Department of Pesticide Regulation

Environmental Monitoring and Pest Management Branch

1020 N Street, Room 161

Sacramento, CA 95814 - 5624

Study 147: Monitoring a Field Application to Determine Air Concentrations of Methomyl and Metalaxyl, San Diego County

June 1996


The application of multiple pesticides to potato fields in proximity to urban areas represents a potential for exposure. The San Diego County Agricultural Commissioner's office has requested that monitoring be conducted to determine air concentrations during and after application. The Commissioner's office will use the data to assess compliance with permit conditions.

The Environmental Hazards Assessment Program (EHAP) will collect air samples associated with an application in order to determine concentrations of two of these chemicals and will estimate concentrations of the additional chemicals, based on a flux estimation procedure (Appendix A).

The plants are usually treated in June and/or July with a tank mix of several different chemicals designed to control a variety of potato pests. Of these chemicals, two have been selected for monitoring, based on available analytical methods, likely magnitude of air concentrations, and existing risk assessment data (Appendix B). Metalaxyl is a broad-spectrum systemic fungicide, and methomyl is an insecticide effective against lepidopterous pests.


The objective of this study is to measure offsite air concentrations of metalaxyl and methomyl during and after a field application. The data will be used by the San Diego County Agricultural Commissioner's office to assess compliance with permit conditions. If possible, the data will be used by the EHAP to model application drift and evaluate a flux estimation procedure (Appendix A).


This study will be conducted by personnel from the Environmental Hazards Assessment Program under the overall supervision of Randy Segawa. Key personnel include:

All questions concerning this project should be addressed to Madeline Brattesani at

(916) 324-4100


Site Selection

Based on information provided by the San Diego County Agricultural Commissioner's Office, the area of concern is in the northeast portion of the county. The monitoring site will be two adjacent potato fields that are 80 and 85-acres each. The site is surrounded by other fields planted mostly with oats, with another 80-acre potato field approximately 0.25 miles northwest. Although the three potato fields are usually treated on the same day, this northern field should be treated at least 24 hours after the end of the monitoring period to ensure that results accurately reflect air concentrations at the field of interest. The application will occur in the morning and usually takes up to 2 hours.

Air Sampling Air will be sampled with High-Volume air samplers capable of drawing 1 m3 of air per minute. A total of eight sampling stations will be located at the treatment site (Figure 1). Four of these (stations 1,2,3, and 4) will be located at the center of each side of the field between 10-20 meters (m) from the edge. The remaining four sampling stations (5,6,7, and 8) will be located 40-80 m from the corners of the field. Additional samplers will be placed at nearby residences and a campground adjacent to the ranch.

Monitoring will take place for approximately 30 hours following the start of application. For 8-12 hours prior to the application, two air samplers will be placed within the treatment site to measure background levels of chlorothalonil and methomyl. Air samples will be collected during the application, and for two consecutive 8-12 hour intervals following application. If possible, the post-application intervals will be split between night and day, to account for differences in air stability and wind speed and direction.

Each air sampler will be positioned approximately 1.2 m (4 ft.) above ground level and will be fitted with a glass jar containing 125-ml of XAD-2 resin rinsed with solvent. Once samples are collected, each jar will be placed in a plastic bag, tightly closed, and stored on dry ice. Samples will be kept frozen until analysis.

Additional Monitoring Meteorological data will be collected at the field site using a Met One/Campbell Scientific weather station (Met One Instruments sensors, Campbell Scientific CR-21XL data logger). Wind direction and wind speed, collected at 10-meter height, ambient air temperature, and relative humidity will be recorded for 24-hours prior to application, and the duration of the monitoring period. Values for these variables will be averaged at one minute intervals.

Application procedure Detailed notes on the application procedure will be made. These notes will describe how the material is applied, how the application rate is determined, the formulations, observations on drift, observations on how and what the spray targeted (ie targeted foliage, blanketed field, etc.), height of application, nozzle type, and nozzle spacing.


Chemical analysis will be performed by the California Department of Food and Agriculture laboratory in Sacramento. Laboratory methodology and analytical verification will be specified in the final report for the study. Results will be reported in micrograms per cubic meter of air, with a reporting limit of 0.7 microgram per sample (µg/sample) for methomyl, and 0.5 µg/sample for metalaxyl.


Monitoring will occur between June and August 1996, depending on the schedule of the proposed applications. A final report will be completed within six months after all analyses are complete.

