24USGS
science for a changing world
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UNIVERSITY OF MICH
08
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The Influence of Ground Water
on Nitrogen Delivery to the
Chesapeake Bay
by Scott W. Phillips and Bruce D. Lindsey
ſ
Summary of Major Findings
• Resource managers need to understand
the "lag time" between implementation of
management practices and improvement
in water quality in the Chesapeake Bay
to help develop future nutrient- and
sediment-reduction strategies. One
factor affecting the lag time is the
influence of ground water on the
transport of nitrogen to streams in the
Bay watershed.
e Ground water supplies a significant
amount (about half) of water and nitrogen
to streams in the watershed and is
therefore an important pathway for
nitrogen to reach the Chesapeake Bay.
• The age of ground water in shallow
aquifers in the Chesapeake Bay
watershed ranges from modern (less
than 1 year) to more than 50 years, with
a median age of 10 years.
• In addition to ground water, stream
water will be influenced by surface
runoff and soil water Runoff and soil
water both have very young ages (hours
to months) and supply, on average, about
half of the water to a stream.
• Proposed water-quality criteria in the
Bay probably will not be met by 2010 due
to the time needed to implement
management practices and the effects of
ground water and other watershed
properties on nutrient transport.
LIBRARIES
JAN 19 2004
DEPOSITED BY
SUD0CS -
COLLECTION
UNIVERSITY OF MICHIGAN
UNITED STATES OF AMERICA
Efforts to Improve Water
Quality in the Chesapeake
Bay
he Chesapeake Bay ecosys-
tem has been impaired by an
Overabundance of nutrients for several
decades. Excess nutrients stimulate algal
blooms, which consume dissolved oxy-
gen as they decompose and cause large
areas of low dissolved-oxygen concentra-
tion in the Bay. The low dissolved-oxy-
gen concentrations have resulted in fish
kills and have affected many bottom-
dwelling organisms in the Bay. The algal
blooms, along with sediment eroding
from the land, also block sunlight need-
ed by underwater grasses. Without sun-
light, the Bay grasses die, removing
important habitat for fish and shellfish,
and food for waterfowl.
In the mid-1980s, the Chesapeake
Bay Program (CBP), a partnership
between jurisdictions (the States in the
watershed and Washington, D.C.), the
Chesapeake Bay Commission, and the
Federal Government, began efforts to
reduce nutrients in the Bay. However,
improvement in water-quality conditions
in the Bay and its rivers (fig. 1) has been
slow. Because of the continued nutrient
and sediment problems, the Bay was list-
ed as an "impaired water body" under
regulatory statutes related to the Clean
Water Act. The CBP has developed
water-quality criteria (U.S. Environmental
Protection Agency, 2003), and is imple-
menting actions to reduce nutrients and
sediment entering the Bay in an attempt
to meet these criteria by 2010. It is
Chesapeake
Bay
Watershed
NEW YORK
unknown how long it will take for water-
quality conditions to improve in the Bay
and its watershed after all the nutrient
and sediment reduction actions are
implemented. Resource managers need
to understand the "lag time" between
implementation of management prac-
tices and improvement in water quality
in the Bay to help develop future nutri-
ent- and sediment-reduction strategies.
One factor affecting the lag time is the
influence of ground water on the trans-
port of nitrogen to streams in the Bay
watershed.
The potential for slow improvements
in stream nitrate levels due to the influ-
ence of ground water has been noted in
previous studies (Shedlock, 1993; Böhlke
and Denver, 1995; Phillips and Bachman,
1996). To better understand the influ-
ence of ground water on water quality in
streams, the U.S. Geological Survey
(USGS) conducted a multi-year study
which addressed the discharge, nitrogen
transport, and age of ground water into
streams in the Chesapeake Bay water-
shed. The purpose of this fact sheet is to
summarize the results of the multi-year
study and its associated reports (Lindsey
and others, 2003, Phillips and others,
1999; Focazio and others, 1998;
Bachman and others, 1998) and discuss
implications for achieving improved
water-quality conditions in the Bay by
2010.
