U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3034

April 2005

People and Water in the Assabet River Basin, 

Eastern Massachusetts

INTRODUCTION

Withdrawals from public-supply wells and reservoirs, wastewater discharges to streams, ground water leaking into sewer 

pipes, and septic-system leachate percolating to the water table—

these are some of the ways in which people alter the hydrologic 

system in a river basin. Ultimately, the effects of these activities 

are seen in streams, as changes in the quantity and quality of 

streamflow. In the Assabet River Basin, water use and population density vary greatly, and people’s activities have altered 

streamflow in the Assabet River and its tributaries to varying 

degrees. The basin is in a rapidly growing area along Interstate 

495 in eastern Massachusetts (fig. 1), where demands for water 

supply and wastewater disposal are increasing. Between 1990 

and 2000, population in the 20 towns in the basin (table 1) 

increased by 15 percent—nearly three times the statewide average—and by more than 30 percent in some towns. 

Ground water supplies much of the water used in the basin. 

Most public-supply wells are in the sand and gravel glacial 

aquifers along the Assabet River and its tributary streams (fig. 1). 

The aquifers drain to the streams, so that, between storms, most 

of the water in the streams consists of ground-water discharge 

(base flow). After use, much of the publicly supplied water 

is transferred to downstream treatment facilities, where it is 

discharged to the Assabet River. Currently (2004), the Assabet 

River is eutrophic during the summer and fails to meet most 

applicable water-quality standards (Massachusetts Department 

of Environmental Protection, 2003). Ground-water withdrawals 

contribute to this problem by reducing the amount of high-quality base flow that would dilute wastewater in the river. In many 

tributaries, reductions in streamflow result in the loss of habitat 

for fish and other aquatic organisms. Less ground water available 

for discharge also may alter wetlands.

To better understand the effects of current and future water 

withdrawals and wastewater discharges on streamflows in the 

Assabet River and its tributaries, the U.S. Geological 

Survey (USGS), in cooperation with the Massachusetts 

Department of Conservation and Recreation (MADCR), recently 

Figure 1. The Assabet River Basin, subbasins, and surficial geology, 

eastern Massachusetts.

An accounting of the inflows, outflows, and uses of water in the rapidly developing Assabet River Basin, along 

Interstate 495 in eastern Massachusetts, was done to quantify how people’s activities alter the hydrologic system. 

The study identified subbasins and seasons in which outflows resulting from people’s activities were relatively 

large percentages of total flows, and quantified the fraction of streamflow in the Assabet River that is treated 

wastewater. Computer models of ground-water flow were also used to test how the components of the hydrologic 

system, particularly streamflow, would change with future development and increased water use. Computer 

simulations showed that, when water use was increased to currently permitted levels, streamflows in tributaries 

would decrease, particularly during the low-flow period. In the Assabet River, increased wastewater discharges 

resulted in a slight increase in total streamflow and an increase in the fraction of streamflow in the river that is 

wastewater, relative to existing conditions. 

Page 1

BOYLSTON

MARLBOROUGH

COL

HAR

AND

HOW

BRO

SUB

HO

BR

SU

ASSABET MAIN STEM

HEADWATERS

SUBBASIN

NASHOBA

BROOK

SUBBASIN

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

From USGS and MassGIS data sources, Massachusetts

State Plane Coordinate System, Mainland Zone.

TOWN BOUNDARY

495

290

495

2

4

Assabet River

73o

00'

42o

30'

42o

00'

41o

30'

72o

00' 71o

00'

70o

00'

Assabet River

Basin
completed a study 

of the basin’s 

water resources 

(DeSimone, 

2004). 

Representatives 

from Federal and 

State agencies, 

towns, a watershed association, 

and other organizations provided 

guidance to the 

study through 

a Technical 

Advisory 

Committee. 

