U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011−5179

Prepared in cooperation with the Massachusetts Department of Environmental Protection

Monitoring to Assess Progress toward Meeting the 

Assabet River, Massachusetts, Phosphorus Total Maximum 

Daily Load—Aquatic Macrophyte Biomass and SedimentPhosphorus Flux
December, a weekday,

no one else crossing

 (by way of the wet path)

the bird sanctuary’s yellow

spongy bottomland,

 no duckweed

any longer willow-green—

for now, the almost smoldering

 gas-lacy water says,

it’s down making turions.

The way to be introduced to it

 is first

to meet nothing. In rain

a thin microscope-specimen rain. 

 One raises a face

to flooded sketchlike

territories of trees,

 sepia, seeping;

to blunt, upward bluffs of ivy,

bared poison oak;

 a soaking place,

fed by springs and floods,

shallow water table

 strained by willows.

In spring, in a more forward month,

yellow-red willow-bud husks

 will sharpen the trail,

their old pen tips,

oleo-spot gulls’ beaks,

 brighten the flat brown pond,

and a man with a knife,

whack, whack,

 righthanded down the path,

will kill new twigs too new

yet to be woody.

 But there’s

no duckweed until the summer

when finally where a creek

 swims in,

 there’s duckweed

barely tugging

the moss-strandy bottom,

 wheatcolored

seed-shrimps

touring in and around

Waiting for Lesser Duckweed: On a Proposal of Issa’s

© Sandra McPherson, with permission of the poet

Published with permission of Wesleyan University Press 
Monitoring to Assess Progress toward 

Meeting the Assabet River, Massachusetts, 

Phosphorus Total Maximum Daily 

Load—Aquatic Macrophyte Biomass and 

Sediment-Phosphorus Flux

By Marc J. Zimmerman,1 Yu Qian,2 and Tian Yong Q.3

Prepared in cooperation with the 

Massachusetts Department of Environmental Protection

Scientific Investigations Report 2011–5179

U.S. Department of the Interior

U.S. Geological Survey

1

U.S. Geological Survey, Massachusetts-Rhode Island Water Science Center, Northborough, Massachusetts.

2

University of Massachusetts, Amherst, Massachusetts.

3

Central Michigan University, Mt. Pleasant, Michigan.
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/

pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Zimmerman, M.J., Yu Qian, and Tian Yong Q., 2011, Monitoring to assess progress toward meeting the Assabet River, 

Massachusetts, phosphorus total maximum daily load—Aquatic macrophyte biomass and sedimentphosphorus flux: U.S. Geological Survey Scientific Investigations Report 2011–5179, 77 p. (Also available at 

http://pubs.usgs.gov/sir/2011/5179/.)
iii

Acknowledgments

Many of our colleagues at the Massachusetts Department of Environmental Protection/Division 

of Watershed Management provided enthusiastic support, assistance, and valuable advice 

for this study; they include Dennis R. Dunn, the late Arthur Screpetis, Brian Friedmann, Alice 

Rojko, Kimberly Groff, and Richard Chase. Suzanne Flint, Staff Scientist of the Organization for 

the Assabet River, provided us with full access to the organization’s large historical database 

and collaborated extensively in the studies of biomass and distribution. Roger and Jeanne Gay 

of Maynard, Massachusetts, Christopher and Maureen Gay of Hudson, Mass., and Stephen 

Angelini of Stow, Mass., provided access to their backyards and boat docks in support of field 

work. Mark Henry (Michigan Department of Environmental Quality) helped in the selection of 

equipment for use in the sediment-phosphorus flux chamber. Christopher Pratt, project manager 

of the Veolia Water North America, (Westborough, Mass.) wastewater-treatment plant, allowed 

us access to the river on the plant site for the collection of water samples. 

The project further benefited greatly from the assistance of U. S. Geological Survey (USGS) 

coop, summer, and volunteer students Larissa Cupp (Northeastern University), Lisa Lach 

(Wesleyan University), and Nicole Rich (Salem State College), respectively. USGS hydrologist 

Gene W. Parker routinely monitored (duckweed) distribution in the impoundments. USGS 

colleague reviewers Stewart Rounds and Nancy Simon provided extensive comments on the 

draft report that led to notable improvements to the text. Other USGS colleagues—Marcus C. 

Waldron, Gerald G. Girouard, Andrew J. Massey, Kimberly W. Campo, Jason R. Sorenson, and 

James M. Duncan—made substantial contributions to the wide variety of logistics that were 

required to successfully complete the field studies. 
THIS PAGE INTENTIONALLY LEFT BLANK
v

Contents

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

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

Purpose and Scope ..............................................................................................................................5

Previous Investigations in the Assabet River Basin .......................................................................5

Part 1: Floating Macrophytes ......................................................................................................................6

Visual Observations of the Distribution of Lemna ...........................................................................6

Multispectral and Hyperspectral Analysis of Lemna Distribution ................................................6

Methods Used to Obtain and Compare Hyperspectral and Multispectral Data ...............7

Application of Remote-Sensing Results to Other Assabet River Impoundments .............7

Estimates of Lemna Biomass Based on Visual Observations .......................................................9

Limitations of Different Methods for Assessing Aquatic-Plant Distribution and Biomass ....19

Part 2: Phosphorus Fluxes From Assabet River Sediments .................................................................19

Previous Assabet River Sediment-Flux Studies ............................................................................20

Flux-Chamber Design and Testing ...................................................................................................20

Chamber Design .........................................................................................................................21

Sampling Methods .....................................................................................................................21

Chamber Testing ........................................................................................................................21

Proof-of-Method Laboratory Test ..................................................................................21

Field Tests ...........................................................................................................................24

Field Test 1 .................................................................................................................24

Field Test 2 .................................................................................................................24

Field Test 3 .................................................................................................................26

Results of Sampling in Free-Flowing and Impounded Reaches .................................................27

Total Phosphorus and Orthophosphorus Concentrations in Free-Flowing Reaches .....28

Total Phosphorus and Orthophosphorus Flux Rates from Sediment in Free-Flowing 

Reaches .........................................................................................................................29

Total Phosphorus and Orthophosphorus Concentrations in Impounded Reaches .........29

Total Phosphorus and Orthophosphorus Flux Rates from Sediment in Impounded 

Reaches .........................................................................................................................41

Comparison of Sediment-Phosphorus Flux Rates between Free-Flowing and 

Impounded Reaches ....................................................................................................41

Comparison of Results of USGS Studies with Results of ENSR International and CDM 

Laboratory Studies ................................................................................................................41

Summary and Conclusions .........................................................................................................................45

Aquatic Macrophytes.........................................................................................................................45

Sediment-Phosphorus Flux ...............................................................................................................45

References Cited..........................................................................................................................................46

Figures 3–9....................................................................................................................................................49
vi

Figures

1. Map showing locations of study-area dams, sampling sites, and 

wastewater-treatment plant discharge points in the Assabet River Basin 

in central Massachusetts............................................................................................................3

2. Photograph showing aquatic macrophytes in 2006 near the Ben Smith Dam ..................4

3. Orthophotomaps showing grid structure used for identifying Lemna distribution in

Assabet River impoundments: A, Allen Street impoundment; B, Hudson

impoundment; C, Gleasondale impoundment; D, Ben Smith impoundment; and 

E, Powdermill impoundment .....................................................................................................50

4. Representative photographs showing differences in Lemna coverage on

comparable dates (August 27, 2007, and August 28, 2008) at sites where Lemna

visual observations were made during this study: A, Allen Street impoundment;

B, Hudson impoundment; C, a cove in the Hudson impoundment; D, Gleasondale

impoundment; E, Ben Smith impoundment near the dam; F, Ben Smith impoundment

downstream from the White Pond Road bridge; G, Ben Smith impoundment

upstream from the White Pond Road bridge; and H, Powdermill impoundment .............55

5. Maps showing percent coverages of Lemna in the Allen Street impoundment

based on visual observations on comparable dates in 2007 and 2008 ..............................58

6. Maps showing percent coverages of Lemna in the Hudson impoundment near Park

Street and the Route 85 dam based on visual observations on comparable dates in

2007 and 2008...............................................................................................................................62

7. Maps showing percent coverages of Lemna in the Gleasondale impoundment

based on visual observations on comparable dates in 2007 and 2008 ..............................66

8. Maps showing percent coverages of Lemna in the Ben Smith impoundment

upstream and downstream from White Pond Road and near the dam based on

visual observations on comparable dates in 2007 and 2008................................................70

9. Maps showing percent coverages of Lemna in the Powdermill impoundment based

on visual observations on comparable dates in 2007 and 2008 ..........................................74

10. Maps showing analyses of estimated aquatic-plant distributions in the Ben Smith

impoundment on August 16, 2007 based on A, IKONOS imagery; B, visual

field-survey observations; C, remote-sensing results classified according to the

visual-survey coverage categories; and D, discrepancies between remote-sensing

and visual surveys for each coverage category .....................................................................8

11. Graphs showing range of Lemna biomass estimates for the Allen Street

impoundment in 2007 and 2008 based on the Massachusetts Department of

Environmental Protection areal-coverage categories.........................................................11

12. Graphs showing range of Lemna biomass estimates for the Hudson impoundment

near Park Street in 2007 and 2008 based on the Massachusetts Department of

Environmental Protection areal-coverage categories.........................................................12

13. Graphs showing range of Lemna biomass estimates for the Hudson impoundment

near the Route 85 dam in 2007 and 2008 based on the Massachusetts Department

of Environmental Protection areal-coverage categories ....................................................13

14. Graphs showing range of Lemna biomass estimates for the Gleasondale

impoundment in 2007 and 2008 based on the Massachusetts Department of

Environmental Protection areal-coverage categories.........................................................14

15. Graphs showing range of Lemna biomass estimates for the Ben Smith

impoundment upstream from the White Pond Road bridge in 2007 and 2008 based

on the Massachusetts Department of Environmental Protection areal-coverage

categories ....................................................................................................................................15
vii

16. Graphs showing range of Lemna biomass estimates for the Ben Smith

impoundment downstream from the White Pond Road bridge in 2007 and 2008

based on the Massachusetts Department of Environmental Protection

areal-coverage categories .......................................................................................................16

17. Graphs showing range of Lemna biomass estimates for the Ben Smith

impoundment near the dam in 2007 and 2008 based on the Massachusetts

Department of Environmental Protection areal-coverage categories ..............................17

18. Graphs showing range of Lemna biomass estimates for the Powdermill

impoundment in 2007 and 2008 based on the Massachusetts Department of

Environmental Protection areal-coverage categories.........................................................18

19. Photograph showing U.S. Geological Survey personnel preparing to deploy the

sediment-phosphorus flux chamber........................................................................................22

20. Photograph showing internal components of the sediment-phosphorus flux 

chamber .......................................................................................................................................23

21. Graph showing changes in specific conductance and dissolved oxygen

concentrations during laboratory testing of the sediment-phosphorus 

flux chamber ...............................................................................................................................24

22. Photograph showing U.S. Geological Survey personnel deploying the

sediment-phosphorus flux chamber for the first field test ..................................................25

23. Graph showing changes in dissolved oxygen concentration and oxidation-reduction 

potential during the first field test of the sediment-phosphorus flux chamber................26

24. Graph showing changes in dissolved oxygen concentration and oxidation-reduction 

potential during the second field test of the sediment-phosphorus flux chamber..........27

25. Graph showing changes in dissolved oxygen concentration and oxidation-reduction 

potential during the third field test of the sediment-phosphorus flux chamber...............28

26. Graphs showing phosphorus and dissolved oxygen concentrations measured in

the first round of sediment-phosphorus flux sampling in five free-flowing reaches

in the Assabet River study area in 2007: A, Westborough wastewater-discharge

point; B, School Street and C, Robin Hill Road in Northborough; D, Cox Street in

Hudson; and E and F, Elks Club in Maynard............................................................................30

27. Graphs showing phosphorus and dissolved oxygen concentrations measured in

the second round of sediment-phosphorus flux sampling in two free-flowing

reaches in the Assabet River study area in 2007: A, Westborough wastewaterdischarge point; and B, School Street, Northborough.........................................................33

28. Graphs showing phosphorus and dissolved oxygen concentrations measured in

the third round of sediment-phosphorus flux sampling in two free-flowing reaches

in the Assabet River study area in 2008: A, Westborough wastewater-discharge

point; and B, School Street, Northborough............................................................................34

29. Graphs showing phosphorus and dissolved oxygen concentrations measured in

the first round of sediment-phosphorus flux sampling in six impounded reaches 

in the Assabet River study area in 2007: A, Hudson site 17; B, Hudson site 20; 

C, Hudson site 29; D, Ben Smith site 32; E, Ben Smith site 38; and F, Ben Smith 

site 41 ...........................................................................................................................................36

30. Graphs showing phosphorus and dissolved oxygen concentrations measured in

the second round of sediment-phosphorus flux sampling in four impounded

reaches in the Assabet River study area in 2007: A, Hudson site 20; B, Hudson site

29; C, Ben Smith site 32; and D, Ben Smith site 41 ................................................................39
viii

31. Graphs showing phosphorus and dissolved oxygen concentrations measured in

the third round of sediment-phosphorus flux sampling in four impounded reaches

in the Assabet River study area in 2008: A, Hudson site 20; B, Hudson site 29; 

C, Ben Smith site 32; and D, Ben Smith site 41 .....................................................................42

Tables

1. Comparison between visual observations and satellite imagery of percentages of

Lemna coverage of surficial grids in the Allen Street, Hudson, and Gleasondale

impoundments, August 16, 2007 ................................................................................................9

2. Comparison between visual observations and satellite imagery of percentages of

Lemna coverage of surficial grids in the Allen Street, Hudson, Gleasondale, and

Ben Smith impoundments, August 16, 2007..............................................................................9

3. Biomass (dry weight) per unit area of Lemna samples collected in 2007.........................10

4. Rates of change in concentrations of total phosphorus and orthophosphorus

sampled inside flux chambers in free-flowing reaches in the study area .......................35

5. Rates of change in concentrations of total phosphorus and orthophosphorus

sampled inside flux chambers in impounded reaches in the study area..........................44

6. Comparisons among flux rates of sediment phosphorus in laboratory studies

reported by ENSR International and Camp Dresser & McKee, Inc., and 

determined in field studies by the U.S. Geological Survey..................................................45
ix

Conversion Factors, Datum, and Abbreviations

Multiply By To obtain

Length

inch (in)

inch (in)

foot (ft)

mile (mi)

yard (yd)

2.54

25.4

 0.3048

1.609

0.9144

centimeter (cm)

millimeter (mm)

meter (m)

kilometer (km)

meter (m)

Area

acre

acre

acre

acre

square foot (ft2

)

square foot (ft2

)

square inch (in2

)

square mile (mi2

)

4,047

0.4047

0.4047

0.004047

929.0

 0.09290

6.452

 2.590

square meter (m2

)

hectare (ha)

square hectometer (hm2

)

square kilometer (km2

)

square centimeter (cm2

)

square meter (m2

)

square centimeter (cm2

)

square kilometer (km2

) 

Volume

gallon (gal)

gallon (gal)

million gallons (Mgal)

ounce, fluid (fl oz.)

 0.003785

 3.785

 3,785

29.57

cubic meter (m3

)

liter (L)

cubic meter (m3

)

milliliter (mL)

Flow rate

acre-foot per day (acre-ft/d)

foot per second (ft/s)

foot per day (ft/d)

cubic foot per second (ft3

/s)

0.01427

 0.3048

0.3048

 0.02832

cubic meter per second (m3

/s)

meter per second (m/s)

meter per day (m/d)

cubic meter per second (m3

/s)

Mass

ounce, avoirdupois (oz)

pound, avoirdupois (lb)

pound per day per square foot 

[(lb/d)ft2

]

28.35

0.4536

4,887,000

gram (g)

kilogram (kg)

milligram per day per square meter 

[(mg/d)m2

]

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F–32)/1.8

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

(NAVD 88).

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

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

or micrograms per liter (μg/L).

Oxidation-reduction potential is given in millivolts.
x

Abbreviations

CDM Camp Dresser & McKee, Inc.

