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

and it has been used by the EPA for describing national environmental concentrations for use in
developing AWQC. The authors recognize that the NAWQA program encountered analytical
challenges with the chemical analysis of Pb in surface waters. The analytical methods available
during the NAWQA program were not as sensitive as methods currently applied today.
Therefore, analytical detection limits are elevated and a large portion of the data set contains
non-detected values. Nevertheless, this data provides a comprehensive overview of Pb
concentrations in U.S. surface waters that is supplemented with data from other relevant studies.
The following sections describe the estimated concentrations of Pb from NAWQA and other
research programs.
NAWQA data are collected during long-term, cyclical investigations wherein study units
undergo intensive sampling for 3 to 4 years, followed by low-intensity monitoring and
assessment of trends every 10 years. The NAWQA program’s first cycle was initiated in 1991;
therefore, all available data are less than 15 years old. The second cycle began in 2001 and is
ongoing; data are currently available through 30 September 2003. The NAWQA program study
units were selected to represent a wide variety of environmental conditions and contaminant
sources; therefore, agricultural, urban, and natural areas were all included. Attention was also
given to selecting sites covering a wide variety of hydrologic and ecological resources.
NAWQA sampling protocols are designed to promote data consistency within and among
study units while minimizing local-scale spatial variability. Water-column sampling is
conducted via continuous monitoring, fixed-interval sampling, extreme-flow sampling, as well as
seasonal, high-frequency sampling in order to characterize spatial, temporal, and seasonal
variability as a function of hydrologic conditions and contaminant sources. Sediment and tissue
samples are collected during low-flow periods during the summer or fall to reduce seasonal
variability. Where possible, sediment grab samples are collected along a 100-m stream reach,
upstream of the location of the water-column sampling. Five to ten depositional zones at various
depths, covering left bank, right bank, and center channel, are sampled to ensure a robust
representation of each site. Fine-grained samples from the surficial 2 to 3 cm of bed sediment at
each depositional zone are sampled and composited. Tissue samples are collected following a
National Target Taxa list and decision trees that help guide selection from that list to
accommodate local variability.
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