Nitrogen (Nitrate)

5.3.4 Nitrogen (Nitrate)

Nitrate is believed by many to be the most widely spread groundwater contaminant worldwide, primarily as a result of agricultural activities utilizing fertilizers. Other sig- nificant and widely spread anthropogenic sources of groundwater contamination with nitrogen forms are the disposal of sewage by centralized and individual systems, leaking sewers, animal feeding operations, and acid rain. Nitrate is the most oxidized form of

inorganic nitrogen. Nitrogen occurs in groundwater as uncharged gas ammonia (NH − − 3 ), which is the most reduced inorganic form, nitrite and nitrate anions (NO + 2 and NO 3 , re- spectively), in cationic form as ammonium (NH 4 ), and at intermediate oxidation states as

a part of organic solutes. Some other forms such as cyanide (CN − ) may occur in ground- water affected by waste disposal (Rees et al., 1995; Hem, 1989). Three gaseous forms of

nitrogen may exist in groundwater: elemental nitrogen (oxidation state of zero), nitrous

oxide (N 2 O; slightly oxidized, +1), and nitric oxide (NO; +2). All three, when dissolved in groundwater, remain uncharged gasses (Rees et al., 1995). Nitrogen can undergo numerous reactions that can lead to storage in the subsurface, or conversion to gaseous forms that can remain in the soil for periods of minutes to many years. The main reactions include (1) immobilization/mineralization, (2) nitrification, (3) denitrification, and (4) plant uptake and recycling (Keeney, 1990). Immobilization is the biological assimilation of inorganic forms of nitrogen by plants and microorganisms to form organic compounds such as amino acids, sugars, proteins, and nucleic acids. Mineralization is the inverse of immobilization. It is the formation of ammonia and ammonium ions during microbial digestion of organic nitrogen. Nitrification is the mi- crobial oxidation of ammonia/ammonium ion first to nitrite, then ultimately to nitrate. Nitrification is a key reaction leading to the movement of nitrogen from the land surface to the water table because it converts the relatively immobile ammonium form (reduced nitrogen) and organic nitrogen forms to a much more mobile nitrate form. Chemosyn- thetic autotrophic soil bacteria of the family Nitrobacteriaceae is believed to be principally responsible for the nitrification process. Ammonium oxidizers, including the genera Ni- trosomonas, Nitrosospira, Nitrosolobus, and Nitrosvibrio, oxidize ammonium to nitrite. The nitrite oxidizing bacteria, which oxidize nitrite to nitrate, include the genus Nitrobacter. Nitrification can also be carried out by heterotrophic bacteria and fungi (Rees et al., 1995). The nitrogen used by plants is largely in the oxidized form. Denitrification is the bio- logical process that utilizes nitrate to oxidize (respire) organic matter into energy usable by microorganisms. This process converts the nitrate to more reduced forms, ultimately yielding nitrogen gas that can diffuse into the atmosphere. Uptake of nitrogen by plants also removes nitrogen from the soil column and converts it to chemicals needed to sustain

360 ChapterFive

the plants. Because the plants eventually die, the nitrogen incorporated into the plant tissues ultimately is released back to the environment, thus completing the cycle (Rees et al., 1995).

Ammonium cations are strongly adsorbed on mineral surfaces, whereas nitrate is readily transported by groundwater and stable over a considerable range of conditions. The nitrite and organic species are unstable in aerated water and easily oxidized. They are generally considered indicators of pollution by sewage or organic waste. The presence of nitrate or ammonium might be indicative of such pollution as well, but generally the pollution would have occurred at a site or time substantially removed from the sampling point. Ammonium and cyanide ions form soluble complexes with some metal ions, and certain types of industrial waste effluents may contain such species (Hem, 1989).

Nitrate is not directly toxic to humans. However, under strongly reducing conditions, such as those in human gut, it transforms to nitrite. Nitrite ions pass from the gut into the blood stream and bond to hemoglobin molecules, converting them to a form that cannot transport oxygen (methemoglobin). Nitrite can also react chemically with amino compounds to form nitrosamides, which are highly carcinogenic (UNESCO, 1998). Ex- cessive consumption of nitrate in drinking water has been associated with the risk of methemoglobinemia or “blue baby syndrome,” an acute effect that is accentuated under poor sanitary conditions such as sewage contamination or dirty drinking vessels (Buss et al., 2005). If left untreated, methemoglobinemia can be fatal for affected infants. The WHO and the European Union have set the standard for nitrate in drinking water at 11.3

mg/L measured as nitrogen (mg N/L) that corresponds to 50 mg NO 3 /L. The standard in the United States, Canada, and Australia is 10 mg N/L. Extensive application of nitrogen fertilizers has caused an increase in nitrate con- centrations over large agricultural areas in many countries. As a worldwide average, pristine waters contain nitrate at approximately 0.1 mg N/L (Heathwaite et al., 1996). This is extremely low compared to typical modern groundwater concentrations. For example, studies of UK aquifers suggest that current natural background or baseline concentrations are more than an order of magnitude above the global average pristine concentration (Buss et al., 2005).

Nitrogen oxides, present in the atmosphere due to the combustion of fossil fuels, + undergo various chemical alterations that produce H and finally leave the nitrogen as nitrate. These processes can lower the pH of rain in the same way sulfur oxides do. Nonindustrially impacted rain may have a total nitrogen concentration of about 6 mg/L, and rainfall of 10 in/yr would yield a nitrogen load to the soil column of about 13 pounds per acre per year in such case. Significant evaporation of such rainwater could result in high concentrations of nitrogen in the infiltration water (Heaton, 1986; Rees et al., 1995). Industrially impacted rain may have a nitrogen concentration higher than 6 mg/L, resulting in higher nitrogen load to the subsurface.