DO dissolved oxygen

MassDEP Massachusetts Department of Environmental Protection

NDVI Normalized Difference Vegetative Index

ORP oxidation-reduction potential

OrthoP orthophosphorus

P phosphorus

RPD relative percent difference

SOD sediment oxygen demand

TMDL total maximum daily load

TP total phosphorus

USACE U.S. Army Corps of Engineers

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
Monitoring to Assess Progress toward Meeting the 

Assabet River, Massachusetts, Phosphorus Total 

Maximum Daily Load—Aquatic Macrophyte Biomass and 

Sediment-Phosphorus Flux

By Marc J. Zimmerman, Yu Qian, and Tian Yong Q.

Abstract

In 2004, the Total Maximum Daily Load (TMDL) for 

Total Phosphorus in the Assabet River, Massachusetts, was 

approved by the U.S. Environmental Protection Agency. 

The goal of the TMDL was to decrease the concentrations 

of the nutrient phosphorus to mitigate some of the instream 

ecological effects of eutrophication on the river; these effects 

were, for the most part, direct consequences of the excessive 

growth of aquatic macrophytes. The primary instrument 

effecting lower concentrations of phosphorus was to be strict 

control of phosphorus releases from four major wastewatertreatment plants in Westborough, Marlborough, Hudson, and 

Maynard, Massachusetts. The improvements to be achieved 

from implementing this control were lower concentrations 

of total and dissolved phosphorus in the river, a 50-percent 

reduction in aquatic-plant biomass, a 30-percent reduction in 

episodes of dissolved oxygen supersaturation, no low-flow 

dissolved oxygen concentrations less than 5.0 milligrams 

per liter, and a 90-percent reduction in sediment releases of 

phosphorus to the overlying water.

In 2007, the U.S. Geological Survey, in cooperation with 

the Massachusetts Department of Environmental Protection, 

initiated studies to evaluate conditions in the Assabet River 

prior to the upgrading of wastewater-treatment plants to 

remove more phosphorus from their effluents. The studies, 

completed in 2008, implemented a visual monitoring plan to 

evaluate the extent and biomass of the floating macrophyte 

Lemna minor (commonly known as lesser duckweed) in five 

impoundments and evaluated the potential for phosphorus flux 

from sediments in impounded and free-flowing reaches of the 

river.

Hydrologically, the two study years 2007 and 2008 were 

quite different. In 2007, summer streamflows, although low, 

were higher than average, and in 2008, the flows were generally higher than in 2007. Visually, the effects of these streamflow differences on the distribution of Lemna were obvious. 

In 2007, large amounts of floating macrophytes accumulated 

behind bridge constrictions and dams; in 2008, high flows 

during the early part of the growing season carried floating 

macrophytes past bridges and over dams, minimizing accumulations. Samples of Lemna were collected and weighed to 

provide an estimate of Lemna biomass based on areal coverage during the summer growing seasons at eight sites in the 

five impoundments. Average estimated biomass during 2007 

was approximately twice the 2008 biomass in each of the areas 

monitored. In 2007, in situ hyperspectral and high-resolution, 

multispectral data from the IKONOS satellite were obtained 

to evaluate the feasibility of using remote sensing to monitor 

the extent of aquatic plant growth in Assabet River impoundments. Three vegetation indices based on light reflectance 

were used to develop metrics with which the hyperspectral and 

satellite data were compared. The results of the comparisons 

confirmed that the high-resolution satellite imagery could differentiate among the common aquatic-plant associations found 

in the impoundments. The use of satellite imagery could counterbalance emphasis on the subjective judgment of a human 

observer, and airborne hyperspectral data can provide higher 

resolution imagery than multispectral satellite data.

In 2007 and 2008, the potential for sediment flux of 

phosphorus was examined in free-flowing reaches of the 

river and in the two largest impoundments—Hudson and Ben 

Smith. These studies were undertaken to determine in situ 

flux rates prior to the implementation of the Assabet River 

Total Maximum Daily Load (TMDL) for phosphorus and to 

compare these rates with those used in the development and 

evaluation of the TMDL. Water samples collected from a 

chamber placed on the river bottom were analyzed for total 

phosphorus and orthophosphorus. Ambient dissolved oxygen 

concentrations and seasonal temperature differences appeared 

to affect the rates of sequestration and sediment release of 

phosphorus. When dissolved oxygen concentrations remained 

relatively high in the chambers and when the temperature was 

relatively low, the tendency was for phosphorus concentrations 

to decrease in the chambers, indicating sediment sequestration 

of phosphorus; when dissolved oxygen concentrations dropped 

to near zero and temperatures were warmest, phosphorus concentrations increased in the chambers, indicating phosphorus 
2 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

flux from the sediment. The rates of release and sequestration 

in the in situ studies were generally comparable with the rates 

determined in laboratory studies of Assabet River sediment 

cores for State and Federal agencies. Sediment-core and chamber studies produced substantial sediment fluxes to the water 

column only under extremely low-DO or anaerobic conditions 

rarely found in the Assabet River impoundments; thus, sediment is not likely to be a major phosphorus source, especially 

when compared to the wastewater effluent, which sustains 

higher ambient concentrations. The regulatory agencies now 

(2011) have substantial laboratory and field data with which 

to determine the required 90-percent reduction in phosphorus 

flux after the completion of upgrades to the wastewater-treatment plants that discharge to the Assabet River.

Introduction

The Assabet River Basin is a rapidly developing area of 

Massachusetts where water and wastewater demands strain 

water resources. The 53-kilometer (km)-long river, draining 

a basin approximately 460 kilometers (km2

) in area, passes 

through nine towns before reaching its confluence with the 

Sudbury River in Concord to form the Concord River (fig. 1). 

Along the way, nine dams affect the river’s flow and morphology. The main stem of the river from Westborough to 

Maynard, Massachusetts, including five of its impoundments 

formed by old mill dams (Allen Street in Northborough, 

Hudson in Hudson, Gleasondale in Stow, Ben Smith in 

Maynard, and Powdermill in Acton), constitute the study area 

discussed here.

In spite of its diversity of wildlife, popularity for 

recreational activities, and designation, in part, as a Wild 

and Scenic River, the Assabet is a highly eutrophic system 

largely because of nutrient enrichment from wastewatertreatment-plant effluent that may dominate up to 90 percent 

of the streamflow during low-flow periods (Massachusetts 

Department of Environmental Protection, 2004). In 

accordance with Massachusetts water-quality standards, 

the Assabet is designated a Class B water body, defined as 

a water body capable of supporting aquatic wildlife habitat 

and primary- and secondary-contact recreation; however, the 

Assabet has been categorized as an impaired water body since 

these classifications were first applied in 1996. Excessive 

nutrients, organic enrichment, and low dissolved oxygen 

concentrations have been cited as the primary causes of this 

classification. The extent of eutrophication is most clearly 

evident in the impounded reaches, where algal mats and 

rooted and floating macrophytes proliferate under low-flow, 

high-nutrient-concentration conditions during the growing 

season (fig. 2). The dams that impound these reaches once 

served to power the sawmills, gristmills, and other industrial 

operations that constituted the economic bases for the towns 

along the river. In the early twentieth century, wool-scouring, 

tannery, and shoe-factory waste, along with inadequately 

treated sewage, contributed to contamination of the Assabet 

(McAdow, 1990). Although those industries are long gone, 

pressure from new demands for regional development and 

growth continues to threaten the river’s water quality.

Although industrial wastes are no longer directly 

discharged to the river without treatment, the Assabet 

nevertheless carries a substantial waste load. This waste load 

comprises the effluents from the four major wastewatertreatment plants in Westborough, Marlborough, Hudson, and 

Maynard, Mass. (fig. 1).

In 2005, the final documentation that served as the basis 

for the Assabet River Phosphorus Total Maximum Daily 

Load (TMDL) was published (ENSR International, 2005). 

A TMDL defines the maximum loads of a particular waterquality constituent that a water body may receive without 

violating State-defined water-quality standards or designated 

uses. The report described the eutrophic condition of the river 

resulting from the excessive amounts of nutrients that led to 

the river’s inclusion, as an impaired water body requiring 

water-quality improvements, on the Integrated List of Waters 

(formerly known as the 303(d) list of impaired waters) by 

the Massachusetts Department of Environmental Protection 

(MassDEP). The listed impairments, in addition to the 

eutrophic state of the river, included aesthetic impairment 

and clogging of river reaches with nuisance growths of 

macrophytes. Phosphorus was designated as the nutrient of 

concern and was the focus of the required TMDL plan for the 

Assabet River. 

To determine means of controlling phosphorus (P) loads 

in the Assabet and developing numerical targets that could 

be monitored, ENSR International calibrated, validated, 

and applied a Hydrologic-Simulation Program—Fortran 

(HSPF) model of the basin (ENSR International, 2005). 

TMDL modeling studies of the Assabet River documented 

the extent of eutrophication of the river’s impoundments and 

free-flowing reaches and suggested that phosphorus releases 

from bed sediments may continue to promote excessive 

plant growth in the river, even following elimination of loads 

from wastewater-treatment plants and other point sources. 

Consequently, the report suggested additional measures 

that might be necessary to attain water-quality standards, 

including removal of the bed sediments, chemical treatment 

to immobilize sediment phosphorus, and dam removal or 

breaching. Selection of the most cost-effective course of action 

would require more detailed information on the distribution, 

transport, and internal cycling of phosphorus between 

sediments and surface water. This information could then be 

used to judge if annual fluxes of phosphorus from sediment 

in the river are of sufficient magnitude to require dredging, or 

if dam removal or breaching could be used to achieve waterquality and other environmental goals.

Based largely on the model simulations, the TMDL plan 

to regulate nutrients in the river was approved by the U.S. 

Environmental Protection Agency (USEPA) (Massachusetts 

Department of Environmental Protection, 2004). Full 

implementation of the Assabet River Phosphorus TMDL 
Introduction 3

71°36'

42°18'

42°24'

42°30'

71°24'

42°36'

EXPLANATION

EXPLANATION

MASSACHUSETTS

42°00'

42°30'

73°00' 72°00'

71°00'

70°00'

41°30'

CAPE COD 

BAY

ATLANTIC

OCEAN

NANTUCKET

SOUND

Basin boundaries

HUDSON BERLIN

NORTHBOROUGH

WESTBOROUGH

BOLTON

STOW

HARVARD

LITTLETON

CLINTON

SHREWSBURY

GRAFTON

MARLBOROUGH

MAYNARD

SUDBURY

CONCORD

ACTON

BOXBOROUGH

WESTFORD

CARLISLE

STUDY BASIN

BOYLSTON

Assabet

River

Assabet River

Basin boundary

Town boundary

Wastewater-discharge point

Dam

Sampling sites used in this study

Map no. Site name and location

1 Westborough wastewater-discharge point,

 Westborough

2 School Street, Northborough

3 Robin Hill Road, Marlborough

4 Hudson site 17, Hudson

5 Hudson site 20, Hudson

6 Hudson site 29, Hudson

7 Cox Street, Hudson

8 Ben Smith site 32, Stow

9 Ben Smith site 38, Stow

10 Ben Smith site 41, Maynard

11 Maynard Elks Club 1 and 2, Maynard

1

2

3

4, 5, 6

7

8 9

10

11

0 5

0 5 MILES

KILOMETERS

From USGS and MassGIS data sources, Massachusetts State Plane

Coordinate System, Mainland Zone. 

Powdermill

Ben Smith

Gleasondale

Hudson

Allen Street 

0 50 MILES

0 50 KILOMETERS

Figure 1. Locations of study-area dams, sampling sites, and wastewater-treatment plant discharge points in the Assabet 

River Basin in central Massachusetts.
4 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

Figure 2. Aquatic macrophytes in 2006 near the Ben Smith Dam (beyond the orange warning barrels). Lemna minor are the palegreen floating plants.

was originally planned for April 2009, but was delayed by 

upgrades to the wastewater-treatment plants. The TMDL 

called for the four wastewater-treatment plants to release 

effluent with total phosphorus (TP) concentrations less than 

or equal to 0.1 milligram per liter (mg/L) based on a 60-day 

rolling average from April through October and for the 

monthly average of TP in wastewater discharge to be less than 

or equal to 1.0 mg/L, and to report only dissolved phosphorus 

concentrations from November through March. These terms 

of the TMDL were based on implicit assumptions that, from 

November through March, all phosphorus discharged from 

the wastewater-treatment plants would be in the dissolved 

form and would pass through the impoundments without 

uptake by organisms or deposition in the sediments. The 

TMDL also required directly observed or measurable waterquality improvements resulting from the upgrades to the 

wastewater-treatment plants. These improvements specifically 

included (1) a 50-percent reduction in the biomass of aquatic 

plants, (2) a 30-percent reduction in the duration of DO 

supersaturation above 125 percent, (3) maintenance of lowflow DO concentrations above 5 mg/L, (4) reduced instream 

concentrations of total and dissolved phosphorus, and (5) a 

90-percent decrease in the rate of phosphorus release from 

sediments.

Restoration of the Assabet River to the standard 

designated “fishable and swimmable,” commensurate with 

part of it having been designated as a Wild and Scenic River, 

is an ambitious goal. The river’s highly eutrophic state and 

its current use in carrying wastewater discharges would 

make such improvements costly. Assessing the effectiveness 

of the TMDL after completion of the wastewater-treatment 

plant upgrades would require knowledge of preexisting 
Introduction 5

conditions. To that end, the U.S. Geological Survey (USGS) 

Massachusetts-Rhode Island Water Science Center, in 

cooperation with the MassDEP, initiated a monitoring program 

in 2007–08 to determine (1) the extent and density of the 

floating macrophyte duckweed (Lemna minor) occurrence in 

the Assabet River and (2) the status of phosphorus (sediment 

flux of phosphorus and phosphorus loads) in Assabet River 

waters and sediments prior to full implementation of the 

TMDL. 

The duckweed studies provide data based on two different approaches that can be used to estimate duckweed extent 

and biomass. These results support straightforward approaches 

to evaluating the effects of improvements in wastewater treatment on aquatic plant growth throughout the State and region.

Phosphorus monitoring yields valuable information 

about the role of sediment flux in the phosphorus budgets of 

impoundments that are strongly affected by nutrients in wastewater effluents. The results of these phosphorus studies provide baseline information for future assessment of the effectiveness of the wastewater-treatment plant TMDL upgrades in 

improving water quality in the Assabet. The study’s data can 

be used to verify assumptions used in the modeling studies 

during the development of the TMDL. 

Purpose and Scope

This report focuses on the Lemna monitoring program 

developed by the USGS, and subsequently taken over by the 

MassDEP, and the potential rates of phosphorus release from 

sediments. The USGS determined the areal extent and biomass 

of Lemna in eight selected areas of five Assabet impoundments 

in 2007–08 before wastewater discharges were upgraded 

to conform to the phosphorus TMDL. The report discusses 

the two basic approaches used to accomplish this objective: 

visual observation combined with manual collection of 

plants, and satellite-based remote sensing. In addition, the 

report describes a series of in situ experiments to determine 

the potential sediment-phosphorus flux rates from freeflowing and impounded reaches of the Assabet River. These 

experiments allowed the comparison of flux rates in the two 

types of reaches and with rates reported in studies by others. 

Comparison of these results could support straightforward 

approaches to evaluating the effects of improvements in 

wastewater treatment on aquatic plant growth throughout the 

State and region.

This study was done concurrently with other studies 

(Camp Dresser & McKee, Inc., 2007) designed to determine 

how best to manage nutrient releases from wastewater-treatment plants and from the river’s sediments. Nuisance growths 

of rooted macrophytes with nutrient requirements met by 

absorption from sediments and from the water column that 

may contribute to the failure of sections of the river to meet 

water-quality standards are noted but not evaluated in this 

report. 

Previous Investigations in the Assabet River 

Basin

DeSimone (2004) used the MODFLOW program to 

investigate the effects of water withdrawals and wastewater 

discharges on the Assabet River’s water resources; the report 

described the loss of aquatic habitat and degradation of water 

quality resulting from increased water-supply withdrawals 

from groundwater sources and increased wastewater 

discharges. Carlson and others (2008) simulated the effects of 

water demands estimated for the year 2030 under a series of 

land-use and water-supply conditions; the estimated effects 

of future development were predicted to cause increases in 

both main-stem and tributary flows as a result of increased 

discharges and increased recharge from septic systems, 

respectively. Parker and others (2004) found that the high 

proportion of tolerant generalist fish species among the fish 

communities in the Assabet River Basin indicated a degraded 

aquatic ecosystem. 

