Most farms have a centralized location to fill spray tanks with pesticides and to flush and clean application equipment. These sites, depending on the frequency of use, could be significant sources of surface and groundwater contamination. One approach to minimize this contamination is to install a treatment system, such as a biobed. This study sought to construct a biobed and test the effects of different biomix materials in enhancing the dissipation of herbicides widely used in crop production. The four types of biomix evaluated had mixing ratios by volume of (1) 12.5% straw:62.5% soil:25% peat, (2) 25% straw:50% soil:25% peat, (3) 12.5% straw:62.5% soil:25% compost, and (4) 25% straw:50% soil:25% compost. The dissipation rates of acetochlor, atrazine, pendimethalin, and trifluralin at different incubation times over 90 d were evaluated. The dissipation of atrazine and pendimethalin in the biomixes were faster than in soil. The half‐lives for atrazine were 27.8 d in soil and 14.3 to 20.2 d in the biomixes and those of pendimethalin were 25.5 d in soil and 11.9 to 14.8 d in the biomixes. The dissipation rates and half‐lives of acetochlor were similar to those in soil; the trifluralin dissipation rates were slower in the biomixes. The phenol oxidase activity was higher in the peat biomixes than in those containing compost. The results showed that biobed materials, especially those with peat, are effective in degrading selected herbicides. Core Ideas Biobed material composition and ratio were evaluated. The dissipation rates of atrazine and pendimethalin were enhanced in biomixes. Phenol oxidase was higher in peat biomixes than in compost. A mobile, low‐cost system was designed to capture contamination on‐farm.