincident with both the regional and 

subregional model grids (fig. 9B). The area-weighted mean for 

each raster cell is overlaid onto the model grid and converted 

into Cartesian coordinates for input into the model. The same 

procedure was used to estimate area-weighted mean flows 

representing disposal at WTFs.

This process was done for both the regional and subregional model grids to ensure consistent return-flow stresses 

between the two models. Return flow and disposal at WTFs 

are represented in the models as enhanced recharge—the sum 

of the natural recharge and the area-weighted mean return 

flow in each model cell. A version of the regional model was 

produced in which return flow in the MPA as represented in 

Walter and Whealan (2004) was removed and replaced with 

recharge representing the area-weighted return flow in each 

cell; return flow was unchanged outside the planning area 
Introduction 19 Figure 9. A, Return-flow rates from MEP vector data and B, the rasterized version of the data mapped to the regional and subregional model grids for the Mashpee Planning Area, western Cape Cod, Massachusetts.

70°28' 70°32'

Popponesset Bay watershed

0 1 2 MILES0 2 KILOMETERS

70°28' 70°32'

41°40'

41°36'

Base from U.S. Geological Survey digital data, 1:24,000, 1983Albers Equal Area projectionStandard parallels 37°50' and 41°10', central meridian -84°

Popponesset Bay watershed

A B

Popponesset Bay subregional model

Popponesset Bay subregional model Popponesset Bay Nantucket Sound Waquoit Bay Waquoit Bay Nantucket Sound Popponesset Bay

Sandwich

Falmouth

Mashpee

Mashpee

Barnstable Sandwich

Falmouth

Barnstable

Return flow from vector data,  in feet per day

Wastewater-treatmentfacility

0.8

EXPLANATION

0.0002

Falmouth, 0.00075Sandwich, 0.00064 Barnstable, 0.00070 

Within Mashpee Planning Area

Return flow rasterized and mapped to model grids, in feet per day

Outside of Mashpee Planning Area

Return flow, in feet/day

0.80.0002

0.80.0002

Regional Subregional

Mashpee PlanningArea boundary

Popponesset Baywatershed boundary

Mashpee Planning Area

Mashpee Planning Area
20 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Figure 10. Examples of A, vector data of parcel-scale return flow, B, vector data mapped to a raster coincident with a model 

grid, and C, area-weighted mean return flow mapped to the model grid.

EXPLANATION

Return flow from vector data, 

 in feet per day

0.000000

0.000198

0.000244

0.000347

0.000381

0.000389

0.000436

0.000495

0.000531

0.000646

0.000747

0.000953

0.000976

0.001046

0.001291

0.001414

Individual parcels mapped 

 to finite-difference grids

Area-weighted mean return flow for 

 subregional model cells, in feet per day

Regional model, 

row 79,

col 124

Subregional 

model, 

rows 53-56,

columns 61-64

Finite-difference cell

0.000000

0.000509

0.000648

0.000726

0.000858

0.000996

0.001112

0.001187

0.001207

0.001442

0.001601

0 100 200 300 400 FEET

Step 1 Step 3

Step 2

0 200 400 METERS

A. Vector data of parcel-scale return

 flow

B. Individual parcels mapped to 

 finite-difference model grid

C. Area-weighted mean return flow 

 assigned to individual model cells

Regional

Subregional
Introduction 21

(fig. 11). The same approach was used to specify enhanced 

recharge in the subregional model, resulting in a more detailed 

recharge distribution (fig. 11). Although enhanced-recharge 

rates may differ between the regional and subregional models 

at a specific point, the enhanced-recharge rate in a regional 

model cell is the same as mean rate for the sixteen subregional 

cells within the cell (fig. 10C).

Currently (1997–2001) about 188,500 cubic feet per day 

(ft3/d) of septic-system effluent are recharged into the aquifer; 

an additional 21,770 ft3/d of treated wastewater are discharged 

at seven WTFs; four of the facilities are in the Popponesset 

Bay watershed (fig. 9B). Two sets of wastewater-management 

actions were evaluated: (1) full sewering of the MPA and 

disposal outside of the MPA and (2) two sequential phases of 

hypothetical but realistic actions that combined partial sewering and centralized disposal of most of the treated wastewater 

at WTFs within the MPA. 

The first set of wastewater-management actions specifies 

the collection of all return flow (about 188,500 ft3/d), the disposal of the treated wastewater offshore or on land at locations 

outside of the MPA, and a maintenance of disposal at existing 

WTFs (21,770 ft3/d); the offshore disposal of treated wastewater is prohibited by law, however, and this set of wastewatermanagement actions is designed only to illustrate the response 

of the system to a full removal of septic-system return flow. 

The first hypothetical phase of the second set of actions 

specifies the collection of 135,934 ft3/d from sewering and 

the disposal of 97,532 ft3/d (72 percent) of treated wastewater 

at seven WTFs in the MPA and the remaining 28 percent at 

unspecified locations outside of the MPA. The second phase 

specifies additional development including a buildout scenario 

as well as additional sewering and centralized disposal at 

WTFs. The net return flow would decrease by 121,581 ft3/d 

as a result of sewering, and the net rate of treated-wastewater 

disposal would increase by 196,198 ft3/d in the MPA as a 

result of additional development. This set of wastewatermanagement actions is realistic because the sewerage 

locations for untreated wastewater and the disposal sites for 

treated wastewater, in addition to the volumes of both, are 

similar to those being considered for the implementation of 

nitrogen TMDLs for ecological receptors within the MPA (Jeff 

Gregg, GHD, Inc., written commun., August 15, 2011).

Recharge inputs representing the collection of return 

flow through sewering and the centralized disposal of treated 

wastewater were developed for each set of wastewatermanagement actions by the same approach that was used to 

produce recharge inputs for current (1997–2001) conditions. 

The combined volumes of return flow and wastewater 

from the vector data summarizing wastewater-return flows 

(including septic systems and WTFs) and the input volumes 

representing those stresses in the model as enhanced recharge 

were within 1 percent for current conditions and all phases of 

wastewater management (fig. 12).

The parcel-scale water data compiled by the MEP 

(fig. 9A) was used to estimate the nitrogen load to the 

aquifer associated with each parcel. Septic-system effluent 

was assumed to have a nitrogen concentration of 35 mg/L 

as N and the nitrogen loss to be about 25 percent in the 

septic system and unsaturated zone prior to recharge at 

the water table (Weiskel and Howes, 1992); the assumed 

nitrogen concentration of 26.25 mg/L as N at the water table 

is consistent with the concentration assumed by the MEP 

in their nitrogen-load estimates (Howes and others, 2004). 

The area-weighted mean volume of septic-system return 

flow determined by the rasterization process (fig. 10) and 

the assumed concentration of 26.25 mg/L as N were used 

to calculate the nitrogen load associated with the estimated 

return flow. For current (1997–2001) conditions, the nitrogenloading rate from septic-system return flow ranged from about 

0.0002 to 1.3 kilograms per day (kg/d) (fig. 13) and had the 

same spatial distribution as the rasterized return flow (fig. 9B). 

Rasterized return flows were used to estimate parcel-scale 

nitrogen loads for the second wastewater-management action 

consisting of partial sewering, disposal at WTFs within the 

MPA, and additional development (Jeff Gregg, GHD, Inc., 

written commun., August 15, 2011) (figs. 14A and B).

The nitrogen loads at WTFs were determined by using 

the specified volume of wastewater disposal to determine 

nitrogen-loading rates. At the seven existing WTFs (fig. 13), 

a nitrogen concentration in treated wastewater of 10 mg/L 

as N was assumed for current conditions and the first phase 

of wastewater management and a concentration of 3 mg/L 

for the second phase of wastewater management; nitrogen 

concentrations in treated wastewater at the two planned WTFs 

(fig. 14) were assumed to be 3 mg/L for the facility in the 

Popponesset Bay watershed (the Rock Landing WTF) and 

10 mg/L for the facility outside the watershed (not shown on 

fig. 14) (Jeff Gregg, written commun., GHD, Inc., August 15, 

2011). The total recharge onto each model cell (fig. 11) and 

the nitrogen-loading rate (including both septic-system and 

WTF components) (figs. 13 and 14) were used to determine 

the concentration of nitrogen flowing into each model cell. 

The inflowing concentration was specified in MT3DMS as a 

specified-concentration boundary condition at the water table 

(recharge) to simulate the transport of nitrogen through the 

aquifer to wells and ecological receptors.

A consistent set of groundwater-flow modeling analyses was done for each hydrologic condition representing a 

wastewater-management condition—current, full sewering, 

and two phases of partial sewering and disposal in the MPA. A 

set of regional-model inputs for recharge was developed from 

the parcel-scale return-flow data, as described above, and used 

in an unconfined-model simulation with the modified, 27-layer 

vertical discretization; simulated heads from that simulation were used to convert the regional model to a linearized 

(confined) model. Results from the confined regional-model 

simulation were rediscretized to produce model inputs for the 

subregional model and to develop the specified-head boundary condition linking the regional and subregional models. 

The subregional model was then run as steady state with the 

recharge inputs developed for the subregional grid from the 

parcel-scale return-flow data.
22 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

70°32' 70°28

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

Popponesset Bay subregional model

Popponesset

 Bay

Nantucket Sound

Waquoit

 Bay

SANDWICH

FALMOUTH

MASHPEE

BARNSTABLE

41°40'

41°36'

Popponesset Bay watershed

0 1 2 MILES

0 1 2 KILOMETERS

Subregional

EXPLANATION

Modeled recharge, in feet per day—within 

Mashpee Planning Area 

Regional

0.8

0.0002

0.00622

0.00686

0.00692

0.00697

0.8

0.0002

Area with no recharge

Modeled recharge, in feet per day—outside 

Mashpee Planning Area 

Figure 11. Distribution of simulated current recharge rates representing septic-system and treated-wastewater return flow derived 

from Massachusetts Estuarine Project data for the Mashpee Planning Area, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on the Hydrologic System 23

The steady-state subregional models were run for a 

stress-period length of 100 years to facilitate the solutetransport simulations. The subregional groundwater-flow 

model is the basis of the solute-transport simulations, 

which use cell-by-cell flows from the subregional model. 

A uniform transport time-step length of 1 day was used for 

all simulations. The specified-concentration boundary at the 

water table (representing the recharge of wastewater) for 

current (1997–2001) conditions (fig. 13) was simulated for a 

transport period of 100 years starting with an initial condition 

of no waste-derived nitrogen in the aquifer. Simulations for all 

wastewater-management actions were also run for 100 years 

with the corresponding specified recharge concentrations 

(figs. 14A and B). Scenarios representing full sewering and 

the first phase of partial sewering and disposal in the MPA 

included an initial condition representing 30 years of transport 

under current (1997–2001) nitrogen-loading rates (fig. 14A); 

the initial condition for the second phase of partial sewering 

and disposal in the MPA represented 30 years of transport 

under phase-1 loading rates (fig. 14B).

The use of a 30-year transport time under current 

loading from surface sources to calculate current nitrogen 

concentrations in the aquifer and loading to receptors was 

arbitrary. The 30-year transport time was used because (1) 

inspection of traveltimes in the watershed suggests that loads 

after 30 years approach steady state (within about 80 percent), 

and (2) the 30-year time period generally is consistent with 

the pace of recent development in the region. It should be 

noted that although the use of different transport times from 

current sources in an initially pristine aquifer would result 

in differing estimates of current (1997–2001) nitrogen loads 

from those sources, the response times of nitrogen loads at 

receptors to wastewater-management actions and the effects of 

simulated attenuation in surface waters—the primary purpose 

of the analysis—would be similar regardless of the simulated 

current loads.

Effects of Wastewater-Management 

Actions on the Hydrologic System

Anthropogenic stresses, such as withdrawal of water 

from pumped wells and the return of the water to the aquifer 

as enhanced recharge from septic systems or WTFs, can affect 

the local hydrologic system. Effects of anthropogenic stresses 

on the hydrologic system of Cape Cod include changes in 

water levels in the aquifer, pond stages, and base flow in 

streams (Walter and Whealan, 2004). Hydraulic gradients in 

the aquifer are affected (Walter and Masterson, 2004) and, 

as a result, recharge areas to ecological receptors also can 

be affected. On average, about 23.7 million gallons per day 

(Mgal/d; about 3.2 million ft3/d) of water recharges the aquifer 

through the Popponesset Bay watershed; about 3 percent of 

the recharge is from wastewater return flow at septic systems 

or WTFs. About 70 percent of the water discharges to streams 

or through pond outflows, and about 27 percent discharges 

to the Popponesset Bay estuarine system. The remaining 

3 percent is withdrawn from wells (table 1).

