Pesticide Trends in Surface and Groundwater in Iowa: 1980 - 1995
M.P. Skopec and B.E. Hoyer
|Figure 1. Summary of IAPEST data for the regions defined by the Groundwater Vulnerability Regions of Iowa (GVRI) map (Hoyer and Hallberg, 1991).|
Good Bedrock Aquifers include those of Cambrian, Ordovician, Silurian, and Devonian (a selected subset) systems. Generally, they have good natural quality and produce adequate quantities for many purposes. Variable Bedrock Aquifers include those of the Devonian (a selected subset), Mississippian and Cretaceous systems. Generally, the natural quality and yields from these aquifers vary considerably. Thin Drift is defined as less than 100 feet in thickness. Moderate Drift cover is defined to be between 100 and 300 feet in thickness. Bedrock Wells with Thick Drift Cover defines those bedrock wells that occur within the identified drift region. Drift Aquifer these are not mapped, per se, on the GVRI. Rather, the GVRI identifies regions where drift aquifers are most commonly used. Drift aquifers are potentially available wherever there is glacial drift or loess of reasonable thickness. They are used where bedrock aquifers are not economical, not reliable, or of poor natural quality. Alluvial Aquifer areas underlain by sand and gravel aquifers situated beneath floodplains along stream valleys and includes alluvial deposits associated with stream terraces and benches, contiguous wind-blown sand deposits, and glacial outwash deposits.
The numbers of samples and sites are not evenly distributed across groundwater vulnerability regions. The most vulnerable areas have received the majority of the research and monitoring efforts. Due to the low number of samples, some of the original GVRI regions were combined for trend analyses.
|Figure 2. Summary of IAPEST data for the interior river basins.|
Seven major surface water basins were identified for Iowa: Northeastern Iowa Rivers basin including Upper Iowa, Maquoketa, Yellow, and the Turkey rivers; Cedar River basin; Iowa River basin; Skunk River basin; Des Moines River basin; Southern Iowa Rivers basin including Chariton, Grand, Nodaway and Nishnabotna rivers; and the Western Iowa Rivers basin including Rock, Little Sioux, Maple, Soldier, and Boyer rivers. The number of samples (and sites) is poorly distributed between and within surface water basins.
|Figure 3. Comparison of
detection frequency in surface and groundwater for the
most commonly found pesticides, 1980 1982.
*ELISA Triazine category represents the enzyme-linked immunoassay test for the triazine family of compounds.
|Figure 4. Precipitation departure from normal for Iowa: 1982 1995 (from Climatological Data for Iowa).|
|Figure 5. Atrazine detection rates in Iowa surface and groundwater: 1982 1995.|
|Figure 6. Alachlor detection rates in Iowa surface and groundwater: 1982 1995.|
|Figure 7. Cyanazine detection rates in Iowa surface and groundwater: 1982 1995.|
|Figure 8. Metolachlor detection rates in Iowa surface and groundwater: 1982 1995.|
Figures 5 - 8. The bar charts show the frequency of detection for four of the most commonly detected pesticides in Iowas waters. Pesticides are detected much more frequently in surface water than in groundwater. All of the detection rates show a decrease in the late 1980s due to the effect of a state-wide drought (see Figure 4). Alachlor detections have dropped in the 1990s as a result of rapidly declining use patterns (see Table 2). Atrazine detection rates also seem to be decreasing despite a relatively stable use pattern. At this time, it appears that management changes in the use of atrazine (including a reduction in maximum use rate, elimination of fall applications, and buffers adjacent to streams and tile intakes) may be responsible for this decrease. Cyanazine detection rates seem to be increasing despite declining use rates. This may be the result of a sampling bias the database contains several recent studies on reservoirs in the southern part of the state where cyanazine is primarily used. Metolachor detection rates have remained stable reflecting its unchanging use patterns.
|Figure 9. Scatterplot of alachlor concentrations in Iowa groundwater.|
|Figure 10. Scatterplot of atrazine concentrations in Iowa groundwater.|
Figures 9 10. Both atrazine and alachlor concentrations in Iowa groundwater have shown significant decreases for the period 1982 1995 (see Table 3). The decreases in alachlor concentrations are most likely due to declining use rates. The atrazine decreases appear to be related to management changes and are not the result of reductions in use.
|Table 1. Pesticides found in
Iowas surface and groundwater
(data from IAPEST).
The same number of products (37) are found in Iowas surface and groundwater, however composition of the list varies slightly. Iowa groundwater has been studied much more extensively with 12,375 samples as compared to surface water with only 5,766 samples.
|Table 2. Herbicide use rates in the state of Iowa: 1979 1995. (Pest Management in Iowa: Planning for the Future, 1996, Iowa State University publication). Use rates are given in 1,000 pounds of active ingredient (a.i.) per year. Rank is based on the number of pounds used in any given year (e.g., the pesticide with the greatest number of pounds used is assigned a rank of "1").|
|Table 3. Trends in herbicide concentrations for Iowa groundwater (1982 1995).|
|Table 4. Trends in herbicide concentrations for Iowa surface water (1980 1995).|
In Tables 3 and 4, linear regression techniques were used to test for significant increases or decreases in herbicide concentrations through time for the various groundwater regions and surface water basins.
The development of the IAPEST database has been supported financially through cooperative agreements with the Iowa Department of Agriculture and Land Stewardship: Laboratory Division, Pesticide Bureau and grants from the U.S. Environmental Protection Agency, Region VII: Water, Wetlands, & Pesticides Division.
Hoyer, B.E., and Hallberg, G.H., 1991, Groundwater Vulnerability Regions of Iowa: Department of Natural Resources, Special Map Series 11.
Mayerfeld, D., Hallberg, G., Miller, G., Wintersteen, W., Hartzler, R., Brown, S., Duffy, M., and DeWitt, J., 1996, Pest Management in Iowa: Planning for the Future: Iowa State University publication IFM 17, University Extension, Iowa State University, Ames, IA, 89 p..
Presented as a poster at the Sixth National Nonpoint-Source Monitoring Workshop, September 21-24, 1998, Sheraton Four Points Hotel, Cedar Rapids, Iowa.