Sediments are an important feature of natural water bodies. The physical, chemical, and biological processes that take place are critically influenced by their presence. A primary interaction is the exchange of solutes between the sediment and the overlying water. The flux - the transport of mass - of dissolved and particulate chemical species to and from the sediments are important components of the chemical and biological cycles that take place. For example, the consumption of dissolved oxygen by organic matter that settles to the sediment in the spring is usually the primary cause, or at least an important cause, of summertime oxygen depletion in the bottom waters of lakes and estuaries. The cycling of nutrients and metals are controlled to a significant extent by processes in sediments. In particular the degree to which they are either trapped or transformed in sediments determines the extent to which they continue to interact with the water column. Dissolved nutrients are returned to the water column where they can be reused by the biological community. Dissolved toxic metals that recycle to the water column are almost always more toxic than their particulate counterparts in the sediment. It is the degree of remobilization that determines the extent to which materials stored in sediments: e.g., organic matter, inorganic phosphorus, toxic and non-toxic metals, become available to interact chemically and biologically in the water column. It determines the extent to which the consequences of historical discharges to water bodies - the materials stored in the sediments - can still exert an influence.
This book addresses the problem of mathematically modeling the processes in sediments that determine the extent to which materials that settle to the sediment are recycled to the water column. It is the flux of materials to and from the water column that is the primary focus of this book. The substances considered are the nutrients: ammonia, nitrate, phosphate, and silica; dissolved oxygen; and the metals: calcium, manganese, iron, and cadmium. They were chosen primarily because of their importance in water quality problems: eutrophication and its consequences (N, P, Si), low dissolved oxygen with its attendant severe biological impacts; excessive concentrations of toxic metals (Mn, Cd), and for their importance to other processes (Ca, Fe).