Prepared in cooperation with the Massachusetts Department of Environmental Protection

Monitoring to Assess Progress toward Meeting the Total 

Maximum Daily Load for Phosphorus in the Assabet River, 

Massachusetts—Phosphorus Loads, 2008 through 2010

Scientific Investigations Report 2013–5140

U.S. Department of the Interior

U.S. Geological Survey
Front cover. Bent oak tree in autumn, reflected by Assabet River waters. 

Back cover. Green heron (top) and great blue heron (bottom). 

All photographs by David Griffin, http://dmg-photography.com/
Monitoring to Assess Progress toward 

Meeting the Total Maximum Daily Load 

for Phosphorus in the Assabet River, 

Massachusetts—Phosphorus Loads, 

2008 through 2010

By Marc J. Zimmerman and Jennifer G. Savoie

Prepared in cooperation with the 

Massachusetts Department of Environmental Protection

Scientific Investigations Report 2013–5140

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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Suggested citation:

Zimmerman, M.J., and Savoie, J.G., 2013, Monitoring to assess progress toward meeting the total maximum daily 

load for phosphorus in the Assabet River, Massachusetts—Phosphorus loads, 2008 through 2010: U.S. Geological 

Survey Scientific Investigations Report 2013–5140, 41 p., http://pubs.usgs.gov/sir/2013/5140/.
iii

Acknowledgments

This study benefitted greatly from the interest, support, and assistance of many colleagues and 

agencies. In particular, Brian Friedmann, Dennis R. Dunn, Alice Rojko, Kimberly Groff, and the 

late Arthur J. Screpetis of the Massachusetts Department of Environmental Protection, Division 

of Watershed Management, were instrumental in guiding the work. Wastewater-treatmentplant managers Harry Bultman of Marlborough’s Westerly plant and Christopher Pratt of Veolia’s 

Westborough plant directly provided monthly monitoring data of great importance in assessing 

loads in the Assabet River. AMVETS Post 208 in Hudson, Massachusetts, gave permission to 

construct a water-quality-monitoring shelter on its property.

The contributions of many U.S. Geological Survey coworkers (Marcus C. Waldron, Joan 

S. Whitley, Joseph F. Whitley, Gerald G. Girouard, Andrew J. Massey, Andrew M. Waite, 

Kimberly W. Campo, Gardner C. Bent, Roy S. Socolow, Timothy R. Driskell, and Britt O. Stock), 

cooperative-education and other student aides (Lisa Lach, Michael Cronin, Andrew Spiers, 

Melissa Weil), and volunteers (Jamie Kendall, Mary Gowins, and Sandra Petrakis) helped make 

the project a success. USGS colleagues Elaine C.T. Trench, Kirk P. Smith, and Leslie A. DeSimone 

reviewed the draft report, making many useful suggestions that substantially improved the text. 
THIS PAGE INTENTIONALLY LEFT BLANK
v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Previous Studies of Phosphorus in the Assabet River...................................................................2

Conceptual Model.................................................................................................................................2

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

Methods for Estimating Phosphorus Loads in the Assabet River .........................................................4

Streamflow Measurements ................................................................................................................4

Monitoring Sites....................................................................................................................................5

Automated Sample Collection ............................................................................................................5

Grab-Sample Collection.......................................................................................................................8

Sample Processing and Chemical Analysis.....................................................................................8

Water-Quality Data...............................................................................................................................8

Phosphorus Concentrations and Loads...................................................................................................11

Phosphorus Loads Upstream and Downstream from Impoundments.......................................11

Hudson Impoundment...............................................................................................................11

Ben Smith Impoundment ..........................................................................................................14

Seasonal Differences between Upstream and Downstream Loads, October 2008 

through December 2010 .......................................................................................................15

Understanding the Effects of Impoundments on Phosphorus Loads in the Assabet River, 

October 2008 through December 2010........................................................................................15

Summary........................................................................................................................................................19

References Cited..........................................................................................................................................20

Appendix 1. Start and end dates and times for composite samples collected at waterquality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and 

Maynard, Massachusetts, October 2008 through December 2010........................................41

Appendix 2. A, Concentrations and relative percent differences of total phosphorus and 

orthophosphorus in concurrent equal-width-increment (EWI) and outside-gage grab 

samples (OGS); B, Concentrations and relative percent differences of total phosphorus 

and orthophosphorus on first and last days of seven-day holding time in sample 

containers........................................................................................................................................41

Appendix 3. Daily mean streamflow in cubic feet per second at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts, October 2008 through December 2010..........................................................41

Figures

1. Map showing locations of water-quality-monitoring stations, wastewater-treatment 

plants, and dams in the study area along the Assabet River Basin in central 

Massachusetts..............................................................................................................................3

2. Orthophotomap identifying the locations of the U.S. Geological Survey water-qualitymonitoring stations 01096835 (Port Street) and 01096840 (AMVETS) upstream and 

downstream from the Hudson impoundment...........................................................................6
vi

3. Orthophotomap identifying the locations of U.S. Geological Survey water-qualitymonitoring stations 01096885 (Stow) and 01097000 (Maynard) upstream and downstream from the Ben Smith impoundment................................................................................7

4. Photographs showing water-quality-monitoring stations along the Assabet River 

in the towns of Hudson, Stow, and Maynard, Massachusetts .............................................9

5. Photograph showing equipment installed in water-quality-monitoring stations along 

the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts ..........10

6. Graphs showing streamflow and estimated weekly loads upstream and downstream 

from the Hudson impoundment of A, total phosphorus and the sum of total phosphorus 

loads from the upstream Westborough and Marlborough Westerly wastewatertreatment plants; B, orthophosphorus; and C, particulate phosphorus, October 2008 

through December 2010.............................................................................................................12

7. Graphs showing streamflow and differences between estimated weekly loads of 

A, total phosphorus; B, orthophosphorus; and C, particulate phosphorus upstream 

and downstream (AMVETS monitoring site) from the Hudson impoundment 

October 2008 through December 2010 ....................................................................................13

8. Graphs showing streamflow and estimated weekly loads of A, total phosphorus and 

the sum of total phosphorus loads from the upstream Westborough, Marlborough 

Westerly, and Hudson wastewater-treatment plants; B, orthophosphorus; and 

C, particulate phosphorus upstream and downstream from the Ben Smith impoundment, October 2008 through December 2010 .........................................................................16

9. Graphs showing streamflow and differences between estimated weekly loads of 

A, total phosphorus; B, orthophosphorus; and C, particulate phosphorus upstream 

and downstream from the Ben Smith impoundment, and streamflow, October 2008 

through December 2010.............................................................................................................17

Tables

1. Location coordinates, drainage-basin areas, and annual mean daily streamflow for 

water-quality- and streamflow-monitoring stations on the Assabet River in the towns 

of Hudson, Stow, and Maynard, Massachusetts, water years 2008 to 2010 ......................5

2. Distances downstream from the Westborough wastewater-treatment plant (WWTP) 

to other WWTPs, impoundment dams, and water-quality-monitoring stations on the 

Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts ...................8

3. Concentrations of total phosphorus and orthophosphorus in weekly composite 

samples collected at water-quality-monitoring stations on the Assabet River in the 

towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through 

December 2010............................................................................................................................24

4. Concentrations of total phosphorus and orthophosphorus in blank samples, point 

samples, and replicate samples collected at water-quality-monitoring stations on 

the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, 

October 2008 through December 2010 ....................................................................................30

5. Estimated weekly total phosphorus and orthophosphorus loads, in kilograms, and 

summary statistics based on weekly composite samples collected at water-qualitymonitoring stations on the Assabet River in the towns of Hudson, Stow, and 

Maynard, Massachusetts, October 2008 through December 2010 ....................................34

6. Estimated weekly total phosphorus loads, in kilograms, in discharges from 

wastewater-treatment plants on the Assabet River in Westborough, Marlborough, 

and Hudson, Massachusetts, October 2008 through December 2010...............................37
vii

7. Summary statistics for the ratios of orthophosphorus concentrations to total phosphorus concentrations in weekly composite samples collected at the Port Street, 

AMVETS, Stow, and Maynard water-quality-monitoring stations on the Assabet River 

in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through 

December 2010............................................................................................................................11

8. A, Summary statistics for nongrowing-season and growing-season loads of total 

phosphorus and orthophosphorus, in kilograms, in wastewater-treatment-plant 

effluent and at water-quality-monitoring stations on the Assabet River, October 2008 

through December 2010; and B, summary statistics for differences between 

particulate phosphorus loads at upstream and downstream water-quality-monitoring 

stations at the Hudson and Ben Smith impoundments on the Assabet River, 

October 2008 through December 2010 ....................................................................................18

Conversion Factors, Datum, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 0.003785 cubic meter (m3)

gallon 3,785 milliliter (mL)

gallon 3.785 liter (L)

million gallons (Mgal) 3,785 cubic meter (m3)

cubic foot (ft3) 0.02832 cubic meter (m3)

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3/s)

foot per second (ft/s) 0.3048 meter per second (m/s)

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

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

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

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

pound, avoirdupois (lb) 0.4536 kilogram (kg)
viii

Mesh diameters of filters are measured in microns (μm). 

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

 °F=(1.8×°C)+32

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), 

unless otherwise indicated.

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).

Abbreviations

EWI equal-width increment

MassDEP Massachusetts Department of Environmental Protection

orthoP orthophosphorus

PP particulate phosphorus

RPD relative percent difference

TMDL total maximum daily load

TP total phosphorus

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

WWTP wastewater-treatment plant
Monitoring to Assess Progress toward Meeting the Total 

Maximum Daily Load for Phosphorus in the Assabet River, 

Massachusetts—Phosphorus Loads, 2008 through 2010

By Marc J. Zimmerman and Jennifer G. Savoie

Abstract

Wastewater discharges to the Assabet River contribute 

substantial amounts of phosphorus, which support accumulations of nuisance aquatic plants that are most evident in the 

river’s impounded reaches during the growing season. To 

restore the Assabet River’s water quality and aesthetics, the 

U.S. Environmental Protection Agency required the major 

wastewater-treatment plants in the drainage basin to reduce the 

amount of phosphorus discharged to the river by 2012. From 

October 2008 to December 2010, the U.S. Geological Survey, 

in cooperation with the Massachusetts Department of 

Environmental Protection and in support of the requirements 

of the Total Maximum Daily Load for Phosphorus, collected 

weekly flow-proportional, composite samples for analysis 

of concentrations of total phosphorus and orthophosphorus 

upstream and downstream from each of the Assabet River’s 

two largest impoundments: Hudson and Ben Smith. The 

purpose of this monitoring effort was to evaluate conditions 

in the river before enhanced treatment-plant technologies had 

effected reductions in phosphorus loads, thereby defining 

baseline conditions for comparison with conditions following the mandated load reductions. The locations of sampling 

sites with respect to the impoundments enabled examination 

of the impoundments’ effects on phosphorus sequestration 

and on the transformation of phosphorus between particulate 

and dissolved forms. The study evaluated the differences 

between loads upstream and downstream from the impoundments throughout the sampling period and compared differences during two seasonal periods of relevance to aquatic 

plants: April 1 through October 31, the growing season, and 

November 1 through March 31, the nongrowing season, when 

existing permit limits allowed average monthly wastewatertreatment-plant-effluent concentrations of 0.75 milligram per 

liter (growing season) or 1.0 milligram per liter (nongrowing 

season) for total phosphorus. At the four sampling sites during 

the growing season, median weekly total phosphorus loads 

ranged from 110 to 190 kilograms (kg) and median weekly 

orthophosphorus loads ranged from 17 to 41 kg. During the 

nongrowing season, median weekly total phosphorus loads 

ranged from 240 to 280 kg and median weekly orthophosphorus loads ranged from 56 to 66 kg.

During periods of low and moderate streamflow, 

estimated loads of total phosphorus upstream from the 

Hudson impoundment generally exceeded those downstream 

during the same sampling periods throughout the study; 

orthophosphorus loads downstream from the impoundment 

were typically larger than those upstream. When storm 

runoff substantially increased the streamflow, loads of total 

phosphorus and orthophosphorus both tended to be larger 

downstream than upstream.

At the Ben Smith impoundment, both total phosphorus 

and orthophosphorus loads were generally larger downstream 

than upstream during low and moderate streamflow, but 

the differences were not as pronounced as they were at the 

Hudson impoundment. High flows were also associated with 

substantially larger total phosphorus and orthophosphorus 

loads downstream than those entering the impoundment 

from upstream. 

In comparing periods of growing- and nongrowingseason loads, the same patterns of loads entering and leaving 

were observed at both impoundments. That is, at the Hudson 

impoundment, total phosphorus loads entering the impoundment were greater than those leaving it, and orthophosphorus 

loads leaving the impoundment were greater than those entering it. At the Ben Smith impoundment, both total phosphorus 

and orthophosphorus loads leaving the impoundment were 

greater than those entering it. However, the loads were greater 

during the nongrowing seasons than during the growing seasons, and the net differences between upstream and downstream loads were about the same.

The results indicate that some of the particulate fraction 

of the total phosphorus loads is sequestered in the Hudson 

impoundment, where particulate phosphorus probably undergoes some physical and biogeochemical transformations to 

the dissolved form orthophosphorus. The orthophosphorus 

may be taken up by aquatic plants or transported out of the 

impoundments. The results for the Ben Smith impoundment 

are less clear and suggest net export of total phosphorus and 

orthophosphorus. Differences between results from the two 

impoundments may be attributable in part to differences in 
2 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

their sizes, morphology, unmonitored tributaries, riparian land 

use, and processes within the impoundments that have not 

been quantified for this study.

Introduction

The Assabet River in central Massachusetts is valued 

for its scenery, recreational access, biological diversity, and 

history. In 1999, the U.S. Congress designated reaches of 

the river’s approximately 33-mile (mi) length as a National 

Wild and Scenic River (National Park Service, 2006). Before 

reaching its confluence with the Sudbury River in the town of 

Concord (fig. 1), the river passes through nine towns. Along 

the way, nine dams affect the river’s flow and morphology.

The Assabet River Basin lies in a rapidly developing 

area of Massachusetts where water and wastewater demands 

have long strained water resources (McAdow, 1990). In 1998, 

the Assabet River was included on the Massachusetts 303d 

list as an impaired river with respect to nutrients, organic 

enrichment, and low dissolved oxygen (Commonwealth of 

Massachusetts, 2004). In particular, eutrophication caused 

by excessive concentrations of phosphorus and nitrogen 

in the river has led to nuisance growth of aquatic plants. 

Municipal wastewater-treatment plant (WWTP) discharges 

in Westborough, Marlborough, and Hudson release nutrientcontaining wastewater directly to the Assabet River in the 

study area. During low-flow periods, treated wastewater 

discharges may constitute 80 percent of the river’s flow 

(Commonwealth of Massachusetts, 2004). Water-supply 

withdrawals and transfers further contribute to the degradation 

of the river’s water quality (DeSimone, 2004).

