Pesticide Trends in Iowa's Surface and Groundwater, 1980 - 1995
by Mary P. Skopec
The 1987 Groundwater Protection Strategy identified pesticide contamination of groundwater as one of the most important issues to Iowans. More than ten years later, this issue remains a primary concern. Since 1991, the U.S. Environmental Protection Agency (EPA) has directed states to take primary responsibility in developing pesticide management plans (PMP) to deal with agricultural chemicals. Because of variations in geology, soil, and landscape features among the states, the PMP strategy allows states to implement a pesticide-specific plan to best protect their water resources. Part of the PMP process includes an inventory and assessment of the current water-quality problems within the state. As a result, the Iowa Department of Natural Resources, Geological Survey Bureau (GSB) in cooperation with the Iowa Department of Agriculture and Land Stewardship (IDALS) has developed a database of pesticide analyses to track the occurrence of agricultural chemicals in Iowas groundwater and surface waters. This database, the Iowa Pesticide Water Resources Database (IAPEST), is a compilation of all the data from water-quality monitoring and research projects conducted in the state during the last 30 years. As such, it is a large, heterogeneous database including thousands of sites covering more than 75 pesticide compounds.
To provide information for the PMP process, the GSB began to evaluate IAPEST to identify trends in the occurrence of pesticides in Iowas waters. Four commonly used corn herbicides, atrazine (Aatrex*), alachlor (Lasso), cyanazine (Bladex) and metolachlor (Dual), were analyzed for changes in detection rate through time. These chemicals were the four most commonly detected pesticides in Iowas waters during the period 1982-1995. The graph above shows the frequencies of detection for these four products in Iowas surface and groundwater. Atrazine was found in 23% of all groundwater samples followed by alachlor (5%), cyanazine (5%), and metolachlor (4%). In surface water samples collected from 1980-1995, the detection rates were: atrazine (81%), alachlor (21%), cyanazine (38%), and metolachlor (28%).
Estimated annual herbicide use in Iowa: 1979 1996.
While these numbers present an overall picture of the occurrence of herbicides in Iowas waters, they do not illustrate how this contamination has changed through time. The table above lists the pounds of pesticide active ingredient (lbs. a.i.) used and the ranking of use (based on pounds) in the state from 1979 through 1996. This table illustrates the continually evolving nature of agriculture as new herbicides enter into the marketplace and replace older products. Such was the case with the herbicides alachlor (Lasso) and acetochlor (Harness). In 1979, alachlor was the most heavily used pesticide in Iowa with more than 15,000,000 lbs. a.i. applied to the states crops. This trend continued through the 1980s, but starting in the early 1990s, alachlor use plummeted as it was rapidly replaced by new herbicides such as acetochlor that entered the market in 1994. Total alachlor use was 728,000 pounds of active ingredient in 1996, roughly 6% of the amount used in 1985. This drop in alachlor use is reflected in the declining frequency of detection (see graphs below). The highest frequency of detection of alachlor in groundwater was 13% in 1991. Since then, this frequency has steadily dropped and in 1995 was less than 1%. For surface water, the rate of alachlor detection peaked in 1990 at just over 40%, but by 1995 was only 3%.
While trends in alachlor detection can be attributed to changes in consumer preference and economic forces that resulted in a drop in alachlor use, the story for atrazine is a bit different. Atrazine use has not changed much since 1979; its annual use has ranged from just over 6,000,000 lbs. a.i. to almost 10,000,000 lbs. active ingredient. During this time, atrazine has remained one of the top-five most used pesticides. Despite this, it appears that atrazine detection rates are decreasing. In groundwater, atrazine detection rates have steadily decreased over the last five years from 28% in 1990 to 12% in 1995. Atrazine detection rates in surface water have dropped by 20% in the last five years. This trend appears to be the result of management factors. State and federal regulations have placed restrictions on atrazine applications including a reduction in the maximum rate of use, elimination of fall applications, and prohibited use near agricultural drainage wells, tile intakes, sinkholes, and lakes. As a result, lower amounts of atrazine are incorporated into herbicide tank mixes with other products. This practice has led to atrazine being applied to more acres, but at lower rates. Therefore even though the total pounds of atrazine have not decreased, the lower application rates appear to be having a positive impact on water quality.
Metolachlor has been the most used pesticide in Iowa (based on total pounds) since 1990. Its annual use has been fairly stable for the past decade and is in the 8,000,000 to 12,000,000 lbs. a.i. range. Not surprisingly, state-wide trends of the rate of metolachlor detection do not show any significant changes.
The trends in cyanazine detection are a bit perplexing. Cyanazine use peaked in 1985 at more than 10,000,000 lbs. of active ingredient. But by 1990, cyanazine use was half of the 1985 value. Use steadily increased from 1990 to 1993, but began to decline again after hitting a maximum use rate of 7,900,000 lbs. a.i. in 1993. Cyanazine use in 1996 was an all-time low of 4,900,000 lbs. active ingredient. This declining use is most likely the result of a manufacturers planned phase-out of this chemical. The cyanazine detection trends do not neatly fit the use patterns, however. While the early 1990s did show a decrease in detection rates for both surface and groundwater, the detection rates have steadily increased since 1993. The reason for this trend is not known at this time and requires further exploration.
Overall, the trends of pesticide detections in Iowa seem to be encouraging. However, it is important to note that these trends are intertwined with management decisions, marketing strategies, consumer preferences, and regional use patterns. As pesticide use patterns and management decisions evolve, so may the pesticide trends within Iowas waters.
The decreases in atrazine and alachlor concentrations in
Iowas surface and groundwater should be seen as
considerable progress toward the goals of the 1987 Groundwater
Protection Act. However, much room for improvement remains. The
everchanging face of contemporary agriculture requires that we
continue to monitor and protect our precious water resources.
* Use of brand names is for reference purposes only and does not constitute an endorsement by the Iowa Department of Natural Resources or the Iowa Department of Agriculture and Land Stewardship.
IAPEST has been supported in part by IDALS, Laboratory Division, Pesticide Bureau, through grants from the U.S. EPA, Region 7 Water, Wetlands, & Pesticides Division.
Adapted from Iowa Geology 1998, Iowa Department of Natural Resources