National Water-Quality Assessment (NAWQA) Project
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This study determined a significant decrease in median atrazine concentration and a significant increase in median metolachlor concentration in Iowa's groundwater. These temporal patterns in median concentration are consistent with their pattern of statewide chemical use and are suggestive of a causal relation. There is, however, an apparent offset between the greatest period of change in chemical use (1982-86 and 1987-91) and that for median concentration (1987-91 and 1992-95). This suggests a lag time likely exists between changes in chemical use at the land surface and measured concentrations in groundwater.
The temporal variability for atrazine and metolachlor decreased with increasing well depth, documenting that changes in chemical concentrations occur most rapidly in the shallowest (youngest) groundwater. The temporal variability for atrazine and metolachlor was found to be significant only for alluvial aquifers. The relative absence of overlying low-permeability material, generally local flowpaths with surface-recharge areas in proximity to wells, and the relatively rapid rates of groundwater movement are all likely contributing factors to the temporal patterns found in alluvial aquifers. However, the relatively small number of wells completed in glacial-drift and bedrock aquifers for the subset of IGWM wells examined decreased our ability to adequately determine temporal patterns for these types of aquifers.
The identification of temporal patterns in median agricultural chemical concentrations in groundwater and their apparent relation to changes in chemical use, demonstrates the importance and benefits of a program, such as the IGWM, to collect long-term water-chemistry data. Only through continued data collection can it be determined if the trends described here represent long-term temporal trends or only short-term changes in groundwater concentrations.
It is also important, however, that such a monitoring program have the flexibility to add compounds to its suite of constituents analyzed as new pesticides are introduced into farming and additional pesticide degradation products are identified. For example, imazethapyr (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid), a soybean herbicide first registered for use around 1988, quickly obtained widespread use in Iowa within several years of registration (Hartzler and Wintersteen, 1991). An even more dramatic example is acetochlor [2-Chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methyl-phenyl) acetamide], a herbicide first registered for use on corn in 1994. In a mere two years, acetochlor went from zero use to the third most heavily used pesticide in Iowa (U.S. Department of Agriculture, 1995). Thus, if these additional compounds are not examined in ground water, a major part of the contemporary pattern of chemical use will be missed.
Furthermore, recent research has shown that pesticide degradation products commonly occur in groundwater, often being detected more frequently than their parent compounds (Kolpin et al., 1996; Kolpin et al., 1997; Potter and Carpenter, 1995; Roy and Krapac, 1994). These degradation products also may contribute to concerns for deleterious effects on human health and the environment (Barrett, 1996; Coats, 1993; Tessier and Clark, 1995) and are requisites for understanding the environmental fate of pesticides. Consequently, if long-term data on pesticides and all their major degradation products are not available, an understanding of the environmental fate of pesticides will remain incomplete.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://water.usgs.gov/nawqa/pnsp/pubs/jeq26/conc.html
Page Contact Information: gs-w_nawqa_whq@usgs.gov
Page Last Modified: Tuesday, 04-Mar-2014 14:44:28 EST
