Nitrate-Nitrite

Nitrogen makes up 78% of the atmosphere as gaseous molecular nitrogen, but most plants can use it only in the fixed forms of nitrate and ammonium (for specific information on ammonium, please refer to Ammonia section). Nitrate and nitrite are inorganic ions occurring naturally as part of the nitrogen cycle (Smith, 1990).

Nitrogen Cycle:

• Nitrogen fixation is the conversion of nitrogen in its gaseous state to ammonia or nitrate. Nitrate is the product of high-energy fixation by lightning, cosmic radiation, and meteorite trails. In high-energy fixation, atmospheric nitrogen and oxygen combine to form nitrates, which are carried to the earth's surface in rainfall as nitric acid. High-energy fixation accounts for little (10%) of the nitrate entering the nitrogen cycle.

  In contrast, biological fixation accounts for 90% of the fixed nitrogen in the cycle. In biological fixation, molecular nitrogen (N2) is split into two free N molecules. The N molecules combine with hydrogen (H) molecules to yield ammonia (NH3).

  The fixation process is accomplished by a series of different microorganisms. The symbiotic bacteria Rhizobium is associated with the roots of legumes. To a lesser extent, some root-noduled nonleguminous plants also exhibit symbiotic relationships with bacteria. Some free-living aerobic bacteria, such as Azobacter and Clostridium, freely fix nitrogen in the soil. Finally, blue-green algae (cyanobacteria) such as Nostoc and Calothrix can fix nitrogen both in the soil and in water, yielding ammonia as the stable end product.

  

• Ammonification is a one-way reaction in which organisms break down amino acids and produce ammonia (NH3).

• Nitrification is the process in which ammonia is oxidized to nitrite and nitrate, yielding energy for decomposer organisms. Two groups of microorganisms are involved in nitrification. Nitrosomonas oxidizes ammonia to nitrite and water. Subsequently, Nitrobacter oxidizes the nitrite ions to nitrate.

• Denitrification is the process in which nitrates are reduced to gaseous nitrogen. This process is used by facultative anaerobes. These organisms flourish in an aerobic environment but are also capable of breaking down oxygen-containing compounds (e.g. NO3-) to obtain oxygen in an anoxic environment. Examples include fungi and the bacteria Pseudomonas (Smith, 1990).

In temperate zones, soil nitrate concentrations will vary seasonally with temperature and moisture levels. Fall and winter rains thoroughly remove all nitrates from the soil. No nitrate is naturally added to the soil during the late fall and winter because the cold weather prohibits mineralization and nitrification processes.

During the spring and summer, the increased nitrogen-fixing activity of organisms and the addition of fertilizer causes the concentration of nitrates in the soil to steadily increase. Most of this nitrate is absorbed by plants. Thus, the removal of crops in the fall increases the chances for large flushes of nitrate from the soil to water bodies. Some leaching may occur in the spring if crops are not well- established enough to absorb the nitrogen (Gower, 1980).

Numerical Categories: Limits Suggested to Maintain Designated Use:
 

Designated Use	Limit (mg/l)(AWWA 1990)
Nitrate (NO3-N):
Human Consumption	10.0
Aquatic Life
Warmwater fish	90.0
Industry
Brewing	30.0
Nitrite (NO2-):
Human Consumption	1.0
Aquatic Life
Warmwater fish	5.0
Nitrate+Nitrite:
Human Consumption	10.0
Agriculture (Livestock etc.)	100.0
Aquatic life
Estuaries (recommended)
maximum diversity	0.1* (and phosphorus 0.01)
moderate diversity	1.0* (and phosphorus 0.1)

Health Effects:

• Nitrate concentrations (as NO3-) > 45 mg/l (or > 10 mg/l NO3-N) may cause Methemoglobinemia (Blue Baby Syndrome) in infants (Straub, 1989). The toxicity of nitrate in humans is a result of the reduction of nitrate (NO3-) to nitrite (NO2-). By reacting with hemoglobin, nitrite forms methemoglobin (MHb), a substance that does not bind and transport oxygen to tissues. Thus, methemoglobin formation may lead to asphyxia. Normally, methemoglobin accounts for 1-2% of the globin in the body. A level greater than 3% is defined as methemoglobinemia. *Note: Pregnant women often have higher levels of methemoglobin during pregnancy, especially after the 30th week. Methemoglobin levels decrease rapidly after birth of the infant (Kubek et al., 1990).

• The USEPA has not yet classified the carcinogenicity of nitrates and nitrites because of inconclusive evidence (AWWA, 1990).

