California Department of Resources Recycling and Recovery (CalRecycle) 

Clopyralid in Compost and Mulch

Sampling Protocol

Suggested Monitoring Protocol for Occurrence and Concentrations of Clopyralid in Feedstocks and Compost at Production Sites in California


This protocol provides a uniform approach to collecting and analyzing samples from compost feedstocks and/or finished compost for clopyralid residues. The protocol designates the analytical sensitivities, laboratory quality control, and quality assurance measures needed to generate useful information. The results can be submitted to California Department of Pesticide Regulation (DPR) as preliminary data or as a formal report when all the background information is available. DPR will ascertain what actions, if any, the information can support in its regulatory programs.

The suggested protocol was developed by a workgroup drawn from individuals participating in a series of stakeholder meetings convened by the DPR and the CIWMB (now known as the Department of Resources Recycling and Recovery, or CalRecycle) in 2002 to address issues concerning clopyralid and compost. The workgroup reviewed a variety of information sources concerned with protocol development and laboratory analysis for environmental contaminants. The workgroup followed a standard format for a research protocol developed by DPR’s Environmental Hazards Assessment Program. 

A staff person at the California Department of Food and Agriculture’s Meadowview Laboratory was consulted regarding the standard laboratory practices used to maintain the quality control and quality assurance of the analytical results. The compost and feedstock sampling strategies were taken from the "test methods examination of composting and compost" (TMECC) document developed by the U.S. Composting Council for the field sampling of compost materials. The "chain of custody" form was also obtained from the TMECC. The specific sampling technique was acquired from a compost sampling project conducted by the composting industry early in 2002. The sample-site background information form was taken from a form developed by the Washington State Department of Agriculture.

Clopyralid is a selective herbicide used to control weeds (clover, dandelions, thistles) in agronomic crops (asparagus, hay, small grains, sugarbeets), lawns, pasture and rangeland, and rights-of way. Commercial lawn maintenance companies, golf course operators, and growers value clopyralid for its ability to control some weed species with only one to two applications per season.

Clopyralid is relatively stable in the environment and does not degrade readily. Green waste collected from treated sites for recycling may contain clopyralid residues. These residues can survive the composting process in trace amounts that are potentially phytotoxic to sensitive crops grown in the compost. This phenomenon was first noted in Washington State in 2000 and 2001. Compost made at a municipal site damaged tomatoes and other garden plants planted in it. The lawn clippings feedstock was tested and found to contain clopyralid residues up to 1.5 parts per million (ppm).

Municipal and private compost producers in California have tested their compost for trace amounts of clopyralid. In 2002, at a City of San Diego site, residues were detected at 11 parts per billion (ppb) and at one site in Los Angeles at 3-4 ppb. The preliminary results from a voluntary survey conducted by the compost industry indicate 13 out of 20 samples tested positive for clopyralid with residue levels ranging from 2-3 ppb. Research by Dow AgroSciences indicates that clopyralid levels as low as 3 ppb can be phytotoxic to garden vegetables like beans, peas, and tomatoes grown in pure compost. At this time, DPR is not aware of any reports of phytotoxicity in California due to compost containing clopyralid residues.

California compost producers and municipalities with greenwaste recycling programs were concerned that the presence of clopyralid residues in compost would damage consumer confidence in compost products. In 2002, a few composters indicated they were starting to lose business as customers became aware of the problem and were afraid to purchase their product. 

The composting industry and CalRecycle view pesticides in compost as a threat to the successful recycling programs for municipal green waste that were initiated in response to Assembly Bill 939. Several actions have been taken to keep clopyralid residues out of compost. In March 2002, DPR initiated a cancellation action on all registered clopyralid products which permit use on residential lawns. Use on residential lawns is considered the least controllable use in relation to keeping clopyralid from entering composting feedstocks. In 2002, Dow AgroSciences initiated a product stewardship program to educate chemical suppliers and users about the persistent nature of clopyralid and the importance of following label restrictions regarding mulching or composting treated lawn clippings. The CIWMB funded statewide testing of compost for clopyralid residues during fall 2003, summer 2004, and fall 2004. The highest levels detected during the three testing events were 6.40 ppb, 5.01 ppb, and 2.75 ppb, respectively. In January 2006, DPR regulations (6576 and 6950) took effect that restrict sale and use of clopyralid for application on lawn and turf.

This sampling protocol can be used to support at least two different objectives:

  • To determine if detectable clopyralid residues continue to occur in finished compost and at what levels they are present.
  • To determine how clopyralid moves with feedstocks to become detectable residues in finished compost during or following high-use periods of clopyralid in turf and landscaping. Obtaining information relevant to this objective is dependent on proper timing of the sampling period and on sufficient financial resources to pay for the collection and analysis of additional samples.

Sampling projects should be managed by a project committee composed of members from the composting industry (municipal and/or private). A field coordinator may be needed to set the calendar period for sampling and to supervise the actual sampling process at the different sites. The feedstocks and/or compost should be sampled by personnel present at the composting sites following the sampling protocol. The samples should be sent to one laboratory contracted by the project committee to analyze all the samples generated by the project.

Study Plan
Project objectives should be pursued by generating information through a limited testing program. For the first objective, for example, this should involve analyzing samples of finished compost from various composting sites for detectable clopyralid residues. For the second objective, this should involve analyzing samples of compost feedstocks and finished compost from these feedstocks during or following high use periods of clopyralid in turf and landscaping. For this latter analysis, since the composting process can take three months, finished compost should be sampled within two months of this high-use period, which typically will be late spring through early summer. For example, in 2000, DPR’s Pesticide Use Report indicated the season of high clopyralid use for landscape maintenance occurred from March-July.

