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Fluorescent tracer technique for measuring the quantity of pesticide deposited to soil following spray applications
Crop Protection 22 (2003) 15–21
Fluorescent tracer technique for measuring the quantity of pesticide deposited to soil following spray applications J.A.S. Barbera,1, C.S. Parkinb,* b
a Cranfield University, Silsoe, Bedford MK45 4QT, UK Silsoe Research Institute, Silsoe, Bedford MK45 4HS, UK
Received 17 December 2001; accepted 18 March 2002
Abstract At present methods for measuring soil deposits from pesticide applications are somewhat limited. As an alternative to using pesticide residue analysis, a simple tracer technique has been developed using the fluorescent dye Tinopal CBS-X to measure deposits directly on the soil. Because there are soil tracer interactions with Tinopal CBS-X, the technique allows for variations in organic matter content by making use of a sorption isotherm as a calibration line. Laboratory measurements showed that tracer deposits on soil were comparable to those on artificial surfaces placed on the soil at the same location. Measurements from laboratory spray simulations and field experiments showed that there was high level of variability in the quantity of tracer recovered on soil. This was due to deposit variability below the crop, across the swath, and between sample locations rather than from the sampling method indicating that shading by the crop canopy has a significant effect on deposition. Because soil samples can be taken without bias, it was concluded that a more accurate indication of the amount reaching the soil could be obtained from soil surface sampling than from the use of artificial surfaces mounted on tables. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pesticide application; Spray; Soil; Tracer; Deposit; Fluorescence; Sampling
1. Introduction It has been estimated that up to one-third of the total amount sprayed onto a crop can be lost to the soil at the time of application (Anon., 1988). The volume deposited onto soil is variable and dependent on the crop canopy (Taylor and Andersen, 1987). Accurate measurements of pesticide deposition to soil are required for models of environmental fate and are imperative if spray accountancy is to be achieved. Soil deposit measurements are also important for soil applied chemicals. However, the literature shows that the predominant method for measuring soil deposits is via artificial surfaces placed on the soil or above the ground on small tables. For quantitative analysis glass slides (Cadogan and Zylstra, 1984), plastic sheeting (Planas and Pons, 1991; Fox et al., 1993), tape, petri dishes (Coates and Palumbo, 1997), *Corresponding author. E-mail address: [email protected] (C.S. Parkin). 1 Present address: Entomology Department, Cornell University, NYSAES, Geneva, NY 14456, USA.
plastic lines, filter or chromatography paper (Ade et al., 2000; Holownicki et al., 2000; Leonard et al., 2000; Mathers et al., 2000) have all been used. For qualitative analysis water-sensitive papers (Cowell et al., 1988; Coates and Palumbo, 1997), silica gel sheets, linograph paper (Maybank et al., 1974), and kromekote cards (Sundaram, 1990; Payne et al., 1996) have been used. It is easy to see why artificial surfaces are popular. They are a simple, effective and direct method but they can never fully represent the natural target (Uk, 1977). Moreover, artificial surfaces need to be positioned prior to application, disturbing the crop and leading to operator bias. For example, a sampler could be placed directly next to the stem base in the shade of canopy (Bryant et al., 1984) or in a more exposed location in tramlines. Also plain artificial targets may not accurately represent the morphology of the soil and not collect the same quantity of spray since it is well known that changes in the topography and angle of a target greatly affect collection efficacy (Lake, 1977; Uk, 1977; Brunskill, 1956; Richardson, 1987). Also, uneven ground with large rough clods of soil is aerodynamically
0261-2194/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 2 ) 0 0 0 6 1 - 3
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dissimilar to the smooth flat surfaces of artificial targets and smooth non-absorbent targets could easily suffer from run-off. Chemical analysis methods are available which directly measure pesticide residues in the soil after application. Techniques include gas chromatography (Sundaram et al., 1985), liquid scintillation counting (Mouvet et al., 1994), HPLC, (Glad et al., 1978), and immunosorbent assay techniques (Peter et al., 1989). However, although all of these techniques are sensitive, the cost and time for analysis often limits the numbers of samples that can be processed. Therefore, it is desirable that a simple technique based on tracers and the use of soil should be developed. The use of fluorescent tracers for deposit measurement in canopies is widespread (Cooke and Hislop, 1993). However, we could find no literature referring to a simple method for directly tracing pesticides depositing on soil surface using fluorescence. This may be due to the perceived inherent complexity of soils, and the possibility of soil/tracer reactions. For example, in hydrology (Quinlan, 1981; Smettem and Trudgill, 1983), the use of fluorescent tracers is limited by their strong sorptive tendencies and photo-degradation (Reynolds, 1966; Corey, 1968; Smart and Laidlaw, 1976; Omoti and Wild, 1979; Davis et al., 1980; Ghodrati and Jury, 1990). The reactive nature of some fluorescent tracers has been exploited by using them as sorbing tracers, to predict qualitatively the presence and breakthrough of pesticides (Sabatini and Austin, 1991; Everts and Kanwar, 1994; Flury and Fluhler, 1995). The fluorochromes fluorescein and rhodamine WT, because of their differing levels of sorption, have been used to delimit the appearance of atrazine and alachlor (Sabatini and Austin, 1991). In this paper, we describe the principle of a fluorescence technique for the direct measurement of spray deposits to soil. We describe the development of the method in the laboratory, and its evaluation under simulated crop and field conditions.