In 2004, the U.S. Environmental Protection Agency 

(USEPA) approved the “Assabet River Total Maximum 

Daily Load for Total Phosphorus” (Commonwealth of 

Massachusetts, 2004), referred to herein as the TMDL. The 

TMDL required the WWTPs to upgrade their facilities so 

as to decrease the loads of total phosphorus (TP) in WWTP 

effluents. The new regulatory requirements would go into 

effect as the facility improvements were completed. All of 

the upgrades were finished in the summer of 2012, after the 

completion of this study’s monitoring period. 

Prior to the TMDL implementation, the Massachusetts 

Department of Environmental Protection (MassDEP) required 

WWTPs to limit effluent concentrations of phosphorus to a 

maximum monthly average of 0.75 milligram per liter (mg/L) 

from April through October, and 1.0 mg/L from November 

through March (Alice M. Rojko, Massachusetts Department of 

Environmental Protection, written commun., 2011). The 2004 

TMDL set new requirements for the November through March 

season at a maximum monthly average TP concentration of 

1.0 mg/L; the April through October average TP concentration limit was set at 0.1 mg/L. If, upon investigation following 

TMDL implementation, the MassDEP determines that the 

limits were inadequate, then the permit limit concentrations 

could be lowered further. 

Previous Studies of Phosphorus in the Assabet 

River

ENSR International (2001) presented the results of field 

and laboratory studies that were used to develop the TMDL for 

TP. The study involved collecting hydrological, water-quality, 

and biological information; sediment cores were collected 

from the river and sent to a laboratory which estimated 

potential phosphorus fluxes experimentally. The data from 

that study were used in a water-quality modeling study (ENSR 

International, 2005). The latter report yielded the quantitative 

results providing the basis for the TMDL that was approved 

by the USEPA for TP in the Assabet River (Commonwealth 

of Massachusetts, 2004). CDM1 (2007) assessed the options 

of dredging and dam removal in conjunction with enhanced 

treatment by the WWTPs for reducing internal phosphorus 

loading—the ability of a water body to retain and release 

phosphorus from the sediment. CDM’s results indicated 

that implementation of WWTP upgrades, dam removal, 

and WWTP limits on winter phosphorus releases that were 

lower than initially planned would have the most beneficial 

effects on water quality. Zimmerman and Sorenson (2005) 

reported on weekly changes in phosphorus concentrations 

in the Hudson impoundment during the summer of 2004; 

nutrient-cycling dynamics in impoundments are complex, and 

this study could not confirm a net phosphorus flux from the 

sediments that might contribute to internal loading. 

Zimmerman and others (2011) presented data on the 

distribution and biomass of the floating aquatic macrophyte, 

Lemna minor (duckweed) and on fluxes of sediment phosphorus in the Hudson and Ben Smith impoundments and freeflowing reaches of the Assabet River. Their study of biomass 

and sediment flux reported that impounded, slow-flowing 

reaches of the river were characterized by larger amounts of 

Lemna and potentially greater sediment fluxes of phosphorus 

than in free-flowing reaches. Highest rates of induced phosphorus flux from the sediments under in situ experimental 

conditions were usually associated with warm summer periods 

and low-oxygen (hypoxic) conditions in the impoundments. 

Conceptual Model

The Assabet River passes through two shallow impoundments, Hudson and Ben Smith, on which this study focuses. 

These impoundments do not stratify thermally during the summer and do not have anoxic hypolimnia—conditions that, if 

they occurred, could lead to the release of dissolved phosphorus from the sediments and its transport downstream. However, hypoxic conditions are sometimes present in the water 

column during warm summer periods in relatively stagnant 

parts of the impoundments. The previous studies cited above 

serve as a basis for a narrative conceptual model of the processes affecting the loads of TP and orthophosphorus (orthoP) 

1

In 2011, CDM (Camp, Dresser & McKee) acquired Wilbur Smith Associates to form CDM Smith.
71°36'

42°18'

42°24'

42°30'

71°24'

42°36'

EXPLANATION

EXPLANATION

MASSACHUSETTS

42o00'

42o

30'

73o

00' 72o

00'

71o

00'

70o

00'

41o

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-treatment plant (WWTP)

Dam

Westborough WWTP

Marlborough

Westerly WWTP

Port Street - 01096835

Stow - 01096885

Maynard - 01097000

From USGS and MassGIS data sources, Massachusetts State Plane

Coordinate System, Mainland Zone. 

Ben Smith dam

Gleasondale dam

Hudson dam

0 50 MILES

0 50 KILOMETERS

AMVETS - 01096840

Hudson WWTP

Water-quality-monitoring station

0 5 MILES

0 5 KILOMETERS

Introduction 3

Figure 1. Locations of water-quality-monitoring stations, wastewater-treatment plants, and dams in the study area along the 

Assabet River Basin in central Massachusetts.
4 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

in the Assabet River study area which, in effect, runs from the 

Westborough WWTP to the U.S. Geological Survey (USGS) 

streamflow-monitoring station in Maynard. 

Phosphorus occurs in dissolved, inorganic (orthoP) and 

particulate forms. Other phosphorus fractions in the form 

of dissolved organic phosphorus and dissolved inorganic 

phosphorus may occur as colloids; for example, in this study, 

any phosphorus fraction that passed through a 0.45-micron 

filter was considered part of the dissolved, orthoP fraction. 

OrthoP is readily available for uptake by aquatic plants that 

include the macrophytes and algae that may form nuisance 

growths in the study impoundments. The particulate form is 

subject to settling to the river bed at any point, including in 

the impoundments. After settling, particulate phosphorus (PP) 

may break down, releasing orthoP into the water column, 

which can then be transported downstream in the river, 

where it may be taken up by aquatic plants. The PP can also 

be resuspended by sufficiently high streamflow velocity. 

Thus, particulate and dissolved phosphorus may enter the 

impoundments and pass through, settle out, break down, 

or be absorbed. By observing the net differences between 

the loads of each of the various forms of phosphorus (TP, 

orthoP, and PP) that are monitored entering and leaving the 

impoundments, inferences can be drawn as to whether the 

impoundments sequester PP that may be converted, at least in 

part, to orthoP that can serve as a plant nutrient.

Purpose and Scope

In 2006, the USGS, in cooperation with the MassDEP, 

undertook studies to obtain baseline information needed to 

determine the eventual effectiveness of improvements in 

phosphorus removal by WWTPs in reducing phosphorus 

loads in the Assabet River. The study area of this report 

included about 22 mi (approximately two-thirds) of the river, 

starting at the WWTP in the town of Westborough, running 

through the towns of Northborough, Marlborough, Hudson, 

and Stow, and ending at the USGS streamgaging station in 

Maynard, Massachusetts (fig. 1). Although the Assabet River 

phosphorus TMDL applies to the entire length of the river, 

the study focused on the two largest impoundments, Hudson 

(in Hudson) and Ben Smith (in Stow and Maynard) that 

were created by old mill dams. The primary objectives of 

this study were to evaluate phosphorus loads in the Assabet 

River prior to implementation of WWTP phosphorus-removal 

improvements and during the seasonal periods (April through 

October and November through March) when permit limits 

at the WWTPs change and to consider the roles of the 

impoundments in the sequestration, transformation, and flux 

of phosphorus. 

This report, the second of two describing conditions in 

the Assabet River prior to the implementation of the TMDLmandated WWTP upgrades (see Zimmerman and others, 

2011), presents the water-quality and streamflow data, estimated loads of phosphorus from treatment-plant effluent, and 

estimated loads of phosphorus downstream from the WWTPs. 

These baseline load estimates of loads will help determine 

the effectiveness of the treatment-plant upgrades in reducing 

phosphorus loads in the Assabet River. 

Methods for Estimating Phosphorus 

Loads in the Assabet River

Typically, monitoring water quality for load estimation 

involves the collection of individual, equal-width-increment 

(EWI) samples on a fixed schedule—for example, quarterly, 

monthly, or weekly. Clearly, the more frequently waterquality samples are collected, the more likely the resulting 

data will represent the variability in stream concentrations 

that must be accurately characterized to estimate constituent 

loads. Robertson and Roerish (1999) found that, for 1-year 

studies, routine monthly sampling, supplemented by storm 

sampling, yielded the best results for estimating loads; for 

studies of 2 or more years, they found that semimonthly 

sampling was most effective, but they did not consider 

continuous, flow-proportional sampling. Because storms and 

other high-discharge events may carry substantial fractions 

of annual loads, collecting individual samples frequently 

following the storm hydrograph would be an obvious solution; 

however, frequent manual sampling could prove extremely 

expensive when the costs of sample collection and analysis 

are considered. Schleppi and others (2006) found that flowproportional sampling gave more accurate and precise flux 

estimates than other methods. Streamflow-data collection 

at the time of water-quality sampling is essential for all 

approaches to constituent-load estimation.

Water-quality-samples that constitute the basis for this 

report were collected from October 2008 to December 2010 

at four monitoring stations in the towns of Hudson (two sites), 

Stow, and Maynard (fig. 1). Field reconnaissance determined 

that these sampling stations were the best available locations 

for continuous monitoring of flows at sites bracketing the 

Hudson and Ben Smith impoundments. A major concern for 

the evaluation of the TMDL’s effectiveness in controlling 

nutrient-induced eutrophication is the determination of 

the portion of the TP loads that may accumulate in the 

impoundments; PP retained in the impoundments may be 

transformed to a dissolved form that may become available 

to support undesirable (nuisance) aquatic-plant growth. This 

study used weekly flow-proportional, composite sampling as 

a cost-effective means to acquire data for estimating loads 

entering and leaving the impoundments.

Streamflow Measurements

Streamflow is calculated as the product of stream velocity 

and stream cross-section area and is typically measured in 

cubic feet per second. Because loads are estimated by multiplying constituent concentration by streamflow volume, accurate measurements of streamflow are essential. Hydrographers 
Methods for Estimating Phosphorus Loads in the Assabet River 5

measure streamflow in the field by using a variety of techniques (Turnipseed and Sauer, 2010) and over a range of 

streamflow conditions. From these measurements, a rating 

curve is developed that relates streamflow to stage (elevation). 

For this study, submersible pressure transducers were 

housed in protective tubing to monitor water levels, which 

were used to estimate flow based on a stage-discharge relation. 

Stream stage was measured nearly continuously (at 15-minute 

intervals) so that streamflow could be calculated from stage 

measurements on the many days when field measurements 

were not made. Outside-reference staff gages were installed 

for use in calibrating the continuous stage measurements.

To ensure accurate stage-discharge relations, streamflows 

at the four Assabet River sampling stations were measured 

routinely (every 6–8 weeks) either by wading or by acoustic 

Doppler technology (Turnipseed and Sauer, 2010). Ratingcurve data were entered into the program stored in a datalogger housed in each monitoring shelter. Each datalogger was 

linked to the pressure transducer, which provided nearly continuous stage readings and calculated streamflow. Data stored 

in the datalogger were transmitted to a database in the USGS 

office in Northborough, Mass., and were subsequently used to 

compute the daily mean streamflow used in load estimation.

Monitoring Sites

Weekly flow-proportional composite samples were 

collected by automated sampling equipment installed in four 

monitoring shelters along the Assabet River: (1) Assabet River 

at Port Street at Hudson, Mass. (short name “Port Street”); (2) 

Assabet River 200 feet (ft) below Rte. 85 at Hudson, Mass. 

(short name “AMVETS”); (3) Assabet River at Sudbury Rd. 

near Stow, Mass. (short name “Stow”); and (4) Assabet River 

at Maynard, Mass. (short name “Maynard”) (fig. 1; table 1; 

appendix 1). The Port Street and AMVETS sites are located 

upstream and downstream, respectively, from the Hudson 

impoundment (fig. 2), and the Stow and Maynard sites are 

located upstream and downstream, respectively, from the 

Ben Smith impoundment (fig. 3). A third impoundment, 

Gleasondale, into which the Hudson WWTP discharges, is 

located between the AMVETS and Stow monitoring sites, 

but it is not a subject of this study. The impoundments are 

located several miles downstream from the largest WWTP 

(in Westborough, fig. 1). The Hudson and Ben Smith 

impoundments are approximately 6 mi apart (table 2). 

Three of the shelters that contain the monitoring and 

sampling equipment were new 4-ft by 6-ft by 10-ft plywood 

sheds, and the fourth (at Maynard) was a long-term concrete 

streamgaging station (fig. 4); all were heated. Automated samplers (fig. 5) drew in water-quality samples through 1⁄2-inch 

(in.) copper sample tubing, which was protected by a 2-in. 

heavy-duty plastic pipe extending from each shelter to the center of flow in the river. The copper tubing was wrapped with 

electrical heating tape to prevent water from freezing in the 

tubing during the winter. The intake was fitted with a plastic 

strainer to prevent debris from clogging the sample tubing. 

Automated Sample Collection

Composite, flow-proportional samples were collected 

with a portable automatic sampler outfitted with a four-bottle 

rack and controlled by a datalogger. The weekly composite target was 32 subsamples (the approximate number of 

100-milliliter (mL) samples that the sample bottle could hold). 

Every week, the average daily streamflow for the upcoming week was estimated; the estimate was used to calculate a 

threshold volume that would trigger the automated sampler. 

If the streamflow volumes were greater on some days than 

others, then samples were collected more frequently on the 

days with greater streamflows; this is what is meant by “flowproportional sampling.”

Table 1. Location coordinates, drainage-basin areas, and annual mean daily streamflow for water-quality- and streamflowmonitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, water years 2008 to 2010.

