Sorption and degradation of alachlor and metolachlor in ground water using green sands

Reactive barriers are used for in situ treatment of contaminated ground water. Waste green sand, a by-product of gray-iron foundries that contains iron particles and organic carbon, was evaluated in this study as a low-cost reactive material for treating ground water contaminated with the herbicides alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide] and metolachlor [2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)-o-acetoluidide]. Batch and column tests were conducted with 11 green sands to determine transport parameters and reaction rate constants for the herbicides. Similar Fe-normalized rate constants (K(SA)) were obtained from the batch and column tests. The K(SA) values obtained for green sand iron were also found to be comparable with or slightly higher than K(SA) values for Peerless iron, a common reactive medium used in reactive barriers. Partition coefficients ranging between 3.6 and 50.2 L/kg were obtained for alachlor and between 1.0 and 54.8 L/kg for metolachlor, indicating that the organic carbon and clay in green sands can significantly retard the movement of the herbicides. Partition coefficients obtained from the batch and column tests were similar (+/-25%), but the batch tests typically yielded higher partition coefficients for green sands exhibiting greater sorption. Calculations made using transport parameters from the column tests indicate that a 1-m-thick reactive barrier will result in a 10-fold reduction in concentration of alachlor and metolachlor for seepage velocities less than 0.1 m/d provided the green sand contains at least 2% iron. This level of reduction generally is sufficient to reduce alachlor and metolachlor concentrations below maximum contaminant levels in the United States.