The purpose of this manual is to provide the reader with an understanding of the factors that govern the fate and transport of naturally-occurring arsenic at sites impacted with petroleum hydrocarbons over the lifetime of the hydrocarbon impact. The central themes of the manual are that arsenic can be mobilized by the biodegradation of petroleum hydrocarbons at concentrations exceeding the natural ambient conditions and that when the hydrocarbon impact is mitigated (spatially or temporally), arsenic concentrations will revert back to the ambient geochemical conditions. Arsenic mobilization and attenuation are governed by simple, fundamental, and understandable principles.