Critical factors that must be considered in the design of field bioaccumulation studies of silver are the selection of appropriately and consistently sized organisms as well as collection of organisms from consistent tidal elevations. The silver concentrations in mussels from reference and outfall-influenced sites vary by a factor of approximately 3-fold over time, reflecting the pattern of ‘natural’ within-site variation. Wastewater mass emission rates from the major southern California wastewater discharges vary by a factor of approximately 3, without significant changes of silver input over the past decade. Concentrations of silver in mussels between reference and outfall-influenced sites in southern California characteristically vary by greater than two orders of magnitude (100-fold). The origin of silver in mussels and its use as a tracer of sewage or other silver-enriched wastes in coastal waters is supported by the following: mussels reflect unique xenobiotic discharges from outfalls (example, DDT compounds); there is strong correlation between zinc/silver outfall discharge and mussel concentrations; rapid uptake and bioconcentration of silver occurs in transplanted mussels in outfall dilution zones; outfall sites have significantly greater concentrations of silver in tissues than reference or non-influenced sites; concentrations of silver in mussels are inversely proportional to distance from major municipal discharges; low correlations exist between silver and other heavy metals known to be introduced by different transport mechanisms (lead). In addition to these factors, there are different sequestration mechanisms in mussels for particular heavy metals; for example, silver behaves very differently in mussels from copper and zinc, which are regulated and apparently are required elements for mussel's metabolism. These factors have led to the use of silver to trace the effects or impacts of a silver-enriched discharge for long distances through biomonitoring of silver concentrations in mussels. A greater than two order-of-magnitude (100-fold) decline in silver concentrations of mussels ( Mytilus californianus) occurs between Point Loma, San Diego, California, USA, and Punta Abreojos, Baja California, Mexico (range of concentrations 59 ppm-0.08 ppm, dry wt).