2. Methods 2.1. Quantifying chemical compounds in soil The fluorochrome chosen was Tinopal CBS-X (distyryl biphenyl DSBP=4,40 -bis(2-sulphostyryl) biphenyl) (Ciba Speciality Chemicals PLC, Charter Way, Macclesfield, Cheshire SK10 2NX, UK). Tinopal CBS-X was considered not only because of its solubility in water (25 g/l at 251C) but because it is capable of being used for measurements in the dry state on leaves and in solution (Last and Parkin, 1987). Although Hall et al. (1993) found that solutions of Tinopal CBS-X degraded in sunlight by 9.4% in 100 min, exposed calibration
standards can be used to calculate the effect in the field (Barber, 2001) making it suitable for field use (Last and Parkin, 1987). Quantifying pesticide soil residues in the field is mainly achieved through the development of sorption isotherms and the subsequent creation of a sorption coefficient (Kd Þ for the chemical and a specific soil type. It is not usually possible to use fluorescent tracers for quantifying leaching because changes in soil composition alter sorption with depth. However, here we are interested only in sampling what immediately lands on the soil surface. What is relevant is that a reliable sorption coefficient is obtained for a relatively homogeneous surface soil type. Once the level of sorption for a given soil type is known, the total volume of chemical deposited can be calculated as long as the amount left in the supernatant is sufficient for accurate analysis. Similar analytical methods to those used to reclaim pesticides from the soil have been adapted for use in this work. It was hypothesised that the volume of spray deposited on the soil could be determined from the sorption characteristics for Tinopal CBS-X. The sorption characteristics of a chemical are usually measured by mixing to equilibrium a specified quantity of soil with an appropriate solution containing a known concentration of the chemical. In this case, to enable samples to be processed at a realistic rate, the concept of reaching equilibrium was discarded and all samples were mixed for 2 h. This also increased the quantity of tracer left in solution and the sensitivity of the technique. To ensure reproducibility calibration standards were processed with each sample batch. After mixing the two phases were separated. To create a clear supernatant, instead of the more usual filtration or centrifugation, gypsum was added as a flocculating agent. The concentration of tracer in solution was measured using a MSE Super D spectro-photometer adjusted to detect an emission of 364 nm wavelength. At the low concentrations involved in spray application, the sorption isotherm was found to be well described by a linear response Cs ¼ Kd Caq ; where Cs is the concentration of the sorbed chemical mg/g, Caq is the concentration in the aqueous phase mg/ ml and Kd is the sorption coefficient ml/g. 2.2. Effect of soil organic matter content Since organic matter is normally the predominant adsorbing component, variations often occur for a given chemical in a range of soils (Singh et al., 1990; Rowell, 1994). To give an indication of possible sorption levels prior to field application, the relative importance of soil type and more significantly organic matter content (OM) on sorption was investigated. Knowledge of
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different levels of sorption will effect the chosen ratio of soil to water used for the analysis. The OM and the Kd coefficients for four generic soil types were measured, as sorption indicators for future use. Samples of four major soil types were collected (sandy loam, clay loam, chalky soil, and compost—representing peat soils). The OM level was determined for each soil type by the weight loss on ignition. Soils were airdried and passed through a 2 mm sieve. Five grams of each soil type was accurately weighed (three replicates) on to a silica dish and placed in a furnace at 8001C for 20 min. Samples were subsequently cooled in a desiccator and moistened with 2% ammonium carbonate solution. Each was partially re-heated over a water bath to drive off the excess carbonate solution and reweighed. The OM is expressed as a percentage of the original soil weight. Sorption isotherms were plotted with soil sample solutions of 10 g soil, 50 ml distilled water and stock solution at three dilutions (0.4 0.8, 1.2 ml stock solution (Tinopal CBS-X 2% w/v)). The soil was weighed into glass jars with the stock solution and distilled water was added. Calibration standards were prepared with stock solutions and distilled water, the dilutions were over a larger dose range (0, 0.2, 0.4, 0.8, 1.4 ml/ml). Samples and standards were mixed on the rotating drum for 2 h in darkness. To clear the solutions after mixing a spatula of the gypsum was added to each sample jar, shaken by hand for 10 s, then left in darkness until the supernatant had cleared. Once the supernatant was clear, fluorescence readings were taken and the Kd calculated. 2.3. Recovery of tracer If the soil deposit method is to be advocated, direct comparisons are necessary to compare the results with those obtained by artificial targets. Ideally, a method that uses the soil directly should be as reliable as one that uses artificial surfaces. However, nature is inherently more variable. The following experiments investigated the variability of tracer recovery from soil under controlled conditions. The recovery from soil and artificial surfaces was compared using two volumes (20, and 40 ml) of stock solutions of (2% w/v) Tinopal CBS-X. Calibration standards were created using 0, 0.4, 0.6, 0.8, 1.2 ml/ml distilled water, and 50 ml was added to 10 g of soil (sandy loam field plot soil). Artificial samplers are normally solid surfaces that do not adsorb the tracer. It was decided that applying the stock solution directly into the glass jars would adequately represent this model. Samples were left in the dark and once dry 50 ml of distilled water was added. The sample was then shaken for 30 s. Calibration standards were prepared by adding the stock solution directly to the distilled water (50 ml). Each treatment was replicated 17 times, and
