A NOVEL TECHNIQUE TO DATE ROAD SEDIMENT USING 210PO:210PB DISEQUILIBRIA: CASE STUDY FROM AN URBAN AREA, DETROIT, MI

Road sediments (also known road dust, <63 µm) have been reported to be a source of heavy metals in reaching the human body via ingestion, direct inhalation and dermal contact. Atmospherically-delivered ²¹⁰Pb, ²¹⁰Po and ⁷Be can be used to track the mobility of road sediment. From the collection and analysis of a suite of road sediment and soil samples, the activities of excess ²¹⁰Pb (²¹⁰Pb_xs = Total ²¹⁰Pb – ²²⁶Ra) and ²¹⁰Po_xs, ²²⁶Ra, ⁷Be and ¹³⁷Cs on bulk and size-fractionated sediment were determined. In sequentially extracted fractions (exchangeable, carbonate, Fe-Mn oxide, organic and residual) from a subset samples as well in the atmospheric deposition, activities of ⁷Be, ²¹⁰Po and ²¹⁰Pb were measured. There is a large range in the activities ²¹⁰Pb_xs, ²¹⁰Po_xs and ⁷Be in road sediments while the activities of ²²⁶Ra and ¹³⁷Cs remain relatively constant. The average activities of ²¹⁰Pb_xs, ⁷Be, and ²¹⁰Po_xs are much higher in road dusts compared to urban soils. Measurements of ⁷Be and ²¹⁰Pb_xs activities in size fractionated road sediment indicate higher activities in <32 µm size compared to 32-45 µm and 45-63 µm size fractions. The average activity ratio of ⁷Be/²¹⁰Pb_xs in road dusts (mean=3.62; range=0.2-9.5, n=45) is approximately half as that of bulk precipitation (mean=7.64; range= 5.5-10.2, n=7). However, the average (²¹⁰Po/²¹⁰Pb)_xs ratio in road sediment (mean 0.59; range=0.23-1.2, n=45, corresponding to an ‘age’ of 178 days) is approximately 10 times higher than that in bulk precipitation (mean=0.062; range=0.015-0.128, n=7). The Fe-Mn oxide fraction of road dust and urban soil contains the largest fraction of ⁷Be and ²¹⁰Pb, although no correlation observed between Fe-Mn content in the soil and radionuclide specific activity. The numerical model developed for the prediction of (²¹⁰Po/²¹⁰Pb)_xs activity ratios suggests that when road sediment is continuously exposed to atmospheric deposition, the (²¹⁰Po/²¹⁰Pb)_xs activity ratio will converge towards a “dynamic equilibrium” (i.e., reaching a plateau value or steady state) value of ~0.59 over a period > 1 year at 10% tagging with longer time period for higher percentage of tagging. The model does predict that the measured (²¹⁰Po/²¹⁰Pb)_xs activity ratio can be reproduced. Recent applications of ²¹⁰Po-²¹⁰Pb disequilibrium as a geochronometer will be summarized.