A Natural Gradient Transport Study of Selected Herbicides

Abstract

AbstractThe behavior of trace quantities of commonly detected herbicides and herbicide degradates was investigated in a shallow sand and gravel aquifer. In 1991, a solute pulse containing approximately 15 mg L−1 bromide, 3 μg L−1 atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine), 2 μg L−1 alachlor (2‐chloro‐2′,6′‐diethyl‐N‐(methoxymethyl)‐acetanilide), 10 μg L−1 cyanazine (2‐[[4‐chloro‐6‐(ethylamino)‐S‐triazin‐2‐yl]amino]‐2‐methylpropionitrile), and 10 μg L−1 metolachlor (2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐1‐methylethyl) acetamide) was injected into a shallow aquifer near Fremont, NE. The plume was monitored for 2 mo following injection using an array of multilevel samplers. A second injection in 1992 included approximately 10 mg L−1 bromide, 1 μg L−1 butachlor (2‐chloro‐2′,6′‐diethyl‐N‐(butoxy methyl) acetanilide), and 3 μg L−1 atrazine, deethylatrazine (DEA) (2‐amino‐4‐chloro‐6‐isopropylamino‐1,3,5 triazine) and deisopropylatrazine (DIA) (2‐amino‐4‐chloro‐6‐ethylamino‐1,3,5 triazine). This plume was monitored for 3 mo. Retardation factors (Rs) were determined for each herbicide by comparing concentration profiles to those of the conservative tracer; Rs were calculated as the ratio of first central moments. Results indicated only slight retention of all injected compounds. Retardation factors for atrazine, DEA, 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 under the breakthrough curves. Atrazine, DEA, DIA, cyanazine, and metolachlor showed no detectable loss. Losses of alachlor and butachlor were approximately 40 and 70%, respectively, in a 2‐mo period.