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analysed for the average volume recovered and the coefficient of variation within each group. 2.4. Laboratory simulation One of the major perceived problems associated with the use of artificial samplers is their placement. This was examined in the laboratory under conditions that simulated the field Spring barley plants were grown in seeding trays in a glasshouse. The plants were sown parallel with the long side of the tray and spaced to simulate a crop inter row. Once the plants had reached growth stage 34 (Tottman, 1987), spraying commenced using a hydraulic track sprayer. It consisted of a dry boom fitted with a nozzle that straddled a track. A trolley pulled by an electric motor and pulley system moved the trays to be treated along the track under the nozzle. The nozzle was aligned over the centre of the long axis of the track, and the boom height adjusted to 40 cm above the crop. The tracer solution was mixed prior to spraying and stored in a light-tight container. The maximum concentration of Tinopal CBS-X (2.5% w/v) was used along with 0.1% w/v Agral (non-ionic surfactant). Two applications, which were likely to produce different levels of soil deposit, were chosen. The nozzles used were a Spraying Systems Cos flat-fan XR100 015 nozzle operated at 4 bar (4 kph) and a flat-fan XR110 06 operated at 1 bar (8 kph). The spray from the former has been classified as a BCPC Fine (Doble et al., 1985) and is more likely to be retained on the crop whilst the latter is classified as a BCPC Coarse spray and is more likely to be deposited on the soil. Forward speed and pressure were adjusted to maintain 200 l/ha application volume for both Fine and Coarse sprays. Acetate sheets (40 cm2) were placed directly on the soil surface prior to spraying and removed once the deposit had dried. Following spraying, once the deposit had dried, soil samples (23 cm2 area) were taken using soil density rings as soil cores. Three different sampling arrangements were used: ‘‘inter-row’’ where both sample types were taken from the inter-row, ‘‘random’’ where both sample types were taken randomly, and ‘‘different’’ where the artificial samples were taken from the inter-row, and soil samples taken randomly. Four trays were placed on the track for each replicate, two for artificial samples, and two for soil sampling. Five samples (soil 23 cm2, acetate 40 cm2) could be taken from each tray. This was repeated four times, hence four replicates with 10 samples from each. 2.5. Field experiment The field experiment formed part of an investigation into the biological effects of different deposit distributions (Barber, 2001). Three different pesticide types were
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assessed at three different doses. Laboratory tests were carried out to detect interactions between the Tinopal CBS-X and the pesticides. HPLC analysis showed that there was no chemical interaction between the tracer and pesticides, and spectroscopy showed that there was no quenching of fluorescence by the pesticides (Barber, 2001). Here, we will present only the results taken from samples located on the ground. We have disregarded chemical type and dose as variables and included only nozzle and sample type. Three conventional and one an air-induction flat-fan nozzle sizes were used (Spraying Systems Cos XR 110 015 (4 bar and 4 kph), XR 110 04 (2 bar and 8 kph), XR 110 06 (1 bar and 8 kph) and AI 110 025 (3 bar and 6 kph)). All were adjusted to apply 200 l/ha. Three single nozzle bodies were mounted at 50 cm intervals on a dry boom extending to one side of an ATV; thus, two passes were necessary to apply the treatments to the 3 3 m plots. The boom was held at 40 cm above the crop, with a nozzle separation of 50 cm as recommended by the manufacturer. Spray function was checked using a patternator. The 36 plots were each sampled twice. For soil measurements, samples were taken, again using 23 cm2 area density rings as soil cores. These were randomly placed in the central strip of plots in positions pre-assigned using a random number generator. Soil samples were left in the density rings and stored in individual light-tight containers for subsequent analysis. Further ground deposit measurements were taken with acetate sheet targets (20 cm2) placed in the pre-assigned plots prior to spraying. To ensure that each sample surface was level, each sheet was mounted on a table stand raised 15 cm above the ground. Two types of stand were used, one with a 15 15 cm table and another with a 5 5 cm table. The smaller table made little impact on the canopy but the larger table significantly opened up the crop. The two table sizes represent the likely minimum and maximum interference of the canopy with artificial targets. After spraying the acetate sheet samples were removed from their tables, placed in watertight plastic bags and stored under blackout conditions.
3. Results and discussion 3.1. Effect of soil type The effect of soil type on the sorption of Tinopal CBS-X is shown in Table 1. The relevant calibrations for the soils are shown on Fig. 1. Tracer sorption appears to be inversely related to the amount of organic matter present. The compost had an extremely high level of organic matter (90%); correspondingly, the amount of tracer in solution was low, creating a shallow calibration line. The lower the OM the higher the sorption coefficient (Table 1) and the steeper the adsorption
Table 1 Percentage organic matter (OM) and sorption co-efficient (Kd ) for Tinopal CBS-X sorbed on each soil type Soil type
OM
Kd coefficient
Sand Clay top Chalk Compost
2.9 6.3 6.6 90.0
0.022 0.030 0.037 0.101
60 Sand
Stock solution recovered (µl)
18
Clay
Chalk
Compost
2
R = 0.9933
50
R2 = 0.9631
40
2
R =0.9396
30 20
2
R = 0.957
10 0
0
10
20 30 40 50 Stock solution applied (µl)
60
70
Fig. 1. Sorption calibrations for Tinopal CBS-X with four different soil types. Table 2 Recovery of Tinopal CBS-X on artificial and soil samples at two doses Sample
Artificial
Soil
Tracer concentration
0.4 ml/ml 0.8 ml/ml 0.4 ml/ml 0.8 ml/ml
Volume Coefficient of variation (%)
0.418 6.93
0.801 3.02
0.401 5.28
0.799 4.26
No significant difference between concentration recovered with soil SED=0.0057, Po0:001; LSD=0.0135.
isotherm (Fig. 1). The sandy soil, with the lowest OM, sorbed a minimal amount of tracer. 3.2. Recovery of Tinopal CBS-X The results of the experiments comparing the recovery of Tinopal CBS-X from artificial samples and soils in Table 2 show that in the laboratory high levels of recovery were achieved with both the samples. Although artificial targets did show a significant difference in recovery this difference is too small to have practical significance. 3.3. Laboratory simulation Statistical analysis showed that overall there was a difference between nozzle (Po0:001), and target type (Po0:048). The Fine spray deposited significantly less (0.760 ml/cm2) to the ground in comparison to the
J.A.S. Barber, C.S. Parkin / Crop Protection 22 (2003) 15–21 Table 3 Mean deposit of Tinopal CBS-X (ml/cm2) for sample type and position Position
Artificial
Soil
Different Inter-row Random
0.925 0.853 0.803
0.791 0.846 0.814
SED=0.0381, P ¼ 0:014; LSD=0.0749.
