1989 - 1994
Pesticide Contamination of Surface Waters Draining Agricultural Fields: Pesticide Contamination Classification
J. Stover and A.S. Hamill
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Agricultural pesticides are effective tools for increasing crop yields and quality. However, their usage has resulted in non-point source contamination of surface waters in the Canadian Great Lakes Basin. In accordance with the terms of Annex 13 of the revised Great Lakes Water Quality Agreement of 1987 (International Joint Commission, 1987), Agriculture Canada has undertaken programs to reduce non-point source pollution arising from agricultural activities. This report deals primarily with the surface runoff component of agricultural drainage, since it is thought to be a significant non-point source of pesticides in surface waters.
In a previous study, Shelton et al. (1988) developed a classification system for assessing the contamination risk posed by pesticides to surface and groundwater systems. Their surface water classification system was based on soil properties and total pesticide usage (i.e. total mass of all active ingredients applied) at the township level. Individual pesticides and their environmental behaviour were not considered in the classification system.
Currently, there is need for a classification system for assessing the relative potential of specific pesticides to be transported from agricultural lands in surface runoff. Simple indices which consider pesticide properties have proven useful for assessing pesticide groundwater pollution potentials. (Gustafson, 1989; 1982; McRae, 1991). Application of a similar approach may prove useful for making preliminary assessments of pesticide losses in surface runoff and can also provide a rationale for the selection of pesticides posing a low risk to surface waters.
This report is intended to provide a background for assessments of the relative potential of pesticides to contaminate surface runoff and to outline practical abatement measures. Toward this goal the report will present the following information:
Pesticides in Surface Waters
Surface water monitoring data provide a valuable source of information for setting basin-wide water quality priorities and assessing the progress of remedial measures. However, river monitoring data can reflect a variety of point and non-point sources of pesticide contamination. Some of the non-point sources which have been documented include; atmospheric deposition, tile drainage, interflow and surface runoff (Bengston et al., 1990; Nutter et al., 1984). At the field-scale though, surface runoff is thought to be one of the most significant sources of pesticides in surface waters (Leonard, 1988; Wauchope, 1978).
The presence of specific pesticides in surface waters is not only a function of their susceptibility to loss in surface runoff but is also affected by pesticide usage volume and transport characteristics within surface water systems, and weather patterns (Frank and Logan, 1988; Frank et al., 1991; Willis et al. 1987;). Section 2 of the report outlines two procedures for assessing the relative susceptibility of specific pesticides to be transported from the site of application in surface runoff.
Watershed Monitoring Data
The PLUARG (Pollution from Land Use Activities Reference Group) studies of the 1970's (Frank et al., 1982) and the studies reported by Frank and Logan (1988) and Frank et al. (1991) provide the most comprehensive analysis of the occurrence of pesticides in agricultural drainage waters in the Canadian portion of the Great Lakes Basin.
Table 1.1 lists the pesticides detected, as well as their frequencies of detection, in the 3 studies (Frank et al., 1982; Frank and Logan, 1988; Frank et al. 1991) . It is clear from the table that the frequency of detection of most pesticides has decreased. Metolachlor is one notable exception to this trend. It's increased presence is probably a reflection of its increased usage. While there was no record of metolachlor use during the PLUARG study (Frank et al., 1982), it currently has the highest usage volume of any agricultural pesticide in Ontario (Moxley, 1989). Although the decreased incidence of some compounds may be linked to soil and water conservation practices (Frank et al., 1991), it is clear from Table 1.1 that deregistration and deleted uses have also been effective in reducing the incidence of specific pesticides. In particular, DDT and dieldrin have been banned and the major use of endosulfan in tobacco was deleted (Frank et al., 1982).
SPISP and Kitchen Table were not developed to be regulatory tools. Rather, they are tools intended for use in making preliminary assessments of pesticide contamination potentials and for providing criteria to producers for selecting pesticides with a low potential to contaminate water systems.
One major technical consideration in the implementation of the runoff components of SPISP and "Kitchen Table" in the Great Lakes Basin concerns the reliability of data required by the soil rating procedure. Data needed for the procedure are available from soil drainage classifications, however, McKeague and Topp (1986) and Chisholm (1992) have advised caution in using the Ontario soil drainage classifications (Chisholm et al., 1984; Irwin, 1984).
McKeague and Topp (1986) attempted to verify the classification with field measurements on a number of soil types in Ontario. They noted significant discrepancies between their soil drainage interpretations and those suggested by the drainage classification. They also commented that the drainage classification was not accurate in many cases since it was based mainly on considerations of soil texture. McKeague and Topp (1986) and Chisholm (1992) have suggested that the drainage classification of Ontario could be improved by considering the structural characteristics of the soils.
Clearly, inaccuracies in the soil drainage classification could have an adverse effect on the soil ratings used by SPISP and "Kitchen Table". It is imperative that this technical question be resolved.
Data requirements for the leaching components of SPISP and "Kitchen Table" were not evaluated.
Increasing concerns over pesticide contamination of both surface and groundwater systems have lead to the development of a group of abatement methods termed Best Management Practices (BMP). BMPs are defined as practical methods, measures or practices which prevent or reduce the amount of pollution generated by nonpoint sources to levels compatible with water quality goals (Novotny and Chesters, 1981; Logan, 1990).
Logan (1990) outlined 4 general classes of BMPs for the control of agricultural non-point source pollution including; 1) structural controls, 2) land management practices, 3) source controls and, 4) pesticide and nutrient management practices. These 4 approaches represent 2 fundamentally different philosophies (Odum, 1987):
While there are effective approaches for reducing the surface runoff losses of pesticides, the implementation of some practices or measures may involve trade-offs.
