Groundwater & Pesticides -- Section 2.4 A Vulnerable Resource

Ground water contamination incidents have been reported in all parts of the United States. Contamination problems vary from region to region and are influenced by climate, population density, intensity of industrial and agricultural activities, the hydrogeology of the region, and the status and enforcement of federal and state regulations that can be used to protect ground water.

Larry Canter ¹

At one time, a common argument -- at times persuasive even with so-called water experts -- was that groundwater was often largely protected from contamination. While a careful review of technical and other literature shows hard data and solid information -- dating back many years -- about the vulnerability of groundwater to contamination, the myth of special protection persisted in many quarters. As late as the 1970s, explains the Environmental Protection Agency, "layers of soil and particles of sand, gravel, crushed rocks, and larger rocks were thought to act as filters, trapping contaminants before they could reach the ground water." ²

Today an overwhelming wealth of data to the contrary has weakened that argument considerably. "We now know," says EPA, "that some contaminants can pass through all of these filtering layers...to contaminate groundwater."²

It's been known since the 30's and 40's that the "filtering action" of soil and rock was often limited or even non-existent for a variety of chemicals. The potential for significant contamination of groundwater is actually a very old story.

Steven Amter ³

Groundwater can be -- and has been -- contaminated by a variety of pollution sources, including landfills and septic tanks, sewage treatment plants and mining operations, chemical waste lagoons and leaking pipelines and tanks, feedlots, golf courses and agricultural practices. Its quality, points out the National Academy of Sciences, ⁴ is affected by virtually all human activity. It is also affected by a wide range of natural processes.

Even before water leaves the atmosphere as rain and snow, it may become contaminated by air pollution. Once on the ground, water running across the land ("runoff") picks up pollutants that these may enter groundwater where surface water seeps into the ground. Other water seeps directly down through the unsaturated zone to the water table. It carries with it contaminants from the earth's surface or encountered in underground pathways.

As the "universal solvent," water traveling underground will dissolve and carry with it constituents from the surrounding soil and rocks. Depending upon the local geology, groundwater can contain naturally high levels of minerals like sodium, chloride, iron, sulfate or arsenic. It may carry naturally occurring radioactivity or bacteria as well. It may also become contaminated by constituents that have been introduced by human activities.

Among the pollutants frequently found and associated with human activity are solvents and cleaning agents like trichloroethylene (TCE) and tetrachlorethylene (PCE), gasoline components such as benzene, toluene and xylene and the fuel additive methyl tertiary butyl ether (MTBE), creosote, nitrates from septic tanks and cesspools, fertilizer application and animal wastes, as well as pesticides like dibromochloropropane (DBCP), atrazine, ethylene dibromide (EDB) and aldicarb. ⁵

First Law of
Ground Water Vulnerability: All ground water is vulnerable.

Second Law of
Ground Water Vulnerability: Uncertainty is inherent in all vulnerability assessments.

National Research Council ⁶

Since groundwater flow is fundamentally different than surface water flow, the behavior of groundwater contamination is different as well. Contaminants in a flowing water body may be rapidly dispersed and diluted; some will volatilize rapidly, moving out of the water and into the air. Some pollution will remain in the "water column," while other pollution settles in "sediments" or is carried through the food chain by aquatic life. Contaminants in the groundwater regime -- for the most part -- will not undergo the degree of dispersion, dilution and evaporation that would occur in a river body.

...detection of a large number of volatile and nonvolatile contaminants...in both shallow and deep ground water has raised questions as to the validity of what has been called the 'filter fantasy,' i.e., that the unsaturated zone acts as a protective buffer.

National Research Council ⁷

Pollutants moving underground into slow-moving groundwater create a zone of contamination or "plume." In most instances, the plume of contamination will follow the groundwater flow direction, responding to a range of physical, chemical and biological factors. At the outer, leading edges of the plume, sometimes referred to as the "mixing zone," concentrations of the moving contaminant may be lower than in the main body and "source area" of the plume.

This diagram shows several contamination plumes. Landfill pollution (upper left) and buried waste (right) has reached the water table and is traveling in the direction of groundwater flow. Wastes injected under pressure into a deeper aquifer are flowing with and against the groundwater flow direction. A smaller plume (middle, top) from non-point activities such as pesticide application has moved into the water table. Adapted from American Institute of Professional Geologists, Ground Water: Issues and Answers, 1983.

Again, as with all aspects of groundwater, the fate and transport of pollutants through both the saturated and unsaturated zones is highly site-specific, with contaminant movement depending on the local hydrogeology as well as the characteristics of the contaminants in question.

The earth's "filtering capacity" is not fail-safe and impenetrable, as often argued, but it clearly affects the movement of various contaminants into and through groundwater. For example, confining layers overlying a critical aquifer or significant depth to groundwater can slow and, in some cases, prevent contaminant entry. High recharge rates in humid regions of the country, on the other hand, or heavy pumping of wells may make contamination problems more pressing. In addition, different contaminants will interact with the natural groundwater environment in different ways. Contaminants also interact and react with other contaminants in groundwater.

The movement of some contaminants can be attenuated or retarded by various physical, chemical and biological interactions with the local environment. Presume, for example, that a tanker truck spills a large volume of water with trace amounts of salt, iron and lead. The wastewater enters a sandy aquifer and a year later, the water itself has moved a horizontal distance of about 200 feet. The salt, moving along with the water, has also traveled about 200 feet. In contrast, the iron has moved only 100 feet in a year and the lead contamination reaches only 30 feet from the spill site.

Some contaminants may volatilize and enter the atmosphere from the shallow, unsaturated zone; some will move through the ground in a vapor state. Depending upon the clay or organic content, some chemicals will adhere or "adsorb" to soil and rock, but, again depending on site-specific variables and the degree of further contamination, some pollutants that initially cling to earthen materials will later be released to contaminate water supplies.

Some contaminants will degrade -- though not always to a less toxic form, their makeup altered over time by chemical or biological reactions. Substances that are lighter than water, such as gasoline, can remain near the top of the water table, while dense, immiscible, low-solubility liquids may move rapidly downward, even, in some cases, against the water's flow direction.

The intrinsic nature of each contaminant -- its solubility, density, propensity to evaporate or to degrade, for example -- along with the detailed variables at play in the local environment -- depth to water table, soil type and texture, recharge rates, topography, vegetation, permeability, porosity, to name a few -- combine to make the accurate prediction of contaminant movement in groundwater a difficult and often costly task.

Ground water vulnerability is not a measurable property, but a probability statement about future contamination that must be inferred from surrogate measurements. ... Like a weather forecast, vulnerability to contamination is best expressed as a probability of an event....

Using vulnerability assessments currently available, it is fairly easy to delineate many areas of high vulnerability, difficult to say for certain that an area has very low vulnerability, and not possible to make fine gradations in between.

National Research Council ⁶

2. US EPA, Office of Water, Citizen's Guide to Ground-Water Protection, EPA 440/6-90-004, 1990.

3. Steven Amter, hydrogeologist, Disposal Safety, personal conversation with author, October 1999.

4. National Research Council, Ground Water Quality Protection: State and Local Strategies, 1986.

5. See, for example, Mackay, Douglas M. and Lynda A. Smith, "Organic Contaminants," in Alley, William A., editor, Regional Ground-Water Quality, 1993.

6. National Research Council, Ground Water Vulnerability Assessment, 1993.