University of California, Riverside Cooperative Extension
	PesticideWise A Quarterly Publication of the Department of Environmental Sciences

Spring 2002

How to Reduce Pesticide Leaching

By Dr. Jay Gan

Cooperative Extension Water Quality Specialist

In the last issue, we mentioned that the vulnerability of groundwater to pesticide contamination is largely a function of the properties of the pesticide itself. After having said that, it is important to note that soil, application, and water management factors can all greatly influence the actual outcome. Conducive conditions and careless uses may allow non-leachers to enter groundwater. On the other hand, improvements in management practices may even prevent leachers from reaching the groundwater.

Four factors govern the possibility of groundwater contamination by pesticides passing through the soil:

• Properties of the soil

• Properties of the pesticide

• Water conditions

• Application and handling

The effect of pesticide properties, through interactions with adsorption and persistence, was discussed in my last article "Pesticides and Groundwater Quality". This publication focuses on the other three processes, with the objective of evaluating mitigation practices that may be used to reduce leaching risk.

Soil Properties and Leaching

Soils whose properties allow rapid transmission of a pesticide to groundwater are called sensitive soils. However, just because a soil is sensitive does not necessarily mean there is a high risk of groundwater contamination. Good water management, low application rates, proper timing of applications, and careful handling of pesticides all compensate for sensitive soils and reduce the risk of groundwater contamination. The opposite of these conditions can increase the risk even on soils that are not particularly sensitive.

Soil sensitivity factors: Soil sensitivity depends on four soil properties:

• Permeability

• Water table conditions

• Organic matter content

• Clay content

Permeability and water table conditions together control the leaching potential. Soils with high leaching potentials are more sensitive than soils with low leaching potentials. Organic matter and clay content together control the adsorption potential. Adsorption of a given pesticide varies dramatically in different soils, and generally increases with soil organic matter and clay content. Soils with low adsorption potentials are more sensitive to groundwater contamination than soils with high adsorption potentials.

Interactions between leaching potential and adsorption potential govern the overall sensitivity of the soil. A soil that has both a high leaching potential and a low adsorption potential is the most sensitive. A soil that has both low leaching potential and a high adsorption potential is the least sensitive.

Permeability: Permeability refers to the rate at which water moves through soil. Permeability is controlled by the size and continuity of the soil pores. Factors that influence soil permeability include:

• Texture

• Organic matter

• Structure

• Root and animal activity

• Density

Coarse-textured sandy and gravelly soils have the largest pores and the most rapid permeabilities. Fine-textured clayey soils have very tiny pores and very slow permeability rates. Soil organic matter helps create and stabilize aggregates of the grains of sand, silt, and clay. These aggregates have relatively large space between them, permitting more rapid water movement. Roots and burrowing insects and animals create large voids, or "macropores", that they are connected to the surface. Heavy rainfall or irrigation events may create temporarily saturated surface soil, which may lead to rapid flow through macropores. If soluble pesticides also are present, they can be carried deep into the soil in a short time. Dense, compact, or cemented soil layers have very slow rates of permeability. Soil permeability rates are published in each county soil survey report.

Water table conditions: Water table conditions include the height and duration of water tables in the soil. Shallow water tables that persist for long periods increase the risk of groundwater contamination. Well-drained soils rarely have water tables that persist for long periods above a depth of 6 ft. They are much less sensitive than poorly drained soils, which may have water tables at or near the surface for several months.

Two types of water tables occur in soils: perched and apparent. A perched water table is the top of a zone of saturation that is separated from permanent groundwater by a soil layer of very low permeability. An apparent water table is the top of a zone of saturation in a soil in which there are no dense or confining layers. Perched water tables do not increase the risk of groundwater contamination as much as apparent water tables do. The soil layer that perches water acts as a barrier to prevent contaminants from moving to the permanent groundwater supply.