Zimmerman and Sorenson (2005) reported the results 

of sediment sampling in six Assabet impoundments and of 

their investigation of phosphorus dynamics in the Hudson 

impoundment. Sediment cores from a predetermined number 

of randomly selected sites in each impoundment were analyzed for bulk chemical properties, including TP, trace metals, 

volatile organic compounds, organochlorine pesticides, polycyclic aromatic compounds, polychlorinated biphenyls, and 

extractable petroleum hydrocarbons. TP concentrations in bulk 

sediment samples ranged from less than 0.01 (undetectable) to 

0.86 percent. The phosphorus-dynamics study did not reveal 

any net change in the mass of phosphorus in the water column 

from upstream to downstream in the Hudson impoundment 

during the summer study period; moreover, no evidence of net 

sediment-phosphorus release was detected.

Nongovernmental organizations, such as the Organization 

for the Assabet River (now OARS; http://www.oars3rivers.

org) and the Sudbury-Assabet-Concord (SuAsCo) Watershed 

Community Council (http://www.sudbury-assabet-concord.

org), advocate for the protection and enhancement of the 

Assabet River Basin. OARS has long been involved in routine 

water-quality monitoring in the basin and, among other 

activities, produces annual reports documenting its findings 

(http://www.oars3rivers.org/river/waterquality/reports). The 

SuAsCo Watershed Community Council serves as an advisory 

group to the National Park Service on matters related to 

permits affecting the rivers’ resources.

To assess the potential effects of dredging sediment and 

removing dams in conjunction with upgrades as a means to 

reduce internal phosphorus loading, the U.S. Army Corps 

of Engineers (USACE) contracted with Camp Dresser & 

McKee (CDM), Inc., to collect data and run model simulations. The evaluations determined that the maximum benefits would be obtained by a combination of wastewatertreatment-plant upgrades and dam removal. Dredging alone 

was not recommended for reducing phosphorus flux from 

sediments, but could serve to limit the redistribution of 
6 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

phosphorus-containing sediments downstream following dam 

removal; in the absence of dam removal, dredged sediments 

would be replenished over time (Camp Dresser & McKee, 

Inc., 2007).

Part 1: Floating Macrophytes

Monitoring Lemna poses some difficult problems. As a 

small floating macrophyte, its distribution in an impoundment 

is subject to the effects of wind and current. Breezes cause 

Lemna to pile up along the shore or even move upstream, 

and high flows cause Lemna to float over dams and out of 

impoundments. Moreover, Lemna generally accumulates in 

shallow, off channel or backwater areas of impoundments, 

where its roots snag onto rooted aquatic macrophytes, other 

plants, and fallen tree limbs. These uncontrollable natural 

factors make it difficult to accurately measure Lemna biomass 

or the extent to which it may cover the water surface. Because 

of these difficulties and uncertainties, two approaches were 

taken to assess Lemna areal extent and distribution in the 

impoundments. 

Visual Observations of the Distribution of Lemna

The first approach required frequent visual observations to monitor the distribution of Lemna in the five Assabet 

impoundments. In each impoundment, zones where Lemna

tended to accumulate were monitored to estimate the extent 

and density of Lemna cover. Using GIS coverages, gridded 

views of the impoundments were created (fig. 3, in back of 

report) to serve as printed field maps for recording observations. A USGS observer visited each site twice weekly during 

the maximum summer growth period (June to September) in 

2007. Because streamflows during the early summer growth 

period were greater in 2008 than in 2007, Lemna did not accumulate to the same extent in 2008 (fig. 4, in back of report), 

and the monitoring frequency was reduced to one time per 

week. In general, visits took place in the early morning, when 

atmospheric conditions usually were calm and had a minimal 

effect on Lemna distribution. 

The observer stood in predetermined locations and 

entered Lemna-areal-coverage data on the grids. The areal 

coverages were estimated visually and classified into one 

of five broad categories prescribed by the MassDEP: not 

observed (0 percent), greater than 0 to 25 percent, greater than 

25 to 50 percent, greater than 50 to 75 percent, and greater 

than 75 to 100 percent. The field data were transferred to 

spreadsheets and GIS databases after the observer returned to 

the office.

Above-average flows characterized water years 2007 and 

2008; however, early in the 2008 growing season, flows were 

greater than in 2007. In many of the locations monitored in 

2008, the areal extent of Lemna was much less than in 2007 

(figs. 5–9, in back of report) because the relatively high flows 

caused Lemna to float above the algae and rooted macrophytes 

on which it would typically snag; the high flows also served 

to carry the free-floating Lemna over the dams before large 

masses could accumulate. 

Although the Lemna coverages were greater in June 2008 

than in June 2007 at the Allen Street impoundment (fig. 5, in

back of report), the coverages in the summer of 2008 soon 

decreased, and the coverage throughout the remainder of the 

summer of 2008 was less than in 2007. The differences in 

coverage were greatest in July.

In 2007, Lemna accumulated more densely along the 

edges of the Hudson impoundment than in 2008 (fig. 6, in

back of report). Periods of dense Lemna coverage were also 

longer in 2007 than in 2008. The small oval-shaped embayment (referred to as the Park Street site) had the greatest 

Lemna coverage in the impoundment for the longest time 

period in 2007; in 2008, this area was covered with Lemna for 

only a short period in late summer.

The Gleasondale impoundment has the steepest gradient 

of the five impoundments studied and tends to have the highest 

velocity flows. This condition results in the general absence of 

Lemna accumulations in the main channel in the impoundment 

(fig. 7, in back of report). During 2007, dense accumulations 

of Lemna developed along the edges and upstream reaches of 

the impoundment. Extensive, long-lasting accumulations did 

not occur in 2008.

The distribution of Lemna was monitored in three sections of the Ben Smith impoundment: upstream and downstream from the White Pond Road bridge (which was replaced 

in midsummer 2007) and near the dam (fig. 8, in back of 

report). From late July through mid-September 2007, the three 

monitored sections of the Ben Smith impoundment were covered extensively with Lemna and other aquatic macrophytes. 

A storm in mid-September 2007 flushed much of the Lemna

over the dam. In 2008, buildups of Lemna were not extensive, 

tended to occur along the margins of the impoundment, and 

did not persist.

During 2007 and 2008, construction at the hydroelectric 

facility at the downstream end of the Powdermill impoundment required a partial drawdown of the impoundment and 

routing most of the river’s flow through a bypass; the effects 

of the drawdown and rerouting on the distribution of Lemna

were not considered as part of this study. Substantial amounts 

of Lemna were not observed in the open water of the impoundment during either study year (fig. 9, in back of report); on 

occasion, for example, on June 22, 2007, August 28, 2008, and 

September 24, 2008, no Lemna were observed at all. 

Multispectral and Hyperspectral Analysis of 

Lemna Distribution

The second approach for estimating the extent and density of Lemna (and other aquatic macrophytes) coverage with 

potential application to biomass estimation involved the use 

of high-resolution, multispectral, satellite-imagery data which 
Part 1: Floating Macrophytes 7

were compared with hyperspectral, ground-truth data obtained 

with a spectroradiometer (for a complete description of this 

work, see Tian and others, 2010). This part of the study was 

intended to determine the utility of a remote-sensing approach 

and was not meant to duplicate or supersede the visual-observation approach. 

Multispectral (red, green, blue, and near-infrared wavelengths) images from the IKONOS satellite were obtained on 

dates close to those during which hyperspectral data were collected. The satellite and hyperspectral data were first compared 

to determine whether the satellite data could be used to monitor the extent of Lemna distribution in the Assabet impoundments. Second, hyperspectral data collected from sites with 

known areal coverages of Lemna and aquatic-plant associations were compared with the multispectral satellite data 

to define relations between multispectral and hyperspectral 

bands; with these data, Lemna coverages and other aquaticplant distributions were determined. Finally, the remote-sensing results were compared with the visual observations.

Hyperspectral data were obtained in situ on August 12 

and September 15, 2007. Through a contract with GeoEye, 

multispectral IKONOS satellite images were obtained that 

included the study impoundments on the Assabet River. Original plans to obtain complete imagery for three satellite passes 

over the area during the August through September 2007 

period were not realized because GeoEye could not obtain 

data of adequate quality; however, partial multispectral images 

that were adequate were obtained on August 15 and 29 and 

September 14, 2007. Routine visual observations used in the 

comparisons with the multispectral data were made on August 

16, 27, and 30 and September 13 and 17, 2007; that the visualobservation dates did not exactly match the multispectral and 

hyperspectral data-collection dates did not affect the interpretation of data because the low-flow conditions during these 

time periods ensured stable distributions of aquatic plants.

Methods Used to Obtain and Compare 

Hyperspectral and Multispectral Data

The methods for obtaining and analyzing the hyperspectral and multispectral data were first developed for application to the Ben Smith Impoundment. A spectroradiometer 

mounted on a boat was used to acquire in situ hyperspectral 

data (579 measurements) to identify six major reflectance 

categories common to the impoundments: (1) Lemna (at 

100-percent areal coverage and with three subclasses based 

on thickness of Lemna accumulation); (2) Wolffia mixed with 

Lemna (approximately 65-percent Wolffia, the world’s smallest 

known flowering plant); (3) Potamogeton, a macrophyte with 

floating leaves; (4) a mixture of Lemna and Wolffia with green 

algal mats, mainly composed of Hydrodictyon and Spirogyra; 

(5) Ceratophyllum, a submerged rooted macrophyte; and 

(6) open water with no aquatic-plant clumps. Three vegetation 

indices (Normalized Difference Vegetative Index, or NDVI; 

Near-Infrared Green Angle Index, or NGAI; and Normalized 

Water Absorption Depth, or DH) based on the hyperspectral 

reflectance data were used to quantify coverage and identify 

plant-species associations (Tian and others, 2010). Multispectral, 4-meter (m)-resolution imagery data from IKONOS were 

processed and transformed using pan sharpening, a method 

that improves relatively coarse color-image resolution by 

cogeoregistering the images with fine resolution black and 

white images, to yield 1-m-resolution pixels which were first 

classified into 10 and then into 6 groups similar to those determined from the hyperspectral data (fig. 10A). Ceratophyllum, 

a submerged macrophyte which is not associated with Lemna,

and was not a focus of the monitoring program, was not 

included in the final evaluations of Lemna distribution.

Next, the satellite data were compared with the results 

of the visual observations (fig. 10B). These comparisons 

(fig. 10C), which are based on the NDVI, reveal that there 

was good agreement between the two methods of identifying 

aquatic-plant associations in the Ben Smith impoundment. 

Overall, the agreement between the satellite and visual assessments of Lemna coverage in the Ben Smith impoundment was 

79 percent based on classification of the grids used in visual 

monitoring (Tian and others, 2010; fig. 10D).

Application of Remote-Sensing Results to Other 

Assabet River Impoundments

To expand the results of the remote-sensing study, the 

satellite images of the aquatic macrophyte associations in 

the Allen Street, Hudson, and Gleasondale impoundments 

were classified in the same manner as for the aquatic macrophytes in the Ben Smith impoundment. (The Powdermill 

impoundment was not included because it was outside of the 

satellite-image boundary.) Then, the NDVI values derived 

from the satellite imagery for the aquatic macrophytes in the 

Ben Smith impoundment were used to categorize the aquatic 

macrophytes in the other three impoundments (as shown in 

fig. 10A). Finally, the percentage of pixels representing Lemna

was determined for each grid, and the grids were categorized 

by percentage of Lemna coverage into the four classes used in 

the visual observations. 

The differences between the two methods of categorizing 

Lemna coverages can be represented with a device called 

a “confusion matrix” (Congalton, 1991) that tabulates how 

the methods differ (tables 1 and 2). For the Allen Street, 

Hudson, and Gleasondale impoundments, the grids have been 

combined into a single matrix (table 1). The values along the 

highlighted diagonal represent the numbers of grids in each 

classification for which the methods agree. For example, the 

third value in the first IKONOS results row, 0–25 percent, 

indicates that for three of the grids, the visual-observation data 

fell in the 51–75-percent range. Comparing the number of 

grids determined by the two methods to have the same percent 

cover (24) with the total number of grids (43) gives a value of 

56-percent agreement (table 1).
8 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

Grid boundary

Open water

Lemna in a thin layer

Lemna in a moderate layer

Potamogeton

Lemna with algae

Lemna in a thick layer

74

90

86

87

78

79

62

70

71

75

83

50

58

91

82

57

53

49 54

65

66 61

92

80

72

73

51

64

68

63

67

48

88

76

84

69

94

85

77

55

59

81

52

89 93

56

60

Lemna areal coverage, in percent

>0–25

>26–50

>51–75

>76–100

No visual survey

74

90

86

87

78

79

62

70

71

75

83

50

58

91

82

57

53

49 54

65

66 61

92

80

72

73

51

64

68

63

67

48

88

76

84

69

94

85

77

55

59

81

52

89 93

56

60

>0–25

>26–50

>51–75

>76–100

No visual survey

70

71

75

83

82

65

61

72

73

94

Error in level 1

Error in level 3

Error in level 4

No errors

No visual survey

Lemna areal coverage, in percent

A. IKONOS imagery B. Visual survey

C. Remote sensing D. Discrepancies

EXPLANATION EXPLANATION

EXPLANATION EXPLANATION

Grid boundary

Grid boundary

Grid boundary

250 METERS

1,000 FEET

0

0 500

250 METERS 125

1,000 FEET

0

0 500

125

250 METERS

1,000 FEET

0

0 500

250 METERS 125

1,000 FEET

0

0 500

125

Figure 10. Analyses of estimated aquatic-plant distributions in the Ben Smith impoundment on August 16, 2007 based 

on A, IKONOS imagery; B, visual field-survey observations; C, remote-sensing results classified according to the visualsurvey coverage categories; and D, discrepancies between remote-sensing and visual surveys for each coverage 

category. Like Lemna, Potamogeton is an aquatic macrophyte with floating leaves. From Tian and others, 2010. Copyright 

Wiley (2010), used by permission.
Part 1: Floating Macrophytes 9

Table 1. Comparison between visual observations and satellite imagery of percentages of Lemna coverage of surficial grids in the 

Allen Street, Hudson, and Gleasondale impoundments, August 16, 2007.

[Values along the highlighted diagonal represent the numbers of grids in each classification for which the visual-observation and satellite-imagery 

methods agree]

Ranges of percent cover shown by 

IKONOS satellite imagery

Number of grids observed visually

0 to 25 percent 26 to 50 percent 51 to 75 percent 76 to 100 percent Total number of grids

0 to 25 percent 7 7 3 1 18

26 to 50 percent 0 0 0 1 1

51 to 75 percent 0 0 0 1 1

76 to 100 percent 0 0 6 17 23

Total number of grids 7 7 9 20 43

To determine the overall agreement of the two methods, 

the corresponding values from the original Ben Smith confusion matrix (Tian and others, 2010) were added to the values 

for the three upstream impoundments to create a new confusion matrix for all four impoundments (table 2). The resulting 

rate of agreement was 68 percent.

Estimates of Lemna Biomass Based on Visual 

Observations

The Gleasondale impoundment was selected for the 

collection of Lemna to determine biomass on July 31, 2007 

because conditions on that date were favorable for the easy 

collection of a substantial number of samples from a small 

area. Lemna were collected from locations with 100-percent 

Lemna coverage by a hula hoop that encompassed an area of 

0.495 m2

 and kept Lemna from drifting away during collection. The floating hula-hoop enclosure was first placed on 

a patch of water completely covered with Lemna; second, 

the Lemna in the enclosure were harvested and separated 

from entangled algae. Two people collected a total of twenty 

samples by this method. The samples were taken to the USGS 

Water Science Center in Northborough, Mass., where they 

were drained, weighed, and then dried in an oven at 105°C 

until their weights were constant, indicating complete dryness. 

