Components of variation in lead, cadmium, copper, and zinc concentration in home drinking water: The seattle study of trace metal exposure

A major difficulty in characterizing the exposure of persons to trace metals in their drinking water at home is the substantial variation in trace metal concentration observed in water samples collected repeatedly from the same home. Interpretation of this variation depends upon knowledge of its sources. In the Seattle study, this variation was studied utilizing open and blind duplicates, samples split for analysis at two laboratories, and samples collected from the same homes several months apart. For each of the metals studied, most of the variation over time is real. A small portion is the result of factors operating after the samples are collected and preserved. Most postcollection variation at higher concentrations is related to intralaboratory factors; most at lower concentrations, to extralaboratory factors.