Table 4 Mean and coefficient of variation (CV) of Tinopal CBS-X deposits collected in each sample location for both nozzle types with artificial and soil targets Nozzle
Position
Volume (ml/cm2)
CV (%)
Artificial
Soil
Artificial
Soil
Coarse
Different Inter-row Random
0.983 0.955 0.873
0.901 0.904 0.887
23 23 32
34 22 34
Fine
Different Inter-row Random
0.867 0.751 0.734
0.681 0.788 0.740
22 22 35
34 21 37
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Table 5 Mean volume (ml//cm2) of Tinopal CBS-X deposited by each nozzle type to the different sample types in the field experiment Nozzle
Sample 5 5 cm Tables 15 15 cm Tables Soil
Air inclusion Coarse Medium Fine Averages
0.541 0.547 0.471 0.416 0.494
0.711 0.761 0.702 0.573 0.687
Averages
0.638 0.631 0.667 0.659 0.656 0.617 0.620 0.548 0.645 —
Average for nozzles: P ¼ 0:079; SED 0.0436, LSD=0.0867; average for sample types: Po0:001; SED=0.324, LSD=0.0645; and averages for the 5 5 cm tables: P ¼ 0:051; SED=0.0749, LSD=0.0645. No significant difference between soil samples and 15 15 cm tables.
fraction of spray deposited to soil and an underestimation of its variability. 3.4. Field experiment
SED=0.0539, P ¼ 0:205; LSD=0.106.
Coarse spray (0.917 ml/cm2). The acetate sheets (0.861 ml/cm2) collected significantly more than the soil samples (0.817 ml/cm2). The results in Table 3 show that the only significant difference (P ¼ 0:014) between the mean deposit found on each sample occurred when they were positioned differently. The soil samples appeared to collect less deposit compared to the acetate sheets placed on the soil. An analysis of the interactions between sample type, position and treatment is shown in Table 4. The difference in retention between artificial surface and soil targets was only apparent for the Fine spray. There was a significant difference between the artificial targets placed inter-row and the randomly taken soil samples. The mean levels of deposit and variation were similar when soil samples and artificial surfaces were in the inter-row. When both sample types were positioned randomly the level of variation rose markedly, but remained similar. Taking the samples from different locations altered the levels of variation. Both target types collected similar spray volumes when they were treated alike. This would indicate that overall there is little difference between the collection efficiencies of soil and table samples under field conditions. Differences occur only when the sampling methodology differed. It appears that larger but less variable deposits were collected when the inter-row was sampled than when sampled randomly. Therefore, the inter-row sampling can lead to an overestimation of the
As can be seen in Table 5, significant differences were observed between both the sample and nozzle types. The Coarse spray deposited significantly more to the soil when compared to the Fine spray (P ¼ 0:079). The smaller table samples collected significantly (Po0:001) lower volumes of spray compared to the soil samples and the larger tables. When nozzle target interactions were analysed, although they all followed similar trends only samples from the larger tables showed any significant difference (P ¼ 0:051) between nozzles, with the Fine spray depositing less compared to the Coarse. The results from the air-inclusion nozzle suggest that it deposited slightly less to the soil compared with the Coarse spray. Since the drop spectra of the Coarse and air-included sprays are likely to be similar, this implies that spray retention on the canopy with the air-included spray is greater.
4. Conclusions Direct measurements of fluorescent tracers on soil were successfully made despite soil complexity. Work with different soil types showed that although sorption differs with type this could be accounted for via a sorption isotherm or calibration. If soil types vary across an area then adequate sampling, mapping and good experimental design could block changes. Under simulated field conditions, differences in the mean quantity retained and variability were observed between sample locations but not between sample types. The field experiment showed the expected trend with the Fine spray distribution depositing less to the ground compared to the Coarse spray. The differences between the soil samples are much less conclusive than those
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from artificial surfaces implying that the latter could overestimate deposit. Also, the results from soil samples are less likely than artificial surfaces to yield significant differences due to much larger variations within groups, caused by the varying levels of shade produced by the crop canopy. Therefore, the main difference between soil sampling and artificial surfaces is the likelihood of sample bias. Artificial samples need to be located within the plot prior to application and the process of target placement can cause crop disturbance. There is a tendency to find an area that is easily accessible to the operator, such as in tramlines or inter-rows. Such areas are invariably more open, and this can lead to an overestimation of the mean deposit and an underestimation of variability. With similar sample placement there is no significant difference in the volume recovered or the level of variation. Random non-intrusive sampling, which is only possible in the field with soil samples, recovers lower volumes of spray with a higher variation, and is a more authentic measurement of the movement of pesticide to soil. Hence, although the use of soil as the sample is slightly more protracted than the artificial surfaces, the advantages gained by being able to sample freely in a completely random manner could negate the disadvantage of increased time for analysis. It, therefore, appears that Tinopal CBS-X could be considered a suitable tracer for the measurement of pesticide deposits both on the crop and directly on the soil.
Acknowledgements We would like to thank the Ministry of Agriculture Fisheries and Food (MAFF) for funding JAS Barber during this work. We would like to thank Bob Walker, Dr Se! amus Murphy and Matthew Fisher for their assistance with the fieldwork. We are also grateful for the discussions held with Professor Paul Miller of SRI and Professor Daryl Joyce of Cranfield University.
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Rowell, D.L., 1994. Soil Science Methods and Applications. Longman Group UK Limited, Harlow, 332pp. Sabatini, D.A., Austin, T.A., 1991. Characteristics of rhodamine WT and fluorescein as adsorbing ground water tracers. Ground Water 29, 341–349. Singh, G., Spencer, W., Genucthen, M.V., 1990. Sorption behaviour of s-triazine and thiocarbamate herbicides on soils. J. Environ. Qual. 106 (19), 91–107. Smart, P.L., Laidlaw, I.M.S., 1976. An evaluation of some fluorescent dyes for water tracing. Water Resour. Res. 13 (1), 15–33. Smettem, K.R.J., Trudgill, S.T., 1983. An evaluation of some fluorescent and non-fluorescent dyes in the identification of water transmission routes in soils. J. Soil Sci. 34, 45–56. Sundaram, K.M.S., 1990. Foliar and ground deposits of fenitrothion following aerial application over a plantation forest. J. Environ. Sci. Health B 25 (5), 643–663. Sundaram, A., Sundaram, K.M.S., Cadogan, B.L., Nott, R., Leung, J.W., 1985. An evaluation of physical properties, droplet spectra, ground deposits and soil residues of aerially applied aminocarb and fenitrothion emulsions in conifer forests in New Brunswick. J. Environ. Sci. Health 20 (6), 665–688. Taylor, W., Andersen, P.G., 1987. Effect of application changes and cereal growth stage on spray deposition. Aspects Appl. Biol. 14, 71–76. Tottman, D.R., 1987. The decimal code for the growth stages of cereals, with illustrations. Ann. Appl. Biol. 110, 441–454. Uk, S., 1977. Tracing insecticide spray droplets by sizes on natural surfaces—The state of the art and its value. Pestic. Sci. 14, 645–658.