Water Levels and Streamflow

Wastewater-management actions can affect both water 

levels and streamflows by altering the distribution of return 

flow through sewering and disposal at WTFs. Sewering in 

highly urbanized watersheds overlying a glacial aquifer on 

Long Island in the 1940s resulted in water-level declines of 

more than 6 ft and decreases exceeding 65 percent in base 

flow to streams (Scorca, 1986). Similar hydrologic effects will 

also likely occur on Cape Cod, although the region generally 

is much less urbanized, and the effects would be less than 

those in more populous, urbanized regions. Septic-system 

return flow and disposal compose about 3.3 percent of total 

recharge within the MPA (table 1). As a result, wastewatermanagement actions have a small effect on groundwater fluxes 

to ecological receptors—ponds, streams, and estuaries—

within the Popponesset Bay watershed (fig. 15A). Recharge 

in the watershed resulting from three wastewater-management 

actions (full sewering and two sequential phases of partial 

sewering and disposal at WTFs) is within 3 percent of current 

Current Phase1 Phase2 Full Sewering

Wastewater-management phase

0.0

0.5

1.0

1.5

2.0

2.5

0.3

Rasterized data, 

subregional model

Rasterized data, 

regional model

EXPLANATION

Vector data

Flow rate, in millions of cubic feet per day

Figure 12. Wastewater return-flow volumes from Mashpee Planning 

Area vector data and rasterized data used as input to both the 

regional and subregional models, Mashpee Planning Area, western 

Cape Cod, Massachusetts.
24 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

70°32' 70°28'

41°36'

41°40'

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection

Standard parallels 37°50' and 41°10', central meridian -84°

Waquoit

 Bay Nantucket 

 Sound

Popponesset

 Bay

SANDWICH

FALMOUTH

MASHPEE

BARNSTABLE

0 1 2 MILES

0 1 2 KILOMETERSPopponesset Bay subregional model

outside Mashpee Planning Area

Wastewater-treatment

facility

Return flow, in feet per day—

Falmouth, 0.00075

Barnstable, 0.00070

Sandwich, 0.00064 

Popponesset Bay 

watershed

Mashpee Planning Area

EXPLANATION

1.3

Nitrogen-loading rate, in kilograms per day—

within Mashpee Planning Area

0.0002

Figure 13. Distribution of nitrogen-loading rates associated with current wastewater return flow in the Mashpee Planning Area, 

western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on the Hydrologic System 25

70°28' 70°32' 70°28' 70°32'

41°40'41°36'

Base from U.S. Geological Survey digital data, 1:24,000, 1983Albers Equal Area projectionStandard parallels 37°50' and 41°10', central meridian -84°

A B

Popponesset Bay subregional model

Popponesset Bay subregional model Popponesset Bay Nantucket Sound Waquoit Bay Waquoit Bay Nantucket Sound

SANDWICH

FALMOUTH

MASHPEE

BARNSTABLE

SANDWICHFALMOUTH

BARNSTABLE

Rock 

Landing WTF

Popponesset Creek

Pinquickset Cove

Popponesset Bay

Popponesset Creek

Pinquickset Cove

MASHPEE

0 1 2 MILES

0 2 1 KILOMETERS

Rock 

Landing WTF

EXPLANATION

Nitrogen-loading rate, in 

kilograms per day—within Mashpee Planning Area

Phase 2 Phase 1

1.40.0002

0.90.0002

Wastewater-treatment facility (WTF)

Falmouth, 0.0056

Sandwich, 0.0046

Barnstable, 0.0052

Nitrogen-loading rate, in 

kilograms per day—outside Mashpee Planning Area

Future sewering

(phase-1 conditions)

Future return flow

(phase-2 conditions)

Mashpee Planning AreaPopponesset Bay watershed

Figure 14. Distribution of nitrogen-loading rates associated with wastewater return flow for A, the first phase and B, the second phase of partial sewering and disposal at wastewater treatment facilities (WTFs) in the Mashpee Planning Area, western Cape Cod, Massachusetts.
26 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

recharge for current conditions (fig. 15A); groundwater 

discharges to ponds, streams, and estuaries are within 2.9, 

1.6, and 2.4 percent of current conditions, respectively, for the 

three wastewater-management actions (fig. 15A). Groundwater 

discharge to all major receptors decreases after full sewering 

and disposal outside of the MPA (fig. 15A). For sequential 

phases of partial sewering and disposal at WTFs, groundwater 

discharge to streams decreases from sewering, whereas 

groundwater discharge to estuaries increases through disposal 

at WTFs (fig. 15A), particularly disposal at the proposed Rock 

Landing site (figs. 14A, B).

Groundwater discharge decreases to all individual 

ecological receptors (streams and estuaries) following full 

sewering (fig. 15B) because all collected return flow is 

transferred outside of the watershed. Groundwater discharge to 

streams decreases slightly (less than 4 percent) following two 

sequential phases of partial sewering and disposal at WTFs 

(fig. 15B). Changes in groundwater discharge to individual 

estuaries vary following the two phases of wastewatermanagement actions. Groundwater discharge to Popponesset 

Bay, Popponesset Creek, and Ockway Bay increased by 8.4, 

28.0, and 14.8 percent, respectively, because of the proximity 

of the Rock Landing site, whereas discharge to Pinquickset 

Cove was almost unchanged (less than 0.2 percent) because 

the distribution of return flow within the watershed generally 

was unchanged (figs. 14A and B). 

Full sewering and the removal of collected return flow 

from the watershed results in a small—about 3 percent—

decrease in recharge within the MPA. As a result, water-table 

altitudes and hydraulic gradients are essentially the same 

as those for current conditions in most of the watershed 

(fig. 16A). The decrease in water-table altitudes (drawdown) 

exceeds 0.2 ft around Snake Pond near the top of the watertable mound (fig. 16B). The mean drawdown from current 

conditions is 0.07 ft and exceeds 0.1 and 0.2 ft in 39.1 and 

4.3 percent of the MPA, respectively. 

Total recharge within the MPA differs by less than 

1 percent between current conditions and conditions following 

the two sequential phases of partial sewering and disposal at 

WTFs and, as a result, water-table altitudes remain similar 

to current conditions in most areas (fig. 16A). Water-table 

altitudes and hydraulic gradients differ the most around an 

area of potential disposal at the Rock Landing site, to the west 

of Popponesset Bay, where most of the treated wastewater 

Table 1. Simulated hydrologic budgets and septic-system return flow and disposal at wastewater-treatment facilities for current 

(1997–2001) conditions and potential future conditions resulting from hypothetical wastewater-management actions in the Popponesset 

Bay watershed and the Mashpee Planning Area, western Cape Cod, Massachusetts.

[All values are in Mgal/d, million gallons per day; WTF, wastewater-treatment facility; --, not applicable]

Components Current conditions Wastewater management Full sewering Phase 1 Phase 2

A. Simulated hydrologic budgets (Mgal/d)

Popponesset Bay watershed

Recharge 23.659 23.411 23.750 23.021

Wells 0.624 0.624 0.624 0.624

Stream discharge 14.720 14.619 14.751 14.579

Pond outflow 1.935 1.706 1.755 1.616

Coastal discharge 6.356 6.441 6.598 6.178

B. Wastewater-management action (Mgal/d)

Popponesset Bay watershed

Septic-system return flow 0.794 0.203 0.261 0.000

Disposal at WTFs 0.117 0.136 0.379 0.117

Sewering (change from current conditions) -- -0.591 -0.531 -0.794

Disposal at WTFs (change from current conditions) -- 0.019 0.262 0.000

Mashpee Planning Area

Septic-system return flow 1.391 0.378 0.484 0.000

Disposal at WTFs 0.163 0.892 1.632 0.163

Sewering (change from current conditions) 0.000 -1.013 -0.907 -1.391

Disposal at WTFs (change from current conditions) 0.000 0.729 1.469 0.000
Effects of Wastewater-Management Actions on the Hydrologic System 27

Major receptor

EXPLANATION

Discharge, in million s of cubic feet per day

0

1

2

3

4

 -30

 -20

 -10

 0

 10

 20

 30

Individual receptor

Change in groundwater discharge, in percent

Ockway Bay

Mashpee River (tidal)

Shoestring Bay

Pinquickset Cove

Popponesset BayPopponesset Creek

Mashpee River

Quaker Run

Santuit River

EXPLANATION

Recharge Stream Coast Ponds

Current conditions

Phase 1 wastewater

management

Phase 2 wastewater

management

Full sewering 

Phase 1 wastewater 

management

Phase 2 wastewater

management

Full sewering 

A

B

Figure 15. A, Groundwaterdischarge to major receptors for 

current wastewater conditions 

and three simulated wastewatermanagement actions, and 

B, changes in groundwater 

discharge from current 

conditions for the three simulated 

wastewater-management actions, 

Popponesset Bay watershed, 

western Cape Cod.
28 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

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70°32' 70°28'

70°28'0"W

41°

40'

0"N

41°

36'

0"N

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70°32' 70°28'

41°40'

41°36'

0 1 2 MILES

0 1 2 KILOMETERS

Popponesset 

Bay

Nantucket 

 Sound

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection standard parallels 37°50' and 41°10' 

central meridian -84°

Nantucket 

Active model domain Sound

Waquoit

 Bay

Waquoit

Bay

Popponesset

 Bay

EXPLANATION

Full sewering, no additional disposal at WTFs

Current return flow and disposal at WTFs

Wastewater-management, partial sewering 

and disposal at WTFs (phase 1)

Water-table altitude, in feet relative 

to NGVD 88, contour interval is 2 feet

Wastewater-management, partial sewering 

and disposal at WTFs (phase 2)

 Drawdown, in feet, contour interval is 0.1 feet—Full sewering, 

no additional disposal at WTFs

60

56

Wakeby Pond

Peters

Pond

Snake

Pond

Mashpee

Pond

Quaker Run 

Santuit River

Mashpee

Falmouth

Barnstable

40

64

50

30

20

10

10

10

8

8

8

Peters

Pond

Snake

Pond

Mashpee

 Pond

Sandwich

Wakeby Pond

0.1

A B

0.2

0.1

0.1

0.1

0.2

Extent of subregional model

Wastewater-treatment facility (WTF)

Rock 

Landing

Site

Santuit

Pond

Active model domain

Mashpee River Extent of subregional model

20

20

20

20

0.2

Figure 16. A, Water-table altitude for current conditions and potential future conditions resulting from three wastewatermanagement actions, and B, drawdown resulting from full sewering within the Mashpee Planning Area, western Cape Cod, 

Massachusetts.
Effects of Wastewater-Management Actions on the Hydrologic System 29

is discharged in this hypothetical scenario (fig. 16A). The 

two phases of wastewater management result in drawdown 

in newly sewered areas and increases in water-table altitudes 

(mounding) near WTFs (fig. 17). After the first phase of 

partial sewering and disposal at WTFs, water-table altitudes 

decrease as a result of sewering across 79.7 percent of the 

MPA with a maximum decrease of 0.21 ft near Snake Pond 

(fig. 17A). Water-table altitudes increase near WTFs across 

13.7 percent of the MPA; mounding exceeds 3.2 ft at the 

Rock Landing site (fig. 17A). After the second phase of 

partial sewering and disposal at WTFs, water-table altitudes 

decrease across 14.8 percent of the MPA with a maximum 

decrease of more than 0.7 ft (fig. 17B). Water-table altitudes 

increase over 24.1 percent of the MPA near WTFs; the 

largest increase exceeds 5.4 ft near the Rock Landing site 

(fig. 17B). Water-table altitudes in the remaining areas are 

essentially unchanged.

Recharge Areas to Receptors

Watershed boundaries in areas dominated by surfacewater drainages, such as those provided by valley-fill aquifers, 

are determined by topography and generally do not change in 

response to changing hydrologic conditions. In groundwaterdominated systems, watersheds are determined by the configuration of the water table, and the resultant hydraulic gradients 

can change in response to changes in hydrologic conditions. 

Anthropogenic stresses, such as pumping at wells and return 

flow at septic systems or WTFs, can alter water-table altitudes 

and hydraulic gradients (fig. 16A) and, therefore, can potentially change the recharge areas to ecological receptors.

Septic-system return flow composes about 3 percent 

of total recharge within the MPA. Removal of return flow 

through full sewering results in water-table altitudes and 

hydraulic gradients that are similar to those for current 

conditions; as a result, recharge areas to wells and ecological 

receptors are essentially the same for the two hydrologic 

conditions (fig. 18A). The two sequential phases of partial 

sewering and disposal at WTFs change recharge by less than 

1 percent from current conditions (fig. 15A) and, as a result, 

water-table altitudes and hydraulic gradients are similar to 

those for current conditions in most areas; an exception is the 

area near the Rock Landing site (fig. 18B), where most of the 

collected return flow is treated and discharged. The recharge 

areas to ponds, streams, and wells are essentially the same for 

current conditions and conditions following the two sequential 

phases of wastewater management. The recharge areas to 

coastal receptors differ between the two conditions to the west 

of Popponesset Bay, where gradients are affected by disposal 

at the Rock Landing site (fig. 18B), but otherwise, are the 

same for current and future conditions.

The recharge areas to Ockway Bay and Popponesset 

Creek are the most affected by discharge at the Rock Landing 

site and the resulting change in hydraulic gradients (fig. 19). 