The average Lemna biomass per unit area was calculated 

to be 29.5 g/m2

, with a standard deviation of 15.8 (table 3). 

This average biomass per unit area was multiplied by the 

area of each impoundment grid and then by the maximum 

and minimum percentage for each density range (0, greater 

than 0–25 percent, greater than 25–50 percent, greater than 

50–75 percent, and greater than 75–100 percent) on each monitoring date. The resulting values gave an estimated biomass 

range and the standard deviation for a given grid on each date. 

Accounting for the sampling error through application of the 

standard deviation may add as much as 50-percent uncertainty 

to the results. The sums of the grids’ biomasses yielded the 

total biomass for each of the eight specific impoundment areas 

(figs. 11–18) studied during the growing seasons.1

 Examining 

1

Because the eight areas monitored for Lemna varied in surface area, comparisons of biomass among the areas could be misleading; that is, a small area 

covered completely with Lemna could have less biomass than another, larger 

area with 1 to 25 percent cover.

Table 2. Comparison between visual observations and satellite imagery of percentages of Lemna coverage of surficial grids in the 

Allen Street, Hudson, Gleasondale, and Ben Smith impoundments, August 16, 2007. 

[Values along the highlighted diagonal represent the numbers of grids in each classification for which the visual-observation and satellite-imagery 

methods agree]

Ranges of percent cover shown by 

IKONOS satellite imagery

Number of grids observed visually

0 to 25 percent 26 to 50 percent 51 to 75 percent 76 to 100 percent Total number of grids

0 to 25 percent 14 7 3 1 25

26 to 50 percent 5 2 0 2 9

51 to 75 percent 1 0 2 3 6

76 to 100 percent 0 0 7 43 50

Total number of grids 20 9 12 49 90
10 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

Table 3. Biomass (dry weight) per unit area of Lemna samples 

collected in 2007.

Sample number Grams per square meter

1 18.0

2 25.3

3 36.0

4 56.4

5 22.0

6 29.9

7 23.2

8 15.4

9 24.0

10 17.8

11 12.1

12 60.4

13 40.2

14 66.9

15 29.9

16 28.9

17 11.1

18 14.7

19 32.3

20 25.9

Average 29.5

Median 25.6

Standard deviation 15.8

the Lemna distribution maps (figs. 5–9, in back of report) provides an indication of which part of each biomass graph best 

represents the actual conditions on a given observation date. 

The graphs of biomass further support the overall visual observation that Lemna coverage was generally greater in 2007 than 

in 2008 (figs. 5–9, in back of report).

In the Allen Street impoundment, the average calculated 

Lemna biomass2 was approximately 200 kg at the start of the 

2007 growing season, declined to about 100 kg by midsummer, and declined further during the remainder of the summer 

(fig. 11). In 2008, the average biomass started at about 100 kg 

and declined to about 50 kg in July. With 0 to 25 percent 

Lemna cover over substantial areas of the impoundment during both years (fig. 5, in back of report), the biomass values 

were probably close to the low ends of their ranges for most of 

the two summers.

At the Park Street site of the Hudson impoundment in 

2007 and 2008, initial average biomasses were about 20 kg 

2

The term “average biomass” used here refers to the midpoint of the biomass 

range at a particular point in time in each figure.

(fig. 12). In 2007, the average biomass increased to about 

100 kg in July and eventually to about 125 kg at the end of the 

summer growing season. In 2008, the biomass for that area 

was less than 50 kg for most of the summer with an increase to 

about 75 kg in August before declining to about 20 kg by the 

end of September. By taking the Lemna cover into consideration (fig. 6, in back of report), it would appear that in midsummer 2007 the biomass was probably closer to the high end 

of its range than it was during the early and late parts of that 

summer. In 2008, the biomass was probably in the low part of 

its calculated range for most of the summer.

In 2007, in the area of the Hudson impoundment near 

the Route 85 dam, the average biomass rose to about 175 kg 

for most of the summer, with some midsummer values greater 

than that amount (fig. 13). In 2008, the average biomass 

remained about 100 kg for the entire summer. The Lemna

coverage was not dense in either 2007 or 2008, and so the 

biomasses were probably close to the low ends of their ranges 

in both years.

At the Gleasondale impoundment, average Lemna biomass increased to about 80 kg in July 2007 and stayed at that 

level through August before declining with some variability in 

September (fig. 14). In 2008, the average biomass remained at 

about 30 kg through the summer.

At the Ben Smith impoundment on the upstream side of 

the White Pond Road bridge, the average Lemna biomass rose 

relatively steadily from about 100 kg in June to about 300 kg 

at the beginning of August 2007; after mid-August, the biomass fluctuated with maxima of approximately 1,000 kg and 

minima less than 100 kg (fig. 15). The fluctuations coincided 

with the demolition and rebuilding of the bridge; during the 

construction phase, rafts and barges were moved back and 

forth, alternately allowing and preventing Lemna to float under 

the bridge or to accumulate on the upstream side of the bridge. 

In 2008, the bridge construction was complete, and the Lemna

biomass remained stable at about 100 kg.

During 2007 and 2008, average Lemna biomass on the 

downstream side of the White Pond Road bridge varied in a 

manner similar to the biomass on the upstream side (fig. 16). 

After rising slowly and steadily until mid-August 2007, the 

biomass oscillated with maxima of about 600 kg and minima 

near 20 kg. In 2008, the average Lemna biomass remained at 

about 50 kg throughout the summer.

The average Lemna biomass in the area just upstream of 

the Ben Smith dam rose gradually from about 75 kg in late 

June to 200 kg in July 2007 before a rapid increase in August 

and the onset of the same oscillations that were observed 

upstream of the White Pond Road bridge (fig. 17). In 2008, the 

biomass did not vary much from 100 kg.

At the Powdermill impoundment, there were no extensive 

accumulations of Lemna in either 2007 or 2008 (fig. 9, in back 

of report). In both years, the average Lemna biomass estimate 

was approximately 75 kg (fig. 18). The effect that the drawdown in the impoundment had on Lemna accumulation was 

not determined. 
Part 1: Floating Macrophytes 11

0

50

100

150

200

250

300

350

6/22/20076/29/2007

7/6/2007

7/13/20077/20/2007

7/27/2007

8/3/2007

8/10/2007

8/17/20078/24/2007

8/31/2007

9/7/2007

9/14/20079/21/2007

9/28/2007

Biomass, in kilograms dry weight

0

50

100

150

200

250

300

350

6/19/20086/26/2008

7/3/2008

7/10/2008

7/17/20087/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/2008

8/28/2008

9/4/2008

9/11/2008

9/18/2008

Biomass, in kilograms dry weight

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 11. Range of Lemna biomass estimates for the Allen Street impoundment in 2007 and 2008 based on the 

Massachusetts Department of Environmental Protection areal-coverage categories.
12 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/20076/29/2007

7/6/2007

7/13/20077/20/20077/27/2007

8/3/2007

8/10/20078/17/20078/24/20078/31/2007

9/7/2007

9/14/20079/21/20079/28/2007

Biomass, in kilograms dry weight Biomass, in kilograms dry weight

0

50

100

150

200

250

7/2/20087/9/2008

7/16/20087/23/2008

7/30/2008

8/6/2008

8/13/20088/20/20088/27/2008

9/3/2008

9/10/20089/17/20089/24/2008

0

50

100

150

200

250

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 12. Range of Lemna biomass estimates for the Hudson impoundment near Park Street in 2007 and 2008 based on 

the Massachusetts Department of Environmental Protection areal-coverage categories.
Part 1: Floating Macrophytes 13

6/22/20076/29/2007

7/6/2007

7/13/20077/20/20077/27/2007

8/3/2007

8/10/20078/17/20078/24/20078/31/2007

9/7/2007

9/14/20079/21/20079/28/2007

Biomass, in kilograms dry weight

6/19/20086/26/2008

7/3/2008

7/10/20087/17/2008

7/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/20088/28/2008

9/4/2008

9/11/20089/18/2008

Biomass, in kilograms dry weight

0

100

200

300

400

500

600

0

100

200

300

400

500

600

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 13. Range of Lemna biomass estimates for the Hudson impoundment near the Route 85 dam in 2007 and 2008 based 

on the Massachusetts Department of Environmental Protection areal-coverage categories.
14 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/20076/29/2007

7/6/2007

7/13/20077/20/20077/27/2007

8/3/2007

8/10/20078/17/20078/24/20078/31/2007

9/7/2007

9/14/20079/21/20079/28/2007

Biomass, in kilograms dry weight

6/19/20086/26/2008

7/3/2008

7/10/20087/17/2008

7/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/20088/28/2008

9/4/2008

9/11/20089/18/2008

Biomass, in kilograms dry weight

0

20

40

60

80

100

120

140

160

0

20

40

60

80

100

120

140

160

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 14. Range of Lemna biomass estimates for the Gleasondale impoundment in 2007 and 2008 based on the 

Massachusetts Department of Environmental Protection areal-coverage categories.
Part 1: Floating Macrophytes 15

6/22/20076/29/2007

7/6/2007

7/13/20077/20/20077/27/2007

8/3/2007

8/10/2007

8/17/2007

8/24/20078/31/2007

9/7/2007

9/14/20079/21/20079/28/2007

Biomass, in kilograms dry weight

6/19/20086/26/2008

7/3/2008

7/10/20087/17/2008

7/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/20088/28/2008

9/4/2008

9/11/20089/18/2008

Biomass, in kilograms dry weight

0

200

400

600

800

1,000

1,200

0

200

400

600

800

1,000

1,200

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 15. Range of Lemna biomass estimates for the Ben Smith impoundment upstream from the White Pond Road bridge 

in 2007 and 2008 based on the Massachusetts Department of Environmental Protection areal-coverage categories.
16 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/20076/29/2007

7/6/2007

7/13/20077/20/20077/27/2007

8/3/2007

8/10/2007

8/17/2007

8/24/20078/31/2007

9/7/2007

9/14/20079/21/20079/28/2007

Biomass, in kilograms dry weight

6/19/20086/26/2008

7/3/2008

7/10/20087/17/2008

7/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/20088/28/2008

9/4/2008

9/11/20089/18/2008

Biomass, in kilograms dry weight

0

100

200

300

400

500

600

700

0

100

200

300

400

500

600

700

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 16. Range of Lemna biomass estimates for the Ben Smith impoundment downstream from the White Pond Road 

bridge in 2007 and 2008 based on the Massachusetts Department of Environmental Protection areal-coverage categories.
Part 1: Floating Macrophytes 17

6/22/20076/29/2007

7/6/2007

7/13/20077/20/20077/27/2007

8/3/2007

8/10/2007

8/17/2007

8/24/20078/31/2007

9/7/2007

9/14/20079/21/20079/28/2007

Biomass, in kilograms dry weight

6/19/20086/26/2008

7/3/2008

7/10/20087/17/2008

7/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/20088/28/2008

9/4/2008

9/11/20089/18/2008

Biomass, in kilograms dry weight

0

200

400

600

800

1,000

1,200

1,400

0

200

400

600

800

1,000

1,200

1,400

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

Figure 17. Range of Lemna biomass estimates for the Ben Smith impoundment near the dam in 2007 and 2008 based on the 

Massachusetts Department of Environmental Protection areal-coverage categories.
18 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/20076/29/2007

7/6/2007

7/13/20077/20/2007

7/27/2007

8/3/2007

8/10/2007

8/17/20078/24/2007

8/31/2007

9/7/2007

9/14/20079/21/2007

9/28/2007

Biomass, in kilograms dry weight

6/19/20086/26/2008

7/3/2008

7/10/20087/17/20087/24/2008

7/31/2008

8/7/2008

8/14/2008

8/21/2008

8/28/2008

9/4/2008

9/11/2008

9/18/2008

Biomass, in kilograms dry weight

Estimated maximum biomass

Estimated minimum biomass

EXPLANATION

2007

2008

0

50

100

150

200

250

300

0

50

100

150

200

250

300

Figure 18. Range of Lemna biomass estimates for the Powdermill impoundment in 2007 and 2008 based on the 

Massachusetts Department of Environmental Protection areal-coverage categories.
Part 2: Phosphorus Fluxes From Assabet River Sediments 19

Limitations of Different Methods for Assessing 

Aquatic-Plant Distribution and Biomass

The degree of agreement between the classifications in 

the Allen Street, Hudson, and Gleasondale impoundments was 

affected by the visual-survey grids—particularly those near 

the riverbanks—which did not match the IKONOS images as 

well as expected. Discrepancies probably were derived from 

two known factors: geometric, georeferencing errors associated with IKONOS imagery and differences in the distributions of macrophyte associations among the impoundments. 

Narrow grids on the edges of small impoundments might have 

been particularly affected by the georeferencing error. The 

visual-survey grids were based on satellite orthophotos taken 

before our field work, and some impoundment boundaries 

had changed since the orthophotos were taken, especially the 

boundaries of the Allen Street impoundment. 

In addition to these imagery-related factors that may have 

affected the percent agreement between the two methods of 

observation, it is also possible that the actual areal coverages 

of the aquatic macrophyte associations in the three upstream 

impoundments were not the same as those in the Ben Smith 

impoundment; as a result, application of Ben Smith NDVI 

values to the data for other impoundments might have produced additional discrepancies. The reflectance values of individual species are unlikely to have changed, but their fractions 

may not have remained the same. In the absence of additional 

ground-truth data for all the impoundments, definitive 

interpretations of the results could not be made.

One of the aims of testing the use of remote sensing to 

assess aquatic-plant distribution was to compare the results 

with those obtained from the visual observer. The results of 

the two approaches agree well when the same grid structure is 

used to classify sections of an impoundment. A degree of subjectivity, however, is associated with the visual observations 

but not the remote sensing approach: small, closely spaced 

patches of coverage in a grid section require the observer to 

mentally distribute the plants over the entire area before classifying the percent coverage. The satellite, with 1-m resolution, can map the distribution much more accurately than a 

human can. For this reason, the remote-sensing technique 

would be expected to provide data that would give better 

estimates of total coverage and therefore biomass at any given 

time. Of course, further development of surface-based data 

on aquatic-plant biomass would improve the interpretation of 

satellite-based estimates. One disadvantage of satellite-based, 

remote-sensing imagery is the absence of any guarantee of 

acquiring timely, high-quality data. Satellites may pass over 

the particular monitoring area at the proper angle during daylight hours only every few days. Furthermore, many users of 

remote-sensing data compete for satellite time, and the scheduling priorities of vendors can cause data-acquisition opportunities to be missed. Cloud cover, particularly in the humid 

summer, can also interfere with data acquisition from satellites. A potential alternative to satellite-based remote sensing 

is airborne imagery collected from an appropriately equipped 

aircraft that can fly under cloud cover and on short notice, if 

necessary. Low-flying aircraft can provide both hyperspectral 

and multispectral imagery with submeter resolution.

Part 2: Phosphorus Fluxes From 

Assabet River Sediments

Many factors may affect the rate of phosphorus flux from 

sediments. The most frequently cited factor is an oxidized 

microzone at the sediment-water interface, where, under 

aerobic conditions, oxidized iron—as an oxyhydroxide, for 

example—may settle or coat particles and effectively block 

phosphorus flux across the sediment-water interface (Wetzel, 

2001). If the oxidized microzone remains intact, phosphorus 

flux to the water column may not occur. Thus, the sedimentoxygen demand (SOD) may control the release of phosphorus 

by depleting oxygen at the microzone and affecting redox conditions. SOD, in turn, is affected by temperature and ambient 

water velocity (Doyle and Rounds, 2003). Cooke and others 

(1977) showed that 65 to 100 percent of summer increases 

in lacustrine phosphorus concentrations could come from 

sediments. In addition, some studies have shown that internal 

loading of phosphorus from lake sediments may continue to 

sustain aquatic-plant growth after the diversion or treatment of 

wastewater (Ryding and Forsberg, 1976); moreover, the loadings could be generated under aerobic conditions, especially in 

shallow lakes that do not stratify. To further confound matters, 

Andersen (1982) determined that the presence of nitrate in 

concentrations exceeding 0.1 mg/L could inhibit the release of 

phosphorus from sediments to the anoxic hypolimnion; Tirén 

and Pettersson (1985) confirmed this observation and added 

that phosphorus flux could be enhanced after denitrification 

because of the increase in bacterial activity. 

