Trends in Nitrogen Concentration and Nitrogen Loads Entering the South Shore
Estuary Reserve from Streams and Ground-Water Discharge in Nassau and
Suffolk Counties, Long Island, New York, 1952-97
by Jack Monti, Jr. and Michael P. Scorca
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 02-4255
ABSTRACT
The 13 major south-shore streams in Nassau and Suffolk Counties,
Long Island, New York with adequate long-term (1971-97) water-quality
records, and 192 south-shore wells with
sufficient water-quality data, were selected for
analysis of geographic, seasonal, and long-term
trends in nitrogen concentration. Annual total
nitrogen loads transported to the South Shore
Estuary Reserve (SSER) from 11 of these
streams were calculated using long-term
discharge records. Nitrogen loads from shallow
and deep ground water also were calculated
using simulated ground-water discharge of
1968-83 hydrologic conditions.
Long-term declines in stream discharge
occurred in East Meadow Brook, Bellmore Creek
and Massapequa Creek in response to extensive
sewering in Nassau County. The smallest longterm
annual discharge to the SSER was from the
westernmost stream, Pines Brook, which is in an
area in which the water table has been lowered by
sewers since 1952. The three largest average
annual discharges to the SSER were from the
Connetquot River, Carlls River, and Carmans
River in Suffolk County; the discharges from each
of these streams were at least twice those of the
other streams considered in this study.
Total nitrogen concentrations in streams show
a geographic trend with a general eastward
increase in median total nitrogen concentration in
Nassau County and a decreasing trend from
Massapequa Creek eastward into Suffolk County.
Total nitrogen concentrations in streams generally
are lowest during summer and highest in winter as
a result of seasonal fluctuations in chemical
reactions and biological activity. The greatest
seasonal difference in median total nitrogen
concentration was at Carlls River with values of
3.4 and 4.2 mg/L (milligrams per liter) as N
during summer (April through September) and
winter (October through March), respectively.
Streams affected by the completion of sewer
districts show long-term (1971-97) trends of
decreasing total nitrogen concentration and
streams showing an increase in total nitrogen
concentration are in unsewered areas with
increased urbanization.
Discharges from shallow ground water (upper
glacial aquifer) and deep ground water (upper part
of Magothy aquifer) were simulated from a
ground-water-flow model calibrated to steadystate
(1968-83) conditions. Simulated discharges
from shallow-ground-water system in Nassau
County were 10,700 Mgal/yr (million gallons per
year) or 40,500,000 m3/yr (cubic meters per year),
and those from Suffolk County were 52,300 Mgal/yr
or 198,000,000 m3/yr. Discharges from
deep-ground-water system in Nassau County
were 4,900 Mgal/yr or 18,500,000 m3/yr, and
those in Suffolk County were 12,700 Mgal/yr or
48,200,000 m3/yr.
Ground-water concentrations of nitrogen
decrease with depth and from west to east. The
shallow ground water median nitrogen
concentration for each county was determined
using 1,155 samples collected at 167 shallow
wells (125 feet deep or less) within 1 mile of the
shore. The deep ground water median nitrate
concentration (nitrate represented almost all of
the total nitrogen) for each county was
determined using 112 samples collected at 25
deep wells (greater than 125 feet deep) within
1 mile of the shore. The median nitrogen
concentration for the shallow and median nitrate
concentration for the deep ground water in
Nassau County were 3.85 and 0.15 mg/L as N,
during 1952-97; the corresponding concentrations
for Suffolk County were 1.74 and <0.10 (less than
0.10) mg/L as N, during 1952-97.
Nitrogen loads discharged from streams to the
SSER for each year during 1972-97 were
calculated as the annual total nitrogen
concentration multiplied by the annual discharge.
These values were calculated only for the seven
streams for which sufficient data were available.
The largest long-term (1972-97) average annual
nitrogen load from Carlls River was 104 ton/yr or
94,300 kg/yr—about twice that of Connetquot
River (54 ton/yr or 48,900 kg/yr) and over three
times that of Carmans River (33 ton/yr or 29,900
kg/yr). The smallest annual mean nitrogen load
was from Pines Brook, which has the lowest
annual mean discharge of all streams analyzed.
The nitrogen load carried to the SSER by
ground-water discharge in shallow-ground-water
system in Nassau and Suffolk Counties
was calculated as the simulated discharge for
each county multiplied by the respective median
nitrogen concentration, and loads from deep-ground-water
system were calculated as the
simulated discharge for each county multiplied
by the respective median nitrate concentration.
All discharges were obtained from the U.S.
Geological Survey's Long Island ground-water-flow
model. The resultant nitrogen loads
discharged to the SSER from shallow ground
water were 172 ton/yr (156,000 kg/yr) from
Nassau County and 380 ton/yr (345,000 kg/yr)
from Suffolk County; equaling 552 ton/yr entering
the SSER. Those from deep ground water were
3 ton/yr (2,700 kg/yr) from Nassau County and
<0.5 ton/yr (480 kg/yr) from Suffolk County;
equaling about 3.5 ton/yr entering the SSER.
The sum of both stream loads and groundwater
loads results in the total load to the SSER.
The largest calculated total nitrogen load entering
the SSER from both streams and ground water
occurred in 1979 with a total load of 1,260 ton/yr
(1,140,000 kg/yr). The smallest calculated
nitrogen load entering the SSER occurred in 1995
with a total load of 725 ton/yr (658,000 kg/yr).
Citation: Monti, Jack Jr., and Scorca, Michael P., 2003,
Trends in Nitrogen Concentration and Nitrogen Loads Entering the South Shore Estuary Reserve from Streams and Ground-Water Discharge in Nassau and Suffolk Counties, Long Island, New York, 1952-97:
U.S. Geological Survey Water-Resources Investigations Report 02-4255, 36 p.
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