THE EFFECT OF MEMBRANE FILTRATION ON DISSOLVED TRACE-ELEMENT CONCENTRATIONS

The almost universally accepted operational definition for dissolved c onstituents is based on processing whole-water samples through a 0.45- mu m membrane filter. Results from field and laboratory experiments in dicate that a number of factors associated with filtration, other than just pore size (e.g., diameter, manufacturer, volume of sample proces sed, amount of suspended sediment in the sample), can produce substant ial variations in the 'dissolved' concentrations of such elements as F e, Al, Cu, Zn, Pb, Co, and Ni These variations result from the inclusi on/exclusion of colloidally-associated trace elements. Thus,'dissolved ' concentrations quantitated by analyzing filtrates generated by proce ssing whole-water through similar pore-sized membrane tilters may not be equal/comparable. As such, simple filtration through a 0.45-mu m me mbrane filter may no longer represent an acceptable operational defini tion for dissolved chemical constituents. This conclusion may have Imp ortant implications for environmental studies and regulatory agencies.