Health Hazards of Nitrates in Drinking Water

Excess nitrate ion in drinking water is a potential health hazard since it can result in methemoglobinemia in newborn infants as well as in adults with a specific enzyme deficiency. The pathological process, in brief, runs as follows.

Bacteria, e.g., in unsterilized milk-feeding bottles or in the baby's stomach, reduce some of the nitrate to nitrite ion, N02~~:

N03^ + 2 H+ + 2 e"->NO,~ + H20

The nitrite combines with and oxidizes the iron ions in the hemoglobin in blood from Fe2+ to Fe3+ and thereby prevents the proper absorption and transfer of oxygen to cells. The baby turns blue and suffers respiratory failure. (In almost all adults, the oxidized hemoglobin is readily reduced back to its oxygen-carrying form, and the nitrite is readily oxidized back to nitrate; also, nitrate is mainly absorbed in the digestive tract of adults before reduction to nitrite can occur.) Methemoglobinemia, or blue-baby syndrome, is now relatively rare in industrialized countries. It was a serious problem in Hungary up until the late 1980s and in Romania.

The U.S. EPA MCL of 10 ppm of nitrate nitrogen was set in order to avoid blue-baby syndrome. Since the syndrome is now almost nonexistent in the United States (only two cases since the mid-1960s), some policy analysts think this value is too stringent.

Recently, an increase in the risk of acquiring non-Hodgkin's lymphoma has been found for persons in some communities in Nebraska who consume drinking water having the highest levels (long-term average of 4 ppm or more of nitrogen as nitrate) of nitrate. As discussed in the next section, excess nitrate ion in drinking water is also of concern because of its potential link with stomach cancer. Recent epidemiological investigations have, however, failed to establish any positive, statistically significant relationship between nitrate levels in drinking water and the incidence of stomach cancer. A study reported in 2001 found that older women in Iowa who drank water from municipal supplies having elevated nitrate levels (> 2.46 ppm) were almost three times as likely to be diagnosed with bladder cancer than those least exposed (< 0.36 ppm in their drinking water). However, a recent large-scale study from the Netherlands failed to find an association between nitrate exposure and the risk of bladder cancer. A review of the current literature concluded that there is also no association between nitrate exposure from drinking water and adverse reproductive effects.

Nitrosamines in Food and Water

Some scientists have warned that excess nitrate ion in drinking water and foods could lead to an increase in the incidence of stomach cancer in humans, since some of it is converted in the stomach to nitrite ion. The nitrites could subsequently react with amines to produce N-nitrosamines, compounds that are known to be carcinogenic in animals. N-nitrosamines are amines in which two organic groups and an —N=0 unit are bonded to the central nitrogen:

N-nitrosamines

NDMA

Of concern not only with respect to its production in the stomach and its occurrence in foods and beverages (e.g., cheeses, fried bacon, smoked and/or cured meat and fish, and beer), but also as an environmental pollutant in drinking water, is the compound in which R in the above structure is the methyl g roup CH3; it is called N-nitrosodimethylamine, or NDMA for short. This organic liquid is somewhat soluble in water (about 4 g/L) and somewhat soluble in organic liquids. It is a probable human carcinogen, and a potent one if extrapolation from animal studies is a reliable guide. It can transfer a methyl group to a nitrogen or oxygen of a DNA base, thereby altering the instructional code for protein synthesis in the cell.

In the early 1980s, it was found that NDMA was present in beer to the extent of about 3000 ppt. Since that time, commercial brewers have modified the drying of malt so that the current levels of NDMA in American and Canadian beers are now only about 70 ppt.

Large quantities of nitrate are used to "cure" pork products such as bacon and hot dogs. In these foods, some of the nitrate ion is biochemically reduced to nitrite ion, which prevents the growth of the organism responsible for botulism. Nitrite ion also gives these meats their characteristic taste and color by combining with hemoproteins in blood. Nitrosamines are produced from excess nitrite during frying (e.g., of bacon) and in the stomach, as discussed. Government agencies have instituted programs to decrease the residual nitrite levels in cured meats. Some manufacturers of these foods now add vitamin C or E to the meat in order to block the formation of nitrosamines. Based upon average levels of NDMA in various foods and the average daily intake for each of them, most of us now ingest more NDMA from consumption of cheese (which is often treated with nitrates) than from any other source.

Even though the commercial production ofNDMA has been phased out, it can be formed as a by-product due to the use of amines in industrial processes such as rubber tire manufacturing, leather tanning, and pesticide production.

