Pesticides ground water transport

In this section, two types of data are briefly summarized 1) environmental transport and persistence, and (2) monitoring. The discussion Is chemical specific. Some Interesting concepts relevant to some of the chemical characteristics are developed later In the Discussion section. Results from this section are also used In the Conclusion section to derive some generalizations about pesticides leaching to ground water. 

Processes and Factors Affecting Transport of Pesticides to Ground Water... 

A major advantage to models such as PRZM or PESTANS is that they are transportable they can simulate a variety of situations with simple changes in weather input and parameters. More Importantly, however, is the fact that in most situations, 90% or more of applied pesticide would have runoff, volatilized, been taken up by the plant, or otherwise decayed before any of it leaches below the root zone. It makes sense, therefore, to develop the capability to predict the fate of pesticides in the root zone, and hence determine the potential for pesticides to contaminate ground water. 

Models can be applied successfully at the transport of pollutants from agricultural areas to surface or ground waters. These approaches for estimating the pollutants residues in food and irrigation waters as a result of pesticides and fertilizers agricultural use, or their storage, are under elaboration and/or perfection. 

Volatilization is an important route of dissipation for pesticides with large vapor pressures or, similarly, large Henry s Law constants (4-6). Volatilization has also been shown to be important for pesticides with low to moderate volatility (7-P). Volatilization reduces the pesticide available to control pests and reduces the potential of ground water contamination, but increases contamination of the atmosphere. This poses an increased risk to persons living near treated fields, since many pesticides are considered to be carcinogenic 10), To protect public health, there is need for more information on the important processes and mechanisms that affect pesticide fate and transport under typical field conditions. 

As noted above, there are cases where we need more accurate representations of how chemical concentration varies with depth. For example, we may be interested in transfers of chemicals from air to shallow ground water or want to consider how long-term applications of pesticides to the soil surface can impact terrestrial ecosystems—including burrowing creatures. However, we also wish to maintain a simple mathematical mass-balance structure of the multimedia model. To illustrate how we can set up a multilayer model that accurately captures soil mass transport processes, we next derive a vertical compartment structure with an air and three soil compartments, but any number of environmental compartments and soil layers can be employed in this scheme. Figure 8.6 provides a schematic of three soil layers linked to an air compartment and carrying pollutants downward to a saturated zone. We represent the inventory in each vertical compartment i, as M, (mol), transformation rate constants as kt, and transfer factors as ky (d ). The latter account for the rate of transfer between each i and j compartment pair. 

In this model from Corwin et al. (1991), the transport of a solute such as a pesticide through the soil is simulated from ground level to the water table. As shown in Fig. 1, the soil column is divided into N elements each of thickness z, with element 1 at the surface and element N directly above the water table. 

From the viewpoint of water management the use of pesticides is a serious danger particularly when these substances are introduced into precipitation, surface, ground-, drainage or wastewaters. The cases of water contamination are often due to the neglect of essential safety measures during their application, purification and decontamination of machines used for their application and transport, and aerial application, particularly in unsuitable periods. 

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