National Water Quality Assessment Program: Eastern Iowa Basins

Temporal Trends Of Selected Agricultural Chemicals In Iowa's Groundwater, 1982-95

Sneck-Fahrer, D. A., and Kolpin, D.W., U.S. Geological Survey, Iowa City, IA
Hallberg, G.R., University of Iowa Hygienic Laboratory, Iowa City, IA
Libra, R.D. Iowa Department of Natural Resources, Geological Survey Bureau, Iowa City, IA

Proceedings of the Geological Society of America North-Central Section meeting, May 2-3, 1996, Ames Iowa.

Because of intense agricultural production, Iowa is one of the largest users of agricultural chemicals in the Nation. Although there has been extensive research on the occurrence of agricultural chemicals in Iowa's groundwater, only a limited amount of data are available to describe regional, long-term trends in the State's groundwater quality. One such source of long-term water-chemistry data is the Iowa Groundwater-Quality-Monitoring program (IGWM), a joint effort among the U.S. Geological Survey, the Iowa Department of Natural Resources Geological Survey Bureau, and the University of Iowa Hygienic Laboratory. This data set was used to determine temporal trends in concentrations of selected agricultural chemicals (nitrate, alachlor, atrazine, cyanazine, and metolachlor). To facilitate the identification of broad temporal trends in chemical concentration, possible seasonal effects were eliminated by using samples collected between April and September from roughly three equal time periods (1982-86; 1987-91; 1992-95).

Based on the selected IGWM data from 1982-95, cyanazine detections significantly increased between 1982-86 and 1987-91 and metolachlor detections significantly increased between 1987- 91 and 1992-95. To test the validity of the observed temporal trends, a subset of 89 wells was selected that had been sampled during both the 1982-87 and 1992-95 time periods. This assured that the same set of wells were examined through time rather than the changing population of wells used in the analysis of the entire IGWM data base. The subset data support findings from the larger data base and also suggest additional trends in cyanazine and atrazine (decreasing concentrations between 1987-91 and 1992- 95). These trends are consistent with temporal patterns in chemical usage and/or application rates for these compounds.

The temporal patterns determined for this study were analyzed statistically for relation to well depth and aquifer type. It was determined that temporal variability in chemical concentrations decreased with increasing well depth. As expected, changes in chemical applications at the land surface should be first reflected by changes in chemical concentrations in the shallowest groundwater. It was determined that the temporal patterns in chemical concentrations were statistically significant only in alluvial aquifers. This is consistent with the increased vulnerability of alluvial aquifers to agricultural-chemical leaching, with characteristics that include 1) a relatively thin mantle of overlying low-permeability material, 2) local flow paths with surface-recharge areas in proximity to wells, and 3) relatively rapid rates of groundwater movement providing lateral recharge from vulnerable areas.

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