U.S. Department of the Interior
U.S. Geological Survey
USGS Fact Sheet FS-091-03
2003










350,000 I I I I I T
300,000
-
250,000 H A
f
4. STREAM DISCHARGE
NITROGEN CONCENTRATION
-
200,000
150,000 H
100,000
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4.
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0
1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Figure 1. Stream discharge and total nitrogen concentrations in the Potomac River at Chain Bridge
at Washington, D.C., 1985-2001. (Concentrations of nitrogen have not shown a significant change over
the past 15 years in the Potomac River, while some rivers including the Susquehanna, James, and
Patuxent have had a slight decrease in nitrogen concentrations. Some of the factors affecting
nitrogen concentrations include the amount of nutrient-source reduction, and watershed
characteristics such as the influence of ground water.)
Ground Water and Nitrogen
Discharge to Streams in the
Chesapeake Bay Watershed
Ground water supplies a significant
amount of water and nitrogen to streams
in the watershed and is therefore an
important pathway for nitrogen to reach
the Chesapeake Bay. Rainfall and
snowmelt percolate through the soil
zone and enter the ground-water system.
Much of this recharge moves slowly
through an aquifer and is discharged to
streams. On average, just over 50 per-
cent of the total volume of water in
streams throughout the Bay watershed is
from ground water, with a range in differ-
ent streams of 16 to 92 percent
(Bachman and others, 1998). Different
lithologic and physiographic settings,
known as “hydrogeomorphic regions"
(HGNMRS), have some influence on the
amount of ground-water discharge to
streams (fig. 2). The Valley and Ridge
Carbonate HGMR had the highest medi-
an (64 percent) and the Mesozoic
Lowland Siliciclastic HGMR had the low-
est median (36 percent) contribution of
ground-water discharge to streams. The
remaining HGMRs had median values of
greater than 50 percent of total stream-
flow contributed by ground water.
Ground water transports a large
amount of nitrogen to streams in the
Chesapeake Bay watershed. Nitrogen
reaches the land surface in rainfall or
through fertilizer application and other
practices associated with agricultural and
suburban areas. Once on the land sur-
face, some of the nitrogen infiltrates into
the underlying soil zone and ground
water. In ground water, nitrogen general-
ly is converted to nitrate and moves
through the aquifer. Much of the nitrate
is discharged to streams and contributes
to the total nitrogen load in a stream.
An average of 48 percent of the nitrogen
load in streams in the Bay watershed was
transported through ground water, with a
range of 17 to 80 percent in different
streams (Bachman and others, 1998).
Additional analysis by Sprague and oth-
ers (2000) found that a similar percent-
age (15 to 65) of the total nitrogen load
in streams was contributed in the form of
nitrate from ground water. The slow
movement of ground water relative to
surface water has a significant impact on
the total time it takes for nitrogen to
move through the Bay watershed.
Age of Ground-Water Discharge
to Streams
To better define the time it takes
ground water to move through an
aquifer from the area of recharge to the
area of discharge (such as a stream,
spring, or well), the USGS Collected Sam-
ples to determine the age of ground
water in shallow aquifers (the uppermost
Several hundred feet of shallow aquifers).
Samples Collected from wells are only
representative of one point in an aquifer,
and streams cannot be sampled directly
for ground-water ages. Therefore, the
USGS sampled springs because they are
discharge points for ground water from
an aquifer (Focazio and others, 1998).
The age of ground water from a spring
can be considered representative, or an
average, of the age of water in an
aquifer. The ages of ground water from
springs were estimated using chemical
isotopic tracers. Chlorofluorocarbons
(CFCs) were the primary tracers used in
this study. Age determinations using this
method are accurate to within several
years (Busenburg and Plummer, 1992).
Based on samples Collected from 46
springs, the age of ground water in shal-
low aquifers in the Chesapeake Bay
watershed ranges from modern (less
than 1 year) to more than 50 years. The
median age of all samples was 10 years,
with 25 percent of the samples having
an age of 7 years or less and 75 percent
of the samples having an age of up to 13
years. Wet and dry periods (1996 and
1997, respectively) showed a small influ-
ence on the ground-water age of most
springs. Samples Collected from Springs
during wet conditions (above average
rainfall) were a few years younger than
samples collected during dry (below
average rainfall) conditions (fig. 3).
