Atrazine transport through a soil/epikarst system

Authors: Lerch, Robert; GROVES, C - Western Kentucky University; POLK, J - Western Kentucky University; MILLER, B - Us Geological Survey (USGS); SHELLEY, J - Western Kentucky University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2018
Publication Date: 6/19/2018
Citation: Lerch, R.N., Groves, C.G., Polk, J.S., Miller, B.V., Shelley, J.A. 2018. Atrazine transport through a soil/epikarst system. Journal of Environmental Quality. doi:10.2134/jeq2017.12.0492.
DOI: https://doi.org/10.2134/jeq2017.12.0492

Interpretive Summary: About 25% of the land area in the United States is covered by karst topography. Karst occurs when surface water infiltrates limestone or dolomite bedrock, eventually forming sinkholes, caves, springs, and losing streams. The Pennyroyal Plateau is a karst sinkhole plain in south-central Kentucky with intensive row crop agriculture. Typical of sinkhole plains, surface drainage is almost entirely lacking within the plateau, as most precipitation directly infiltrates, moving through the soil to the epikarst (soil-bedrock boundary) and then into the deeper groundwater aquifer. The well drained soils, permeable bedrock, and internal drainage of these sinkhole plains is an especially vulnerable setting for groundwater contamination. In this study, we investigated the movement of the common corn herbicide, atrazine, from a treated field to the epikarst drains within a cave adjacent to the field. The Crumps Cave study has a recharge area of about 2.5 acres that contains 2 epikarst drains (designated WF-1 and WF-2) which were monitored for atrazine, and 2 of its breakdown products, deethylatrazine (DEA), and deisopropylatrazine (DIA), from Jan 2011 to May 2012. Atrazine, DEA, and DIA were detected in 100% of the samples collected. Atrazine concentrations in both drains did not increase above winter background levels for nearly two months following application when levels spiked and reach peak concentrations in May 2011. Atrazine concentrations at WF-1 exceeded the drinking water standard of 3 parts per billion in 12% of the samples, but the proposed standard for aquatic life (3.4 parts per billion average over 60 days) was not exceeded in either drain. The two breakdown products were generally present at higher concentrations than atrazine throughout the study, and the seasonal pattern of DEA and DIA concentrations corresponded to changes in soil temperature. The study supported the hypothesis that a combination of atrazine binding and microbial breakdown in the soil column above the epikarst controlled the movement of atrazine and its breakdown products. This resulted in the observed delay in atrazine transport following application and the prolonged transport of atrazine and its breakdown products to the epikarst. Losses of atrazine and breakdown products from the field accounted for about 1% of the applied atrazine, an amount similar to that of runoff losses in other areas of the Corn Belt. Management practices that reduce herbicide usage, such as diverse crop rotations, weed suppressing cover crops, and low application rate herbicides, would improve groundwater quality in areas of the Corn Belt with intensive row cropping on karst topography. This study benefits conservation agencies and growers by demonstrating that atrazine movement to groundwater in karst areas is comparable to that of surface losses in other agricultural areas and allow for implementation of management practices tailored to karst areas to reduce the impacts of row crop agriculture on groundwater resources.

Technical Abstract: Row crop and livestock production contaminate soils and groundwater of the karst aquifer systems within south-central Kentucky’s Pennyroyal Plateau. We investigated the transport of atrazine from field application to the epikarstic drainage system beneath a field with active row-crop farming. Because of the thick residuum soils (~3 m) that overlie the St Louis Limestone at the site, the working hypothesis of the study was that the soils would exert control on the degradation and transport of atrazine to the epikarst drains. The Crumps Cave study site is a shallow autogenic drainage system with a recharge area of ~1 ha that contains two epikarst drains (WF-1 and WF-2) which were monitored for atrazine, deethylatrazine (DEA), and deisopropylatrazine (DIA) concentrations from Jan 2011 to May 2012. Atrazine concentrations in both drains did not increase above winter background levels for nearly two months following application when levels spiked and reach peak concentrations (38.5 µg/L at WF-1 and 0.83 µg/L at WF-2) during an early May 2011 event. Atrazine, DEA, and DIA were detected in 100% of the samples, and metabolites accounted for 54-94% of the monthly total loads, except in May 2011. Median dealkylated metabolite to atrazine ratios (DMAR) were ~5:1 at both sites, and seasonal DMAR patterns corresponded with changes in soil temperature. These data support the hypothesis that a combination of sorption and degradation in the soil column above the epikarst controlled the transport of atrazine and its metabolites, resulting in delayed atrazine transport following application and continual transport of the weakly sorbed metabolites to the epikarst aquifer. Management practices that improve soil quality, leading to increased retention and degradation of herbicides, and the use of strongly sorbed, non-persistent herbicides would improve groundwater quality in this region.