Most of these studies were conducted in lakes, not in 

shallow, run-of-the-river impoundments, such as those that 

constitute much of the Assabet River. The Assabet impoundments exhibit only weak thermal stratification in the summer, 

and their surface waters have not been found to become anaerobic (Brian Friedmann, Environmental Engineer, MassDEP, 

oral commun., 2009; ENSR International, 2001). Even under 

summer low-flow conditions, there is enough advective water 

movement to keep the water column mixed. Moreover, nitrate 

concentrations in the river exceed the concentration reported 

to inhibit phosphorus flux from sediments.

Phosphorus flux is typically measured in sediment cores 

retrieved from the field and returned to the laboratory for 

experimentation. Although these studies simplify manipulation 

for experimental purposes, making it possible to obtain high 

vertical-resolution results from sample collection and analysis, 

the cores are disturbed on removal and may not represent conditions at the sediment-water interface. In addition, temperature differences and atmospheric-gas (oxygen) exchange in 

open-top cores in a laboratory environment may affect experimental results. Operating in situ in closed chambers set on the 
20 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

sediment surface eliminates some of these concerns, but does 

not guarantee that conditions inside the chamber will remain 

natural; flows that replenish DO may be cut off, and the sediment surface will be disturbed to some extent. Kuwabara and 

others (2000) studied fluxes of metals and nutrients from sediments into the water column of Lake Coeur d’Alene in Idaho. 

Their study compared changes in benthic fluxes of orthophosphorus (orthoP) between incubated core samples and samples 

collected in situ. The in situ fluxes were substantially less than 

those from the incubated cores.

Previous Assabet River Sediment-Flux Studies

The USGS studied phosphorus fluxes to and from the 

sediments in the Assabet River periodically from July 2007 

to June 2008 before the full implementation of the TMDL. 

Because the studies on which the TMDL was based (ENSR 

International, 2001) hypothesized that sediment served as a 

source of the phosphorus and because the TMDL, based on 

model simulations (ENSR International, 2005), called for a 

90-percent decrease in the amount of phosphorus entering the 

water column from the sediment, it was important to determine the potential magnitude of the sediment-phosphorus 

contribution and the effects of the sediment-phosphorus flux 

on the overall phosphorus status of the Assabet River. 

To monitor sediment-phosphorus flux from undisturbed 

sediments for the purpose of comparing these fluxes with 

those determined in laboratories and used in developing the 

TMDL, an in situ approach was chosen. Like the laboratory 

tests in which the sediment cores were manipulated to create 

anoxic conditions that would maximize phosphorus flux, 

these in situ studies were designed to generate anoxia and to 

measure the associated flux of phosphorus. If the improvements in phosphorus treatment at the wastewater-treatment 

plants prove successful, as predicted by modeling studies 

(ENSR International, 2005; Camp Dresser & McKee, Inc., 

2007), then the potential phosphorus flux to be measured 

after the upgrades are fully implemented should decrease 

commensurately.

Overall, 11 sampling locations were selected to include 

sites most likely to release phosphorus (areas with low DO 

concentrations) as well as areas which traditional limnological considerations would not indicate as sediment-phosphorus sources—namely, locations characterized as having 

relatively high DO concentrations. Water samples were 

collected from six sites in two impoundments and from five 

free-flowing river reaches (fig. 1). DO, other field parameters 

(temperature, specific conductance, pH, and, initially, oxidation-reduction potential, or ORP), and phosphorus forms (TP 

and orthoP) were monitored routinely to provide data about 

the physical and chemical processes controlling phosphorus release. Most of the samples were collected during the 

summer (July and August 2007), but some samples were 

collected in the late spring (June 2008) and fall (November 

2007). Hazardous icy conditions in the winter precluded the 

collection of samples.

To isolate the potential sediment contributions of phosphorus, flux chambers were manually placed on the sediment 

surface. Water samples were withdrawn from the chambers 

by using methods slightly modified from Zimmerman and 

others (2005) and from pore water below the sediment 

surface under the chambers for periods ranging from a few 

hours to a few days. Phosphorus concentrations in samples 

collected from the surrounding surface water were compared 

with the potential magnitude of the sediment-phosphorus 

contribution. TP and orthoP were analyzed from all samples 

collected. 

ENSR International (2001) collected bottom-sediment 

samples from the Assabet River and studied the rates of 

phosphorus release or sequestration. ENSR International 

(2005) also used computer simulations to predict the decreases 

in phosphorus fluxes that would be required for the river 

to achieve the water quality associated with its designated 

use. For conditions preceding wastewater-treatment-plant 

upgrades, the simulations used 12.0 mg phosphorus per 

square meter per day (mg P/m2

/day) for orthoP fluxes from the 

upstream impoundments and 21.6 mg P/m2

/day for fluxes from 

the downstream impoundments, regardless of specific local 

characteristics such as depth, sediment characteristics, stream 

velocity, water chemistry, dissolved oxygen concentrations, 

and temperature. The Gleasondale Dam in Stow was designated the dividing line between the upstream and downstream 

sections of the river.

CDM, in an additional modeling study for the USACE, 

considered the effects of wastewater-treatment-plant upgrades, 

dredging, and dam removal on the reduction of phosphorus 

in the Assabet River impoundments. Of relevance to the 

present study, CDM found that wastewater-treatment-plant 

upgrades, by reducing phosphorus concentrations in effluents, 

could result in a 60-percent reduction in sediment release of 

phosphorus.

Flux-Chamber Design and Testing

The studies done to provide data for the development of 

the TMDL used sediment cores collected from the Assabet 

River and incubated in laboratories under varied temperature 

regimes (ENSR International, 2001); some of the cores were 

capped to initiate the development of anaerobic conditions. 

In the present study, flux chambers were placed on the river 

bottom, and water-quality parameters were monitored. During 

initial equipment testing, some conditions in the chambers 

were manipulated; during the study, however, to prevent or 

minimize effects on the flux rates, conditions in the chambers 

were not deliberately manipulated. Of course, deploying a 

chamber that isolated an area of sediment and its overlying 

water modifies the conditions that would affect sedimentphosphorus flux in open water.
Part 2: Phosphorus Fluxes From Assabet River Sediments 21

Chamber Design

Conceptually, the flux chamber used in this study may 

be thought of as a hybrid that combines features of an SOD 

chamber (Sonzogni and others 1977; Caldwell and Doyle, 

1995; Rounds and Doyle, 1997; and Wood, 2001) and a seepage meter (Lee, 1977). Based on the SOD-chamber design 

described by Caldwell and Doyle (1995), the instrumentation 

monitors DO depletion over time as organic matter in the sediments and chamber is oxidized. Similar to a seepage meter, 

the flux chamber provides a means for sampling water-quality 

constituents transported across the sediment-water interface 

by possible groundwater inflow and diffusion (Lee, 1977). 

Advective transport of water-quality constituents is blocked; 

however, with the design that was implemented, ambient 

concentrations were measured according to the sampling 

schedules.

The chambers used in this study consisted of a pair of 

inverted 19-L (5-gal) plastic buckets, one inside of the other, 

creating a sampling chamber inside the inner bucket and a 

space between the bottoms of the buckets (figs. 19 and 20). 

An external metal frame held the buckets together and 

provided sufficient mass to hold the chamber in place on the 

sediment surface. A toilet flap valve allowed water to displace 

air from the sampling chamber (the inner bucket) when the 

chamber was deployed; the exterior bucket was vented to 

allow the air to leave the apparatus. Two 10-foot (ft)-long 

SedPoint samplers (M.H.E. Products, East Tawas, Michigan, 

http://www.mheproducts.com/SedPoints.pdf; figs. 19 and 

20) passed through the bottoms of the buckets and into the 

sampling chamber. (The SedPoint sampler is a 0.63-cm (1⁄4-in.) 

polyethylene tube with a polypropylene screen at the tip; the 

screen’s mesh has square openings 0.0003 m (0.0117 in.) on 

a side.) When inverted, the rim of the chamber rested on the 

river bottom with the screened tip of one SedPoint sampler 

penetrating the sediment approximately 5 cm to sample 

pore-water phosphorus and the other SedPoint sampler in the 

middle of the sampling chamber to enable monitoring of phosphorus flux. A third SedPoint sampler, fastened to the chamber’s external frame, was used to sample ambient phosphorus 

concentrations. A small submersible pump, attached to the top 

of the sampling chamber and powered by an external battery 

pack, circulated water in the chamber. To provide a better 

understanding of the changes taking place in the chamber near 

the sediment-water interface, a YSI 600XL Multi-Parameter 

Water Quality Sonde that was installed through the bucket 

bottoms recorded DO, temperature, specific conductance, pH, 

and, on occasion, ORP data.

Sampling Methods

Samples were collected by attaching a peristaltic pump to 

the three SedPoint samplers. After purging with approximately 

1 L of ambient water, 125-mL samples were collected for 

analysis of TP and orthoP. The TP sample was acidified immediately with 1 mL 0.45 N sulfuric acid. The orthoP sample 

was filtered through a 0.45-micron syringe filter before being 

collected in the sample bottle. All samples were immediately 

placed on ice before refrigerator storage and shipping to the 

USGS National Water Quality Laboratory for analysis. All 

chemical water-quality data are accessible through the USGS 

National Water Information System (http://waterdata.usgs.gov/

ma/nwis/nwis).

In addition to the environmental phosphorus samples, 

quality-control equipment-blank and sequential duplicate 

samples were collected. In accordance with the Quality 

Assurance Project Plan prepared for the MassDEP, qualitycontrol samples composed about 10 percent of the total 

environmental samples collected. TP and orthoP blank-sample 

concentrations were all less than the minimum reporting 

levels of 0.008 and 0.006 mg/L, respectively. The median 

relative percent differences (RPD) between the sequential 

duplicate TP quality-control samples, the sequential duplicate 

orthoP quality-control samples, and their corresponding 

environmental samples were 7.5 percent and 6.2 percent, 

respectively; 19 pairs of these quality-control samples were 

analyzed. RPD values for TP ranged from 0.5 to 158 percent 

and, for orthoP, from 0.0 to 155 percent. If the two highest 

values for both TP and orthoP are dropped, the TP median 

declines to 7.0, and the orthoP median declines slightly to 

6.15 percent. The range of the RPD for TP becomes 0.5 to 

50.8 percent and, for orthoP, 0.0 to 56.5 percent. Five of the 

TP RPDs and three of the orthoP RPDs exceeded the QAPP 

maximum of 10 percent. 

Chamber Testing

Prior to deploying the flux chambers, a series of laboratory and field tests were run to evaluate the functionality of the 

chambers. The multiparameter sonde was used to monitor the 

chamber’s operation during these tests. 

Proof-of-Method Laboratory Test

The first experiment used a tub of water to which top soil 

had been added to simulate sediment. When the top soil had 

settled, one of the flux chambers was submerged in the tub, 

and DO and specific conductance were monitored for more 

than 7 hours (h) (fig. 21). After approximately 5 h, when the 

DO concentration was just above zero, about 20 L of deionized water was pumped into the chamber through one of the 

SedPoint tubes over a period of 30 minutes. The specific 

conductance dropped, and the DO concentration increased. 

After another hour, the DO concentrations started to fall 

rapidly, and the specific conductance started to increase. These 

results demonstrated that, even within a relatively short time 

period, the decay of organic matter inside the chamber could 

deplete the DO and create conditions favorable for the release 

of dissolved ionic material, such as orthoP, from sediments.
22 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

Sampling chamber

SedPoint sampling tubes

Multiparameter sonde

Figure 19. U.S. Geological Survey personnel preparing to deploy the sediment-phosphorus flux chamber.
Part 2: Phosphorus Fluxes From Assabet River Sediments 23

Circulation pump

Flap valve

Multiparameter 

sonde

SedPoint for

sampling water

inside chamber

SedPoint for sampling

pore water

Figure 20. Internal components of the sediment-phosphorus flux chamber.
24 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

0 100 200 300 400

Elapsed time, minutes

0

400

800

1,200

1,600

Specific conductance, in microsiemens per centimeter

0.0

0.5

1.0

1.5

2.0

2.5

Dissolved oxygen, in milligrams per liter

500

3.0

Figure 21. Changes in specific conductance and dissolved oxygen concentrations during laboratory testing of the 

sediment-phosphorus flux chamber. 

Field Tests

Following successful completion of the laboratory testing, the chambers were taken to field locations for further 

testing. Field procedures were similar to those used in the 

laboratory.

Field Test 1

The first field-test site was next to a private dock on the 

Hudson impoundment (fig. 22). In this test, the DO concentration and ORP were monitored for about 2 h. During this 

period, the DO concentration decreased from 6.6 mg/L to 

about 1 to 1.5 mg/L. ORP decreased from about -30 to -90 mV 

(fig. 23). The water temperature in the chamber declined from 

20.25 to 19.28 oC. The DO and ORP results indicate conditions approaching anoxia—a state favorable for reduction 

reactions and sediment flux of phosphorus.

Field Test 2

The second test was conducted at the same Hudson 

impoundment site. This test lasted for slightly more than 3 

h. Unlike the first field test, ORP remained at about its initial 

level between –30 and –40 mV until the last reading, when 

it dropped to about –85 mV (fig. 24). The DO concentration 

started to decline from the outset, increased slightly after 

about 45 minutes, and was nearly zero at the end of the test. 

The water temperature in the chamber declined slightly—from 

17.11 to 17.05 oC. The cause of the ORP stability is uncertain, 

but it could have been a problem with the probe that corrected 

itself near the end of the test when the ORP dropped rapidly. 

The brief increase in the DO concentration may have been 

caused by movement of the chamber, but that also is uncertain.
Part 2: Phosphorus Fluxes From Assabet River Sediments 25

Figure 22. U.S. Geological Survey personnel deploying the sediment-phosphorus flux chamber for the first field test.
26 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

11:45 12:00 12:15 12:30 12:45 1:00 1:15 1:30

Eastern Daylight Time

1

2

3

4

5

6

Dissolved oxgygen, in milligrams per liter

-100

-80

-60

-40

-20

Oxidation-reduction potential, in millivolts

TEST 1

0

11:30 1:45

7

Figure 23. Changes in dissolved oxygen concentration and oxidation-reduction potential during the first field test of the sedimentphosphorus flux chamber. 

Field Test 3

In this test that lasted about 5 h and was conducted at a 

private dock on the Ben Smith impoundment, DO and ORP 

were monitored continuously, and water in the chamber was 

flushed out with deionized water after about 4 h (fig. 25). The 

DO concentration decreased from 7 mg/L to less than 1 mg/L 

in about 3.5 h, after which deionized water was pumped in, 

and the DO concentration returned to about 7 mg/L before 

decreasing to less than 2 mg/L at the end of the experiment. 

The decline in the initial DO concentration accelerated after 

the chamber’s position on the sediment was adjusted slightly, 

possibly preventing infiltration of oxygenated ambient water, 

after about 0.5 h. The ORP varied greatly, dropping from 

approximately 100 mV to less than 20 mV in about 0.5 h 

and then rising to greater than 80 mV before falling again to 

almost 0 mV. Prior to the addition of the deionized water, the 

water temperature inside the chamber declined from 21.74 

to 19.54oC. When the deionized water was pumped into the 

chamber, the ORP rose to 100 mV but was starting to decline 

when the experiment was terminated. Like the DO concentration, the ORP decreased relatively rapidly after the chamber’s 

placement was adjusted; the reason for its subsequent increase 

after about 1 h is not understood. 

The field and laboratory tests satisfactorily demonstrated 

the utility of the flux chamber for monitoring phosphorus flux 

from Assabet River sediments; moreover, the additional information obtained from the continuous sonde measurements 

supplied ancillary data useful in interpreting the phosphorusflux data.
Part 2: Phosphorus Fluxes From Assabet River Sediments 27

11:00 11:15 11:30 11:45 12:00 12:15 12:30 12:45 1:00 1:15 1:30 1:45 2:00

Eastern Daylight Time

0

1

2

3

4

5

Dissolved oxgygen, in milligrams per liter

Oxidation-reduction potential, in millivolts

-90

-80

-70

-60

-50

-40

-30

TEST 2

10:45 2:15

Figure 24. Changes in dissolved oxygen concentration and oxidation-reduction potential during the second field test of the sedimentphosphorus flux chamber.