[Locations of monitoring stations are shown in figures 1 to 3. Latitude and longitude are given in degrees (°), minutes (ʹ), and seconds (′′) relative to the 

North American Datum of 1983 (NAD 83), except for station number 01097000, whose coordinates are relative to NAD 27; USGS, U.S. Geological Survey; 

mi2, square mile; ft3/s, cubic foot per second; --, no data; ft, foot]

USGS station name Short 

name

USGS 

station 

number

Latitude Longitude

Drainage 

basin area

(mi2)

Annual mean daily 

streamflow 

(ft3/s)

2008 2009 2010

Assabet River at Port Street at Hudson Port Street 01096835 42°23ʹ07′′ 71°34ʹ40′′ 60.5 -- 162 163

Assabet River 200 ft below Route 85 at Hudson AMVETS 01096840 42°23ʹ24′′ 71°34ʹ11′′ 64.2 -- 171 172

Assabet River at Sudbury Road near Stow Stow 01096885 42°24ʹ42′′ 71°30ʹ29′′ 88.0 -- 202 219

Assabet River at Maynard Maynard 01097000 42°25ʹ55′′ 71°27ʹ01′′ 116 247 266 275
71°34'30" 71°34'0"

42°23'30"

42°23'

Base map from MassGIS Color Ortho Imagery 1:5,000

Massachusetts State Plane projection, NAD83 0 0.05 0.1 0.2 MILES

0 0.05 0.1 0.2 KILOMETERS

Port Street—01096835

AMVETS— 01096840

Hudson impoundment dam

Port Street—01096835

AMVETS— 01096840

Hudson impoundment dam

6 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

Figure 2. The locations of the U.S. Geological Survey water-quality-monitoring stations 01096835 (Port Street) and 01096840 (AMVETS) 

upstream and downstream from the Hudson impoundment.
Ben Smith

impoundment dam

Stow—01096885

Maynard—01097000

Stow—01096885

Maynard—01097000

71°30' 71°29' 71°28' 71°27'

42°26'

42°25'

Base map from MassGIS Color Ortho Imagery 1:5,000

Massachusetts State Plane projection, NAD83 0 0.25 0.5 1 MILES

0 0.25 0.5 1 KILOMETERS

Methods for Estimating Phosphorus Loads in the Assabet River 7

Figure 3. The locations of U.S. Geological Survey water-quality-monitoring stations 01096885 (Stow) and 01097000 (Maynard) 

upstream and downstream from the Ben Smith impoundment.
8 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

Table 2. Distances downstream from the Westborough 

wastewater-treatment plant (WWTP) to other WWTPs, 

impoundment dams, and water-quality-monitoring stations on 

the Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts.

[Monitoring station and number, wastewater-treatment plants (WWTPs) and 

dams are shown in figure 1]

Location Distance

(miles)

Westborough WWTP 0

Marlborough-Westerly WWTP 6

Port Street - 01096835 11.1

Hudson dam 11.8

AMVETS - 01096840 11.9

Hudson WWTP 14.1

Gleasondale dam 15.6

Stow - 01096885 18

Ben Smith dam 30.6

Maynard - 01097000 31.9

When the threshold was exceeded, a signal was transmitted to the automated sampler, causing it to purge the 

suction line and collect two 100-mL aliquots of water. The 

first aliquot, the TP sample, was pumped into a 3.8-liter (L) 

polyethylene bottle previously acidified with 10 mL of sulfuric 

acid. The second aliquot, the orthoP sample, was pumped into 

a second bottle, which was modified so that the water sample 

could be completely pumped into a refrigerator by a peristaltic 

pump. Inside the refrigerator, the tubing connected to an inline 

polysulfone capsule filter (Pall AquaPrep 4270) with a pore 

size of 0.45 micron, polyester reinforcement, and an effective 

filtration area of 20 square centimeters (cm2

). After passing 

through the filter, the sample entered a 9-L polyethylene storage bottle. 

In the event that the week’s streamflow was underestimated, and more than 32 TP samples were needed, a second 

3.8-L acidified bottle collected any additional TP samples. The 

9-L storage bottle had ample excess capacity for additional 

orthoP samples. If the total weekly streamflow was overestimated, fewer than 32 TP and orthoP samples were collected, 

but the sample numbers were nevertheless collected proportionally to flow.

Grab-Sample Collection

Based on cross-sectional conductance data and scientific 

judgment prior to the start of sampling, the sampling intake 

locations were regarded as representing well-mixed zones of 

the river. For quality control to confirm that the automated 

sampling system collected representative samples, outside 

grab samples were collected from bridges near the stations by 

using a weighted-bottle sampler at approximately the same 

time as an inside grab sample was collected by the automated 

sampler. The phosphorus concentrations in the outside grab 

samples that were collected manually from the river were 

compared with phosphorus concentrations in individual 

samples collected by the automated sampler. 

Sample Processing and Chemical Analysis

Samples were processed at the USGS MassachusettsRhode Island Water Science Center laboratory in 

Northborough, Mass. The weekly composite samples were 

retrieved from the shelters every 7 days. Sample bottles were 

replaced with clean ones, and new filters were installed at 

each site. In addition to collecting samples, USGS personnel 

recorded measurements of river stage, battery voltage, 

streamflow, and refrigerator temperature. The collection 

bottle for TP, preacidified with 10 mL of sulfuric acid, usually 

needed additional acid to preserve the sample to a pH less than 

2. The volume of additional acid was calculated based on the 

weight of the sample. Samples were chilled and stored prior to 

shipping to the USGS National Water Quality Laboratory for 

analysis (Fishman, M.J., 1993; U.S. Environmental Protection 

Agency, 1993). 

Water-Quality Data

Weekly flow-proportional composite samples collected in 

2008 to 2010 from four monitoring stations along the Assabet 

River were analyzed for TP and orthoP (table 3, in back of 

report). Occasional equipment failures—for example, power 

failures and frozen and disconnected sample tubing—led to 

missing data. The WWTPs provided additional phosphorus 

data for treated effluent discharged to the Assabet River.

In addition to the environmental phosphorus samples 

collected from the stream by the automated sampler, blanks, 

grab samples, replicate (split-composite) samples, and EWI 

samples were collected. Phosphorus and orthoP were not 

detected in the blanks. Concurrent grab samples were collected 

25 times, replicates were collected for about 10 percent of 

the total number of environmental samples, and blanks were 

collected 7 times (table 4, in back of report). Concentrations of 

phosphorus in grab samples collected manually from the river 

correlated well with concentrations in grab samples collected 

by the automated sampler. Relative percent differences2

 (RPD) 

for the grab samples averaged about 6.6 percent for TP and 

3.8 percent for orthoP, with medians of 2.7 and 3.0 percent, 

respectively. EWI samples were collected concurrently with 

inside grab samples (see appendix 2, part A); the average RPD 

2

Relative percent difference is the absolute difference between two numbers 

divided by the average of the two numbers and multiplied by 100 percent. 

Measurement-performance criteria for TP and orthoP were RPDs less than 

20 percent.
Port Street—01096835 AMVETS—01096840

Stow—01096885 Maynard—01097000

Methods for Estimating Phosphorus Loads in the Assabet River 9

Figure 4. Water-quality-monitoring stations along the Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts.

values for TP and orthoP samples were 8.2 and 5.6 percent, 

respectively. These quality-control-sampling results indicated 

that the automated samplers were collecting representative 

samples from these well-mixed sections of the river. RPDs for 

the replicate samples averaged about 1.6 percent for TP and 

5.1 percent for orthoP, with medians of 1.1 and 0.0 percent, 

respectively. These replicate quality-control data demonstrated 

that sample processing yielded consistent results.

In a test to determine whether one week’s holding time 

led to adsorption of phosphorus to the sides of the sampling 

containers, acidified and filtered samples collected at each 

monitoring station were retained in their containers for an 

additional week after initial processing for analysis. At the 

end of the week, a second set of samples was collected for 

analysis. Decreasing concentrations would indicate that 

the containers adsorbed some phosphorus. Concentrations 

increased slightly during the week for all but one sample (see 

appendix 2, part B). The maximum RPD was about 10 percent, 

and the minimum was about 1 percent.

The value of composite-sample concentrations is in their 

use in load-estimate calculations. Weekly TP loads (table 5, 

in back of report) were calculated at the four stations by 

multiplying the composite sample concentrations (table 3, in 

back of report) by the total weekly flow derived from mean 

daily streamflows (appendix 3). When loads were calculated, no attempts were made to estimate the weekly loads 

when water-quality data were missing or when sampling was 

incomplete because of equipment problems. Weekly TP loads 

during the study ranged from 16 to 950 kilograms (kg) at Port 

Street (median 220 kg), 17 to 1,100 kg at AMVETS (median 
Refrigerator

Automatic sampler

Filter

Peristaltic pump Battery charger

Electronics

control box

9-liter bottle for 

orthophosphorus

samples

10 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

Figure 5. Equipment installed in water-quality-monitoring stations along the Assabet River in the 

towns of Hudson, Stow, and Maynard, Massachusetts.
Phosphorus Concentrations and Loads 11

170 kg), 8.3 to 1,500 kg at Stow (median 214 kg), and 16 to 

2,000 kg at Maynard (median 240 kg). OrthoP loads ranged 

from 2.5 to 170 kg at Port Street (median 25 kg), 2.1 to 270 kg 

at AMVETS (median 36 kg), 1.0 to 310 kg at Stow (median 

44 kg), and 2.9 to 280 kg at Maynard (median 53 kg).

WWTP operators are required to provide basic information about their operations to the USEPA and the MassDEP. 

Of particular relevance to this study are the requirements 

to provide water-quality data. At least once a week, WWTP 

staffs analyze plant effluents for phosphorus and nitrogen. 

These data are tabulated monthly and, along with discharge 

volume and precipitation data, are reported to the USEPA and 

the MassDEP. The USGS obtained TP data either directly 

from individual WWTPs or from the MassDEP. Because the 

WWTPs do not provide daily phosphorus data, the loads for 

each week were estimates that used the average TP sample 

concentrations during the same one-week period as the USGS 

composite samples, regardless of the number of days on which 

concentrations were reported (generally, only once per week). 

This average concentration was multiplied by the reported 

total weekly effluent discharge volume to yield an estimated 

weekly load (table 6, in back of report). These data should be 

viewed with caution because a single WWTP phosphorus sample may not accurately represent the concentrations of weekly 

effluent discharges in the same manner that flow-proportional, 

composite samples represent the stream concentrations. 

Phosphorus Concentrations and Loads

The USGS phosphorus-load estimates were based on the 

concentrations of total phosphorus and of orthoP analyzed 

from weekly, flow-proportional, composite samples. Loads 

were calculated by multiplying sample concentrations by the 

flow volume for the sampling week. The net effects of the 

impoundments on the phosphorus loads were determined by 

comparing the differences in loads upstream and downstream 

from each impoundment. The particulate component of TP can 

settle in the impoundments and elsewhere along the river; in 

situ, phosphorus can desorb from the sediments, and dissolved 

phosphorus can move downstream or be taken up by aquatic 

macrophytes and contribute to nuisance growth of plants that 

are associated with eutrophication. 

Evaluating the analytically based estimates of TP loads 

alone can lead to misunderstandings or misinterpretations of 

the load estimates. TP loads and orthoP loads constitute the 

basic results presented in this report, and the TMDL primarily 

addresses TP loads. However, TP consists of PP and dissolved (orthoP) fractions, which have different properties, 

interactions, cycles, and pathways in the aquatic environment. 

Focusing on TP does not account for the varying proportions 

of these two components among samples. Thus, evaluating 

the particulate fraction may clarify the overall effects of the 

impoundments on the distribution and deposition of phosphorus loads. In this study, ratios of orthoP to TP were calculated 

for all weekly load estimates. Maxima for these ratios ranged 

between 0.30 and 0.39; the minima ranged from 0.04 to 0.09; 

medians ranged between 0.12 and 0.23; and means ranged 

between 0.14 and 0.22 (table 7). These ratios are based on 

sampling dates for which both orthoP and TP data were available. These results show that, on the average, about 80 percent 

of the estimated TP loads consisted of PP, and 20 percent was 

in the dissolved form; under some conditions, however, the 

ratios varied considerably.

Phosphorus Loads Upstream and Downstream 

from Impoundments

A simple means of comparison between loads upstream 

and downstream from each impoundment is subtraction. This 

report uses graphs of the loads and the differences between 

upstream and downstream loads at various times to enhance 

visualization of the study’s results.

Hudson Impoundment

Estimated TP loads entering the Hudson impoundment 

near the Port Street sampling station frequently exceeded loads 

leaving the impoundment during sampling periods from 2008 

to 2010 (fig. 6A). Subtracting the downstream loads from the 

upstream loads demonstrates that upstream loads exceeded 

downstream loads during most of the study’s sampling periods 

(fig. 7A). The larger upstream loads are most apparent when 

loads were moderate (250 to 500 kg); fig. 6A). In association with short-lived streamflow peaks in December 2008, 

July 2009, and March 2010, the downstream loads exceeded 

upstream loads, with peak loads at the downstream AMVETS 

station in excess of 600 kg. During periods marked by low 

flows in September to October 2009 and July to September 

2010, the upstream and downstream phosphorus loads were 

about equal and less than 200 kg. 

Table 7. Summary statistics for the ratios of orthophosphorus 

concentrations to total phosphorus concentrations in weekly 

composite samples collected at the Port Street, AMVETS, 

Stow, and Maynard water-quality-monitoring stations on the 

Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts, October 2008 through December 2010.

Port Street 

01096835

AMVETS

01096840

Stow

01096885

Maynard

01097000

Maximum 0.30 0.39 0.42 0.39

Minimum 0.04 0.07 0.04 0.09

Median 0.12 0.23 0.20 0.22

Mean 0.14 0.22 0.19 0.22
A

B

C

0

400

800

1,200

1,600

2,000

0

100

200

300

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Orthophosphorus, in kilograms

0

400

800

1,200

1,600

2,000

0

200

400

600

800

1,000

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Particulate phosphorus, in kilograms

0

400

800

1,200

1,600

2,000

0

200

400

600

800

1,000

1,200

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Total phosphorus, in kilograms

AMVETS streamflow

Port Street loads (upstream)

AMVETS loads (downstream)

Westborough + Marlborough

 Westerly WWTP loads

EXPLANATION

Time

12 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

Figure 6. Streamflow and estimated weekly loads upstream and downstream from the Hudson impoundment of 

A, total phosphorus and the sum of total phosphorus loads from the upstream Westborough and Marlborough Westerly 

wastewater-treatment plants; B, orthophosphorus; and C, particulate phosphorus, October 2008 through December 2010.
A

B

C

0

400

800

1,200

1,600

2,000

-200

0

200

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Orthophosphorus, in kilograms

0

400

800

1,200

1,600

2,000

-300

0

300

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Particulate phosphorus, in kilograms

0

400

800

1,200

1,600

2,000

-600

0

600

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Total phosphorus, in kilograms

AMVETS streamflow

Upstream load minus downstream load

EXPLANATION

Time

Phosphorus Concentrations and Loads 13

Figure 7. Streamflow and differences between estimated weekly loads of A, total phosphorus; B, orthophosphorus; 

and C, particulate phosphorus upstream and downstream (AMVETS monitoring site) from the Hudson impoundment 

October 2008 through December 2010.
14 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

The sums of the weekly estimated TP loads discharged 

by the upstream WWTPs in Westborough and Marlborough 

were about 250 to 300 kg in late 2008 through mid-2009 

(fig. 6A; table 6, in back of report). During late 2009 through 

early 2010, TP loads decreased to less than 200 kg. From 

July through October 2010, the loads were less than 100 kg. 