For instance, there are indications that practices resulting in increased infiltration rates (such as no-till or terracing) may result in higher pesticide loadings to groundwater (Donigian and Carsel, 1987; Logan, 1990). In addition, although conservation tillage can reduce the total loadings of pesticide in runoff, some studies have indicated that conservation tillage can result in increased pesticide concentrations in runoff (Fawcett, 1992). It has been suggested that concerns over these transient pesticide concentrations may be more relevant than concerns over total loadings (Haith, 1987).
Minimizing Pesticide Movement in Surface Runoff
There have been relatively few studies on the effectiveness of practices specifically designed to reduce non-point sources of pesticide contamination. Many of the approaches which have been proposed, borrow from a group of structural controls and land management practices which were originally associated with reducing the transport of sediment and nutrients into surface waters.
Baker and Johnson (1983) proposed a 3 point approach for the reduction of chemical runoff losses using a number of BMPs:
Reducing the Volumes of Runoff and Sediment
Considerable interest and research has been focused on differences in pesticide losses between soil tillage systems. In particular, it has generally been assumed that conservation tillage systems, which have been widely promoted as practices for reducing runoff volumes, soil erosion and nutrient losses, would also reduce pesticide losses.
In an extensive review of tillage effects on pesticide losses Fawcett et al. (1992) concluded that conservation tillage practices generally result in reduced pesticide losses, except in cases where infiltration is limited by soil type, internal drainage or problems such as compaction. Under these conditions pesticide losses from conservation tillage may exceed or be comparable to losses from conventional tillage systems.
Contouring is a soil conservation practice applicable to sloping lands. With contouring, tillage and planting operations are carried out perpendicular to the slope of the land as opposed to up-and-down the slope.
c) Tile Drainage
Tile drainage is normally regarded as a crop production tool rather than a measure for improving water quality. However, studies have shown that tile drainage also has the potential to decrease runoff losses of pesticides. Tile drainage lowers surface moisture levels allowing more storm water to infiltrate, thereby resulting in lower surface runoff volumes. To some extent, reductions in surface runoff losses may be compensated by increased losses of pesticide in tile drainage. The extent to which this occurs is dependant on i) pesticide travel times from the soil's surface to tile drainage and ii) pesticide transformation rates in the soil (Baker and Johnson, 1983).
Reducing the Delivery of Pesticides From Field to Stream
Terracing involves the construction of a ridge or embankment across a slope to control erosion. Terraces reduce slope lengths and divert or store surface runoff. Excess water is removed by grassed outlets, subsurface drains or by infiltration.
b) Grassed Waterways
Grassed waterways are used to conduct excess surface water from croplands. They are defined as broad and shallow waterways covered in erosion resistant grasses. Grassed waterways improve water quality by retarding the transport of sediment and water from field to stream.
Clearly, grassed waterways are effective measures for reducing the movement of both water and sediment associated pesticides. However, Fawcett et al. (1992) pointed out that the capacity of grassed waterways must also be considered.
c) Buffer Strips
Buffer strips are untreated areas bordering fields or bodies of water. These areas may or may not consist of permanent vegetation or may be planted in densely grown crops such as a forage or small grain.
Reducing the Concentrations of Pesticide in Runoff
Incorporation or subsurface application of pesticides reduces chemical concentrations and loadings in surface runoff by decreasing the amounts of chemical initially present in the soil's surface active zone.
A variety of environmental considerations can increase the magnitude of pesticide loss in runoff including; i) a high antecedent soil moisture at the time of application, ii) a high probability of rainfall/runoff occurring shortly after application and iii) a high seasonal risk of rainfall/runoff following application. Optimization involves making adjustments in application timing such that one or more of these conditions are avoided.
c) Product Selection
Baker and Johnson (1983) suggested that pesticides and formulations less susceptible to losses in surface runoff could be substituted for those with a high potential for loss.
Reducing the Usage of Pesticides
a) Integrated Pest Management (IPM)
Precise definitions of IPM are still vague and a number of interpretations are possible (Higham, 1990). In their simplest form IPM programs are targeted at eliminating unnecessary pesticide usage. This approach is an alternative to prophylactic or calendar-based pesticide applications.
The most advanced forms of IPM use combinations of chemical, mechanical, biological, cultural and varietal controls to manage pest populations.
b) Crop Rotation
Crop rotation is an effective technique for managing some pest problems - it can also reduce the risks of surface water contamination. Fawcett et al. (1992) commented that the greater diversity of crops grown in watersheds managed under crop rotation will result in a greater diversity in the timing and types of pesticides applied.
c) Reducing Recommended Rates
Lowering the recommended application rates of pesticides has the potential to lower the quantities of pesticides applied, thereby decreasing the quantities of pesticide available for transport in runoff.
Currently, recommended rates are based on 80% or better pest control across a broad geographic area, under widely varying climatic conditions and reasonably high infestation levels. Although excellent control has been achieved with lower rates under ideal circumstances, current legislation prohibits recommendations for the use of a pesticide below the labelled rates. Until liability and legal concerns can be resolved relative to this issue, one of the most direct measures for reducing chemical inputs will go unimplemented.
d) Source Controls
Source controls include legislative actions to ban or restrict the use of specific pesticides. Although these measures represent a highly effective means of eliminating the threat of pesticide contamination to both surface and ground waters, it is clear that water quality objectives must be balanced against economic realities. In order to maintain productivity, suitable alternatives must be available to producers. Such alternatives may include substitute compounds and/or non-chemical pest control options.
5.0 GAPS/NEEDS FOR FUTURE RESEARCH
b) Pesticide Movement Abatement
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