Disclosed are novel solutions in which a preexisting sediment capping system is integrated with improvements such as a cap ingredient processing and delivery apparatus, cap processing method and novel compositional cap product, whereby a hazardous site is remediated by delivering at least one novel cap composition layer over the contaminated subaqueous waterway or terrestrial substrate floor. One embodiment includes a powdered activated carbon (“PAC”) based cap layer produced by the system and methodology disclosed, the cap layer ingredients comprising predetermined ratio amounts of dry PAC and dry powdered 2-Hydroxypropyl guar gum (“HGG”) used as a safe and effective delivery ingredient, blended in a cap processing and delivering apparatus with a dewatered sand semi-slurry, the processed resulting capping composition being delivered in a manner such that a novel PAC cap layer is provided to a cap placement area in need of remediation.