The relationships between rock-derived lead and iron in natural waters

The relationships between Fe and Pb released to the hydrocycle via rock weathering is examined in terms of field observations and laboratory experiments. Rock, soil, and unpolluted water samples from granodiorite, basalt, and carbonate terrains were analyzed for their Pb, Fe, and other element (Ca, Mg, Na, K, Si, Al, Mn) concentrations. Samples of granodiorite, soil developed on the granodiorite, basalt, and dolomite rocks, as well as biotite and feldspar minerals, were also used for laboratory leaching experiments. Fe/Pb ratios found in granodiorite, basalt, and insoluble residue of carbonate bedrocks are preserved within an order of magnitude in surface and shallow groundwater reservoirs draining these rocks. With the exception of carbonate rocks, leachates of these rocks have Fe/Pb ratios similar to that of the bedrock. Laboratory experiments suggest that Pb is adsorbed and coprecipitates with Fe-rich particles even in the presence of higher concentrations of competing ions (e.g., Mg²⁺). Complexation of Pb by ethylenediaminetetraacetate (EDTA), however, substantially decreases the amount of Pb bound to surface sites (ligand competition). Based on our results we suggest that in early phases of the weathering cycle, Pb both coprecipitates and is adsorbed on surfaces of Fe-rich particles. Because of complementary transport mechanisms and comparable rates of release from common source rock and soil minerals, the molar ratio of Fe to natural (rock-derived) Pb in the particulate load of rivers (in many cases it includes particles which pass through 0.45 μm pore size filters) is similar to that of the upper continental crustal rock, approximately 6500. This ratio can vary in watersheds which drain rocks with different Fe Pb ratios than the average upper continental crust, in rivers loaded with dissolved natural organic matter, or in rivers polluted with Pb or with anthropogenic chelators (secondary pollution effect).