Occasionally, spikes in the loads (notably, March and April 

2010) substantially exceeded 300 kg. These spikes may be 

artifacts of the calculation method, which generally relied on a 

single concentration to represent an entire week (an unusually 

high concentration could lead to a substantial overestimation 

of a load), or they could reflect increased effluent discharge in 

association with a major storm. For example, in March 2010, 

the Westborough WWTP discharged atypically large volumes 

of effluent (more than 8 million gallons per day (Mgal/d)) 

during a period when more than 12 in. (30 centimeters (cm)) 

of precipitation resulted in the fifth-largest streamflow of 

record at Maynard (for the period of record, 1941 to present). 

In December 2010, the high load was associated with a 

relatively high concentration of TP (almost 4 mg/L) in the 

sample used to represent the load. 

During the first half of the 2009 water year3, wastewater 

loads equaled 50 to 100 percent of the estimated TP loads 

entering and leaving the Hudson impoundment (fig. 6A). 

During the summer of 2009, the wastewater loads remained 

the same, but their fraction of the total stream loads decreased 

substantially because the total stream loads increased in association with relatively high streamflows. During the winter and 

spring of 2010, the WWTP cumulative loads steadily increased 

before decreasing sharply following the high streamflows in 

March and April. The WWTP loads remained relatively small 

until they peaked again in late 2010 at approximately the same 

time that loads at Port Street were peaking. 

Estimated orthoP loads at the Hudson impoundment 

sampling sites did not demonstrate the same dynamics as the 

TP loads. OrthoP loads at the upstream Port Street sampling 

station rarely exceeded those at the downstream AMVETS 

station (fig. 6B); the calculated differences in the loads further 

support that observation (fig. 7B). However, in a manner 

similar to that of TP loads under high-flow conditions associated with storms and substantial runoff, such as in the summer of 2009 and the spring of 2010, the orthoP loads at the 

downstream AMVETS station increased substantially relative 

to the loads at the upstream Port Street station (figs. 6B, 7B), 

indicating that orthoP had been transported downstream out of 

the impoundment during storms.

Results of the application of the Wilcoxon signed-rank 

test further support the interpretation that loads upstream and 

downstream from the Hudson impoundment were significantly 

different (p less than 0.001 for both TP and orthoP). Upstream 

loads of TP were most often larger than downstream loads, 

and downstream loads of orthoP were most often larger than 

upstream loads (figs. 7A, B).

3

A water year is a continuous period from October 1 through September 30 

and is designated by the year in which it ends.

Because TP is the sum of particulate and dissolved 

phosphorus fractions, further examination of this relation can 

clarify the effects of impoundments on stream phosphorus 

loads. Ratios of orthoP to TP varied considerably among 

samples, with the ratios for the AMVETS station almost 

always exceeding the ratios at the Port Street station (table 7). 

That is, orthoP generally represented a higher proportion of 

the TP load at the downstream AMVETs station than at the 

upstream Port Street station. Subtraction of the orthoP loads 

from the TP loads to obtain weekly PP loads shows that the 

differences between upstream and downstream TP loads 

resulted from PP sequestration in the Hudson impoundment 

(figs. 6C and 7C). That is, in a pattern similar to that of the 

TP loads, the PP loads at the upstream Port Street station 

generally exceeded the loads at the downstream AMVETS 

station (fig. 7C), indicating retention in the impoundment. 

Ben Smith Impoundment

Estimated TP and orthoP loads at the Ben Smith 

impoundment did not show the same pronounced upstream 

versus downstream relations as demonstrated for the Hudson 

impoundment. Starting in July 2009, TP and orthoP loads were 

more often greater at the downstream (Maynard) sampling 

site than at the upstream (Stow) sampling site; the differences 

are rarely large (figs. 8A, B and 9A, B). From October 2008 

until July 2009, TP loads were usually larger at Stow than at 

Maynard; however, after that period, the loads were either 

greater at Stow or approximately equal at the two stations. 

OrthoP loads at Stow also tended to be larger than those at 

Maynard from October 2008 through March 2009, with loads 

after that period either greater at Maynard or approximately 

equal at both stations. The greatest downstream loads were 

generally associated with high flows, as in the summer of 2009 

and spring of 2010. In August and September 2010, the river 

stage and its associated flow in the Assabet River at Maynard 

were so low that the water level did not reach the sampling 

intake, and no samples were collected. In a pattern similar to 

that observed in the Hudson-impoundment load estimates, 

TP and orthoP loads at the corresponding upstream and 

downstream sampling stations were not greatly different when 

streamflow was low or moderate. Results of the application 

of the Wilcoxon signed-rank test indicate that the downstream 

loads at Maynard were significantly greater than those 

upstream (p less than 0.001 for TP and orthoP.)

In general, the estimated TP loads at the Stow and 

Maynard sampling sites closely tracked the estimated TP loads 

from upstream WWTPs during periods of low and moderate 

flow, such as summer 2010, when wastewater discharges 

frequently were in excess of 90 percent of the total streamflow 

entering the impoundment. During summer 2009, however, 

there was a disparity between the estimated loads at the 

sampling sites and the WWTP loads; the WWTP TP loads 

remained low, at about 200 kg, while the loads at Stow and 

Maynard increased substantially, reaching more than 1,000 kg 
Understanding the Effects of Impoundments on Phosphorus Loads in the Assabet River, October 2008–December 2010 15

during a period of increased streamflow. By contrast, in winter 

2010, both estimated stream loads and WWTP TP loads 

increased with increasing streamflow.

Considering the differences in estimated upstream and 

downstream loads of PP at the Ben Smith impoundment further supports the interpretation associated with the TP loads. 

During most of the 2009 water year, estimated PP loads entering the Ben Smith impoundment at the Stow sampling station 

were usually greater than particulate loads downstream from 

the impoundment, indicating retention in the impoundment 

(figs. 8C and 9C). From July 2009 through December 2010, 

by contrast, estimated PP loads downstream were generally 

greater than or approximately equal to the loads upstream at 

the Stow sampling station. The flooding in March and April 

2010 undoubtedly transported great amounts of suspended 

materials downstream and redistributed PP throughout the 

basin. These interpretations demonstrate why it is important 

to evaluate the loads of PP when considering the TP loads that 

are sequestered in the impoundment, where the phosphorus 

may remain, may be transformed to a dissolved form, and 

may be mobilized and transported downstream during extreme 

events, such as the floods in March and April 2010. 

Seasonal Differences between Upstream and 

Downstream Loads, October 2008 through 

December 2010

From April 1 through October 31 in 2009 and 2010, 

the WWTPs’ permit limits for TP were monthly average 

concentrations of 0.75 mg/L; the April 1 through October 

31 period roughly corresponded to the growing season for 

aquatic plants. From November 1 through March 31 (the 

nongrowing season), when aquatic plant growth was minimal 

because of low temperatures and short daily photosynthesis 

periods, the permit limit for TP at the WWTPs was a monthly 

average concentration of 1.0 mg/L. Phosphorus loads at 

the WWTPs and at the sampling stations were compared 

for differences between the two seasons (median values for 

two years, table 8A). As expected, because of the seasonal 

differences in permit limits, study-period median and average 

estimated TP and orthoP loads at all treatment plants and 

at the sampling stations were larger during the nongrowing 

season than during the growing season. Maximum dischargeload values were larger at all WWTPs during the nongrowing 

season than during the growing season and, except for some 

orthoP loads, all minimum values were larger, too. With the 

exception of Maynard, the medians at the sampling stations 

were roughly 75 to 150 percent larger during the nongrowing 

season than during the growing season. At Maynard, median 

TP and orthoP loads were only about 42 and 54 percent larger, 

respectively, during the nongrowing season than during the 

growing season. 

Summary statistics for seasonal PP loads do not reveal 

the same magnitudes of differences as for TP and orthoP loads 

(table 8B). During the nongrowing season, the median (2-year) 

difference between the PP weekly loads at the Port Street 

and AMVETS stations (Hudson impoundment) was 26 kg; 

during the growing season, the difference was 28 kg. The 

median differences between the PP weekly loads at Stow and 

Maynard (Ben Smith impoundment) during the nongrowing 

and growing seasons are -8 and -18 kg, respectively (negative 

values indicate that median downstream loads are greater than 

median upstream loads). Although these PP load estimates 

may not show substantial differences between the seasons at 

each impoundment, they still reflect previous observations 

that PP is sequestered in the Hudson impoundment, and that 

there are relatively small net losses of PP from the Ben Smith 

impoundment.

Understanding the Effects of 

Impoundments on Phosphorus Loads 

in the Assabet River, October 2008 

through December 2010

The data interpretations presented here focus on the 

effects of the Hudson and Ben Smith impoundments on TP, 

orthoP, and PP loads in the Assabet River. The central questions are (1) “Do the impoundments sequester TP?” (2) “Do 

the forms of phosphorus change in the impoundments?” 

and (3) “Are there seasonal changes in the net effects of the 

impoundments on the proportions of particulate and orthoP in 

stream loads?” 

Load estimates indicate that the phosphorus dynamics 

in the two impoundments differ. For the Hudson impoundment, TP loads entering the impoundment exceed the loads 

leaving it, and orthoP loads leaving the impoundment exceed 

the loads entering; in the Hudson impoundment, some of the 

PP is retained, and orthoP is produced. These differences are 

generally associated with moderate streamflow. The simplest 

interpretation for these observations is that PP deposited in 

the impoundment is subjected to physical and (or) biogeochemical processes that transform a fraction of the particulate 

matter into orthoP. The particulate fraction is likely to be 

quite heterogeneous with some of its constituents more or less 

labile (likely to change) than others; those (labile) constituents 

would be converted to orthoP at different rates. The internally 

produced orthoP is added to the inflowing orthoP, resulting 

in a net gain measured downstream from the impoundment. 

During the growing season, substantial amounts of orthoP may 

be recycled in the impoundments and either sequestered as 

living biomass or converted into PP. The magnitude of orthoP 

uptake, transformation, and release in the impoundments cannot be determined without a sampling program that would be 

substantially more complex (and expensive) than the existing 

one and would be designed to evaluate phosphorus cycling 

and impoundment hydrodynamics. 
A

B

C

0

500

1,000

1,500

2,000

2,500

0

100

200

300

400

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Orthophosphorus, in kilograms

0

400

800

1,200

1,600

2,000

2,400

0

500

1,000

1,500

2,000

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Particulate phosphorus, in kilograms

0

500

1,000

1,500

2,000

2,500

0

500

1000

1,500

2,000

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Total phosphorus, in kilograms

Maynard streamflow

Stow loads (upstream)

Maynard loads (downstream)

Westborough + Marlborough

 Westerly + Hudson WWTP loads

Time

16 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

Figure 8. Streamflow and estimated weekly loads of A, total phosphorus and the sum of total phosphorus loads from 

the upstream Westborough, Marlborough Westerly, and Hudson wastewater-treatment plants; B, orthophosphorus; 

and C, particulate phosphorus upstream and downstream from the Ben Smith impoundment, October 2008 through 

December 2010.
0

500

1,000

1,500

2,000

2,500

-800

0

800

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Total phosphorus, in kilograms

Maynard streamflow

Upstream load minus downstream load

0

500

1,000

1,500

2,000

2,500

-200

0

200

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Orthophosphorus, in kilograms

0

500

1,000

1,500

2,000

2,500

-600

0

600

10/01/0811/01/0812/01/0801/01/0902/01/0903/01/0904/01/0905/01/0906/01/0907/01/0908/01/0909/01/0910/01/0911/01/0912/01/0901/01/1002/01/1003/01/1004/01/1005/01/1006/01/1007/01/1008/01/1009/01/1010/01/1011/01/1012/01/10

Streamflow, in cubic feet per second

Particulate phosphorus, in kilograms

Time

EXPLANATION

Figure 9. Streamflow and differences between estimated weekly loads of A, total phosphorus; B, orthophosphorus; 

and C, particulate phosphorus upstream and downstream from the Ben Smith impoundment, and streamflow, 

October 2008 through December 2010.

Understanding the Effects of Impoundments on Phosphorus Loads in the Assabet River, October 2008–December 2010 17
18 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts Table 8. A, Summary statistics for nongrowing-season and growing-season loads of total phosphorus and orthophosphorus, in kilograms, in wastewater-treatment-plant effluent and at water-quality-monitoring stations on the Assabet River, October 2008 through December 2010; and B, summary statistics for differences between particulate phosphorus loads at upstream and downstream water-quality-monitoring stations at the Hudson and Ben Smith impoundments on the Assabet River, October 2008 through December 2010. [Locations of monitoring stations are shown in figures 1–3. All data are rounded to two significant figures. Values in part B were derived by using the differences in loads from individual weekly sampling periods. Negative values indicate that the downstream-load value was larger than the upstream-load value. WWTP, wastewater-treatment plant; TP, total phosphorus; OrthoP, orthophosphorus; --, no data] A. Total phosphorus and orthophosphorus, in kilograms Westborough WWTP Marlborough Westerly WWTP Hudson WWTP Port Street 01096835 AMVETS 01096840 Stow 01096885 Maynard 01097000 TP OrthoP TP OrthoP TP OrthoP TP OrthoP TP OrthoP TP OrthoP TP OrthoP Nongrowing season (November 1 through March 31) Maximum 500 360 530 160 260 57 960 220 1,300 200 1,400 250 1,700 230 Minimum 23 24 10 1.4 2.3 3.0 71 18 74 14 74 14 110 14 Median 98 87 50 23 44 16 240 56 280 66 280 66 270 63 Mean 120 95 78 39 49 19 280 66 340 72 340 73 360 73 Growing season (April 1 through October 31) Maximum 260 200 160 81 180 -- 540 63 620 200 1,100 240 1,100 230 Minimum 6.9 28 7.9 1.8 1.9 -- 16 2.5 17 2.1 8.3 1.0 16 2.9 Median 73 67 24 9.0 16 -- 140 17 110 25 140 27 190 41 Mean 78 75 28 14 30 -- 180 19 150 33 210 45 280 66 B. Particulate phosphorus Hudson impoundment Ben Smith impoundment Nongrowing season (November 1 through March 31)

Maximum 290 85Minimum -190 -510Median 26 -8.0Mean 28 -40

Growing season (April 1 through October 31)

Maximum 230 140Minimum -71 -96Median 28 -18Mean 45 -7
Summary 19

Under conditions associated with large increases in 

streamflow through the Hudson impoundment, there are 

marked net decreases of both PP and orthoP loads in the 

impoundments during sampling periods, probably as a result 

of scouring caused by the increased stream velocities: PP that 

may have settled in the impoundment is resuspended, and 

dissolved orthoP that may have accumulated in elevated concentrations in isolated parts of the impoundment is exported. 

More orthoP would likely be entrained from relatively stagnant areas of the impoundments under high flow conditions 

than under a normal flow regime.

