Simazine and metolachlor removal by subsurface flow constructed wetlands

A subsurface flow constructed wetlands system was used in this pilot-scale field study to treat runoff from a container nursery. The goal of the study was to determine the statistical significance of certain factors on the removal of simazine (2-chloro-4,6-bis[ethylamino]-s-triazine) and metolachlor (2-chloro-N-[2-ethyl-6-methylphenyl]-N-[2-methoxy-1-methylethyl] acetamide). The factors analyzed included the hydraulic flowrate, media depth, cell aspect ratio, and the presence or absence of Scirpus validus. Analysis of variance using the general linear model procedure determined that vegetation and flowrate significantly (alpha = 0.05) affected herbicide mass removal. The results showed that vegetated subsurface flow cells generally remove more simazine and metolachlor than nonvegetated cells. Decreasing the hydraulic loading rate and, therefore, the areal simazine mass loading rate to vegetated cells from 1659 to 415 mg/m2 caused an increase in the percentage of total mass removed from approximately 60 to 96%. Similarly, metolachlor mass removal percentage increased from approximately 62 to 96% when the areal mass loading to vegetated cells decreased from 1037 to 260 mg/m2. Media depth and aspect ratio did not significantly (alpha = 0.05) affect effluent simazine and metolachlor concentrations. Peak effluent metolachlor concentrations were less than 300 microg/L from vegetated cells receiving hydraulic loading rates of 204 and 102 m3/d x ha. Peak simazine concentrations in the effluent of cells receiving similar hydraulic loadings were approximately 1400 and 750 microg/L (204 and 102 m3/d x ha), respectively.