Differences between the HGNARS in
the Bay watershed (shown in fig. 2) did
not significantly affect ground-water ages
from springs. The range of ground-water
ages (modern to more than 50 years)
was similar in large HGMRs above the
Fall Line, and only two small HGMRs
(the Piedmont Carbonate and Mesozoic
Lowland) had ground-water age ranges
of modern to 10 years. The median val-
ues of ground-water ages in all HGMRs
ranged from 7 to 11 years, but not
enough samples were collected to detect
statistical differences between the
HGMRs. Only two springs were sam-
pled in the Coastal Plain, but data from
wells showed the range of ages (modern
to 50 years) was similar to that in the
large HGMRs above the Fall Line. The
lack of age differences in ground water
between HGMRS indicates that other
factors influence the age of water in
aquifers. The most important factor, as
determined by the study, is the distance
between the recharge and discharge
area, which in turn will be influenced
locally by aquifer geometry, permeability,
and hydraulic gradient.
2 The Influence of Ground Water on Nitrogen Delivery to the Chesapeake Bay
EXPLANATION
CHESAPEAKE BAY 76°
WATERSHEDS SELECTED FOR WATERSHED BOUNDARY 1-2
LOCAL-SCALE STUDY \
ea SPRINGS ANALYZED FOR AGE-
DATING TRACERS
HYDROGEOMORPHIC
REGIONS (HGMRs)
APPALACHIAN PLATEAU
CARBONATE
APPALACHIAN PLATEAU
SILICICLASTIC
VALLEY AND RIDGE
CARBONATE
VALLEY AND RIDGE
SILICICLASTIC
BLUE RIDGE
CRYSTALLINE
MESOZOIC LOWLAND
SILICICLASTIC
PIEDMONT o
CARBONATE 40 —
PIEDMONT
CRYSTALLINE
COASTAL PLAIN
DISSECTED UPLAND
COASTAL PLAIN
LOWLAND
COASTAL PLAIN
UNDISSECTED
UPLAND
VIRGINA’.”
| Zºº
Figure 2. Location of springs and local-scale
// - - ſ a.
- ſ J
UDDY CREEk
21WAFSHF)
VIRGINA
-n.
PQLECATCHEEKº Y
WATERSHED
Pºnswana -
e-
º
- EAST
MAHANTANGO CREEK
WATERSHED
UPPER
POCOMOKE
RIVER
WATERSHED
50 MILES
50 KILOMETERS
watersheds sampled in different hydrogeomorphic
regions (HGMRs) in the Chesapeake Bay watershed (modified from Lindsey and others, 2003).
Age of Flow Components to
Streams and Effect on Water
Quality
In addition to ages of the ground
water, the water in a stream will be influ-
enced by the ages of the surface runoff
and soil water (fig. 4). Surface runoff is
water entering a stream after flowing
across the land surface during or shortly
after storms. A portion of the soil water
enters a stream without reaching the
water table and is delivered during
Storms and periods of high moisture Con-
tent. Runoff and soil water both have
very young ages (hours to months,
respectively) and supply, on average,
about half of the water to a stream (fig.
5). The remainder of the water supplied
to a stream moves through the ground-
water system and has a median age of
10 years. The overall result is that about
half of the water entering a typical
stream in the Bay watershed can be con-
sidered modern, and about 75 percent is
less than 10 years old (fig. 5).
