Considering the longevity of biochar in agroecosystems, biochar application must be carefully evaluated and engineered to achieve agricultural (e.g., minimum impact on efficacy of agrochemicals) and environmental (minimum runoff contamination) benefits. Heavy metals and agrochemicals are the key targets for biochar-induced mitigation of runoff/groundwater contamination. Inorganic and organic contaminants interact differently with biochars as well as soil components. Mechanistic understandings are needed on sorption, desorption, and competitive sorption of inorganic and organic contaminants together and separately. This study utilized (1) attenuation of pyrolysis conditions, (2) in situ and ex situ characterization of biomass pyrolysis, and (3) batch sorption-desorption experiments to provide qualitative and quantitative selection criteria for various mitigation scenarios. For heavy metals (Pb, Cu, Ni, Cd), surface ligand (e.g., oxygen-containing) and ash (e.g., phosphorus-containing) components of biochar were the primary controlling parrameters in both contaminated and agricultural soils. For triazine and organophosphorus agrochemicals, in contrast, specific surface area of biochar determined the extent of sorption. Depending on the properties of soil and biochar, complex competitive sorption behaviors were observed between organic and inorganic contaminants. In vitro bioaccessibility testing results indicated that biochar’s impact on Pb mobility does not always reflect in bioavailability. Depending on the metal speciation, the level of contamination exceeded the toxicity/mortality threshold, regardless of biochar amendment.