The water-use 

and management issues of 

concern in the 

Assabet River 

Basin are common 

to many other 

basins in eastern 

Massachusetts and 

adjacent States, 

where communities are striving 

to balance growth 

and the available water resources. This Fact Sheet summarizes 

some of the important findings of the cooperative study.

WATER USE AND THE MANAGED HYDROLOGIC SYSTEM, 

1997–2001

Through infrastructure such as wells, water-distribution and 

sewer systems, and treatment facilities, water use and management moves water around in the Assabet River Basin and changes 

water quality (fig. 2). Water used in the basin is provided by 

public-supply and private wells, a few surface-water reservoirs, 

and water imported from outside the basin. Public-water systems 

supply most of the water that is used in the basin, serving about 

80 percent of the basin population and about half of its area 

(table 1 and fig. 3). Water withdrawals for public supply and large 

nonmunicipal users, 9.9 Mgal/d in 1997–2001, are mostly 

(74 percent) from ground water. Nonmunicipal users include 

industrial, agricultural, and golf-course users. Nearly half of the 

ground water withdrawn for public supply is discharged to the 

Assabet River after treatment (3.6 Mgal/d) and nearly half is 

returned to the ground-water-flow system through on-site septic 

systems (3.5 Mgal/d; fig. 2). Septic-system return flow occurs in 

areas that are supplied by public water but are not served by public sewers (fig. 3). Water use and disposal, therefore, redistribute 

water from supply-well locations to areas served by public-water 

systems, and they accelerate the natural flow of ground water to 

the main stem of the Assabet River. 

Water is imported into the basin for public supply 

(1.7 Mgal/d), and as wastewater (3.7 Mgal/d) for treatment at 

a regional facility near the basin headwaters (fig. 3). Imported 

0 1 2 3 4 5 MILES

0 2 1 3 4 5 KILOMETERS

From USGS and MassGIS data sources, Massachusetts

State Plane Coordinate System, Mainland Zone.

PUBLIC-SUPPLY WITHDRAWAL

PERMITTED NONMUNICIPAL 

WITHDRAWAL

WASTEWATER DISCHARGE

42

Figure 3. Public-water and sewer systems and locations of permitted water 

withdrawals and wastewater discharges in the Assabet River Basin, 2001.

Page 2

Town Total

population

Population on public

water and sewer 

(percent), 2003

Water Sewer

Acton 100 20,331 94 0

Berlin 100 2,380 0 0

Bolton 72 4,148 0 0

Boxborough 66 4,868 0 0

Carlisle 29 4,717 0 0

Clinton 15 13,435 100 100

Concord 36 15,537 95 38

Grafton 7 14,894 10 10

Harvard 22 5,981 10 0

Hudson 94 18,113 94 82

Littleton 42 8,184 80 0

Marlborough 43 36,255 99 92

Maynard 100 10,433 100 95

Northborough 94 14,013 85 20

Shrewsbury 37 31,640 97 85

Stow 100 5,902 0 0

Sudbury 9 16,841 1100 0

Westborough 41 17,997 95 85

Westford 24 20,754 75 0

Proportion

of town

area in

basin

(percent)

1 Value applies to area of town in basin.

Boylston 24 4,008 10

Littleton,

Shrewsbury,

Westford

Massachusetts

Water 

Resources 

Authority

WastewaterTreatment 

Facilities

Consumptive

Use

Septic 

Systems

Ground Water

Surface Water

Water Use

Hopkinton,

Shrewsbury,

Westborough

Imported

Water

Supply

Impo

Wast

wate

EXPLANATION

WATER TRANSFER—Arrow line thickness is proportional to volumetric flow rate

from 0.5 to 11 million gallons per day. 

