MNR is a remedy that uses ongoing, naturally-occurring processes to contain, destroy, or reduce the bioavailability or toxicity of contaminants in sediment (EPA, 2005a; National Research Council (NRC), 1997). MNR typically relies on decreases in contaminant bioavailability and toxicity in surface sediments. Throughout this document, surface sediment is defined as the site-specific biologically active benthic layer at the sediment-water interface. This layer typically is 5 to 10 cm thick in fresh water systems (EPA, 2005a) and can be as much as 1 m thick in estuarine and marine environments (Apitz et al., 2002; Murdoch and Azcue, 1995). Bioavailability may be generally defined as the extent to which living organisms can uptake chemical contaminants by active (biological) or passive (physical or chemical) processes. Natural physical, chemical, and biological processes that contribute to natural recovery of contaminated sediments may include sediment burial, sediment erosion/dispersion, and contaminant sequestration/degradation (e.g., precipitation, adsorption, and transformation). Each of these processes, discussed in greater detail below, can directly affect risk to site-specific receptors associated with site-specific contaminants, and consideration should, therefore, be given to understanding how each may apply to a given site and the ecological and human receptors associated with specific sites.