Results of Sampling in Free-Flowing and 

Impounded Reaches

Water-quality samples were collected on a routine basis 

according to the duration of the field work in July, August, 

and November 2007, and in June 2008. At first, two or three 

sites were visited on a circuit three times during the course 

of a single day. Sample collection required driving either a 

car to wadeable sites (on free-flowing reaches) or a boat on 

an impoundment (impounded reaches). After the first round 

of sampling, the chambers were left unattended and samples 

were collected daily Monday through Friday. In addition to TP 

and orthoP samples, temperature, specific conductance, DO, 

pH, and occasionally ORP data were collected.

Sediment-phosphorus-flux rates were calculated using 

changes in concentrations and the physical characteristics of 

the chamber. The concentration values were multiplied by 

the water volume in the chamber to give the total mass of 

phosphorus in the chamber; these results were divided by the 

sediment-surface area and the time between samples to yield a 

flux rate.

The sediment-phosphorus-flux data from the National 

Water Quality Laboratory indicated that initial phosphorus 

concentrations were consistently high—an artifact caused 

by the initial disturbance of the sediment associated with 

emplacement of the flux chamber (see figs. 26–31); similar 

results have been reported elsewhere in association with initial 

rates of SOD (Doyle and Lynch, 2005). For this reason, the 

phosphorus-flux-rate data reported here do not include the 

changes between the first and second sample collections at 

each site during each round of sampling. Two other calculated 

fluxes were excluded because of evidence that the flux chamber had been disturbed.
28 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

11:30 12:00 12:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30

Eastern Daylight Time

0

2

4

6

0

20

40

60

80

100

Oxidation-reduction potential, in millivolts

TEST 3

Dissolved oxgygen, in milligrams per liter

8 120

Figure 25. Changes in dissolved oxygen concentration and oxidation-reduction potential during the third field test of the 

sediment-phosphorus flux chamber.

Total Phosphorus and Orthophosphorus 

Concentrations in Free-Flowing Reaches

The first round of sampling in the free-flowing reaches of 

the Assabet River in July and August 2007 took place at five 

sites (fig. 1): one each in Westborough (at the Westborough 

wastewater-treatment plant, upstream from the effluent 

discharge), Northborough (at School Street, the first study 

site downstream from the Westborough wastewater-treatment 

plant), Marlborough (at Robin Hill Road, near a public 

access point downstream from the Marlborough Westerly 

wastewater-treatment plant), Hudson (at Cox Street, upstream 

from the Hudson wastewater-treatment plant ), and at two sites 

in Maynard (both sites near the Maynard Elks Club, between 

the Ben Smith dam and the Maynard wastewater-treatment 

plant). Upon further evaluation, for reasons of security, 

accessibility, and safety, the number of sites in free-flowing 

reaches during the second and third rounds of sampling in 

November 2007 and June 2008 was reduced to the two sites in 

Westborough and Northborough.

In sediment pore water where SOD creates reducing 

conditions, more phosphorus would be expected to be in 

the dissolved form than in the water column. This dissolved 

phosphorus and any phosphorus associated with ferric oxyhydroxides would serve as a dissolved-phosphorus reservoir that 

could be released into the water column if anoxic conditions 

develop and could break down the oxidized microzone at the 

sediment-water interface. (Although the existence of iron-rich, 

oxidized microzones in the Assabet River impoundments was 

not investigated here, Zimmerman and Sorenson (2005) found 

that surficial-sediment samples in the impoundments generally had iron concentrations higher than 1 percent by mass. 

In all likelihood, a substantial portion of this iron would have 

been in the oxidized form.) Surficial sandy sediments with low 

organic content would be less likely to become anoxic than 

organic-rich sediments, and aerated water could circulate in 

the hyporheic zone and inhibit phosphorus flux.
Part 2: Phosphorus Fluxes From Assabet River Sediments 29

In general, the initial round of short-period studies 

showed that the highest concentrations of TP and orthoP at 

each sampling site were usually measured in samples collected 

from pore water, and that the pore-water TP concentrations 

generally decreased in the downstream direction (figs. 26A–

F). Ambient concentrations (in the water column, outside the 

chamber) were generally the lowest. 

Inside the chambers, DO concentrations declined from 

initial conditions at all sites, but the final concentrations did 

not drop below 2.5 mg/L at any of the sites. The short duration 

of these studies and concomitant maintenance of relatively 

high DO concentrations probably combined to suppress phosphorus flux into the chambers. The free-flowing condition may 

also have enhanced transport of DO through the hyporheic 

zone and into the chambers.

The patterns in the data from the second round of sampling in November 2007 (fig. 27) were similar to those from 

the first round. The highest TP concentrations were detected 

in the pore water, and the other concentrations changed little. 

In general, the DO dropped—to about 1.5 mg/L upstream 

from the Westborough wastewater-discharge point (fig. 27A), 

but only to about 3.5 mg/L at the downstream School Street 

site (fig. 27B; the increase in DO concentration at the School 

Street site was a result of the chamber falling over between the 

November 6 and 7 sample collections. After the chamber was 

righted, the DO resumed its decrease in concentration).

During the final round of sampling from June 6–12, 

2008, concentrations of orthophosphorus in the pore water 

and inside the chamber upstream from the Westborough 

wastewater-discharge point increased after anoxic conditions 

developed (fig. 28A). At School Street during this period, DO 

concentrations declined but did not reach near-zero levels, 

and no consistent increases in concentrations were observed 

(fig. 28B). During chamber deployment, it was noted that the 

sediment quality at the sampling location upstream from the 

Westborough wastewater-discharge point had changed since 

the November 2007 assessments were made: more organic 

matter covered the surface. Decay of this organic matter 

isolated in the chamber likely created an increased SOD that 

depleted DO. Reduction of iron in oxyhydroxides at the sediment surface could also have contributed to DO depletion.

Total Phosphorus and Orthophosphorus Flux 

Rates from Sediment in Free-Flowing Reaches

For each chamber deployment, sediment-phosphorus 

flux rates for TP and orthoP were calculated. The difference 

in phosphorus concentration in the chamber between two consecutive samples in milligrams per liter was multiplied 

by the chamber’s volume in liters and divided by the chamber’s area in square meters to yield the change in phosphorus 

mass per square meter. This value was divided by the time 

between samplings in days to yield the flux rate in milligrams 

of phosphorus per square meter per day.

At the free-flowing sites, flux rates did not demonstrate 

any particular temporal or seasonal pattern (table 4). Overall, 

flux rates for both TP and orthoP were highest in June 2008 

and were associated with the site upstream from the Westborough wastewater-discharge point and the downstream site at 

School Street in Northborough. These rates were substantially 

higher than those determined in July and November 2007 at 

the same sites. In the period between the November 2007 

and June 2008 sample collections, the surficial sediment quality was observed to have changed from primarily sandy to primarily organic. This change in substrate could account for an 

increased rate of oxygen consumption and phosphorus release. 

Additional factors likely affected the relatively high values in 

June 2008 at the sampling site upstream from the wastewaterdischarge point and at the School Street sampling site: first, 

the duration of the June 2008 study was longer than the previous studies, allowing more time for DO depletion; second, 

the water temperatures in June 2008 were slightly higher than 

they were during the studies in July and August 2007 and 

substantially higher than those in November 2007, possibly 

affecting reaction rates. A substantial proportion of the net TP 

flux rates during the first sampling round were negative, indicating that TP was lost from the surface water in the chamber 

to the sediment. Net losses of TP at some sites—in Maynard, 

in particular—may be associated with settling of particles, a 

sandy substrate, an absence of the sufficiently low DO conditions needed to initiate sediment release, microbial uptake, or 

sorption. The relatively short duration of these initial studies in 

July 2007 (several hours) probably matched the settling time 

for the TP particles. Because it is in solution, orthoP would 

not be subject to settling unless it became bound to particulate 

material. As a result, relatively few negative orthoP flux values 

appear in the results.

Total Phosphorus and Orthophosphorus 

Concentrations in Impounded Reaches

Several sites in the two largest impoundments, Hudson 

and Ben Smith, were selected for investigation (fig. 29). 

During the first round of sediment-phosphorus flux sampling, 

three sites were monitored in each impoundment. In following 

rounds, only two sites were sampled in each impoundment. 

During the first round of sampling at the Hudson 

impoundment, from July 25 to 26, 2007, samples were collected twice each day; at the Ben Smith impoundment, from 

August 21 to 23, 2007, samples were collected once each 

day for three days. At the Hudson impoundment, DO concentrations in the two chambers at sites 203 and 29 (figs. 29B, 

C) declined to about 0 mg/L, whereas at the third (site 17; 

fig. 29A), the DO concentration dropped to about 4.5 mg/L 

3

Site numbers and their locations in the Hudson and Ben Smith impoundments 

are the same as previously used in Zimmerman and Sorenson (2005); the Hudson impoundment site numbers were those used in the phosphorus dynamics part of the study, and the Ben Smith site numbers were used to identify 

sediment-sampling sites.
30 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

B. School Street 07/12/07

3.5

4.0

4.5

0

0.1

0.2

0.3

0.4

0.5

0.6

9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 6:00

Eastern Standard Time

A. Westborough wastewater-discharge point 07/12/07

0

1

2

3

4

5

6

7

0

0.1

0.2

0.3

0.4

0.5

0.6

9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 6:00

Figure 26. Phosphorus and dissolved oxygen concentrations measured in the first round of sediment-phosphorus flux sampling 

in five free-flowing reaches in the Assabet River study area in 2007: A, Westborough wastewater-discharge point; B, School 

Street and C, Robin Hill Road in Northborough; D, Cox Street in Hudson; and E and F, Elks Club in Maynard. At the School Street 

site, pore-water orthoP concentrations exceeded TP concentrations in one sample, possibly because an alternate analytical 

method with a higher analytical range was used (Kathleen M. Bryant, U.S. Geological Survey, written commun., 2010).
Part 2: Phosphorus Fluxes From Assabet River Sediments 31

C. Robin Hill Road 07/12/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

D. Cox Street 08/16/07

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

6.9

7.0

7.1

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

12:00 1:00 2:00 3:00 4:00 5:00 6:00

Eastern Standard Time

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 6:00

Figure 26.—Continued.
32 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts Phosphorus, in milligrams per liter as P Dissolved oxygen, in milligrams per liter

E. Elks Club 08/16/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

Eastern Standard Time

F. Elks Club 2 08/16/07

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

0.02

0.04

0.06

0.08

0.10

0.12

9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 6:00

0

1

2

3

4

5

6

7

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 6:00

Figure 26.—Continued.
Part 2: Phosphorus Fluxes From Assabet River Sediments 33

A. Westborough wastewater-discharge point 11/06/07–11/10/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

B. School Street 11/06/07–11/10/07

0

0.1

0.2

0.3

0.4

0.5

0.6

11/6/07 11/7/07 11/8/07 11/9/07 11/10/07

Phosphorus, in milligrams per liter as P

0

1

2

3

4

5

6

7

8

9

Dissolved oxygen, in milligrams per liter

0

0.5

1.0

1.5

2.0

2.5

3.0

11/6/07 11/7/07 11/8/07 11/9/07 11/10/07

Date

Phosphorus, in milligrams per liter as P

0

1

2

3

4

5

6

7

8

Dissolved oxygen, in milligrams per liter

Figure 27. Phosphorus and dissolved oxygen concentrations measured in the second round of sediment-phosphorus flux sampling 

in two free-flowing reaches in the Assabet River study area in 2007: A, Westborough wastewater-discharge point; and B, School 

Street, Northborough.
34 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

6/9/08 6/10/08 6/11/08 6/12/08 6/13/08 6/14/08

Phosphorus, in milligrams per liter as P

0

1

2

3

4

5

6

7

8

9

Dissolved oxygen, in milligrams per liter

Date

B. School Street 06/09/08–06/13/08

A. Westborough wastewater-discharge point 06/09/2008–06/12/2008

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

6/9/08 6/10/08 6/11/08 6/12/08 6/13/08 6/14/08

Phosphorus, in milligrams per liter as P

0

1

2

3

4

5

6

7

8

9

Dissolved oxygen, in milligrams per liter

Figure 28. Phosphorus and dissolved oxygen concentrations measured in the third round of sediment-phosphorus flux 

sampling in two free-flowing reaches in the Assabet River study area in 2008: A, Westborough wastewater-discharge point; 

and B, School Street, Northborough. At the School Street site, pore-water orthoP concentrations exceeded TP concentrations 

in several samples, possibly because an alternate analytical method with a higher analytical range was used (Kathleen M. 

Bryant, U.S. Geological Survey, written commun., 2010).
Part 2: Phosphorus Fluxes From Assabet River Sediments 35

Table 4. Rates of change in concentrations of total phosphorus and orthophosphorus sampled inside flux chambers in free-flowing 

reaches in the study area. 

[Values are derived from differences in phosphorus concentrations between consecutive samples. A negative flux rate indicates a decrease in phosphorus 

concentration. The shaded area indicates low dissolved oxygen concentrations in the chamber. WDP, near wastewater discharge point]

Sampling site Sampling period

(month/day/year)

Flux rate (milligrams of phosphorus per square meter per day)

Total phosphorus Orthophosphorus

First sampling round

Westborough WDP, Westborough

School Street, Northborough

Robin Hill Road, Marlborough

Cox Street, Hudson

Cox Street, Hudson

Elks Club #1, Maynard

Elks Club #1, Maynard

Elks Club #2, Maynard

Elks Club #2, Maynard

07/12/2007

07/12/2007

07/12/2007

08/16/2007

08/16/2007

08/16/2007

08/16/2007

08/16/2007

08/16/2007

1.98

–24.0

0

3.97

4.96

–2.72

–15.7

–2.42

–5.45

–1.98

–19.6

–7.90

1.98

4.96

0

0

2.42

5.45

Second sampling round

Westborough WDP, Westborough

Westborough WDP, Westborough

School Street, Northborough

11/7–8/2007

11/8–9/2007

11/8–9/2007

–1.59

0.211

1.8

–0.227

0.211

–1.13

Third sampling round

Westborough WDP, Westborough

Westborough WDP, Westborough

Westborough WDP, Westborough

School Street, Northborough

School Street, Northborough

School Street, Northborough

06/10–11/2008

06/11–12/2008

06/12–13/2008

06/10–11/2008

06/11–12/2008

06/12–13/2008

8.16

10.2

6.38

–4.62

14.2

7.62

4.88

8.66

6.07

–9.830

18.0

25.1

on July 25 and did not change thereafter. It is possible that the 

sediment at site 17 consisted primarily of sand, which would 

have allowed aerated surface water to circulate into the bottom 

of the chamber. Another less likely possibility is that an undetected submerged object wedged under the chamber may have 

allowed ambient water to enter and would thus also account 

for the stable DO concentrations and for the relatively small 

changes in TP and orthoP concentrations inside the chamber. 

In contrast, at Hudson sites 20 and 29 and at the three sites in 

the Ben Smith impoundment (figs. 29D–F), DO concentrations dropped to nearly 0 mg/L during the sampling periods. 

After near-anaerobic conditions were reached, increases were 

detected in TP and orthoP in the chambers; when DO concentrations were closer to 0, TP and orthoP concentrations 

occasionally exceeded the concentrations in the pore water. 

In general, and particularly when DO concentrations were 

not close to 0, TP and orthoP concentrations in the pore water 

were greater than in the chamber or in the ambient water. 

When the first samples were collected during the second (late-fall) sampling round at the Hudson and Ben Smith 

impoundments, initial DO concentrations were generally 

higher than during other parts of the study’s field work 

(fig. 30); water temperatures were lower in the late fall than 

at other times, and this condition increases the saturation 

concentration. The two relatively high initial DO concentrations (greater than 12 mg/L) measured during summer 

sampling in 2007 were probably a result of supersaturation 

caused by photosynthesis. DO concentrations inside the 

chambers fell to near zero during the sampling period at all 

of the sites (figs. 30A–C), with the exception of Ben Smith 

site 41 (fig. 30D);it is possible that the chamber at site 41 

did not deploy properly, allowing DO to enter the chamber. 

