Identifying the Major Sources of Nutrient Water Pollution A national watershed-based analysis connects nonpoint and point sources of nitrogen and phosphorus with regional land use and other factors. L A R R Y 1. P U C K E T T
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ince the passage of the Clean Water Act (CWA] in 1972, more than $540 billion has heen spent on water pollution controls. In spite of this massive commitment of funds, in 1992 approximately44% of US. river miles tested still did not fully support the uses designated by the states (11. One wellrecognized problem is the lack of controls in the CWA for nonpoint-source pollution (21. Preventing or controlling pollution from nonpoint sources has been stymied, in part, because little was known about the relative magnitudes of nonpoint and point sources of nutrients at the national level. Only recently has it heen possible to estimate the major sources of nutrients entering watersheds and relate these inputs and resulting stream loads to land use patterns and regional settings. Analysis of these new data reveals that nutrient inputs into watersheds vary according to land use practices, that atmospheric nitrogen may he a more important source of nutrient contamination than previously believed, and that in some localities point sources are still the major water quality problem. These findings underscore the need for individualized watershed management plans for preventing and controlling water pollution in the United States. The CWA has traditionally focused on reducing discharges of pollutants, particularly nitrogen and phosphorus, to surface waters from sewage treatment plants and other point sources. Because of the Act’s point-source focus, nearly 90% of the monies allocated to water pollution prevention have been targeted at point sources I J ) . Congress has been considering revisions to the Act since 1993, but it is unclear at this time how nonpoint-source pollution will be addressed. In spite of Congressional uncertainty, many state and federal water quality programs have begun to tackle nonpoint sources by focusing on watershedbased management plans. However, to develop ef-
VOL. 29, NO. 9.1995 iENVIRONMENTAL SCIENCE &TECHNOLOGY
fective plans, it is necessary to identify and quantify the dominant sources of nonpoint-source pollution at the watershed scale, and then eliminate, to the greatest extent possible, those sources contributing most to water quality problems. Using information from various databases, such as the U.S. Geological Survey’s National Water Quality Assessment program, Census of Agriculture, and the National Trends Network, it is now possible to estimate the relative magnitudes of the major nonpoint and point sources of nutrients. The estimates allow three key points to be addressed the proportion of nonpoint and point sources of nutrients in US. watersheds, the continuing importance of point sources of nutrients, and the relative importance of atmospheric inputs as a nonpoint source of nitrogen.
Identifying the major sources Among the various sources of water quality impairment, agricultural practices are ranked as the most impoaant factor for rivers and lakes and third in importance for estuaries (1). Going one step further and delineating agriculturalpractices with respect to specific pollutants, nutrient contamination is identified as the second most important contribution to river and lake pollution and first in importance for estuaries. Commercial fertilizer is the primary agricultural nonpoint source of nitrogen and phosphorus. Between 1945 and 1993, the use of nitrogen in commercial fertilizers in the United States increased 20fold, rising from about 0.5 million to nearly 10.3 million metric tons per year. Phosphorus use has increased as well, kom about 0.5 million to nearly 1.8 million metric tons per year [Figure 1). Much of the fertilizer is applied in the upper Midwest “corn belt” states (Figure 21. Studies have estimated that farmers may apply 24 to 38% more fertilizers than crops require because of uncertainties 0013-936W95/0929-408A109.00iO
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1995 American Chemical Society
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Agricultural practices are the moa important factor in water quality impairment for rivers and lakes (above). but in lage urban centers point sources remain a major nutrient source (below).