Domestic sewage in sparsely populated areas of the United States is disposed of primarily in on-site septic systems. In 1980, 20.9 million residences (about 24 percent of the total in the United States) disposed of about 4 million acre-ft of domestic sewage in on- site septic systems (Reneau et al., 1989). Inherent in this method is the discharge of effluent to the local groundwater. To avoid contamination problems in an area, treated sewage effluent can be removed from a basin and discharged elsewhere if wastewater treatment is centralized. Unfortunately, removal is not possible with on-site septic systems, and even properly designed and constructed on-site septic systems frequently cause nitrate concentrations to exceed the MCL in the underlying groundwater (Wilhelm et al., 1994).

GroundwaterQuality

Total nitrogen concentrations in septic-tank effluent range from 25 mg/L to as much as 100 mg/L, and the average is in the range 35 to 45 mg/L (USEPA, 1980), of which about 75 percent is ammonium and 25 percent is organic. Wilhelm et al. (1994) report that nitrate concentrations in the effluent below a septic field can be two to seven times the MCL, and distinct plumes of nitrate-contaminated groundwater may extend from the septic system. Seiler (1996) estimates that septic systems contribution of nitrogen to groundwater in the East Lemmon subarea of Washoe County, Nevada, is between 16,500 and 42,000 kg (18 to 46 tons) of nitrogen annually.

In animal feeding lots, wastes may lose much of the nitrogen by ammonia volatiliza- tion, particularly in corrals that are not subject to water application; water can transport the nitrogen to the subsurface before substantial volatilization has occurred. The amount of nitrate from animal wastes that percolates to the groundwater depends on the amount of nitrate formed from the wastes, the infiltration rate, the frequency of manure removal, the animal density, the soil texture, and the ambient temperature (National Research Council, 1978).

The decay of natural organic material in the ground can contribute substantial amounts of nitrogen to groundwater. For example, in the late 1960s in west-central Texas, several cattle died from drinking groundwater containing high concentrations of nitrate; the source of the nitrate was determined to be naturally occurring organic material in the soil (Kreitler and Jones, 1975; from Seiler, 1996). The average nitrate concentration (as

NO 3 ) for 230 wells was 250 mg/L, and the highest concentration exceeded 3000 mg/L. Native vegetation, which included a nitrogen-fixing plant, was destroyed by plowing of the soil for dryland farming. This increased oxygen delivery to the soil and the nitrate causing the contamination were formed by oxidation of the naturally occurring organic material in the soil.

The stable isotope composition of nitrate is known to be indicative of its source and can also be used to indicate that biological denitrification is occurring (Buss et al., 2005). The variable used is δ 15 N, which compares the fraction of 15 N/ 14 N of the sample to that of an internationally accepted standard (the air in the case of nitrogen):

sample − 15 15 N/ 14 δ N standard N(‰) =

N) standard

When tracing the origins of contamination, some sources have characteristic isotopic signatures. For instance, the δ 15 N values for inorganic nitrate fertilizers tend to be in the range –7‰to +5‰, for ammonium fertilizers –16‰to –6‰, for natural soil –3‰to +8‰, for sewage, +7‰to +25‰, and for precipitation –3‰(Fukada et al., 2004; Widory et al., 2004; BGS, 1999; Heaton, 1986). This approach is often combined with information from

other species of interest: Barrett et al. (1999) used δ 15 N and microbiological indicators to identify sewage nitrogen, while Widory et al. (2004) used δ 15 N, δ 11 B, and 87 Sr/ 86 Sr to discriminate between mineral fertilizers, sewage, and pig, cattle, and poultry manure.

B ¨olke and Denver (1995) use δ 15 N with δ 13 34 S, chloroflurocarbons, tritium, and major C, δ ion chemistry to determine the application history and fate of nitrate contamination in agricultural catchments (Buss et al., 2005).

Isotopic effects, caused by slight differences in the mass of two isotopes, tend to cause the heavier isotope to remain in the starting material of a chemical reaction. Denitrifica- tion, for example, causes the nitrate of the starting material to become isotopically heavier.

362 ChapterFive

Volatilization of ammonia results in the lighter isotope preferentially being lost to the atmosphere, and the ammonia that remains behind becomes isotopically heavier. These isotopic effects mean that, depending on its origin, the same compound may have differ- ent isotopic compositions. Even when the stable-isotope composition of the source ma- terial is known, what reactions occur after its deposition and how they affect its isotopic composition also must be known, if the source of nitrate in groundwater is to be iden- tified. Because fractionation after deposition blurs the isotopic signatures of the source

materials, the use of 15 N data alone may not be sufficient to differentiate among sources. Because of the many ways human activities influence various forms of nitrogen in the environment, and the public health concerns associated with elevated concentrations of nitrite and nitrate in potable groundwater, many scientific investigations on the sources of nitrogen, the nitrogen cycle, and related groundwater impacts are available (e.g., Feth, 1966; National Research Council, 1978; Zwirnmann, 1982; Keeney, 1990; Spalding and Exner, 1993; Puckett, 1994; Rees et al., 1995; Mueller et al., 1995; Buss et al., 2005).