As part of the study, the relation
between nutrient sources, ground-water
age, and response in nitrate Concentra-
tions in a stream was analyzed in four
local-scale watersheds (locations shown
in figure 2). The watersheds were select-
ed to represent areas of agricultural land
use that are underlain by different
lithologies. Of the major nitrogen
Sources (atmospheric, urban, and agricul-
tural) in the watershed, multiple studies
(Ator and Ferrari, 1997; Lindsey and oth-
ers, 1997, Shedlock and others, 1999)
have shown that agricultural land use has
the greatest impact on nitrogen concen-
trations in ground water. In the East -
Mahantango Creek watershed, a pre- -
dominantly agricultural basin underlain
by fractured rock, a model was devel-
oped to predict nitrate concentrations in
the stream over time (Lindsey and oth-
ers, 2003). The model used information
on the amount of nitrogen applied to the
land surface Over time in the basin,
assumed a ground-water age of about 10
years, and estimated a response of the
base-flow (the amount from ground
water) nitrate concentrations in a stream
(fig. 6). The model results indicate that
the base-flow nitrate concentration of
the stream increased during the last sev-
eral decades (curve a) because of
increases in the concentrations discharg-
ing from ground water. The increase in
nitrogen sources used in the model is
typical of many agricultural regions with-
in the Chesapeake Bay watershed. Two
future scenarios were examined with the
model: (1) nitrogen applications continu-
ing at current levels, and (2) elimination
of all nitrogen applications. The scenario
with nitrogen applications at Current lev-
els results in a continued increase in Con-
centration of base-flow nitrate in the
stream Over the next several decades
(curve b in fig. 6). The scenario with
complete elimination of nitrogen applica-
tions shows that a 50-percent reduction
in nitrate base-flow concentrations could
occur in about 5 years, with a decrease
likely to continue until 2040 (curve C in
fig. 6). Base-flow nitrate concentrations
Over time in many streams in the
Chesapeake Bay watershed likely will be
bounded by these two scenarios.
Effects of Lag Time on Water-
Quality Improvement in the Bay
The information presented in this
fact sheet is designed to improve under-
standing of the effects of ground water
on nitrogen delivery to the Chesapeake
Bay. However, this factor should be con-









































3 The Influence of Ground Water on Nitrogen Delivery to the Chesapeake Bay
sidered in conjunction with all of the fac-
tors affecting the lag time between
reductions of nutrient sources in the Bay
watershed and improvement of water
quality by 2010. The major factors
affecting lag time include (a) the type of
nutrient source (point and nonpoint) and
time between planning and implementa-
60
50 H - --
WET YEAR (1996)
DRY YEAR (1997)
40 H -
30 H -
20 H -
10 H | -
O Jill | |
SPRING-WATER SAMPLES
Figure 3. Ground-water age from springs in the Chesapeake Bay watershed, 1996-1997 [The ages
of ground water ranged from modern (less than 1 year) to over 50 years, with a median age of 10 years.
The ages of samples collected during "wet" conditions (periods of above average rainfall) were slightly
younger than those collected during "dry" conditions (periods of lower than average rainfall).]
(modified from Lindsey and others, 2003).
Discharge º
stream sº
* º º
Soil water | º f
- " __Fungi-
Ground-Water Discharge soil water
to Stream Mºnths
Nºliº
Nº º
Decades
Figure 4. Conceptual watershed showing the relation between nutrient sources, streams, and
ground water (Nitrogen moves from point and nonpoint sources and is transported to a stream
through runoff soil water, and ground water. Once in the ground-water system, nitrogen may take
less than a year to decades to be transported to a stream. The distribution of flow paths in the ground-
water system will affect the nitrogen concentration and the "lag time" between implementation of
management practices in a watershed and the decrease of nitrogen concentrations in a stream.)
(modified from Phillips and others, 1999).
tion of a management practice, (b) the
time it takes to transport nutrients and
sediment through surface and ground
water in the watershed, and (c) the time
between a nutrient- and sediment-source
reduction and an actual improvement in
Bay water quality. This information can
be used to help develop strategies to
reduce nutrients and sediment to the
Bay and its tributaries.
Point and nonpoint sources con-
tribute about 20 and 80 percent, respec-
tively, of the nitrogen that is delivered to
the Chesapeake Bay (U.S. Environmental
Protection Agency, 2002). Planning and
implementing a management practice to
reduce these sources can take several
years and contribute to the lag time of
water-quality improvement in the Bay.
