Nonpoint pollution of surface waters with phosphorus and nitrogen
Summary (3 min read)
INTRODUCTION
- People are attracted to lakes, rivers, and coastlines for diverse reasons.
- Water shortage and poor water quality are linked, because contamination reduces the supply of water and increases the costs of treating water for use.
- Eutrophication caused by excessive inputs of phosphorus (P) and nitrogen (N) is the most common impairment of surface waters in the United States (U.S. EPA 1990), with impairment measured as the area of surface water not suitable for designated uses such as drinking, irrigation, industry, recreation, or fishing.
- Nonpoint inputs often derive from extensive areas of land and are transported overland, underground, or through the atmosphere to receiving waters.
- In many cases, point sources of water pollution have been reduced, owing to their relative ease of identification and control.
Eutrophication
- —Eutrophication, caused by excessive inputs of P and N, is a common and growing problem in lakes, rivers, estuaries, and coastal oceans (Smith 1998).
- Eutrophication is also widespread and rapidly expanding in estuaries and coastal seas of the developed world (NRC 1993a, Nixon 1995).
- The blooms have severe negative impacts on aquaculture and shellfisheries (Shumway 1990).
- —Nonpoint sources are now the dominant inputs of P and N to most U.S. surface waters (Table 3).
- Recovery can sometimes be accelerated by combining input controls with other management methods (Sas 1989, NRC 1992, Cooke et al. 1993).
Direct health effects
- Because of this, no drinking water standards have been established for P (U.S. EPA 1990).
- The proximal cause is toxic algal blooms or anoxic conditions stimulated by P pollution.
- This age group is most sensitive because bacteria that live in an infant’s digestive tract can reduce NO3 to nitrite, causing conversion of hemoglobin into methemoglobin, which interferes with the oxygen-carrying ability of blood (Amdur et al. 1991).
- Nitrate can also be toxic to livestock if reduced to nitrite, which causes methemoglobinemia and abortions in cattle.
- NO3-N levels of 40–100 mg/L in drinking water are considered risky unless the feed is low in NO3 and fortified with vitamin A (Sandstedt 1990).
WHAT ARE THE SOURCES OF NONPOINT POLLUTION?
- Nonpoint P and N pollution is caused primarily by agricultural and urban activities (Novotny and Olem 1994, Sharpley et al. 1994).
- Atmospheric deposition from diverse sources can add significant amounts of N to surface waters (Howarth et al. 1996).
- Agriculture is the predominant source of nonpoint nutrient pollution in the United States (NRC 1992, U.S. EPA 1996).
Agriculture
- On the world’s agricultural lands, nutrient transport by farming systems has overwhelmed natural nutrient cycles (Fig. 1).
- Fertilizers are moved from areas of manufacture to areas of crop production.
- N is added to the atmosphere through volatilization of NH3 (Schlesinger and Hartley 1992) and microbial generation of N2O (Eichner 1990).
- Manure yields from concentrated livestock operations often exceed the capacity of croplands to sequester the nutrients (NRC 1993b).
- The amount of P lost to surface waters increases with the P content of the soil (Fig. 2).
Urban runoff
- A significant amount of P and N enters surface waters from urban nonpoint sources, such as construction sites, runoff of lawn fertilizers and pet wastes, and inputs from unsewered developments.
- Urban runoff is the third most important cause of lake deterioration in the United States (U.S. EPA 1990), affecting ;28% of the lake area that does not meet water quality standards.
- Erosion rates for construction sites can be extremely high, so the total nonpoint pollution yield is large.
- This eroded material contributes to siltation of water bodies as well as to eutrophication (U.S. EPA 1990).
Atmospheric deposition of N
- N deposited to aquatic ecosystems from the atmosphere has several origins, including gases released from agriculture and burning of fossil fuels (Vitousek et al. 1997).
- Combustion releases significant quantities of fixed N into the atmosphere, both from the oxidation of organic N stored in the fossil fuels and from the ‘‘fixation’’ of molecular N2 during high-temperature, high-pressure combustion.
- This is only onefourth of the amount of N used in inorganic N fertilizer and perhaps one-seventh of the total amount of N fixed globally through human activity, including inorganic fertilizers and N fixation by agricultural crops (Vitousek et al. 1997).
- In a comparative study of N fluxes from 33 rivers in the northeastern United States, Jaworski et al. (1997) found that the fluxes of both NO3 and total N in the rivers were correlated with the atmospheric deposition of oxidized N (which comes largely from fossil fuel combustion) onto the watersheds of these rivers (Fig. 3).
- Nitrate concentrations increased in all of the rivers over that time period.
Landscape management
- Riparian vegetation can significantly reduce nonpoint nutrient flows to surface waters (Lowrance et al.
- This vegetation also makes important contributions to fish and wildlife habitat and regional biodiversity.
- Interest in the use of riparian vegetation for nonpoint pollution control has grown rapidly in recent years, as evidenced by more than 500 publications on the subject (Correll 1997 and www.serc.si.edu).
- Wetlands, lakes, and rivers are sites of denitrification (conversion of the nutrient NO3 to atmospheric N2) that decrease the flow of N to downstream ecosystems (Jansson et al. 1994).
Agricultural P and N management
- The ultimate causes of nonpoint pollution from agricultural lands are excessive fertilizer use and highdensity livestock operations.
- Defining thresholds has been controversial, in part because data are insufficient.
- A stronger scientific foundation can, and should, be developed for soil nutrient thresholds so that scientifically based standards can be promulgated and defended.
- —Transport of P and N to surface waters by erosion and runoff may be reduced by riparian zones, buffer strips, conservation tillage, terracing, contour tillage, cover crops, and retention ponds (Osborne and Kovacic 1993, Sharpley et al.
Control of urban runoff
- Control of urban nonpoint pollution is a well-developed branch of civil engineering, with an extensive and sophisticated literature (Novotny and Olem 1994).
- Other approaches used to varying degrees include the creation of retention ponds, wetlands, and greenways as an integrated component of urban stormwater management systems; litter control and street sweeping; reduction of impervious areas; and reduction of erosion, especially from construction sites.
Atmospheric deposition
- Thus, steps needed to reduce surface transport of agricultural N will also reduce atmospheric transport.
- Reductions in fossil fuel combustion and improved interception of NO3 from fossil fuel combustion will also reduce atmospheric deposition of N (Vitousek et al. 1997).
CONCLUSION
- A sound fundamental understanding of the processes that cause nonpoint pollution and eutrophication exists.
- At regional and global scales, the causes and consequences of nonpoint pollution are clear.
- There is need for site-specific analyses, and the capacity for such analyses is well developed and improving.
- The most critical need in reducing nonpoint pollution and mitigating eutrophication may be creative institutional mechanisms that match scientific understanding with social realities (Gunderson et al. 1995).
- Their evaluation of the literature suggests that the necessary scientific understanding is well developed and could be readily mobilized in the search for solutions.
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References
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...In recent decades, N transport to the oceans has increased (Howarth et al. 1996)....
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...Atmospheric deposition from diverse sources can add significant amounts of N to surface waters (Howarth et al. 1996)....
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...However, volatilization from agricultural land is the source of a significant fraction of this N (Howarth et al. 1996, Vitousek et al. 1997)....
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