Diel cycling of trace metals in streams

Historically, scientists have assumed that a properly collected water sample will provide an accurate assessment of constituent concentrations in a water body on a given day assuming constant hydrologic conditions.  However, an increasing body of evidence indicates that the concentration of many potentially toxic trace elements (such as arsenic, cadmium, copper, manganese, zinc) in streams can vary over a large range (up to 500 percent) during a 24-hour period irrespective of changes in streamflow.  These diel concentration cycles have been shown to be robust and reproducible, having been documented in many streams separated by large distances, in different geologic environments, and over a large range of metal concentrations.