Biochar can have unique benefits to terrestrial and aquatic ecosystems. Investigations of biochar effectiveness within these environments often come from homogenous feedstocks, such as plant biomass, which have simple thermochemical processing methods and produce physically and chemically stable biochar. Current methods to increase biochar production include the addition of oil-derived products such as plastics, which produces a more heterogenous feedstock. This feedstock is similar to materials from waste recycling streams. The adoption of more heterogenous feedstocks produces additional challenges to biochar production and use. This can result in pollution contained within the feedstock being transferred to the biochar or the creation of pollutants during the processing. With the current climate emergency, it is essential to eliminate environmental contamination arising from biochar production. It is critical to understand the physiochemical composition of biochar, where detailed analysis of contaminants is often overlooked. Contamination is common from heterogenous feedstocks but on commercial scales, even homogeneous biochar will contain organic pollutants. This chapter investigates biochar produced from various waste feedstocks and the challenges faced in thermochemical processing. Using Automotive Shredder Residue (ASR) as an example of a heterogeneous feedstock, the levels of contamination are explored. Potential solutions are reviewed while assessing the environmental and economic benefits of using biochar from mixed sources.