Skip Links
Scientific Investigations Report 2011–5179
Prepared in cooperation with the Massachusetts Department of Environmental Protection
Monitoring to Assess Progress toward Meeting the Assabet River, Massachusetts, Phosphorus Total Maximum Daily Load—Aquatic Macrophyte Biomass and Sediment-Phosphorus Flux
By Marc J. Zimmerman,1 Yu Qian,2 and Tian Yong Q.3
1U.S. Geological Survey, Massachusetts-Rhode Island Water Science Center, Northborough, Massachusetts.
2University of Massachusetts, Amherst, Massachusetts.
3Central Michigan University, Mt. Pleasant, Michigan.
Abstract
In 2004, the Total Maximum Daily Load (TMDL) for
Total Phosphorus in the Assabet River, Massachusetts, was
approved by the U.S. Environmental Protection Agency.
The goal of the TMDL was to decrease the concentrations
of the nutrient phosphorus to mitigate some of the instream
ecological effects of eutrophication on the river; these effects
were, for the most part, direct consequences of the excessive
growth of aquatic macrophytes. The primary instrument
effecting lower concentrations of phosphorus was to be strict
control of phosphorus releases from four major wastewatertreatment
plants in Westborough, Marlborough, Hudson, and
Maynard, Massachusetts. The improvements to be achieved
from implementing this control were lower concentrations
of total and dissolved phosphorus in the river, a 50-percent
reduction in aquatic-plant biomass, a 30-percent reduction in
episodes of dissolved oxygen supersaturation, no low-flow
dissolved oxygen concentrations less than 5.0 milligrams
per liter, and a 90-percent reduction in sediment releases of
phosphorus to the overlying water.
In 2007, the U.S. Geological Survey, in cooperation with
the Massachusetts Department of Environmental Protection,
initiated studies to evaluate conditions in the Assabet River
prior to the upgrading of wastewater-treatment plants to
remove more phosphorus from their effluents. The studies,
completed in 2008, implemented a visual monitoring plan to
evaluate the extent and biomass of the floating macrophyte
Lemna minor (commonly known as lesser duckweed) in five
impoundments and evaluated the potential for phosphorus flux
from sediments in impounded and free-flowing reaches of the
river.
Hydrologically, the two study years 2007 and 2008 were
quite different. In 2007, summer streamflows, although low,
were higher than average, and in 2008, the flows were generally
higher than in 2007. Visually, the effects of these streamflow
differences on the distribution of Lemna were obvious.
In 2007, large amounts of floating macrophytes accumulated
behind bridge constrictions and dams; in 2008, high flows during the early part of the growing season carried floating
macrophytes past bridges and over dams, minimizing accumulations.
Samples of Lemna were collected and weighed to
provide an estimate of Lemna biomass based on areal coverage
during the summer growing seasons at eight sites in the
five impoundments. Average estimated biomass during 2007
was approximately twice the 2008 biomass in each of the areas
monitored. In 2007, in situ hyperspectral and high-resolution,
multispectral data from the IKONOS satellite were obtained
to evaluate the feasibility of using remote sensing to monitor
the extent of aquatic plant growth in Assabet River impoundments.
Three vegetation indices based on light reflectance
were used to develop metrics with which the hyperspectral and
satellite data were compared. The results of the comparisons
confirmed that the high-resolution satellite imagery could differentiate
among the common aquatic-plant associations found
in the impoundments. The use of satellite imagery could counterbalance
emphasis on the subjective judgment of a human
observer, and airborne hyperspectral data can provide higher
resolution imagery than multispectral satellite data.
In 2007 and 2008, the potential for sediment flux of
phosphorus was examined in free-flowing reaches of the
river and in the two largest impoundments—Hudson and Ben
Smith. These studies were undertaken to determine in situ
flux rates prior to the implementation of the Assabet River
Total Maximum Daily Load (TMDL) for phosphorus and to
compare these rates with those used in the development and
evaluation of the TMDL. Water samples collected from a
chamber placed on the river bottom were analyzed for total
phosphorus and orthophosphorus. Ambient dissolved oxygen
concentrations and seasonal temperature differences appeared
to affect the rates of sequestration and sediment release of
phosphorus. When dissolved oxygen concentrations remained
relatively high in the chambers and when the temperature was
relatively low, the tendency was for phosphorus concentrations
to decrease in the chambers, indicating sediment sequestration
of phosphorus; when dissolved oxygen concentrations dropped
to near zero and temperatures were warmest, phosphorus concentrations
increased in the chambers, indicating phosphorus flux from the sediment. The rates of release and sequestration
in the in situ studies were generally comparable with the rates
determined in laboratory studies of Assabet River sediment
cores for State and Federal agencies. Sediment-core and chamber
studies produced substantial sediment fluxes to the water
column only under extremely low-DO or anaerobic conditions
rarely found in the Assabet River impoundments; thus, sediment
is not likely to be a major phosphorus source, especially
when compared to the wastewater effluent, which sustains
higher ambient concentrations. The regulatory agencies now
(2011) have substantial laboratory and field data with which
to determine the required 90-percent reduction in phosphorus
flux after the completion of upgrades to the wastewater-treatment
plants that discharge to the Assabet River.
|
Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge. |
Suggested citation:
Zimmerman, M.J., Yu Qian, and Tian Yong Q., 2011, Monitoring to assess progress toward meeting the Assabet River,
Massachusetts, phosphorus total maximum daily load—Aquatic macrophyte biomass and sediment-phosphorus
flux: U.S. Geological Survey Scientific Investigations Report 2011–5179, 77 p. (Also available at
http://pubs.usgs.gov/sir/2011/5179/.)
Contents
Abstract
Introduction
Purpose and Scope
Previous Investigations in the Assabet River Basin
Part 1: Floating Macrophytes
Visual Observations of the Distribution of Lemna
Multispectral and Hyperspectral Analysis of Lemna Distribution
Methods Used to Obtain and Compare Hyperspectral and Multispectral Data
Application of Remote-Sensing Results to Other Assabet River Impoundments
Estimates of Lemna Biomass Based on Visual Observations
Limitations of Different Methods for Assessing Aquatic-Plant Distribution and Biomass
Part 2: Phosphorus Fluxes From Assabet River Sediments
Previous Assabet River Sediment-Flux Studies
Flux-Chamber Design and Testing
Chamber Design
Sampling Methods
Chamber Testing
Proof-of-Method Laboratory Test
Field Tests
Field Test 1
Field Test 2
Field Test 3
Results of Sampling in Free-Flowing and Impounded Reaches
Total Phosphorus and Orthophosphorus Concentrations in Free-Flowing Reaches
Total Phosphorus and Orthophosphorus Flux Rates from Sediment in Free-Flowing Reaches
Total Phosphorus and Orthophosphorus Concentrations in Impounded Reaches
Total Phosphorus and Orthophosphorus Flux Rates from Sediment in Impounded Reaches
Comparison of Sediment-Phosphorus Flux Rates between Free-Flowing and Impounded Reaches
Comparison of Results of USGS Studies with Results of ENSR International and CDM Laboratory Studies
Summary and Conclusions
Aquatic Macrophytes
Sediment-Phosphorus Flux
References Cited
Figures 3–9