Desert varnish as an indicator of modern-day air pollution in southern Nevada.

摘要

Rock varnish, often called desert varnish, is a slow-growing, manganese-rich coating that accumulates on exposed rock surfaces. The mechanism of varnish formation is not fully understood, however, most authors agree that varnishes derive their components from the atmosphere. The main goal of this study was to demonstrate the potential use of desert varnish as a passive environmental monitor of present and past atmospheric pollution. Analysis of varnishes is a new field that can potentially provide records of pre-anthropogenic levels of atmospheric metals and other environmental pollutants. To evaluate the potential of desert varnish as an environmental monitoring tool, the following hypotheses were tested: (1) rock varnish accumulates and preserves a record of airborne heavy elements and can be used as a passive environmental monitor of relatively recent events (2) anthropogenic pollutants are deposited in the varnish's outermost layers and can be traced to their sources, such as ore smelters or coal--fired power plants and (3) heavy metals and radionuclides can be quantified in the varnish coatings using field portable X-ray fluorescence spectroscopy (FPXRF), inductively coupled plasma mass spectroscopy (ICPMS), laser ablation-inductively coupled plasma mass spectroscopy (LA-ICPMS), and cold vapor atomic absorption spectrometry (CVAA).Desert varnish samples were collected in the areas surrounding four point sources of air pollution: the Nevada Test Site (NTS), Nye County, NV the Mohave Power Project (MPP), Laughlin, NV the Reid Gardner Power Plant (RGPP), Moapa, NV and the Titanium Metal Corporation (TIMET), Henderson, NV. The chemical composition of rock varnishes was examined with FPXRF, LA-ICPMS, ICPMS, and CVAA.Both FPXRF and LA-ICPMS results show that many trace elements in the analyzed varnishes appear to be enriched relative to the upper continental crust (UCC). Notably, elements that show the highest levels of enrichment relative to the UCC are commonly found in atmospheric emissions from coal-fired power plants. Abundances of these elements plotted against the distance from the power plant show general patterns consistent with the predictions of the Gaussian Plume model for transport and diffusion of the pollutants. The model predicts that lower concentrations of pollutants will be observed at the point of origin followed by a maximum peak concentration and gradual decrease with distance from the source.To confirm correlation of contamination to distance from the power plant, total concentrations of the elements in desert varnish films were determined. Varnishes collected in the downwind locations from MPP and RGPP were stripped from the base rock with concentrated HCl. The resulting solutions were analyzed by quantitative ICPMS for 27 isotopes (9Be, 51V, 52Cr, 59Co, 60Ni, 63Cu, 66Zn, 75As, 88Sr, 98Mo, 102Ru, 103Rh, 106Pd, 111Cd, 118Sn, 121Sb, 133Cs, 184W, 187Re, 195Pt, 205Tl, 206Pb, 207Pb, 208Pb, 209Bi, 232Th, and 238U). Additionally, mercury was analyzed by CVAA. The average elemental concentrations in the varnish samples were plotted as a function of the distance from each power plant. The ICPMS data confirm that many trace elements have a deposition patterns consistent with the Gaussian Plume model.The results of this study provided basic knowledge of the chemical and radiochemical composition of varnish coatings and, thus, additional evidence to help understand the mechanism of varnish formation. In conclusion, the results of this study will have implications (1) for the mapping of the distribution of some of the components of air contamination, (2) for identifying the sources of air pollution, (3) for deciphering the history of atmospheric pollution, (4) for contributing to our understanding of desert varnish formation, and (5) for use as a prospecting tool.