Soil survey reports contain information on water table conditions in soil. The depth to the water table, the months during which it persists, and whether it is perched or apparent all are given in tabular format. This information is very useful in assessing soil sensitivity. Indications of shallow groundwater include riparian vegetation; persistently green, unirrigated grass or herbaceous vegetation; springs; evidence of seasonal flooding; or low topographic position in relation to nearby surface water, springs, and riparian vegetation.

Soil adsorption potential: Adsorption of a pesticide may vary greatly as a function of soil type. Adsorption refers to the binding of chemicals to particles of organic matter and clay in the soil. Adsorption retains pesticides in the soil, where they can be degraded. Thus the higher the adsorption potential for a soil, the lower the risk of groundwater contamination. Adsorption potential depends on organic matter and clay content.

Organic matter content is the most important variable affecting adsorption of pesticides. Organic matter provides the greatest number of binding sites because it has an extremely large surface area and is very reactive chemically. Organic matter content in soil depends on climate, vegetation, soil texture, and farming practices. Desert soils have very low organic matter contents. Farming practices that return crop residues and animal wastes to soils help maintain soil organic matter content.

Clay content refers to the percentage of microscopic plate-shaped grains in the soil. These tiny, flat particles have a tremendous amount of surface area per unit weight of soil, and their surfaces are chemically reactive. The higher the clay content, the greater the number of binding sites for pesticide retention. Clay content is particularly important in the subsoil, where the organic matter content is generally much less than in the surface soil. Data on clay content are readily available in soil survey reports. For evaluation of sorption potential, it is sufficient to classify soils in generalized groups ranging from low adsorption for the coarse-textured sands and gravels to high adsorption for the fine-textured silty clays and clays.

Water Conditions and Leaching

The total amount of water applied to the soil, or hydraulic loading, is also important in determining the risk of groundwater contamination by pesticides. No matter how permeable the soil, the leaching risk remains low if there is insufficient water to move completely through the soil.

Leaching occurs where rainfall exceeds both plant consumptive use and the soil's ability to store water. Water moving below the root zone ultimately reaches groundwater, carrying with it soluble soil constituents. In these soils, the leaching potential is highly correlated with soil permeability.

Irrigation compensates for water deficits in dry areas. Irrigation is especially important in the production of traditional and specialty crops in California due to relative scarcity of precipitation. Most irrigation is taken up by plants, but some usually passes through the soil out of the root zone. Thus irrigation can increase groundwater vulnerability. Careful management of the amount and timing of irrigation water applications can be very effective in reducing the risk of groundwater contamination.

The position of a soil in the landscape often influences its hydraulic loading. Soils near a hilltop often shed water, either by runoff over the surface or by lateral flow within the soil. Soils lower on the hillside and where the slope begins to flatten out often receive excess water from the highest positions. These soils are more susceptible to leaching from the added hydraulic loading.

Mitigation Practices

Now, with an appreciation of the importance of pesticide properties, soil properties and water conditions for pesticide leaching, let's look at what can be done to reduce the risk. The mitigation practices can be summarized into three categories: site restrictions, pesticide selection, and water management.

Site restriction: The importance of soil properties is shown in DPR's approach of adopting the Pesticide Management Zone (PMZ) program throughout the state of California. A PMZ is a land area that is considered sensitive to pesticide leaching. Although the identification of PMZs relies on previous positive detections of pesticide residues in the groundwater, PMZs are generally soils that are susceptible to pesticide contamination due to high leaching potential, shallow groundwater tables, and/or low adsorption potential. In PMZs, the use of certain pesticides, mostly pesticides that are included in the Groundwater Protection List, is restricted. DPR works with the county's agriculture departments or commissioners to enforce PMZs. The information on the precise location of PMZs in each county is available from the county offices.