The levels of NDMA in drinking water drawn from groundwater is of concern in some localities that have industrial sources of the compound. For example, following the discovery that the water supply of one town had been contaminated by up to 100 ppt NDMA from a tire factory, Ontario, Canada, adopted a guideline maximum of 9 ppt of NDMA in drinking water, which corresponds to a lifetime cancer risk of 1 in 100,000. By contrast, the guideline for water in the United States is set at 0.68 ppt, which corresponds to a cancer risk of 1 in a million, but which actually lies considerably below the detection limit (about 5 ppt) for the compound.

PROBLEM 14-6

Write balanced redox half-reactions (assuming acidic conditions) for the conversion of NH4+ to N03^, and of N02™ to N2.

Perchlorates

Perchlorate ion, C104 , is analogous to nitrate ion in that both oxyanions involve nonmetals in their highest common oxidation numbers ( + 7 in the case of perchlorate; see Additional Problem 13 to generate a summary of chlorine with its many oxidation numbers). For that reason, both ions are oxidizing agents, and both have been used in explosives and propeHants. Both have both natural and anthropogenic sources. Perchlorate is a newly discovered (late 1990s) pollutant in the drinking-water supply of about 15 million Americans. Large quantities of ammonium perchlorate, NH4C104, are manufactured for use as oxidizing agents in solid rocket propellants, fireworks, batteries, and automobile air bags. Because rocket fuel has a limited shelf life, it must be replaced regularly. Large amounts of perchlorates were washed out of missiles and rocket boosters onto the ground or into holding lagoons in the second half of the twentieth century.

Perchlorate contamination has been established in 23 U.S. states, including much of the Colorado River and aquifers in the deserts of the Southwest. Concentrations of perchlorate in drinking water in the U.S. Southwest range from 5 to 20 ppb. The map (Figure 14-5) of perchlorate releases in the United States indicates that most occur in the south-central and western states, especially California. Perchlorate has also been found in garden fertilizers at concentrations approaching 1%. k occurs naturally in some C Chilean deposits of nitrate which are exported to the United States and elsewhere as fertilizers. Perchlorate also exists naturally in some minerals

Perchlorate Colorado River

FIGURE 14-5 Regions of perchlorate use and contamination in the United States. [Source: B. E. Logan, "Assessing the Outlook for Perchlorate Remediation," Environmental Science and Technology (1 December 2001): 484A.)

Major rivers

[_Affected states

Major rivers

FIGURE 14-5 Regions of perchlorate use and contamination in the United States. [Source: B. E. Logan, "Assessing the Outlook for Perchlorate Remediation," Environmental Science and Technology (1 December 2001): 484A.)

[_Affected states found in the U.S. Southwest. A recent analysis indicates that both its use as an oxidizer, in fireworks, rockets, etc., and its presence in fertilizer make important contributions to its contamination of foodstuffs in the United States.

At high doses, perchlorate affects human health by reducing hormone production in the thyroid, where it competes with iodide ion. Its hazard at low concentrations, if any, is not known, making the development of a drinking-water standard for the ion a difficult problem. No federal MCL for the ion has yet been set, but several states have set their own limits, ranging from 1 to 18 ppb; e.g., California's limit is 6 ppb. In 2002, a draft report by the U.S. EPA proposed a drinking-water standard of 1 ppb as safe for human health, but this value has been criticized as too low by the Department of Defense and companies that make or use perchlorates. Research reported in 2002 on volunteers indicated that the no-effect level (NOEL; Chapter 10) for the inhibition of iodine uptake corresponds to a drinking-water concentration of at least 180 ppb. The EPA based its 1-ppb recommendation on studies indicating that mothers who drink perchlorate-contaminated water could give birth to children whose IQs would be affected negatively because correct maternal thyroid hormone levels are vital to fetal brain development.

Like nitrate, perchlorate is a difficult ion to remove from water supplies since it is a highly water-soluble anion that is very inert and does not adsorb readily either to mineral surfaces or to activated carbon. Barrier methods, using elemental iron, etc. are not successful because the anion is so unreac-tive. The primary technologies currently in use to remediate perchlorate in water are ion exchange and biological treatment. Some ion exchange resins successfully remove perchlorate, though it tends to remain in solution until all other anions have been absorbed. Ion exchange is used especially when perchlorate concentrations are low to begin with.

Certain bacteria found naturally in many soils, sediments, and natural waters biodegrade perchlorate by reduction to chloride ion:

Continue reading here: Groundwater Contamination by Organic Chemicals

Was this article helpful?

0 0