Protecting Groundwater in North Carolina—A Pesticide and Soil Ranking System

Table of Contents

Prepared by
R. A. McLaughlin, Extension Specialist, Pesticides/Water Quality; J. B. Weber, Professor of Soil Science; R. L. Warren, Soil Science Graduate Student

Publication AG-439-31
December 1994 (JMG)

Last Web Update:
December 1997 (DBL)

Groundwater is an accumulation of water that seeps through the soil until downward movement is blocked or slowed by a barrier, such as clay or solid rock. Groundwater is the source of drinking water for more than half the people of North Carolina and nearly all its rural residents. So when many of us turn on the faucet in our homes, we are actually drawing water from this underground "pool " of accumulated water. Preventing pesticides from seeping into our groundwater is critical if we are to keep the water safe to drink.

Recent studies show that pesticides can be detected in groundwater in North Carolina, although the levels found are usually very low and below current health standards. Pesticides in groundwater may result from problems that occur when pesticides are mixed or loaded, such as spills or backsiphoning. These are "point sources," or small areas of high concentrations of pesticides that can contaminate large areas of groundwater over time. Point sources can be located and cleaned up. Good construction and maintenance of the pesticide mixing and loading area can prevent most of these problems. However, pesticides may also be making their way into groundwater from fields where they were applied. By a process called leaching, some of the applied pesticide moves through the soil with water as it percolates down to groundwater.

Soil normally filters water as the water moves downward. This filtration leaves the water relatively free of contaminants by the time it reaches groundwater. Soils and pesticides both have properties that influence pesticide movement through the soil to the groundwater. These properties can be combined to rank the ability of each soil type to filter out pesticides, as well as to rank the tendency of each pesticide to leach through the soil.

In this fact sheet, we describe methods of determining soil leaching potential and pesticide leaching potential. We then use both of these values to determine the contamination potential of pesticide-soil combinations. Because the concern for leaching is greater in the coastal plain than in the piedmont or in the mountains of North Carolina, we focus on the soils of the coastal plain.

Soil Properties and Leaching Potential

The following soil properties affect pesticide leaching:

Organic Matter (OM)

When plant and animal material decomposes in or on the soil, a small part of the material remains in the soil as very slowly degradable organic matter. This soil organic matter binds most pesticides very effectively, so the more organic matter in the soil, the less likely a pesticide will leach through the soil.

Texture

The percentage of sand, silt, and clay in a soil determines its texture. Soil texture influences how fast water can move through soil. The more sand there is in the soil the easier it is for water and any contaminants to move to groundwater.

Acidity (pH)

Soil acidity, or pH, affects the chemical properties of many pesticides. As soil pH decreases, pesticides bind more to the clay in the soil and are filtered out of the percolating water. Also, pesticides are usually less soluble in water at lower pH values. Acidity is more important with some types of pesticides than others and is less important overall than organic matter and texture.

Other geologic and environmental factors also affect pesticide leaching to groundwater. Depth from the soil surface to groundwater is very important. The closer the water is to the surface, the less chance there is for a pesticide to be filtered and degraded in the soil. Weather plays an important role in many ways. Pesticides break down faster in warm, moist soil than in cooler or drier soil. The timing and amount of rainfall or irrigation influence how much water percolates through the soil. If heavy rainfall or irrigation occurs soon after a pesticide application, the percolating water can carry the pesticide deep into the soil where it breaks down more slowly. Also, the type of tillage practiced can affect soil temperature, moisture, and water infiltration, all of which have an impact on pesticide degradation and leaching.

Soil Leaching Potential

The three soil properties that affect pesticide leaching—organic matter, texture, and pH—can be combined in an equation to rank soils according to their susceptibility to leaching. The following is an explanation of how we made these calculations, which are presented in Table 1.

The first step in determining soil leaching potential (SLP) is to use the value for each property to place it into a rating category. For example, a soil with an organic matter content of more than 2 percent would have a rating of 10, the highest. The rating is then multiplied by an importance factor relative to leaching. The importance factors are 10, 6, and 3 for organic matter, texture, and pH, respectively. These factors are used to emphasize the relative importance of each property. Once the ratings are multiplied by the importance factor for each property, the numbers are added to obtain the SLP, as in the following equation:

Soil Leaching Potential (SLP) = Organic Matter + Texture + pH

The SLP values for many coastal plain soils of North Carolina and the southeastern United States are given in Table 1.

