ROCK VARNISH: A PASSIVE FORENSIC TOOL FOR MONITORING RECENT AIR POLLUTION AND SOURCE IDENTIFICATION

Rock varnish samples were collected near three point sources of air pollution to determine if the varnish contained a record of recent air pollution. Samples were collected as follows: downwind of the Nevada Test Site (NTS); in the fallout pattern of the shuttered Mohave Power Plant, located in Laughlin, Nevada; and, near the operating Reid-Gardner Power Plant, just east of Las Vegas, Nevada. Analysis of the NTS rock varnish shows (240)Pu/(239)Pu mass ratios as low as 0.0592 +/- 0.0003 and (241)Pu/(239)Pu ratios as low as 0.00063 +/- 0.00004, compared to worldwide values of 0.18 +/- 0.01 and 0.009 +/- 0.002, respectively, clearly indicating that the varnish can be used as a forensic tool for identifying the source of air pollution, in this case the NTS. The samples collected in the plumes of the coal-fired power plants contain thorium and uranium, and have (232)Th/(238)U mass ratios from 1 to 30, and concentrations from 5 to 755 ppm for Th and 1 to 578 ppm for U. The highest concentrations of these elements occur together at locations that implicate the power plants; however, additional samples would be required to demonstrate unequivocally that the power plants are the sources. Overall, it is apparent that rock varnish can be utilized as a passive monitor to investigate recent air pollution.