Protecting The Watershed Is Everyone’s Job
August 1, 2013 When I teach about ways to protect our water quality, I use a watershed model. The big plastic tabletop-sized relief map includes a lake, streams, a farm, a factory, a subdivision, a golf course, roads, bridges, sewage treatment plant, etc. — a typical assortment of anthropogenic additions one will find in most developed areas on earth. This model represents a watershed; all the people living and working there share the same drinking water. I use the model to clearly illustrate that everything that everyone does above ground can influence the quality of the water underground.
During the lesson, I call for volunteers. I ask for someone to role-play a farmer, a wealthy polluting industrialist, a greedy land developer, and a road/bridge builder. Then I give each of them a pile of scale-sized accoutrements to live out their newly acquired occupation within the model. The farmer places his barn, tractor, livestock and fencing; tilled ground meets the water’s edge. The industrialist builds his factory; polluted sludge drains into the adjacent stream. The developer puts houses on streetside lots; storm drains run debris directly to the lake. The road builder bulldozes and flattens; soil erodes and oil drips. Once the model pieces are in place, I give little shakers of powders to the volunteers to represent the fertilizers, pesticides and oils that are then liberally applied to the farms, lawns, roadways and golf courses. The thin layer of powder is harmless enough until along comes the big spray bottle to represent a crashing thunderstorm of Florida proportions. The water drops that form on the plastic land turn red and green, quickly making way downhill to the spring fed lake. I pull the plug from the lake bottom and the dyed water, now a muddled mess, drains into a bowl below: the Florida aquifer. My offer of a refreshing cup of the brown sludgy water is unanimously rejected. Imagine that. It’s only kool-aid and cocoa, but in our minds it has become glyphosate, imidacloprid, urea formaldyhede and petroleum residues — pollutants that have destroyed our precious, life-giving water supply.
Water covers almost two-thirds of the earth’s surface, and much of its subsurface, too. Water is a common element that links ecosystems, transporting food, nutrients and organisms. Fresh water — critical for human consumption — is very limited with less than 1% of the planet’s water in lakes, streams, rivers, and groundwater. Fortunately, we do not live on plastic land that allows all runoff to go directly to our drinking water source. But, in spite of the earth’s best efforts at self-defense, many harmful particles and residues do end up as runoff into our local water bodies, and can have a profound effect on the quality of our water.
U.S. Geological Survey (USGS) research[1] has shown that some fungicides are moved from areas of intense use to nearby streams and groundwater at concentrations of concern for environmental health. For instance, the Toxic Substances Hydrology Program study documented the presence of bascalid in groundwater, zoxaminde in sediments and pyraclostrobin in suspended sediments.
Some studies suggest that fungicides may be more toxic to freshwater biological communities, including beneficial fungi, than previously expected. The results of this study indicate the importance of including fungicides in pesticide-monitoring programs, particularly in areas where crops are grown that require frequent treatments to prevent fungal diseases. There are more than 3,600 pesticide products containing fungicides registered with the U.S. Environmental Protection Agency, yet very few studies have investigated fungicide occurrence in the environment. When USGS scientists recently researched this, they found at least one fungicide in 75% of the surface waters sampled and in 58% of the groundwater wells sampled. They also found at least two fungicides in 55% of the bed sediment samples and in 83 percent of the suspended sediment samples that were collected.
| Summary of Fungicides Detected | |
| 12 fungicides were detected in streams, including | |
| Boscalid | 72% |
| Azoxystrobin | 51% |
| Pyraclostrobin | 40% |
| Chlorothalonil | 38% |
| Pyrimenthanil | 28% |
| Six fungicides were detected in sediments, including: | |
| Pyraclostrobin | 75% |
| Boscalid | 53% |
| Chlorothalonil | 41% |
| Zoxmide | 22% |
Source: USGS Toxic Substances Hydrology Program
The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) defines a pesticide as “any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest.” This can include plant growth regulators, defoliators and desiccants, and categories of insecticides, fungicides, herbicides, rodenticides and more. The physical properties of pesticides are the factors that influence the fate of pesticides in our environment.
[1] “Crosscutting Topics: Surface-Water Contaminant Transport,” USGS Toxic Substances Hydrology Program, last modified April 12, 2013, http://toxics.usgs.gov/topics/sw_contamtransport.html.
A Pesticide’s Staying Power
Pesticide persistence, or half-life, is the length of time required for one-half of the original quantity to break down. Three categories are used to describe persistence:
| Pesticide Persistance Category | Typical soil half-lives |
| Nonpersistent | Less than 30 days |
| Moderately persistent | 30 – 100 days |
| Persistent | More than 100 days |
The processes that affect persistence include degradation by sun, chemical and microbial activity. Breakdown of pesticides by sunlight is referred to as photodegradation. Chemical degradation occurs when a pesticide reacts with oxygen, water or other chemicals in the environment. Microorganisms breakdown pesticides, called microbial degradation.
On The Move
Pesticides can move within the application site or off-site. This mobility is affected by a number of factors: the pesticide’s sorption, water solubility and vapor pressure as well as the site characteristics such as weather, topography, canopy, ground cover, rainfall, flooding, irrigation and tillage practices. It also is influenced by soil texture, structure, and organic matter content.
Water solubility describes the amount of mg/L of a pesticide that will dissolve in water, affected by temperature and other chemicals present. Highly soluble pesticides are more likely to move off-site by way of leaching or runoff.
Volatilization is the ability of the pesticide to evaporate from the plant or soil surfaces, determined by the pesticide’s vapor pressure. In general, the higher the vapor pressure, the more likely it is to volatilize. Site factors that affect volatilization include temperature, humidity and wind.
