Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityDecember, 1997
The Fate of Pesticides in the Environment
Keywords: environment, regulations, water quality, herbicides
I recently read an article entitled ĪPesticide Fears May Only Be a Smoke Screenā (smoke screen for what they didnāt say). Everything said in the article was true; further I can appreciate the motivation for the article, which was perhaps to calm the publicās fears about pesticides. Nevertheless, I think the author dealt with the issue of pesticides in the environment too shallowly. In my opinion, claiming that pesticides, when properly used, pose no more risk than aspirin or flea collars isnāt a good argument; after all, nobodyās applying those to millions of acres of farmland (just imagine that, if you can)! Also the qualifying statement - when properly used - is the major weakness in the argument. My guess is that many folks worry the that pesticides may be used haphazardly and unnecessarily.
Letās adress the publicās concerns by reassuring them that we are responsible pesticide users and that we are also concerned about what happens to pesticides in the environment. Understanding the processes that remove pesticides from the environment will not only help us use them safely, but also more effectively. This article will focus on the fate of herbicides in the environment, but the mechanisms discussed are valid for all pesticides.
Drift. Drift is the movement of spray droplets or even granular pesticides to non-target sites. Drift is particularly a problem when the material is carried into a sensitive area (e.g., a neighborās backyard) and when the chemical is foliar-active (e.g., 2,4-D). Proper choice of nozzles to control droplet size, choice of spray formulation (low-volatility), and application under calm weather conditions will go a long way to control drift.
Volatilization. Some chemicals are volatile, which means they may be lost as a gas. The potential for volatilization increases with increasing temperature. With the exception of soil fumigants, we generally want to prevent volatilization of herbicides to prevent loss of activity. This is the reason why the herbicide Casoron is used in the winter and fall seasons. Treflan is another volatile herbicide; although used in the warm season, it should always be followed by irrigation or mechanical incorporation to prevent loss. With the exception of certain volatile fumigants like methyl bromide (which may deplete the ozone layer), I donāt know of any research on the risk that volatile pesticides may pose to the environment. Probably most rapidly dissipate in the air to non-toxic levels.
Uptake by plants and other living things. Pesticides that are taken up by plants may be metabolized and broken down to harmless substances within the plant, or once the plant dies, may enter the soil (this pathway discussed below). Great concern arises when pesticides are consumed by animals, remain intact, and work their way up the food chain. Pesticides that have this trait have been removed from the US market. Many of these chemicals were very useful, and actually had very low acute toxicities (means you could ingest relatively a lot and survive!). However, scientists lacked the means to determine effects of very long term exposures to low concentrations of these chemicals in the body.
What about direct human or pet contact with pesticide-treated foliage or soil? While there is potential for an undesirable level of exposure if a treated site is entered before the spray dries, research with lawn chemicals has shown that the amounts picked up on clothing, feet, hands, etc., once the spray has dried are well within safe limits.
Soil-pesticide interactions. The soil and the thatch in turf are key sites for processes that immobilize or break down pesticides. Many pesticides are strongly attracted (adsorbed) to the surfaces of clay or organic matter particles. Heavy clay soils or highly organic soils have strong adsorptive capacities; coarse textured soils low in organic matter have low adsorptive capacities. Adsorption occurs rapidly, such that strongly adsorbed chemicals essentially have no soil activity. Thus pesticides that are strongly adsorbed are not available for root uptake and have a low leaching potential. However, adsorbed chemicals still can move off-site by soil erosion. Postemergence herbicides that are used to clear vegetation prior to reseeding usually are strongly adsorbed to soils; Roundup and diquat are prime examples. Once chemicals are adsorbed, they may be broken down by soil organisms.
Soil microbes are the most important actors in the fate of pesticides since microbial degradation is the ultimate fate of most pesticides. This process occurs more slowly than adsorption; the actual rate depends on the nature of the chemical and how readily microbes can digest it. Rates are described in terms of half-lives which may be days, weeks, or months depending on the chemical. Warm temperatures and moisture increase the rate of microbial decomposition. The process occurs slowly in winter; this is why preemergence herbicides applied in late fall are still active in early spring.
Relatively fewer pesticides are broken down by purely chemical (non-living) reactions. One example of a chemical process is photodegradation, which is destruction by UV light. Devrinol is an example of a herbicide that is destroyed by UV radiation. This herbicide is most effective when used in early spring. Under the high light intensity conditions of summer, as much as 50% of the chemical may be lost from the soil surface after four days.
With the knowledge that most pesticides are either immobilized or broken down by microbes in the soil, it becomes clear why the best means of disposing of pesticides is through their proper use. Dumping chemicals in one place will overload the biological filtering capacity of the soil as well as increase the risk of leaching into groundwater. If too much chemical is prepared, the best action is to find an appropriate use for the remaining chemical.
Leaching. Leaching is loss by percolation through the soil, and is undesirable since chemicals that leach may enter the groundwater. Leaching is more of a problem in coarse, sandy soils. Many preemergence herbicides used in our industry have very low water-solubility, thus are not subject to leaching. A key example of low-solubility chemicals are the dinitroanaline class of preemergence herbicides (e.g., Surflan, Factor (Barricade), Treflan, and Pendimethalin). In contrast, Princep and Pennant are two very important herbicides in our industry, but have a greater capacity to leach. These two chemicals are currently under review by the EPA because of their potential to leach into groundwater. Responsible use of these chemicals involves staying within the maximum annual application rate, and choosing this rate based on your soil texture. In areas where the soils are high in clay content, leaching of these chemicals is not much of a problem.
Unfortunately, when chemicals from any source reach the groundwater, there is less potential for degradation because this environment is cold, dark, airless, and nearly lifeless. Thus it is important to prevent chemicals from getting there. Key methods of protecting the groundwater include properly constructed chemical mixing and storage areas; backflow prevention devices; and wellhead protection.
Runoff. Runoff is a loose term for water that leaves your property. Some common ways pesticides can enter runoff include: erosion of soil; siting production areas too close to natural bodies of water; and directly applying pesticides to the banks of drainage ditches or too close to bodies of water. There are many preventative options, starting with controlling, eliminating, and/or recycling runoff. While complete elimination of runoff may be a serious challenge to nurseries; the use of grass-lined drainage channels and vegetated filter strips between production areas and natural bodies of water are options that should not be difficult to adopt. These Ībiological filtersā slow down the flow of water and sediment from the nursery, allowing more time for removal of substances from the runoff water by natural processes.
For the latest information on pesticide applicator training and current regulations, check out the OSU Extension Pesticide Education Website: http://www.ag.ohio-state.edu/~pested