The recharge areas to the two receptors increased by 19 and 

21 percent, respectively, following the two phases of wastewater-management actions. The recharge areas extend farther 

to the west toward the Rock Landing site than the recharge 

areas for current conditions. The recharge area to Ockway Bay 

expands about 2,000 ft farther to the south, and the recharge 

area to Popponesset Creek expands about 1,200 ft farther to 

the southwest, as a result of disposal at the Rock Landing site 

(fig. 19). The larger recharge areas are consistent with the 

increases in groundwater discharge to the two receptors of 

14.8 and 28.0 percent, respectively (fig. 15B). The groundwater-discharge rate to a receptor is a function of the size 

of the recharge area and recharge rates within the area. The 

results indicate that, given the volume of redistributed return 

flow relative to natural recharge to the aquifer, the removal of 

return flow through sewering does not greatly affect hydraulic 

gradients and recharge areas, whereas disposal at WTFs can 

affect hydraulic gradients and recharge areas—the effects are 

functions of the volume of disposal, hydrologic setting, and 

proximity of the discharge to a recharge-area boundary.

Pumping from wells for water supply represents another 

hydraulic stress that can affect hydraulic gradients and 

potentially recharge areas to ecological receptors. The Upper 

Cape Cooperative (UCC) wells are on the MMR in Sandwich 

to the north of the Popponesset Bay watershed (fig. 6A); 

the wells are an auxiliary source of water to communities 

near the MMR during dry periods and were pumped at a 

rate of 1.7 Mgal/d in 2003 (Walter and Whealan, 2004). 

Operation of the UCC wells, which are near the top of the 

water table mound, decreases water levels in the upper part 

of the Popponesset Bay watershed by about 1.5 ft, changes 

hydraulic gradients, and shifts the regional groundwater 

divide as much as 1,000 ft to the south (fig. 20). As a result, 

the northern extent of the Popponesset Bay watershed shifts 

southward, and the recharge area to the ponds in the northern 

part of the watershed decreases in size by about 8 percent 

(fig. 20). Groundwater fluxes to ponds, which are functions of 

the sizes of the recharge areas, decrease by about 11 percent. 

The recharge areas to streams and estuaries in the watershed 

generally are unaffected by operation of the UCC wells.
30 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

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Popponesset

 Bay

NANTUCKET 

 SOUND

NANTUCKET 

Active model domain SOUND

Waquoit

 Bay

Waquoit

 Bay

Popponesset

 Bay

Wakeby Pond

Peters

Pond

Snake

Pond

Mashpee

Pond

Mashpee River

Quaker Run 

Santuit River

Mashpee

Falmouth

Barnstable

Peters

Pond

Snake

Pond

Mashpee

Pond

Sandwich

Wakeby Pond

0.1

A B

0.1

0.1

-0.1

Extent of subregional model Extent of subregional model

Mashpee River

Run 

Quaker

Santuit River

-0.4

-5.4

-3.0

-1.0

-0.1

-0.4

-0.2

Active model domain

-3.2

-1.0

0.1

0.1

0.1

0.1

 Rock 

Landing

 Site

Falmouth

 Rock 

Landing

 Site

0.2

0.6

Santuit

Pond

Santuit

Pond

Mashpee

Barnstable

Sandwich

EXPLANATION

Drawdown relative to current water-table altitudes, in feet, 

(-) indicates mounding—contour interval is variable 

Partial sewering 

and disposal at

WTFs (phase 1)

Partial sewering 

and disposal at 

WTFs (phase 1)

0.1

-1.0

Partial sewering 

and disposal at

WTFs (phase 2)

Partial sewering 

and disposal at

WTFs (phase 2)

0.1

-1.0

Wastewater-treatment

facility (WTF)

70°32' 70°28' 70°32' 70°28'

41°40'

41°36'

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection Standard parallels 37°50' and 41°10'

central meridian -84°

0 1 2 MILES

0 1 2 KILOMETERS

Figure 17. Drawdown resulting from A, the first phase and B, the second phase of partial sewering and disposal at wastewater 

treatment facilities (WTFs) within the Mashpee Planning Area, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on the Hydrologic System 31 Base from U.S. Geological Survey digital data, 1:24,000, 1983 Albers Equal Area projection Standard parallels 37°50' and 41°10', central meridian -84°

0 1 2 MILES0 1 2 KILOMETERS

Popponesset

 Bay

NANTUCKET 

 SOUND Waquoit Bay

Mashpee River

Quaker Run 

Santuit River

PetersPond

SnakePond

Wakeby Pond

Mashpee Pond

30

60

50

40

20

10

SantuitPond

 Rock 

Landing

 Site

30

60

50

40

20

10

PetersPond

SnakePond

Wakeby Pond

Mashpee Pond

Mashpee River

Quaker Run 

Santuit River

Popponesset

 Bay

NANTUCKET 

 SOUND Waquoit Bay

A

B EXPLANATION

 Water table, number 

is altitude, in feet— contour interval is 5 feet 

Recharge areas to

major receptors

Current conditions

Future conditions

Current return

flow and disposalat WTFs

Full sewering and no

additional disposalat WTFs

Future (phase 2)

sewering and disposal at WTFs

Wastewater-treatment

facility (WTF)

Production well

Coastal watersStreamsPondsWellsCoastal watersStreamsPondsWells

404040

70°28' 70°30'

41°40'41°36'

70°30' 70°28'

Figure 18. Recharge areas to wells and ecological receptors for current conditions and A, full sewering and B, two sequential phases of wastewater management, Popponesset Bay watershed, western Cape Cod, Massachusetts.
32 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts POPPONESSET BAY Ockway Bay Popponesset Creek

12

10

8

10

8

6

6

10

4

 Rock 

Landing

 Site

70°28' 70°29'

41°36'41°35'

Base from U.S. Geological Survey digital data, 1:24,000, 1983Albers Equal Area projection Standard parallels 37°50' and 41°10', central meridian -84°

0 0.5 MILES 0.250 0.5 KILOMETERS 0.25

EXPLANATION

Current nitrogen-loading rate from 

septic-system return flow, in kilograms per day

0.90.0002

Recharge area to Popponesset Creek

Future hydrologic

conditions (phase 2)

Current hydrologic

conditions 

Recharge area to Ockway Bay

Future hydrologic 

conditions (phase 2)

Current hydrologic

conditions 

Wastewater-treatment facility

Current return

flow and disposal

Partial sewering 

and disposal (phase 2)

Water-table altitude, in feet— 

contour interval is 2 feet 

1010

Figure 19. Recharge areas to Popponesset Creek and Ockway Bay for current hydrologic conditions and potential future conditions resulting from two phases of wastewater management following partial sewering and disposal, and current nitrogen-loading rates from parcels within the Mashpee Planning Area, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on the Hydrologic System 33

68

66

Snake

Pond

Mashpee

Pond

Wakeby

Pond

Santuit

Pond

Peters

Pond

66

64

62

60

64

62

60

62

62

60

60

58

58

52

56 56

48

70°30' 70°28'

41°42'

41°40'

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection Standard parallels 37°50' and 41°10', 

central meridian -84° 0 1 KILOMETER

0 1 MILE

Nitrogen loading rate, in

kilograms per day

1.3

0.0002

EXPLANATION

Water table, number is altitude, in feet— 

contour interval is 2 feet 

Recharge areas for current conditions

Ponds

Other receptors

Upper Cape Cooperative 

wells operating

Upper Cape Cooperative

wells operating

Upper Cape Cooperative 

wells not operating

Ponds

Other receptors

Upper Cape Cooperative 

wells not operating

60

60

Figure 20. Recharge areas to ponds in the Popponesset Bay watershed with operation of the Upper Cape Cooperative wells (current 

conditions) and without operation of the wells (potential future conditions), western Cape Cod, Massachusetts.
34 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Effects of Wastewater-Management 

Actions on Nitrogen Loads to 

Receptors

Daily loads of waste-derived nitrogen to ecological receptors can be determined in two ways: (1) implicitly by using 

model-delineated recharge areas and vector data on nitrogen 

sources and (2) explicitly by using a solute-transport model 

that represents the same sources as inflowing concentrations at 

the water table. Nitrogen loads estimated from recharge areas 

are referred to as “instantaneous” loads because loads from 

sources at the land surface (recharging at the water table) are 

assumed to be equal to those discharging to the receptor at the 

same time. The method does not take into account subsurface 

transport times or changes in recharge areas in response to 

hydrologic stresses.

Nitrogen Loads Estimated by Using Recharge 

Areas

Estimates of waste-derived nitrogen loads obtained by 

using recharge areas involves three steps: (1) identification of 

individual ecological receptors within an estuarine system, (2) 

delineations of recharge areas to those receptors, (3) mapping 

of delineated recharge areas to spatial data regarding sources 

of nitrogen, and (4) summation of loads from individual 

sources in the recharge areas. The identification of individual 

estuarine receptors typically is based on a set of hydrodynamic, water-quality, and ecological criteria; recharge areas to 

those identified receptors are determined by using a groundwater-flow model and particle tracking as discussed in the section 

entitled “Simulation of Advective Transport.” Recharge areas 

are mapped to spatial (vector) source data, and the loads are 

estimated by using a GIS.

Recharge Areas and Traveltimes

The Popponesset Bay system is separated into six 

separate estuaries—Popponesset Creek, Ockway Bay, the tidal 

portion of the Mashpee River, Shoestring Bay, Pinquickset 

Cove, and Popponesset Bay—and three freshwater streams—

Mashpee River, Quaker Run, and Santuit River (fig. 21). The 

sizes of the individual recharge areas to the estuaries range 

from 0.35 mi2 for Popponesset Creek to 1.83 mi2 for the tidal 

portion of the Mashpee River; recharge areas to streams range 

in size from 0.60 mi2 for Quaker Run to 8.11 mi2 for the 

Mashpee River (fig. 21). 

Traveltimes within the Popponesset Bay watershed range 

from essentially instantaneous at discharge boundaries to 

more than 100 years, but generally traveltimes from the water 

table to the receptors in most parts of the watershed are small 

(fig. 22). The median traveltime to all receptors (including 

ponds) is about 4.5 years; the median traveltimes to estuaries, 

streams, and ponds are 5.7, 3.7, and 4.5 years, respectively 

(fig. 23). Traveltimes from the water table to a receptor can 

be divided into specific intervals to allow nitrogen loads to be 

estimated for these specific periods of advective transport. As 

an example, nitrogen loads in MEP analyses are differentiated 

for areas with traveltimes of greater than or equal to and less 

than 10 years. In the Popponesset Bay watershed, 63.4 percent 

of the traveltimes to estuaries are less than 10 years; 76.4 and 

65.8 percent of the traveltimes to streams and ponds, respectively, are less than 10 years (fig. 22). Traveltimes are analogous to advective-transport times; nitrogen transported conservatively from sources in most areas reaches a receptor within 

10 years. Recharged water that flows deeper into the system 

can require longer traveltimes to reach a receptor. Traveltimes 

exceed 100 years from 8.9, 2.8, and 3.3 percent of the recharge 

areas to estuaries, streams, and ponds, respectively (fig. 23). 

The percentage of long traveltimes from the recharge areas 

to estuaries is largest (fig. 23) because estuaries, as the most 

downgradient component of the flow system, generally receive 

the deepest (and oldest) recharged water. This water comprises 

water recharged close to the regional groundwater divide, near 

the edge of the watershed boundary, and between recharge 

areas to streams (fig. 22).

Estimates of Nitrogen Loads for Current and 

Future Wastewater Conditions

Model-delineated recharge areas and traveltimes can be 

used in conjunction with spatial data on nitrogen sources to 

estimate daily nitrogen loads by adding loads from individual 

sources within an entire recharge area or within specific 

ranges of traveltimes. The recharge area to the tidal portion 

of the Mashpee River is the largest area of direct groundwater 

discharge to an estuary within the Popponesset Bay watershed. 

Traveltimes from the water table to this receptor range from 

essentially instantaneous at the estuary to more than 100 years 

from near the western edge of the recharge area. Current 

(1997–2001) nitrogen loads in septic-system return flow from 

individual parcels range from 0.0002 to 0.8 kg/d (fig. 24). 

The total load of waste-derived nitrogen to the tidal Mashpee 

River is 8.9 kg/d, determined by summation of the loads 

from the nitrogen sources within the recharge area by a GIS. 

Traveltimes from the water table in about 62.8 percent of the 

recharge area to the estuary are 10 years or less (fig. 24). Of 

the total waste-derived nitrogen load to the estuary, about 

56 percent originates within an area with a traveltime of 

10 years or less to the estuary. It should be noted that the 

estuary also receives a load of waste-derived nitrogen from 

surface-water inflow from the Mashpee River (fig. 24).

