Groundwater & Pesticides -- Section 4.3 The Pesticide

Overlain on the complex of natural environment factors are another set of factors that affect pesticide movement. These relate to the chemical itself, and it is in this arena that the experts were for a time misled.

After the early "miracle" pesticides like DDT were found to be so injurious to the environment, pesticide makers worked to create a newer generation of chemicals that would not persist so stubbornly in the environment. The laboratory chemistry of some newer, short-lived compounds, however, proved unreliable in the field. Scientists today appreciate that a number of factors related to the pesticide itself affect its movement, and they understand that the interplay of those factors with the natural environment can be difficult to predict.

In looking at the chemical and physical attributes of pesticides that may indicate a propensity for a given substance to leach into groundwater, a few factors stand out. The propensity for a pesticide to dissolve in water is, of course, extremely important. Its solubility is also directly and inversely related to its capacity for soil adsorption.

Researchers at the Florida Cooperative Extension Service ¹ explain that as a pesticide enters the soil, some of the chemical will adhere to soil particles in a process called "adsorption." The rest will remain dissolved in the water held between the soil particles. When more water passes through the soil, some of the adsorbed pesticide molecules may detach or "desorb" and be carried along with the moving water. Generally speaking, however, the more water-soluble the pesticide, the lesser its tendency to adsorb to the soil, and thus the greater its tendency to migrate through the soil and in groundwater.

The stability and persistence of pesticide residues in Long Island groundwater is clearly demonstrated by the fact that six of the 10 chemicals found to exceed drinking water MCLs [Maximum Contaminant Levels] have been banned from sale or use for 10 to 20 years.

New York State Department of Environmental Conservation ²

Measures of soil adsorption propensity distinguish between different mechanisms of adsorption -- adherence to clay versus organic matter. Combined with measures of solubility, these factors can indicate which pesticides are more likely to present problems as "leachers" or pesticides that move readily into and through the groundwater environment.

Other important pesticide factors include "volatility" or the propensity for a pesticide to disperse into the air. In general, one would expect volatile compounds to pose less of a problem, since they would be expected to volatilize quickly from the soil surface.

Groundwater contamination by some highly volatile chemicals like EDB and DBCP, however, ran counter to this expectation and helped to teach researchers two important lessons: First, that the method of application, such as direct injection into the soil, can be critical to leaching, and second that the actual volatility can change when the pesticide reaches water. Notes EPA, "[h]igh water solubility can cause high vapor pressure chemicals to remain in the soil, particularly when these pesticides are applied just prior to irrigation or rainfall." ³

Another critical factor involves a pesticide's persistence or as the Florida Cooperative Extension Service calls it "lasting power."¹ As noted earlier, pesticides can be degraded or broken down in the subsurface environment by chemical and/or microbial action. Transformations may lead to benign constituents or to metabolites of varying toxicity. Microbial degradation will depend upon the presence of bacteria, fungi and other microorganisms that use the pesticides as a food source. Chemical degradation will depend upon temperature, moisture, pH and other factors.

Pesticides that resist these sorts of decomposition processes and do not readily evaporate are said to have a long "half-life" -- the time period in which half of a given amount of pesticide will degrade.

Pesticide Persistence in Soils ⁴

Low Persistence (half-life <30 days)	Moderate Persistence (half-life 30-100 days)	High Persistence (half-life >100 days)
Aldicarb	Aldrin	Bromacil
Captan	Atrazine	Chlordane
Dalapon	Carbaryl	Lindane
Dicamba	Carbofuran	Paraquat
Malathion	Diazinon	Picloram
Methyl Parathion	Endrin	Trifluralin
Oxamyl	Fonofos
2,4-D	Glyphosate
2,4,5-T	Heptachlor
	Linuron
	Parathion
	Phorate
	Simazine
	Terbacil
	TCA

As was the case with volatility, information on persistence led to assumptions that proved erroneous in some cases. Pesticides that were considered to have short half-lives based on laboratory data were not anticipated to be a groundwater problem. As it turned out, the degradation rates varied in the natural environment, with much slower degradation occurring once a pesticide reached the water table and varying degradation rates found depending upon the site-specific conditions.

Although highly persistent pesticides that are also water soluble might generally be expected to threaten groundwater, as can be seen from the above table, reliance on that generalization alone is not fully justified. Aldicarb, with its short half-life, for example, was not expected to pose any problem to groundwater.

"Based on the environmental fate studies reported at the time of registration," explains Patrick Holden, author of the National Academy of Sciences' Pesticides and Groundwater Quality, "aldicarb was expected to break down to the non-toxic by-products long before it could leach to the groundwater in a toxic form." Nonetheless, it was found above health levels in wells in a number of states. ⁵

2. New York State Department of Environmental Conservation, Water Quality Monitoring Program to Detect Pesticide Contamination in Groundwaters of Nassau and Suffolk Counties, NY, 1999 available at <http://www.co.suffolk.ny.us/health/summary.html>.

3. U.S. EPA, Office of Ground-Water Protection, Pesticides in Ground Water: Background Document, 1986.

5. Holden, Patrick W., Pesticides and Groundwater Quality: Issues and Problems in Four States, 1986.