The present invention discloses the formulation and use of advanced solid-media chemical compositions in the preferred forms of pellets, tablets, capsules, or other similar forms which are designed and intended to enhance the removal of a broad range of recalcitrant organic and inorganic contaminants from a variety of difficult-to-treat environments, in particular, sediments beneath water bodies, by providing an improved means of promoting the anaerobic, biologically mediated degradation, transformation, and / or detoxification of the contaminants. Specific properties of the pellet, tablets, capsules, or other similar forms of the compositions are disclosed which enable the variation in the settling velocity of the compositions and hence the depth to which the compositions will penetrate the underlying contaminated sediments. The compositions comprise carbonaceous co-substrates, inorganic and organic anaerobic electron acceptors, organic and inorganic nutrients to promote the growth of contaminant-degrading microorganisms, and inoculum of naturally occurring microorganisms which act to promote the biodegradation of contaminants.