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MA-100
--- Abstract
Nutrient
Sources and Loads in the Connecticut, Housatonic, and Thames River
Basins
Water-Resources
Investigations Report 99-4236
By Elaine C. Todd Trench
Sources
and loads of total nitrogen and total phosphorus in streams of the
Connecticut, Housatonic, and Thames River Basins study unit were
evaluated as part of the U.S. Geological Survey’s (USGS)
National Water-Quality Assessment Program. The study area
encompasses 15,758 square miles and extends from northern New
Hampshire and Vermont to coastal Connecticut.
Annual nutrient
loads and yields were estimated using data from 25 USGS
water-quality monitoring stations in and near New England. Major
sources of nutrients include atmospheric deposition, agricultural
fertilizer and manure, urban nonpoint runoff, and municipal
wastewater discharges. Nutrient yields for drainage basins with
relatively homogenous land use, along with data on point-source
loads, were used to estimate nutrient yields for general
categories of forested, agricultural, and urban land in selected
basins.
Yields
of total nitrogen in undeveloped forested drainage basins, for
which data are limited, are generally less than about 2,000 pounds
per square mile per year (lb/mi2/yr), and yields of
total phosphorus are generally less than about 100 lb/mi2/yr.
Nutrient yields are somewhat higher in forested drainage basins
that have larger percentages of developed land or that receive
point-source discharges, with total nitrogen yields generally
less than 4,000 lb/mi2/yr and total phosphorus yields
generally less than 400 lb/mi2/yr. Nonpoint agricultural sources, including fertilizer and manure,
constitute substantial nutrient inputs in some drainage basins but
do not always result in high stream nutrient loads. Average basin
yields of total nitrogen for three agricultural drainage basins
were generally less than 4,000 lb/mi2/yr, similar to
forested basins, and total phosphorus yields were generally less
than 300 lb/mi2/yr. Total nitrogen yields for the small
Broad Brook Basin, however, exceeded 10,000 lb/mi2/yr,
similar to nitrogen yields in urban basins in Connecticut that
receive point discharges. Conditions in the Broad Brook Basin may
not be representative of typical agricultural areas.
Municipal
wastewater discharges are major sources of stream nutrient loads
in urban areas of Massachusetts and Connecticut. Point-source
discharges are the most important source of nutrient loads in four
highly urban basins—the Pequabuck, Hockanum, Quinnipiac,
and Naugatuck River Basins in Connecticut—even though 46 to 67
percent of the land in these drainage basins is undeveloped. These
findings emphasize the importance of point discharges in urban
coastal areas as sources of nutrients to Long Island Sound.
As
wastewater flows increase in proportion to streamflow in urban
basins, basin yields of total nitrogen and total phosphorus
increase. The largest total nitrogen and total phosphorus yields
were found in the Pequabuck, Hockanum, Quinnipiac, and Naugatuck
River Basins, where wastewater discharges constitute from 9 to 19
percent of the annual mean streamflow in a median flow year. Basin
yields of total nitrogen generally ranged from 8,000 to 14,000
lb/mi2/yr, and basin yields of total phosphorus
generally ranged from 1,000 to 1,400 lb/mi2/yr.
Nutrient yields
from urban drainage basins with no point sources cannot be
adequately characterized with the limited amount of monitoring
data available, but yields appear to span a wide range, based on
different types and intensities of urban land use. Yields for two
urban basins that receive no point discharges were much smaller
than yields in drainage basins that receive point discharges.
Although yields
are moderately low in large, primarily forested drainage basins,
rivers draining these large basins transport most of the nutrient
load. The Connecticut River, measured at the Thompsonville
station, transports about two-thirds of the total nitrogen
exported from the major monitored drainage basins and about
three-fifths of the total phosphorus.
The major sources
of nutrients differ in the northern and southern parts of the
study area. Forested land accounts for most of the stream nutrient
load in northern areas, whereas municipal wastewater discharges
are the dominant source of nutrients in urban areas of
Massachusetts and Connecticut.
Export of total
nitrogen from the major monitored drainage basins in the study
area has ranged from about 32,000,000 to 63,000,000 lb/yr during
the 1980’s and 1990’s, and in recent years has typically been
about 37,000,000 lb/yr. Export of total phosphorus from major
monitored basins during the 1980’s and 1990’s ranged from
about 2,300,000 to 4,700,000 lb/yr and in recent years has
typically been about 2,400,000 lb/yr. Historical maximum loads of
total phosphorus do not represent currently expected maximum
conditions because of the observed downward trends in phosphorus
concentrations on many streams.
Although the 25
drainage basins represent about 86 percent of the study area, the
load from these basins probably represents considerably less than
86 percent of the total nutrient load because (1) the monitored
area contains only 55 percent of the population in the study area,
(2) the monitored area includes essentially all of the least
developed land with low nutrient yields, and (3) many point
discharges in urban coastal areas are either in unmonitored basins
or discharge directly to Long Island Sound.
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