Movement and aquifer contamination potential of atrazine and inorganic chemicals in central Kansas croplands
By Marios Sophocleous, M. A. Townsend, Costas Orountiotis, R. A. Evenson, D. O. Whittemore, C. E. Watts, and E. T. Marks
Originally published in 1990 as Kansas Geological Survey Ground Water Series 12. This is, in general, the original text as published. The information has not been updated. This publication is also available as an Acrobat PDF file (4 MB).
Recent incidents of chemical contamination of ground water by pesticides and other agricultural chemicals, such as nitrogen fertilizers, in surrounding states and in Kansas have increased concern for long-term environmental consequences of current agricultural practices. The Great Bend Prairie is a prime agricultural area in Kansas where agrichemicals have been used for decades. However, until recently, no study had been conducted in that area to assess the impact of these practices on the Great Bend aquifer, which yields water for irrigation, industrial, municipal, and domestic uses. Our initial conception was that the shallow aquifer of the region would either contain appreciable amounts of various agrichemicals, such as nitrogen fertilizer derivatives and atrazine, or the agrichemicals would be held "in transit" in the unsaturated zone on top of the shallow clay layers in the region if they were not present in the aquifer. Therefore the purpose of this project was to determine the fate of agrichemicals commonly used in Kansas croplands in general and in the Great Bend Prairie in particular and to evaluate their potential to move through the soil profile into the underlying aquifer. We outfitted three sites in the Great Bend Prairie with instruments to monitor the processes involved in the movement and concentration of agrichemicals in soils and the underlying aquifers. Two sites (site 6, in Stafford County, and site 7, in Pratt County), characterized by different soil types but used for growing the same crop (irrigated corn), were examined in detail. A third site (site 10, in Edwards County), which is not cultivated (pastureland) and on which agrichemicals have never been used, was selected as the control site. We studied experimentally the simultaneous movement of both atrazine and bromide by flooding the study sites. Other inorganic chemicals, such as nitrates and chlorides, were also studied. The purpose of the flooding experiments was not necessarily to simulate actual field conditions but to evaluate the potential of atrazine to be leached into the water table under highly favorable conditions. Because of the importance of the soil environment in controlling the movement and degradation of atrazine, a detailed study of the soil horizons was conducted before the flooding experiments. Although we observed relatively high nitrate concentrations in both the unsaturated zone and the aquifer zone in one of our study sites, atrazine was not detected in either the deeper unsaturated zone or the underlying aquifer in any of the study sites. We also have no evidence of atrazine accumulation on top of clay layers. We found that atrazine readily breaks down to other often nontoxic by-products, thus explaining why atrazine was not detected deeper in the soil profile and in the underlying shallow aquifer. Based on the limited results of this study, the probability that atrazine will leach through the soils to the underlying aquifer in the Great Bend Prairie under present conditions seems remote. The nitrate data from site 7 indicate a close relationship between irrigation and nitrate movement into the subsurface; that is, nitrate is moving into the deeper soil profile and into the ground water as a consequence of irrigation. However, low nitrate levels in both the soil and the ground water were observed at site 6. The total dissolved solids content (the amount of dissolved material in a sample of water) in the ground water from sites 6 and 7 was much higher than that from the nonirrigated site 10, again demonstrating the impact of continuous irrigation on ground-water quality. The classical processes of chemical movement did not fit the data well at the studied sites. The significance of such observations to agricultural chemical movement is that, during periods of unusually high rainfall and ponding or during excessive flood irrigation, downward movement of chemicals concentrated in soils might be triggered and dissolved chemicals might bypass certain soil zones and move to greater depths than expected.
We gratefully acknowledge Anna Corcoran, the KGS Geohydrology Section secretary, who typed the entire manuscript, and Mark Schoneweis, who drafted all the figures. Robin Garabay, Susan Snyder, Oliver Bennett, and William Connors, under the supervision of Max Foster, Kansas State Board of Agriculture, Division of Laboratories, conducted the bulk of the atrazine determinations in soil and water. The Soil Conservation Service performed most of the soil analyses and soil descriptions. William Roth and Dennis Nettleton, among others, were the SCS representatives most closely associated with our project. The Big Bend Ground-water Management District No. 5 and its manager at the time, Ralph Davis, provided logistic and other support for the project. The Analytical Services Section of the Kansas Geological Survey, under the direction of Larry Hathaway, conducted most of the inorganic water analyses. Roy Spalding, of the University of Nebraska, Lincoln; Milton Meyer, of the Soil Conservation Service, Washington D.C.; and Dennis Nettleton, of the Soil Conservation Service, Lincoln, Nebraska, reviewed the manuscript. Several anonymous reviewers of a summary extract from this report, to appear in the Journal of Hydrology, also provided useful reviews.
We conducted flooding experiments at two sites with different soils to study the transport and fate of the commonly used herbicide atrazine and inorganic chemicals in the Great Bend Prairie croplands. To characterize in situ the hydraulic and chemical properties of the appropriately instrumented field sites, we used the instantaneous profile method supplemented by means of an organic (atrazine) and an inorganic (bromide) tracer chemical. Atrazine readily hydrolyzed into hydroxyatrazine and biodegraded to ethylatrazine and isopropylatrazine, thus explaining why atrazine was not detected deeper in the soil profile and in the underlying shallow aquifer. The classical processes of chemical movement based on equilibrium conditions and diffuse flow through porous media did not fit the data well at either site. Incompletely mixed slug flow predominates at one of the sites, and preferential flow predominates at the other. As a result of the observed slug movement, piston-type displacement of more saline solutions in the soil profile to the shallow water table occurred. Relatively high nitrate concentrations in both the vadose and the saturated zone were observed in one of the sites, which is characterized by sandy soil. The nitrate data indicate a close relationship between nitrogen fertilizer application, irrigation, and nitrate movement into the subsurface.
Kansas Geological Survey, Geohydrology
Placed on web Aug. 23, 2010; originally published 1990.
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