Agricultural contaminants in Quaternary aquitards: A review of occurrence and fate in North America

The intensity of agriculture has increased significantly during the past 30 years, resulting in increased detection of agricultural contaminants (nutr ients, pesticides, salts, trace elements, and pathogens) in groundwater. Ti ll, 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 organ ic carbon (SOC, as much as 1.4%), dissolved or ganic carbon (DOC, as much a s 205 mg/L), and reduced sulfur (as much as 0.9%) than do aquifers. Their p otential to sorb pesticides increases with the percent of older SOC, becaus e diagenesis increases K-OC. Denitrification consistently reduces nitrate t o non-detectable levels in unweathered Quaternary aquitards. Organic carbon of Quaternary age is a more labile electron donor than carbon from shale c lasts. 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 hydraul ic conductivity. In contrast, agricultural contaminants are common in weath ered Quaternary aquitards. Lower reserves of reduced sulfur and sorptive/la bile organic carbon, and a higher bulk K due to fractures, limit their abil ity to attenuate nitrate and pesticides. Subsurface drainage, which is comm on in Quaternary aquitards because of high water tables, bypasses the atten uation capacity of Quaternary aquitards and facilitates the transport of ag ricultural contaminants to surface water.