State of California 

M e m o r a n d u m

To : Kean S. Goh, Ag. Program Supervisor             Date: 25 July 1994 
     Environmental Monitoring and 
     Pest Management Place: Sacramento 
                                                     Phone: 324-3904 

From : Department of Pesticide Regulation - J. Jesus Leyva 
       APRS (Chemistry) 
       Pesticide Registration 
  
Subject: Dissipation of Amitrole on California Rights-of-Way 


INTRODUCTION 

Amitrole (3-amino-lH-1,2,4-triazole) is an herbicide used to control a broad spectrum of annual and perennial grasses, and broad-leaf weeds. Since 1986 amitrole use has been restricted by the U.S. Environmental Protection Agency (USEPA) to non-cropland areas only. In California, total reported amitrole use has declined from 48,800 kg active ingredient (ai) in 1986 to 8,996 kg ai in 1990 (California Pesticide Use Report 1987 and 1991). 

Technical amitrole is an off-white powder at room temperature with a vapor pressure of less than 1 mPa at 20oC and a water solubility of 280 g/L at 25oC (Hartley and Kidd 1992). Half-lives in non-sterile soil ranged from less than 1 to greater than 56 days(Burschel and Freed 1959; Day et al. 1961; Riepma 1962, Weed Science Society of America 1983). Amitrole disappearance in soil has been attributed to oxidative chemical processes (abiooxidation) (Kaufman et al. 1968), and microbial degradation (Riepma 1962). Hydrolysis or volatilization of amitrole are much less significant dissipation processes (Reinert and Rodgers 1987; Weed Science Society of America 1983). 

In California, amitrole was placed on the list of chemicals known to cause cancer pursuant to Proposition 65, the Safe Drinking Water and Toxic Enforcement Act of 1986. A study was undertaken in the spring of 1988 to provide environmental fate information for typical amitrole applications to highway rights-of-ways in California. Specific objectives were to measure amitrole concentrations in air, soil, vegetation, and in simulated rain runoff following applications on both sloped and flat surfaces. 


Site Description and Amitrole Application 

Experimental roadside sites were selected in two California Counties, Merced and Monterey. Three experimental sites were chosen in Merced County designated as sites 1, 2 and 3, and one in Monterey County designated as site 4. Sites designated 1, 2, and 4 were both 25 m in length but site 1 was flat and site 2 was sloped at 45o-60o. Site 3 was 36 m long and had a 45o-60o slope away from the pavement. The flat sites, 1 and 4, were representative of an area with low runoff potential whereas the two sloped sites, 2 and 3, represented areas with a high potential for runoff. Air sampling during application was conducted at all sites, dissipation rates in soil were measured at sites 1, 2 and 4,and simulated rain was applied to site 3 to collect runoff water samples. 

The sites were adjacent to paved highway roads with applications made to approximately 4 m wide strips. Amitrole (Amizol) was applied in the spring of 1988 to all sites at the rate of 4.48 kg ai/ha using a truck-mounted spray rig. During application, two tank samples were collected at each site. 

Mass Deposition 

Amitrole mass deposition was measured with 8 mass deposition cards(Kimberly-Clark Corp.) per site. Mass deposition cards were placed 3-m apart along a diagonal transect across each treatment plot. The 0.9 m2 cards were placed horizontal to the ground at aheight of 0.5 m. 

Air Sampling 

During amitrole application, 1-hour air samples were collected at sites 1, 2 and 4 with Anderson low-volume samplers. The air sampling apparatus consisted of a plastic cassette containing a 37-mm diameter glass fiber filter as the trapping medium with the cassette attached to the pump inlet with Tygon tubing. Each cassette was placed upright on a metal rod at 1.5 m above the ground. Eight air samplers were located as follows: two at site 1and three at sites 2 and 4. Air samplers were spaced equidistantly along the downwind edge of each site. The samplers were calibrated at 17 L/min and were started 5 minutes before amitrole application began and ran for 1 hour. Samples were frozen until analysis. 