Loads of waste-derived nitrogen vary among receptors as 

functions of the sizes and the unique distributions of nitrogen 

sources within the recharge area to the receptor. For current 

(1997–2001) conditions, the largest loads of waste-derived 

nitrogen to estuaries are for the tidal portion of the Mashpee 

River and Shoestring Bay (fig. 25), which also have the two 
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 35

Popponesset

 Bay

NANTUCKET 

 SOUND

Ockway Bay

Mashpee River

Quaker Run 

Santuit River

Pinquickset

 Cove

Mashpee Santuit Pond

 Pond

Shoestring Bay

Popponesset 

 Creek 

Mashpee 

 River

 (tidal)

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection Standard parallels 37°50' and 41°10', 

central meridian -84°

0 0.5 1 MILES

0 0.5 1 KILOMETERS

70°30' 70°28'

41°38'

41°36'

EXPLANATION

Recharge areas to

individual receptors

Parcel with nitrogen source 

from septic-system return flow

Production well

Popponesset Bay

Popponesset Creek

Ockway Bay

Mashpee River (tidal)

Shoestring Bay

Pinquickset Cove

Mashpee River

Quaker Run

Santuit River

Pond

Well

Figure 21. Distribution of recharge areas to wells and individual ecological receptors—streams and estuaries—and parcels 

containing sources of septic-system return flow, Popponesset Bay watershed, western Cape Cod, Massachusetts.
36 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Popponesset

 Bay NANTUCKET 

 SOUND

Waquoit

 Bay

Mashpee River

Quaker Run 

Santuit River

Mashpee

Falmouth

Barnstable

Peters

Pond

Snake

Pond

Mashpee

Pond

Sandwich

Wakeby

Pond

65

30

60

50

40

10

20

Santuit

Pond

Recharge to

Nantucket Sound

Recharge to

Nantucket Sound

Recharge to

Nantucket Sound

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection Standard parallels 37°50' and 41°10', 

central meridian -84°

0 1 MILES

0 1 KILOMETERS

70°30' 70°28'

41°40'

41°36'

EXPLANATION

Traveltime, in years

Coast

Area with traveltime to 

receptor 10 years or less

Water table for current conditions, number 

is altitude, in feet. Contour interval is 5 feet 

>0–0.25

>0.25–0.5

>0.5–1

>1–2

>2–3

>3–4

>4–5

>5–7

>7–10

>10–15

>15–20

>20–25

>25–30

>30–40

>40–50

>50–60

>60–70

>70–80

>80–90

>90–100

>100

Streams Ponds Wells

Production well

10

Figure 22. Traveltimes to wells and ecological receptors within the Popponesset Bay watershed for current conditions, Popponesset 

Bay watershed, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 37

largest recharge areas (fig. 21); both estuaries also receive an 

additional nitrogen load from surface-water inflows. The load 

of waste-derived nitrogen for traveltimes less than 10 years 

to Shoestring Bay is larger, despite the smaller recharge 

area, because of intense development in the recharge area 

(fig. 21). The smallest load of waste-derived nitrogen for 

traveltimes less than 10 years is to Pinquickset Cove, which 

has the smallest recharge area and relatively sparse residential 

development (fig. 21). The largest load of waste-derived 

nitrogen to a freshwater stream for traveltimes less than 

10 years is to the Santuit River (fig. 25). Although the recharge 

area to the Santuit River is about 40 percent smaller than the 

recharge area to the Mashpee River, there are more sources of 

waste-derived nitrogen within its recharge area (fig. 21). 

The total load of nitrogen to the estuary derives from all 

of the surficial sources within the watershed and is referred to 

as the “instantaneous” load in MEP analyses. The portion of 

the total nitrogen load that discharges within a certain period 

of transport is a function of the distribution of traveltimes and 

nitrogen sources within the recharge area. Most of the total 

load of waste-derived nitrogen travels from the water table 

to an estuary within 10 years (fig. 25) because most of the 

recharge areas are within a 10-year traveltime to a receptor 

(fig. 22). Also, residential development generally is densest 

nearer to the coast (fig. 21). The portion of nitrogen loads 

arriving within 10 years of traveltime varies among individual 

estuaries. Loads from about 70 percent of the recharge area 

to Popponesset Creek arrive within a 10-year traveltime, 

but more than 90 percent of the total load of waste-derived 

nitrogen to the estuary arrives within a traveltime of 10 years 

(fig. 25). Most of the load of waste-derived nitrogen to 

Pinquickset Cove—about 67 percent—is from source areas 

with traveltimes greater than 10 years (fig. 25) because of 

the large undeveloped areas near the shore (fig. 21). About 

84 percent of the total load of waste-derived nitrogen arrives at 

the Santuit River within a traveltime of 10 years or less. Only 

Traveltime to receptors, in years

Percentile

EXPLANATION

0 20 40 60 80 100 120 140 160 180 200

0

20

40

60

80

100

10 years

100 years

Estuaries

Streams

Ponds

Wells

Figure 23. Distribution of traveltimes from the water table to wells and ecological receptors, Popponesset Bay watershed, 

western Cape Cod, Massachusetts.
38 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Surface-water

inflow

Mashpee River

(tidal)

Mashpee

 River 

0 0.5 MILE

0 0.5 KILOMETER

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection Standard parallels 37°50' and 41°10', 

central meridian -84°

70°30' 70°28'

41°37'

41°36'

EXPLANATION

Traveltime, in years

Area for which 

traveltime to receptor 

is less than 10 years

>0–0.25

>0.25–0.5

>0.5–1

>1–2

>2–3

>3–4

>4–5

>5–7

>7–10

>10–15

>15–20

>20–25

>25–30

>30–40

>40–50

>50–60

>60–70

>70–80

>80–90

>90–100

>100

Nitrogen loading rate,

in kilograms per day

0.8

0.0002

0

Recharge area to

Mashpee River

Recharge area to 

Mashpee River (tidal)

Figure 24. Distribution of traveltimes and nitrogen-loading rates within the recharge area to the tidal portion of the Mashpee River for 

current wastewater conditions, Popponesset Bay Watershed, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 39

about 13.9 percent of the total load to Quaker Run is from 

areas with traveltimes of 10 years (fig. 25) because of the large 

amount of undeveloped land near the stream (fig. 21).

The same methods were used to estimate loads resulting from hydrologic stresses caused by a set of wastewatermanagement actions by using spatial data on potential future 

sources of waste-derived nitrogen in recharge areas. Full sewering within the MPA resulted in a 94.7-percent decrease in 

the total load of waste-derived nitrogen to wells and ecological 

receptors (fig. 26); the remaining nitrogen load was from existing WTFs. Full sewering resulted in no load of waste-derived 

nitrogen to wells and ponds because septic-system return flow, 

which would be completely removed by the action, was the 

only source of waste-derived nitrogen in recharge areas to 

those receptors. Loads to estuaries and streams decreased by 

about 90.2 and 93.0 percent, respectively, as a result of full 

sewering. The first phase of partial sewering and disposal at 

WTFs resulted in a 70.2-percent decrease in the total load of 

waste-derived nitrogen to receptors; loads of waste-derived 

nitrogen to estuaries decreased the most, by about 67.0 percent. Waste-derived nitrogen loads to streams and ponds 

decreased by 77.1 and 63.1 percent, respectively (fig. 26). 

The second phase of wastewater management resulted in a 

22.0-percent increase relative to the first phase in total loads of 

waste-derived nitrogen to receptors because of the additional 

residential development within the watershed. Nitrogen loads 

to estuaries increased by about 10.3 percent; nitrogen loads to 

streams and ponds increased by 24.4 and 33.8 percent, respectively (fig. 26).

Receptor

Nitrogen-loading rate, in kilograms per day

0

2

4

6

8

10

12

Ockway Bay

Mashpee River 

 (tidal)

Shoestring Bay

Pinquickset 

 Cove

Popponesset

 Bay

Popponesset

 Creek

Mashpee River

Quaker Run

Santuit River

EXPLANATION

Streams

Estuaries

Traveltime of 

10 years or less

Traveltime greater

than 10 years

Traveltime of 

10 years or less

Traveltime greater

than 10 years

Figure 25. Nitrogen-loading rates from recharge areas to individual ecological receptors—streams and estuaries—estimated 

for current (1997–2001) conditions, Popponesset Bay watershed, western Cape Cod, Massachusetts.
40 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

The Effect of Dynamic Recharge Areas on 

Estimates of Nitrogen Loads

Recharge areas in groundwater-dominated aquifers, such 

as on Cape Cod, are functions of hydraulic gradients and can 

change in response to disposal at WTFs (fig. 19) and pumping 

from production wells (fig. 20). A change in a recharge area 

could result in a different estimated nitrogen load, depending 

on the amount of change in the recharge area and the unique 

distribution of nitrogen sources within near the recharge area. 

Operation of the UCC wells shifts the regional groundwater 

divide southward. As a result, the northern extent of the 

Popponesset Bay watershed shifts southward by as much 

1,000 ft, and the recharge area to the ponds is smaller by about 

8 percent (fig. 20). The result is a decrease of about 9.5 percent 

in the total current (1997–2001) load of waste-derived 

nitrogen to ponds in the northern part of the watershed.

Hypothetical disposal at the Rock Landing site, to the 

west of Popponesset Bay, increases the sizes of the recharge 

areas to Popponesset Creek and Ockway Bay and extends 

them southward in the direction of the site (figs. 19, 27). 

Using the recharge areas delineated for current (1997–2001) 

conditions and current sources of waste-derived nitrogen, so 

that recharge stresses are consistent with return-flow rates, 

results in loads to Popponesset Creek and Ockway Bay of 

about 3.22 and 1.83 kg/d, respectively. The hypothetical 

wastewater-management action includes the sewering of 

almost all of the recharge areas to the two estuaries, changing 

the hydraulic stresses and nitrogen sources (fig. 27). Using 

the future nitrogen sources and recharge areas for current 

conditions results in waste-derived nitrogen loads to the 

two estuaries of 0.44 and 0.08 kg/d, which are equal to 

decreases from current loading of about 86 and 96 percent, 

respectively. Using future nitrogen sources and recharge areas 

Nitrogen-loading condition

Nitrogen-loading rate, in kilograms per day

EXPLANATION

0

20

40

60

80

100

Estuaries

Streams

Ponds

Wells

Total

Current Phase 1 Phase 2 Full sewering

Figure 26. Nitrogen-loading rates estimated for recharge areas to wells and major ecological receptors under current (1997–2001) 

conditions and conditions resulting from full sewering and two phases of partial sewering and disposal at wastewater treatment 

facilities, Popponesset Bay watershed, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 41

POPPONESSET

 BAY

Ockway 

 Bay

Popponesset 

 Creek

12 12

10

8

10

8

6

6

10

4

 Rock 

Landing

 Site

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection standard parallels 37°50' and 41°10', 

central meridian -84°

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

70°29' 70°28'

41°36'

41°35'

Current return

flow and disposal

at wastewater

treatment facility

Wastewatermanagement, 

partial sewering 

and disposal 

(phase 2)

Water-table altitude, 

in feet; contour interval

is 2 feet

EXPLANATION

Future (phase 2) nitrogenloading rate from septicsystem return flow, in

kilograms per day

0.9

0.0002

Recharge area to 

Popponesset Creek

Current hydrologic

conditions

Future hydrologic

conditions 

(phase 2) 

Recharge area to

Ockway Bay

Current hydrologic

conditions

Future hydrologic

conditions 

(phase 2) 

Potential area of 

wastewater disposal

10

10

Figure 27. Recharge areas to Popponesset Creek and Ockway Bay for current hydrologic conditions and potential future conditions 

resulting from two phases of wastewater management following partial sewering and disposal, and future nitrogen-loading rates from 

parcels within the Mashpee Planning Area, western Cape Cod, Massachusetts.
42 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

for hydrologic stresses consistent with those future sources, 

however, yields nitrogen-load estimates for Popponesset Creek 

and Ockway Bay of 1.45 and 0.77 kg/d—3.3 and 9.6 times 

larger than the estimates obtained by using current recharge 

areas. Disposal at the Rock Landing site, which is close to 

the western boundary of the watershed, would raise water 

levels in the area by about 5 ft, change hydraulic gradients, 

and expand the recharge areas to include unsewered areas 

assumed to be outside of the watershed (figs. 5, 27). The 

nitrogen loads estimated by using recharge areas consistent 

with the estimated changes in nitrogen sources are nonetheless 

much less than current loads—about 55 and 58 percent less, 

respectively, for Popponesset Creek and Ockway Bay.

The use of recharge areas that are inconsistent with the 

nitrogen sources that cause a change in hydrologic stresses 

has the potential to be problematic if the stressed area is close 

to the boundary of a watershed. Estimates of current loads 

obtained by using recharge areas for current conditions, as has 

been done by the MEP, likely are valid. Furthermore, partial 

sewering and disposal at other WTFs did not affect recharge 

areas, and, therefore, presumably nitrogen-load estimates to 

the other four estuaries and three streams in the Popponesset 

Bay watershed, suggesting that the use of current recharge 

areas to estimate future loads probably is reasonable in those 

cases. However, when recharge areas are used to implicitly 

estimate nitrogen loads to ecological receptors, recharge areas 

must be consistent with wastewater-management actions if 

those actions or changes in groundwater withdrawals from 

pumped wells could alter hydraulic gradients and recharge 

areas. Inconsistencies are most likely to be problematic if large 

point sources, such as large WTFs, are proposed for locations 

near watershed boundaries.

