Nitrate Contamination

During the last few decades agricultural productivity has increased enough to keep pace with the rapid increase in global population. This dramatic increase in productivity is largely due to improvements in crop varieties and associated increases in the use of fertilizers and pesticides, technical innovations sometimes collectively referred to as the "Green revolution". The benefits of these innovations are apparent and relatively easy to document. What is not so apparent are the costs, both to human health and to the environment in general. Concerns about these costs have generated debates and have led to a number of regulations, especially in the developed countries. Many of the issues which arise in the case of fertilizer application can be illustrated with the case of nitrate use in Germany.

The increased use of nitrogen fertilizer for agriculture introduces additional nitrate into the biosphere. In order to put the issue in perspective, the nitrogen balance for Germany is summarized in Table 7.1.19

This table clearly reveals that the biological fixation of nitrogen is dwarfed by the inputs from nitrogen fertilizer and manure. On a global scale, nitrogen fixation associated with fertilizer production is already comparable to the total amount of natural (preindustrial)

nitrogen fixation.20 This means that the flux through the nitrogen cycle has been doubled by human activities. The harvesting of crops removes only little more than one-half of the total input. The remainder ends up in different compartments of the environment. (Table 7.2).

About one-third leaches into groundwater and eventually enriches surface waters before it gets back into the cycle in the form of nitrogen gas or is stored in the nitrogen pool of the soil. The ecological consequences of this intervention are still not fully understood. It is, however, well known that nitrogen enrichment increases the likelihood and extent of eutrophication, particularly in estuarine environments.

The denitrification which occurs in the aquifers under anaerobic conditions gets rid of nitrate, but utilizes resources such as organic carbon on the aquifer matrix which when used up is no longer available to maintain the aquifer function of denitrifying filtration. On the basis of a recharge rate in Germany of about 200 mm/yr and a maximum allowable concentration of 50 mg NO3/ l, and assuming that there is no denitrifica-tion, a maximum tolerable nitrate flux to groundwater of 20 kg N/ha/yr can be computed. The actual flux of nitrate to the groundwater with about 40 kg N/ha/yr is already twice as high.

The bulk of nitrate excess is due to intensive agriculture where the farmer fertilizes in the expectation of maximum yield. This yield is not achieved every year. In fact, in the majority of years the extraction of nitrogen with the crop will be less than the input of nitrogen by fertilization. Over the years, the accumulation of excess nitrate leads to a large pool of organically bound nitrogen in the soil, which makes the system more prone to produce episodically high temporary inputs of nitrate to groundwater by washout. A solution to this dilemma could come from precision agriculture, which monitors the soil's demands much more closely and makes it feasible to apply small amounts of fertilizer frequently in an economic fashion. The time required for such technical innovations to have an effect on the scale of nitrate concentrations could be quite large. This time

Table 7.1. Nitrate balance for Germany

(1990)

Units: kg N/(ha

agricultural land)

Input

Mineral

135

fertilizer

N-fixation

14

by legumes

Manure

78

Deposition from

30

atmosphere

Sum

257

Output

Withdrawal

141

at harvest

Excess

116

includes the travel time in the unsaturated zone, the travel time in the saturated zone, and the time to deplete the pool of organically bound nitrogen in the topsoil. In a typical German watershed, this amounts to several decades

The potentially adverse effects of nitrate on human health have been the primary focus of efforts to regulate nitrogen fertilizer use in Germany. The World Health Organization and the European Community have both adopted standards that require that nitrate concentrations in drinking water should not exceed a standard of 50 mg/l NO3. Concentrations in excess of this value are found frequently in groundwater. In European countries such as Germany or Denmark, about 10% of all water works are above or close to this value. The fact that the percentage has been virtually constant over the last decade is misleading. It is not due to a leveling off of the problem, but rather to the continuous shutting down of polluted wells which then do not appear in the statistics any more. If one takes the "warning value" of 25

Table 7.2. Fate of nitrate excess

(ha agricultural land) = 100%

Direct surface runoff

9

Atmosphere as N2O

5

Atmosphere as NH3

18

N-pool of soil

17

Denitrification in soil

17

Leaching to groundwater

34

mg/l NO3 as a threshold, the percentage of water works affected is almost twice as high. In the German State of Baden-Wuerttemberg, where an extensive monitoring program has been undertaken, regions with high concentrations show a leveling off, while formerly cleaner regions are still on the rise.

In Europe, non-point nitrate pollution of groundwater presents more of a problem than industrial pollution of groundwater. This is because the source is much more pervasive and the nitrate ion is very mobile and not easily removed by standard water processing techniques. Nitrate removal to meet existing standards could almost double present prices of drinking water in Germany. While such removal may be feasible in the centralized treatment plants of the industrialized world, it is much more difficult to implement in a cost effective way in the small decentralized water systems of the third world. As fertilizer use and domestic water demands increase in the developing world, it is quite possible that nitrate-induced health problems could become more pervasive. In the short term this will lead to increased costs, either for farmers who must reduce fertilizer application and accept lower yields, or for domestic water users, who must pay for nitrate removal. In the longer term, it may be possible to maintain high yields while reducing the amount of nitrogen leached into groundwater, e.g., through the use of

"recession agriculture" or genetic manipulation of crops to provide for direct nitrogen fixation from the soil air.

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