PROBLEMS ASSOCIATED WITH USING FILTRATION TO DEFINE DISSOLVED TRACE-ELEMENT CONCENTRATIONS IN NATURAL-WATER SAMPLES

Field and laboratory experiments indicate that a number of factors ass ociated with filtration other than just pore size (e.g., diameter, man ufacturer, volume of sample processed, amount of suspended sediment in the sample) can produce significant variations in the ''dissolved'' c oncentrations of such elements as Fe, Al, Cu, Zn, Pb, Co, and Ni. The bulk of these variations result from the inclusion/exclusion of colloi dally associated trace elements in the filtrate, although dilution and sorption/desorption from filters also may be factors. Thus, dissolved trace element concentrations quantitated by analyzing filtrates gener ated by processing whole water through similar pore-sized filters may not be equal or comparable. As such, simple filtration of unspecified volumes of natural water through unspecified 0.45-mu m membrane filter s may no longer represent an acceptable operational definition for a n umber of dissolved chemical constituents.