Interpretation of the phosphorus dynamics is less 

clear for the Ben Smith impoundment than for the Hudson 

impoundment. TP and orthoP loads are usually larger downstream from the Ben Smith impoundment than upstream. In a 

manner similar to that observed in the Hudson impoundment, 

however, under high-flow conditions, both TP and orthoP 

loads downstream from the Ben Smith impoundment increase 

substantially over the upstream loads, indicating that stormrelated streamflows play an important role in the distribution 

of riverine phosphorus loads, including the redistribution of 

loads previously retained in impoundments.

Differences in net loading patterns (retention versus 

loss) between the two impoundments may be explained, 

at least in part, by physical characteristics: (1) the Hudson 

impoundment has only one small tributary; (2) the Ben 

Smith impoundment has several tributaries; among them, the 

largest drains about 20 square miles (mi2) and another drains 

about 4.6 mi2; (3) retention times for the two impoundments 

would be expected to differ given their differences in size 

and morphology; and (4) the Assabet River downstream 

from the Ben Smith impoundment flows through the town of 

Maynard before reaching the downstream monitoring station. 

Runoff from the unmonitored tributaries and from downtown 

Maynard may transport nonpoint-source phosphorus loads 

into the river that contribute to the downstream increases in 

TP and orthoP loads determined from samples collected at the 

downstream monitoring station (Maynard).

A large proportion of the TP load in the river, roughly 

80 percent, is in the form of PP. The PP may settle out of the 

water column to the river bed in free-flowing and impounded 

reaches. When stream velocities at the river bed in freeflowing reaches become fast enough, the particles may be 

resuspended and transported farther downstream. To mobilize 

or scour particulates in the deeper areas of the river, primarily 

in its impoundments, stream velocities there would also have 

to increase. 

Although orthoP is not subject to settling, it may be 

absorbed by aquatic plants and thus removed from the water 

column. During the warm parts of the growing season, when 

conditions are optimal for growth and accumulation of floating 

macrophytes, substantial amounts of dissolved phosphorus 

may be rapidly recycled in the impoundments and either 

sequestered as living biomass or converted into PP. These 

warm periods may be characterized by extremely low streamflow dominated by WWTP discharges that may support large 

amounts nuisance-plant growth and accumulation. Without 

additional sampling, the magnitudes of orthoP uptake, transformation, and release in the impoundments and along the 

free-flowing reaches of the river can only be hypothesized.

Finally, for the two nongrowing seasons that were part 

of this study, seasonal changes in loadings from the WWTPs 

did not seem to affect the patterns of loads entering and leaving the two impoundments. During both of the nongrowing 

seasons, the loads of TP entering the Hudson impoundment 

were usually greater than the loads leaving it, and the orthoP 

loads leaving the impoundment were usually greater than 

those entering. At the Ben Smith impoundment, TP and orthoP 

loads leaving the impoundment were slightly greater than 

loads entering the impoundment during both the nongrowing 

and growing seasons. However, at both impoundments during 

the nongrowing seasons, the loads themselves are larger, especially the maximum loads, than in the growing season, but the 

net differences do not change very much between seasons. 

Regarding the questions posed at the beginning of 

this section, it is likely that the impoundments do sequester 

phosphorus, but the size and complexity of the Ben Smith 

impoundment and its watershed in conjunction with multiple 

potential instances of settling and resuspension of PP make 

such an interpretation that impoundment difficult on the basis 

of existing data. Similarly, it is also likely that phosphorus 

forms change in the impoundments, as exemplified by the data 

from the Hudson impoundment. Seasonally, no net change in 

the patterns of sequestration or transformation could be determined for the two impoundments studied. These questions 

could be reevaluated after the effects of changes in phosphorus 

loadings associated with the upgraded WWTPs are monitored 

and interpreted. Implications of the findings from the current 

study point to the importance of the impoundments in affecting the transport, retention, and transformation of phosphorus 

loads in the Assabet River.

Summary

Although the Assabet River has, in part, been designated 

as a National Wild and Scenic River, the Massachusetts 

Department of Environmental Protection (MassDEP) has 

also assessed the Assabet River as having impaired water 

quality—an effect of excessive nutrient-containing effluents 

from municipal wastewater-treatment plants in the towns 

of Westborough, Marlborough, Hudson, and Maynard, 

Massachusetts. In 2004, the U.S. Environmental Protection 

Agency approved the “Assabet River Total Maximum Daily 

Load for Total Phosphorus” developed by the MassDEP, 

which required that the wastewater-treatment plants decrease 

the loads of phosphorus in their discharges to the river, and the 

Department set the limits for those phosphorus discharges. 

To document baseline phosphorus loads prior to the 

implementation of the restrictions and to determine if 

impoundments affected the phosphorus concentrations and 
20 Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts

loads in the river, the U.S. Geological Survey, in cooperation 

with the MassDEP, measured streamflow and collected flowproportional total phosphorus and orthophosphorus samples, 

composited for one-week periods, at four locations along the 

Assabet River in the towns of Hudson, Stow, and Maynard 

from October 2008 to December 2010. The sampling sites 

were located upstream and downstream from each of two 

impoundments, and the differences in total phosphorus and 

orthophosphorus loads upstream and downstream from each 

impoundment were assessed.

For most of the study period, the results for the comparatively small upstream impoundment (Hudson) indicated that 

median estimated weekly loads of total phosphorus entering 

the impoundment (220 kilograms (kg)) were larger than loads 

leaving it (170 kg); median weekly estimated orthophosphorus 

loads leaving the impoundment were larger (36 kg) than those 

entering it (25 kg). At the larger downstream impoundment 

(Ben Smith), the differences were not as substantial relative 

to those of the smaller Hudson impoundment, and median 

weekly total phosphorus and orthophosphorus loads leaving 

the impoundment (240 and 53 kg, respectively) usually were 

larger than those entering (214 and 44 kg, respectively). At 

both impoundments, under high flow conditions, total phosphorus and orthophosphorus loads leaving the impoundments 

were greater than upstream loads. 

During nongrowing-season periods when limits on treatment-plant-discharge concentrations were higher than growing-season limits, median loads at the Hudson impoundment 

were roughly double those during growing-season periods, but 

the net differences in upstream and downstream loads did not 

change substantially between those seasons. At the Ben Smith 

impoundment, the median loads at the upstream Stow monitoring station during the nongrowing seasons were also double 

the median loads during growing seasons. At the downstream 

Maynard monitoring station, the differences in median total 

phosphorus and orthophosphorus loads between nongrowing 

and growing seasons were smaller, but nongrowing-season 

loads were still larger than growing-season loads for both total 

phosphorus and orthophosphorus. 

The differences in loads upstream and downstream from 

the Hudson impoundment provided evidence that particulate phosphorus was sequestered in the impoundment during 

most of the sampling period. Dissolved orthophosphorus was 

produced in the impoundment by physical and (or) biogeochemical processes and, in combination with orthophosphorus 

transported into and out of the impoundment, created a net 

export of orthophosphorus from the impoundment. The small 

differences between inflowing and outflowing loads at the 

comparatively large downstream impoundment (Ben Smith) 

suggest that additional factors may have had effects on the 

observed load at that location. Such factors could include a 

possible longer retention time at the Ben Smith impoundment 

than at the Hudson impoundment, loads from unmonitored 

tributaries, and possible nonpoint sources of phosphorus in the 

town of Maynard, which is downstream from the impoundment and upstream from the monitoring station.

This project has provided baseline information on total 

phosphorus, orthophosphorus, and particulate phosphorus 

loads for comparison with future assessments of phosphorus 

loads in the Assabet River under new, lower concentration 

limits for total phosphorus. In addition, the study has demonstrated the value of using flow-proportional, composite 

monitoring in evaluating nutrient loads in rivers. Phosphorus 

dynamics within impoundments on the Assabet River, including the effects of complex physical, hydrologic, chemical, and 

biological factors, play an important—but variable—role in 

affecting the form, timing, and magnitude of phosphorus loads 

in the river.

References Cited

Camp, Dresser, and McKee (CDM), 2007, Assabet River 

sediment and dam removal study, draft modeling report: 

Cambridge, Mass., CDM, variously paged.

Commonwealth of Massachusetts, 2004, Assabet River total 

maximum daily load for total phosphorus: Boston, Mass., 

Massachusetts Department of Environmental Protection, 

report no. MA82B–01–2004–01, 104 p., accessed 

January 15, 2013, at http://www.mass.gov/dep/water/

resources/anuttmdl.pdf.

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. (Also 

available at http://pubs.usgs.gov/sir/2004/5114/.)

ENSR International, 2001, SuAsCo watershed Assabet River 

TMDL study phase one—Assessment final report: Westford, 

Mass., ENSR International, variously paged.

ENSR International, 2005, SuAsCo watershed Assabet River 

TMDL study phase two—Analysis final report: Westford, 

Mass., ENSR International, variously paged.

Fishman, M.J., ed., 1993, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of inorganic and organic constituents in 

water and fluvial sediments: U.S. Geological Survey OpenFile Report 93–125, 217 p.

McAdow, Ron, 1990, The Concord, Sudbury, and Assabet 

Rivers—A guide to canoeing, wildlife, and history: 

Marlborough, Mass., Bliss Publishing Company, Inc., 

223 p.

Mueller, D.K., and Titus, C.J., 2005, Quality of nutrient data 

from streams and ground water sampled during water years 

1992–2001: U.S Geological Survey Scientific Investigations 

Report 2005–5106, 27 p. (Also available at http://pubs.usgs.

gov/sir/2005/5106/.)
References Cited 21

National Park Service, 2006, The Sudbury, Assabet and 

Concord (SUASCO) Wild and Scenic Rivers: National Park 

Service, accessed August 24, 2012, at http://www.nps.gov/

ncrc/programs/pwsr/suasco_pwsr_sub.html.

Robertson, D.M., and Roerish, E.D., 1999, Influence of vari-

ous water quality sampling strategies on load estimates for 

small streams: Water Resources Research, v. 35, no. 12, 

p. 3747–3759.

Schleppi, Patrick, Waldner, P.A., and Stähli, Manfred, 2006, 

Errors of flux methods for solutes in grab samples of runoff 

water, as compared to flow-proportional sampling: Journal 

of Hydrology, v. 319, p. 266–281.

Turnipseed, D.P., and Sauer, V.B., 2010, Discharge 

measurements at gaging stations: U.S. Geological Survey 

Techniques and Methods, book 3, chap. A8, 87 p. (Also 

available at http://pubs.usgs.gov/tm/tm3-a8/.)

U.S. Environmental Protection Agency, 1993, Method 

365.1, Determination of phosphorus by semi-automated 

colorimetry revision 2.0—Methods for the determination 

of inorganic substances in environmental samples: U.S. 

Environmental Protection Agency, EPA/600/R–93/100, 

172 p.

Winter, T.C., 1981, Uncertainties in estimating the water 

balance of lakes: Water Resources Bulletin, v. 17, no. 1, 

p. 82–115.

Zimmerman, M.J., and Sorenson, J.R., 2005, Sediment stud-

ies in the Assabet River, central Massachusetts, 2003: 

U.S. Geological Survey Scientific Investigations Report 

2005–5131, 87 p. (Also available at http://pubs.usgs.gov/

sir/2005/5131/.)

Zimmerman, M.J., Yu, Qian, and Tian, Y.Q., 2011, Monitoring 

to assess progress toward meeting the Assabet River, 

Massachusetts, phosphorus total maximum daily load—

Aquatic macrophyte biomass and sediment-phosphorus 

flux: U.S. Geological Survey Scientific Investigations 

Report 2011–5179, 77 p. (Also available at http://pubs.usgs.

gov/sir/2011/5179/.)
THIS PAGE INTENTIONALLY LEFT BLANK
Tables 3–6
24 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 3. Concentrations of total phosphorus and orthophosphorus in weekly composite samples collected at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through December 2010.

[Locations of monitoring stations are shown in figures 1 to 3. mg/L, milligrams per liter; P, phosphorus; --, no data; E, estimated concentration at or less than 

method detection level; <, less than value shown]

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Port Street - 01096835

10/01/08 0.077 --

10/07/08 0.086 --

10/15/08 0.080 E 0.008

11/11/08 0.134 0.038

11/18/08 0.189 0.056

11/25/08 0.116 0.029

12/02/08 0.099 0.024

12/09/08 0.084 0.020

12/16/08 0.054 E 0.005

12/23/08 0.048 0.011

12/30/08 0.042 0.011

01/06/09 0.054 0.012

01/13/09 0.100 0.022

01/20/09 0.105 0.025

01/27/09 0.121 0.026

02/03/09 0.101 0.018

02/10/09 0.085 0.009

02/17/09 0.082 0.018

02/24/09 0.071 0.011

03/03/09 0.080 0.013

03/10/09 0.064 E 0.007

03/17/09 0.100 0.017

03/24/09 0.092 E 0.008

03/31/09 0.084 0.007

04/07/09 0.068 E 0.006

04/14/09 0.108 0.009

04/21/09 0.098 E 0.005

04/28/09 0.167 0.010

05/05/09 0.163 E 0.008

05/12/09 0.182 0.013

05/19/09 0.162 E 0.007

05/26/09 0.140 --

06/02/09 0.135 E 0.007

06/09/09 0.156 0.013

06/15/09 0.205 0.020

06/23/09 0.190 0.019

06/30/09 0.155 --

07/06/09 0.088 --

07/13/09 0.101 0.009

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Port Street - 01096835—Continued

07/21/09 0.085 0.010

07/28/09 0.088 0.010

08/04/09 0.110 0.012

08/11/09 0.104 0.018

08/18/09 0.102 0.012

08/25/09 0.104 0.025

09/01/09 0.084 0.014

09/08/09 0.079 0.014

09/15/09 0.070 0.015

09/22/09 0.069 0.012

09/29/09 0.074 0.015

10/06/09 0.067 0.012

10/13/09 0.070 E 0.006

10/20/09 0.085 0.012

10/27/09 0.101 0.016

11/03/09 0.110 0.027

11/10/09 0.194 0.056

11/17/09 0.109 0.024

11/24/09 0.100 0.016

12/01/09 0.122 0.015

12/08/09 0.134 0.013

12/15/09 0.081 0.010

12/22/09 0.083 E 0.008

12/29/09 0.077 0.011

01/05/10 0.079 0.011

01/12/10 0.106 0.013

01/19/10 0.176 0.017

01/26/10 0.070 0.014

02/02/10 0.086 0.014

02/09/10 0.109 0.017

02/16/10 0.082 0.012

02/23/10 0.085 0.012

03/02/10 0.050 0.009

03/09/10 0.052 0.008

03/16/10 0.035 0.008

03/23/10 0.036 0.008

03/30/10 0.033 E 0.007

04/06/10 0.053 E 0.008

04/13/10 0.077 E 0.008
Table 3 25

Table 3. Concentrations of total phosphorus and orthophosphorus in weekly composite samples collected at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through December 2010.