Soil Sampling 

Amitrole dissipation was determined in surface soil at Merced sites 1 and 2 and Monterey site 4 which were not exposed to simulate drain. Four background soil samples were collected at each site prior to amitrole application and eight samples were collected on the day of application and 1, 2, 4, 8, and 16 days later. A sample consisted of a composite of three random subsamples which were 
collected using a 6.03-cm internal diameter stainless steel cylinder inserted to a depth of 4 cm. In addition to pesticide analysis background samples were analyzed for particle size, pH, organic matter content, and bulk density. Soil that was sampled after application was only analyzed for the presence of amitrole.  Samples were stored at -70oC until extraction and results were reported on a dry weight basis. 

Vegetative Sampling 

Roadside vegetation samples were also collected from Merced sites 1 and 2 and Monterey site 4 to measure residue levels on plant material. Two background samples were collected prior to application and three samples were collected on the day of application, and again on 1, 2, 4, 8, and 16 days after application. Each vegetative sample consisted of three randomly selected sub samples that were clipped just above ground level using stainless steel shears. Samples were stored at 4oC until extraction and results were reported on a dry weight basis. 

Simulated Rain Runoff Sampling 

The amount of amitrole in runoff water from simulated rain events was determined at the Merced site 3. The site was divided into 24 subplots, each 4 m x 1.5 m in dimension. Sampling periods were at O (immediately after the spray), 1, 2, 4, 8, and 16 days post application during each period, simulated rainfall was applied to four randomly selected subplots. The simulator was designed according to Meyer and Harmon (1979) and it applied artificial rainfall at a rate of 189 mm/hr. Simulated rainfall was applied for 2 min to each subplot after which two rain runoff samples were collected using a hand-pump with teflon tubing. Samples were stored at 40C until extraction. A subplot was not resampled once it was exposed to simulated rain. 

Chemical Analysis 

Chemical analyses for amitrole were conducted by the California Department of Food and Agriculture (CDFA), Chemistry Laboratory Services. Amitrole concentrations in air and water samples were determined by diazotization-coupling with N-(l-naphtyl)ethylenediamine (NEDA) under acidic conditions and measured by spectrophotometry (S. C. Dual Channel-Colorimeter, Technicon Instrument Corp., Tarrytown, N. Y.) at a wavelength of 520 nm.  Minimum detection levels (MDL) were 50 ng/g for air and 0.1 ug/g for water. Amitrole in vegetation and soil samples was determined by ethanol extraction, diazotization-coupling with NEDA under acidic conditions, then measured by spectrophotometry (Spectronic20, Bausch & Lomb) at 455 nm. MDL for soil and vegetation was 0.1 ug/g. 

Statistical analyses to determine dissipation rates of amitrole in soil and in vegetation were determined using SASS PROC REGRESSION to fit a linear regression to mean amitrole concentration by day for each site (SASS Institute, 1987). Data were transformed to base 10 logarithms. 

RESULTS AND DISCUSSION 

Mass Deposition 

Average amitrole percent in tank samples was 71% of the theoretical rate at the Merced site 1, 90% at the Merced site 2, and 108% at the Monterey site 4. 

Average amitrole applied, as measured in mass deposition cards, was 30% of the theoretical rate at the Merced site 1, 55% at the Mercedsite 2, and 148% at the Monterey site 4. 

Air Concentrations 

Amitrole residues were not detected in ambient air samples taken during application. Since amitrole's Henry's Law Constant estimated from vapor pressure and water solubility is less than 3 X 10-7 KPa-L/mol, volatilization was not expected as a significant route for dissipation (WSSA 1983). 

Soil Dissipation 

Dissipation of residues were anomalous in Monterey County site 4.  Mean amitrole residues in soil prior to application were 0.27 ug/g. But average amitrole residues increased significantly from day 0 to day 16 (Figure 5, Table 1). 

Amitrole residues in soil collected prior to application averaged 0.12 ug/g and 0.17 ,ug/g at Merced sites 1 and 2, respectively.  Since amitrole applications had occurred at these sites in previous years, it was expected that some residues would be present. Soil bulk density and organic matter content were higher at site 1 (Table 2). Amitrole dissipation during the 16-day post application period followed a first order exponential decay pattern at both sites. At site 1, amitrole soil half-life was estimated at 11 days(Figure 3, Table 1) and at site 2 soil half-life was estimated at 12 days (Figure 4 and Table 1). 

