Elemental Analysis of Desert Varnish Samples in the Vicinity of Coal-Fired Power Plants and the Nevada Test Site Using Laser Ablation ICPMS

Abstract

Desert varnish is a thin manganese-iron rich dark coating that forms on rocks most often in arid climates. This coating may be up to a few hundred microns in thickness and is thought to grow by approximately one micron per thousand years. Recent research has shown that many elements are incorporated into this coating, and, in spite of its slow growth, it may serve as a passive environmental monitor for recent and historical air pollution. To test this hypothesis, small pieces of varnished rock were probed directly by laser ablation inductively coupled plasma mass spectrometry (LA- ICPMS). Results were obtained for 11 elements (13 isotopes: 9 Be, 107 Ag, 111 Cd, 118 Sn, 121 Sb, 138 Ba, 184 W, 205 Tl, 206 Pb, 207 Pb, 208 Pb, 232 Th, and 238 U) by magnetic sector ICPMS and for 23 elements (25 isotopes: same as before plus 51 V, 52 Cr, 59 Co, 60 Ni, 65 Cu, 66 Zn, 88 Sr, 98 Mo, 115 In, 133 Cs, 197 Au, and 209 Bi) by quadrupole ICPMS. Varnished rock samples were collected mainly in the fallout patterns of two coal-fired power plants and downwind from the Nevada Test Site (NTS). The LA-ICPMS data indicate that many of the elements commonly found in fly ash from coal-fired power plants (e.g., Cr, Co, Ni, Cu, Zn, Cd, Sn, Sb, Tl and Pb) were highly enriched in the rock coatings in relation to the upper continental crust (UCC). For one of the power plants, where tracer plume studies had been carried out, the highest concentrations in the rock varnish coincide with the peak tracer concentration locations. Thus, these elements in rock varnish hold promise for identifying those geographical regions impacted by nearby power plants. Overall, the results indicate that rock varnish can be used as a passive environmental monitor of relatively recent events, and that the trace element fingerprints held in rock varnish may also hold much potential in forensic air pollution studies.