(8) Van Hoof, P. Ph.D. Dissertation, University of Wisconsin-Madison, 1989.
Received for reuiew July 19, 1991. Accepted August 13, 1991. This work was funded by the U S . Air Force Office of Scientific
Research, Grant AFOSR-88-0301, and by the University of Wisconsin Sea Grant College Program under grants f r o m the Office of Sea Grant, National Oceanic and Atmospheric Administration, U S . Department of Commerce, and from the State of Wisconsin (Federal Grant NA84AA-D-00065, Project R / MW-44).
CORRESPONDENCE Comment on “Inadequacy of NASQAN Data for Assessing Metal Trends in the Nation’s Rivers” SIR: The article Windom et al. (I)correctly points to the fact that until recently, dissolved metals data have been biased by contamination during sampling, storage, and analysis. However, I disagree with the article’s assumptions and conclusions regarding the source of metal “contamination” in suspended sediments from east coast rivers. The authors show (Figure 5) that metal concentrations in total suspended solids (TSS) are increasingly enriched relative to crustal material with decreasing TSS concentration. The authors incorrectly assume that higher metal enrichments at low TSS concentrations are due to anthropogenic contributions which are subsequently diluted by “natural” contributions at higher TSS concentrations. In fact, metal enrichments a t low TSS concentration can be entirely accounted for by grain size effects without invoking anthropogenic enrichment as described below. In most cases, as river discharge increases, the TSS concentration and percentage of coarser grained material also increase. As discharge decreases, the coarser grained material is deposited, TSS concentration decreases, and the finer grained material remains in suspension. These fiier grained materials have a much higher specific surface area than the coarse-grained material. The finer grained materials with the larger specific surface area will obviously adsorb more surface-active metals, resulting in higher metal concentrations. Normalization using average crustal abundances to produce enrichment factors will factor out the bulk crystalline component, revealing the surface component of metal enrichment. Therefore, we can expect to observe higher metal enrichments in low TSS concentration material due to the grain size effect alone, regardless of the source of the surface component metals. A useful approach, which could help eliminate this problem and uncover potential anthropogenic enrichments, would be to normalize for grain size by comparing metal and TSS concentrations from similar size fractions (e.g.,