Estimates of Time-Varying Nitrogen Loads by 

Using a Solute-Transport Model

The use of recharge areas to implicitly estimate wastederived nitrogen loads from surface sources has limitations that include (1) the reliance on static recharge areas to 

represent dynamic conditions, (2) inherent limits in the ability 

of the resultant instantaneous analysis to assess the temporal 

response of the system to changing nitrogen sources, and (3) 

the needs to delineate potentially large numbers of recharge 

areas and to use a GIS for additional postprocessing to 

determine loads. Solute-transport models, on the other hand, 

explicitly represent the transport of nitrogen from sources to 

receptors. Modeled quantities include time-varying concentrations within the aquifer and at receptors and mass-loading 

rates determined by adding the products of concentrations and 

flow rates for groups of model cells representing a receptor. 

The only preprocessing required is the conversion of source 

information into the representation of modeled sources.

Estimates of Current Loads

Current (1997–2001) nitrogen loads can be explicitly 

simulated by using nitrogen sources rasterized from parcelscale water-use data as the source term in a solute-transport 

model (fig. 9B). An inherent assumption of this approach is 

that current residential land uses—and the associated nitrogen 

sources—have not changed historically, and that current loads 

can be simulated by using the current nitrogen sources and 

the assumed initial condition wherein the aquifer contains no 

nitrogen. The simulated loads will approach a steady-state load 

approximating the instantaneous loads over sufficiently long 

transport times. Although residential development has changed 

over time, there is little historical data available regarding 

residential water use at the parcel scale. Given the short traveltimes in the aquifer, current (1997–2001) loading rates can be 

assumed to be reasonable approximations of historical loading 

rates; this assumption also is inherent in the use of current 

recharge areas to estimate instantaneous loads. Concentrations of nitrogen in the aquifer between 0 and -10 ft, the range 

which generally corresponds to the altitude of groundwater 

discharging into coastal waters, exceed 5 mg/L as N in several 

areas after 30 years of transport (fig. 28A); patterns of concentrations generally are similar to the distribution of septicsystem sources (fig. 13). Concentrations generally decrease 

with depth (increasing vertical transport distance from surface 

sources), but exceed 1 mg/L at an altitude of -100 ft in several 

areas (fig. 28B). Most nitrogen generally is in shallow parts of 

the aquifer (less than about -100 ft) after 30 years of transport from surficial sources to receptors (fig. 28C). Nitrogen is 

transported to deeper parts of the aquifer (altitude higher than 

-100 ft) primarily downgradient of ponds where the aquifer 

is recharged by pond water, and downward vertical gradients 

are greater; nitrogen is transported downward almost to the 

bedrock surface downgradient of Mashpee Pond (fig. 28C). 

Nitrogen concentrations downgradient of ponds—areas 

referred to as pond shadows—generally are more homogenous 

than concentrations upgradient of ponds because of mixing 

within the ponds (fig. 28C).

The simulated nitrogen load to a receptor would approach 

the instantaneous load over sufficiently long transport times, 

with some negligible difference caused by transverse and 

vertical dispersion—the rate at which the simulated load 

approaches the instantaneous load would be a function 

of groundwater velocities and likely would be greater in 

advection-dominated systems. The combined simulated load 

to all receptors in the Popponesset Bay watershed is within 

ranges of 50 percent of the instantaneous load estimated from 

sources in recharge areas after 7 years and 80 percent after 

30 years (fig. 29) under the assumption that the aquifer initially contains no nitrogen. The rapid rate at which simulated 

loads approach instantaneous loads estimated from recharge 

areas suggests that, given the absence of historical land-use 

data, it is reasonable to use (1) current (1997–2001) sources 

for estimating current loads to receptors and (2) instantaneous 

loads estimated from recharge areas as approximations of 
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 43

current loads. Changes in nitrogen loads differ by the type of 

receptor. Loads to estuarine receptors reach 50 and 80 percent 

of the instantaneous loads estimated from recharge areas after 

about 7 and 25 years; similarly, loads to wells reach 90 percent 

of instantaneous loads estimated from recharge areas within 

50 years (fig. 29). Nitrogen loads to streams exceed those 

estimated from direct recharge areas to the streams within 

12 years and, after 100 years of transport, are 65 percent 

greater (fig. 29). This excess is a result of the additional load 

of nitrogen entering the streams from groundwater that has 

flowed through ponds prior to discharge and is not accounted 

for in the model-calculated recharge areas to the streams.

Effect of Surface Waters

Most (about 70 percent) of recharged water in the 

Popponesset Bay watershed discharges into a stream or pond 

(fig. 18A). As a result, attenuation of nitrogen concentrations 

in surface waters affects nitrogen loads to estuaries. Streams, 

estuaries, and wells are features where groundwater leaves 

the aquifer system, whereas ponds are flow-through features 

that receive water and associated nitrogen from recharge areas 

upgradient of the pond and contribute water and associated 

nitrogen to the aquifer downgradient from the ponds. In some 

cases, water and associated nitrogen may also exit a pond as 

surface-water outflow (Walter and Masterson, 2011). About 

40 percent of the Popponesset Bay watershed is within the 

recharge area to a pond (fig. 18A), and about 34 percent of 

the total waste-derived nitrogen from current (1997–2001) 

sources in the watershed (79.1 kg/d; fig. 29) passes through 

a pond prior to discharging from the system at a stream, 

estuary, or well (26.9 kg/d). After 100 years of transport, 

about 0.01 and 0.21 kg/d of that nitrogen load is discharging 

to estuaries and wells, respectively (fig. 30A), representing 

less than 0.1 percent of the total mass of nitrogen passing 

through ponds. About 15.1 kg/d nitrogen (56 percent of the 

total) passing through ponds is discharging to streams after 

100 years of transport (fig. 30A); another 6.2 kg/d (23 percent) 

is discharging into streams as surface-water outflow directly 

from the pond (fig. 30A).

After 100 years of transport, about 20 percent of the total 

discharging load of waste-derived nitrogen (represented as a 

mass-loading rate) flowing through a pond is not yet discharging to a receptor and is entrained in the aquifer (fig. 30A). 

Flow-through ponds cause steep vertical gradients that are 

upwards in areas upgradient and downward in areas downgradient from the ponds. As a result, advective flow paths 

downgradient from a pond pass deeper into the aquifer where 

groundwater fluxes are smaller and traveltimes are longer 

(Walter and others, 2003). Simulated concentrations at an 

altitude of -90 to -100 ft indicate areas with higher concentrations (exceeding 0.5 mg/L as N) downgradient from Mashpee 

Pond, Peters Pond, and Santuit Pond (fig. 28B); in these areas, 

pond water containing nitrogen has recharged the aquifer at 

the pond bottom and flowed to substantial depth. The large 

traveltimes of this deep water are the reason why a significant 

amount (about 20 percent) of the solute mass passing through 

the ponds has not arrived at a receptor after 100 years of 

transport; over sufficiently long transport times, the mass 

entrained deeper in the aquifer will discharge into a receptor, 

and nitrogen loads to the receptors will approach the instantaneous load.

Streams can receive water and an associated load of 

waste-derived nitrogen through (1) groundwater originating 

from direct recharge at the water table, (2) groundwater 

originating from a flow-through pond, and (3) surface-water 

inflow from a pond. The portion of the total nitrogen load 

from each component is a function of the hydrologic setting 

of the stream (fig. 30B). In the Popponesset Bay watershed, 

the simplest example is Quaker Run, which has no surfacewater inflow and receives almost all (more than 99 percent) of 

its load of waste-derived nitrogen directly from groundwater 

recharged at the water table (fig. 30B). The Mashpee and 

Santuit Rivers both receive loads of waste-derived nitrogen 

from ponds through surface-water and groundwater inflows. 

The Santuit River, which receives the largest load of nitrogen 

of any receptor in the watershed (fig. 25), receives most 

(about 64 percent) of its nitrogen load from groundwater 

recharged directly at the water table. Nitrogen from Santuit 

Pond composes the remaining 36 percent of the load to the 

river—comprising about 24 percent groundwater originating 

from the pond and 12 percent from surface-water outflow 

from the pond. About half (51 percent) of the nitrogen load to 

the Mashpee River is from Mashpee Pond—about 40 percent 

from groundwater and 14 percent from surface-water 

outflow (fig. 30B). The remaining 49 percent is directly from 

groundwater recharged at the water table.

There likely is a loss of nitrogen in fresh surface waters 

resulting from biological processes; in determining loads to 

estuaries, the MEP assumes a loss of 50 percent within ponds 

and 30 percent within streams (Howes and others, 2003). 

The loss can be represented by using either of two methods: 

adjusting (1) the source terms or (2) loads at the receptors by 

a desired loss term. A simple loss term can be incorporated 

into estimates of nitrogen loads to receptors by decreasing 

the loads associated with sources within a recharge area by 

the desired amount. The same result can be achieved in a 

solute-transport model by adjusting the simulated loads at 

the receptor by the same loss term and thus requiring no 

delineation of the recharge area or other source adjustments. 

With an assumed loss of 50 percent in ponds and 30 percent 

in streams, the loads simulated by adjusting source terms by 

the appropriate loss term for the corresponding recharge areas 

are the same as the loads at the receptor adjusted directly by 

the same loss term at the receptor (fig. 31A). The effect of 

mass moving through more than one pond was not evaluated 

as part of this analysis, but the same concept of additive loss 

likely would apply. As an example, nitrogen entering the 

aquifer upgradient of Peters Pond (the northernmost pond in 

the Popponesset Bay watershed; fig. 3) would flow through 

the pond prior to reentering the aquifer and discharging into 
44 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts Column 124

A

A'

Popponesset Bay watershed

A

B

Planning Area

Mashpee 

 Pond

NANTUCKET 

 SOUND

Popponesset

 Bay

Mashpee 

 Pond

Mashpee River

Wakeby Pond

Santuit River

Quaker Run Popponesset

 Bay

NANTUCKET 

 SOUND

Snake

Pond

Peters

Pond

Snake

Pond

Popponesset Bay watershed

Nitrogen from pond outflow

Wakeby Pond

Peters

Pond

Mashpee River

Santuit River

Quaker Run

Nitrogen from pond outflow Santuit Pond

Nitrogen from pond outflow

Santuit 

 Pond

Base from U.S. Geological Survey digital data, 1:24,000, 1983Albers Equal Area projection standard parallels 37°50' and 41°10', central meridian -84°

0 1 2 MILES0 1 2 KILOMETERS

70°28' 70°32'

41°40'41°36'

70°28' 70°32'

Nitrogen concentration, 

in milligrams per liter as N

EXPLANATION

0.00–0.1>0.1–0.2>0.2–0.3>0.3–0.4>0.4–0.5>0.5–0.6>0.6–0.7>0.7–0.8>0.8–0.9>0.9–1.0>1.0–1.2>1.2–1.4>1.4–1.6>1.6–1.8>1.8–2.0>2.0–2.4

>2.4–2.8>2.8–3.2>3.2–3.6>3.6–4.0>4.0–4.5>4.5–5.0>5.0–5.5>5.5–6.0>6.0–7.0>7.0–8.0>8.0–9.0>9.0–10.0>10.0–11.0>11.0–12.0>12.0–13.0

>13.0–15.4

Nitrogen source from 

return flow in MashpeePlanning Area

Nitrogen source from 

return flow outside of Mashpee Planning Area

Nitrogen source from 

wastewater-treatmentfacility

Figure 28. Simulated nitrogen concentrations after 30 years of transport from current nitrogen sources for altitude ranges A, 0 to -10 feet, B, -90 to -100 feet, and C, along vertical section A–A’, near Popponesset Bay watershed, western Cape Cod, Massachusetts. Bedrock topography from Fairchild and others, 2013.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 45

Mashpee—Wakeby Pond; the sum of the loss terms for the 

two ponds could apply to that component of the nitrogen load.

Attenuation of nitrogen in surface waters—ponds and 

streams—was implicitly represented in MEP load calculations 

as assumed loss factors of 0.5 and 0.3, respectively. 

The potential for loss in fresh surface waters makes it 

advantageous to explicitly account for the mechanisms by 

which nitrogen enters an estuary. As an example, the Mashpee 

River receives most of its load of waste-derived nitrogen 

from recharged groundwater (fig. 30B). The load of nitrogen 

directly from groundwater recharged at the water table (about 

49 percent) can be decreased by 30 percent (as assumed by 

the MEP) to implicitly represent attenuation in the river; the 

load of nitrogen in groundwater that passed through a pond 

prior to discharging into the river could be decreased by 

50 percent (as assumed by the MEP) to represent attenuation 

in the pond and, if the loss terms are assumed to be sequential, 

decreased by an additional 30 percent to represent the final 

degree of attenuation in the stream. With assumed losses of 

50 and 30 percent for nitrogen originating within recharge 

areas to ponds and streams, respectively (consistent with 

MEP assumptions), the attenuation of nitrogen entering the 

tidal portion of the Mashpee River as surface-water inflow 

(the difference between attenuated and unattenuated loads) 

is estimated as 10.5 kg/d (40 percent) after 100 years of 

transport (fig. 31B). The nitrogen load can be attenuated by 

an additional 2 kg/d if an additional loss of 30 percent is 

assumed for nitrogen in groundwater entering the stream that 

had originated from a pond—a total loss of 65 percent for that 

component of the load (fig. 31B).

