Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

concentration (120.11 and 109.07 µg/g, respectively). Similarly, the median ambient Pb
concentration was higher than the median natural Pb concentration (28.00 and 22.00 µg/g,
respectively) and the ambient 95th percentile was higher than the natural 95th percentile
(200.00 and 161.50 µg/g, respectively). While the natural and ambient surface water Pb
distributions differed only at the extremes, the natural sediment Pb percentiles were consistently
lower than the ambient percentiles throughout the distributions (Figure AX7-2.2.8). Unlike the
surface water dataset, because the sediment dataset was not heavily censored, assessing national
trends in sediment Pb concentrations was possible. The data were mapped and categorized into
the four quartiles of the frequency distribution (Figure AX7-2.2.9). The following observations
were made:
• Sediment Pb concentrations generally increased from west to east (the majority of
sites along East Coast had Pb concentrations in the fourth quartile of the sediment Pb
concentration frequency distribution).
• Several “hot spots” of concentrated sites with elevated sediment Pb concentrations
were apparent in various western states.
• Sediment Pb concentrations were generally lowest in the midwestern states
(the majority of sites in North Dakota, Nebraska, Minnesota, and Iowa had Pb
concentrations in the first or second quartile of sediment Pb concentration
frequency distribution).
As was seen with surface water Pb concentrations, the highest measured sediment Pb
concentrations were found in Idaho, Utah, and Colorado. Not surprisingly, of the top 10
sediment Pb concentrations recorded, 7 were measured at sites classified as mining land use.
Tissue
As was true for the surface water data, there were a high number of tissue samples below
the detection limit (47/93 natural whole organism ND, 130/332 ambient whole organism ND,
74/83 natural liver ND, 398/559 ambient liver ND; Table AX7-2.2.4). In general, more
non-censored data were available for whole organism samples than liver samples, and for
ambient sites than natural sites. As expected, for whole organism samples, the 95th percentile Pb
concentration measured at ambient sites was higher than that measured at natural sites (3.24 and
2.50 µg/g, respectively); however, Pb liver concentration 95th percentiles for ambient and
natural samples were very similar, with the natural 95th percentile actually higher than the
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