Pesticide Properties and Leaching Potential

Pesticides have several properties that affect their ability to leach to groundwater, as follows:

K_oc

This property refers to how tightly and quickly the pesticide binds to organic particles in the soil. A higher number indicates a greater tendency for the pesticide to bind to organic matter and a lesser tendency to leach with the soil water.

Persistence (T_1/2)

Pesticides are degraded primarily by sunlight, soil microbes, and chemicals in the soil. The combination of these factors determines persistence, or how long the pesticide remains in the soil. Persistence is usually measured in terms of half-life (T_1/2), or the time it takes for half of the applied chemical to break down. The greater the persistence of a pesticide, the more likely it is to leach to groundwater.

Rate of Application (R)

Different amounts of each pesticide are required to control target weeds, insects, or diseases. Generally, the chance of leaching increases when pesticides are applied at a higher rate.

Application Method (F)

Pesticides may be incorporated into the soil by mixing, applied to the soil surface, or applied to growing plants. To leach through the soil, a chemical first has to reach the soil. Pesticides applied to plants can be absorbed by the plant or broken down by sunlight, reducing the potential for leaching. Pesticides applied to the soil surface can also be broken down by sunlight before reaching the soil surface. Of the three methods of application, soil incorporation provides the greatest opportunity for leaching because all of the chemical is placed in the soil.

Pesticide Leaching Potential

The pesticide properties previously described are combined in the following equation to estimate their impact on leaching potential:

Pesticide Leaching Potential (PLP) = (T_1/2 x R x F)/K_oc

where

T_1/2 = Persistence of the pesticide, measured as half-life in days

R = Rate of application (pounds of active ingredient per acre)

F = Fraction of pesticide reaching the soil during application (1 for soil applications, less for postemergent applications, depending on row width and canopy size.)

K_oc = Affinity for soil organic matter

The PLP index has been calculated for most pesticides registered for use in North Carolina. The results are presented in Table 2. The rate of application used to calculate PLP is the median of the range recommended by extension specialists in North Carolina.

Groundwater Contamination Potential

The potential of a pesticide to leach into groundwater results from a combination of the soil and pesticide properties described previously in this fact sheet. The groundwater contamination potential (GWCP) index was developed to rank the relative risk of applying a specific pesticide to a specific soil (Table 3). To use the GWCP, first determine the dominant soil series for a field and find its SLP number from Table 1. Then list the pesticide alternatives for handling the weed, insect, or disease problems for the crop planned for that field and find their PLP numbers from Table 2. Finally, go to Table 3 and match the SLP category for your soil to the PLP categories for your pest control alternatives. If all other factors are equal, choose the least "risky" pesticide to handle your problem.

For example, assume that your field contains Norfolk and Goldsboro soils, and that you plan to grow soybeans. According to Table 1, the SLP values for Norfolk and Goldsboro soils are 74 and 70, respectively. These soils fall into the "High" SLP category of Table 3. You decide that you can use alachlor (Lasso), pendimethalin (Prowl), or trifluralin (Treflan) as a preplant, incorporated weed-control strategy. In Table 2, the PLP index is 52 for alachlor, 29 for pendimethalin, and 28 for trifluralin. The PLP ratings in Table 3. Table 3 for these three pesticides are "Moderate," "Low," and"Low," respectively. Lastly, to rate alachlor on this soil, use Table 3. Look down the "High" column for SLP and across the "Moderate" row for PLP to obtain "High Risk" as the GWCP rating. For pendimethalin or trifluralin, look down the same "High" SLP column and across the "Low" PLP row to get "Moderate Risk." So using either pendimethalin or trifluralin instead of alachlor reduces the risk of groundwater contamination for these two soil types.

This procedure is one part of an overall approach to preventing pesticides from contaminating groundwater. Other Best Management Practices (BMPs), such as proper handling, storage, and disposal of pesticides and their containers, and integrated pest management, are also part of the preventive approach. A complete BMP program that includes all these elements is critical to protect the groundwater on which we all depend.