Assessing System Response to WastewaterManagement Actions

Changes in loads of waste-derived nitrogen at receptors 

in response to wastewater-management actions are functions 

of advective traveltimes in an advection-dominated aquifer 

(fig. 23); these traveltimes are small (less than 10 years from 

the water table to receptors) in most of the Popponesset Bay 

watershed (fig. 22). Nitrogen concentrations in the elevation 

horizon between 0 and -10 ft are less than 0.2 mg/L as N in 

most of the watershed after 30 years of transport following full 

sewering and removal of the collected and treated wastewater 

from the watershed (fig. 32). Concentrations exceed 5 mg/L 

as N downgradient from existing WTFs and 0.5 mg/L downgradient from Peters Pond and Mashpee Pond (fig. 32). The 

nitrogen downgradient from the ponds—one of the components of nitrogen loads to the Mashpee River—is dissolved in 

deep groundwater recharged from the ponds; traveltimes for 

groundwater flowing deeper in the aquifer are longer, and the 

nitrogen load to streams from groundwater that has passed 

through a pond is still increasing after 90 years of loading at 

the assumed rate for current sources (1997–2001) (fig. 30B).

A A' FEET

VERTICAL SCALE GREATLY EXAGGERATED

C

Mashpee Pond

Peters 

 Pond

Column 124

-100

-440

-150

Pond

outflow

Pond

outflow

NORTH AMERICAN 

VERTICAL DATUM 

OF 1988

65

Bedrock

0 1 2 MILES

0 1 2 KILOMETERS

Nitrogen concentration, in milligrams per liter as N

EXPLANATION

>0.0–0.1

>0.1–0.2

>0.2–0.3

>0.3–0.4

>0.4–0.5

>0.5–0.6

>0.6–0.7

>0.7–0.8

>0.8–0.9

>0.9–1.0

>1.0–1.2

>1.2–1.4

>1.4–1.6

>1.6–1.8

>1.8–2.0

>2.0–2.4

>2.4–2.8

>2.8–3.2

>3.2–3.6

>3.6–4.0

>4.0–4.5

>4.5–5.0

>5.0–5.5

>5.5–6.0

Figure 28. Simulated nitrogen concentrations after 30 years of transport from current nitrogen sources for altitude 

ranges A, 0 to -10 feet, B, -90 to -100 feet, and C, along vertical section A–A’, near Popponesset Bay watershed, 

western Cape Cod, Massachusetts. Bedrock topography from Fairchild and others, 2013.—Continued
46 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Time, in years

Nitrogen-loading rate, in kilograms per day

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

EXPLANATION

Total

Estuaries

Streams

Surface-water outflow 

from ponds

Wells

Groundwater outflow 

from ponds

Total load to receptors from recharge area

Load to streams estimated from recharge area

Load to estuaries estimated from recharge area

Load to wells estimated from recharge area

50% of

total load

80% of

total load

90% of total load

50% of 

total load

80% of

total load

90% of 

total load

100% of 

total load

90% of 

total load

Figure 29. Time-varying loads of waste-derived nitrogen to wells and ecological receptors over 100 years of transport from 

current nitrogen sources, Popponesset Bay watershed, western Cape Cod, Massachusetts. %, percent.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 47

Time, in years

0 10 20 30 40 50 60 70 80 90 100

Nitrogen-loading rate, in kilograms per day

0

5

10

15

20

25

30

0

3

6

9

12

15

Nitrogen-loading rate, in kilograms per day

A

B

Stream

Estuary

Surface-water outlflow from ponds (pond outflow)

Wells

Load to ponds estimated from recharge area

Total load through ponds

Nitrogen load remaining 

in the aquifer (20 percent)

Mashpee River

Santuit River

Quaker Run

EXPLANATION

EXPLANATION

Total

Estuaries

Streams

Surface-water 

outflow from 

pond

Wells

Mashpee River

 Direct recharge

 Surface-water 

outflow from 

ponds

 Pond groundwater

Quaker Run

 Direct recharge

 Pond groundwater 

Santuit River

 Direct recharge

 Surface-water 

outflow from 

ponds

 Pond groundwater 

Figure 30. A, Time-varying loads of waste-derived nitrogen that have passed through ponds prior to discharging at receptors and 

B, components of nitrogen loads to three streams, Popponesset Bay watershed, western Cape Cod, Massachusetts.
48 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

0

10

20

30

40

50 EXPLANATION

Streams

 Unattenuated

 30-percent 

attenuation

at source

 Load corrected for 

30-percent loss

Ponds

 Unattenuated

 50-percent 

attenuation

at source

 Load corrected for 

50-percent loss

ANitrogen-loading rate, in kilograms per day

0

5

10

15

20

25

30 EXPLANATION

Nitrogen load to 

Mashpee River

 Unattenuated

 Attenuated with 

loss terms for 

the river

 Attenuated with 

loss terms for 

groundwater 

outflow from 

ponds to the river 

Nitrogen-loading rate, in kilograms per day

B

Time, in years

0 10 20 30 40 50 60 70 80 90 100

Figure 31. A, Unattenuated and attenuated loads to streams and ponds calculated by using loss terms and B, nitrogen loads to the tidal 

reach of the Mashpee River calculated under different loss assumptions, Popponesset Bay watershed, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 49

Nitrogen 

 from

 pond 

 outflow

Nitrogen from

 pond outflow

Mashpee 

 Pond

Mashpee River

Wakeby Pond

Santuit River

Quaker Run

Popponesset

 Bay

NANTUCKET 

 SOUND

Snake

Pond

Peters

Pond

Popponesset Bay 

watershed

70°30' 70°28'

41°40'

41°36'

Base from U.S. Geological Survey digital data, 1:24,000, 1983

Albers Equal Area projection standard parallels 37°50' and 41°10', 

central meridian -84°

0 0.5 1 MILE

0 0.5 1 KILOMETER

Nitrogen concentration, 

in mg/L as N

EXPLANATION

Nitrogen sources: 

Return flow from outside 

Mashpee planning area

Wastewater-treatment

facility

0.00–0.1

>0.1–0.2

>0.2–0.3

>0.3–0.4

>0.4–0.5

>0.5–0.6

>0.6–0.7

>0.7–0.8

>0.8–0.9

>0.9–1.0

>1.0–1.2

>1.2–1.4

>1.4–1.6

>1.6–1.8

>1.8–2.0

>2.0–2.4

>2.4–2.8

>2.8–3.2

>3.2–3.6

>3.6–4.0

>4.0–4.5

>4.5–5.0

>5.0–5.5

>5.5–6.0

>6.0–7.0

>7.0–8.0

>8.0–9.0

>9.0–10.0

>10.0–11.0

>11.0–12.0

>12.0–13.0

>13.0–15.4

Boundary of 

Popponesset Bay

watershed

Figure 32. Concentrations of nitrogen at an altitude between 0 and -10 feet following 30 years of full sewering in the Mashpee 

Planning Area, western Cape Cod, Massachusetts.
50 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

The response of the system to wastewater-management 

actions, including nitrogen loads to receptors, involves 

adjusting nitrogen sources to reflect the action and using 

the simulated concentration field after a desired period of 

transport from pervious sources as the initial condition for 

the solute-transport simulation. The load to receptors in 

the Popponesset Bay watershed decreases from about 64 to 

6 kg/d after 90 years of transport following full sewering 

of the MPA (fig. 33). About 50 percent of the total decrease 

occurs by about 5 years after full sewering and 80 percent 

by about 15 years after full sewering (fig. 33). These results 

are consistent with the median advective traveltimes from 

the water table to receptors, which range from 3.7 years 

for streams to 5.7 years for estuaries (fig. 23). It should be 

noted that the predicted loads presented in figures 33–36 are 

not corrected for surface-water attenuation; these predicted 

(unattenuated) loads can be corrected by a specified loss term 

as discussed in the section entitled “Effect of Surface Waters.” 

Correcting for loss in surface waters proportionally decreases 

the response curves but does not affect the proportional 

response of loads to wastewater-management actions.

The response of nitrogen loads at receptors to partial sewering and disposal within the planning area is similar to that 

for full sewering (fig. 34). Loads of waste-derived nitrogen 

decrease from about 64 to 27 kg/d after 90 years of transport 

and 30 years after implementation of the second phase of 

wastewater management; these rates of decrease approximate 

a near steady state with respect to nitrogen-loading rates. 

About 50 percent of the total decrease occurs within 3 years 

and about 80 percent within 10 years after implementation 

of the first phase of wastewater management (fig. 34). Thirty 

years after the phase-1 action has been in place, the total load 

of waste-derived nitrogen has decreased to about 26 kg/d. 

Loads of waste-derived nitrogen increase slightly—to about 

27 kg/d—after implementation of the second phase of wastewater management; the second phase includes additional 

return flow from new residential development (fig. 14B). The 

load of waste-derived nitrogen to the coast (Nantucket Sound) 

increases following the two phases of wastewater management from about 12 to 21 kg/d. Open coastal waters have a 

large amount of hydrodynamic circulation and flushing, and, 

therefore, nitrogen inflow to the coastal waters likely does not 

adversely affect ecosystems in the waters around Cape Cod; 

Howes and others (2004) report that Nantucket Sound has a 

much lower mean nitrogen concentration than the Popponesset 

Bay estuarine system.

Wastewater-management and TMDL implementation, 

as currently (2003–2012) evaluated on Cape Cod, are based 

on the assumption that open coastal waters, like Nantucket 

Sound, are suitable as receiving waters for treated wastewater. 

As a result, the direct-recharge areas to open coastal waters, 

such as Nantucket Sound, are generally assumed to be suitable locations for the future disposal of treated wastewater. 

Discharge to these areas effectively transfers the potential 

nitrogen load from more sensitive receptors, such as streams 

and inland estuaries, directly to open coastal waters.

The load of waste-derived nitrogen comes from both 

septic-system return flow and treated wastewater (with lower 

nitrogen concentrations); partial sewering and disposal from 

WTFs in the watershed can partially offset each other, with 

sewering decreasing the load and disposal at WTFs within 

the watershed increasing the loads. The desired result is a 

net decrease in the total load to a rate that is compliant with 

a given TMDL. The two phases of wastewater management 

decrease the nitrogen load to receptors by about 37 kg/d 

(fig. 35A), whereas disposal at WTFs within the watershed 

results in an increase of about 4 kg/d. The load of nitrogen 

from WTFs is about 29 percent of the total load to receptors 

within the Popponesset Bay watershed (fig. 35A) after 

90 years of wastewater management if treatment at WTFs is 

assumed to reduce nitrogen concentrations in the collected 

wastewater to 3 mg/L. Estuaries are the type of receptor that 

receives the largest amount of treated wastewater—about 

55 percent of the total load after 90 years of wastewater 

management (fig. 35A, B); the load of treated wastewater 

to streams is about 18 percent of their total load from WTFs 

and septic systems after the same time interval (fig. 35B). 

Determining the relative effects of sewering and disposal at 

WTFs can allow planners to determine the efficacies of these 

actions in lowering effluent concentrations at WTFs if larger 

decreases in nitrogen loads are needed for TMDL compliance.

Examination of total loads to different types of receptors 

improves understanding of the dynamics of the aquifer system 

as it relates to nitrogen transport and the response to wastewater-management actions; however, acceptable loads to individual estuaries, which are based on a set of water-quality, ecological, and hydrodynamic criteria established by the MEP, are 

needed to evaluate TMDL compliance. The MPA includes the 

6 estuaries within the Popponesset Bay system and 10 of the 

15 estuaries within the eastern part of the Waquoit Bay system 

(table 2); the location of each modeled estuary is shown in 

figure 8 (corresponding identifiers are listed in table 2). The 

response of nitrogen-loading rates from direct groundwater 

discharge varies among the estuaries (fig. 36) and is a function 

of the hydrologic setting of the estuary, traveltimes within the 

recharge area, and the unique set of current and future nitrogen 

sources within the recharge area. The nitrogen loads presented 

in figure 36 are from direct groundwater discharge and do not 

include nitrogen loads from surface-water inflows.

