| The behavior of trace quantities of commonly-detected herbicides and herbicide degradates was investigated in a shallow sand and gravel aquifer. Two injection studies were conducted, including low concentrations {dollar}(sim{dollar}MCL) of the herbicides atrazine, alachlor, cyanazine, metolachlor, butachlor, and the triazine degradates deethylatrazine (DEA) and deisopropylatrazine (DIA), along with a conservative tracer (bromide). The plumes were monitored for 2 to 3 months following injection using an array of multilevel samplers.; Advection-dispersion and exponentially modified Gaussian (EMG) equations were fitted to breakthrough curves to determine transport parameters. Comparison of the breakthrough curves for the conservative tracer and each herbicide provides a measure of retardation in the aquifer. A retardation factor (R) may be calculated from the first central moment of the data or from peak parameters obtained from the fitted curves. 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 {dollar}rm (R>1.6).{dollar} 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-month period.; Laboratory batch equilibrium sorption estimates are often used in groundwater transport models to predict field transport of solutes. Laboratory retention results were compared with those from a natural-gradient field transport study. Batch equilibration assuming a linear isotherm tended to overestimate retention as compared to that observed in the field, and also failed to predict differences in herbicide sorption observed in the field. A nonlinear isotherm (Freundlich model) gave a much more accurate description of herbicide transport, despite small deviations from linearity.; Laboratory and field studies were conducted to evaluate the effects of well casing material on aqueous concentrations of these herbicides and nitrate-N. Results indicate that representative ground water samples for these analytes are not dependent on the materials used for well construction. |