I/I

UNACC

Figure 2. Schematic diagram showing water use, transfer, and disposal in the 

Assabet River Basin, 1997-2001. Blue lines indicate withdrawal or import of 

water for use; brown lines indicate discharge, disposal, or import of wastewater; and orange lines indicate consumptive use. (UNACC, unaccounted-for 

water; I/I, infiltration to sewers)

Table 1. Population on public water and sewer in 

towns in the Assabet River Basin. Yellow shading 

indicates towns with public-water supply and 

wastewater disposal (sewers); green shading 

indicates towns with public-water supply and 

private-wastewater disposal (septic systems); and 

brown shading indicates towns with private-water 

supply and wastewater disposal.

[Total population: From U.S. Census Bureau, 2003. 

Population on public water and sewer: From U.S. Census 

Bureau, 2003, and town water departments] 
wastewater accounted for about one-third of the total wastewater discharged to the Assabet River, which was 11.0 Mgal/d in 

1997–2001. About one-quarter of the discharged wastewater 

(2.6 Mgal/d) originated as ground water leaking into sewers 

(ground-water infiltration). Ground-water infiltration and wastewater discharges are greatest in the spring (fig. 4), when groundwater levels are high. Sewer systems and wastewater-treatment 

facilities (fig. 3) serve about 50 percent of the basin population.

Water withdrawals were greatest in May, June, and July 

in 1997–2001 (fig. 4). Per capita water-use rates averaged 30 

percent greater in these months (82 gallons per person per day) 

than in November through March (61 gallons per person per day; 

annual average is 73 gallons per person per day). Greater withdrawals in the late spring and summer probably supply outdoor 

water use, which is partly or wholly consumptive. Consumptive 

use is water that is evaporated, transpired by plants, incorporated 

into products or crops, consumed by people or animals, or otherwise removed from the immediate environment (Hutson, 2004). 

Consumptive use by publicly and privately supplied residential 

users was estimated at 1.4 Mgal/d in 1997–2001, and averaged 

about 15 percent of total residential use annually; agriculture 

and golf courses consumptively used an estimated additional 

0.6 Mgal/d. In basin areas where private wells supply water use, 

consumptive use is the only component of the water withdrawals 

that is a net outflow of water from the ground-water-flow system; 

water withdrawn from private wells that is not used consumptively (85 percent of total withdrawals) is returned to the aquifer 

after use, although degraded in quality, through septic systems. 

BASIN WATER BUDGETS

Water budgets provide the framework for understanding how 

much water is available in a basin and how much of that water is 

being used or manipulated by people’s activities. Knowing water 

budgets for the basin and its subbasins allows water-resource 

managers to identify areas of greatest concern in the basin and 

make more informed decisions about permitting, conservation, 

and remediation. The water-use information described above 

was used in conjunction with computer models of ground-water 

flow to calculate water budgets for the Assabet River Basin and 

its tributary subbasins. The calibrated models represent average 

annual and monthly conditions during 1997–2001. The models 

simulate water flow, and account for processes that affect flow 

in the saturated zones (that is, below the water table) of the sand 

and gravel glacial aquifers, glacial till, and underlying bedrock, 

in the basin. Flow in streams also is simulated, except for the 

component of streamflow that results from storms (direct runoff). 

The streamflows calculated with the models are average nonstorm 

streamflows, which are equal to base flow plus any wastewater 

discharges to the streams. More information about the models can 

be found in the report by DeSimone (2004). 

Basinwide Budget

Annually, total nonstorm water flow through the study area 

averaged 195 Mgal/d. Recharge from precipitation (180 Mgal/d) 

and ground-water discharge to streams (129 Mgal/d) were the 

major inflow and outflow components of the hydrologic system, 

respectively (fig. 5). Evapotranspiration of ground water from 

wetlands (29 Mgal/d) and nonwetland areas (13 Mgal/d) 

also were important losses (outflows) from the hydrologic system. Basinwide, water-supply withdrawals (9.9 Mgal/d) were 

5 percent of total annual outflows; septic-system return flows (4.3 

Mgal/d, which includes septic-system return flow from imported 

public supply) accounted for 2 percent of total annual inflows. 