In the Popponesset Bay system, most of the estuaries 

show similar changes in nitrogen loads in response to 

the two phases of wastewater management: a decrease 

following implementation of sewering in the first phase of 

wastewater management followed by a small increase after 

implementation of the second phase because of additional 

residential development (fig. 36A). The load to Pinquickset 

Cove increases steadily but remains less than 1 kg/d 90 years 

after implementation of the wastewater-management 

actions (fig. 36A) probably because the recharge area to the 

estuary is small and relatively undeveloped with no planned 

sewering (fig. 14A). Nitrogen loads decrease the most (by 

about 90 percent) in the tidal portion of the Mashpee River 
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 51

Current Full sewering

Time, in years

0 10 20 30 40 50 60 70 80 90 100 110 120

Nitrogen-loading rate, in kilograms per day

Stream

Estuaries

 Surface-water 

outflow from ponds

Total

0

10

20

30

40

50

60

70

80

EXPLANATION

Full sewering

 Total

 Estuaries

 Streams

 Surface-water

outflow from

ponds

No action

 Total

 Estuaries

 Streams

 Surface-water

outflow from

ponds

50 percent of total decrease

80 percent of total decrease

90 percent of total decrease

Figure 33. Changes in loads of waste-derived nitrogen to wells and ecological receptors in the Popponesset Bay watershed 

following full sewering of the Mashpee Planning Area, western Cape Cod, Massachusetts.
52 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts Nitrogen-loading rate, in kilograms per day

Stream

Estuaries

Total

0

10

20

30

40

50

60

70

Coast

Current Phase 1 Phase 2

50 percent 

of total

decrease

80 percent

of total

decrease 90 percent 

of total 

decrease

 Surface-water 

 outflow from 

 ponds

EXPLANATION

Total

Total (no action)

Estuaries

Coast

Streams

Surface-water

outflow from

ponds

Well

Well

Time, in years

0 10 20 30 40 50 60 70 80 90 100 110 120

Figure 34. Changes in loads of waste-derived nitrogen at wells and ecological receptors in the Popponesset Bay watershed 

following two phases of partial sewering and disposal of treated wastewater at wastewater-treatment facilities within the 

Mashpee Planning Area, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 53

Nitrogen-loading rate, in kilograms per day

Stream

Estuaries

0

5

10

15

20

25

30

35

40 EXPLANATION

Septic-system 

return flow

 Estuaries

 Streams

Treated wastewater

 Estuaries

 Streams

Current Phase 1 Phase 2

BNitrogen-loading rate, in kilograms per day

Septic-system return flow

Treated wastewater

Total

0

10

20

30

40

50

60

70

EXPLANATION

Total

Septic-system 

return flow

Treated wastewater

A

Current Phase 1 Phase 2

Time, in years

0 10 20 30 40 50 60 70 80 90 100 110 120

Figure 35. Changes in nitrogen loads in wastewater from partial sewering, septic-system return flow, and disposal at wastewatertreatment facilities in the Mashpee Planning Area for A, all receptors and B, streams and estuaries in the Popponesset Bay watershed, 

western Cape Cod, Massachusetts.
54 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

(fig. 36A). The load to the estuary decreases following 

implementation of the first phase of sewering, but more 

gradually than at other estuaries (fig. 36A), partly because 

most of the residential areas to be sewered (fig. 14A) are in 

areas with longer traveltimes (fig. 24). The result is a slower 

response of loads to sewering within the recharge area. Also, 

loads decrease substantially following implementation of the 

second phase of wastewater management (fig. 36A) as a result 

of the cessation of wastewater discharge at a WTF within the 

recharge area (fig. 14B). The responses of nitrogen loads to 

estuaries in the eastern part of the Waquoit Bay system, where 

current loading rates generally are lower, are similar: a large 

decrease following the first phase of wastewater management 

and little change resulting from the second phase (fig. 36B). 

The first phase would include nearly full sewering of the 

recharge areas to those estuaries (fig. 14A).

Two estuaries in the Popponesset Bay system, the tidal 

reach of the Mashpee River and Shoestring Bay, receive loads 

of waste-derived nitrogen from surface-water inflows—77 and 

75 percent, respectively—prior to the implementation of 

wastewater-management actions. The total load of wastederived nitrogen to the tidal portion of the Mashpee River 

decreases by about 17.4 kg/d after 90 years of wastewater 

management (fig. 37A). Decreases in loads from direct 

groundwater discharge and surface-water inflow from the 

Mashpee River compose about 31 and 61 percent of the 

total decrease in nitrogen load, respectively; the remaining 

8 percent is from a decrease in the load from surface-water 

outflow from Mashpee Pond (fig. 37A). Wastewatermanagement actions result in a total decrease in nitrogen 

load to Shoestring Bay of about 13.4 kg/d (fig. 37B). About 

21 percent of the total decrease is from a decrease in the load 

from direct groundwater discharge (fig. 36B). Decreases in 

loads in surface-water inflow from the Santuit River and 

Quaker Run compose about 68 and 6 percent, respectively, of 

the total decrease; the remaining decrease—about 5 percent—

is from surface-water outflow from Santuit Pond (fig. 37B). 

Decreases in loads to surface waters account for 79 and 

69 percent, respectively, of the total decrease in loads to 

Shoestring Bay and the tidal portion of the Mashpee River.

Explicitly representing the mass of nitrogen in a transport 

simulation allows for attenuated loads to be determined for a 

given set of assumed loss factors; the model-calculated loads 

can be adjusted by loss factors to implicitly represent attenuation in surface waters. With losses of 50 percent in ponds and 

30 percent in streams, which are consistent with MEP assumptions, the attenuated loads (uncorrected for pond groundwater) 

to the tidal portion of the Mashpee River and Shoestring Bay 

are both about 4 kg/d lower than the unattenuated load after 

the 30 years of partial sewering and disposal at WTFs (phase 

1; fig. 37). The loss of nitrogen in groundwater that has passed 

through a pond prior to discharge into a stream would include 

both loss terms—for a total of loss of about 65 percent. Including the additional loss for groundwater that has passed through 

a pond prior to discharge would decrease the load of wastederived nitrogen to the tidal portions of the Mashpee River and 

Table 2. Names and corresponding identifiers for freshwater 

streams, estuaries, and open coastal waters that receive nitrogen 

from terrestrial sources, Popponesset Bay watershed and the 

eastern part of Waquoit Bay watershed, western Cape Cod, 

Massachusetts.

[Nantucket Sound, identifier 666/777, is the only open coastal-water receptor 

and is not listed; identifiers are shown on figure 8]

Identifier Name Watershed

Mashpee 

Planning 

Area

Freshwater streams

6 Childs River Childs River

7 Quashnet River Quashnet River

8 Little River Little River

9 Mashpee River Mashpee River X

10 Quaker Run Quaker Run X

11 Santuit River Santuit River X

12 Marston Mills River Marston Mills River

Estuaries

41 Ockway Bay Popponesset Bay X

63 Eel Pond Waquoit Bay

64 Upper Quashnet River 

(tidal)

Waquoit Bay X

65 Childs River (tidal) Waquoit Bay

66 Sage Lot Pond Waquoit Bay X

67 Hamblin Pond Waquoit Bay X

68 Great River Waquoit Bay X

76 Jehu’s Pond Waquoit Bay X

78 Flat Pond Waquoit Bay X

79 Mashpee River (tidal) Popponesset Bay X

80 Shoestring Bay Popponesset Bay X

81 Pinquickset Cove Popponesset Bay X

82 Rushy Marsh Pond Three Bays

83 Popponesset Bay Popponesset Bay X

99 Seapit River Waquoit Bay

129 Popponesset Creek Popponesset Bay X

130 Waquoit Bay Waquoit Bay

142 Little River (tidal) Waquoit Bay X

143 Upper Seapit River Waquoit Bay

144 Red Brook Waquoit Bay X

178 Lower Quashnet River Waquoit Bay X

179 Lower Great River Waquoit Bay X
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 55

Nitrogen-loading rate, in kilograms per day

Current Phase 1 Phase 2

Nitrogen-loading rate, in kilograms per day

0

1

2

3

4

5

6

7 EXPLANATION

0

1

2

3

4

5

6

7 EXPLANATION

Current Phase 1 Phase 2

A

B

Ockway Bay

Mashpee River 

(tidal)

Shoestring Bay

Pinquickset Cove

Popponesset Bay

Popponesset Creek

Upper Quashnet 

River

Sage Lot Pond

Hamblin Pond

Great River

Jehu’s Pond

Flat Pond

Little River

Red Brook

Lower Quashnet 

River

Lower Great River

Time, in years

0 10 20 30 40 50 60 70 80 90 100 110 120

Figure 36. Changes in nitrogen loads from direct groundwater discharge resulting from partial sewering and disposal at wastewatertreatment facilities in the Mashpee Planning Area for individual receptors in the A, Popponesset Bay system and B, the eastern part of the 

Waquoit Bay system, western Cape Cod, Massachusetts.
56 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts Nitrogen-loading rate, in kilograms per day

0

5

10

15

20

25

30

Current Phase 1 Phase 2

B

0

5

10

15

20

25

30

Nitrogen-loading rate, in kilograms per day

Current Phase 1 Phase 2

A

EXPLANATION

EXPLANATION

Groundwater

discharge

Mashpee River

Outflow from 

Mashpee Pond

Total

Total (attenuated)

Total (attenuated and

corrected for 

groundwater flow 

through ponds)

Groundwater

discharge

Santuit River

Outflow from

Santuit Pond

Quaker Run

Total

Total (attenuated)

Total (attenuated and

corrected for 

groundwater flow 

through ponds)

Time, in years

0 10 20 30 40 50 60 70 80 90 100 110 120

Figure 37. Changes in nitrogen-loading rates from septic-system return flow and treated wastewater in response to two sequential 

phases of partial sewering and disposal at wastewater-treatment facilities for different load sources in A, the Mashpee River and 

B, Shoestring Bay, Popponesset Bay watershed, western Cape Cod, Massachusetts.
Effects of Wastewater-Management Actions on Nitrogen Loads to Receptors 57

table to receptors. Changes in the transport time affect the rate 

at which the load of nitrogen at a receptor changes in response 

to a change in nitrogen sources; however, the total mass 

discharging to the receptor after sufficiently long transport 

times would be the same. Dispersivity describes the effect of 

heterogeneity on the spreading of mass in an aquifer and is 

a function of aquifer characteristics and transport distance. 

Numerical dispersion refers to the spreading of simulated 

mass that arises from the numerical solution of the transport 

equation in discretized space beyond the amount of spreading 

explicitly related to heterogeneity. Longitudinal dispersion—in 

the direction of groundwater flow—is the largest component 

of numerical dispersion. In this analysis, a model-cell 

discretization was chosen that resulted in a longitudinal 

dispersion generally within the range of dispersivities reported 

for similar transport distances in other studies. Longitudinal 

dispersion affects the rate at which nitrogen loading changes 

in response to a change in nitrogen sources, but like porosity, 

does not affect the total mass discharging to a receptor over 

sufficiently long transport times.

Differences between predicted and actual loads of 

waste-derived nitrogen to a receptor also can arise from the 

assumptions in the analyses regarding nitrogen sources and 

transport processes. The assumption assumed to be reasonable 

in a generally oxic aquifer like that on Cape Cod that nitrogen 

is transported conservatively as nitrate allows for the use of 

groundwater and solute-transport models to explicitly represent mass transport and loading rates. Attenuation processes, 

such as denitrification, may occur locally in the aquifer and 

will result in actual loads that are lower than those predicted 

by the models; these processes, which generally operate within 

fine-grained estuarine muds that are characterized by reducing 

environments, further decrease actual loads to an estuary.

Assumptions are also made with regard to the definitions 

of nitrogen sources within a watershed. Little direct information regarding the amount and character of septic-system 

return flow is available, and as a result, the nitrogen input from 

return flow is estimated implicitly from water-use data, which 

are often combined from different periods. Nitrogen concentrations in septic-system return flow were assumed to be equal 

to 35 mg/L for these analyses, and losses associated with 

attenuation in the unsaturated zone were represented implicitly as loss factors applied to surface sources. Inaccuracies in 

all of these assumptions propagate into the model-calculated 

estimates of nitrogen-loading rates at receptors. An additional 

uncertainty arises from the assumption that the calculated 

loads are in equilibrium with the loads originating from the 

defined sources. It was assumed that current sources reasonably represent the current distribution of mass in the aquifer, 

thereby discounting temporal changes in nitrogen sources, for 

which there generally was little information; changes in nitrogen sources resulting from wastewater-management actions 

were also assumed to be instantaneous. Although both of these 

assumptions are approximate, the small traveltimes in this 

aquifer suggest that the two assumptions are reasonable.

Shoestring Bay by an additional 2.3 and 2.1 kg/d, respectively 

(fig. 37).

