Persistence of Selected Pesticides in Ground‐Water Samples

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Abstract
This study was conducted to determine the stability of five commonly used herbicides fortified into freshly collected ground water. No aquifer materials were present. Ground‐water samples obtained from three sites in eastern Arkansas were fortified with an aqueous mixture containing either the chloroacetamide herbicides, alachlor, metolachlor, and propanil, or the phenoxy herbicides, 2,4‐D and dichlorprop. Experiments were conducted at tempersitures comparable to Arkansas' extreme ground‐water temperatures, 15 and 22° C, and at initial concentrations of 1 and 5 μg/1. Degradation occurring for each herbicide was determined at two‐month intervals, for a period of 18 months.Results indicated that degradation did not begin immediately, but generally required a lag period of several months. Of the five herbicides studied, propanil degraded the fastest and to the greatest extent. After eight months, 44% to 69% of propanil at the lower fortification level and 85% to 99% at the higher level had degraded, The remaining four herbicides exhibited little or no degradation at the lower concentration during the first 12 months. By 18 months, however, significant degradation of each herbicide had occurred. Degradation generally occurred earlier for each herbicide at the higher fortification level. With the exception of 2,4‐D, a higher percentage of herbicide was broken down at the greater fortification level. Degradation was more extensive at the elevated storage temperature. In general, the degradation rate was faster in ground water taken from shallower depths. Zero‐order rate constants and half‐lives are presented. Half‐lives, including lag periods, ranged from 196 to 1907 days.