Origin of lead in the Gulf of California Ecoregion using stable isotope analysis

摘要

The magnitude and sources of lead (Pb) pollution in the Gulf of California Ecoregion (GCE) in northwest Mexico were evaluated using various samples collected from urban and rural areas around two typical subtropical coastal ecosystems. Lead concentrations and isotopic compositions (~(206)Pb/~(207)Pb, ~(208)Pb/~(207)Pb, ~(206)Pb/~(204)Pb and ~(208)Pb/~(204)Pb) were measured using high resolution inductively-coupled plasma mass spectrometry (HR-ICP-MS) and thermal ionization mass spectrometry (TIMS). Urban street dust (157 ± 10.1 ug g~') was heavily enriched with Pb, compared to the Pb enrichment of agricultural soils (29.0 ±16.0 ug g~(-1)) and surface estuary sediments (35.6 ±15.4 ug g~(-1)), all of which contained higher Pb concentrations than found in the natural bedrock (16.0 ±5.0 ug g~(-1)). Pb concentrations in SPM (>95% of total Pb) were significantly higher in sewage effluent (132±49.9 ug g~(-1)) than in agricultural effluents (29.3±5.9 ug g~(-1)), and river runoff (73 ± 4.2 ug g~(-1)). SPM in estuary water column averaged 68.3 ± 48.0 ug g~(-1). The isotopic composition of Pb (~(206)Pb/~(207)Pb,~(208)Pb/~(207)Pb) in rural samples of aerosols (1.181 ±0.001,2.444±0.003) and soil runoff(1.181 ±0.003, 2.441 ±0.004) was comparable to that of natural Pb-bearing bedrock (1.188±0.005,2.455 ±0.008); while urban samples of aerosols, street dust, and sewage (1.190-1.207, 2.452-2.467) showed a significant contribution from automotive emissions from past leaded gasoline combustion (1.201 ±0.006, 2.475±0.005). The absence of lead from fertilizer (1.387 ± 0.008,2.892 ± 0.005) suggests that this mixture is not representative of the GCE. A mixing model revealed that the Pb content in the environmental samples is predominantly derived from natural weathering and the past leaded gasoline combustion with the later influence of inputs from a more radiogenic source related with anthropogenic lead of North American origin (1.21 ± 0.02; 2.455 ± 0.02).