Sorption and transport of atrazine, alachlor and fluorescent dyes in alluvial aquifer sands
The ability to predict the movement of pesticides in groundwater aquifers is important to formulating response plans to the detection of small concentrations of pesticides in groundwater. This study utilized batch, column and modeling efforts to investigate the movement of atrazine and alachlor in low organic carbon content alluvial aquifer sands. The ability of fluorescent dyes (fluorescein and rhodamine WT) to act as surrogates (adsorbing tracers) for the pesticides was evaluated;Batch and column studies showed the following sequence of increasing adsorption on the alluvial sands: fluorescein, atrazine, alachlor and rhodamine WT. Existing empirical estimation techniques (based on K[subscript] ow and f[subscript] oc) were successful in predicting the level of adsorption for the pesticides (within 50%) while the levels of adsorption experienced by the dyes were several orders of magnitude greater than those estimated. The pesticide isotherms were linear in the [mu]g/L range and no competitive adsorption was evidenced when the two pesticides were present jointly. The dye isotherms were linear in the [mu]g/L range but became nonlinear in the mg/L range. Increasing valency and concentration of background ions were observed to increase the level of adsorption for rhodamine WT while no effect was observed for the pesticides;Increasing pore water velocities for the column runs resulted in earlier appearance of the pesticide breakthrough curves. Equilibrium models were not able to predict the nonequilibrium shapes of the observed breakthrough curves or account for the earlier appearance or increased nonequilibrium shape with increasing pore water velocity. Use of a Fickian physical nonequilibrium model improved the ability to describe the breakthrough curves (by increasing the aggregate radius or increasing the diffusion resistance). Calibration of the model to atrazine data at one pore water velocity resulted in fairly good predictions for atrazine at a second pore water velocity and for alachlor at two pore water velocities (including the earlier appearance of the breakthrough curves). Hysteresis of desorption was observed during column studies for both the pesticides and the dyes. Column studies with rhodamine WT resulted in a plateau in the rhodamine WT breakthrough curves (C/CO remained at 0.5 for a number of pore volumes prior to increasing again).