Determination of trace element concentrations in natural freshwaters: how low is "low", and how low do we need to go?

There is an on-going need for reliable concentration data for trace elements in natural freshwaters, including soil solutions and groundwaters, surface waters (wetlands, streams, rivers, and lakes), precipitation (fog, rain, and snow), and drinking water (including natural spring water as well as bottled water and tap water). Some of the trace elements of interest may be present in these waters at elevated concentrations due either to natural processes such as mineral weathering (e.g. As and U in groundwater), or because of human activities (e.g. atmospheric contamination of snow with Pb and Sb).