Transport and persistence of nitrate, atrazine, and alachlor in large intact soil columns under two levels of moisture content

The objective of this study was to determine the relative mobility and persistence of atrazine, alachlor, and nitrate (NO3 -) in soil columns at two levels of moisture content, high saturation (approximately 70%) and low saturation (approximately 40%). Large intact soil columns of (i) Naron fine sandy loam (fine, loamy, mixed, thermic Udic Argiustoll) and (ii) Pratt loamy fine sand (sandy, mixed, thermic Psammentic Haplustalf), were obtained from two sites in Kansas. Liquid flow was traced by bromide (Br-), a relatively nonreactive chemical. Transport behavior of Br- and NO3- was comparable, i.e., concentrations of both solutes in leachates peaked at about the same pore volume (PV). In high moisture columns, concentrations of anions in effluents peaked after eluting 0.7 and 1.2 PV through the Naron and Pratt columns, respectively. Under low moisture conditions, Br- and NO3- effluent concentrations reached maxima after about 1 PV, and breakthrough curves (BTCs) were nearly symmetrical around the center of mass. Bromide recoveries were generally high (>=0.90 mg mg-1 applied) in both moisture regimes. Regardless of moisture regime, initial breakthrough of atrazine and alachlor in leachates occurred much later than did the anions, suggesting chemical interaction between herbicides and solid phase. Atrazine and alachlor BTCs showed tailing, an indication of possible nonequilibrium sorption. Degradation of atrazine to deethylatrazine and deisopropylatrazine was evident in low moisture soil columns, with deethylatrazine detected in greater concentrations. The order of mobility for the triazines was deethylatrazine >= atrazine >= deisopropylatrazine. Atrazine recovery was greater in high-than in low moisture columns, likely due to increased residence times conducive to biotic and/or abiotic transformation. Alachlor was less mobile and less persistent than atrazine. Recoveries of alachlor in high moisture columns were 0.130 and 0.098 mg mg-1 in Naron and Pratt soils, respectively. Alachlor was not detected in effluents of low moisture columns. [PUBLICATION ABSTRACT]