Both the retention and release of nutrients and contaminants by sediments are controlled by complex physical and biogeochemical interactions. Sediments, porewaters, and overlying waters are dynamic matrices that alternatively serve as sources and sinks for elements and compounds, as physical and biogeochemical conditions vary. Some of those conditions markedly change over small spatial scales, from nanometers to millimeters, and over time, from nanoseconds to millennia. Consequently, rates of scavenging and mobilization of elements and compounds to and from sediments, respectively, may also exhibit pronounced spatial and temporal variability.

This summary, therefore, briefly describes some of the principal factors involved in the uptake and release of nutrients and contaminants from sediments, as well as the diffusive and advective dispersion of those constituents in sediment porewaters and overlying waters. This includes a synopsis of chemical reactions that affect the speciation, cycling, and bioavailability of constituents in sediments that can result from the degradation of organic matter, other redox transformations, the dissolution and precipitation of mineral phases, and sorption processes, including cation exchange. It also includes brief comments on transport processes related to geo-chemical and physical characteristics of the media, as well as the influence of benthic organisms on the distribution and dispersion of those constituents in sediments and associated waters.

The complex integration of all the preceding factors determines the physical and biogeochemical cycling of nutrients and contaminants in sediments. Directions and rates of those cycles are governed by prevailing chemical and physical conditions, and are often mediated or driven by microbial communities. While those constituents may initially be entrained in sediments by geochemical sorption, authigenic precipitation, and biological scavenging, they may subsequently be remobilized from the sediments into dissolved and colloidal phases by mineral weathering, desorption, and decomposition.

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