REFERENCES (including references for Appendices)

Hsieh, Dennis P.H, James N. Seiber and James E. Woodrow. 1995. Pesticides in air. Part II.

Development of predictive methods for estimating pesticide flux to air. Final Report Prepared for the California Air Resources Board Under Contract #92-313. September 25, 1995.

Johnson, Bruce. 1995. Memorandum to John Sanders on Proposal to regulate 1,3-d using township/range cap. Dated December 7, 1995.

Kollman, Wynetta and Randall Segawa. 1995. Interim report of the pesticide chemistry database 1995. Environmental Hazards Assessment Program. State of California, Environmental Protection Agency, Department of Pesticide Regulation, Environmental Monitoring and Pest Management Branch. EH95-04.

Ross, L.J., B. Johnson, K.D. Kim, and J. Hsu. 1995. Prediction of methyl bromide flux from area sources using the ISCST model EH 95­03 February 1995 mebr, volatilization, @2061 model soil fumigant

Ross, L.J., B. Johnson, K.D. Kim, and J. Hsu. 1996. Prediction of methyl bromide flux from area sources using the ISCST model. Journal of Environmental Quality 25(4): (in press)

Sine, Charlotte (ed.) 1995. Farm Chemical Handbook '95. Meister Publishing Company.

Willoughby, Ohio.

Teske, M.E., T.B. Curbishley and J.W. Barry. 1994. Forest service aerial spray computer model FSCBG 4.3 User Manual Extension. Continuum Dynamics, Inc. C.D.I. Technical Note No. 93-19.

Wagner, Curtis P. 1987. Industrial source complex (ISC) dispersion model user's guide - second edition (revised -- Volume I. EPA-450/4-88-002a. Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency

Ware, George W. 1986. Fundamentals of pesticides - A self instruction book. Second Edition. Thomson Publications. Fresno, CA.

Wauchope, R. Don, Arthur G. Hornsby, Don W. Goss and John P. Burt. 1991. The SCS/ARS/CES pesticide properties database: I A set of parameter values for first-tier comparative water pollution risk analysis. p. 455-470 IN Diana L. Weigmann (ed.) 1991. Pesticides in the next decade: The challenges ahead. Proceedings of the Third National Research Conference on Pesticides. November 8-9, 1990. Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University. Blacksburg, Virginia.


This appendix describes two additional analyses which may be conducted if monitoring is successful.

Application Drift Modeling The Forest Service Aerial Spray Computer Model - FSCBG 4.3 (Teske, Curbishley and Barry, 1994) will be used to model the airborne drift of pesticides during and immediately following the application process. This model combines an aircraft wake effects model and a tilted gaussian plume model to simulate the deposition of aerially applied pesticides both on and off site. Input parameters required include the aircraft model (both fixed wing and helicopters may be represented), pesticide formulation characteristics, application system characteristics, site description and weather data. In addition, the flight path followed during the application must be specified. The model can simulate either air concentrations or droplet deposition. The model prediction of air concentrations will be compared to the air concentrations measured during the application event.

Flux Estimation

One method for estimating air concentrations is to use the Industrial Source Complex Short Term (ISCST) and flux rates. Flux rates have been difficult to obtain because their measurement is expensive and requires field personnel experienced in this type of measurement. However, because of the importance of obtaining flux rates for the purposes of estimating ambient air concentrations, the Air Resources Board (ARB) funded a study to derive an empirical approach towards estimation of flux rates based on available published literature (Hsieh et al. 1995).

The fruits of this work were, in part, two empirical equations for estimating pesticide flux rates based on the physicochemical properties of the pesticide (Hsieh et al. 1995). One equation estimates flux from plant surfaces and another estimates flux from the soil. The former equation uses vapor pressure. The latter, in addition, uses solubility, soil adsorption and application rate to determine the emission rate (flux) for each chemical.

The application to young potato plants of a mixture of several pesticides offers a unique opportunity to investigate the validity of the approach provided in Hsieh et al. (1995). The simultaneous application of several different pesticides eliminates many potentially confounding variables such as temperature, humidity, moisture, insolation, application technique, and so on. A reasonable correspondence between flux rates predicted by the empirical model of Hsieh et al. (1995) and flux rates back calculated from offsite air concentrations, as described in Ross et al. (1996), would raise confidence in this empirical technique and open the door for possible regulatory applications. The goal of this additional work is to use the monitoring data in conjunction with a back calculation procedure (Ross et al. 1996) to compare to flux rates derived from the procedures of Hsieh et al. (1995).