Once implemented, point-source reduc-
tions would provide the most immediate
improvement in water quality in the
receiving stream and subsequently the
Bay. Nonpoint-source reductions would
have less of an immediate effect on
water quality because of the slower
reduction of Sources and the influence
of watershed properties affecting the
transport of nutrients through a water-
shed.
Typically, nutrients in a watershed
are transported either directly in water
(dissolved form) or they attach them-
selves (primarily phosphorus) to sedi-
ment (table 1). Dissolved nitrogen and
phosphorus discharged from point
Sources into surface water have a short
transport time (days) through the water-
shed and to the Bay. Dissolved nitrogen
from nonpoint sources is transported
almost equally between two major path-
ways: (a) runoff and soil water, or (b)
ground water. As previously discussed,
dissolved nitrogen associated with runoff
and soil water will have a transport time
of hours to months in the watershed.
Dissolved nitrogen associated with
ground water may have a transport time
of years to decades, with a median time
of about 10 years. Nutrients associated
with sediment can have much longer
transport times (several decades) in the
watershed because of their storage in
soil and stream Corridors, both of which
are greatly influenced by yearly rainfall.
Information on the transport time of
nutrients in a watershed can be used by
resource managers to develop manage-
ment strategies to achieve the most rapid

4. The Influence of Ground Water on Nitrogen Delivery to the Chesapeake Bay
Ground
Water
Runoff and
Soil Water
Modern
(less than
1 year)
Figure 5. Ages of water being discharged to
a stream. (Runoff and soil water contribute
about one half of the Water to a stream, and
the water can be considered "modern" as it
moves to a stream in hours to months.
Ground water contributes the remaining
half of the Water to a stream, with most
of the water being less than 13 years old.
Overall, about 50 percent of the water
discharged to a stream is "modern" and
75 percent is less than 10 years old.)
improvement in water quality of the Bay.
Additionally, the location of the source
area in the watershed will influence the
lag time between implementation of
management practices and improvement
in water quality in the Bay. In general,
designing management practices that tar-
get the highest nutrient sources in areas
closest to the Bay may provide a more
rapid water-quality benefit. The CBP
watershed model can be used in con-
junction with results of the USGS
SPAtially Referenced Regressions On
Watershed attributes (SPARROW) model
(Brakebill and others, 2001) to help iden-
tify these target areas. Figures 7A and
7B show SPARROW model predictions
of areas of highest nitrogen delivery to
streams (fig. 7A) and the Bay (fig. 7B).
Information on sources contributing to
the nutrient loads in different areas is
available (Brakebill and others, 2001) to
help resource managers determine the
strategies needed in different basins that
would achieve the most rapid improve-
ment in the water quality of the
Chesapeake Bay.
Based on the time needed to imple-
ment nutrient-source reductions and fac-
tors affecting the transport time of nutri-
ents in the watershed, the proposed
water-quality criteria for the Bay proba-
1
4
i I I I i I i I i I I
2. [
33 12H
-
: É 10 - … curve h-hypothetical base--
Dr. E - curve a - base-flow nitrate * …” \ flow nitrate Concentration -
E ºf [ concentrations 2^ \ (constant input) -
III iſ 8 H … " w T
9 Cl- - 2’ \ -
33 6. _* \– T
O 3: - 2^ -
LL, Dr. I 2 * \ I
º CD 4 H - / v --
ºf - [ _2^ curve c - hypothetical base- \ I
E 5 - - * flow nitrate concentration ^ -
z 2 |- - - - - - ~ *- -
z [ (no input) * ~
0 | l | l | l | I | I | l | l | l - TI - - -
1940 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040
YEAR OF DISCHARGE
Figure 6. Predicted nitrate concentrations of base flow to a stream in the East Mahantango Creek
watershed in Pennsylvania. [A model was used to predict the stream concentrations based on
nitrogen-source reductions and the influence of ground water in an agricultural watershed. The
model results indicate that the base-flow nitrate concentration of the stream increased during the
last several decades (curve a) because of increases in the concentrations discharging from ground
water that are related to increases in nitrogen sources. Two future scenarios were examined with
the model: (1) continuation of nitrogen applications at current levels, and (2) elimination of all nitrogen
applications. Scenario (1) results in a continued increase in concentration of base-flow nitrate in
the stream over the next several decades (curve b), while scenario (2) shows that a 50-percent
reduction in nitrate base-flow concentrations could occur in about 5 years, with a decrease likely
to continue until 2040 (curve cy. Base-flow nitrate concentrations over time in many streams in the
Chesapeake Bay watershed likely will be bounded by these two scenarios.]