—Continued

[Locations of monitoring stations are shown in figures 1 to 3. mg/L, milligrams per liter; P, phosphorus; --, no data; E, estimated concentration at or less than 

method detection level; <, less than value shown]

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Port Street - 01096835—Continued

04/20/10 0.069 E 0.007

04/27/10 0.129 0.011

05/04/10 0.168 0.015

05/11/10 0.126 0.013

05/18/10 0.183 0.014

05/25/10 0.166 0.016

06/01/10 0.196 0.018

06/08/10 0.207 0.023

06/15/10 0.154 --

06/22/10 0.131 0.014

07/06/10 0.119 0.012

07/13/10 0.110 0.011

07/20/10 0.163 --

07/27/10 0.198 --

08/03/10 0.126 0.021

08/10/10 0.138 0.018

08/17/10 0.107 0.016

08/24/10 0.128 0.025

08/31/10 0.127 0.015

09/07/10 0.127 0.018

09/14/10 0.176 0.020

09/21/10 0.189 0.020

09/28/10 0.165 0.041

10/05/10 0.138 --

10/12/10 0.125 0.011

10/19/10 0.133 0.015

10/26/10 0.180 0.014

11/02/10 0.166 --

11/09/10 0.208 0.018

11/16/10 0.321 0.026

11/23/10 0.180 0.023

11/30/10 0.224 0.014

12/07/10 0.187 --

12/14/10 0.176 0.014

12/21/10 0.160 --

12/28/10 0.179 0.021

AMVETS - 01096840

10/01/08 0.082 0.020

10/07/08 0.079 0.019

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

AMVETS - 01096840—Continued

10/15/08 0.081 0.015

10/21/08 0.091 0.027

10/28/08 0.071 0.021

11/04/08 0.098 0.028

11/11/08 0.131 0.048

11/18/08 0.171 0.067

11/25/08 0.098 0.035

12/02/08 0.093 0.027

12/09/08 0.105 0.026

12/16/08 0.065 0.011

12/23/08 0.052 0.014

12/30/08 0.045 0.012

01/06/09 0.055 0.012

01/13/09 0.115 0.035

01/20/09 0.097 0.014

01/27/09 0.112 0.017

02/03/09 0.086 0.024

02/10/09 0.066 0.015

02/17/09 0.068 0.016

02/24/09 0.057 0.015

03/03/09 0.062 0.015

03/10/09 0.052 0.015

03/17/09 0.083 0.023

03/24/09 0.077 0.023

03/31/09 0.064 0.015

04/07/09 0.054 0.009

04/14/09 0.082 0.009

04/21/09 0.066 0.013

04/28/09 0.107 0.013

05/05/09 0.089 0.021

05/12/09 0.100 0.024

05/19/09 0.096 0.017

05/26/09 0.082 0.015

06/02/09 0.089 0.014

06/09/09 0.114 0.027

06/15/09 0.121 0.025

06/23/09 0.129 0.038

06/30/09 0.142 0.028

07/06/09 0.109 0.027
26 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 3. Concentrations of total phosphorus and orthophosphorus in weekly composite samples collected at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through December 2010.

—Continued

[Locations of monitoring stations are shown in figures 1 to 3. mg/L, milligrams per liter; P, phosphorus; --, no data; E, estimated concentration at or less than 

method detection level; <, less than value shown]

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

AMVETS - 01096840—Continued

07/13/09 0.094 0.016

07/21/09 0.086 0.027

07/28/09 0.096 0.032

08/04/09 0.098 0.017

08/11/09 0.090 0.025

08/18/09 0.090 0.023

08/25/09 0.095 --

09/01/09 0.081 0.021

09/08/09 0.076 0.023

09/15/09 0.076 --

09/22/09 0.075 0.014

09/29/09 0.067 0.016

10/06/09 0.070 0.016

10/13/09 0.059 0.010

10/20/09 0.070 0.016

10/27/09 0.077 0.012

11/03/09 0.096 0.015

11/10/09 0.142 0.018

11/17/09 0.082 --

11/24/09 0.064 0.017

12/01/09 0.083 0.019

12/08/09 0.066 0.015

12/15/09 0.063 --

12/22/09 0.047 0.008

01/05/10 0.089 0.013

01/12/10 0.082 0.019

01/19/10 0.086 0.020

01/26/10 0.072 0.015

02/09/10 0.094 0.020

02/16/10 0.068 0.011

02/23/10 0.086 0.012

03/02/10 0.045 0.010

03/09/10 0.054 0.009

03/16/10 0.038 0.009

03/23/10 0.036 0.010

03/30/10 0.044 0.011

04/06/10 0.043 0.012

04/13/10 0.052 0.009

04/20/10 0.085 0.013

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

AMVETS - 01096840—Continued

04/27/10 0.055 E 0.008

05/04/10 0.091 0.015

05/11/10 0.076 0.013

05/18/10 0.093 0.016

05/25/10 0.113 0.017

06/01/10 0.124 0.024

06/08/10 0.110 --

06/15/10 0.111 0.022

06/22/10 0.105 0.018

06/29/10 0.123 --

07/06/10 0.120 0.013

07/13/10 0.121 0.015

07/20/10 0.149 --

07/27/10 0.191 0.025

08/03/10 0.162 0.024

08/10/10 0.125 0.014

08/17/10 0.100 0.016

08/24/10 0.098 0.026

08/31/10 0.146 0.019

09/07/10 0.369 0.026

09/14/10 0.150 0.024

09/21/10 0.249 0.021

09/28/10 0.169 0.040

10/05/10 0.101 0.026

10/12/10 0.092 0.014

10/19/10 0.067 0.018

10/26/10 0.073 0.018

11/02/10 0.078 0.022

11/09/10 0.085 0.022

11/16/10 0.115 0.032

11/23/10 0.118 0.033

11/30/10 0.101 0.020

Stow - 01096885

10/01/08 0.092 0.026

10/07/08 0.073 0.015

10/15/08 0.080 0.021

10/21/08 0.080 --

10/28/08 0.074 0.020

11/04/08 0.094 0.024
Table 3 27

Table 3. Concentrations of total phosphorus and orthophosphorus in weekly composite samples collected at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through December 2010.

—Continued

[Locations of monitoring stations are shown in figures 1 to 3. mg/L, milligrams per liter; P, phosphorus; --, no data; E, estimated concentration at or less than 

method detection level; <, less than value shown]

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Stow - 01096885—Continued

11/11/08 0.112 0.032

11/18/08 0.135 0.056

11/25/08 0.107 0.027

12/02/08 0.086 0.022

12/09/08 0.119 0.022

12/16/08 0.057 0.017

12/23/08 0.054 0.017

12/30/08 0.043 0.015

01/06/09 0.063 0.015

01/13/09 0.080 0.024

01/20/09 0.082 0.027

01/27/09 0.093 0.032

02/03/09 0.078 0.022

02/10/09 0.069 <0.008

02/17/09 0.062 0.018

02/24/09 0.066 0.018

03/03/09 0.068 0.017

03/10/09 0.066 E 0.007

03/17/09 0.087 0.022

03/24/09 0.083 0.017

03/31/09 0.084 0.021

04/07/09 0.063 0.012

04/14/09 0.093 0.011

04/21/09 0.092 0.009

04/28/09 0.149 --

05/05/09 0.147 0.012

05/12/09 0.153 0.033

05/19/09 0.132 0.012

05/26/09 0.125 0.012

06/02/09 0.114 0.014

06/09/09 0.130 0.029

06/15/09 0.156 0.036

06/23/09 0.176 --

06/30/09 0.186 0.069

07/06/09 0.142 0.032

07/13/09 0.114 0.023

07/21/09 0.109 0.028

07/28/09 0.118 0.027

08/04/09 0.115 0.020

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Stow - 01096885—Continued

08/11/09 0.106 0.038

08/18/09 0.092 0.039

08/25/09 0.099 0.038

09/01/09 0.076 0.030

09/08/09 0.059 --

09/15/09 0.067 --

09/22/09 0.059 0.018

09/29/09 0.054 0.014

10/06/09 0.074 --

10/13/09 0.058 E 0.005

10/20/09 0.063 0.016

10/27/09 0.076 0.014

11/10/09 0.126 0.040

11/17/09 0.084 0.027

11/24/09 0.074 0.026

12/01/09 0.107 E 0.021

12/08/09 0.082 0.016

12/15/09 0.057 0.014

12/22/09 0.067 0.012

12/29/09 0.048 0.011

01/05/10 0.053 0.013

01/12/10 0.079 0.015

01/19/10 0.085 0.011

01/26/10 0.067 0.009

02/02/10 0.060 0.014

02/09/10 0.075 0.013

02/16/10 0.064 0.008

02/23/10 0.099 0.008

03/02/10 0.045 0.008

03/09/10 0.068 E 0.007

03/16/10 0.039 0.008

03/23/10 0.035 0.009

03/30/10 0.043 0.009

04/06/10 0.054 0.012

04/13/10 0.056 E 0.007

04/20/10 0.058 E 0.006

04/27/10 0.079 E 0.007

05/04/10 0.093 0.021

05/11/10 0.074 0.013
28 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 3. Concentrations of total phosphorus and orthophosphorus in weekly composite samples collected at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through December 2010.

—Continued

[Locations of monitoring stations are shown in figures 1 to 3. mg/L, milligrams per liter; P, phosphorus; --, no data; E, estimated concentration at or less than 

method detection level; <, less than value shown]

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Stow - 01096885—Continued

05/18/10 0.082 0.015

05/25/10 0.070 0.017

06/01/10 0.097 0.015

06/08/10 0.117 0.035

06/15/10 0.105 0.025

06/22/10 -- 0.016

06/29/10 0.070 --

07/06/10 0.068 0.013

07/13/10 0.063 0.012

07/20/10 0.062 0.009

07/27/10 0.086 0.009

08/03/10 0.068 E 0.006

08/10/10 0.050 E 0.006

08/17/10 0.043 E 0.007

08/24/10 0.059 0.010

08/31/10 0.041 0.010

09/07/10 0.045 E 0.005

09/14/10 0.089 < 0.008

09/21/10 0.109 0.009

09/28/10 0.113 0.020

10/05/10 0.083 --

10/12/10 0.074 0.010

10/19/10 0.054 0.011

10/26/10 0.057 0.011

11/02/10 0.064 0.013

11/09/10 0.066 0.012

11/16/10 0.086 0.016

11/23/10 0.087 0.019

11/30/10 0.074 0.013

12/07/10 0.112 0.013

12/14/10 0.105 0.015

12/21/10 0.098 0.010

12/28/10 0.105 0.014

Maynard - 01097000

10/02/08 0.090 --

10/07/08 0.064 0.018

10/15/08 0.080 0.015

10/21/08 0.067 0.010

10/28/08 0.059 0.015

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Maynard - 01097000—Continued

11/04/08 0.071 0.016

11/11/08 0.078 0.023

11/18/08 0.088 0.019

11/25/08 0.091 0.024

12/02/08 0.065 0.015

12/23/08 0.045 0.013

12/30/08 0.033 0.013

01/06/09 0.041 0.009

01/13/09 -- 0.010

01/27/09 0.076 0.021

02/03/09 0.067 0.021

02/10/09 0.050 0.013

02/17/09 0.050 0.013

02/24/09 0.045 0.010

03/03/09 0.045 0.010

03/10/09 0.040 E 0.004

03/17/09 0.058 E 0.021

03/24/09 0.070 0.017

03/31/09 0.063 0.016

04/07/09 0.050 0.009

04/14/09 0.066 0.015

04/21/09 0.060 0.012

04/28/09 0.091 0.021

05/05/09 0.093 0.024

05/12/09 0.091 0.027

05/19/09 0.084 0.021

05/26/09 0.088 0.021

06/02/09 0.070 0.019

06/09/09 0.102 0.022

06/15/09 0.109 0.034

06/23/09 0.358 0.064

06/30/09 0.138 0.039

07/06/09 0.110 0.026

07/13/09 0.107 0.026

07/21/09 0.112 0.027

07/28/09 0.115 0.025

08/04/09 0.114 0.036

08/11/09 0.096 0.035

08/18/09 0.078 --
Table 3 29

Table 3. Concentrations of total phosphorus and orthophosphorus in weekly composite samples collected at water-quality-monitoring 

stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 through December 2010.

—Continued

[Locations of monitoring stations are shown in figures 1 to 3. mg/L, milligrams per liter; P, phosphorus; --, no data; E, estimated concentration at or less than 

method detection level; <, less than value shown]

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Maynard - 01097000—Continued

08/25/09 0.090 0.031

09/01/09 0.065 0.025

09/08/09 0.056 0.018

09/15/09 0.049 --

09/22/09 0.043 0.016

09/29/09 0.058 0.009

10/06/09 0.066 0.018

10/13/09 0.069 0.014

10/20/09 0.060 0.009

10/27/09 0.073 0.012

11/03/09 0.070 0.023

11/10/09 0.109 0.026

11/17/09 0.070 0.022

11/24/09 0.071 0.020

12/01/09 0.076 0.013

12/08/09 0.056 0.011

12/15/09 0.044 0.008

12/22/09 0.052 0.011

12/29/09 0.041 --

01/05/10 0.042 0.009

01/12/10 0.060 0.016

01/19/10 0.063 0.009

01/26/10 0.059 0.011

02/09/10 0.057 0.017

02/16/10 0.052 0.011

02/23/10 0.110 0.012

03/02/10 0.039 --

03/09/10 0.082 0.009

03/16/10 0.049 E 0.007

Weekly start date Total phosphorus

(mg/L)

Orthophosphorus 

(mg/L as P)

Maynard - 01097000—Continued

03/23/10 0.036 0.009

03/30/10 0.057 0.010

04/06/10 0.049 0.013

04/13/10 0.051 0.009

04/20/10 0.052 0.008

04/27/10 0.065 0.011

05/04/10 0.081 0.016

05/11/10 0.065 0.015

05/18/10 0.065 0.013

05/25/10 0.077 --

06/01/10 0.091 0.011

06/15/10 0.080 0.019

06/22/10 -- 0.011

06/29/10 0.046 --

07/06/10 0.052 E 0.008

07/13/10 0.048 --

07/27/10 0.045 <0.008

09/28/10 0.086 0.012

10/05/10 0.078 0.007

10/12/10 0.077 0.009

11/02/10 0.073 0.009

11/09/10 0.055 0.010

11/16/10 0.082 0.010

11/23/10 0.069 0.014

11/30/10 0.061 0.011

12/07/10 0.092 --

12/21/10 0.074 0.013

12/28/10 0.076 0.009
30 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 4. Concentrations of total phosphorus and orthophosphorus in blank samples, point samples, and replicate samples collected 

at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 

through December 2010.