Seasonally, inflows and outflows to the hydrologic system 

varied substantially. In March, the highest flow month, total 

nonstorm water flow averaged 592 Mgal/d. Precipitation recharge 

(578 Mgal/d) was still the major inflow. Much of the recharge 

was stored in the aquifer, as the water table rose (261 Mgal/d, 

fig. 5; flow to aquifer storage is shown as an outflow, although it 

remains within the hydrologic system). Most of the 

remaining outflow was to ground-water discharge to streams 

(297 Mgal/d). Water-supply withdrawals were a smaller fraction 

of total outflows (1.6 percent, or 9.3 Mgal/d), as was infiltration to sewers (less than 1 percent of total flows, or 4.2 Mgal/d), 

although infiltration is larger in March than annually. 

In September, the lowest flow month, total nonstorm water 

flow averaged 112 Mgal/d. The source of nearly all of the water 

flowing through the hydrologic system was aquifer storage, as the 

water table declined and water was released from storage 

(93 Mgal/d, fig. 5; flow from aquifer storage is shown as an 

inflow). Outflows were about equal for ground-water discharge 

to streams (49 Mgal/d) and evapotranspiration of ground water 

from wetlands, ponds, and nonwetland areas (49 Mgal/d; fig. 5). 

Jan. Feb. Mar. Apr.MayJune July Aug.Sept. Oct. Nov.Dec.

RATE, IN MILLION GALLONS PER DAY 16

14

12

10

8

6

0

2

4

PUBLIC-SUPPLY 

WITHDRAWALS

PERMITTED INDUSTRIAL, 

AGRICULTURAL, AND GOLFCOURSE WITHDRAWALS

PUBLIC WATER SUPPLIED 

BY MASSACHUSETTS 

WATER-RESOURCES 

AUTHORITY

WASTEWATER 

DISCHARGES

EXPLANATION

MONTH

PRECIPITATION RECHARGE

SEPTIC-SYSTEM RETURN FLOW

GW DISCHARGE TO STREAMS

FLOW FROM WETLANDS AND PONDS

FLOW FROM NONWETLAND AREAS

GW WITHDRAWALS

INFILTRATION TO SEWERS

CU IN PRIVATELY SUPPLIED AREAS

CU BY UNPERMITTED AGRICULTURE

EXPLANATION

FLOW, IN MILLION GALLONS PER DAY

INFLOWS

OUTFLOWS

250

100

-100

-200

0

200

150

50

-50

-150

-250

600

300

200

-300

-500

0

500

400

100

-100

-200

-400

-600

150

0

100

50

-50

-100

-150

A. ANNUAL B. MARCH C. SEPTEMBER

FLOW FROM STORAGE

FLOW TO STORAGE

SOME INFLOWS AND OUTFLOWS ARE TOO 

SMALL TO BE SEEN AT THE SCALE SHOWN. 

SCALE VARIES FROM A TO C

Figure 4. Monthly average permitted withdrawals, wastewater discharges, 

and imported water for public supply, 1997–2001, in the Assabet River Basin.

Figure 5. Model-calculated water budgets for the ground-water-flow 

system in the Assabet River Basin. The budgets represent average annual, 

high-flow (March), and low-flow (September) conditions during 1997–2001. 

(GW, ground water; CU, consumptive use)

Page 3
Inflows and outflows from people’s activities were higher 

percentages of total flows in September than in March or annually, averaging 4 percent of total inflows for septic-system return 

flow (4.3 Mgal/d) and 9 percent of total outflows for watersupply withdrawals (10.5 Mgal/d). 

Subbasin Budgets

Major inflows and outflows of water in subbasins of the 

Assabet River Basin were similar to those for the basin as a 

whole. However, water withdrawals, septic-system return flow, 

and other flows caused by people’s activity varied among subbasins, from nearly zero in the Elizabeth Brook subbasin in Stow, 

to relatively large fractions of total flows, as in the Assabet Main 

Stem Upper and Fort Meadow subbasins in Westborough, 

Marlborough, and Hudson (fig. 6). Septic-system return flow 

was largest in subbasins where much of the population was 

served by public water and private disposal, such as the Fort 

Pond and Nashoba Brook subbasins in Acton (figs. 1 and 3). 

