Environmental Movement of Pesticides
My students actually work through two different agriscience pathways in my Environmental Landscape Technology class. They not only get a landscaping background, they also get an environmental science background. This makes for excellent opportunities to look at things in an interdisciplinary nature. This next section does just that; students will explore how to protect the environment from the chemicals used in the landscape industry. The goal is to have the students always asking themselves two key questions before applying a pesticide to treat the landscape. Where is the pesticide going to go in the environment after it leaves its container or application equipment? What effects can this pesticide have on non-target sites it may reach in the environment?
Proper application procedures are essential to reducing the environmental impact of pesticide use. Pesticides can move away from the application site by wind or air currents by a process called drift. Drift is a major problem and something that must be controlled when applying pesticides. Studies have shown that a significant percentage of pesticides may never reach the intended target due to drift. This can be a significant factor in contaminating the environment by contaminating sensitive plants, poisoning bees, posing health risks to humans and animals, and contaminating soil and water adjacent to the area being treated. Students need to know that they are legally responsible for the damages resulting from an off-target movement of pesticides. This can be avoided by paying close attention to spray droplet size and the wind direction and speed. I am fortunate to have quite a bit of equipment. We will look at a spray table that I have where we can swap out nozzles and look at the effects of different pressures and orifices on droplet size. You may want to delve into this if you can do something hands on. The premise is just common sense, the larger the droplet, the less likely to drift. To get a larger droplet, you typically use a larger orifice in the nozzle and lower the pressure. Other ways to do this are by viscosity and you can buy different adjuvants, additives intended to reduce drift; you just have to check their compatibility with the chemical you are using. Weather is a big determining factor when trying to minimize your impact when applying chemicals. The wind is affected by the temperature difference between the ground and the air above it. The best time to apply chemicals is when there is cooler air near the soil surface. The warm air on the ground is usually when the sun is higher in the sky and shining on the soil. So the conditions in which you have cooler air and less wind will be in the early morning and in the evening and these are the best times to apply these chemicals to the site.[19] This is why I love this type of work, it is so interdisciplinary, and you must be a soil scientist, chemist, entomologist, and meteorologist all at once in order to be effective at pesticide application!
Once the pesticide is applied to the landscape, the potential for pesticide runoff is greater for some pesticides than others, and my students need to be aware of those that are most susceptible to runoff. Solubility and persistence are two important factors that contribute to the runoff potential. This information is on the MSDS available from the pesticide manufacturer. All certified applicators are required to have the MSDS for each of the chemicals they have on hand. Danger to aquatic organisms is a factor when choosing a pesticide, as well as the danger to animals and the applicator themselves. The effectiveness is obviously another consideration when choosing a chemical to use for an application; many pesticides are not particularly effective under certain weather conditions, so that must be understood as well before applying a chemical.[19] Applying pesticides at a time or in an amount that is not effective is a wasted application and, therefore, an environmental hazard! Students also need to understand that it is illegal to apply chemicals at a different rate than listed on the label, especially a more concentrated application. Proper application is important for optimal pest control and safety for my students and the environment.
In order to have the students fully understand how these chemicals may move in the environment, they need to understand the physical and chemical characteristics of pesticides and how that drives the movement they undergo in the environment. The characteristics that they need to explore are solubility, adsorption, and persistence. Solubility is the measure of the ability of a pesticide to dissolve in a solvent, usually water. Pesticides that are highly soluble in water are more likely to move with water into surface and groundwater supplies compared to those that are less soluble in water. This characteristic was looked at when the students worked through the lesson on pesticide labels and MSDS and derived the water solubility number that is now in their Excel sheet. This is due to the charge of the pesticide molecules and the constituents present in the underlying soils. Adsorption is the process by which a pesticide binds to soil particles. Solubility and adsorption are inversely related characteristics. Students have done quite a bit with soil science by this point in my class so they have explored physical properties of soils, chemical properties of soils and even cation exchange capacity. They should come with the knowledge that most soils have a net negative charge and so the pesticides that have molecules with positive charges will bind readily with the soil and are, therefore, less likely to move from the site of application. Another thing to discuss is that typically oil-soluble pesticides are more attracted to clay particles and organic matter than are the water-soluble pesticides. A partition coefficient (K O C) is the most useful way to quantify pesticide sorption. The K O C value is the ratio of pesticide concentration in the adsorbed state (bound to soil) and the solution phase (dissolved in water). This means that the greater the concentration of pesticide in solution, the smaller the K O C value. You can find K O C values in tables that are determined by the chemical properties of the pesticide such as its solubility and melting point.[22]
Pesticide persistence is described as its half-life in the environment, the time that is required for half of the original quantity to break down. They are divided into three categories based on half-lives: non-persistent pesticides with a soil half-life of less than 30 days, moderately persistent pesticides with a soil half-life of 30-100 days, or persistent pesticides which have a soil half-life of more than 100 days. Organic chemicals will ultimately degrade into water, carbon dioxide, and minerals, but the intermediate degradation products of pesticides are of concern. Pesticides degrade by soil microbial activity of fungi and bacteria, chemical activity usually through hydrolysis with water, and/or photodegradation as the chemical reacts with sunlight.
To estimate a pesticide's potential to contaminate the environment, you need to look at the half-life and the partition coefficient together. Pesticides with a small K O C (less than 100) and a long half-life (more than 100 days) pose a considerable threat to groundwater resources through leaching. Pesticides with intermediate to large (500-1000) K O C values and short half-lives are the safest in terms of groundwater protection. Non-volatile pesticides with large K O C values (1000 or more) and long half-lives are likely to remain on or near the soil surface, increasing their chance of being carried to surface water bodies through sediment runoff. If a pesticide has a short half-life, the possibility of it polluting groundwater depends primarily on site characteristics, such as, permeability and the depth to the water table, and weather issues such as rain and irrigation timing after application. Without water to move them, pesticides with short half-lives remain in the root zone and they may be degraded rapidly. Ideally the best pesticide are ones with intermediate to large K O C values and short half-lives since they are retained in the soil and then degrade rather rapidly. This pesticide movement rating can be calculated using the Groundwater Ubiquity Score (GUS). This can be calculated to rank pesticides for their potential to move toward groundwater. The formula is GUS = log 1 0 (half-life in days) X [4 - log 1 0 (K O C)]. Extremely low potential to move toward groundwater values are less than 0.1, values 1.0-2.0 are low, 2.0-3.0 are moderate, 3.0-4.0 are high, and values greater than 4.0 have a very high potential to move toward groundwater.[20]
By correctly identifying the pests, knowing the site conditions, and using the persistence, solubility and sorption rates of the different choices of pesticides to be used, you can make very informed decisions on pesticide use. This is most important when you are working on a site that has soils with low organic material and with very permeable subsoils or high water tables. Here in the Delaware Coastal Plain, we have these conditions. Many of our soils are sandy in nature and there are many areas where the water table is not far from the surface. Most of our drinking water here in New Castle County is derived from groundwater, so it is critical to protect the aquifers we utilize for this resource. This is why I devised this unit to be an in-depth look at the deleterious effects of pesticide applications, and why I feel it is critical to empower students with the knowledge they need to make informed decisions on pesticide selection and application.
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