Koc, solubility, and vapor pressure for the seven pesticides were obtained and are shown in

Table 1 (Kollman & Segawa 1995, Wauchope et al. 1991). Application rates were obtained from the pesticide use report. There was reasonably good agreement between the two databases where both provided estimates. The largest difference was for vapor pressure for chlorothalonil, where the estimates differed by 3 orders of magnitude.

According to informal reports, the material is applied at a phenological stage of the potato when some, but not all of the ground is covered by vegetation. Therefore, volatilization following application may derive from both soil and plant surfaces. The procedures of Hsieh et al. (1995) provide an equation for estimating volatilization (flux) from each type of surface. They are

Equation for Estimating Volatilization

Equation for Estimating Volatilization

where Fs and Fp are flux rates from soil and plant surfaces, respectively (µg/m2h); VP is vapor pressure (torr), Koc is organic carbon partition coefficient (ml/g), Sw is water solubility (ppm). Utilizing these equations results in an estimate of flux based on soil volatilization or plant surface volatilization (Table 2). No estimate is possible for mancozeb because it has no vapor pressure.

Since air concentration will be proportional to flux rate, the flux rates in Table 2 provide an index of air concentration. The three highest flux rates under both estimation equations are chlorothalonil, methamidophos and methomyl.

These measurements will be utilized in the ISCST model to both back calculate flux and to determine air concentrations of the additional pesticides in the tank mix.

Vegetation analysis

To evaluate the empirical estimation procedure of Hsieh et al. (1995) it may be necessary to quantify the fraction of volatilization from soil surface versus plant surface. To estimate these fractions, at 10 randomly selected locations, the percent of bare ground will be estimated in a 3x3m area. At 10 randomly selected locations, the number of potato plants for 2 meters will be counted. Distance between row centers will be measured. Fifty potato plants will be randomly selected, plant height measured and aboveground biomass harvested. For each of 10 randomly subselected plants, the leaf area will be measured. This subsampling scheme will enable establishment of a regression relationship between plant mass and leaf area. For all 50 plants the above ground wet weight will be measured.

Table 1. Physicochemical properties used in flux estimation procedure. First line for each chemical is from Pestchem Data Base. Second line is from USDA/ARS database. Note that 1torr=1mmHg.

Vapor Pressure (torr)

Koc (ml/g)

Solubility (mg/L)

Application Rate (kg/ha)




































Table 2. Estimated flux rates in ug/m2h and the equivalent value in ug/m2s utilizing physicochemical parameters and equations 1 and 2 (Hsieh et al. 1995).

Flux from Soil

Flux from Plant Surface








































Pesticide Selection

Seven pesticides were found in 92-93 pesticide use report, as candidate pesticides applied simultaneously to potatoes in San Diego, Section 9S/3E-16: chlorothalonil, esfenvalerate, mancozeb, metalaxyl, methamidophos, methomyl, and metribuzin. The pesticides include fungicides, insecticides and a single herbicide (Table 3). Part of the criteria for selecting which pesticide to sample for includes ranking the pesticides according to the likely magnitude of any resulting air concentration. Monitoring for pesticides with potentially higher air concentrations will result in higher likelihood of successful monitoring. The techniques of Hsieh et al. (1995) were used to index the pesticides according to expected air concentration (Appendix).

Table 3. Brief description of candidate pesticides (Ware 1986 and Farm Chem 95).

Chlorthalonil: Substituted aromatic benzene derivative, broad-spectrum fungicide.

Esfenvalerate: Synthetic pyrethroid, broad spectrum insecticide.

Mancozeb: Coordination product of zinc ion, manganese ethylene bisdithiocarbamate, related to both maneb and zineb. In the family ethylene bisdithiocarbamate. Fungicide.

Metalaxyl: Acylalanine, soil and foliar fungicide. Used for mildew and soil born diseases.

Methamidophos: Aliphatic organophosphate, insecticide.

Methomyl: Carbamate. Insecticide.

Metribuzin: Triazinone. Herbicide.

Chemical Selection

The chemicals chosen for analysis will be determined based on several factors, including environmental fate characteristics, potential toxicity, and prior monitoring. Of the pesticides applied, the two with the greatest likelihood of detection will be selected. Based on prior calculations, certain pesticides are projected to occur at higher concentration, based on vapor pressure, solubility, and soil adsorption values (Appendix A). These pesticides will be more likely to be detected with the available sampling and analytical methods. In addition, priority will be given to those chemicals which have been reviewed for potential human toxicity, and have also been monitored during previous air studies.