Measured and Predicted Herbicide Removal by Mulch

The removal of the herbicides diuron, isoxaben, oryzalin, and clopyralid by shredded cedar mulch was studied in laboratory batch and column experiments. The distribution coefficients (Kd) for the herbicides diuron, isoxaben, and oryzalin measured in batch experiments ranged from 129 to 187 L/kg for raw mulch and from 153 to 341 L/kg for ground mulch. Kd could not be accurately determined for clopyralid because of its weak sorption. A reactive transport model that described adsorption-desorption using a linear two-rate (site) reversible submodel provided a good fit for the experimental breakthrough data for isoxaben from column studies; adjusting the exchange rate coefficients using molecular diffusion coefficient ratios allowed breakthrough curves for oryzalin and diuron to be predicted effectively using no adjustable parameters. Model sensitivity analysis indicates that over 80% herbicide removal efficiency can be obtained for detention times greater than 8 min and storm durations of less than 100 min for the ground mulch particles. Widespread use of mulch as landscape material and the short detention times required for reasonable removal efficiency suggests that a mulch treatment system may be an efficient best management practice; these experimental and model results provide a basis for future pilot testing.