Nitrate Contamination of Groundwater

The inorganic contaminant of greatest concern in groundwater is the nitrate ion, NO3-, which commonly occurs in both rural and suburban aquifers. Although uncontaminated groundwater generally has nitrate nitrogen levels of 4-9 ppm, about 9% of shallow aquifers—from which water is often extracted via privately owned wells—in the United States now have nitrate levels that exceed the 10-ppm nitrogen MCL value. Indeed, elevated levels of about 100 ppm can result from agricultural activity. The location of areas in the United States that have a high risk of nitrate contamination of groundwater is shown in Figure 14-4. Exceeding the 10-ppm MCL limit is much rarer (1%) for public U.S. groundwater supplies, partially because they are drawn from deeper aquifers; these are generally less contaminated because of their depth, because their location is remote from large sources of contamination, and because natural remediation via denitrification of nitrate in the low-oxygen conditions can occur.

The expenditure of public money on nitrate-level reductions in drinking water has become a controversial subject. In Great Britain, in particular, hundreds of millions of dollars have been spent on achieving the 50-ppm maximum level of nitrate ion set by the European Union. Because nitrate removal from well water is very expensive, water contaminated with high levels of the ion are not normally used for human consumption, at least in public supplies.

PROBLEM 14-4

Convert the EU nitrate standard of 50 ppm to its nitrogen content alone. Is the EU standard more or less stringent than the U.S. regulatory limit of 10 ppm nitrogen as nitrate per liter?

FIGURE 14-4 The risk of nitrate contamination of the groundwaters in the United States. ¡Source: B. T. Nolan et al., "Risk of Nitrate in Groundwaters of the United Stales A National Perspective," Environmental Science and Technology 31 (1997): 2229.1

Nitrate in groundwater originates mainly from four sources:

• application of nitrogen fertilizers, both inorganic and animal manure, to cropland,

• cultivation of the soil,

• human sewage deposited in septic systems, and

• atmospheric deposition.

Concern has been expressed about the increasing levels of nitrate ion in drinking water, particularly in well water in rural locations; the main source of this NO3- is runoff from agricultural lands into rivers and streams. Almost 12 million tons of nitrogen is applied annually as fertilizer for agriculture in the United States, and manure production contributes almost 7 million tons more. Initially, oxidized animal wastes (manure), unabsorbed ammonium nitrate, NH4NQ3, and other nitrogen fertilizers were thought to be the culprits in nitrate contamination of groundwater, since reduced nitrogen unused by plants is converted naturally to nitrate, which is highly soluble in water arid can easily leach down into groundwater. It now appears that intensive cultivation of land, even without the application of fertilizer or manure, also facilitates the oxidation of reduced nitrogen to nitrate in decomposed organic matter in the soil by providing aeration and moisture. The original, reduced forms of nitrogen become oxidized in the soil to nitrate, which, being mobile, then migrates down to the groundwater, where it dissolves in water and is diluted. Denitrifkation of nitrate to nitrogen gas (see Chapter 6) and uptake of nitrate by plants can occur in forested areas that separate agricultural farms from streams, thereby lowering the risk of contamination in areas with significant woodland. However, rural areas with high nitrogen input, well-drained soil, and little woodland are at particular risk for nitrate contamination of groundwater.

The atmospheric deposition of nitrate results from its production in the atmosphere when NOx emissions from vehicles and power plants, and its natural sources in thunderstorms, are oxidized in air to nitric acid and then neutralized to ammonium nitrate (see Chapters 3 and 5).

In urban areas, the use of nitrogen fertilizers on domestic lawns and golf courses, parks, etc. contributes nitrate to groundwater. Septic tanks and cesspools also are significant contributors where they exist.

Excess nitrate ion in wastewater flowing into seawater, e.g., the Baltic Sea, has resulted in algal blooms that pollute the water after they die. Nitrate ion normally does not cause this effect in bodies of fresh water, where phosphorus rather than nitrogen is usually the limiting nutrient; increasing the nitrate concentration there without an increase in phosphate levels does not lead to an increased amount of plant growth. There are, however, instances where nitrogen rather than .phosphorus temporarily becomes the limiting nutrient even in fresh waters.

PROBLEM 14-5

The nitrate concentration in an aquifer is 20 ppm, and its volume is 10 million liters. What mass of ammonia upon oxidation would have produced this mass of nitrate?

Continue reading here: Health Hazards of Nitrates in Drinking Water

Was this article helpful?

0 0