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

Lead Distributions Generated from the NAWQA Database
Natural versus Ambient Groups
There were four to eight times more ambient surface water (Table AX7-2.2.2) and bulk
sediment (Table AX7-2.2.3) samples in the compiled dataset than natural samples. This is most
likely a function of both the NAWQA program site selection process and the fact that sites
unaffected by human activities are extremely limited. The spatial distributions of natural and
ambient surface water/sediment sites were fairly comparable, with natural samples located in
almost all of the same areas as ambient samples except in the Midwest (Ohio, Illinois, Iowa, and
Michigan), where natural sites were not present (Figure AX7-2.2.3). This exception may be
because these areas are dominated by agricultural and urban areas. The same spatial
distributions were observed for the natural and ambient liver and whole organism tissue
samples (Figure AX7-2.2.4 and Figure AX7-2.2.5).
Table AX7-2.2.2. Summary Statistics of Ambient and Natural Levels of
Dissolved Lead in Surface Water
AX7-126
Surface Water Dissolved Pb (µg/L)
Statistic Natural Ambient
% Censorship 87.91 85.66
N 430 3445
Minimum 0.04 0.04
Maximum 8.40 29.78
Mean 0.52 0.66
Standard Deviation 0.59 1.20
Median 0.50 0.50
90th Percentile 0.50 0.50
95th Percentile 0.50 1.10
96th Percentile 0.67 2.00
97th Percentile 1.00 2.34
98th Percentile 1.79 3.58
99th Percentile 2.48 5.44