the
Chesapeake Bay
Table 1. Factors affecting lag times, and implications for improvement in water quality in
Nutrient
source
Management practice
implementation time
residence
Watershed
Implications for load reductions
time in the Chesapeake Bay
Point sources
(About 20 percent
of nutrient load
delivered to
the Bay)
Several years
Hours to weeks
Would provide the most rapid improvement
of water quality due to immediate
reduction of source.
Nonpoint sources
(About 80 percent
of nutrient load
delivered to
the Bay)
Dissolved
nutrients
Nutrients
associated
with
sediment
Several years
Several years
decades if
Decades or
on location
watershed
Hours to months
if associated
With runoff/soil
Water, years to
associated with
ground water,
with a median
time of 10 years
longer depending
Improvement in water quality would
depend on the rate of implementation of
nonpoint-source reduction and amount of
nutrients still in soils. Once fully
implemented, there would be a fairly rapid
reduction of the load associated with runoff
and soil water. Nitrogen load associated
with ground water would have a median
time of 10 years for water-quality
improvements to be evident.
Load reductions would be greatly
influenced by streamflow variability. Storm
events would deliver sediment and
associated nutrients contained on land and
in stream corridors. Loads to the Bay may
not show reductions for decades due to
long residence times.
in
bly will not be met by 2010. However,
the drought from 1999 through 2002
provided some insights into the response
of the Bay to reduced nutrient loads.
Freshwater flow into the Bay during the
period was below average for all 4 years,
which resulted in fewer nutrients enter-
ing the Bay. Information compiled by
the CBP showed improvement in the vol-
ume of Bay water having dissolved oxy-
gen above 5 milligrams per liter during
this period, suggesting that water quality
probably will improve fairly rapidly as
nutrient loads to the Bay are reduced.


5 The Influence of Ground Water on Nitrogen Delivery to the Chesapeake Bay
EXPLANATION
TOTAL NITROGEN
INCREMENTAL YIELD,
in kilograms per hectare
per year
LESS THAN 1.5
1.5-3
3-4.5
4.5-5
5-6.5
6.5-8
8-9.5
9.5-11
11-12.5
GREATER THAN
12.5
Figure 7. Predicted total nitrogen yields to (A) streams and (B) the Chesapeake Bay (In-stream processes account for the difference between
nitrogen delivered to streams and the amount delivered to the Chesapeake Bay. Targeting nutrient-reduction strategies in the areas of high
nitrogen loading would probably provide the maximum benefit for the Chesapeake Bay.) (modified from Brakebill and others, 2001).
References Cited
Ator, S.W., and Ferrari, M.J., 1997, Nitrate
and selected pesticides in ground water
of the Mid-Atlantic Region: U.S.
Geological Survey Water-Resources
Investigations Report 97-4139, 8 p.
Bachman, L.J., Lindsey, B.D., Brakebill, J.W.,
and Powars, D.S., 1998, Ground-water
discharge and base-flow nitrate loads of
nontidal streams, and their relation to a
hydrogeomorphic classification of the
Chesapeake Bay Watershed, Middle
Atlantic Coast: U.S. Geological Survey
Water-Resources Investigations Report 98-
4059, 71 p.
Böhlke, J.K., and Denver, J.M., 1995,
Combined use of ground-water dating,
chemical, and isotopic analyses to resolve
the history and fate of nitrate contamina-
tion in two agricultural watersheds,
Atlantic Coastal Plain, Maryland: Water
Resources Research, v. 31, no. 9, p.
2,319-2,339.