Evapotranspiration of ground water also varied among subbasins, 

depending to a large extent on the areal distribution of wetlands. 

By comparing total outflows from people’s activities (groundwater withdrawals, surface-water withdrawals, and ground-water 

infiltration to sewers) to total water flows through the subbasin, 

the degree of alteration to the hydrologic system in a subbasin 

can be characterized. On an annual average basis, outflows from 

people’s activities were more than 10 percent of total flows 

through four subbasins: Taylor Brook, Fort Meadow Brook, Stirrup Brook, and the Assabet Main Stem Upper subbasin 

(fig. 7). Outflows from people’s activities also were relatively 

large percentages (5 to 10 percent) of total flows in the Assabet 

Main Stem Headwaters and Lower subbasins. In September, 

flows from people’s activities accounted for more than 20 percent of total flows in the Fort Meadow and Assabet Main Stem 

Upper subbasins and for more than 10 percent of total flows in 

seven basins (fig. 7).

Wastewater in the Assabet River

Treated wastewater was discharged to the Assabet River 

from four municipal treatment facilities and one nonmunicipal facility in 2001, which are distributed along the length of 

the river (fig. 1). Thus, wastewater accounted for a variable 

percentage of total nonstorm streamflow in the Assabet River 

from its headwaters downstream. At its maximum, immediately downstream of the wastewater-treatment facility that is 

closest to the headwaters of the river (facility in Westborough, 

figs. 1 and 3), wastewater accounted for 60 percent of average annual streamflow in the Assabet River, and for 93 percent 

of average September streamflow (figs. 8A and B). In March, 

when overall nonstorm streamflows were much higher, wastewater accounted for smaller fractions of flow. Surface-water 

withdrawals were insignificant fractions of total nonstorm 

streamflow in the main stem Assabet River in all seasons.

UNDERSTANDING THE PRESENT AND SIMULATING 

THE FUTURE

One way to understand how people’s activities affect conditions in the Assabet River Basin is to simulate the removal 

of all the water-management activities in the hydrologic system. Streamflows that result from this scenario approximate 

streamflows that might have occurred before water-resource 

development in the basin (that is, predevelopment conditions). 

A way to understand how people’s activities may further change 

hydrologic conditions is to simulate future water-use activities. 

The computer flow models were used to simulate these two conditions. In the first scenario (S1), water withdrawals, wastewater 

discharges, infiltration to sewers, septic-system return flows, and 

consumptive uses were set to zero, to simulate predevelopment 

conditions. In the second scenario (S2), water withdrawals and 

wastewater discharges were increased to levels currently 

Figure 6. Model-calculated water budgets for the ground-water-flow system in subbasins of the Assabet River Basin, for average annual and average low-flow 

(September) conditions during 1997–2001 (gw, ground water; cu, consumptive use). 