At Hudson site 29 (fig. 30B), the DO concentration dropped 
36 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts Phosphorus, in milligrams per liter as P Dissolved oxygen, in milligrams per liter

A. Hudson site 17 07/25/07–07/27/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

B. Hudson site 20 07/25/07–07/26/07

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

7/25/2007 7/26/2007 7/27/2007

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

7/25/07 7/26/07 7/27/07

Figure 29. Phosphorus and dissolved oxygen concentrations measured in the first round of sediment-phosphorus flux 

sampling in six impounded reaches in the Assabet River study area in 2007: A, Hudson site 17; B, Hudson site 20; C, Hudson 

site 29; D, Ben Smith site 32; E, Ben Smith site 38; and F, Ben Smith site 41.
Part 2: Phosphorus Fluxes From Assabet River Sediments 37

Figure 29.—Continued. Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

C. Hudson site 29 07/25/07–07/26/07

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

1

2

3

4

5

6

7

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

D. Ben Smith site 32 08/21/07–08/23/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

0

2

4

6

8

10

12

0

0.1

0.2

0.3

0.4

0.5

0.6

7/25 7/26 7/27

8/21/07 8/22/07 8/23/07 8/24/07
38 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

0

0.1

0.2

0.3

0.4

0.5

0.6

0

2

4

6

8

10

12

14

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

E. Ben Smith site 38 08/21/07–08/23/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0

1

2

3

4

5

6

7

8

9

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

F. Ben Smith site 41 08/21/07–08/23/07

8/21/07 8/22/07 8/23/07 8/24/07

Date

8/21/07 8/22/07 8/23/07 8/24/07

Figure 29.—Continued.
Part 2: Phosphorus Fluxes From Assabet River Sediments 39

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

A. Hudson site 20 11/13/07–11/17/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

11/13/07 11/14/07 11/15/07 11/16/07 11/17/07

0

2

4

6

8

10

12

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

B. Hudson site 29 11/13/07–11/17/07

0

0.2

0.4

0.6

0.8

1.0

1.2

11/13/07 11/14/07 11/15/07 11/16/07 11/17/07

0

2

4

6

8

10

12

Figure 30. Phosphorus and dissolved oxygen concentrations measured in the second round of sediment-phosphorus flux sampling in 

four impounded reaches in the Assabet River study area in 2007: A, Hudson site 20; B, Hudson site 29; C, Ben Smith site 32; and 

D, Ben Smith site 41. 
40 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

11/26/07 11/27/07 11/28/07 11/29/07 11/30/07

0

2

4

6

8

10

12

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

C. Ben Smith site 32 11/26/07–11/30/07

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

D. Ben Smith site 41 11/26/07–11/30/07

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

11/26/07 11/27/07 11/28/07 11/29/07 11/30/07

0

2

4

6

8

10

12

14

16

Figure 30.—Continued.
Part 2: Phosphorus Fluxes From Assabet River Sediments 41

at first, and then rose briefly on November 14 before falling 

again; this fluctuation may have been caused by a physical 

disturbance during the sampling process. During the sampling period, no systematic increases were observed in any of 

the TP or orthoP concentrations in the pore water, inside the 

chamber, or in the ambient water. Water temperatures were no 

greater than 5.4°C in November, whereas they exceeded 20°C 

for the other samples. These results indicate that, in addition 

to the relation to DO concentrations, there is a temperature 

component to sediment-phosphorus flux in the Assabet River 

system that probably has diffusion-rate and biological-process 

components.

During the final round of sampling in June 2008, samples 

from the pore water in the Hudson impoundment could be 

retrieved only at site 20 (fig. 31A). The clayey sediments at 

the other sites in the Hudson and Ben Smith impoundments 

clogged the screens, preventing water from entering the SedPoints. That pore water could be collected on previous sampling dates demonstrates the areal heterogeneity of the river 

sediments, even among samples collected no more than a few 

meters apart. At all sites, except Hudson site 29 (fig. 31B), DO 

concentrations dropped to less than 1 mg/L (figs. 31A, C, and 

D). In spite of DO concentrations higher than 0 at Hudson site 

29 (fig. 31B) and Ben Smith site 32 (fig. 31C), TP and orthoP 

generally increased over time after the DO minimum was 

reached at each site. The same sonde was used at Hudson site 

29 and Ben Smith site 32, and it may have had an undiagnosed 

problem that could have affected DO readings. The results of 

previous studies at these two sites indicate that the sonde DO 

data were incorrect, and that the DO concentrations were actually close to 0 mg/L.

Total Phosphorus and Orthophosphorus Flux 

Rates from Sediment in Impounded Reaches

Rates of phosphorus release were, for the most part, 

greater in the warm months of July and August 2007 and June 

2008 than in November 2007 (table 5). Warm temperatures 

would be more conducive to increased SOD that would drive 

down the DO concentrations, facilitate the disruption of the 

sediment-water interface by reducing ferric oxyhydroxides, 

and allow phosphorus to diffuse from the sediment pore 

water into the chamber. In July 2007, the highest TP flux rate, 

514 milligrams per square meter per day (mg/m2

/d), and the 

highest orthoP flux rate, 396 mg/m2

/d, were determined from 

data for sites 20 and 29, respectively, in the Hudson impoundment. The highest sequestration rate for TP, –8.65 mg/m2

/d, 

was measured at Hudson site 17 in July 2007, and the highest 

sequestration rate for orthoP, –18.8, mg/m2

/d, was measured at 

Hudson site 29 in November 2007 (table 5).

Comparison of Sediment-Phosphorus Flux Rates 

between Free-Flowing and Impounded Reaches

In general, the flux rates from sediments in free-flowing 

reaches of the river were smaller than the flux rates from 

impounded reaches (tables 4 and 5). Sequestration was more 

common in the free-flowing reaches than in the impounded 

reaches. Similar to rates in the impounded reaches, the rates 

in the free-flowing reaches were smallest during the second 

round of sampling in November 2007, when water temperatures were lowest and DO concentrations inside the chambers 

decreased the least. Knowledge of seasonal and hydrological 

conditions may help regulators to identify the conditions that 

are likely to produce a 90-percent reduction in phosphorus flux 

rates as required by the TMDL. 

Comparison of Results of USGS Studies with 

Results of ENSR International and CDM 

Laboratory Studies

ENSR International used sediment-phosphorus flux rates 

of 21.6 and 12.0 mg P/m2

/day in downstream and upstream 

sections of the river, respectively, in its computer simulations 

(David Pincumbe, U.S. Environmental Protection Agency, 

written commun., 2009). Regardless of the fact that the same 

flux rates were respectively assigned to all reaches within the 

upstream and downstream sections, both sections include freeflowing and impounded reaches.

Taking into consideration the differences in methodologies used in the sediment-phosphorus-flux measurements, 

the flux rates used by ENSR International were reasonably 

comparable to the largest release rates determined in this study 

for free-flowing reaches (table 6); with the exception of the 

rates determined in the second (late fall) sampling round of 

this study, the ENSR International sediment-phosphorus flux 

rates were substantially less than the rates determined for the 

impounded reaches in this study. On the other hand, the rates 

used by ENSR International in the modeling study (ENSR 

International, 2005) were higher than determined in their 

laboratory studies. 

The sediment-phosphorus release rates determined by 

CDM in its December 2006 studies of sediment-phosphorus 

flux from sediment cores taken from impounded reaches had 

wide ranges of values that, unlike the USGS results, did not 

exceed zero (table 6); the CDM-determined rates ranged from 

-156 (net sequestration rate) to 0 mg P/m2

/day (no net flux of 

P into or out of sediment). In data derived from samples from 

cores collected at the free-flowing sites, the range of CDM 

sediment-phosphorus-flux rates exceeded the entire range, 

encompassing both sequestration and release, of sediment 

phosphorus-flux rates indicated by the USGS data (table 6).
42 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

6/16/08

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

6/17/08 6/18/08 6/19/08 6/20/08 6/21/08

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

A. Hudson site 20 06/16/08–06/20/08

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

6/16/08 6/17/08 6/18/08 6/19/08 6/20/08 6/21/08

0

1

2

3

4

5

6

7

8

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

B. Hudson site 29 06/16/08–06/20/08

Figure 31. Phosphorus and dissolved oxygen concentrations measured in the third round of sedimentphosphorus flux sampling in four impounded reaches in the Assabet River study area in 2008: A, Hudson site 20; 

B, Hudson site 29; C, Ben Smith site 32; and D, Ben Smith site 41.
Part 2: Phosphorus Fluxes From Assabet River Sediments 43

0

0.2

0.4

0.6

0.8

1.0

1.2

6/23/08 6/24/08 6/25/08 6/26/08 6/27/08 6/28/08

0

1

2

3

4

5

6

7

8

Phosphorus, in milligrams per liter as P

Dissolved oxygen, in milligrams per liter

C. Ben Smith site 32 06/23/08–06/27/08

EXPLANATION

Total phosphorus in pore water

Orthophosphorus in pore water

Total phosphorus inside chamber

Orthophosphorus inside chamber

Total phosphorus outside chamber

Orthophosphorus outside chamber

Dissolved oxygen concentration

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

6/23/08 6/24/08 6/25/08 6/26/08 6/27/08 6/28/08

0

1

2

3

4

5

6

7

8

Phosphorus, in milligrams per liter as P

Date

Dissolved oxygen, in milligrams per liter

D. Ben Smith site 41 06/23/08–06/27/08

Figure 31.—Continued.
44 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

Table 5. Rates of change in concentrations of total phosphorus and orthophosphorus sampled inside flux chambers in impounded 

reaches in the study area. 

[Values are derived from differences in phosphorus concentrations between consecutive samples. A negative flux rate indicates a decrease in phosphorus concentration. The shaded areas indicate low dissolved oxygen concentrations in the chamber]

Sampling site Sampling period

(month/day/year)

Flux rate (milligrams of phosphorus per square meter per day)

Total phosphorus Orthophosphorus

First sampling round

Hudson Site 17 07/25–26/2007 –8.65 –3.46

Hudson Site 20 07/25–26/2007 514 167

Hudson Site 29 07/25–26/2007 253 396

Ben Smith 32 08/22–23/2007 48.2 59.6

Ben Smith 38 08/22–23/2007 21.9 10.4

Ben Smith 41 08/22–23/2007 25.6 13.6

Second sampling round

Hudson Site 20 11/14–15/2007 –4.97 –4.44

Hudson Site 20 11/15–16/2007 –0.383 –0.383

Hudson Site 29 11/14–15/2007 3.40 12.80

Hudson Site 29 11/15–16/2007 15.2 –18.8

Ben Smith 32 11/27–28/2007 6.16 8.69

Ben Smith 32 11/28–29/2007 1.03 –10.1

Ben Smith 41 11/27–28/2007 3.49 2.13

Ben Smith 41 11/28–29/2007 3.26 0.502

Third sampling round

Hudson Site 20 06/17–18/2008 122 148

Hudson Site 20 06/18–19/2008 274 82.30

Hudson Site 20 06/19–20/2008 30.4 29.4

Hudson Site 29 06/17–18/2008 27.3 33.3

Hudson Site 29 06/18–19/2008 62.2 27.9

Ben Smith 32 6/24–25/2008 81.6 85.2

Ben Smith 32 6/25–26/2008 34.4 23.3

Ben Smith 32 6/26–27/2008 46.0 30.2

Ben Smith 41 6/24–25/2008 20.9 5.58

Ben Smith 41 6/25–26/2008 42.3 63.0

Ben Smith 41 6/26–27/2008 37.6 44.0
Summary and Conclusions 45

Table 6. Comparisons among flux rates of sediment phosphorus 

in laboratory studies reported by ENSR International and Camp 

Dresser & McKee, Inc., and determined in field studies by the U.S. 

Geological Survey. 

[All values are in units of milligrams per square meter per day; negative 

values indicate flux into the sediment. USGS, U.S. Geological Survey; CDM, 

Camp Dresser & McKee, Inc.]

ENSR USGS

Maximum, total phosphorus 13.3 514

Minimum, total phosphorus –1.7 –24.0

Maximum, orthophosphorus 8.3 396

Minimum, orthophosphorus –2.1 –19.6

CDM USGS

Impounded sites, total phosphorus –156 to 0 –8.65 to 514

Riverine (free-flowing) sites, total 

phosphorus –87 to 35 –24.0 to 14.2

Attempts to apply the data out of context could result 

in conflicting modeling results. Given that the samples were 

collected at different times and locations, that the experimental 

systems were manipulated in a variety of different ways, that 

the results were within about two orders of magnitude, and 

that the cool-weather fluxes were generally from the water 

column to the sediment, the results of the two studies are 

generally consistent with one another. Moreover, that sediment 

fluxes to the water column were measured in the sedimentcore and chamber studies only under extremely low-DO to 

anaerobic conditions rarely, if ever, found in the Assabet 

River impoundments suggests that sediment is not likely to 

be a major phosphorus source, especially when phosphorus 

concentrations from sediments are compared to the ambient 

concentrations sustained by wastewater effluent.

Summary and Conclusions

The Assabet River Phosphorus Total Maximum Daily 

Load, approved by the U.S. Environmental Protection Agency 

in 2004, prescribed reductions in phosphorus concentrations 

for wastewater-treatment plant discharges along the Assabet 

River. The eventual results of these reductions were to include 

(1) lower concentrations of total and dissolved phosphorus 

in the river; (2) a 50-percent reduction in aquatic-plant 

biomass; (3) a 30-percent reduction in instances of dissolved 

oxygen supersaturation; (4) no low-flow dissolved oxygen 

concentrations less than 5.0 milligrams per liter; and (5) and 

a 90-percent reduction in sediment releases of phosphorus to 

the overlying water. In 2007 and 2008, the U.S. Geological 

Survey, in cooperation with the Massachusetts Department 

of Environmental Protection, undertook studies to monitor 

aquatic-plant distribution and biomass and the potential 

rates of phosphorus release from sediments in free-flowing 

and impounded reaches of the Assabet River prior to the 

reductions in phosphorus concentrations.

Aquatic Macrophytes

As a surrogate for the monitoring of all aquatic macrophytes, the U.S. Geological Survey developed and implemented a monitoring program for Lemna minor, commonly 

known as lesser duckweed. Lemna was monitored at eight 

selected locations in the five largest Assabet River impoundments, where it was known to accumulate. A trained observer 

estimated the Lemna coverage at each location on a weekly 

or biweekly basis. During the 2007 and 2008 growing seasons, hydrological conditions differed substantially; in 2007, 

although streamflows were somewhat greater than usual, 

Lemna accumulated heavily in most locations monitored; in 

2008, streamflows were even higher than in 2007, and there 

were no heavy Lemna accumulations. At most of the locations 

monitored, estimated biomass in 2007 was approximately 

twice as great as in 2008. The effects of year-to-year variability in summer streamflow on estimated biomass and aesthetic 

impairment of the impoundments underscored the importance 

of maintaining long-term records to evaluate the effects of the 

implementation of phosphorus-reduction improvements.

In addition to the visual monitoring of Lemna, the use 

of satellite-based, multispectral, remote-sensing data was 

investigated as a potential nonsubjective means for assessing the extent of aquatic-plant growth. By comparing in situ 

hyperspectral data obtained by a field spectroradiometer with 

multispectral satellite data, it was determined that the satellite 

data could be used to differentiate among the various aquaticplant associations common to the river’s impoundments. The 

1-m resolution of the satellite data affords substantially more 

accuracy than the estimates made by a land-based observer 

and, with postprocessing, can be expected to yield unbiased 

results. Furthermore, airborne hyperspectral data acquisition can more reliably provide higher-resolution imagery for 

assessing aquatic-macrophyte distribution than the satellitebased remote sensing.