associated with weather and soil nutrient status (3.4). Animal manure is another agricultural pollution source. Each year in the United States the manure from 7.5 billion farm animals produces an estimated 5.9 million and 1.8 million metric tons of nitrogen and phosphorus, respectively The upper Midwest has the greatest manure nutrient input (Figure 3).Where farm animals are allowed to graze freely, the manure is distributed over the landscape and represents a nonpoint source of nutrients. However, where animals are confined to feedlots, barns, or sheds, they may become more of a point source nutrient problem. lfthe nutrient content of applied ma-
nure is not accounted for when computing crop requirements, the excess may move into surface and ground waters. Atmospheric inputs have been largely ignored as nonpoint-source pollution because they originate primarily at point sources. For example, releases of nitrogen oxides into the air from point sources become nonpoint sources ofwater pollution when that nitrogen reaches water bodies as rainfall. More than 2.9 million metric tons of nitrogen are deposited in the United States each year from the atmosphere (3, primarily in the northeastern states (Figure 4). About 53%of these nitrogen emissions come from coal- and VOL. 29. NO. 9.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY. 409 A
have been developed. The data were examined for differences and similaities according to land use clasChanges in fertilizer nitrogen and phosphorus use $ificationand geographical region; differences that U S data show that from 1945 to 1993 nrtrogen use jumped 20-fold and ire cited are statisticallysignificant. Land use classes ihorus use tripled Table 1) identify the dominant use of land within he watershed hut do not eliminate other uses that can affect water quality Fertilizer nitrogen inputs per unit area to agricultural land (Table 2) were largest in watersheds dominated by agriculture, Less in forested watersheds, and lowest in urban-dominated watersheds. nputs of fertilizer in mixed watersheds were greater than in urban watersheds, hut there was no statistically significant difference from inputs in agriculural and forested watersheds, There were no statistically significant differences among phosphorus inputs in agricultural, forested, and mixed-land-use watersheds. However, all Yhowed significantly greater inputs than did urban Natersheds. The higher fertilizer inputs of both nitrogen and 3hosphorus per unit area to agricultural lands in agricultural- versus urban-dominated watersheds suggest a systematic difference in agricultural practices as well as higher application rates in predominantly agricultural watersheds, This could also mean that more of the agricultural land in agricultural-dominated watersheds is used for intenWatershed class tion by dominant land u sive crop production. Conversely, the significantlylow Values under the land use classifications are the percent; ~’ fertilizer application rates in urhan-dominated waeach category in the watershed, except for populi e tersheds suggest less intensive use of agricultural which is persons per square kilometrlands or possibly conversion to other uses. The higher inputs of nitrogen and phosphorus m forested and Dominant land Cmp and ropuit mixed watersheds relative to urban watersheds reU o C C l ~ partun ~ ~ Form urban dsns tlect the importance of agricultural land as a source of nutrients in those watersheds. Agricultur 40 C40 < 10 Nitrogen inputs to agricultural land from fertilUrban 30 >3 Forest 40 > 50 < 10 izer (Table 2) were largest in western and southeastMixed watersheds not meeting the above crit ern watersheds and lowest in northeastern watersheds. Inputs in the central watersheds were not statistically significantlydifferent from the other watersheds. oil-burning electric utilities and large industries (6); Phosphorus inputs to agricultural land from ferapproximately 38% from automobiles, trucks, buses, tilizer were significantly larger in western than in cenand other forms of transportation; and the remain- tral watersheds. Inputs in southeastern and northeastern watersheds were not statisticallysignificantly ing 9% from miscellaneous sources. Ammonium can also he an important form of ni- different from western and central watersheds. trogen in precipitation. However, its predominant The consistently large nitrogen and phosphorus sources are volatilization from fertilizer and animal fertilizer inputs in western watersheds reflect the inmanure. tensive agriculturalpractices in areas like the San JoaOverall, the most recent nationwide estimates of quin Valley CA (Figure 21, where multiple crops in point sources of pollution [compiled in 1984) indi- the same year may require multiple fertilizer applicated that from 1978 to 1981 point sources dis- cations. charged approximately 1.2 million metric tons of niThere were no significant differences in nitrotrogen and 260,000 metric tons of phosphorus gen or phosphorus inputs to agricultural land from annually [7).Thesevalues are dwarfed by the 19.4 mil- animal manure based on land use (Table 21. On a relion tons of nitrogen and 5.7 million metric tons of gional basis nitrogen and phosphorus inputs were phosphorus from nonpoint sources. Yet, in spite of largest in southeastern and northeastern waterthe billions of dollars spent on point source pollu- sheds and lowest in central watersheds. Inputs in tion controls, point sources may still represent a sig- western watersheds were not statistically significantly different from those in other regions [Figure nificant local impact (I). ~
