A composition for in situ remediation of soil and groundwater contaminated with hydrocarbons. The composition includes an adsorbent, such as activated carbon, capable of adsorbing the hydrocarbons. The composition also includes a sulfate-containing compound that releases sulfate over a period of time, e.g., a time-release compound that may include calcium sulfate. The composition includes a nutrient system for promoting growth of facultative anaerobes, in the soil or provided in the composition itself. In some embodiments, the nutrient system includes a sulfide scavenging agent such as iron sulfate. In the same or other embodiments, the nutrient system includes at least one of a nitrogen source and a phosphorous source.