Fluorescent tracer technique for measuring the quantity of pesticide deposited to soil following spray applications J.A.S. Barbera,1, C.S. Parkinb,* b
a Cranfield University, Silsoe, Bedford MK45 4QT, UK Silsoe Research Institute, Silsoe, Bedford MK45 4HS, UK
Received 17 December 2001; accepted 18 March 2002
Abstract At present methods for measuring soil deposits from pesticide applications are somewhat limited. As an alternative to using pesticide residue analysis, a simple tracer technique has been developed using the fluorescent dye Tinopal CBS-X to measure deposits directly on the soil. Because there are soil tracer interactions with Tinopal CBS-X, the technique allows for variations in organic matter content by making use of a sorption isotherm as a calibration line. Laboratory measurements showed that tracer deposits on soil were comparable to those on artificial surfaces placed on the soil at the same location. Measurements from laboratory spray simulations and field experiments showed that there was high level of variability in the quantity of tracer recovered on soil. This was due to deposit variability below the crop, across the swath, and between sample locations rather than from the sampling method indicating that shading by the crop canopy has a significant effect on deposition. Because soil samples can be taken without bias, it was concluded that a more accurate indication of the amount reaching the soil could be obtained from soil surface sampling than from the use of artificial surfaces mounted on tables. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pesticide application; Spray; Soil; Tracer; Deposit; Fluorescence; Sampling
1. Introduction It has been estimated that up to one-third of the total amount sprayed onto a crop can be lost to the soil at the time of application (Anon., 1988). The volume deposited onto soil is variable and dependent on the crop canopy (Taylor and Andersen, 1987). Accurate measurements of pesticide deposition to soil are required for models of environmental fate and are imperative if spray accountancy is to be achieved. Soil deposit measurements are also important for soil applied chemicals. However, the literature shows that the predominant method for measuring soil deposits is via artificial surfaces placed on the soil or above the ground on small tables. For quantitative analysis glass slides (Cadogan and Zylstra, 1984), plastic sheeting (Planas and Pons, 1991; Fox et al., 1993), tape, petri dishes (Coates and Palumbo, 1997), *Corresponding author. E-mail address: [email protected] (C.S. Parkin). 1 Present address: Entomology Department, Cornell University, NYSAES, Geneva, NY 14456, USA.
plastic lines, filter or chromatography paper (Ade et al., 2000; Holownicki et al., 2000; Leonard et al., 2000; Mathers et al., 2000) have all been used. For qualitative analysis water-sensitive papers (Cowell et al., 1988; Coates and Palumbo, 1997), silica gel sheets, linograph paper (Maybank et al., 1974), and kromekote cards (Sundaram, 1990; Payne et al., 1996) have been used. It is easy to see why artificial surfaces are popular. They are a simple, effective and direct method but they can never fully represent the natural target (Uk, 1977). Moreover, artificial surfaces need to be positioned prior to application, disturbing the crop and leading to operator bias. For example, a sampler could be placed directly next to the stem base in the shade of canopy (Bryant et al., 1984) or in a more exposed location in tramlines. Also plain artificial targets may not accurately represent the morphology of the soil and not collect the same quantity of spray since it is well known that changes in the topography and angle of a target greatly affect collection efficacy (Lake, 1977; Uk, 1977; Brunskill, 1956; Richardson, 1987). Also, uneven ground with large rough clods of soil is aerodynamically
0261-2194/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 2 ) 0 0 0 6 1 - 3
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dissimilar to the smooth flat surfaces of artificial targets and smooth non-absorbent targets could easily suffer from run-off. Chemical analysis methods are available which directly measure pesticide residues in the soil after application. Techniques include gas chromatography (Sundaram et al., 1985), liquid scintillation counting (Mouvet et al., 1994), HPLC, (Glad et al., 1978), and immunosorbent assay techniques (Peter et al., 1989). However, although all of these techniques are sensitive, the cost and time for analysis often limits the numbers of samples that can be processed. Therefore, it is desirable that a simple technique based on tracers and the use of soil should be developed. The use of fluorescent tracers for deposit measurement in canopies is widespread (Cooke and Hislop, 1993). However, we could find no literature referring to a simple method for directly tracing pesticides depositing on soil surface using fluorescence. This may be due to the perceived inherent complexity of soils, and the possibility of soil/tracer reactions. For example, in hydrology (Quinlan, 1981; Smettem and Trudgill, 1983), the use of fluorescent tracers is limited by their strong sorptive tendencies and photo-degradation (Reynolds, 1966; Corey, 1968; Smart and Laidlaw, 1976; Omoti and Wild, 1979; Davis et al., 1980; Ghodrati and Jury, 1990). The reactive nature of some fluorescent tracers has been exploited by using them as sorbing tracers, to predict qualitatively the presence and breakthrough of pesticides (Sabatini and Austin, 1991; Everts and Kanwar, 1994; Flury and Fluhler, 1995). The fluorochromes fluorescein and rhodamine WT, because of their differing levels of sorption, have been used to delimit the appearance of atrazine and alachlor (Sabatini and Austin, 1991). In this paper, we describe the principle of a fluorescence technique for the direct measurement of spray deposits to soil. We describe the development of the method in the laboratory, and its evaluation under simulated crop and field conditions.
