Groundwater & Pesticides -- Section 4.2 The Natural Environment

Important variables affecting pesticide "leachability" (the propensity to move into and through groundwater) include not only the amount of precipitation but also its timing and other weather factors such as air and soil temperature and evaporation rates. In general, humid areas are more prone to contamination than are dry or arid areas, and periods of heavy rainfall -- particularly directly following pesticide application -- may initiate pulses of contamination moving through the subsurface environment.

The landscape itself can affect rates of contamination, with sloping areas frequently more susceptible to runoff after rain events. Flatter areas, on the other hand, may experience more leaching as the water percolates through the ground rather than running off.

The presence and types of vegetation may also affect pesticide movement, with more deeply rooted crops and plants capable of taking up more of a pesticide that resides in the root zone above the water table following application.

root zone
Adapted from Raymond, Lyle S., Jr., Ground Water Contamination, New York State Water Resources Institute, 1988

In considering the natural characteristics at the ground surface, it may be useful to think of essentially two zones -- first, the ground and upper layers of earth above the water table; second, that saturated area below the water table. What happens in the upper "unsaturated zone" that contains both air and water is critical in determining whether a given pesticide will reach groundwater; what happens below that -- in the "saturated zone" -- will determine where, when and in what concentrations pesticide-contaminated groundwater will reach a point of discharge -- a well, a spring or a surface water body.

At the "ground level," critical components affecting pesticide movement include numerous soil characteristics, such as texture -- the relative proportions of sand, silt and clay, and structure -- how these component parts are held together, the amount of organic matter, mineral content, acidity or pH, moisture content and permeability. The depth of the soil layer, likewise, is a key factor. In general, deep dense soils with a high level of clay and organic matter will allow for certain pesticides and other contaminants to bind or adsorb to soil particles rather than move quickly to the water table.

...it is natural to expect large variations in soil properties vertically and horizontally in different areas. It would be necessary to have site-specific data on the soil type to indicate soil structure, mineralogy, chemistry, and texture before making detailed predictions on the potential for contaminating groundwater with agrichemicals.

Office of Technology Assessment ¹

In-depth site investigations have revealed that soil organisms or biota can play an important role as well. On the one hand, soils with rich populations of beneficial bacteria, fungi and invertebrates may provide the type of environment in which pesticides can be transformed by microbial and chemical degradation.

On the other hand, such a soil may contain voids or "macropores" that allow for the "preferential flow" of contaminants through the soil zone. These soil "bypasses" as it were can be created by earthworms and insects, burrowing animals, decaying roots or simply the cracking and fracturing that occurs in certain soils. "The use of flow models that do not account for preferential flow," cautions researchers from the University of North Dakota, "could severely underestimate the rapidity and depth of pesticide movement in soils...."²

...in some environments a combination of soil and hydrologic characteristics will make it practically impossible to prevent the movement of highly soluble organic chemicals through the vadose zone to groundwater.

Patrick Holden ³

Once pesticides move beyond the root zone, they are no longer available to the plant and will continue to move through the environment. At this point, notes the Office of Technology Assessment, "there is little to stop them from moving downward to groundwater."¹

After reaching groundwater, variations of the factors that affected movement in the unsaturated zone will continue to affect the chemical's movement. The characteristics of each encountered geologic zone -- be it a less permeable aquitard or a more permeable aquifer -- will impact the movement of the pesticide until it degrades or reaches a point of discharge.

As noted in the previous section, the geology of an area can be complex, with numerous formations of differing composition -- and therefore differing levels of porosity and permeability and capacity to degrade or attenuate a pesticide in transit.

Where low permeability or confining layers extend for some distance laterally and horizontally over an important aquifer, that aquifer will have some degree of protection. Unconsolidated deposits consisting largely of clay may exhibit high porosity -- holding a significant amount of water, but they may hold that water tightly and lack permeability, thus retarding pesticide movement. Consolidated deposits such as granite or slate can act to slow movement, but again generalities are not to be trusted in the field. Hard crystalline materials, for example, may exhibit an overall low permeability, but cracks, joints or fractures can allow for unseen "preferential flow." The chart below is but one example of how dramatically geology can affect the flow of water underground.

Variation in Estimated Permeability
of Typical Geologic Materials in Illinois ¹

Geologic Material	Flow Rate
Clean sand and gravel	100 ft/yr
Fine sand and silty sand	100 ft/yr -- 1 ft/yr
Silt	10 ft/yr - 1ft/10yrs
Gravelly till	1 ft/yr - 1 ft/100yrs
Clayey tills, greater than 25% clay	1ft/100yrs - 1 ft/10,000yrs
Sandstone	10 ft/yr
Fractured rock	10 ft/yr
Shale	1ft/100yr - 1 ft/1,000,000yrs
Dense unfractured limestone	1 ft/1,000yrs - 1 ft/1,000,000yrs

Citation in OTA is to 1984 data from the Illinois State Geological Survey.

Pesticide-contaminated groundwater will move through the underground environment and may eventually discharge to low-lying surface water, emerge in a spring, or be drawn into a drinking water well. Groundwater flow will, as a general rule, follow the path of gravity and the architecture of the geologic formations, but the natural flow can be altered significantly by the pumping of wells.

Think back to the cone of depression in the water table that is created by a pumping well. Imagine two moderately pumping wells pulling water from the same aquifer. Their cones of depression are moderate as well. Well A is "upgradient" from Well B. Assume Well A lies directly in the path of groundwater contamination, and the pumping of this well either increases or stops entirely. In the first case -- increased pumping, Well A pulls the contaminated groundwater through the aquifer at a faster rate. In the case of well closure, the contaminated groundwater in the natural environment will travel to beyond Well A to Well B or another downgradient point. Any effort to predict groundwater vulnerability and likely contamination, then, must consider the various pumping scenarios that are possible.

1. Office of Technology Assessment, Beneath the Bottom Line: Agricultural Approaches to Reduce Agrichemical Contamination of Groundwater, 1990.

2. University of North Dakota, Energy and Environmental Research Center, "Impacts of Agricultural Chemicals on Groundwater Quality," 1990.

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

The Natural Environment

Variation in Estimated Permeabilityof Typical Geologic Materials in Illinois 1

Variation in Estimated Permeability
of Typical Geologic Materials in Illinois ¹