The main function of any sediment capping project is to physically separate the contaminated sediments from the creatures living on and burrowing through those sediments. These organisms are typically at the bottom of the food chain, so the contamination will quickly spread from these organisms to all the fish, birds and mammals that are eating them.
Traditionally sediments are capped by placing clean - uncontaminated
sand or sediment on top of the contaminated sediment. This capping layer may
range from 3 to 5 feet thick depending upon the types of contaminants, capping
material available, water currents, and creatures living on the sediments. These
caps are deployed using many different techniques and often require large equipment
to move and spread that much material over the sediment bottom. Some equipment,
such as split-hull barges, can result in a large variation of cap thickness
from the designed thickness. This results in even more capping materials being
required to ensure the minimum coverage. The typical design life for these sediment
caps range from 30 years up.
Reactive barrier capping systems use both the chemical and physical properties of a material to separate the living creatures from the contaminated sediments. Because of this, they can be deployed in much thinner layers 2 to 6 inches and still be effective. The bottom dwellers still need a place to live and burrow and would poke holes in the reactive barriers if there was not a clean layer of sand or sediment over the top of the reactive layer. However, even with this second layer, a reactive barrier's thickness would only range from 1.5 to 3 feet.