[Locations of monitoring stations are shown in figures 1 to 3. Relative percent differences compare concentrations in automated grab samples with concentrations in corresponding manually collected grab samples and in split-replicate composite samples. Time is given as military (24-hour) time. Sample type: OGS, 

outside grab sample; IGS, inside grab sample; C, composite sample; R, replicate (split composite sample); mg/L, milligrams per liter; P, phosphorus; --, no data; 

<, less than value shown]

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

11/04/08 1549 Blank <0.008 --

01/06/09 1134 Blank <0.008 <0.006

03/10/09 1139 Blank <0.008 <0.008

06/15/09 1304 Blank <0.008 <0.008

08/11/09 1349 Blank <0.008 <0.008

03/02/10 1214 Blank <0.008 --

06/15/10 1229 Blank <0.008 <0.008

Port Street - 01096835

10/01/08 930 IGS 0.083 0.046

10/01/08 940 OGS 0.078 0.041

Relative percent difference 6.2 11.5

10/15/08 1055 IGS 0.081 0.031

10/15/08 1100 OGS 0.083 0.034

Relative percent difference 2.4 9.2

11/26/08 1325 IGS 0.144 0.070

11/26/08 1335 OGS 0.143 0.069

Relative percent difference 0.7 1.4

12/09/08 1040 IGS 0.134 0.094

12/09/08 1050 OGS 0.140 0.102

Relative percent difference 4.4 8.2

03/31/09 1135 IGS 0.083 0.040

03/31/09 1145 OGS 0.085 0.044

Relative percent difference 2.4 9.5

04/28/09 1130 IGS 0.103 0.037

04/28/09 1140 OGS 0.087 0.040

Relative percent difference 16.8 7.8

11/11/08 1040 C 0.134 0.038

11/11/08 1041 R 0.133 0.038

Relative percent difference 0.7 0

12/09/08 1035 C 0.084 0.020

12/09/08 1036 R 0.085 0.020

Relative percent difference 1.2 0

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

Port Street - 01096835—Continued

01/27/09 1030 C 0.121 0.026

01/27/09 1031 R 0.121 0.026

Relative percent difference 0 0

05/12/09 1055 C 0.182 0.013

05/12/09 1056 R 0.172 0.013

Relative percent difference 5.6 0

06/23/09 1145 C 0.190 0.019

06/23/09 1146 R 0.194 0.018

Relative percent difference 2.1 5.4

07/13/09 1505 C 0.101 0.009

07/13/09 1506 R 0.100 0.009

Relative percent difference 1.0 0

08/04/09 925 C 0.110 0.012

08/04/09 926 R 0.109 0.011

Relative percent difference 0.9 8.7

09/15/09 1020 C 0.070 0.015

09/15/09 1021 R 0.072 0.016

Relative percent difference 2.8 6.5

04/27/10 1045 C 0.129 0.011

04/27/10 1046 R 0.126 0.011

Relative percent difference 2.4 0

08/03/10 1130 C 0.126 0.021

08/03/10 1131 R 0.124 0.021

Relative percent difference 1.6 0

08/24/10 950 C 0.128 0.025

08/24/10 951 R 0.128 0.023

Relative percent difference 0 8.3

11/16/10 945 C 0.321 0.026

11/16/10 946 R 0.320 0.026

Relative percent difference 0.3 0
Table 4 31

Table 4. Concentrations of total phosphorus and orthophosphorus in blank samples, point samples, and replicate samples collected 

at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 

through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3. Relative percent differences compare concentrations in automated grab samples with concentrations in corresponding manually collected grab samples and in split-replicate composite samples. Time is given as military (24-hour) time. Sample type: OGS, 

outside grab sample; IGS, inside grab sample; C, composite sample; R, replicate (split composite sample); mg/L, milligrams per liter; P, phosphorus; --, no data; 

<, less than value shown]

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

AMVETS - 01096840

10/01/08 1020 IGS 0.085 0.046

10/01/08 1010 OGS 0.095 0.049

Relative percent difference 11.1 6.3

10/15/08 1130 IGS 0.089 --

10/15/08 1120 OGS 0.096 0.036

Relative percent difference 7.6 --

11/04/08 1615 IGS 0.071 0.035

11/04/08 1610 OGS 0.071 0.037

Relative percent difference 0 5.6

11/04/08 1616 IGS-R 0.072 0.037

11/04/08 1611 OGS-R 0.072 0.036

Relative percent difference 0 2.7

11/26/08 1305 IGS 0.132 0.070

11/26/08 1255 OGS 0.143 0.069

Relative percent difference 8.0 1.4

03/31/09 1210 IGS 0.076 0.040

03/31/09 1200 OGS 0.077 0.042

Relative percent difference 1.3 4.9

04/28/09 1210 IGS 0.096 0.040

04/28/09 1200 OGS 0.092 0.040

Relative percent difference 4.3 0

10/07/08 1110 C 0.079 0.019

10/07/08 1111 R 0.078 0.018

Relative percent difference 1.3 5.4

10/21/08 1425 C 0.091 0.027

10/21/08 1426 R 0.090 0.026

Relative percent difference 1.1 3.8

11/18/08 1000 C 0.171 0.067

11/18/08 1001 R 0.170 0.068

Relative percent difference 0.6 1.5

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

AMVETS - 01096840—Continued

12/23/08 1135 C 0.052 0.014

12/23/08 1136 R 0.053 0.014

Relative percent difference 1.9 0

03/17/09 1155 C 0.083 0.023

03/17/09 1156 R 0.083 0.023

Relative percent difference 0 0

05/26/09 1345 C 0.082 0.015

05/26/09 1346 R 0.082 0.014

Relative percent difference 0 6.9

07/06/09 1230 C 0.109 0.027

07/06/09 1231 R 0.108 0.029

Relative percent difference 0.9 7.1

08/11/09 1300 C 0.090 0.025

08/11/09 1301 R 0.090 0.025

Relative percent difference 0 0

07/13/10 1005 C 0.121 0.015

07/13/10 1006 R 0.126 0.016

Relative percent difference 4.0 6.5

10/12/10 1010 C 0.092 0.014

10/12/10 1011 R 0.093 0.014

Relative percent difference 1.1 0

10/26/10 1005 C 0.073 0.018

10/26/10 1006 R 0.073 0.018

Relative percent difference 0 0

Stow - 01096885

10/01/08 1050 IGS 0.106 0.051

10/01/08 1053 OGS 0.107 0.053

Relative percent difference 0.9 3.8

10/15/08 1240 IGS 0.081 0.039

10/15/08 1245 OGS 0.081 0.037

Relative percent difference 0 5.3
32 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 4. Concentrations of total phosphorus and orthophosphorus in blank samples, point samples, and replicate samples collected 

at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 

through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3. Relative percent differences compare concentrations in automated grab samples with concentrations in corresponding manually collected grab samples and in split-replicate composite samples. Time is given as military (24-hour) time. Sample type: OGS, 

outside grab sample; IGS, inside grab sample; C, composite sample; R, replicate (split composite sample); mg/L, milligrams per liter; P, phosphorus; --, no data; 

<, less than value shown]

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

Stow - 01096885—Continued

11/04/08 1530 IGS 0.074 0.037

11/04/08 1535 OGS 0.072 0.037

Relative percent difference 2.7 0

11/26/08 1110 IGS 0.148 0.063

11/26/08 1120 OGS 0.144 0.062

Relative percent difference 2.7 1.6

11/26/08 1111 IGS-R 0.142 0.062

11/26/08 1121 OGS-R 0.147 0.061

Relative percent difference 2.1 1.6

12/16/08 1115 IGS 0.070 0.026

12/16/08 1125 OGS 0.058 0.026

Relative percent difference 18.8 0

03/31/09 1235 IGS 0.088 0.032

03/31/09 1245 OGS 0.075 0.032

Relative percent difference 16.0 0

03/31/09 1236 IGS-R 0.073 0.032

03/31/09 1246 OGS-R 0.072 0.032

Relative percent difference 1.4 0

10/07/08 1140 C 0.073 0.015

10/07/08 1141 R 0.075 0.015

Relative percent difference 2.7 0

10/29/08 925 C 0.074 0.020

10/29/08 926 R 0.073 0.020

Relative percent difference 1.4 0

11/25/08 1340 C 0.107 0.027

11/25/08 1341 R 0.109 0.033

Relative percent difference 1.9 20.0

01/13/09 1410 C 0.080 0.024

01/13/09 1411 R 0.082 0.023

Relative percent difference 2.5 4.3

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

Stow - 01096885—Continued

03/31/09 1230 C 0.084 0.021

03/31/09 1231 R 0.084 0.021

Relative percent difference 0 0

04/14/09 1120 C 0.093 0.011

04/14/09 1121 R 0.093 0.011

Relative percent difference 0 0

06/15/09 1400 C 0.156 0.036

06/15/09 1401 R 0.156 0.035

Relative percent difference 0 2.8

07/21/09 1140 C 0.109 0.028

07/21/09 1141 R 0.108 0.029

Relative percent difference 0.9 3.5

08/25/09 1135 C 0.099 0.038

08/25/09 1136 R 0.097 0.038

Relative percent difference 2.0 0

03/02/10 1305 C 0.045 0.008

03/02/10 1306 R 0.045 0.009

Relative percent difference 0 11.8

06/15/10 1200 C 0.105 0.025

06/15/10 1201 R 0.105 0.026

Relative percent difference 0 3.9

09/07/10 1135 C 0.045 E .005

09/07/10 1136 R 0.044 E .004

Relative percent difference 2.2 --

12/07/10 1005 C 0.112 0.013

12/07/10 1006 R 0.111 0.014

Relative percent difference 0.9 7.4

12/14/10 1100 C 0.105 0.015

12/14/10 1101 R 0.104 0.014

Relative percent difference 1.0 6.9
Table 4 33

Table 4. Concentrations of total phosphorus and orthophosphorus in blank samples, point samples, and replicate samples collected 

at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, Massachusetts, October 2008 

through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3. Relative percent differences compare concentrations in automated grab samples with concentrations in corresponding manually collected grab samples and in split-replicate composite samples. Time is given as military (24-hour) time. Sample type: OGS, 

outside grab sample; IGS, inside grab sample; C, composite sample; R, replicate (split composite sample); mg/L, milligrams per liter; P, phosphorus; --, no data; 

<, less than value shown]

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

Maynard - 01097000

10/01/08 1028 IGS 0.125 0.037

10/01/08 1035 OGS 0.087 0.038

Relative percent difference 35.8 2.7

10/15/08 1310 IGS 0.066 0.029

10/15/08 1315 OGS 0.074 0.029

Relative percent difference 11.4 0

11/04/08 1530 IGS 0.058 0.025

11/04/08 1435 OGS 0.057 0.025

Relative percent difference 1.7 0

11/26/08 1145 IGS 0.135 0.060

11/26/08 1155 OGS 0.141 0.062

Relative percent difference 4.3 3.3

03/31/09 1310 IGS 0.060 0.025

03/31/09 1320 OGS 0.059 0.024

Relative percent difference 1.7 4.1

05/12/09 1225 IGS 0.094 0.040

05/12/09 1235 OGS 0.096 0.039

Relative percent difference 2.1 2.5

10/15/08 1320 C 0.080 0.015

10/15/08 1311 R 0.082 0.015

Relative percent difference 2.5 0

11/04/08 1500 C 0.071 0.016

11/04/08 1501 R 0.072 0.015

Relative percent difference 1.4 6.5

12/02/08 1325 C 0.065 0.015

12/02/08 1326 R 0.064 0.015

Relative percent difference 1.6 0

Date Time Sample 

type

Total 

phosphorus 

(mg/L)

Orthophosphorus 

(mg/L as P)

Maynard - 01097000—Continued

02/10/09 1335 C 0.050 0.013

02/10/09 1336 R 0.054 0.013

Relative percent difference 7.7 0

05/19/09 1230 C 0.084 0.021

05/19/09 1231 R 0.084 0.022

Relative percent difference 0 4.7

06/30/09 1315 C 0.138 0.039

06/30/09 1316 R 0.141 0.039

Relative percent difference 2.2 0

07/28/09 1210 C 0.115 0.025

07/28/09 1211 R 0.111 0.040

Relative percent difference 3.5 46.2

09/28/10 1130 C 0.086 0.012

09/28/10 1131 R 0.086 --

Relative percent difference 0 --

11/09/10 1110 C 0.055 0.010

11/09/10 1111 R 0.056 0.010

Relative percent difference 1.8 0

12/21/10 1110 C 0.074 0.013

12/21/10 1111 R 0.082 0.022

Relative percent difference 10.3 51.4
34 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 5. Estimated weekly total phosphorus and orthophosphorus loads, in kilograms, and summary statistics based on weekly 

composite samples collected at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts, October 2008 through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3; --, no data; summary statistics, rounded to two significant figures, are at the bottom of the table]

Weekly 

start date

Port Street - 01096835 AMVETS - 01096840 Stow - 01096885 Maynard - 01097000

Total

phosphorus

OrthophosphorusTotal

phosphorus

OrthophosphorusTotal

phosphorus

OrthophosphorusTotal

phosphorus

Orthophosphorus

10/1/2008 146 -- 1681 411 2991 851 3791 --

10/7/2008 1211 -- 1191 291 1701 351 1891 531

10/15/2008 671 71 721 131 1031 271 1251 231

10/21/2008 -- -- 140 42 197 39 161 24

10/28/2008 -- -- 126 62 176 -- 193 49

11/4/2008 -- -- 187 53 212 54 206 46

11/11/2008 203 57 208 76 222 63 199 59

11/18/2008 234 69 272 106 224 93 200 43

11/25/2008 397 99 354 126 450 113 496 131

12/2/2008 290 70 290 84 329 84 343 79

12/9/2008 713 170 8821 2181 1,0811 2001 -- --

12/16/2008 332 31 447 76 496 148 -- --

12/23/2008 306 70 369 99 432 136 450 130

12/30/2008 202 53 238 64 301 105 297 117

1/6/2009 213 47 232 51 359 86 298 65

1/13/2009 224 49 271 83 300 90 -- 48

1/20/2009 198 47 190 27 250 82 -- --

1/27/2009 247 53 239 36 288 99 299 83

2/3/2009 173 31 155 43 212 60 228 71

2/10/2009 242 26 200 45 273 32 249 65

2/17/2009 223 49 193 45 218 63 241 63

2/24/2009 241 37 209 55 306 83 270 60

3/3/2009 285 46 233 56 334 84 280 62

3/10/2009 290 32 270 78 426 45 329 33

3/17/2009 273 46 246 68 327 83 300 109

3/24/2009 227 20 201 60 271 56 303 74

3/31/2009 289 24 239 56 407 102 383 97

4/7/2009 264 23 224 37 326 62 340 61

4/14/2009 258 21 203 22 293 35 281 64

4/21/2009 339 17 245 48 420 41 357 71

4/28/2009 307 18 204 25 354 -- 297 69

5/5/2009 461 23 270 64 502 41 434 112

5/12/2009 344 25 194 47 336 72 279 83

5/19/2009 163 7 108 19 182 17 150 38

5/26/2009 139 -- 90 17 166 16 147 35

6/2/2009 88 5 68 11 95 12 76 21

6/9/2009 1971 161 1521 361 2041 451 1881 411

6/15/2009 422 41 224 46 327 75 339 106

6/23/2009 359 36 254 75 373 -- 1,114 199

6/30/2009 4271 -- 2841 561 6031 2241 6531 1841

7/6/2009 4921 -- 3611 891 1,078 243 924 218

7/13/2009 4161 371 4121 701 5561 1121 6031 1461
Table 5 35

Table 5. Estimated weekly total phosphorus and orthophosphorus loads, in kilograms, and summary statistics based on weekly 

composite samples collected at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts, October 2008 through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3; --, no data; summary statistics, rounded to two significant figures, are at the bottom of the table]