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Agricultural practices and regional differences Using information from various databases (see sidebar, Data sources and methodology), national estimates based on nutrient inputs and stream loads 4 1 0 A m VOL. 29. NO,
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The large manure nutrient inputs to agricultural land in the northeast reflect the higher density animal production in that region, particularly for the dairy industry. Confined feeder operations for the pro-
Median values for annual loadings - to watersheds Values followed by the same letter are not significantlydiflerent (a= 0.051 and apply only u1 multiple comparisons among land use classes or regions listed in the rows. Retention is defned as the traction of total nnrogen or phosphorus inpuis not accounted for b stream loads Laad un clan
Elmad
Atmosphere Fertilizer Manure Point source Stream load c
Retention
Nitrogen Nitrogen Phosphorus Nitrogen Phosphorus Nitrogen Phosphorus Nitrogen Phosphorus Nnmgen Phosphorus
Apricubrel
Region
Ford
Mixsd
Urban
0.35a 6.06 a 0.75 a
0.68 a,b
0.78 a,b
2.88 b 0.69a
3.24a.b 0.76a
0.87 b 0.01 c 0.00 b 2.85 a 0.53 a 0.18~ 0.05 b
1.50 a
3.18a
2.10a
0.65 a 0.03 a,b
0.73a 0.00 a
0.70a 0.14 b,c
0.01 a,b 0.61 a,b 0.03 a 0.93 a 0.98 a
O.00a 0.31 a 0.02s
0.02 b 0.59 b 0.06 b
0.82 b 0.93 b
0.84a.b 0.93a.b
0.39 a,b 0.03 a,b 0.77 b 0.86 b
Watcm
0.17 a 7.29 a 0.84 a
2.63 a,b 0.63 a,b 0.00 a 0.00 a 0.30 a 0.02 a 0.86 a,b
0.96 a
Csml
0.39 b 3.42 a,b 0.38 b
Souheartom
Norheaalo
0.64 b
0.99 c
4.04a 0.46 a,b
1.41 a
1.91 b
O37a 0.16 b 0.03b
0.72 b 0.06 a,b 001 a.b 0.33a
2.30 b 0.68 a.1 3.63 b 0.84 b 0.10 b 0.01 b 0.66 b
0.02 a
0.04 a
0.89 a 0.95 a
0.76 b
0.30 a 0.05 a 0.90 a 0.93 a
0.93
s(
tl
duaion of pouliq and hogs also may account for pari of this pattern.
Atmospheric deposition and point sources Atmospheric inputs of nitrogen (Table 2) were l a est in predominantly urbanwatersheds and smaliest in agricultural watersheds, whereas forested and mixed-land-usewatersheds were not statisticallysignilicantly different from other types of land use. There was a pronounced west-east trend of increasing nitrogen inputs, from the smallest inputs in western watersheds to the largest inputs in noaheastern watersheds. The large inputs in predominantly urban watersheds are due, in part, to large emissions of nitrogen oxides in urban areas. Also, many of the urban watersheds are located in the northeast, where atmosphericinputs are larger (Figure 4). On the other hand.. a -e r i c u l t d v dominated watersheds mav be located at some distance from urban areas or in the western pari of the nation, where atmospheric inputs are not as great. Atmospheric inputs in forested and mixed-land-use watersheds fall between the extremes of urban and agricultural watersheds, because they ate scattered throughout the nation and are influenced less by strong regional trends. Nitrogen inputs from point sources (Table2) were greatest in urban watersheds and smallest in forested watersheds Values for agricultural and mixedland-use watersheds fell between urban and forested watersheds. Phosphorus inputs followed nitrogen trends, with the exception that phosphoIUS & not significantlyp a t e ; in urban than & agricultural watersheds. It was not surprising that point source nutrient inputs were greatest in urban watersheds because generally there are more point source dischargers in urban areas. Mixed-land-use watersheds may incorporate large urban areas, accounting for their frequently large point source inputs. Conversely, predominantly forested watersheds would be expected to have the smallest point source inputs because major point sources are less likely to be located there. On a regional basis nitrogen and phosphorus in-
rogen, phosphorus ferb'liier inputs cording to 1987 data, most fertilizer is applied in upper midweste tes. Inputs are in memc tons per square kilometer, by county. trogon
.essthan 0.35 1.35to 1.05
2.45to 10.5
I
puts from point sources (Table2) were largest in central and northeastern watersheds, and smallest in western watersheds. The southeast inputs were not significantly different from watersheds in other regions. The large inputs in the northeast result from VOL. 2s. NO. s, 19% i ENVIRONMENTALSCIENCE &TECHNOLOGY
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3
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Nitrogen, phosptorus inputs from animal'manure
'
'
Data collected for 1987 showthatthe upper midwest has the greatest manure nutrient input Inputs are in metric tons par square kilometer. by counoh.
