RESUMO
Nitrogen from agriculture is known to be a primary source of groundwater NO-N. Research was conducted in a northeastern Missouri watershed to assess the impact of cropping systems on NO-N for a loess and fractured glacial till aquifer underlying claypan soils. Three cropped fields with 10 yr of similar management were each instrumented with 20 to 25 monitoring wells, 3 to 15 m in depth, in 1991 to 1992. Wells were sampled and analyzed for NO-N at least annually from 1991 to 2004. Initial NO-N concentrations were variable, ranging from undetectable to >24 mg L but averaged 7.0 mg L. Groundwater NO-N was significantly higher in Field 3, probably the result of concurrent applications of manure and N fertilizer before 1980. Overall changes in NO-N levels in Fields 1 and 2 were generally small; however, NO-N levels for Field 3 have decreased an average of 0.28 mg L yr. Excessive loading of N into the matrix of the glacial till may have had a long-term impact on NO-N for this field. Despite the presence of dissolved O in the aquifer, evidence of denitrification in some upper-landscape groundwater wells was found. The greatest decreases in NO-N concentration occurred as groundwater moved through an in-field tree line or through a riparian zone. While overall conclusions were complicated by the long-term impact of past management, the capacity of the till to buffer changes, hydrogeologic variability found among wells, and the activity of biological processes, we conclude that cropping practices during this study did not increase glacial till NO-N.
RESUMO
Hydroxyatrazine (HA) is the major metabolite of atrazine in most surface soils. Knowledge of HA sorption to soils, and its pattern of stream water contamination suggest that it is persistent in the environment. Soils with different atrazine use histories were collected from four sites, and sediments were collected from an agricultural watershed. Samples were exhaustively extracted with a mixed-mode extractant, and HA was quantitated using high performance liquid chromatography with UV detection. Atrazine, deethylatrazine (DEA), and deisopropylatrazine (DIA) were also measured in all samples. Concentrations of HA were considerably greater than concentrations of atrazine, DEA, and DIA in all soils and sediments studied. Soil concentrations of HA ranged from 14 to 640 µg/kg with a median concentration of 84 µg/kg. Sediment concentrations of HA ranged from 11 to 96 µg/kg, with a median concentration of 14 µg/kg. Correlations of HA and atrazine concentrations to soil properties indicated that HA levels in soils were controlled by sorption of atrazine. Because atrazine hydrolysis is known to be enhanced by sorption and pH extremes, soils with high organic matter (OM) and clay content and low pH will result in greater atrazine sorption and subsequent hydrolysis. Significant correlation of HA concentrations to OM, pH, and cation exchange capacity of sediments indicated that mixed-mode sorption (i.e., binding by cation exchange and hydrophobic interactions) was the mechanism controlling HA levels in sediment. The presence of HA in soils and stream sediments at the levels observed support existing hypotheses regarding its transport in surface runoff. These results also indicated that persistence of HA in terrestrial and aquatic ecosystems is an additional risk factor associated with atrazine usage.