2. Methods 2.1. Quantifying chemical compounds in soil The fluorochrome chosen was Tinopal CBS-X (distyryl biphenyl DSBP=4,40 -bis(2-sulphostyryl) biphenyl) (Ciba Speciality Chemicals PLC, Charter Way, Macclesfield, Cheshire SK10 2NX, UK). Tinopal CBS-X was considered not only because of its solubility in water (25 g/l at 251C) but because it is capable of being used for measurements in the dry state on leaves and in solution (Last and Parkin, 1987). Although Hall et al. (1993) found that solutions of Tinopal CBS-X degraded in sunlight by 9.4% in 100 min, exposed calibration
standards can be used to calculate the effect in the field (Barber, 2001) making it suitable for field use (Last and Parkin, 1987). Quantifying pesticide soil residues in the field is mainly achieved through the development of sorption isotherms and the subsequent creation of a sorption coefficient (Kd Þ for the chemical and a specific soil type. It is not usually possible to use fluorescent tracers for quantifying leaching because changes in soil composition alter sorption with depth. However, here we are interested only in sampling what immediately lands on the soil surface. What is relevant is that a reliable sorption coefficient is obtained for a relatively homogeneous surface soil type. Once the level of sorption for a given soil type is known, the total volume of chemical deposited can be calculated as long as the amount left in the supernatant is sufficient for accurate analysis. Similar analytical methods to those used to reclaim pesticides from the soil have been adapted for use in this work. It was hypothesised that the volume of spray deposited on the soil could be determined from the sorption characteristics for Tinopal CBS-X. The sorption characteristics of a chemical are usually measured by mixing to equilibrium a specified quantity of soil with an appropriate solution containing a known concentration of the chemical. In this case, to enable samples to be processed at a realistic rate, the concept of reaching equilibrium was discarded and all samples were mixed for 2 h. This also increased the quantity of tracer left in solution and the sensitivity of the technique. To ensure reproducibility calibration standards were processed with each sample batch. After mixing the two phases were separated. To create a clear supernatant, instead of the more usual filtration or centrifugation, gypsum was added as a flocculating agent. The concentration of tracer in solution was measured using a MSE Super D spectro-photometer adjusted to detect an emission of 364 nm wavelength. At the low concentrations involved in spray application, the sorption isotherm was found to be well described by a linear response Cs ¼ Kd Caq ; where Cs is the concentration of the sorbed chemical mg/g, Caq is the concentration in the aqueous phase mg/ ml and Kd is the sorption coefficient ml/g. 2.2. Effect of soil organic matter content Since organic matter is normally the predominant adsorbing component, variations often occur for a given chemical in a range of soils (Singh et al., 1990; Rowell, 1994). To give an indication of possible sorption levels prior to field application, the relative importance of soil type and more significantly organic matter content (OM) on sorption was investigated. Knowledge of
J.A.S. Barber, C.S. Parkin / Crop Protection 22 (2003) 15–21
different levels of sorption will effect the chosen ratio of soil to water used for the analysis. The OM and the Kd coefficients for four generic soil types were measured, as sorption indicators for future use. Samples of four major soil types were collected (sandy loam, clay loam, chalky soil, and compost—representing peat soils). The OM level was determined for each soil type by the weight loss on ignition. Soils were airdried and passed through a 2 mm sieve. Five grams of each soil type was accurately weighed (three replicates) on to a silica dish and placed in a furnace at 8001C for 20 min. Samples were subsequently cooled in a desiccator and moistened with 2% ammonium carbonate solution. Each was partially re-heated over a water bath to drive off the excess carbonate solution and reweighed. The OM is expressed as a percentage of the original soil weight. Sorption isotherms were plotted with soil sample solutions of 10 g soil, 50 ml distilled water and stock solution at three dilutions (0.4 0.8, 1.2 ml stock solution (Tinopal CBS-X 2% w/v)). The soil was weighed into glass jars with the stock solution and distilled water was added. Calibration standards were prepared with stock solutions and distilled water, the dilutions were over a larger dose range (0, 0.2, 0.4, 0.8, 1.4 ml/ml). Samples and standards were mixed on the rotating drum for 2 h in darkness. To clear the solutions after mixing a spatula of the gypsum was added to each sample jar, shaken by hand for 10 s, then left in darkness until the supernatant had cleared. Once the supernatant was clear, fluorescence readings were taken and the Kd calculated. 2.3. Recovery of tracer If the soil deposit method is to be advocated, direct comparisons are necessary to compare the results with those obtained by artificial targets. Ideally, a method that uses the soil directly should be as reliable as one that uses artificial surfaces. However, nature is inherently more variable. The following experiments investigated the variability of tracer recovery from soil under controlled conditions. The recovery from soil and artificial surfaces was compared using two volumes (20, and 40 ml) of stock solutions of (2% w/v) Tinopal CBS-X. Calibration standards were created using 0, 0.4, 0.6, 0.8, 1.2 ml/ml distilled water, and 50 ml was added to 10 g of soil (sandy loam field plot soil). Artificial samplers are normally solid surfaces that do not adsorb the tracer. It was decided that applying the stock solution directly into the glass jars would adequately represent this model. Samples were left in the dark and once dry 50 ml of distilled water was added. The sample was then shaken for 30 s. Calibration standards were prepared by adding the stock solution directly to the distilled water (50 ml). Each treatment was replicated 17 times, and
17