  Note: Water contaminated with nitrate is very difficult and costly to treat. Thus, if contamination affects a large water supply, the best alternative may be a new water source (Kubek et al., 1990).

• Freshwater system impacts: Generally, phosphorus is the limiting nutrient in freshwater aquatic systems. That is, if all phosphorous is used, plant growth will cease, no matter the amount of nitrogen available.

  Many bodies of freshwater are currently experiencing influxes of nitrogen and phosphorus from outside sources. The increasing concentration of available phosphorus allows plants to assimilate more nitrogen before the phosphorus is depleted. Thus, if sufficient phosphorus is available, high concentrations of nitrates will lead to phytoplankton (algae) and macrophyte (aquatic plant) production.

• Estuarine system impacts: In contrast to freshwater, nitrogen is the primary limiting nutrient in the seaward portions of most estuarine systems (Paerl, 1993). Thus, nitrogen levels control the rate of primary production. If a nitrogen limited system is supplied with high levels of nitrogen, significant increases in phytoplankton (algae) and macrophyte (larger aquatic plants) production may occur.

  The recommended level of nitrogen in estuaries to avoid algal blooms is 0.1 to 1 mg/l, while the phosphorus concentration is .01 to .1 mg/l. Higher concentrations of both will support less diversity (NOAA/EPA, 1988).

  It has been observed that if dissolved inorganic nitrogen levels in Chesapeake Bay tributary watersheds are maintained at less than 0.15 mg/l and dissolved inorganic phosphorus concentrations are less than 0.02 mg/l, submerged aquatic vegetation nutrient requirements are met and summer chlorophyll-a levels remain less than 15 micrograms per liter (Batiuk et al. 1992). Submerged aquatic vegetation provide food and/or habitat for estuarine organisms, including shellfish, finfish, and waterfowl (Batiuk et al., 1992). When inorganic nitrogen concentrations remain between 0.3 to 0.5 mg/l in the upper estuary when concentrations of inorganic phosphorus are less than 0.1 mg/l, chlorophyll-a levels can be maintained at 25 micrograms/l under normal summer conditions (Jaworski and Villa, 1981).

1. Algal mats, decaying algal clumps, odors, and discoloration of the water will interfere with recreational and aesthetic water uses.

2. Extensive growth of rooted aquatic macrophytes will interfere with navigation, aeration, and channel capacity.

3. Dead macrophytes and phytoplankton settle to the bottom of a water body, stimulating microbial breakdown processes that require oxygen. Eventually,dissolved oxygen will be depleted.

4. Aquatic life uses may be hampered when the entire water body experiences daily fluctuations in dissolved oxygen levels as a result of nightly plant respiration. Extreme oxygen depletion can lead to death of desirable fish species.

5. Siliceous diatoms and filamentous algae may clog water treatment plant filters and result in reduced time between backwashing (process of reversing water flow through the water filter in order to remove debris).

6. Toxic algae (occurrence of "red tide") have been associated with eutrophication in coastal regions and may result in paralytic shellfish poisoning (Mueller et al., 1987).

7. Algal blooms shade submersed aquatic vegetation, reducing or eliminating photosynthesis and productivity (Dennison et al., 1993; Batiuk et al., 1992)

Sources:

1. Nonpoint:
   • Agriculture: Primary agricultural sources of nitrate include livestock excrement (from barnyards, pastures, rangeland, feedlots, and uncontrolled manure storage areas); nitrogenous fertilizers; irrigation return flows; and decomposing plant debris (Straub, 1989).

   • Residential and Urban: Primary residential sources of nitrate include nitrogenous fertilizer used on lawn and garden, leaky on-site wastewater disposal/septic systems, sewage treatment system outfalls, sewage treatment bypass outfalls, and domestic pet excreta.

   • Other: The combustion of fossil fuels, industrial and agricultural discharges of nitrogen- containing gases, aerosols, and air-borne particles contribute to the atmospheric nitrogen load. Evidence suggests that the atmospheric deposition of nitrogen in water bodies (directly and via rainfall) constitutes a large portion of total nitrogenous inputs to estuarine and marine systems and a somewhat lesser portion of total nitrogen inputs to freshwater systems (Paerl, 1993). Additional nitrate sources include excreta both from wild animals in the surrounding watersheds, excreta from wildfowl congregating on the water body and boats that discharge raw sewage overboard.

