Nutrient Concentrations in Streams from Nonpoint Sources
by
James M. Omernik*
Introduction
The enclosed maps (3) and graphs (2) were compiled to provide a national
overview of nutrient concentrations in streams from nonpoint sources.
These materials are being incorporated into an Ecological Research Series
(U.S. Environmental Protection Agency, Office of Research and Development)
report to be issued this summer. Meanwhile, we hope they will be useful
to water resource planners and managers for making general assessments
of stream nutrient levels attributable to nonpoint sources.
The enclosed materials are based on (1) the mean annual nutrient concen-
trations from a nationwide set of 928 National Eutrophication Survey (NES)
stream sites associated with watersheds impacted only by nonpoint sources
and (2) the relationships of the NES nonpoint data to general land use
categories and other macro-watershed characteristics.
Although there is considerable disagreement about what exactly comprises
a "point source" as compared to a "nonpoint source", very generally speak-
ing, point sources are considered to be municipal and industrial waste
discharges and nonpoint sources are everything else. Exceptions, which
are roughly defined in section 502 of Public Law 92-500 (U.S. Congress,
1972), include concentrated animal feeding operations and also other
operations which are or may be discharging pollutants through a pipe,
ditch, channel, etc. For the purpose of this study, nonpoint source
watersheds are those without municipal and industrial waste discharges
and animal feedlots identified as point sources.
It should be clearly understood that the values illustrated on the maps
are those of mean annual concentrations representative of existing non-
point source areal characteristics, which include both natural and anthro-
pogenic sources. The NES stream samples were taken on the average of once
a month for one year (see map insert for years of sampling initiation and
distribution of sites). Land use and other drainage area characteristics
data were compiled from available aerial photography and other materials
dated as closely to the sampling years as possible. Sampling and laboratory
methods were uniform throughout the entire data set. For more detailed
information on the sampling and laboratory methods, the history, objectives
and overall design of the NES, and the land use study appendage to the NES,
* Corvallis Environmental Research Laboratory, Office of Research and
Development, U.S. Environmental Protection Agency, Corvallis, OR 97330
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see the following publications: NES Working Paper No. 175 (U.S. Environ-
mental Protection Agency, 1975), and The Influence of Land Use on Stream
Nutrient Levels (Omernik, 1976).
Map Development
Basically, the development of each of the three stream nutrient concentra-
tion maps involved several preliminary processes. First, the actual mean
annual nutrient concentrations were assigned to the representative posi-
tions of their true sampling site locations on a 1:3,168,000 scale base
map. Then, an enlargement of Anderson's Major Land Uses map (U.S. Geologi-
cal Survey, 1970) was prepared in color at the 1:3,168,000 scale. It
should be noted that the land use category scheme used on Anderson's map
is compatible, with some transposition, with that used for the enclosed
graphs. Next, a blank drafting film overlay was attached to the base
map on which the actual concentrations for a given nutrient form had
been annotated. These in turn were superimposed on, and registered to,
the enlarged land use map. Then a 1:3,168,000 map illustrating all of
the study watersheds by color coded dots, indicating their respective
land use categories, was compared to the above map to enable the compiler
to determine whether or not data points were representative of typical
general land use in their respective regions.
By studying these mapped data and knowing the general relationships
shown by the graphs, one could visualize the general land use patterns
and the spatial relationships between observed stream nutrient concen-
trations and land use. Additional help in .understanding the part other
macro-watershed characteristics play was provided by comparison with
various other maps including: distribution maps of fertilizer expendi-
tures, cattle and other agricultural products or activities, (U.S. Dept.
of Commerce, Bureau of Census, 1973); isometric maps of acid precipita-
tion observations (Likens, 1975); and an "ecoregions" map, which in
itself provides a regional breakdown of a synthesis of the macro-water-
shed characteristics relative to forest and rangeland resources (Bailey,
1976). Therefore, the actual drawing of the stream nutrient concentra-
tion map units was guided to a great extent by the alignment of Anderson's
land use map units. However, as the observed values and their apparent
relationships and interrelationships with existing land use and other
phenomenon varied regionally, the map units were drawn to reflect these
variations.
The nutrient concentration map units (each representing a range of concen-
trations) were determined mainly by analysis of the frequency distributions
of the 928 values for each nutrient form. The objective was to obtain a
fairly even distribution of values (observations) throughout each map's
range of map units. Understandably, the map unit sizes were adjusted
slightly to allow for even, easy-to-understand intervals.
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The categories shown in the reliability map inset reflect several factors.