Pesticide selection: The importance of pesticide properties is shown in pesticide labeling and selective registrations. In labeling pesticides for uses under certain conditions, EPA achieves groundwater protection by preventing the use of leaching pesticides in sensitive areas. As general rules, users should follow the following guidelines:

• Select pesticides that are not known or suspected to be groundwater contaminants, especially when applications are planned for the rainy season.

• Select herbicides not on the Groundwater Protection List for soil applications in areas of shallow groundwater. This practice is especially important in areas of high rainfall or where the soil has low organic matter content.

• Use pesticides most selective for the target pest species to enhance natural population control mechanisms and reduce pesticide need.

Water management: Water movement is the driving force for pesticide leaching. In California, managing irrigation practices is especially important in preventing groundwater contamination by pesticides. The following should be observed to reduce pesticide leaching:

• Improve irrigation uniformity: Poor uniformity leads to over irrigation in certain areas, which can result in active water movement and enhanced leaching.

• Select proper irrigation methods: Furrow irrigation can lead to non-uniform water distribution or ponded water in many instances, which can increase leaching risk. Generally speaking, the leaching risk follows the order furrow > basin > sprinkler.

• Reduce irrigation rates: Apply water in amounts that would limit percolation over the root zone. Keep water budgeting in mind, and use ETo, or the baseline evapotranspiration value, as a reference when determining irrigation rates. Frequent irrigations with low rates are safer than infrequent irrigations with high rates.

• Improve irrigation timing and frequency: Schedule irrigation events in a manner to minimize deep percolation beyond the root zone.

• Watch out for collection ponds: In areas with shallow groundwater table, tail water collection ponds may become a recharge system for groundwater. When surface water and groundwater exchange occurs, pesticide residues may get purged into the groundwater. Construction of collection ponds may be improved to prevent this from happening.

In addition to irrigation, a significant portion of water loading is from rainfall. In California, rainfalls often occur in the winter when vegetation is thin, which can lead to a large amount of water to percolate deep into the soil. While it is impossible to control rain events, it is advisable to avoid timing pesticide application with a forecasted rain storm. Timing applications to let rain wash the pesticide into the soil can be a bad idea, especially in sensitive areas, and particularly because the amount of precipitation is not at all predictable each time.

Pesticide Handling

Detection of pesticides in wells in the 1970-80s was often a consequence of careless pesticide handling. In many cases, pesticides were directly introduced from the well opening into the well through drift, back siphoning, or spills of spray solutions or rinse water. This type of contamination cannot be easily differentiated from the symptoms of groundwater contamination through leaching after legal pesticide applications. The following precautions should be exercised:

• Reduce drift by applying pesticides only when wind speed is less likely to result in drift. Use low delivery pressure and nozzles that do not create ultra-small droplets than can easily drift off-target.

• Equip each service rig and piece of application equipment that handles pesticides and draws water from an outside source with an air-gap separation, a reduced pressure principle backflow prevention device, or a double check valve assembly. Backflow protection must be acceptable to both the water purveyor and the local department.

• Mix, load, and store pesticides at least 100 feet away from water sources, pumps, well heads and sink holes. Store pesticides in a secure and dry place.

• Properly rinse spray equipment and use closed mixing systems and a triple rinse of the empty pesticide container, and safely apply the rinsate to the target field or dispose of safely.

• Use returnable refillable liquid pesticide containers when available. Properly dispose of pesticide containers.

• Prepare an emergency spill and response plan for each chemical tank truck.

References

CDPR (California Department of Pesticide Regulation). California Pesticide Management Plan for Water Quality. DPR, Environmental Monitoring and Pest Management Branch, Sacramento, CA, 1997.

Huddleston, J.H. How soil properties affect groundwater vulnerability to pesticide contamination. Oregon State Extension Service, 1996.

Troiano, J., C. Garretson, C. Krauter, J. Brownell, and J. Hutson. Influence of amount and method of irrigation water application on leaching of atrazine. J. Environ. Qual. 22: 290-298 (1993).

Coming up:

Pesticides in Surface Water Quality