Sediment-Phosphorus Flux

Two objectives of monitoring sediment-phosphorus flux 

were achieved: first, in situ measurements of flux were compared with those made by other investigators who collected 

sediment cores taken from the river and studied sedimentphosphorus flux under laboratory conditions; and second, data 

describing conditions prior to wastewater-treatment upgrades 

were assembled for comparison to data describing conditions after completion of the upgrades. Sediment-phosphorus 

flux was monitored in July, August, and November 2007, 
46 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

and in June 2008, by using sampling chambers placed on the 

river-bottom sediments in free-flowing and impounded river 

reaches. The chambers effectively isolated sediment and overlying water from their surroundings and allowed the dissolved 

oxygen inside the chambers to be consumed at rates probably 

affected by temperature and the amount of decomposable 

organic matter in the chamber. 

Analysis of sampling results showed that the highest rates 

of phosphorus flux from the sediments were determined in 

samples from the chambers in July 2007 and June 2008 in the 

Hudson impoundment. In November 2007, phosphorus concentrations, for the most part, were lower in the water column 

inside the chambers than at other sampling times. In general, 

phosphorus fluxes from the sediments were substantially 

greater in impounded reaches than in free-flowing reaches. In 

free-flowing reaches, the generally sandy quality of bottom 

sediments caused by higher water velocities probably contributed to lower sediment oxygen demand, lower phosphorus 

flux rates, and to increased phosphorus sequestration than in 

impounded reaches. 

Sediment-phosphorus flux results reported by other investigators of the Assabet River were broadly consistent with 

results of this study. All studies reported losses of phosphorus 

from the water to the sediments (sequestration) during the 

cold parts of the year, but the magnitudes of the rates varied 

considerably among the studies. The conditions required to 

generate phosphorus release from any of the laboratory or in 

situ experimental systems rarely occur in the Assabet River 

impoundments, indicating that the sediment probably does not 

serve as a major phosphorus source.

The interannual variability in hydrologic conditions and 

the general heterogeneity of the sediments in the Assabet 

River impoundments support the need for long-term assessments of water quality. Rate coefficients, such as those used in 

modeling studies, and the modeling studies themselves could 

yield realistic results if they incorporated more of the variability inherent in the system under study. Regulatory agencies 

responsible for evaluating the success of the Assabet River 

Total Maximum Daily Load for Total Phosphorus now have 

more baseline information to use in determining whether river 

sediments are likely to be a source of phosphorus to the water 

column, and whether the 90-percent reduction in sedimentphosphorus flux has been achieved. 

References Cited

Andersen, J.M., 1982, Effect of nitrate concentration in lake 

water on phosphate release from the sediment: Water 

Research, v. 16, no. 7, p. 1119–1126.

Caldwell, J.M., and Doyle, M.C., 1995, Sediment oxygen 

demand in the Lower Willamette River, Oregon, 1994: U.S. 

Geological Survey Water-Resources Investigations Report 

95–4196, 14 p.

Carlson, C.S., DeSimone, L.A., and Weiskel, P.K., 2008, 

Simulated effects of year 2030 water-use and land-use 

changes on streamflow near the Interstate-495 Corridor, 

Assabet and Upper Charles River Basins, Eastern Massachusetts: U.S. Geological Survey Scientific Investigations 

Report 2008–5132, 108 p.

Camp Dresser & McKee, Inc., 2007, Assabet River sediment 

and dam removal study, draft modeling report: Cambridge, 

Massachusetts, Camp Dresser & McKee, Inc., variously 

paged.

Congalton, R.G., 1991, A review of assessing the accuracy of 

classifications of remotely sensed data: Remote Sensing of 

Environment, v. 37, p. 35–46.

Cooke, G.D., McComas, M.R., Waller, D.W., and Kennedy, 

R.H., 1977, The occurrence of internal phosphorus loading 

in two small, eutrophic, glacial lakes in northeastern Ohio: 

Hydrobiologia, v. 56, p. 129–135.

DeSimone, L.A., 2004, Simulation of ground-water flow and 

evaluation of water-management alternatives in the Assabet 

River Basin, Eastern Massachusetts: U.S. Geological Survey Scientific Investigations Report 2004–5114, 133 p.

Doyle, M.C., and Lynch, D.D., 2005, Sediment oxygen 

demand in Lake Ewauna and the Klamath River, Oregon, 

June 2003: U.S. Geological Survey Scientific Investigations Report 2005–5228, 14 p.

Doyle, M.C., and Rounds, S.A., 2003, The effect of chamber 

mixing velocity on bias in measurement of sediment oxygen 

demand rates in the Tualatin River Basin, Oregon: U.S. 

Geological Survey Water-Resources Investigations Report 

03–4097, 16 p.

ENSR International, 2001, SuAsCo Watershed Assabet River 

TMDL Study Phase One—Assessment final report: Westford, Massachusetts, ENSR International, variously paged.

ENSR International, 2005, SuAsCo Watershed Assabet River 

TMDL Study Phase Two—Analysis final report: Westford, 

Massachusetts, ENSR International, variously paged.

Kuwabara, J.S., Berelson, W.M., Balistrieri, L.S., Woods, P.F., 

Topping, B.R., Steding, D.J., and Krabbenhoft, D.P., 2000, 

Benthic flux of metals and nutrients into the water column 

of Lake Coeur d’Alene, Idaho—Report of an August, 1999, 

pilot study: U.S. Geological Survey Water-Resources 

Investigations Report 00–4132, 74 p.

Lee, D.R., 1977, A device for measuring seepage flux in lakes 

and estuaries: Limnology and Oceanography, v. 22, no. 1, 

p. 140–147.
References Cited 47

Commonwealth of Massachusetts, 2004, Assabet River total 

maximum daily load for total phosphorus: Boston, Commonwealth of Massachusetts, Department of Environmental 

Protection, report no. MA82B–01–2004–01, 104 p.

McAdow, Ron, 1990, The Concord, Sudbury, and Assabet 

Rivers—A guide to canoeing, wildlife, and history: Marlborough, Massachusetts, Bliss Publishing Company, Inc., 

223 p.

Parker, G.W., Armstrong, D.S., and Richards, T.A., 2004, 

Comparison of methods for determining streamflow requirements for aquatic habitat protection at selected sites on the 

Assabet and Charles Rivers: U.S. Geological Survey Scientific Investigations Report 2004–5092, 72 p. 

Rounds, S.A., and Doyle, M.C., 1997, Sediment oxygen 

demand in the Tualatin River Basin, Oregon, 1992–96: 

U.S. Geological Survey Water-Resources Investigations 

Report 97–4103, 19 p.

Ryding, S-O., and Forsberg, Curt, 1976, Six polluted lakes—A 

preliminary evaluation of the treatment and recovery processes: Ambio, v. 5, no. 4, p. 151–156.

Sonzogni, W.C., Larsen, D.P., Malueg, K.W., and Schuldt, 

M.D., 1977, Use of large submerged chambers to measure 

sediment water interactions: Water Research, v. 11, 

p. 461–464.

Tian Y.Q., Yu Qian, Zimmerman, M.J., Flint, Suzanne, and 

Waldron, M.C., 2010, Differentiating aquatic plant communities in a eutrophic river using hyperspectral and multispectral remote sensing: Freshwater Biology, v. 55, no. 8, 

p. 1658–1673.

Tirén, Torbjörn, and Pettersson, Kurt, 1985, The influence of 

nitrate on the phosphorus flux to and from oxygen depleted 

lake sediments: Hydrobiologia, v. 120, no. 3, p. 207–223.

Wetzel, R.G., 2001, Limnology, lake and river ecosystems (3d 

ed.): San Diego, California, Academic Press, 1006 p.

Wood, T.M., 2001, Sediment oxygen demand in Upper Klamath and Agency Lakes, Oregon, 1999: U.S. Geological Survey Water-Resources Investigations Report 01–4080, 13 p.

Zimmerman, M.J., Massey, A.J., and Campo, K.W., 2005, 

Pushpoint sampling for defining spatial and temporal variations in contaminant concentrations in sediment pore water 

near the ground-water/surface-water interface: U.S. Geological Survey Scientific Investigations Report 2005–5036, 

70 p.

Zimmerman, M.J., and Sorenson, J.R., 2005, Sediment studies 

in the Assabet River, central Massachusetts, 2003: U.S. 

Geological Survey Scientific Investigations Report 2005–

5131, 87 p.
THIS PAGE INTENTIONALLY LEFT BLANK
Figures 3–9 49

Figures 3–9
50 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

A. Allen Street

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

71°38'04" 71°38' 71°37'56" 71°37'52" 71°37'48" 71°37'44" 71°37'40" 71°37'36"

42°19'52"

42°19'48"

42°19'44"

42°19'40"

42°19'36"

0 300 600 FEET

0 75 150 METERS

Figure 3. Grid structure used for identifying Lemna distribution in Assabet River impoundments: A, Allen Street impoundment; 

B, Hudson impoundment; C, Gleasondale impoundment; D, Ben Smith impoundment; and E, Powdermill impoundment. Red dots 

represent observation points.
Figures 3–9 51

B. Hudson

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

71°34'38" 71°34'34" 71°34'30" 71°34'26" 71°34'22" 71°34'18" 71°34'14" 71°34'10" 71°34'06"

42°23'34"

42°23'30"

42°23'26"

42°23'22"

42°23'18"

42°23'14"

0 300

0 75 150 METERS

600 FEET

Figure 3.—Continued.
52 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

C. Gleasondale

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

71°31'50" 71°31'46" 71°31'42" 71°31'38" 71°31'34" 71°31'30" 71°31'26" 71°31'22"

42°24'38"

42°24'34"

42°24'30"

42°24'26"

42°24'22"

42°24'18"

0 300

0 75 150 METERS

600 FEET

Figure 3.—Continued.
Figures 3–9 53

Figure 3.—Continued.

D. Ben Smith

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

71°28'50" 71°28'42" 71°28'34" 71°28'26" 71°28'18" 71°28'10" 71°28'02" 71°27'54" 71°27'46"

42°25'46"

42°25'42"

42°25'38"

42°25'34"

42°25'30"

42°25'26"

42°25'22"

42°25'18"

42°25'14"

42°25'10"

0 500

0 100 200 METERS

1,000 FEET
54 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

E. Powdermill

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

71°26'16" 71°26'12" 71°26'8" 71°26'4" 71°26' 71°25'56" 71°25'52" 71°25'48" 71°25'44"

42°26'34"

42°26'30"

42°26'26"

42°26'22"

42°26'18"

0 300

0 75 150 METERS

600 FEET

Figure 3.—Continued.
Figures 3–9 55

2007 2008 A Allen Street impoundment

2007 2008 B

C

Hudson impoundment

Cove in Hudson impoundment

Figure 4. Differences in Lemna coverage on comparable dates (August 27, 2007, and August 28, 2008) at sites where Lemna visual observations were 

made during this study: A, Allen Street impoundment; B, Hudson impoundment; C, a cove in the Hudson impoundment; D, Gleasondale impoundment; E, 

Ben Smith impoundment near the dam; F, Ben Smith impoundment downstream from the White Pond Road bridge; G, Ben Smith impoundment upstream 

from the White Pond Road bridge; and H, Powdermill impoundment.
56 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

2007 2008 D

E

Gleasondale impoundment

Ben Smith impoundment near dam

Figure 4.—Continued.
Figures 3–9 57

2007 2008 F

G Ben Smith impoundment upstream

from White Pond Road bridge

Ben Smith impoundment downstream

from White Pond Road bridge 

2007 Powdermill impoundment 2008 H

Figure 4.—Continued.
58 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/2007 6/19/2008

7/2/2007 7/2/2008

A 2007 2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 400 FEET

0 50 100 METERS

200

Figure 5. Percent coverages of Lemna in the Allen Street impoundment based on visual observations on comparable 

dates in 2007 and 2008. >, greater than.
Figures 3–9 59

7/16/2007 7/16/2008

7/31/2007 8/1/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

B 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 400 FEET

0 50 100 METERS

200

Figure 5.—Continued.
60 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

8/13/2007 8/13/2008

8/27/2007 8/28/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

C 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 400 FEET

0 50 100 METERS

200

Figure 5.—Continued.
Figures 3–9 61

9/10/2007 9/10/2008

9/25/2007 9/24/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

D 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 400 FEET

0 50 100 METERS

200

Figure 5.—Continued.
62 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/2007 6/19/2008

7/2/2007 7/2/2008

Route 85 dam

monitoring area

Park Street

monitoring area

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

A 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 450 900 FEET

0 100 200 METERS

Figure 6. Percent coverages of Lemna in the Hudson impoundment near Park Street and the Route 85 dam based on visual 

observations on comparable dates in 2007 and 2008. >, greater than.
Figures 3–9 63

7/16/2007 7/16/2008

7/31/2007 8/1/2008

Route 85 dam

monitoring area

Park Street

monitoring area

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

B 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 450 900 FEET

0 100 200 METERS

Figure 6.—Continued.
64 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

8/13/2007 8/13/2008

8/27/2007 8/28/2008

Route 85 dam

monitoring area

Park Street

monitoring area

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

C 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 450 900 FEET

0 100 200 METERS

Figure 6.—Continued.
Figures 3–9 65

9/10/2007 9/10/2008

9/25/2007 9/24/2008

Route 85 dam

monitoring area

Park Street

monitoring area

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

D 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0 450 900 FEET

0 100 200 METERS

Figure 6.—Continued.
66 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/2007 6/19/2008

7/2/2007 7/2/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

A 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0

25

250 FEET

0

125

50 METERS

Figure 7. Percent coverages of Lemna in the Gleasondale impoundment based on visual observations on comparable dates 

in 2007 and 2008. >, greater than.
Figures 3–9 67

7/16/2007 7/16/2008

7/31/2007 8/1/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

B 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0

25

250 FEET

0

125

50 METERS

Figure 7.—Continued.
68 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

8/13/2007 8/13/2008

8/27/2007 8/28/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

C 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0

25

250 FEET

0

125

50 METERS

Figure 7.—Continued.
Figures 3–9 69

9/10/2007 9/10/2008

9/25/2007 9/24/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

D 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 0

25

250 FEET

0

125

50 METERS

Figure 7.—Continued.
70 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/07 6/19/08

7/2/07 7/2/08

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

White Pond Road

bridge

A 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 100

0 500

200 METERS

1,000 FEET

Figure 8. Percent coverages of Lemna in the Ben Smith impoundment upstream and downstream from White Pond Road 

and near the dam based on visual observations on comparable dates in 2007 and 2008. >, greater than.
Figures 3–9 71

7/31/2007 8/1/2008

7/16/07 7/16/08

B 2007 2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

White Pond Road

bridge

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 100

0 500

200 METERS

1,000 FEET

Figure 8.—Continued.
72 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

C 2007 2008

8/27/2007 8/28/2008

8/13/2007 8/13/2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

White Pond Road

bridge

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 100

0 500

200 METERS

1,000 FEET

Figure 8.—Continued.
Figures 3–9 73

9/25/2008

9/10/2007 9/10/2008

9/25/2007 9/24/2008

White Pond Road

bridge

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

D 2007 2008

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 100

0 500

200 METERS

1,000 FEET

Figure 8.—Continued.
74 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

6/22/2007 6/19/2008

7/2/2007 7/2/2008

A 2007 2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 50 METERS

0 125 250 FEET

25

Figure 9. Percent coverages of Lemna in the Powdermill impoundment based on visual observations on comparable dates 

in 2007 and 2008. >, greater than.
Figures 3–9 75

7/16/2007 7/16/2008

7/31/2007 8/1/2008

B 2007 2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 50 METERS

0 125 250 FEET

25

Figure 9.—Continued.
76 Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts

8/13/2007 8/13/2008

8/27/2007 8/28/2008

C 2007 2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 50 METERS

0 125 250 FEET

25

Figure 9.—Continued.
Figures 3–9 77

9/10/2007 9/10/2008

9/25/2007 9/24/2008

D 2007 2008

>75 to 100

>50 to 75

>25 to 50

>0 to 25

Not observed

EXPLANATION

Lemna coverage, 

in percent

Base orthophoto from MassGIS, Commonwealth of Massachusetts, Information Technology Division 

0 50 METERS

0 125 250 FEET

25

Figure 9.—Continued.
Prepared by the Pembroke and Sacramento Publishing 

Service Centers.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Massachusetts-Rhode Island Water Science Center

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at:

http://ma.water.usgs.gov
Zimmerman and others—Monitoring Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux, Assabet River, Massachusetts—SIR 2011−5179

Printed on recycled paper