The two most important are (1) the distribution of data points and (2)
the types and homogeneity of land use in a given region together with the
probable applicability of land use-stream nutrient concentrations rela-
tionships to that region. Also important were the significance of surface
runoff in determining stream nutrient concentrations and the distinguish-
ability of nonpoint from point source impact on streams. Examples of
where the latter becomes a problem can be found in the flat tidal reaches
of the Atlantic Coastal Plain and throughout much of Florida. Obviously
regions where NES stream sampling sites were concentrated and where land
use and watersheds were well defined, the reliability would be categorized
as good. On the other hand, arid areas where stream data (where streams
exist) are difficult to obtain and/or where surface runoff is an insigni-
ficant factor, the reliability would be categorized as poor. Areas cate-
gorized fair were generally those where NES tributary sampling data were
scarce or lacking, but where land use and other macro-drainage area charac-
teristics were such that reasonable estimates could be made based on the
relationships and interrelationships observed in similar areas.
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Bibliography
Bailey, R. G. 1976. Ecoregions of the United States. Map, scale -
1:7,500,000. U.S. Forest Service, Ogden, Utah.
Likens, G. E. 1975. Acid Precipitation: Our Understanding of the
Phenomenon. In: Acid Precipitation. Proceedings of a Conference
on Emerging Environmental Problems. EPA-902/9-75-001, U.S. Environ-
mental Protection Agency, Region II, New York, New York. pp. 45-75.
Omernik, J. M. 1976. The Influence of Land Use on Stream Nutrient
Levels. EPA Ecological Research Series, EPA-600/3-76-014. U.S.
Environmental Protection Agency, Con/all is Environmental Research
Laboratory, Corvallis, Oregon. 105 pp.
U.S. Congress. 1972. An Act to Amend the Federal Uater Pollution
Control Act. Public Law 92-500, 92nd Congress, Washington, D.C.
U.S. Department of Commerce, Bureau of Census. 1973. 1969 Census of
Agriculture; Graphic Summary Vol. V, Part 15. U.S. Government Print-
ing Office, Washington, D.C. 145 pp.
U.S. Environmental Protection Agency. 1975. National Eutrophication
Survey Methods, 1973-1976. National Eutrophication Survey Working
Paper No. 175. U.S. Environmental Protection Agency, National Eutro-
phication Research Program, Corvallis, Oregon. 91 pp.
•
U.S. Geological Survey. 1970. The National Atlas of the United States,
U.S. Government Printing Office, Washington, D.C. 417 pp.
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.009
68
77
295
5
16
103
12
17
10
144
11
72
74
>90% Forest
>75% Forest
> 50% Forest
> 75% Cleared
Unproductive
> 50% Cleared
Unproductive
Mixed
>50% Range:
Remainder predominantly
forest
> 75% Range
> 50% Range:
Remainder predominantly
agriculture
> 50% Agriculture
>40% Urban
> 75% Agriculture
590% Agriculture
Land Use
vs.
Mean Total Phosphorus and Mean
Orthophosphorus Stream Concentrations
Data from 904 "Nonpoint source-type" watersheds
distributed throughout the United States
orthophosphorus concentration
.034
4
I— total phosphorus concentration
.02 .04 .06 .08 .10 .12
Milligrams per Liter
.14
.16
.18
.20
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68 >90% Forest
77 > 75% Forest
295 > 50% Forest
> 75% Cleared
5 Unproductive
> 50% Cleared
Unproductive
103 Mixed
£ 50% Range:
12 Remainder predominantly
forest
17 £ 75% Range
250% Range:
IQ Remainder predominantly
agriculture
144 > 50% Agriculture
11 > 40% Urban
72 > 75% Agriculture
74 > 90% Agriculture
.598
Land Use
vs.
Mean Total Nitrogen and Mean
Inorganic Nitrogen Stream Concentrations
Data from 904 "Npnpoint source-type" watersheds
distributed throughout the United States
inorganic nitrogen concentration
.294
J839
L- total nitrogen concentration
4.233
] 5.354
1.0
2.0
3.0 4.0
Milligrams per Liter
50
6.0
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ERRATA NOTICE
A few errors are present on the total phosphorus and total inorganic
nitrogen maps because the wrong negatives were used in printing.
These mistakes will probably not affect your use of the maps since
all of the errors except one involve less than 0.2% of the total
area. Corrected maps will be included with the final project report
which will be issued later this year.
The more significant corrections are listed below:
Total Inorganic Nitrogen
(1) Map unit 6 is incorrectly represented by the same
color as that used for map unit 7; therefore, use the
unit number to differentiate between areas. This is
the only error involving a significant portion of the
map.
(2) A few small areas are not categorized and there-
fore appear as white. These will be categorized in
the corrected maps.
Total Phosphorus
(1) There are no errors involving significant portions
of the map. Great Salt Lake should appear as white
rather than represented by map unit 7.
(2) The white area shown for the western edge of Utah
should be represented by map unit 7.
(3) A small area in the extreme northwestern tip of
Florida and southern edge of Alabama is incorrectly
categorized.
Any questions regarding the correction should be directed to
James M. Omernik (FTS 420-4613 or 4611/Commercial 503-757-4613
or 4611) at the Corvallis Environmental Research Laboratory,
200 S. W. 35th St., Corvallis, Oregon 97330.
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