Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/1997
Publication Date: N/A
Interpretive Summary: The general public is concerned that normal herbicide use for field crop production in the Corn Belt is polluting ground water making it unfit for drinking water and other uses. This research showed only a very slight potential for ground water contamination by atrazine and alachlor herbicides within the Goodwater Creek watershed, a claypan soil watershed in north- central MO. Claypan soils cover over 10 million acres in MO and surrounding states. Over a 5-yr period (1991-1996), atrazine and alachlor concentrations were detected in only 7.2% and 0.4% of over 1100 ground water samples. Concentrations were always well below the EPA's maximum contaminant level for drinking water (3 and 2 ppb for atrazine and alachlor, respectively). Why? Most of the ground water recharge occurs during the fall and winter period, months after these herbicides are applied by producers. By this time, herbicide compounds have degraded to where soil concentrations are very low. The degradation process, however, can produce breakdown products that are more stable than the parent herbicide. For example, alachlor ESA, a breakdown product of alachlor, was found in ground water much more frequently than the parent compound, but concentrations were still generally less than 2 ppb. These results show that herbicides applied by producers to their fields for weed control are not polluting ground water underlying claypan soils.
Technical Abstract: The principle objective of this study was to determine the influence of three different farming systems on herbicide contamination of the ground water aquifer underlying claypan soils. The claypan is a naturally occurring argillic soil horizon that limits percolation to ground water, particularly in the spring. The study area was within the 7250-ha Goodwater rCreek watershed in north-central MO. Three row-cropped fields with 10 years of similar prior management history were selected and instrumented with 20 to 25 monitoring wells each in 1991. During the 5-yr study, the fields were treated with differing amounts of atrazine [2-chloro-(4-ethylamino)-6-(isopropylamino)-s-triazine] and alachlor [2-chloro-2', 6'-diethyl-N-methoxymethyl)acetanilide]. Over 1100 samples were collected on a quarterly basis from 1991 to 1996. Atrazine was detected in 7.2% of the samples, with a maximum concentration of 0.12 ppb. Alachlor was detected in 0.4% of the samples, with a maximum concentration of 0.14 ppb. Differences in hydrology among fields were more important than differences in herbicide application rates, since the field treated with the least atrazine and alachlor had the most frequent detections of atrazine, the atrazine metabolite deethylatrazine, and an alachlor metabolite. On claypan soils, most of the ground water recharge occurs during the fall and winter when soil solution concentrations of the parent herbicides are very low. The alachlor metabolite leached the most, but concentrations were generally less than 2 ppb. These results indicate that herbicides used by producers for weed control are not significantly contaminating the ground water aquifer underlying the 4-million ha of claypan soils in MO and surrounding states.