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 (nutrients, pesticides , salts, trace elements, and pathogens) in groundwater. Till, glaciolacustrine, and loess deposits of Quaterna-ry 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 K oc. 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 Quater-nary 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. Résumé L'agriculture s'est significativement accrue au cours des dernières 30 années, provoquant une détection plus fréquente dans les eaux souterraines de contaminants agricoles (engrais, pesticides, sels, éléments en traces et germes pathogènes). Les moraines, les formations glacio-lacustres et les loess d'âge quaternaire sont les dé-pôts superficiels les plus courants sur lesquels sont im-plantées les régions agricoles d'Amérique du Nord. Les imperméables quaternaires contiennent généralement des concentrations en carbone organique particulaire (COP), jusqu'à 1,4%, en carbone organique dissous, jusqu'à 205 mg/L et en soufre réduit, jusqu'à 0,9%, plus élevées que dans les aquifères. Leur potentiel à adsorber les pesticides augmente en même temps que le pourcentage en COP plus ancien, parce que la diagenèse fait croître K OC. La dénitrification réduit logiquement les nitrates à des niveaux non détectables dans les imperméables quater-naires non altérés. Le carbone organique d'âge quater-naire est un donneur d'électrons plus labile que le car-bone des débris schisteux. La pyrite est un donneur d'électrons plus labile que le carbone dans …