Some pesticides are highly adsorbed to soil particles; some are not. Properly designed buffers are effective in trapping eroded sediment and runoff losses may be reduced by the soil in conservation buffers. However, several common pesticides are only moderately adsorbed to soil particles and are carried with runoff from fields in the dissolved phase. An effective buffer will slow rainfall’s runoff and increase infiltration. This will help a pesticide to get trapped and eventually degraded in the buffer’s soil and vegetation. A sheet flow — a shallow and wide flow of water — is substantially more effective than a concentrated flow at trapping the pesticides. Think of the current in a river or stream — the wide areas slow down, and the narrow areas flow more quickly. Maintenance of the buffers is critical to maintain the desired shallow sheet flow.
Conservation buffers are not a substitute for careful pesticide selection and use. One important consideration is to determine if applying a pesticide is even necessary. Sometimes a mechanical control is adequate. Changing our perception of what we can tolerate in a landscape could be especially beneficial. Is a monoculture of turf really needed, or can we adjust to some areas that comprise a neatly mown mix of greens? Are the insect pests only doing damage or are they participating in the greater ecosystem? Is that fungus going to destroy the entire landscape, or will conditions change quickly that render it innocuous?
Know The Potential For Pollution
K values, half-life and water-solubility values for specific pesticides, which are useful to estimate persistence, can be found on the USDA database at http://www.ars.usda.gov/Services/docs.htm?docid=6433.
USDA’s “Conservation Buffers to Reduce Pesticide Losses” publication provides a wealth of useful information for pesticide applicators who are concerned about protecting the watershed.
Adsorption can refer to the ability of a pesticide to be held on to the surface of soil particles. Absorption refers to the the ability of a pesticide to be taken inside a material such as a plant. The term sorption is used to refer to both processes. The interaction of pesticides with soil particles is primarily adsorption.
Pesticides vary greatly in how tightly they are adsorbed to soil particles. The degree of soil binding is measured by coefficients, or K values. The K value of organic carbon is the measure of adsorption to the organic matter or carbon content of soil, with higher values indicating more binding. Pesticides will also bind with clay particles. The degree of binding to organic matter is a useful predictor of pesticide behavior and movement in the soil. For example, the K value for dicamba is two, meaning it is held very loosely to the soil versus paraquat, which has a coefficient of one million.
K values greater than 1,000 indicate that pesticides are highly adsorbed to soil, and the pesticides tend to be carried off of fields on eroded soil particles. If conservation buffers are effective in trapping the sediment particles, they will consequently trap this type of pesticide. Pesticides with lower K values tend to move on water more than sediments. The concentration of pesticides held on sediments is higher than the concentration carried in water, but because water quantities running off of fields are so much greater than eroded soil quantities, water accounts for the majority of chemicals leaving fields. To trap these low K pesticides effectively, buffers need to increase water infiltration and maximize runoff contact with soil and vegetation that can adsorb pesticides. The lower the K value, the more time the pesticide needs to have contact with soil and plants to keep it out of the watershed.
Rainfall or irrigation can cause pesticides to run off the surface of treated areas. Losses are greatest when severe rainstorms occur soon after pesticide application. It is the responsibility of the pest control operator to time the application of pesticides with the weather in mind, avoiding the risk of runoff whenever possible.
Setbacks are untreated areas where surface runoff enters streams, and are an important part of preventing water pollution with pesticides. Read the label on the pesticide for a description of the size of the area next to a water body that should be left untreated.
Agriculture Nutrient Management Methods
USDA research confirms that vegetated drainage ditches can help capture pesticide and nutrient loads in field runoff, giving farmers a low-cost alternative for managing agricultural pollutants and protecting natural resources. The primary function of many ditches was previously thought to be for channeling excess water away from crop fields. Research conducted by Agricultural Research Service ecologist Matt Moore proved the additional function of capture of pesticides. Moore studied a 160-foot section of drainage ditch for 28 days for the capture of the herbicide atrazine and the insecticide lambda-cyhalothrin. One hour after starting a simulated runoff event, 61% of the atrazine and 87% of the lambda-cyhalothrin had transferred from the water to the ditch vegetation. At the end of the ditch, runoff pesticide concentrations were decreased to levels generally thought to be non-toxic to downstream aquatic fauna.
Some pesticides are relatively short-lived in water and will degrade while sequestered in wetlands, thereby reducing contaminants reaching streams. Wetlands can also serve to attenuate pulses of concentrated runoff before it enters streams. Some pesticides are relatively persistent once they reach water. However, the high organic matter content of wetland sediment binds these pesticides, removing them from water.
| Highly adsorbed pesticides | |
| Pesticide | K value – organic carbon |
| Chlorpyrifos | 6070 |
| Diflufenican | 1990 |
| Lindane | 1100 |
| Trifluralin | 8000 |
| Moderately adsorbed pesticides | |
| Acetohlor | 150 |
| Alachlor | 170 |
| Atrazine | 100 |
| Cyanazine | 190 |
| 2,4-D* | 20 |
| Dicamba* | 2 |
| Mecoprop* | 20 |
*These herbicides with very low adsorbtion values are commonly recommended for use on Florida turfgrasses.
Use of integrated pest management is critically important in protecting our watershed from the potential pollutants we apply as pesticides. Do you really want to risk sending that chemical into our drinking water? Please consider long-term effects and accept the responsibility to carefully select and apply chemicals according to the label — and only if other alternatives are not acceptable.
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