OUTFLOWS

EXPLANATION

INFLOWS

PRECIPITATION RECHARGE

SEPTIC-SYSTEM RETURN FLOW

FLOW FROM OTHER SUBBASINS

FLOW FROM UNDERLYING BEDROCK

FLOW FROM STORAGE

OUTFLOWS

FLOW TO STORAGE

FLOW TO OTHER SUBBASINS

FLOW TO UNDERLYING BEDROCK

GW DISCHARGE TO STREAMS

FLOW FROM WETLANDS AND PONDS

FLOW FROM NONWETLAND AREAS

GW WITHDRAWALS

INFILTRATION TO SEWERS

CU IN PRIVATELY SUPPLIED AREAS

CU BY UNPERMITTED AGRICULTURE

OUTFLOWS

INFLOWS

B. SEPTEMBER

INFLOWS

30

-30

20

10

-10

-20

0

30

20

10

-10

-20

-30

0

Headwaters

Upper

Middle

Lower

Hop

Cold Harbor

 & Howard

North

Danforth

Fort Meadow

Elizabeth

Taylor

Fort Pond

Nashoba

Spencer

Stirrup

TRIBUTARIES

Headwaters

Upper

Middle

Lower

Hop

Cold Harbor

 & Howard

North

Danforth

Fort Meadow

Elizabeth

Taylor

Fort Pond

Nashoba

Spencer

Stirrup

ASSABET MAIN TRIBUTARIES

STEM

ASSABET MAIN 

STEM

FLOW, IN MILLION GALLONS PER DAY

A. ANNUAL

Page 4
permitted under the Massachusetts Water Management Act 

and the National Pollution Discharge Elimination System; 

the effects of new sewers also were simulated in the second 

scenario. Other scenarios simulated in the study are described by 

DeSimone (2004).

Predevelopment Conditions

In scenario S1, elimination of withdrawals, discharges, and 

other flows associated with people’s activities resulted in more 

streamflow in tributaries and less streamflow in the Assabet 

River. Increased average annual streamflows in tributaries 

ranged from 2 percent, in the Elizabeth Brook subbasin, to 44 

percent, in the Fort Meadow Brook subbasin. In most subbasins, 

streamflow increases resulted from a variable combination of 

decreased withdrawals and decreased ground-water infiltration 

to sewers (table 2). Decreased withdrawals contributed most to 

the streamflow increases in the Assabet Main Stem Headwaters 

(not shown in table 2), Cold Harbor and Howard Brooks, Stirrup 

Brook, Fort Meadow Brook, and Taylor Brook subbasins. In the 

Hop Brook and Danforth Brook subbasins, decreased infiltration 

to sewers contributed most to increases in streamflow 

(table 2). In many subbasins, decreases in withdrawals and 

infiltration to sewers were offset by decreases in septic-system 

return flow, which was an inflow to the aquifer and augmented 

streamflow under simulated existing conditions. This happened 

in the Fort Pond Brook and Nashoba Brook subbasins, which 

have primarily public-water supply and private disposal. In the 

Elizabeth Brook subbasin, where private-water supply and disposal serve most of the population, the small streamflow increase 

resulted mostly from decreased consumptive use (table 2). A 

small decrease in streamflow in the Spencer Brook subbasin 

resulted from the elimination of a wastewater discharge.

Changes in average monthly flows in scenario S1 relative to 

existing conditions varied during the annual cycle and among 

subbasins, depending on the variable monthly rates of 

Tributary subbasin

Change in inflows and outflows from people’s

activities 

(cubic feet per second)

Inflows Outflows

Septicsystem 

return

flow

Watersupply

withdrawals

Groundwater

infiltration

to sewers

Consumptive 

use

Scenario 1: Predevelopment Conditions

Hop Brook -0.17 -0.1 0.45 0.36 (+5)

Cold Harbor & 

Howard Brooks

-.13 -.45 -.20 -.05 +.6

Stirrup Broo .1 .0 +.22

North Broo -.05 -.13 +.10

Danforth Broo -.01 -.12 -.05 +.20

Fort Meadow

Brook

-.11 -1.4 +1.3

Elizabeth Broo -.08 .2

Taylor Broo .0 +.37

Fort Pond Broo 1.04 -.71 -.06 -.03

Nashoba Broo -.93 .1 .3

Spencer Broo .0 .0

Scenario 2: Future Conditions –Maximum Permitted Withdrawals and Discharges

Hop Broo .0 -.67

Cold Harbor & 

Howard Brooks

-.13 .0 -.20

Stirrup Broo +.03 .1

North Broo .0 -.17

Danforth Broo -.03 -.03

Fort Meadow

Brook

-.09 +.02 .3

Elizabeth Broo

Taylor Broo -.04

Fort Pond Broo .0

Nashoba Broo .0

Spencer Broo .0

(Number refers 

to label 

on figure 7)