Limitations of Analysis

Sources of uncertainty associated with predictions of 

waste-derived nitrogen loads to receptors and of changes in 

those loads in response to a set of wastewater-management 

actions arise from (1) conceptual and numerical models of 

groundwater flow, (2) simplifying assumptions of conservative transport and values of transport parameters, and (3) 

limitations in the accuracy of data related to nitrogen sources 

and characteristics. The existing regional model was useful in 

determining the effect of time-varying recharge and pumping on the surface waters and the sources of water to pumped 

wells on western Cape Cod (Walter and Whealan, 2004) and 

recharge areas to ecological receptors—ponds, streams, and 

estuaries (Walter and others, 2004). With some modifications, 

namely an increase in vertical discretization, the model also 

allowed for the development of more finely discretized subregional models capable of simulating the transport of nitrogen in the aquifer. The existing regional groundwater-flow 

model used in this analysis was calibrated by trial and error to 

observations of water levels and streamflows; natural recharge, 

hydraulic conductivity, and boundary leakances were adjusted 

manually until there was a reasonable match to observed water 

levels and streamflows (Walter and Whealan, 2004). Changes 

in these three parameters could affect advective transport 

patterns and, therefore, predictions of the physical transport 

of nitrogen from sources to receptors. Because the calibrated 

flow model reasonably matched observed hydrologic conditions, model-calculated predictions of hydraulic gradients and 

advective-flow patterns were assumed to be reasonable. 

The assumed quantities pertaining to nitrogen transport, 

such as the rates of dispersion and attenuation and the 

representation of surface waters, are considered valid for 

this analysis as discussed in the text. The effects of these 

assumptions, which were designed to be consistent with the 

MEP process, on nitrogen transport and loads to receptors 

could be readily evaluated with sensitivity analyses or the 

use of multiple-model realizations, but these processes were 

beyond the scope of this effort. An example of the effect 

of assumed dispersivities on nitrogen transport and loads 

is presented in Walter (2006). Also, physical and chemical 

processes that could affect nitrogen transport and attenuation 

in local environments, such as ponds and organic estuarine 

muds, were beyond the scope of and not explicitly represented 

in this analysis; those processes could be more explicitly 

simulated using a reactive-transport model. 

Predictions of waste-derived nitrogen loads also are 

affected by transport parameters, such as porosity and 

dispersivity. Groundwater velocities are inversely proportional 

to porosity, which was assumed to be 0.3 in these analyses. 

A range in porosity of 0.2 to 0.4 would represent a twofold 

difference in velocities and transport times from the water 
58 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Summary and Conclusions

The discharge of excess nitrogen into estuaries on Cape 

Cod has led to the degradation of water quality and clarity, 

algal blooms, and the loss of critical eel-grass habitat in 

several estuarine systems. The largest source of nitrogen is 

from wastewater, primarily from residential septic systems. 

The Massachusetts Estuaries Project (MEP) was initiated 

by the Massachusetts Department of Environmental 

Protection (MassDEP) to establish Total Maximum Daily 

Loads (TMDLs) for nitrogen in estuaries and assist local 

communities in the restoration of marine ecosystems. The 

Popponesset Bay ecosystem consists of six estuaries that 

receive excess nitrogen primarily from residential septic 

systems within its 19-mi2 watershed. TMDLs for nitrogen 

have been developed for the system, and the town of Mashpee 

currently (2012) is developing wastewater-management 

strategies to implement those TMDLs.

Development of the TMDLs included the implicit 

estimation of current nitrogen loads from parcel-scale wateruse data and model-delineated recharge areas; these loads 

are referred to as “instantaneous” loads. A hydrodynamic 

model used the nitrogen loads to estimate the resultant waterquality conditions at specific locations within the estuary; this 

process is referred to as the “Linked Watershed-Embayment 

Management Modeling Approach.” Implementation of the 

TMDLs involved determination of nitrogen loads from the 

same set of model-delineated recharge areas for a set of 

nitrogen sources altered or emplaced through wastewatermanagement actions. The same linked-model approach was 

used to determine if a specific wastewater-management action 

would result in a nitrogen load and resultant water-quality 

conditions in the estuary suitable for the growth of eel grass. 

The use of static recharge areas delineated by a groundwaterflow model to estimate nitrogen loads introduces limitations, 

including the inability to (1) account for the effects of dynamic 

recharge areas on nitrogen loads, (2) explicitly represent the 

potential effects of surface-water bodies on nitrogen loads, 

and (3) evaluate the time-varying responses of waste-derived 

nitrogen loads to wastewater-management actions. Solutetransport models, which explicitly represent the transport 

of mass through an aquifer from sources to receptors, can 

enhance implementation of the TMDLs by (1) eliminating 

the reliance on static recharge areas, (2) more explicitly 

representing the transport of mass through surface waters, 

and (3) allowing for the evaluation of changing waste-derived 

nitrogen loads in response to wastewater-management actions.

The U.S. Geological Survey, in cooperation with 

MassDEP, has evaluated the efficacy of using a solutetransport model to assist in the implementation of the TMDLs 

for nitrogen in the Popponesset Bay ecosystem. Parcel-scale 

water-use data assembled by the MEP were used to represent 

current nitrogen sources within the watershed. Three other 

source terms were developed representing full sewering of 

the watershed and two sequential phases of hypothetical 

wastewater management within the Mashpee Planning Area 

(MPA), which is composed of the Popponesset Bay watershed 

and parts of the Waquoit Bay watershed. The nitrogen-source 

terms were used as input into a subregional model of the 

watershed designed to represent the conservative transport 

of nitrate in the aquifer and linked to an existing regional 

model of western Cape Cod. The subregional model was used 

to evaluate the effects of different wastewater-management 

actions—hypothetical but consistent with those that could be 

considered for the watershed—on water levels, groundwaterdischarge rates, recharge areas, and time-varying loads of 

waste-derived nitrogen to wells and ecological receptors. 

Wastewater-management actions within the MPA 

included sewering, treatment, and disposal at wastewatertreatment facilities (WTFs). The resulting changes in 

hydrologic conditions included changes in water levels, 

groundwater discharges to streams and coastal waters, and 

hydraulic gradients. Changes in hydraulic gradients, in turn, 

can change recharge areas to wells and ecological receptors. 

Significant findings are summarized below.

• Return flow from septic systems is a dispersed hydraulic stress and represented only about 3 percent of 

recharge within the MPA. Groundwater discharge to 

ponds, streams, and estuaries in the Popponesset Bay 

watershed changed by less than 3 percent as a result 

of full sewering. The mean water-level decrease in the 

watershed as a result of full sewering was 0.07 ft; the 

maximum drawdown exceeded 0.2 ft near the northern 

extent of the watershed.

• The centralized disposal of treated return flow at WTFs 

is a more focused hydraulic stress that can cause localized water-level increases and changes in hydraulic 

gradients. Hypothetical disposal at a WTF site west of 

Popponesset Bay resulted in a water-level increase of 

more than 5 ft after two sequential phases of wastewater management and larger hydraulic gradients near 

Popponesset Bay. Groundwater discharges to two estuaries near the WTF increased by 15 and 28 percent as a 

result of the disposal of treated wastewater at this site.

• Recharge areas to receptors changed very little as a 

result of sewering because changes in the dispersed 

stress do not significantly change hydraulic gradients. 

However, the disposal of large amounts of treated 

wastewater at a WTF can cause large localized changes 

in water levels and hydraulic gradients. As a result, 

recharge areas to nearby receptors can change. Simulated recharge areas to two estuaries increased by about 

20 percent and expanded 1,200–2,000 ft upgradient 

as a result of hypothetical disposal at a WTF site west 

of Popponesset Bay. Recharge areas also are affected 

by large groundwater withdrawals from wells; simulated pumping of three wells north of the watershed 

shifted the regional groundwater divide southward and 
Summary and Conclusions 59

decreased the recharge areas to ponds in the watershed 

by about 8 percent.

Estuaries receive waste-derived nitrogen from direct 

groundwater discharge and, in some cases, surface-water 

inflow from streams. Ponds are hydrologic features that both 

receive water from and contribute water to the aquifer; as a 

result, some waste-derived nitrogen passes through ponds 

prior to discharge into estuaries. Recharge areas to estuaries, 

streams, and ponds respectively compose about 27, 33, and 

40 percent of the watershed. Wastewater-management actions 

that included sewering and centralized disposal at WTFs redistributed nitrogen sources within the watershed and changed 

loads of waste-derived nitrogen to receptors. Significant findings are summarized below.

• Loads of waste-derived nitrogen resulting from a set 

of wastewater-management actions can be implicitly 

estimated on the basis of recharge areas. However, 

recharge areas delineated on the basis of current 

hydrologic conditions were different from recharge 

areas delineated on the basis of hydrologic stresses 

and nitrogen sources resulting from wastewatermanagement actions. In most areas of the Popponesset 

Bay watershed, hydraulic gradients and recharge 

areas did not change substantially in response to 

wastewater-management actions, and inconsistencies 

between current and future recharge areas and nitrogen 

sources likely did not affect estimates of waste-derived 

nitrogen loads. Hydraulic gradients and recharge areas 

to two estuaries did change in response to hypothetical 

disposal at a WTF site to the west of Popponesset Bay. 

Loads of waste-derived nitrogen to these two estuaries 

estimated by using recharge areas consistent with 

the simulated hydraulic gradients were 3 to 9 times 

larger than loads estimated by using current-condition 

recharge areas. 

• Current nitrogen loads explicitly simulated by using a 

solute-transport model were similar to those estimated 

on the basis of recharge areas over sufficiently long 

transport times when simulated loads approach a 

steady-state condition with respect to with surface 

sources. Total nitrogen loads estimated from current 

sources by a solute-transport model were within 50 and 

80 percent of the instantaneous loads by about 7 and 

30 years, respectively. These results are consistent with 

the small advective traveltimes in the aquifer—median 

traveltimes were less than 6 years to ponds, streams, 

and estuaries—and suggest that the use of currentcondition recharge areas is suitable for estimating 

current nitrogen loads to receptors.

• Most (about 70 percent) of recharged water in the 

Popponesset Bay watershed enters the estuaries 

as surface-water inflow, with about 40 percent of 

recharged water flowing through a pond prior to 

discharging into a stream or estuary. Surface waters 

attenuate nitrogen; attenuation losses are assumed 

to be 30 and 50 percent in streams and ponds, 

respectively, for evaluating TMDL compliance. 

Explicitly representing the transport of mass improved 

the estimation of attenuation within surface waters. 

After 100 years of transport from current sources, 

about 56 percent of the simulated mass of nitrogen 

passing through a pond discharged to a stream, and 

about 23 percent left the pond as surface-water outflow 

to streams; less than 0.1 percent discharged directly 

to estuaries. The remaining 20 percent of the nitrogen 

daily load passing through a pond was still stored in 

deeper parts of the aquifer after 100 years of simulated 

transport but would be expected to eventually 

discharge to a receptor after sufficiently long transport 

times. Accounting for the mass of nitrogen passing 

through ponds prior to discharging to a stream resulted 

in total attenuated loads to the estuaries that were 

lower than loads determined without accounting for 

that process.

• The response of waste-derived nitrogen loads at 

receptors to wastewater-management actions is a 

function of traveltimes, which generally are small 

in the Cape Cod aquifer—median traveltimes 

were less than 6 years to the natural receptors. Full 

sewering of the MPA decreased loads to receptors 

in the Popponesset Bay watershed from about 64 to 

6 kg/d. About 50 and 80 percent of the total decrease 

in nitrogen load occurred within 5 and 15 years, 

respectively, following implementation of full 

sewering. Nitrogen loads to receptors decreased from 

about 64 to 27 kg/d following the implementation of 

two sequential phases of partial sewering and disposal 

at WTFs; the decrease in loads was about 50 and 

80 percent of the final amount within 3 and 10 years, 

respectively, of complete implementation of the 

two phases.

• The response of an individual estuary is a function 

of traveltimes to the estuary, the mechanisms by 

which nitrogen enters the estuary, and the specific 

wastewater-management actions implemented within 

the recharge area to the receptor. Changes in nitrogen 

loads from direct groundwater discharge to individual estuaries in the Popponesset Bay watershed 

ranged from a decrease of 90 percent to an increase of 

80 percent depending on the wastewater-management 

actions that were implemented. Two estuaries in the 

Popponesset Bay ecosystem also receive loads from 

surface-water inflows. Most of the 69- and 79-percent 

decreases in the total nitrogen loads to these estuaries 

was from decreases in loads delivered by the surfacewater inflows. 
60 The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts

Solute-transport models explicitly represent the mass 

transport of nitrogen from sources at the water table to 

receptors. The detailed parcel-scale data used to characterize 

nitrogen sources can be accurately represented as a source 

term in the models, and the effects of changes in the source 

terms on nitrogen loads to receptors of interest can be 

evaluated. This method can improve implementation of 

TMDLs for nitrogen in estuaries on Cape Cod by (1) avoiding 

the limitations associated with implicit methods for evaluating 

nitrogen loads, (2) improving the method used to incorporate 

nitrogen attenuation in surface waters, and (3) providing 

an understanding of the complex response of the system to 

wastewater-management actions.

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Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

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U.S. Geological Survey

New England Water Science Center 

Massachusetts-Rhode Island Office

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at:

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Walter—The Simulated Effects of Wastewater-Management Actions on the Hydrologic System, Cape Cod, Massachusetts—Scientific Investigations Report 2013–5060

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