2. Point source: Industries that use nitrates in manufacturing may release nitrate in the effluent water. Nitrate is used in the following processes: meat curing, production of fertilizer, explosives, glass, heat-transfer fluid, and heat-storage medium for solar-heating applications (Kubek et al., 1990). Additional nitrates may be contributed by sewage treatment systems and sewage treatment bypass outfalls (during high flow periods). Estuaries may be particularly susceptible to nutrient enrichment from offshore sewage pipe outfalls (Kennish, 1992).

Regional Trends of Nonpoint Source Pollution in the United States:

• The primary nitrogen source in the western United States is agricultural fertilizers. Atmospheric deposition is the second-most prevalent source.

• The central and southeastern United States also primarily derive nitrogen pollution from agricultural fertilizers. The second-most prevalent source is animal manure.

• The northeastern United States is considerably more urban than the rest of the U.S. As a consequence, atmospheric deposition is the primary source of nitrogen (accounts for one-third of the N load to watersheds). Animal manure is the second-most prevalent source (Puckett, 1994).

Mode of Transport:

• Transport in water: Water carries nitrates to surface water systems in 1) overland flow (runoff), 2) unsaturated flow, and 3) ground water flow. Overland flow is the most direct route for water transportation. Underground flow is less direct because water flow is impeded by soil permeability and porosity constraints.

• Transport in air: The combustion of fossil fuels and the discharge of agricultural and industrial nitrogen-containing compounds into the atmosphere has allowed gases, aerosols, and fine particles to be borne by wind and deposited either directly into the water body or carried from the atmosphere to the water body via precipitation. Studies have shown that rainfall is the chief means by which biologically available nitrogen (nitrate, nitrate, ammonia, some organic N) is transported to aquatic systems from the atmosphere (Paerl et al., 1990).

• Transport in soil: Nitrites in soil are readily oxidized to nitrate. Nitrates do not readily sorb to soil particles and can be removed quickly from the soil profile, Nitrates are removed by 1) the leaching action of infiltrating water, 2) plant uptake, or 3) denitrification (Gower, 1980).

A. Nitrate-Nitrogen (APHA, 1992)

1. Ultraviolet Spectrophotometric Screening Method:
   • Detection limits: Used to screen non-contaminated samples (low inorganic matter) to determine most suitable method.

   • Interferences: Dissolved organic matter, surfactants, NO2(-) and Cr(+).

2. Ion Chromatography Method:
   • Detection limits: 0.1 mg/l nitrate

   • Interferences: N/A

3. Nitrate Electrode Method:
   • Detection limits: NO3(-) ion activity between 0.00001 and 0.1 M (0.14 to 1400 mg/l)

   • Interferences: Chloride and bicarbonate, when their weight ratios to nitrate are >10, or >5, respectively.

4. Cadmium Reduction Method: Nitrate is reduced to nitrite in the presence of cadmium. The nitrite concentration is determined by diazotizing with sulfanilamide and coupling with NED dihydrochloride to form a colored azo dye that is measured colorimetrically.
   • Detection limits: 0.01 mg/l to 1.0 mg/l nitrate. Recommended especially for nitrate concentrations below 0.1 mg/l, when other methods lack sufficient sensitivity.

   • Interferences: Suspended matter in the column will restrict sample flow.

5. Automated Cadmium Reduction Method:
   • Detection limits: 0.5 mg/l to 10 mg/l nitrate.

   • Interferences: Turbidity, color

6. Titanous Chloride Method: Nitrate is determined potentiometrically using an NH3 gas-sensing electrode after nitrate is reduced to NH3 by a titanous chloride reagent. (Proposed 1992)
   • Detection limits: 0.01 mg/l to 10 mg/l nitrate.

   • Interferences: NH3 and NO2(-), if present, are measured with NO3(-). Measure separately and subtract.

7. Automated Hydrazine Reduction Method: Nitrate is reduced to nitrite by hydrazine sulfate. The nitrite concentrations is determined by diazotizing with sulfanilamide and coupling with NED dihydrochloride to form a colored azo dye that is measured colorimetrically. (Proposed 1992)
   • Detection limits: 0.01 mg/l to 10 mg/l nitrate.

   • Interferences: Color, sulfide ion concentrations of less than 10 mg/l.

1. Digestion followed by distillation.

2. Automated Phenate Colorimetric Method: Reaction produces indophenol, an intensely blue compound.
   • Detection limits: 0.05 mg/l to 2.0 mg/l.

   • Interferences: Iron and chromium ions tend to catalyze, while copper ions will inhibit the color reaction.