A NATURAL GRADIENT TRANSPORT STUDY OF SELECTED HERBICIDES

The behavior of trace quantities of commonly detected herbicides and h erbicide degradates was investigated in a shallow sand and gravel aqui fer. In 1991, a solute pulse containing approximately 15 mg L(-1) brom ide, 3 mu g L(-1) atrazine loro-4-ethylaminod-isopropylamino-1,3,5-tri azine), 2 mu g L(-1) alachlor loro-2',6'-diethyl-N-(methoxymethyl)-ace tanilide), 10 mu g L(-1) cyanazine ino)-S-triazin-2-yl]amino]-2-methyl propionitrile), and 10 mu g L(-1) metolachlor -ethyl-6-methylphenyl)-N -(2-methoxy-1-methylethyl) acetamide) was injected into a shallow aqui fer near Fremont, NE. The plume was monitored for 2 mo following injec tion using an array of multilevel samplers. A second injection in 1992 included approximately 10 mg L(-1) bromide, 1 mu g L(-1) butachlor (2 -chloro-2',6'-diethyl-N-(butoxy methyl) acetanilide), and 3 mu g L(-1) atrazine, deethylatrazine (DEA) (2-amino-4-chloro-6-isopropylamino-1, 3,5 triazine) and deisopropylatrazine (DIA) (2-amino-4-chloro-6-ethyla mino-1,3,5 triazine). This plume was monitored for 3 mo. Retardation f actors (Rs) were determined for each herbicide by comparing concentrat ion profiles to those of the conservative tracer; Rs were calculated a s the ratio of first central moments. Results indicated only slight re tention of all injected compounds. Retardation factors for atrazine, D EA, DIA, cyanazine, alachlor, and metolachlor were less than 1.3, with butachlor being slightly more retained (R = 1.65). Persistence of the injected compounds was estimated by the decrease with time in area un der the breakthrough curves. Atrazine, DEA, DIA, cyanazine, and metola chlor showed no detectable loss. Losses of alachlor and butachlor were approximately 40 and 70%, respectively, in a 2-mo period.