Regardless of the specific objective, each sample should include background information on the feedstocks handled at the site and a brief description of the composting process, the percent of each feedstock used, and the composting capacity of the site. A chain of custody form should be completed at each site where samples are taken. The chain of custody form is also available at U.S. Composting Council website. To provide anonymity for the individual composting sites, the samples should be identified by a site code that is kept confidential and known only to the project leader. Examples of these information sheets are attached as an appendix to this protocol:

With either sampling objective, composting sites that have a history of detectable clopyralid residues should be included in the sampling project. These samples can provide information on the continuity of clopyralid residues at these sites.

Sampling Methods
. Each sample shall consist of a minimum of 20 or more cup-size sub-samples, taken at a depth of one foot into the feedstock or compost pile at various locations. A slotted pipe, shovel, or garden trowel can be used to take the samples. These sub-samples will be combined and mixed thoroughly in a five-gallon plastic bucket. From this mixture a one-gallon zip-lock plastic bag will be filled to make the composite sample. This sample will be placed inside a second one-gallon zip-lock plastic bag and placed in an insulated container provided by the analytical laboratory and packed with ice packs or blue ice. The sample shall be maintained on ice and shipped to the laboratory within 24 hours.

B. The project will utilize one of the following sampling methods, depending on whether feedstocks or finished compost are being sampled:

  • Feedstock—Because the heterogeneity of most feedstocks is unknown, a stratified sampling method will be used to obtain representative samples for feedstock testing. One sample will be taken from each horizontal layer in the feedstock. A minimum of five of these stratified composite samples will be needed for each feedstock. The feedstock shall be sampled after the routine removal of recyclable and/or problem material has occurred. The samples should be taken before shredding or size reduction, and before supplemental nutrients, bulking agents, or water have been added.
  • Finished Compost—Finished compost is considered to be more homogenous than a feedstock, so one composite sample should be adequate to generate an estimate of the average or median amount of clopyralid residues present. The finished compost sample shall be taken from the actual product or products that are released to the end-user at each site. Compost for sampling is defined as organic material which has been composting for at least 15 days at a temperature of 131 degrees Fahrenheit or higher during which time the pile has been turned at least five times.

C. All samples shall be taken from areas of the feedstock or compost pile that are representative in general appearance. Avoid collecting atypical moist samples, because an overly wet sample will not mix well to make the composite sample.

D. All sampling equipment shall be thoroughly cleaned between sampling each feedstock or compost pile, including an alcohol rinse. Disposable gloves should be worn and disposed of after taking and processing each sample. All samples shall be labeled as directed by the project leader and a logbook shall be maintained on the sampling history at each site.

E. Field spiking of various feedstock and compost materials will not be conducted at the composting sites. Laboratory studies indicate clopyralid is stable under environmental conditions. Clopyralid losses during sampling and shipping are not expected to be significant if samples are kept frozen. The analytical laboratory shall conduct a storage stability study if the field samples are kept longer than a month in storage.

Sample Analysis
The samples shall be analyzed by a contract laboratory capable of detecting clopyralid in various materials down to 1 ppb. This level of sensitivity is necessary because sensitive plant species have shown a phytotoxic response to clopyralid residues in compost as low as 3 ppb. The laboratory shall use gas chromatography with electron capture detection or mass spectroscopy to analyze the samples. The laboratory shall use an established referenced analytical method or an in-house method that is described in detail.

The project leader should review the contract laboratory’s proposed analytical protocol for doing the sample analysis and its standard operating procedures (SOPs) used to establish the limit of detection (LOD) and/or the limit of quantitation (LOQ). The project leader shall reserve the right to consult with knowledgeable entities and recommend protocol changes if problems arise. 

A copy of the following laboratory quality control and quality assurance procedures and analytical results shall be provided to the project leader for inclusion in a final report. Information should be provided about how and how often the laboratory’s instruments are standardized. The analytical method should be validated with the recovery of sample matrices spiked with clopyralid in the range of 5-100 ppb. If the feedstocks vary greatly in composition, two or three overspikes should be analyzed in place of the validation. 

During the analysis of the field samples, aliquots from the samples need to be spiked in the range of 10-50 ppb in the laboratory and run with field samples to verify the recoveries. The quality assurance procedures should include analyzing reagent and matrix blanks to demonstrate the absence of interference or contamination. Duplicate runs of 10 percent of the samples should be conducted in order to demonstrate the precision of the analysis.

Data Analysis
The statistical methods used for any analysis of the sampling results should be listed.

Submission of Preliminary Sampling Results and Study Report
The preliminary results from this type of study can be submitted by the project leader to DPR as "additional data" to provide DPR’s Registration Branch with information on detection trends of clopyralid residues in compost feedstocks and finished compost. This information can be used by DPR to evaluate regulatory actions regarding clopyralid and product stewardship programs by the registrant. The project leader can submit this information to David Haskell for review and routing to the proper evaluation stations. A formal report should be submitted at a later date that includes sampling details, raw data, analytical results, and the quality control and quality assurance from the laboratory analysis.

If references are cited, they should be listed on a separate page using the CalRecycle Contractor Publication Guide as an example for citing sources.

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Last updated: May 10, 2017
Organic Materials Management