Several classes of pesticides have been shown to impair water quality in California, including organophosphates, pyrethroids and neonicotinoids.  Vegetative treatment systems (VTS) can reduce pesticide loads and associated toxicity in agricultural runoff, but many water soluble pesticides such as neonicotinoids are not effectively treated by VTS, and VTS installation is not always an option for growers required to remove non-crop vegetation for food safety concerns.  Recent studies have shown that biochar filtration can be used to remove soluble contaminants, especially when coupled with other VTS components.  We evaluated a mobile carbon filter system consisting of a trailer-mounted tank containing approximately 600L (~180 kg) of biochar.  Input water from a 437 hectare agricultural drainage was pre-filtered and treated with biochar during two multi-week study periods.  Laboratory toxicity tests and chemical and nutrient analyses were conducted on input and output water.  Pesticide concentrations were initially reduced by greater than 99%.  Treatment efficacy declined linearly and was expected to remain at least 50% effective for up to 34 weeks.  Toxicity was assessed with Ceriodaphnia dubia, Hyalella azteca and Chironomus dilutus.  Significant input toxicity was reduced to non-toxic levels in 6 of 16 samples.  Some input concentrations of the neonicotinoid imidacloprid and the pyrethroid cypermethrin exceeded organism-specific toxicity thresholds and benchmarks, but the overall causes of toxicity were complex mixtures of agricultural chemicals.  Nutrients were not reduced by the biochar.  Results demonstrate the utility of biochar in treating agricultural runoff and provide measures of the longevity of biochar under field conditions.