Groundwater & Pesticides -- Section 4.4 The Method of Application

A final set of complex but important factors revolve around the methods of applying an agricultural chemical in the field.

The volume of the chemical used is, naturally, critically important, and one common response to contamination problems has been to lower pesticide application rates, reducing the amount of chemical used per acre. Allowable application rates are specified for each use of a pesticide, but rates may vary from region to region and even applicator to applicator. A 1998 paper by Robert Kellogg of USDA, for example, reports that application rates for the weedkiller atrazine within the state of Illinois ranged from .9 to 1.4 pounds per acre, in Indiana from 1.1 to 1.7 pounds and in Iowa, from .3 to 1.2 pounds of atrazine per acre. ¹

While reduced application rates of leaching pesticides can lead to reduced groundwater contamination, it should not be assumed that low application rates always correlate with low risk. In fact, some newer generation pesticides are standardly applied at low rates because they are highly toxic.

Another important variable is the timing of applications -- both at what stage in a crop's life cycle the chemical is used and during what type of weather conditions.

Pesticides applied before a rainstorm or during a rainy season may either wash off the site or be carried into the groundwater. If the ground is fully saturated when pesticides are applied, those chemicals that might naturally adhere to soil particles may not have an opportunity to do so. This effect of weather can be seen in groundwater monitoring results, with observed seasonal variation in detection levels. The presence or stage of vegetation, likewise, impacts pesticide movement. Deep-rooted, water loving plants can help to retard pesticide flow, while shallow-rooted plants like corn allow for more rapid movement.

Different formulations -- granule, soluble or wettable powders, emulsifiable concentrates, etc. -- may have differing impacts on pesticide movement, and methods of application -- aerial sprays, chemigation, soil injection, etc. -- can be important as well. The proper calibration and upkeep of equipment likewise affects contamination potential, and the use of back-siphoning prevention devices can reduce the danger of accidental backflow from a pesticide mixing tank back down into a water well.

In a number of instances in which pesticides have been detected in groundwater, regulators, applicators, pesticide-makers and others have joined together in a search for more protective methods of application. Reduced application rates, well setbacks, alterations in application timing, changes in drainage practices and other approaches have been adopted in response to problems.

There were various attempts to develop new ways of using aldicarb on Long Island at different times, at lower rates, or in controlled release formulations, but none of these proved to be effective, and it was finally concluded that it would be virtually impossible to use aldicarb on Long Island in such a way as to avoid causing groundwater residues above the state's 7 ug/l guideline.

Patrick Holden ²

Given the slow movement of groundwater, it will always be difficult to discern the true impact of such changes, but in some instances these types of changes may indeed make a critical difference. In other instances in particularly vulnerable settings, application restrictions may not work as envisioned. Such was the case, for example, with the problem of aldicarb in the potato-growing region of Long Island.

1. Kellogg, Robert L., et al, Natural Resources Conservation Service, U.S. Department of Agriculture, "An Information Aid for Assessing Possible NRCS Involvement in the State Management Plan Process for Regulation of Pesticides," 1998 available at <http://www.nhq.nrcs.usda.gov/land/pubs/pestsmp.html>.

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

The Methods of Application