(1)

 (2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

Change in 

streamflow,

in cubic feet 

per second

(change in percent)

( 9)

( +4)

( 2)

( -.1)

( 2)

( -3)

( 8)

( -1)

( -1)

( .3)

( 0 )

( -3)

( +12)

( +.6)

( +44)

( +23)

( -10)

( -.05)

( .7)

Figure 7. Outflows from people’s activities (ground-water withdrawals, surface-water withdrawals, and ground-water infiltration to sewers) relative to total 

model-calculated outflows from the ground-water-flow system in subbasins of the Assabet River Basin, for average annual and average low-flow (September) 

conditions during 1997–2001. Subbasin numbers refer to subbasin names as listed in table 2. 

Table 2. Changes in average annual inflows and outflows from people’s 

activities and the resulting model-calculated streamflow in tributary subbasins of the Assabet River Basin in simulated scenarios of (1) predevelopment 

conditions and (2) future conditions, in which withdrawals and discharges 

are increased to maximum permitted levels. Changes are relative to existing 

(1997–2001) conditions. 

71o36'

71o24'

42o18'

42o24'

42o30'

0 5 MILES

0 5 KILOMETERS

B. SEPTEMBER

71o36'

71o24'

42o18'

42o24'

42o30'

A. ANNUAL

OUTFLOWS FROM PEOPLE'S 

ACTIVITIES—Shading indicates magnitude 

of outflows as percentages of total modelcalculated flows through the subbasin

 Less than 2 percent

Greater or equal to 2 percent to 5 

percent

Greater than 5 percent to 

10 percent

Greater than 10 percent to 

20 percent

Greater than 20 percent

BASIN AND SUBBASIN 

BOUNDARY

EXPLANATION

1

2 3

4

5

7

8

10

9

11

6

1

2 3

4

5

7

8

10

9

11

6

Page 5
existing conditions (fig. 8A). These streamflows were reduced 

because wastewater discharges, which augment streamflow 

under existing conditions, were eliminated in S1. In September, 

decreases in streamflow in the Assabet River ranged from more 

than 60 percent, downstream of the headwaters wastewatertreatment facility, to about 7 percent, at the confluence with the 

Sudbury River (fig. 8B). 

Increased Withdrawals and Discharges

In scenario S2, water withdrawals were increased from 

existing sources and a new source (supply well) was added in 

Westborough so that total withdrawals increased from 

9.9 Mgal/d to maximum permitted levels of 12.2 Mgal/d. Wastewater discharges at existing treatment facilities were increased to 

the maximum permitted levels, and a new facility was added in 

Acton to increase total discharges from 11.0 Mgal/d to permitted levels of 15.0 Mgal/d. The effects of new sewers, mostly in 

Acton and Northborough, also were simulated, by decreasing 

septic-system return flow and increasing ground-water infiltration to sewers in these areas. Basinwide, septic-system return 

flow decreased from 4.3 Mgal/d to 3.0 Mgal/d, and infiltration to 

sewers increased from 2.6 Mgal/d to 2.9 Mgal/d. In this scenario, 

streamflow decreased in nearly all tributaries and increased in 

the Assabet River, relative to existing conditions (table 2 and 

figs. 8A and 8B). Decreases relative to existing conditions ranged 

from less than 1 percent, in the Danforth and Nashoba Brook 

subbasins, to about 10 percent, in the Hop and Fort Meadow 

Brook subbasins. In Hop, Fort Meadow, and Taylor Brook subbasins, increased withdrawals accounted for most of the decrease 

in streamflow; decreased septic-system return flow accounted for 

most of the decreased streamflow in other subbasins 

(table 2). As in scenario S1, changes in monthly model-calculated streamflows were largest relative to 1997–2001 flows in 

September, when decreases in model-calculated streamflows 

ranged from about 1 to 98 percent. 

