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

Figure AX7-2.2.6. Frequency distribution of ambient and natural levels of surface water
dissolved lead (µg/L).
Due to the preponderance of non-detectable (ND) measurements, assessing national trends
in surface water-dissolved Pb concentrations was not possible. However, areas with elevated Pb
concentrations were identified by classifying the data with detectable Pb concentrations above
and below the 99th percentile. The 99th percentile (versus the 95th percentile) was chosen in
this instance to represent extreme conditions given the small window of variability in the dataset.
By convention, the 95th percentile was used in subsequent analyses of this type. Areas with high
surface water Pb concentrations were observed in Washington, Idaho, Utah, Colorado, Arkansas,
and Missouri (Figure AX7-2.2.7). The maximum measured Pb concentration was located in
Canyon Creek at Woodland Park, ID, a site classified as mining land use.
Sediment
There were approximately one-half of the number of surface water data available for
sediments (N = 1466). In contrast to the surface water data, however, very few sediment data
were below the detection limit (7/1466 ambient ND, 3/258 natural ND; Table AX7-2.2.3).
As expected, the mean ambient Pb concentration was higher than the mean natural Pb
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