Agricultural contaminants in Quaternary aquitards: A review of occurrence and fate in North America |
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Authors: | S Rodvang W Simpkins |
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Institution: | (1) Groundwater Unit, Irrigation Branch, Alberta Agriculture, Food and Rural Development, Agriculture Center, 100, 5401-1st Ave. S., Lethbridge, Alberta T1J 4V6, Canada,;(2) Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011, USA, |
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Abstract: | The intensity of agriculture has increased significantly during the past 30 years, resulting in increased detection of agricultural
contaminants (nutrients, pesticides, salts, trace elements, and pathogens) in groundwater. Till, glaciolacustrine, and loess
deposits of Quaternary age compose the most common surficial deposits underlying agricultural areas in North America. Quaternary
aquitards generally contain higher concentrations of solid organic carbon (SOC, as much as 1.4%), dissolved organic carbon
(DOC, as much as 205 mg/L), and reduced sulfur (as much as 0.9%) than do aquifers. Their potential to sorb pesticides increases
with the percent of older SOC, because diagenesis increases Koc. Denitrification consistently reduces nitrate to non-detectable levels in unweathered Quaternary aquitards. Organic carbon
of Quaternary age is a more labile electron donor than carbon from shale clasts. Pyrite is a more labile electron donor than
carbon in many instances. Unweathered Quaternary aquitards provide a high degree of protection for underlying aquifers, due
to their large reserves of SOC and reduced sulfur for sorption and denitrification, combined with their typically low hydraulic
conductivity. In contrast, agricultural contaminants are common in weathered Quaternary aquitards. Lower reserves of reduced
sulfur and sorptive/labile organic carbon, and a higher bulk K due to fractures, limit their ability to attenuate nitrate
and pesticides. Subsurface drainage, which is common in Quaternary aquitards because of high water tables, bypasses the attenuation
capacity of Quaternary aquitards and facilitates the transport of agricultural contaminants to surface water.
Electronic Publication |
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Keywords: | groundwater agricultural contamination glacial till nitrate pesticide sorption |
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