In the Assabet River, other than in the main stem Headwaters subbasin (fig. 1), model-calculated streamflows for the S2 

scenario increased to relative existing conditions (figs. 8A and 

B), because of increased wastewater discharges. In this scenario, 

the wastewater fraction of streamflow ranged from 69 to 10 

percent annually (decreasing downstream from the headwaters 

wastewater-treatment facility, number 1 in fig. 8), and from 94 to 

26 percent in September, both of which are increases of several 

percentage points relative to existing conditions. 

REFERENCES

DeSimone, L.A., 2004, Simulation of ground-water flow and evaluation of watermanagement alternatives in the Assabet River Basin, eastern Massachusetts: 

U.S. Geological Survey Scientific Investigations Report 2004-5114, 133 p. 

(Available online at http://water.usgs.gov/pubs/sir/2004/5114/.)

Hutson, S.S., Barber, N.L., Kenny, J.F., Linsey, K.S., Lumia, D.S., and Maupin, 

M.A., 2004, Estimated use of water in the United States in 2000: U.S. Geological Survey Circular 1268, 46 pages.

Massachusetts Department of Environmental Protection, 2003, Massachusetts 

Year 2002 integrated list of waters, Part 2—Final listing of individual categories 

of waters: Boston, MA, Massachusetts Department of Environmental Protection, 

variously paged. 

U.S. Census Bureau, 2003, American Factfinder Census 2000 data sets: accessed 

January 2003 at http://factfinder.census.gov

250

200

150

50

100

EXPLANATION

TOTAL FLOW, 1997–2001

TOTAL FLOW, SCENARIO 1

TOTAL FLOW, SCENARIO 2

WASTEWATER, 1997–2001

WASTEWATER, SCENARIO 2

FLOW, IN CUBIC FEET PER SECOND

Hop Brook

Cold Harbor and Howard Brooks

North Brook

Stirrup Brook

Nashoba Brook

Spencer Brook

Elizabeth Brook

Taylor Brook

Fort Meadow Brook

Danforth Brook

1

2

3

4

5

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

STREAM MILE

80

60

40

20

0

0

70

50

30

10

Hop Brook

Cold Harbor and Howard Brooks

North Brook

Stirrup Brook

Nashoba Brook

Spencer Brook

Elizabeth Brook

Taylor Brook

Fort Meadow Brook

Danforth Brook

A. ANNUAL

B. SEPTEMBER

1

2

3

4

5

Figure 8. Model-calculated nonstorm streamflow in the Assabet River and 

the component of flow that originated as wastewater for existing conditions (1997–2001) and for two hypothetical scenarios. Scenario 1 represents 

predevelopment conditions, in which flows caused by people’s activities are 

eliminated. Scenario 2 represents conditions in which water withdrawals 

and wastewater discharges are increased to the maximum currently permitted levels. Average annual and average low-flow (September) conditions 

are shown. Numbers indicate the locations of municipal (1-4) and

nonmunicipal wastewater (5) discharges to the river. 

withdrawals, infiltration to sewers, and other flows and on 

the response time of streamflow to these changes. Generally, changes were smaller fractions of total model-calculated 

streamflow during March, when overall flows were high, and 

changes were larger fractions of total streamflow during September, during the low-flow period. In March, increases in tributary 

streamflows ranged from less than 1 to about 20 percent. In 

September, increases ranged from 4 to more than 100 percent.

Along the main stem Assabet River downstream of the Headwaters subbasin (and downstream of the wastewater-treatment 

facility near the basin headwaters), model-calculated annual 

streamflows were slightly less (2 to 3 percent) in S1 than under 

For more information about this or other USGS investigations in Massachusetts, contact the USGS Massachusetts-Rhode Island Water Science Center 

1-508-490-5000 or visit our Web site at http://ma.water.usgs.gov/.

By Leslie DeSimone. Graphic design by Christine Mendelsohn.

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