analysed for the average volume recovered and the coefficient of variation within each group. 2.4. Laboratory simulation One of the major perceived problems associated with the use of artificial samplers is their placement. This was examined in the laboratory under conditions that simulated the field Spring barley plants were grown in seeding trays in a glasshouse. The plants were sown parallel with the long side of the tray and spaced to simulate a crop inter row. Once the plants had reached growth stage 34 (Tottman, 1987), spraying commenced using a hydraulic track sprayer. It consisted of a dry boom fitted with a nozzle that straddled a track. A trolley pulled by an electric motor and pulley system moved the trays to be treated along the track under the nozzle. The nozzle was aligned over the centre of the long axis of the track, and the boom height adjusted to 40 cm above the crop. The tracer solution was mixed prior to spraying and stored in a light-tight container. The maximum concentration of Tinopal CBS-X (2.5% w/v) was used along with 0.1% w/v Agral (non-ionic surfactant). Two applications, which were likely to produce different levels of soil deposit, were chosen. The nozzles used were a Spraying Systems Cos flat-fan XR100 015 nozzle operated at 4 bar (4 kph) and a flat-fan XR110 06 operated at 1 bar (8 kph). The spray from the former has been classified as a BCPC Fine (Doble et al., 1985) and is more likely to be retained on the crop whilst the latter is classified as a BCPC Coarse spray and is more likely to be deposited on the soil. Forward speed and pressure were adjusted to maintain 200 l/ha application volume for both Fine and Coarse sprays. Acetate sheets (40 cm2) were placed directly on the soil surface prior to spraying and removed once the deposit had dried. Following spraying, once the deposit had dried, soil samples (23 cm2 area) were taken using soil density rings as soil cores. Three different sampling arrangements were used: ‘‘inter-row’’ where both sample types were taken from the inter-row, ‘‘random’’ where both sample types were taken randomly, and ‘‘different’’ where the artificial samples were taken from the inter-row, and soil samples taken randomly. Four trays were placed on the track for each replicate, two for artificial samples, and two for soil sampling. Five samples (soil 23 cm2, acetate 40 cm2) could be taken from each tray. This was repeated four times, hence four replicates with 10 samples from each. 2.5. Field experiment The field experiment formed part of an investigation into the biological effects of different deposit distributions (Barber, 2001). Three different pesticide types were
J.A.S. Barber, C.S. Parkin / Crop Protection 22 (2003) 15–21
assessed at three different doses. Laboratory tests were carried out to detect interactions between the Tinopal CBS-X and the pesticides. HPLC analysis showed that there was no chemical interaction between the tracer and pesticides, and spectroscopy showed that there was no quenching of fluorescence by the pesticides (Barber, 2001). Here, we will present only the results taken from samples located on the ground. We have disregarded chemical type and dose as variables and included only nozzle and sample type. Three conventional and one an air-induction flat-fan nozzle sizes were used (Spraying Systems Cos XR 110 015 (4 bar and 4 kph), XR 110 04 (2 bar and 8 kph), XR 110 06 (1 bar and 8 kph) and AI 110 025 (3 bar and 6 kph)). All were adjusted to apply 200 l/ha. Three single nozzle bodies were mounted at 50 cm intervals on a dry boom extending to one side of an ATV; thus, two passes were necessary to apply the treatments to the 3 3 m plots. The boom was held at 40 cm above the crop, with a nozzle separation of 50 cm as recommended by the manufacturer. Spray function was checked using a patternator. The 36 plots were each sampled twice. For soil measurements, samples were taken, again using 23 cm2 area density rings as soil cores. These were randomly placed in the central strip of plots in positions pre-assigned using a random number generator. Soil samples were left in the density rings and stored in individual light-tight containers for subsequent analysis. Further ground deposit measurements were taken with acetate sheet targets (20 cm2) placed in the pre-assigned plots prior to spraying. To ensure that each sample surface was level, each sheet was mounted on a table stand raised 15 cm above the ground. Two types of stand were used, one with a 15 15 cm table and another with a 5 5 cm table. The smaller table made little impact on the canopy but the larger table significantly opened up the crop. The two table sizes represent the likely minimum and maximum interference of the canopy with artificial targets. After spraying the acetate sheet samples were removed from their tables, placed in watertight plastic bags and stored under blackout conditions.
3. Results and discussion 3.1. Effect of soil type The effect of soil type on the sorption of Tinopal CBS-X is shown in Table 1. The relevant calibrations for the soils are shown on Fig. 1. Tracer sorption appears to be inversely related to the amount of organic matter present. The compost had an extremely high level of organic matter (90%); correspondingly, the amount of tracer in solution was low, creating a shallow calibration line. The lower the OM the higher the sorption coefficient (Table 1) and the steeper the adsorption
Table 1 Percentage organic matter (OM) and sorption co-efficient (Kd ) for Tinopal CBS-X sorbed on each soil type Soil type
OM
Kd coefficient
Sand Clay top Chalk Compost
2.9 6.3 6.6 90.0
0.022 0.030 0.037 0.101
60 Sand
Stock solution recovered (µl)
18
Clay
Chalk
Compost
2
R = 0.9933
50
R2 = 0.9631
40
2
R =0.9396
30 20
2
R = 0.957
10 0
0
10
20 30 40 50 Stock solution applied (µl)
60
70
Fig. 1. Sorption calibrations for Tinopal CBS-X with four different soil types. Table 2 Recovery of Tinopal CBS-X on artificial and soil samples at two doses Sample
Artificial
Soil
Tracer concentration
0.4 ml/ml 0.8 ml/ml 0.4 ml/ml 0.8 ml/ml
Volume Coefficient of variation (%)
0.418 6.93
0.801 3.02
0.401 5.28
0.799 4.26
No significant difference between concentration recovered with soil SED=0.0057, Po0:001; LSD=0.0135.
isotherm (Fig. 1). The sandy soil, with the lowest OM, sorbed a minimal amount of tracer. 3.2. Recovery of Tinopal CBS-X The results of the experiments comparing the recovery of Tinopal CBS-X from artificial samples and soils in Table 2 show that in the laboratory high levels of recovery were achieved with both the samples. Although artificial targets did show a significant difference in recovery this difference is too small to have practical significance. 3.3. Laboratory simulation Statistical analysis showed that overall there was a difference between nozzle (Po0:001), and target type (Po0:048). The Fine spray deposited significantly less (0.760 ml/cm2) to the ground in comparison to the
J.A.S. Barber, C.S. Parkin / Crop Protection 22 (2003) 15–21 Table 3 Mean deposit of Tinopal CBS-X (ml/cm2) for sample type and position Position
Artificial
Soil
Different Inter-row Random
0.925 0.853 0.803
0.791 0.846 0.814
SED=0.0381, P ¼ 0:014; LSD=0.0749.
