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

In open waters of the North Atlantic the decline of Pb concentrations has been associated
with the phasing out of leaded gasoline in North America and western Europe (Véron et al.,
1998). Likewise, Pb restrictions in gasoline appear to have been effective in reducing
atmospheric Pb loading to the Okefenokee Swamp in southern Georgia/northern Florida
(Jackson et al., 2004). Based on sediment cores from the Okefenokee Swamp, Pb concentrations
were approximately 0.5 mg/kg prior to industrial development, reached a maximum of
approximately 31 mg/kg from about 1935 to 1965, and following passage of the Clean Air Act in
1970 concentrations have declined to about 18 mg/kg in 1990 (Jackson et al., 2004). Trends in
metals concentrations (roughly 1970-2001) in sediment cores from 35 reservoirs and lakes in
urban and reference settings were analyzed by Mahler et al. (2006) to determine the effects of
three decades of legislation, regulation, and changing demographics and industrial practices in
the United States on concentrations of metals in the environment. The researchers found that
decreasing trends outnumbered increasing trends for all seven metals analyzed (Cd, Cr, Cu, Pb,
Hg, Ni, and Zn). The most consistent trends were for Pb and Cr: For Pb, 83% of the lakes had
decreasing trends and 6% had increasing trends; for Cr, 54% of the lakes had decreasing trends
and none had increasing trends. Mass accumulation rates of metals in cores, adjusted for
background concentrations, decreased from the 1970s to the 1990s, where median changes
ranged from 246% (Pb) to 23% (Hg and Zn). The largest decreases were found in lakes located
in dense urban watersheds where the overall metals contamination in recently deposited
sediments decreased to on-half its 1970s median value. However, Mahler et al. (2006) found
that anthropogenic mass accumulation rates in dense urban lakes remained elevated over those in
lakes in undeveloped watersheds, in some cases by as much as two orders of magnitude (Cr, Cu,
and Zn), indicating that urban fluvial source signals can overwhelm those from regional
atmospheric sources. In estuarine systems, however, it appears that similar declines following
the phase-out of leaded gasoline are not necessarily as rapid. Steding et al. (2000) used isotopic
evidence to demonstrate the continued cycling of Pb in the San Francisco Bay estuary. In the
southern arm of San Francisco Bay, which has an average depth of