Many of the issues encountered in the nitrate case study discussed above also arise with pesticides. The major differences lie in the nature of the human health and environmental costs incurred and in the types of technical solutions needed to reduce these costs. Also, the uncertainties associated with pesticide transport processes, and with the human health and ecological effects of pesticides, are probably even greater than the comparable uncertainties about nitrate.
By any measure used, whether volume applied, hectares treated or market value, global pesticide use is large and still increasing. In 1995, world consumption reached 2.6 million metric tons of so-called active ingredients (the biologically active compounds) with a market value of US $38 billion. About 75% of pesticide use occurs in developed countries, mostly in North America, Western Europe and Japan, where high pesticide application rates are common. In these countries, herbicide use dominates. Herbicides are generally less toxic than insecticides, which are more widely used in developing countries. In fact, pesticide applications in developing countries are growing steadily, especially where export crops such as cotton, bananas, coffee, vegetables and flowers are predominant.21
It is estimated that annual world consumption of herbicides was around 1 million tons in 1993. Arable land and permanent crops cover around 1.4 billion hectares worldwide. Assuming that between 80% and 85% of herbicides are for agricultural use and that they are distributed uniformly on cultivated land, an average of about 0.5 kg/ha is used every year.22 However, local applications can be much higher in areas where intensive agriculture is practiced. Table 7.3 gives more location-specific figures on the average herbicide doses applied to corn, soybeans, rice, wheat and sugarbeets in the US (Illinois), France and Italy. These are well above the global average.
Cereals account for 0.7 billion ha, i.e., 50% of total global arable land, of which wheat, rice and corn are the most widespread. Among other crops, soybean is the most widely cultivated, with 56 million ha. This explains why more than 50% of herbicides are used on only a few crops.
Pesticides can move through the soil with water as it percolates down to groundwater. This process is called leaching. Both soil and pesticide properties must be considered when evaluating the tendency of a pesticide to leach in a particular location. The type of degrad-able organic matter (OM), the soil texture, and the soil acidity (pH) are the most important parameters determining the soil leaching potential, while the mobility, the persistence (usually given in terms of half-life), the rate of application and the application method are the relevant pesticide parameters. The strong heterogeneity of natural soils makes predictions of leaching potential difficult. Homogeneous soil column studies miss the most important point of pesticide transport in the field, which is preferential flow. Therefore, laboratory studies tend to underestimate the pollution potential of a pesticide.
The severity and global extent of ground-water contamination by pesticides cannot be adequately assessed. Data are only available in those isolated areas where monitoring programs have been carried out, i. e., almost exclusively in developed countries. The rapid introduction of pesticides into less developed countries has not been accompanied by monitoring, as the measurement of these compounds is still costly and requires a strong laboratory infrastructure.
Although pesticides can be removed from drinking water (e.g., with activated carbon), this becomes more difficult as the pesticide (or pesticide metabolites produced by biodegradation) becomes more polar. As with nitrate, the difficulties and expenses associated with removal fall more heavily on small water suppliers who do not benefit from economies of scale. Since most pesticide contamination comes from non-point sources, the effects can be widespread. When taken together, these two factors make pesticide contamination particularly problematic in agricultural regions, such as rural Denmark, where drinking water is obtained from many small suppliers.
Denmark's decentralized water supply system consists of 3,470 waterworks. A recent monitoring program examined pesticide concentration in groundwater samples from 976 monitoring wells and 2,798 abstraction wells.26 This program revealed that 3 to 4% of the samples exceeded the maximum limit for individual pesticides in drinking water (0.1 ^g/l). By way of comparison, in the same survey no samples of chlorinated hydrocarbons exceeded the comparable threshold (25 ^g/l). In about 10 percent of the samples, one or more of a target group of eight pesticides were detected. These pesticides were found down to a depth of 60-70 m, most frequently in younger shallow groundwaters, with the occurrence generally decreasing with depth. The pesticides dichlorprop, mechlorprop (phenoxyacids) and atrazine (triazine) were most frequently found. Phenoxy acids have been found exclusively in anaerobic aquifers, often under bedded thick till and clay layers. In unconfined aerobic sand aquifers, triazines are found.27
Some of the Danish counties analyzed groundwater samples from monitoring wells for more than the 8 pesticides in the target group. These detailed studies revealed the presence of 35 pesticides or metabolites in Danish groundwater. Moreover, 22 of these were found with concentrations above the maximum admissible concentration in drinking water.26 Understandably, these sampling results have been the source of great concern in Denmark, and wells with pesticide concentrations above the 0.1 ^g/l threshold will be closed. It is worth noting that the threshold which prompted this action is based on historic detection limits rather than toxicological risk considerations.
In any case, it is likely that more Danish wells will be closed in the future, as nonde-gradable pesticides from more remote source areas arrive at additional pumping and monitoring sites. If this process continues, it is likely that Danish water supply system will have to become more centralized. In Denmark this does not necessarily pose a great problem, since water is plentiful and it is unlikely that pesticide contamination will create serious supply shortages. This may not be the case in more arid regions in developing countries, where localized treatment and centralization of water supplies may be technically difficult
Table 7.3. Average herbicide doses (g ha-1) applied in different crops (1988)
Corn Soybeans Rice Wheat Sugarbeets
Illinois (USA) 3125 1400
France 2000 1600-2480 - 1140 3300-3800
Italy 3090 - 6500 1500-4100 7210
Adapted from Catizone, Fougeroux and Bourdet, Pike et al.25
and/or may impose significant economic penalties on local populations.
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