The amitrole dissipation rates of 11 and 12 days measured at the Merced County sites were similar to previous reports. A range of 2 to 4 weeks is reported by WSSA (1983). Burschel and Freed (1959)reported first-order dissipation half-life rates of 3 to 4 weeks in a Chchalis loam soil whereas Riepma (1962) reported a first-order dissipation rate of 7 days in a sandy soil. (Kaufman (1968)proposed that amitrole-5-l9C dissipation occurred in nonsterilized soil (Hagerstown silty clay loam) by nonbiological reactions, 69 %of the 5-carbon from amitrole was released as 14CO2 in 20 days.) 

Vegetative Samples  

In Merced County, mean amitrole total residues in vegetation prior to application were less than 1 ug/g at both sites. Amitrole total residues in vegetation at site 1 were highly variable with the data indicating a trend toward a decrease in residues over time (Figure6 and Table 2). Amitrole dissipation at site 2 fit a first-order exponential decay pattern with a dissipation half-life estimated at 7 days (Figure 7 and Table 2). 

At Monterey site 4, mean amitrole residue in vegetation prior to application was 0.41 ug/g. Amitrole dissipation also tended to follow a first-order exponential decay pattern with a dissipation half-life estimated at 6.5 days (Figure 8 and Table 1). 

Simulated Rain Runoff  

Amitrole was detected in simulated rain runoff water collected at site 3 in Merced County. The highest mean amitrole concentrationwas 5.48 ug/g on the day of application and the lowest mean concentration was 0.11 ug/g at 16 days after application (Figure 9 and Table 3). 

Quality Control and Storage Stability  

Average amitrole percent recovery (continuing quality control) was 92% for mass deposition cards, 88% for air (glass fiber filter),62% for soil, 62% for vegetation, and 95% for water (Table 4). 

A storage stability study was conducted to determine the potential breakdown of amitrole in soil, vegetation, and water. In soil amitrole began to breakdown around day 14 with continued gradual breakdown over a 56-day period (Table 5), amitrole in vegetation was stable up to 42 days (Table 6) and amitrole was stable in water up to 14 days (Table 7). 

CONCLUSION 

Dissipation of amitrole in soil was measured at two roadside sites in Merced County but it was not observed at the site in Monterey County. The estimated dissipation rate of amitrole in soil was approximately 12 days which was similar to previous reports. The cause for differences at the Monterey County site is unknown. 

Data for dissipation in vegetation was also variable but a trend was evident for disappearance over time. Half-life was estimated at 7 days. 

Amitrole residues were measured in water runoff samples generated from simulated rain events indicating that movement of residues in runoff water is another path of dissipation of residues from the initial site of application. 

Since the registration of amitrole was voluntarily withdrawn in 1991, no further studies will be conducted. 


cc: Bob Rollins 
Tom Leffingwell 


REFERENCES

Burschel P. Freed V H (1959) The Decomposition of Herbicides inSoil. Weeds 7:157-161. 

California Pesticide Use Report (1987) Pesticide Use ReportAnnual 1986, Department of Food and Agriculture (Now Departmentof Pesticide Regulation), 1220 N Street Sacramento, California95814 

California Pesticide Use Report (1991) Pesticide Use ReportAnnual 1990 Department of Food and Agriculture (Now Departmentof Pesticide Regulation), 1220 N Street Sacramento, California95814 

Day B E, Jordan L S. Hendrixson R T (1961) The Decomposition ofAmitrole in California Soils. Weeds 9:443-456. 

Kaufman D D, J R Plimmer, P C Kearney, J Blake, and F S Guardia(1968) Chemical Versus Microbial Decomposition of Amitrole inSoil Weed Science 16:226-272 

Meyer L D, Harmon W C (1979) Multiple-Intensity Rainfall Simulatorfor Erosion Research on Row Slideslopes. Trans of the ASAE pp100-103 

Riepma, P (1962) Preliminary Observations on the Breakdown of3-Amino-1,2,4-Triazole in Soil. Weed Res. 2:41-50 

SAS Institute, Inc. (1987) SAS/STAT guide for personalcomputers, Version 6 edition, Cary, NC, SAS Institute, Inc. 

Weed Science Society of America (1983) Herbicide Handbook Fifthedition, Champaign, IL pp 19-21