Nitrate in Surface Water

Summary

Surface water safe to drink and aquatic ecosystem healthy, low/background concentrations of nitrate.

General Information
What is it?

Many macro and micro nutrients are essential to primary productivity, food-web dynamics, and ecological function. In particular, almost all organisms require nutrients (e.g., nitrogen and phosphorus) in some form for physiological processes (Ryther and Dunstan 1971, Vitousek et al. 1997b, Carpenter et al. 1998). The availability and forms of these elements play important roles in shaping communities, and organisms are frequently nitrogen or phosphorus limited (Galloway et al. 1995, Vitousek et al. 1997a). However, human alteration of nutrient cycles has resulted in many watersheds being highly enriched in certain elements, specifically nitrogen, phosphorus, and/or sulfur. Among other reasons, this is frequently a result of agricultural practices which use nitrogen and phosphorus enriched fertilizers to increase crop yield (Vitousek et al. 1997a, Vitousek et al. 1997b) or sulfur-based fungicides, which subsequently wash into the riverine systems. While artificial fertilization can increase ecosystem productivity, it can also decrease biological diversity (Tilman 1987). Currently, human activity adds as much fixed nitrogen to terrestrial ecosystems as do all natural sources combined (Vitousek et al. 1997a, Vitousek et al. 1997b). Nutrients, which are necessary for aquatic life, are toxic at high concentrations, which can sometimes be found in groundwater used for drinking. Secondary impacts of nutrient enrichment are important in surface waters (e.g., low dissolved oxygen, disruption of nutrient cycling) that cause concern (SWRCB 2010). Thus, these natural elements can become a significant form of pollution in aquatic ecosystems by upsetting natural nutrient cycles and can result in eutrophication and fuel harmful algal blooms (Carpenter et al. 1998). Nitrogen is often of the greatest concern as a nutrient pollutant and has as a result more monitoring data.

What can Influence or Stress Condition?

Nitrogen is one of the most essential elements for plant reproduction and growth, so the amount of available nitrogen can strongly limit plant productivity. At first glance, nitrogen limitation may be counterintuitive, since 78% of the atmosphere is composed of nitrogen gas (N2). Yet N2, and most forms of nitrogen found in terrestrial ecosystems, is not directly available to plants. Plants therefore rely on nitrogen fixing organisms to transform N2 into bioavailable forms. Consequently, although nitrogen is abundant, many natural systems are nitrogen limited. To get around this limitation, humans developed the Haber-Bosch process to artificially produce bioavailable nitrogen that can be used in fertilizers to increase productivity in cultivated crops. Fertilizers must be continuously reapplied because crops take up some the added nitrogen, and leaching causes the movement of nutrients out of the soil via irrigation or storm-water runoff. As this nitrogen moves into nearby waterways, it can accumulate and at higher than natural concentrations. High levels of nitrogen in waters can also produce harmful algal blooms. In turn, these blooms can produce "dead zones" in water bodies where dissolved oxygen levels are so low that most aquatic life cannot survive (EPA 2010). Through these processes along with other sources such as urban effluent and atmospheric deposition, humans have roughly doubled the amount of fixed nitrogen, and this alteration may have drastic consequences for ecological systems (Houlton).

Target or Desired Condition

Nutrients (nitrogen and phosphorus) were consistently one of the top pollutants on the CWA Section 303(D) Lists to Congress Reports beginning in the early 1990’s, but "excess" concentrations of nutrients vary by waterbody type, climate, geologic areas, and other local risk cofactors (e.g., degraded riparian). Therefore, Nutrient Criteria cannot be developed as a single number for the U.S., or CA, due to variability in background conditions and the role of other risk co-factors which affect nutrient processing within ecosystems. TMDL levels for nutrients are being developed by the EPA and the state of California for various waterways, however specific numeric values for criteria thresholds are not available for all waterbodies for ecological concerns. California does have a threshold for drinking water nitrate of 10 µg/L (ppm) in surface and groundwater sources. For nitrate we used 1 ppm as the target for a good condition (score = 100) and 10 ppm for a target for poor condition (score = 0).

References

Carpenter, S. R., N. F. Caraco, D. L. Correll, R. W. Howarth, A. N. Sharpley, and V. H. Smith. 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8:559-568.

Coats, R.N. and C.R. Goldman. 2001. Patterns of nitrogen transport in streams of the Lake Tahoe basin, California-Nevada. Water Resources Bulletin, 37(2): 405-415.

EPA (Environmental Protection Agency). 2010. http://www.epa.gov/waterscience/criteria/nutrient/index.htm. Accessed April 2nd, 2010.

Galloway, J. N., W. H. Schlesinger, H. Levy, A. Michaels, and J. L. Schooner. 1995. Nitrogen fixation - anthropogenic enhancement - environmental response. Global Biogeochemical Cycles 9:235-252.

Galloway et al. 2003. The nitrogen cascade. Bioscience 53 (4): 341-356.

Ryther, J.H. and Dunstan, W.M. 1971. Nitrogen, phosphorus, and euthrophication in coastal marine environment. Science 171(3975): 1008-1013.

Tilman, D. 1987. Secondary succession and the pattern of plant dominance along experimental nitrogen gradients. Ecological Monographs 57:189-214.

Vitousek, P. M., J. D. Aber, R. W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and G. D. Tilman. 1997a. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7:737-750.

Vitousek, P. M., H. A. Mooney, J. Lubchenco, and J. M. Melillo. 1997b. Human domination of Earth's ecosystems. Science 277:494-499.