r ---
Nitrogen
I Less than 0.35
00.35 to 0.70
I 0.70 to 1.4 I 1.40 to 12.
nitrogen
nitrogen deposited from the r square kilometer, by count\ ss than 0.32 32t00.46
- J.46 to 0.63 IO.63tol.l6
: *,
the high numbers of urban watersheds, whereas the large inputs in the central region are traced to mixedland-use watersheds. The small inputs in the weste m watersheds are due to the predominance of forested watersheds in that sample population, and the intermediate values in the southeast reflect the large number of forested watersheds there. Based on land use, nitrogen and phosphorus loads (Table 2 ) followed similar patterns. The largest loads were seen in mixed watersheds, the smallest in forested watersheds; loads in @cultural and urban watersheds were not statistically significantly different from others. As before, nitrogen loads were significantly larger in the northeast than in other regions. Phosphorus loads did not differ significantlyby region. The large nitrogen and phosphorus loads in mixed watersheds, and intermediate loads in agricultural and urban watersheds, suggest that a mixture of nonpoint and point sources of nutrients, as in the predominantly mixed-land-use watersheds, produced the largest loads. The large nitrogen stream loads in northeastern watersheds (as compared to the results for phosphorus. which did not vary significantly by region) may result from a combination of the significantly larger point-source and atmospheric inputs of nitrogen in the northeast. Retention of nitrogen and phosphorus (defined as the fraction of total nutrient input from fertilizers, manure, atmospheric deposition, and point sources not found in stream loads [see Table 21) was greater in agriculturally dominated watersheds than in the forest and urban watersheds. Mixed-landuse watersheds were not statistically different. The retention results for land use comparisons were consistent with the assumption that more of the nutrients from fertilizer, manure, or atmospheric deposition would be retained by crops in the agricultural watersheds. Lower retention may he seen in forested watersheds hecause forests are located at higher elevations where atmospheric inputs are greater, or lands are steeper and rockier with shallower soils. Urban watersheds have large areas of pavement and other impermeable materials that result in faster runoff of atmospheric inputs. They also provide a greater potential for direct inputs of nutrients to streams from point sources. Regionally, there were no significant differences For phosphorus retention (Table 2). However, nitrogen retention decreased from central and southeastern watersheds, to western watersheds, to northeastern watersheds. The smaller retention values for nitrogen in the northeast compared with the rest of the watersheds are consistent with the finding of significantly larger stream loads there as well. Other contributing factors include the larger percentage of urban- and forest-dominated watersheds in the northeast as well as the significantly larger atmospheric inputs compared with the other regions.
