Republication or reproduction of this report or its storage and/or dissemination by electronic means is permittedSynopsis: Pesticide use is an important component of agricultural and non-agricultural pest control in tropical areas. However, the fate of pesticides in tropical soils is not as well understood as that for soils from temperate regions. Tropical soils defy easy generalizations, but they are typically very old soils characterized by year-round uniformity of temperature regime. Although only a few studies have directly compared pesticide fate in tropical and temperate soils, there is no evidence that pesticides degrade more slowly under tropical conditions. Laboratory studies in which soils have been held under standardized conditions reveal that pesticide degradation rate and pathway are comparable between tropical and temperate soils. However, field investigations of tropical pesticide soil fate indicate that dissipation occurs more rapidly, in some cases much more rapidly, than for pesticides used under similar temperate conditions. The most prominent mechanisms for this acceleration in pesticide dissipation appear to be related to the effect of tropical climates, and would include increased volatility and enhanced chemical and microbial degradation rates on an annualized basis.

Three simplistic models are developed for evaluating the transport of organic pollutants through soil to ground water. The models consider mobility and first‐order degradation. The first calculates linear sorption/desorption of the pollutant and first‐order degradation without considering dispersion. The second is similar to the first but also considers dispersion. The third considers nonlinear sorption following a Freundlich equation and first‐order degradation but does not consider dispersion. The models are compared to field data for the pesticides aldicarb and DDT. The models projected a lower mobility for DDT than was observed in the field.

At least 17 pesticides have been found in ground water in a total of 23 states as a result of agricultural practice. These results have been obtained through three different types of monitoring studies: (1) large-scale retrospective, (2) smallscale retrospective, and (3) small-scale prospective. The first two types of studies survey areas where the pesticide(s) in question has already been used. The third type of study is an intensive field study where the pesticide is applied and monitoring begins at time zero. Often, soil core data are at least as important as ground-water data. The ability to draw meaningful conclusions from large-scale studies is greatly diminished unless the studies have a statistical, stratified design. The purpose of this paper is threefold: to describe the three study types; suggest guidelines for ground-water sampling, soil sampling and well construction; and update the data summary of pesticides in ground water from agricultural practice.In 1984, Cohen et al. reviewed leaching and monitoring data on 12 different pesticides found in ground water in a total of 18 different states as a result of agricultural practice (V) · They also established criteria for predicting whether certain pesticides could leach to ground water as a result of normal use.In the less than 2 years since that paper was published, the numbers of pesticides found in ground water have increased significantly as have the number of states found to have pesticides in ground water. Seventeen pesticides have now been found in the ground water of 23 states as a result of agricultural practice (Figure 1, Table I). (As described in the "Occurrence" section at the end of this paper, many additional findings can be attributed to poor disposal practices, mixing-loading operations, etc. and are not included in this count.) This significant increase is more likely due to an increase in the quality and quantity of studies rather than an increase in This chapter not subject to U.S.