Table 4 Mean and coefficient of variation (CV) of Tinopal CBS-X deposits collected in each sample location for both nozzle types with artificial and soil targets Nozzle
Position
Volume (ml/cm2)
CV (%)
Artificial
Soil
Artificial
Soil
Coarse
Different Inter-row Random
0.983 0.955 0.873
0.901 0.904 0.887
23 23 32
34 22 34
Fine
Different Inter-row Random
0.867 0.751 0.734
0.681 0.788 0.740
22 22 35
34 21 37
19
Table 5 Mean volume (ml//cm2) of Tinopal CBS-X deposited by each nozzle type to the different sample types in the field experiment Nozzle
Sample 5 5 cm Tables 15 15 cm Tables Soil
Air inclusion Coarse Medium Fine Averages
0.541 0.547 0.471 0.416 0.494
0.711 0.761 0.702 0.573 0.687
Averages
0.638 0.631 0.667 0.659 0.656 0.617 0.620 0.548 0.645 —
Average for nozzles: P ¼ 0:079; SED 0.0436, LSD=0.0867; average for sample types: Po0:001; SED=0.324, LSD=0.0645; and averages for the 5 5 cm tables: P ¼ 0:051; SED=0.0749, LSD=0.0645. No significant difference between soil samples and 15 15 cm tables.
fraction of spray deposited to soil and an underestimation of its variability. 3.4. Field experiment
SED=0.0539, P ¼ 0:205; LSD=0.106.
Coarse spray (0.917 ml/cm2). The acetate sheets (0.861 ml/cm2) collected significantly more than the soil samples (0.817 ml/cm2). The results in Table 3 show that the only significant difference (P ¼ 0:014) between the mean deposit found on each sample occurred when they were positioned differently. The soil samples appeared to collect less deposit compared to the acetate sheets placed on the soil. An analysis of the interactions between sample type, position and treatment is shown in Table 4. The difference in retention between artificial surface and soil targets was only apparent for the Fine spray. There was a significant difference between the artificial targets placed inter-row and the randomly taken soil samples. The mean levels of deposit and variation were similar when soil samples and artificial surfaces were in the inter-row. When both sample types were positioned randomly the level of variation rose markedly, but remained similar. Taking the samples from different locations altered the levels of variation. Both target types collected similar spray volumes when they were treated alike. This would indicate that overall there is little difference between the collection efficiencies of soil and table samples under field conditions. Differences occur only when the sampling methodology differed. It appears that larger but less variable deposits were collected when the inter-row was sampled than when sampled randomly. Therefore, the inter-row sampling can lead to an overestimation of the
As can be seen in Table 5, significant differences were observed between both the sample and nozzle types. The Coarse spray deposited significantly more to the soil when compared to the Fine spray (P ¼ 0:079). The smaller table samples collected significantly (Po0:001) lower volumes of spray compared to the soil samples and the larger tables. When nozzle target interactions were analysed, although they all followed similar trends only samples from the larger tables showed any significant difference (P ¼ 0:051) between nozzles, with the Fine spray depositing less compared to the Coarse. The results from the air-inclusion nozzle suggest that it deposited slightly less to the soil compared with the Coarse spray. Since the drop spectra of the Coarse and air-included sprays are likely to be similar, this implies that spray retention on the canopy with the air-included spray is greater.
4. Conclusions Direct measurements of fluorescent tracers on soil were successfully made despite soil complexity. Work with different soil types showed that although sorption differs with type this could be accounted for via a sorption isotherm or calibration. If soil types vary across an area then adequate sampling, mapping and good experimental design could block changes. Under simulated field conditions, differences in the mean quantity retained and variability were observed between sample locations but not between sample types. The field experiment showed the expected trend with the Fine spray distribution depositing less to the ground compared to the Coarse spray. The differences between the soil samples are much less conclusive than those
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from artificial surfaces implying that the latter could overestimate deposit. Also, the results from soil samples are less likely than artificial surfaces to yield significant differences due to much larger variations within groups, caused by the varying levels of shade produced by the crop canopy. Therefore, the main difference between soil sampling and artificial surfaces is the likelihood of sample bias. Artificial samples need to be located within the plot prior to application and the process of target placement can cause crop disturbance. There is a tendency to find an area that is easily accessible to the operator, such as in tramlines or inter-rows. Such areas are invariably more open, and this can lead to an overestimation of the mean deposit and an underestimation of variability. With similar sample placement there is no significant difference in the volume recovered or the level of variation. Random non-intrusive sampling, which is only possible in the field with soil samples, recovers lower volumes of spray with a higher variation, and is a more authentic measurement of the movement of pesticide to soil. Hence, although the use of soil as the sample is slightly more protracted than the artificial surfaces, the advantages gained by being able to sample freely in a completely random manner could negate the disadvantage of increased time for analysis. It, therefore, appears that Tinopal CBS-X could be considered a suitable tracer for the measurement of pesticide deposits both on the crop and directly on the soil.
Acknowledgements We would like to thank the Ministry of Agriculture Fisheries and Food (MAFF) for funding JAS Barber during this work. We would like to thank Bob Walker, Dr Se! amus Murphy and Matthew Fisher for their assistance with the fieldwork. We are also grateful for the discussions held with Professor Paul Miller of SRI and Professor Daryl Joyce of Cranfield University.
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