Toward a Better Understanding of Contamination of Ground Water by Pesticides

Abstract

Public awareness of pesticides in ground water has been heightened by numerous reports in newspapers, in magazines, and on television. Yet, the understanding of processes causing contamination of ground water by pesticides remains insufficient. Indeed, the literature on pesticide fate and transport decreases exponentially with increasing depth below the soil surface. Considering the entire continuum of the unsaturated zone (not only the soil but also the whole depth below the root zone) and the saturated zone, the transport and fate of not a single pesticide has been accurately simulated, not even within an order of magnitude of the concentration observed. The problem is exacerbated by the quasi-systematic presence of several substances in any given sample. What pesticide must we look for, where, and when? Will the present contamination increase or decrease? What is the economic cost of the contamination? None of these questions has a unique, straightforward answer widely accepted by the scientific community. This situation is unacceptable not only for earth scientists but also primarily for water managers who are charged with setting up policies to reduce existing contamination and prevent future occurrences. Hundreds of pesticides are registered worldwide, and metabolites are often found with a greater frequency and higher concentration than the parent compound. Current procedures to assess the potential of a pesticide to leach to ground water are not sufficient to decide which pesticides should be monitored in each specific situation. Indeed, the data bases on pesticide loads at the soil surface derive from agricultural statistics or manufacturer or distributor inquiries at the regional (or national) scale and do not correspond to the spatial boundaries of the aquifers or local agricultural practices. Furthermore, these data bases do not enable the location of the applications at plot scale, even though such a precise location is needed to link observed concentrations in a well to loading at the soil surface. We urgently need appropriate data bases at the aquifer scale to identify the suite of substances to be analyzed and to quantify pesticide input functions. In parallel, monitoring networks must be developed and operated over long time frames in view of the slow time response of many aquifers. Preferential transport of pesticides through the soil has been a hot topic over the past decade, but its actual influence on observed ground water concentrations has not been adequately demonstrated. Depending on the hydrodynamics in the unsaturated zone below the root zone and in the saturated zone, concentration peaks from preferential transport in the soil zone may be smeared or remain unaltered in the saturated zone. Dedicated field studies that focus on the structure, hydrodynamic, and biogeochemical properties of the subsurface are needed. Additionally, models are needed that dynamically link unsaturated and saturated zone processes to represent the effect of preferential transport of pesticides in the soil on ground water contamination. More information is required on the adsorption and (bio)degradation properties of a range of pesticides (and their metabolites) along with the solids in the subsurface with which they are in contact. Great care is needed to conduct these studies under (sub)-μg/L concentrations and incubation times up to years. The potential for adaptation of microorganisms to degradation of pesticides must also be considered. The mass of pesticides present in the unsaturated zone and its availability for mobilization by recharge water also remain largely unknown. Analytic chemists must reach quantification limits less than 0.1 μg/kg for pesticides in water saturated and unsaturated solids, and sampling procedures appropriate for such pesticide determinations must be developed. We also lack data on the economic cost of pesticide contamination of ground water. Neither the total damage costs nor the potential benefits of alternative management practices have been adequately assessed at regional or national scales. The assessment of effectiveness is absolutely required to verify the potential benefits of mitigation policies, which are sometimes described as disproportionately costly by farmers and agrochemical lobbies. Field studies at the aquifer scale are needed and the obstacles to such studies, not all of which are simply financial, must be overcome at least for a selection of sites. Otherwise, we will continue to understand little about pesticides in ground water for quite awhile.