Port Street - 01096835 AMVETS - 01096840 Stow - 01096885 Maynard - 01097000 Weekly Total Ortho- Total Ortho- Total Ortho- Total Ortho- start date

phosphorus phosphorus phosphorus phosphorus phosphorus phosphorus phosphorus phosphorus

7/21/2009 535 63 621 195 845 217 934 225

7/28/2009 476 54 604 201 786 180 897 195

8/4/2009 295 32 276 48 342 60 453 143

8/11/2009 197 34 173 48 221 79 258 94

8/18/2009 127 15 114 29 124 52 126 --

8/25/2009 215 52 209 -- 238 91 238 82

9/1/2009 121 20 122 32 145 57 151 58

9/8/2009 99 18 98 30 84 -- 87 28

9/15/2009 71 15 78 -- 87 -- 68 --

9/22/2009 49 9 56 11 48 15 37 14

9/29/2009 95 19 89 21 90 23 113 17

10/6/2009 87 16 95 22 135 -- 149 41

10/13/2009 78 7 68 12 82 7 131 27

10/20/2009 132 19 114 26 119 30 166 25

10/27/2009 212 34 171 27 201 37 273 45

11/3/2009 129 32 117 18 -- -- 153 50

11/10/2009 3431 991 2701 341 438 139 3061 731

11/17/2009 310 68 245 -- 336 108 365 115

11/24/2009 226 36 150 40 199 70 265 75

12/1/2009 389 48 273 62 409 80 389 67

12/8/2009 489 47 250 57 367 72 326 64

12/15/2009 301 37 248 -- 247 61 256 47

12/22/2009 250 24 148 25 245 44 240 51

12/29/2009 262 37 -- -- 218 50 223 --

1/5/2010 167 23 202 29 155 38 146 31

1/12/2010 195 24 159 37 195 37 173 46

1/19/2010 419 41 215 50 240 31 237 34

1/26/2010 407 81 440 92 491 66 534 100

2/2/2010 205 33 -- -- 176 41 -- --

2/9/2010 191 30 171 36 152 26 166 49

2/16/2010 132 19 112 18 111 14 146 31

2/23/2010 949 134 9621 1341 1,2971 1051 1,6771 1831

3/2/2010 325 59 304 68 373 66 446 --

3/9/2010 403 62 496 83 706 73 1,047 115

3/16/2010 474 108 607 144 6791 1391 1,1461 1641

3/23/2010 326 72 365 101 477 123 597 149

3/30/2010 514 109 7931 1981 7061 1481 1,2941 2271

4/6/2010 286 41 265 73 480 110 550 142

4/13/2010 246 25 174 30 270 36 331 57

4/20/2010 159 17 209 33 196 21 248 39

4/27/2010 226 20 102 14 190 18 232 40

5/4/2010 279 24 157 26 200 45 269 53
36 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 5. Estimated weekly total phosphorus and orthophosphorus loads, in kilograms, and summary statistics based on weekly 

composite samples collected at water-quality-monitoring stations on the Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts, October 2008 through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3; --, no data; summary statistics, rounded to two significant figures, are at the bottom of the table]

Weekly 

start date

Port Street - 01096835 AMVETS - 01096840 Stow - 01096885 Maynard - 01097000

Total

phosphorus

OrthophosphorusTotal

phosphorus

OrthophosphorusTotal

phosphorus

OrthophosphorusTotal

phosphorus

Orthophosphorus

5/11/2010 150 16 95 16 133 23 171 39

5/18/2010 279 22 149 25 180 34 203 41

5/25/2010 162 16 116 18 108 26 155 --

6/1/2010 200 18 133 26 136 22 168 21

6/8/2010 287 32 162 -- 216 65 -- --

6/15/2010 130 -- 100 20 123 29 121 29

6/22/2010 73 8 62 10 -- 13 -- 11

6/29/2010 -- -- 47 -- 34 -- 26 --

7/6/2010 36 3 37 4 25 5 22 3

7/13/2010 37 4 43 5 27 5 23 --

7/20/2010 62 -- 59 -- 30 4 -- --

7/27/2010 45 -- 47 6 24 3 16 3

8/3/2010 25 4 34 5 17 1 -- --

8/10/2010 20 3 20 2 9 1 -- --

8/17/2010 16 3 17 3 8 1 -- --

8/24/2010 57 11 49 13 51 9 -- --

8/31/2010 36 4 43 6 15 4 -- --

9/7/2010 35 5 109 8 14 1 -- --

9/14/2010 56 6 48 8 34 3 -- --

9/21/2010 43 5 58 5 28 2 -- --

9/28/2010 111 28 125 29 106 18 96 13

10/5/2010 107 -- 86 22 93 -- 110 11

10/12/2010 103 9 83 13 80 11 120 14

10/19/2010 60 7 32 9 33 7 -- --

10/26/2010 85 7 36 9 34 6 -- --

11/2/2010 142 -- 71 20 74 15 112 14

11/9/2010 226 20 97 25 109 20 120 22

11/16/2010 451 37 170 48 173 31 222 27

11/23/2010 144 18 100 28 113 24 112 23

11/30/2010 230 15 109 22 120 21 120 22

12/7/2010 244 -- -- -- 224 26 206 --

12/14/2010 416 33 -- -- 439 61 -- --

12/21/2010 165 19 -- -- 153 16 177 31

12/28/2010 187 22 -- -- 167 22 171 21

Maximum 950 170 1,100 270 1,500 310 2,000 280

Minimum 20 2.5 17 2.1 8.3 1.0 16 2.9

Median 220 25 170 36 210 44 240 53

Mean 240 35 210 50 270 60 320 71

1

Weekly composite is greater or less than 7 days; see appendix 2.
Table 6 37

Table 6. Estimated weekly total phosphorus loads, in kilograms, in discharges from wastewater-treatment plants on the Assabet 

River in Westborough, Marlborough, and Hudson, Massachusetts, October 2008 through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3. --, missing data, load not calculated; WWTP, wastewater-treatment plant]

Date

Total phosphorus loads, in kilograms

Westborough 

WWTP

Marlborough Westerly 

WWTP

Hudson 

WWTP

Sum of phosphorus loads from 

Westborough and 

Marlborough Westerly 

WWTP

Sum of all total 

phosphorus loads

10/07/08 89 13 39 102 141

10/15/08 66 31 68 98 165

10/21/08 46 22 35 68 103

10/28/08 97 18 38 115 153

11/04/08 91 32 40 123 163

11/11/08 112 75 56 188 244

11/18/08 118 149 257 267 524

11/25/08 137 156 74 294 368

12/02/08 135 194 68 330 398

12/09/08 95 206 71 300 371

12/16/08 -- 265 31 -- --

12/23/08 108 169 118 278 395

12/30/08 160 179 67 339 405

01/06/09 87 116 46 203 248

01/13/09 101 105 51 206 258

01/20/09 86 96 43 182 225

01/27/09 162 108 61 271 332

02/03/09 98 111 44 209 253

02/10/09 -- 101 48 -- --

02/17/09 84 112 48 196 244

02/24/09 75 93 41 168 209

03/03/09 85 77 133 163 295

03/10/09 188 93 41 281 322

03/17/09 156 113 66 269 335

03/24/09 117 95 76 212 288

03/31/09 133 47 165 180 345

04/07/09 100 43 52 143 195

04/14/09 103 30 44 132 177

04/21/09 147 32 179 179 358

04/28/09 105 38 67 143 211

05/05/09 162 31 80 193 273

05/12/09 260 25 58 286 344

05/19/09 183 24 55 207 261

05/26/09 57 24 48 81 128

06/02/09 38 21 65 59 124

06/09/09 38 40 50 77 127

06/15/09 89 26 45 115 161

06/23/09 138 18 52 156 208

06/30/09 102 23 68 125 194
38 Phosphorus Loads, 2008 through 2010 in the Assabet River, Massachusetts

Table 6. Estimated weekly total phosphorus loads, in kilograms, in discharges from wastewater-treatment plants on the Assabet 

River in Westborough, Marlborough, and Hudson, Massachusetts, October 2008 through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3. --, missing data, load not calculated; WWTP, wastewater-treatment plant]

Date

Total phosphorus loads, in kilograms

Westborough 

WWTP

Sum of phosphorus loads from 

Marlborough Westerly Hudson Westborough and 

WWTP WWTP Marlborough Westerly 

WWTP

Sum of all total 

phosphorus loads

07/06/09

07/13/09

07/21/09

07/28/09

08/04/09

08/11/09

08/18/09

08/25/09

09/01/09

09/08/09

09/15/09

09/22/09

09/29/09

10/06/09

10/13/09

10/20/09

10/27/09

11/03/09

11/10/09

11/17/09

11/24/09

12/01/09

12/08/09

12/15/09

12/22/09

12/29/09

01/05/10

01/12/10

01/19/10

01/26/10

02/02/10

02/09/10

02/16/10

02/23/10

03/02/10

03/09/10

03/16/10

03/23/10

03/30/10

128

142

80

193

112

91

107

--

110

81

85

84

76

69

82

57

49

98

74

93

--

71

133

--

48

83

114

97

98

98

137

135

184

146

251

187

102

178

404

--

14

--

30

41

34

13

11

14

36

21

12

21

10

19

17

29

24

63

161

49

29

58

51

26

17

19

23

33

34

75

57

30

32

51

56

183

532

145

--

63

--

61

69

76

57

29

16

25

15

13

26

35

53

31

22

30

26

16

37

34

33

51

82

54

44

41

86

55

36

33

62

48

118

48

46

122

33

--

156

--

223

153

124

120

--

124

118

105

96

97

79

101

74

78

122

137

254

--

100

191

--

74

101

133

120

131

133

212

193

213

178

301

243

285

710

549

--

219

--

284

222

201

177

--

140

143

121

109

123

114

154

105

100

152

163

270

--

134

223

--

156

155

177

161

217

188

248

226

275

226

420

291

331

832

582
Table 6 39

Table 6. Estimated weekly total phosphorus loads, in kilograms, in discharges from wastewater-treatment plants on the Assabet 

River in Westborough, Marlborough, and Hudson, Massachusetts, October 2008 through December 2010.—Continued

[Locations of monitoring stations are shown in figures 1 to 3. --, missing data, load not calculated; WWTP, wastewater-treatment plant]

Date

Total phosphorus loads, in kilograms

Westborough 

WWTP

Marlborough Westerly 

WWTP

Hudson 

WWTP

Sum of phosphorus loads from 

Westborough and 

Marlborough Westerly 

WWTP

Sum of all total 

phosphorus loads

04/06/10 174 165 27 338 365

04/13/10 -- 36 16 -- --

04/20/10 70 54 16 124 140

04/27/10 43 41 9 83 93

05/04/10 67 42 31 109 140

05/11/10 85 18 19 104 123

05/18/10 56 51 12 107 119

05/25/10 48 29 4 76 80

06/01/10 -- 26 7 -- --

06/08/10 11 20 4 31 35

06/15/10 29 30 3 59 62

06/22/10 -- 15 3 -- --

06/29/10 7 13 4 20 24

07/06/10 17 8 5 24 30

07/13/10 27 16 4 43 46

07/20/10 15 12 5 27 32

07/27/10 20 45 3 65 68

08/03/10 21 59 2 80 82

08/10/10 44 9 4 54 57

08/17/10 14 9 4 23 27

08/24/10 9 10 5 19 24

08/31/10 11 11 4 22 25

09/07/10 -- 12 3 -- --

09/14/10 31 12 3 43 47

09/21/10 20 25 3 45 48

09/28/10 51 29 4 80 84

10/05/10 40 12 4 52 56

10/12/10 35 26 3 61 64

10/19/10 31 20 4 51 55

10/26/10 -- 14 3 -- --

11/02/10 36 11 8 46 54

11/09/10 23 16 7 39 46

11/16/10 31 20 4 51 56

11/23/10 35 22 2 57 60

11/30/10 34 22 3 56 59

12/07/10 500 35 5 535 540

12/14/10 -- 32 6 -- --

12/21/10 247 32 6 279 285

12/28/10 231 30 5 261 267
THIS PAGE INTENTIONALLY LEFT BLANK
Appendixes 1–3 41

Appendix 1. Start and end dates and times for composite 

samples collected at water-quality-monitoring stations 

on the Assabet River in the towns of Hudson, Stow, and 

Maynard, Massachusetts, October 2008 through December 

2010.

[Available separately at http://pubs.usgs.gov/sir/2013/5140/]

Appendix 2. A, Concentrations and relative percent 

differences of total phosphorus and orthophosphorus in 

concurrent equal-width-increment (EWI) and outside-gage 

grab samples (OGS); B, Concentrations and relative percent 

differences of total phosphorus and orthophosphorus on 

first and last days of seven-day holding time in sample 

containers.

[Available separately at http://pubs.usgs.gov/sir/2013/5140/]

Appendix 3. Daily mean streamflow in cubic feet per 

second at water-quality-monitoring stations on the 

Assabet River in the towns of Hudson, Stow, and Maynard, 

Massachusetts, October 2008 through December 2010.

[Available separately at http://pubs.usgs.gov/sir/2013/5140/]
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Office Chief

U.S. Geological Survey

New England Water Science Center 

Massachusetts-Rhode Island Office

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at:

http://ma.water.usgs.gov
Zimmerman and Savoie—Phosphorus Loads, 2008 through 2010, in the Assabet River, Massachusetts—Scientific Investigations Report 2013–5140

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