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

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Figure AX7-2.2.14. <strong>Lead</strong> cycle in an aquatic ecosystem.<br />

Sediment Core Dating and Source Tracing<br />

In addition to directly measuring Pb concentrations in various aquatic compartments (see<br />

Section AX7.2.3.3), it is useful to study the vertical distribution <strong>of</strong> Pb. Sediment pr<strong>of</strong>iling and<br />

core dating is a method used to determine the extent <strong>of</strong> accumulation <strong>of</strong> atmospheric Pb and<br />

provide in<strong>for</strong>mation on potential anthropogenic sources. Sediment concentration pr<strong>of</strong>iles are<br />

typically coupled with Pb isotopic analysis. The isotope fingerprinting method utilizes<br />

measurements <strong>of</strong> the abundance <strong>of</strong> common Pb isotopes (i.e., 204 Pb, 206 Pb, 207 Pb, 208 Pb) to<br />

distinguish between natural Pb over geologic time and potential anthropogenic sources. Details<br />

<strong>of</strong> this method were described in Section AX7.1.2. The concentration <strong>of</strong> isotope 204 Pb has<br />

remained constant throughout time, while the other isotope species can be linked to various<br />

anthropogenic Pb sources. Typically, the ratios or signatures <strong>of</strong> isotopes (e.g., 207 Pb: 206 Pb) are<br />

compared between environmental samples to indicate similarities or differences in the site being<br />

investigated and the potential known sources.<br />

Generally, Pb concentrations in sediment vary with depth. For example, Chow et al.<br />

(1973) examined sediment Pb pr<strong>of</strong>iles in southern Cali<strong>for</strong>nia. <strong>Lead</strong> concentrations were<br />

increased in the shallower sediment depths and comparatively decreased at greater depths. These<br />

changes in sediment vertical concentration were attributed to higher anthropogenic Pb fluxes<br />

AX7-143

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