Brakebill, J.W., Preston, S.D., and Martucci,
S.K., 2001, Digital data used to relate
nutrient inputs to water quality in the
Chesapeake Bay, version 2.0: U.S.
Geological Survey Open-File Report 01-
251, variously paged].
Busenberg, E. and Plummer, L.N., 1992, Use
of chlorofluorocarbons (CCl3F and
CCI2F2) as hydrologic tracers and age-
dating tools: The alluvium and terrace sys-
tem of central Oklahoma: Water
Resources Research, v. 28, no. 9, p.
2,257-2,283.
Focazio, M.J., Plummer, L.N., Böhlke, J.K.,
Busenburg, E., Bachman, L.J., and
Powars, D.S., 1998, Preliminary estimates
of residence times and apparent ages of
ground water in the Chesapeake Bay
Watershed, and water quality data from a
EXPLANATION
TOTAL NITROGEN
DELIVERED YIELD,
per year
LESS THAN 1.5
1.5-3
3-4.5
4.5-5
5-6.5
6.5-8
8-9.5
9.5-11
11-12.5
12.5
in kilograms per hectare
GREATER THAN
survey of springs: U.S. Geological Survey
Water-Resources Investigations Report 97-
4225, 75 p.
Lindsey, B.D., Loper, C.A., and Hainly, R.A.,
1997, Nitrate in ground water and stream
base flow in the Lower Susquehanna
River Basin, Pennsylvania and Maryland:
U.S. Geological Survey Water-Resources
Investigations Report 97-4146, 66 p.
Lindsey, B.D., Phillips, S.W., Donnelly, C.A.,
Speiran, G.K., Plummer, L.N., Böhlke,
J.K., Focazio, M.J., and Burton, W.C.,
2003, Residence time and nitrate trans-
port in ground water discharging to
streams in the Chesapeake Bay
Watershed: U.S. Geological Survey Water-
Resources Investigations Report 03-4035,
202 p.
Phillips, P.J., and Bachman, L.J., 1996,
Hydrologic landscapes on the Delmarva
Peninsula Part 1: Drainage basin type and
base-flow chemistry: Water Resources
Bulletin, v. 32, no. 4, p. 767-778.
Phillips, S.W., Focazio, M.J., and Bachman,
L.J., 1999, Discharge, nitrate load, and
residence time of ground water in the
Chesapeake Bay Watershed: U.S.
Geological Survey Fact Sheet FS-150-99,
6 p.
Shedlock, R.J., 1993, The Delmarva study:
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p. 12-14.
Shedlock, R.J., Denver, J.M., Hayes, M.A.,
Hamilton, P.A., Koterba, M.T., Bachman,
L.J., Phillips, P.J., and Banks, W.S.L.,
1999, Water-quality assessment of the
Delmarva Peninsula, Delaware, Maryland,
and Virginia: Results of investigations,
1987-91, U.S. Geological Survey Water-
Supply Paper 2355-A, 41 p.
Sprague, L.A., Langland, M.J., Yochum, S.E.,
Edwards, R.E., Blomquist, J.D., Phillips,
S.W., Shenk, G.W., and Preston, S.D.,
2000, Factors affecting nutrient trends in
major rivers of the Chesapeake Bay
Watershed: U.S. Geological Survey Water-
Resources Investigations Report 00-4218,
109 p.
U.S. Environmental Protection Agency, 2002,
The state of the Chesapeake Bay: U.S.
Environmental Protection Agency Report
903-R-02-002, 60 p.
, 2003, Ambient water-quality criteria
for dissolved oxygen, water clarity, and
chlorophyll a for Chesapeake Bay and its
tidal tributaries: U.S. Environmental
Protection Agency Report 903-R-03-002,
|variously paged].
Editor: Valerie M. Gaine
Graphics and design: Timothy W. Auer
For further information contact:
Chesapeake Bay Programs Coordinator
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore, Maryland 21237
or visit the USGS Chesapeake Bay
Homepage on the World Wide Web at
http://chesapeake.usgs.gov
For more information on the Chesapeake
Bay Program:
http://www.chesapeakebay.net
2USGS
science for a changing world
º-S
FS-091-03
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