Fertilizers, water quality, and human health.

Fertilizer use has increased 10-fold since World War II. Current application rates are staggering and greatly exceed the amounts absorbed by plants. For example, the average amount of nitrogen applied to corn in the Midwest is approximately 160 kg/hectare/year, and in California > 200 kg/hectare/year is added to more than 3 million cultivated hectares [U.S. Department of Agriculture (USDA) 1991, 2003].

The geochemical fate of this excess nitrate is complex, but it is evident that much of it becomes non-point source pollution that degrades both surface waters and valuable groundwater supplies. Large increases in nitrate loads accompany flood pulses in midwestern rivers, and these loads clearly originate in the "Corn Belt," where nitrate application is highest (Goolsby et al. 1993; Meade 1995; Winston and Criss 2003). Much of this load ends up in the Gulf of Mexico, where a hypoxic "dead zone" of up to 20,000 [km.sup.2] develops each year, much to the detriment of the aqueous environment (e.g., Goolsby 2000; Mclsaac et al. 2001).

Other paths convey nitrate downward into shallow groundwater, an essential resource that provides the domestic water supply for nearly 50% of Americans. Nitrate contamination of well water is now widespread in the United States (Nolan et al. 1998). Numerous case histories document that the change from potable to nonpotable nitrate contents can occur very rapidly (e.g., Snow et al. 1988). Moreover, well contamination is aggravated by low groundwater levels (Davisson and Criss 1993), so one can predict that further rapid degradation of drinking-water supplies will accompany the current drought conditions in the western United States. This problem has been most severe in California, where several municipalities in the Central Valley and coastal valleys such as the Salinas have been forced to abandon entire well fields because nitrate levels have risen sharply above the maximum contaminant level (Davisson and Criss 1996).

These contaminant levels were established to prevent acute exposure that leads to methemoglobinemia, or "blue baby syndrome," in infants, which can be potentially fatal (Knobeloch et al. 2000). Further, chronic exposure to nitrate-contaminated drinking water in Spain, China, and Taiwan has more recently been linked to increased risk of gastric cancer (Knobeloch et al. 2000; Morales-Suarez-Varela et al. 1995; Xu et al. 1992; Yang et al. 1998), which may have profound implications for potential health risks from our widespread non-point source nitrate contamination in the United States.

Widespread nitrate contamination is not necessary. Nitrate contamination is one of many problems that can arise when profits are based on short-term economics, in this case on maximized crop yields. Such faulty cost analysis neglects the long-term consequences of commercial activities and the fact that those consequences need not be confined to the properties where problems originate. We can do better.

REFERENCES

Davisson ML, Criss RE. 1993. Stable isotope imaging of a dynamic groundwater system in the southwestern Sacramento Valley, California (USA). J Hydrol 144:213-246.

Davisson ML, Criss RE. 1996. Stable isotope and groundwater flow dynamics of agricultural irrigation recharge into groundwater resources of the Central Valley, California. In: International Symposium on Isotopes in Water Resources Management. IAEA-SM336/14. Vienna:International Atomic Energy Agency, 405-418.

Goolsby DA. 2000. Mississippi basin nitrogen flux believed to cause Bulf hypoxia. E0S Transactions, American Geophysical Union 81(29):321-327.

Goolsby DA, Battaglin WA, Thurman EM 1993. Occurrence and Transport of Agricultural Chemicals in the Mississippi River Basin, July through August 1993. U.S. Geological Survey Circular 1120-C. Reston, VA:U.S. Geological Survey.

Knobeloch L, Salna 8, Hogan A, Postle J, Anderson H. 2000. Blue babies and nitrate-contaminated well water. Environ Health Perspect 108:675-678.

McIsaac OF, David MB, Gertner GZ, Goolsby DA. 2001. Eutrophication: nitrate flux in the Mississippi River. Nature 414:166-187.

Meade RH, ed. 1995. Contaminants in the Mississippi River, 1967-1992. U.S. Geological Survey Circular 1133. Reston, VA:U.S. Geological Survey.

Morales-Suarez-Varela MM, Llopis-Gonzalez A, Tejerizo-Perez ML. 1995. Impact of nitrates in drinking water on cancer mortality in Valencia, Spain. Eur 3 Epidemiol 11(1):16-21.

Nolan BT, Ruddy BC, Hitt K J, Dennis R, Helsel DR. 1998. A national look at nitrate contamination of ground water. Water Conditioning Purification 39:76-79. Available: http://water.usgs.gov/nawqa/wcp/index.html [accessed 18 May 2004].

Snow J, Mills T, Zidar M. 1988. Nitrates in Ground Water, Salinas Valley, California. Salinas, CA:Monterey County Flood Control and Water Conservation District.

USDA. 1991. Agricultural Chemical Usage, California Vegetable Summary 1990. U.S. Department of Agriculture. Available: http://usda.mannlib.cornell.edu/reports/ nassr/other/pcu-bb/cach0692.txt [accessed 3 June 2004].

USDA. 2003. Agricultural Chemical Usage, 2002 Field Crop Summary. Available: http://usda.mannlib.cornell.edu/reports/nassr/other/pcu-bb/agcs0503.pdf [accessed 3 June 2004].

Winston WE, Criss RE. 2003. Oxygen isotope and geochemical variations in the Missouri River. Environ Geol 43:546-556.

Xu G, Song P, Reed PI. 1992. The relationship between gastric mucosal changes and nitrate intake via drinking water in a high-risk population for gastric cancer in Moping county, China. Eur J Cancer Prev 1(6):437-443.

Yang CY, Cheng MF, Tsai SS, Hsieh YL. 1998. Calcium, magnesium, and nitrate in drinking water and gastric cancer mortality. Jpn J Cancer Res 89(2):124-130.

Robert E. Criss

Washington University

St. Louis, Missouri

E-mail: [e-mail]

M. Lee Davisson

Lawrence Livermore National Laboratory 

Livermore, California

Robert E. Criss is a professor of earth and planetary sciences at Washington University. His research focuses on stable isotope studies of natural waters and water-rock interactions.

M. Lee Davisson is a staff researcher at Lawrence Livermore National Laboratory, where he conducts numerous studies tracing sources and ages of water and contaminants using analytical chemistry and isotope geochemistry methods.