Watershed-by-watershed variability This study shows that different land use practices re5ult in variable inputs of nutrients to watersheds around the nation. Major differences in the magnitudes of the various nonpoint and point sources of nutrients are found even in neighboring water412
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watershed and then multiplied by the area of the watershed to estimate total inputs. Adlustments were made for droplet depo: Watersheds were selected from among those investigated in the lion. urban effects, and dry deposition 16). Point-source astmates were developed from data collectec U.S. Geological Survey's (USGS) National Water Quality Assessoy E W s Permit Compliance System. Effluent volumes were mu ment program. These program study units were first impletiplied by the reported concentrations of nitrogen and phospho mented in 1991. rJs for each facility located n the watersned to yield estimate! A modified version of the scheme employed by Smith and of me total mput to a given stream ( 1 0. others (8)was used to classify 114 watersheds based on the Mass transport of total n:trogen and total phosphorus n percent of various land use classes (agriculture, forest, urban, mixed). This provided an indication of the land use in the water- streams was calculated for typical-flow years iusing the minimum variance Jnbiased estimator method 112. Mi witn conce shed that would be most likely to have the greatest impact on water quality (Table 1).Watersheds were also classified accord- tration oata available from USGS and the EPA databases. Retention was calculated as the fraction of the total nutrv ing to region (Western, Central, Southeastern, Northeastern). ent mputs to the watershed from manure, fertilizer, air, and providing a means of grouping watersheds along broad hydropo;nt soLrces that could not be accounted for n the stream geologic, climatic, and vegetation zones. Estimates of nitrogen and phosphorus in commercial fertilizer loao. Althougn tnese four sources are not the only sources 01 nitrogen an0 phospnorus, they are believe0 to represent the and in animal manure were derived from national databases of majority t felrilizer sales for 1985-1991 (9and from Census of Agriculture
Data sources and methodology
d stream loads wer rces were appor-
sheds. For example, animal manure accounted for more than half of the nitrogen inputs in Virginia's Shenandoah River watershed whereas, across the Potomac River in Maryland, fertilizer accounted for more than half of the nitrogen in the Monocacy River watershed. One interesting finding was that fertilizer nitrogen inputs per unit area of agricultural land were related to the dominant land use in the watershed. In general, agricultural land in those watersheds classified as predominantly agricultural received larger nitrogen inputs from fertilizer than agricultural land in predominantly urban watersheds, probably as a result of differences in the intensity of agricultural practices. lkese findings underscore that nutrient inputs to watersheds are dependent on variations in land use practices as well as the mixture of land use in the watershed. It is not enough to know what the land is used for. It is equally important to know the type and intensity of that use. Considerable uncertainty remains over the relative importance of the major nonpoint sources with respect to their contributions to stream loads. Mass balance studies of individual watersheds have suggested that although animal manure may represent a potentially large nutrient input to the watershed, it may contribute only a small portion of stream load (15-17).However, statistical studies of water quality trends have suggested that increases in total nitrate and phosphorus are associated with increases in livestock population densities and fertilized acreage (8.18). There is mounting evidence that atmospheric nitrogen inputs may be a more important nitrogen s o w e than has been previously suggested (19).Fisher and Oppenheimer (15) and Jaworski and his colVOL. 29. NO. 9.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY rn 4 1 3 A
leagues (16) demonstrated that nitrogen inputs from the atmosphere account for 34-40% of the nitrogen in the Chesapeake Bay and 28% in the Potomac River basins. Fu and Winchester (17) found similar results for three watersheds in Georgia and Florida. Recently, Paerl (20) associated atmospheric deposition of nitrogen with eutrophication in coastal areas of the North Atlantic, Baltic, Mediterranean, and North seas. Smith and others (18)found strong statistical association between atmospheric deposition and total nitrate conceneations in northern midwestern and northeastern streams, particularly in forested watersheds. Stoddard's suggestion that forested watersheds in many areas of the eastern United States may have already reached a saturation point with respect to atmospheric nitrogen inputs (19) could explain the significantly larger stream loads in the northeastern watersheds, where about 39% of nitrogen inputs were attributed to atmospheric deposition. In some streams, particularly in or near large urban centers, point sources remain a major nutrient source. During the past two decades, many of the point source nutrient-control efforts have concentrated on removing phosphorus because it was believed that the nutrient was most limiting to eutrophication. Furthermore, because of the high cost of total nitrogen removal, nitrogen controls have focused primarily on converting unoxidized forms of nitrogen to a less harmful form such as nitrate. This approach has reduced nitrogen oxygen demand and the toxic effects of ammonia, but it has not reduced the total amount of nitrogen released by many point sources. In most watersheds studied, point sources contributed relatively small portions of nutrients to total stream loads (Figure 5). Point sources accounted for less than 10% of the stream loads of nitrogen and phosphorus in 56 and 36% of the watersheds studied, and less than 50% of the stream loads of nitrogen and phosphorus in 91 and 63% of watersheds studied, respectively. This means that in the majority of the streams studied, nonpoint sources were the dominant source of nutrients. The results of this study support the argument that watershed management plans need to be developed on an individual watershed basis, taking into account the various hydrologic, geologic, climatic, and land use-related factors that influence water quality. Developing a more complete understanding of which factors have the greatest impact on water quality, and the timing of those impacts, is essential to providing effective pollution prevention and control programs. Although point sources may represent only a small portion of the nitrogen and phos-
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phorus transported by streams, their local impacts may exceed those of nonpoint sources scattered throughout a watershed. Consequently, any shift in focus of pollution prevention efforts to nonpoint sources must also be accompanied by continuing efforts to address point source impacts.
