A Review of Herbicide Leaching Studies in Sweden: Field, Lysimeter

Chapter 6

A Review of Herbicide Leaching Studies in Sweden: Field, Lysimeter, and Laboratory Measurements 1

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L . Bergström and A. Shirmohammadi Downloaded by STANFORD UNIV GREEN LIBR on September 28, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0699.ch006

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Department of Soil Sciences, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden Department of Biological Resources Engineering, University of Maryland, College Park, MD 20742 2

To characterize the mobility of pesticides in soil, results obtained in short-term laboratory tests are commonly used, along with data on degradation rates and physico/chemical properties of pesticides. However, the environmental conditions in such tests are quite different from natural field conditions in which biological, chemical, and physical processes occur in a complex soil structure, mostly under non -equilibrium conditions. Outdoor lysimeter experiments conducted under non-steady state flow conditions, are good alternatives to laboratory tests, which have demonstrated that they can simulate field situations quite well with respect to pesticide leaching. In this paper, results from a number of Swedish leaching studies with selected pesticides, carried out in monolith lysimeters, are described. Attention has been focused on comparing these leaching estimates with what one could expect in terms of mobility based on the inherent properties of the pesticides. Also, the impact of soil properties on pesticide leaching is discussed, as well as the importance of correctly evaluating the significance of measured concentrations and loads of pesticides obtained in lysimeter experiments in terms of actual field situations. Transport of agricultural chemicals through soils has become a problem of international concern, due to its potential for causing deterioration in surface water and groundwater quality. Although the concentrations in these environments are usually very low and typically below levels of toxicological concern (1), a significant number of pesticides have been detected (e.g. 1,2,3), which are often attributed to leaching through the unsaturated zone. Along with the increasing concern about pesticide contamination of various water bodies, much emphasis has been put on designing suitable methods to characterize leachability in soil. In development of regulatory decision schemes, such considerations are extremely critical. This far, we have relied heavily on results obtained in short-term laboratory leaching tests, along with data on physico/chemical 76

©1998 American Chemical Society

In The Lysimeter Concept; Führ, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

Downloaded by STANFORD UNIV GREEN LIBR on September 28, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0699.ch006

77 properties of pesticides (4). However, such leaching tests are mostly performed in homogeneous, uniformly packed sand under saturated steady-state flow conditions and are therefore not typical of natural field situations. In contrast, outdoor lysimeter experiments, which are normally conducted with unsaturated soil over much longer periods, have demonstrated that they can simulate actual field conditions quite well with respect to leaching of pesticides in soil (5). The small size and therefore easiness to control water flows and environmental conditions in lysimeter experiments, contribute to make them very suitable for characterization of pesticide leaching (6). However, one should also be aware of the limitations of lysimeters, which may impact on the results. For example, in freely drained lysimeters, the zero-tension bottom boundary will result in formation of a water-saturated zone at the bottom of the soil profile. This may certainly modify the soil-water conditions throughout the profile, especially for shallow lysimeters. Cutting off the monolith from the underlying soil prevents upward migration of water from layers that would typically support the profile with water in a field situation. This will also to some extent affect soil moisture conditions inside a lysimeter, especially during the growing season when the evapotranspirational demand is high. In addition to direct measurements, mathematical simulation models are now increasingly being used to predict pesticide transport in soil. Models provide a relatively inexpensive way of estimating likely leaching behavior for a variety of environmental conditions, which would not be feasible with costly field studies. Accordingly, several simulation models are now available for prediction of pesticide fate in soil (7), and they are also at an increasing rate being built into various regulatory assessment procedures. However, one limitation for extensive use, especially of detailed mechanistic models, is the difficulty in determining appropriate parameter values. Also, in line with what is mentioned above; can we really trust pesticide parameter values determined in the laboratory, that are used in models, and are appropriate response functions for temperature and moisture included in the models. In the following presentation, we discuss some of the major reasons for the commonly poor resemblance between leaching estimates which are based on laboratory studies and those that are obtained in field experiments. Also, the importance of correctly evaluating the significance of measured concentrations and loads of pesticides in lysimeter leachate in terms of actual field conditions is discussed. The presentation is based on results obtained in a selection of pesticide leaching studies carried out in undisturbed field lysimeters in Sweden. Materials and Methods Experimental Setup. The results presented here are based on measurements performed in 0.3-m diameter and 1-m deep monoliths, which were collected using a coring technique described by Persson and Bergstrôm (8). With this method a standard PVC sewer pipe is gently pressed into the soil by a steel cylinder, equipped with four cutting teeth, which rotates slowly around the pipe as it penetrates the soil. After collection, the soil cores were prepared for free drainage and transported to a lysimeter station in Uppsala, equipped for collection of leachate. A schematic representation of this type of lysimeter is shown in Figure 1.



Figure 1. Lysimeter placed in a below-ground pipe (dimensions in meters; from 33).


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The active ingredients included in this overview are: bentazon chlorsulfuron, clopyralid, dichlorprop, fluroxypyr, metsulfuron methyl, and a non-registered molecule (Table I). Treatments included three soils ranging from loamy sand/sand to clay, different precipitation regimes resembling normal and worst-case conditions for Sweden, and normal and double the normal application rates of spring applied pesticides (Table Π). Application of pesticides occurred in early June each year. A l l lysimeters were cropped with spring barley (Hordeum distchum L.), grown according to normal agricultural practices, i.e. sown in May and harvested in August/ September.

Table I. Chemical Names of the Included Compounds Common Name

Chemical Name

Bentazon

3-( 1 -methylethyl)-( 17/)-2,1,3-benzothiadiazin-4(37ï)-one 2,2dioxide 2-chloro-^-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbony]benzenesulfonamide 3,6-dichloro-2-pyridinecarboxylic acid (±)-2-(2,4-dichlorophenoxy)propanoic acid [(4-amino-3,5-dichloro-6-fluoro-2-pyridinyl)oxy]acetic acid 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl]amino]sulfonyl]benzoic acid

Chlorsulfuron Clopyralid Dichlorprop Fluroxypyr Metsulfuron methyl

Analytical Methods. The acidic compounds (dichlorprop, bentazon, clopyralid, fluroxypyr, and the non-registered molecule) were extracted with dichloromethane after acidification (9), except fluroxypyr, which was extracted with ethyl acetate before being transferred to dichloromethane. Quantitation of these compounds was by gas chromatography (70) with detection limits between 0.1 and 1 μg L" . Chlorsulfuron and metsulfuron methyl were analyzed with ELISA (Enzymelinked Immunosorbent Assay) Microplate Immunoassays (11,12), with detection limits of 0.0125 and 0.010 μg L" for chlorsulfuron and metsulfuron methyl, respectively. 1

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Results and Discussion Pesticide Properties as Predictors for Pesticide Leaching. A l l the compounds included here are considered to be fairly mobile based on the information obtained in laboratory tests (Table ΙΠ). However, when considering the amounts of the pesticides that actually leached out during periods from 7 to 11 months in the lysimeter studies, all seemed relatively non-leachable; i.e,

A Review of Herbicide Leaching Studies in Sweden: Field, Lysimeter

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