Acknowledgments This study was conducted as part of the US. Geological Survey, National Water Quality Assessment Program (NAWQA). Reviews from Charles Kratzer, Owen Bricker, Dale Robertson, Tudor Davies, and a n anonymous reviewer were very helpful in revision of the manuscript. Most important, I want to recognize all the NAWQA personnel who were involved in assembling and analyzing the data used in this study.
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c:res; U.S.
Environmental Protection Agency: Washingt on, DC, 1994; 841-R-94-001. Knopman, D. S.; Smith, R. A. Environment 1993, 35, 1& L11. Babcock, B. A,; Blackmer,A. M. j.Agric. Resour. Econ. 1992, 17, 335-47. rrachtenberg, E.; Ogg, C. WaterResour. Bull. 1994,30, 1109118. Sisterson, D. L. NAPAP Report 8, Acidic Deposition: State $Science and Technoloo, Appendix A National Acid Preipitation Assessment Program: Washington, DC, 1990. Kohout, E. J. et al. Month and State Current Emission i7md.s br NO,, SO,, and VOC: Methodology and Results; Ar;onne National Laboratory: Argonne, IL, 1990. Gianessi, L. E; Peskin, H. M. An Overview of the Enuiron-
nental Data Inventory: Methods, Sources, and Preliminary i'esults; Resources for the Future: Washington, DC, 1984. Smith, R. A.; Alexander, R. B.; Lanfear, K. J. In National Mater Summary 1990-91-Hydrologic Events and Water 7ualitY; Paulson, R. W. et al., Eds.; U.S. Geological Surrey: Washington, DC, 1993;Water Supply Paper 2400. U.S. Environmental Protection Agency County-Level Ferilizer Sales Data; U.S. Environmental Protection Agency: Washington, DC, 1990; PM-221.
National Atmospheric Deposition Program (NRSP-3)l National Trends Network NADP/NTN Coordination Office: Fort Collins, CO, 1992.
Point Source Methods Document; National Oceanic and Atmospheric Administration; Silver Spring, MD, 1993. Cohn, T. A. et al. Water Resour. Res. 1989, 25, 937-42. Cohn, T. A. et al. Water Resour. Res. 1992, 28, 2353-63. Helsel, D. R.; Hirsch, R. M. Statistical Methods in Water Resources; Eisevier: New York, 1992. Fisher, D. C.; Oppenheimer, M. Ambio 1991, 20, 10208. Jaworski, N. A. et al. Estuaries 1992, 15, 83-95. Fu, J.; Winchester, J. W. Geochem. Cosmochem. Acta 1994, 58, 1581-90. Smith, R. A.; Alexander, R. B.; Wolman, M. G. Science 1987, 235, 1607-15. Stoddard, J. L. In Environmental Chemistry of Lakes and Reservoirs; Baker, L. A., Ed.; Advances in Chemistry Series 237; American Chemical Society: Washington, DC, 1994; pp. 223-84. Paerl, H. A. Can. J. Fish. Aquat. Sci. 1993, 50, 2254-69. Larry J. Puckett is a n ecologist with the US.Geological Survey Water Resources Division i n Reston, VA.
