Biomonitoring of elemental atmospheric deposition with the lichen Xanthoparmelia spp. in Arizona, USA

The overall objective of this study was to document the spatial pattern of elemental deposition as reflected in lichens (Xanthoparmelia spp. ) as of 2006 within the area encompassing Maricopa County and the greater Grand Canyon region, Arizona, USA. Because the patterns present in the accumulation of nonessential elements by lichens are the result of the lichens' life history, especially the use of the atmosphere for inorganic nutrients, I predicted that changes in the atmosphere from human activities would be correlated with changes in the accumulation patterns of elements in lichens. Lichens of the genus Xanthoparmelia spp. were collected from rural and urban areas of Maricopa County and the greater Grand Canyon region of Arizona, USA and the levels of 47 elements determined using high resolution inductively coupled mass spectroscopy (HR-ICP-MS) and cold vapor analysis for mercury. Spatial analyses of the elements in both data sets as well as a subset of elements in Maricopa County based on previous research were accomplished using geographical information software (ArcGIS), and analyzed in relation to anthropogenic and geologic sources. Temporal analysis was accomplished with the Maricopa County data set using previous research that did extensive sampling in 1998 and used herbarium specimens collected in 1973. Multivariate statistical techniques were applied to both data sets to attempt to determine underlying patterns and relative contributions of the local soil geology and anthropogenic inputs. For both locations overall variation across sites within each region was relatively small, reflecting the fact that few sites were close to known point industrial sources of elemental emissions such as smelters, mines and factories. The major exception to this is copper, which is found in elevated amounts in parts of Maricopa County from mining and smelting activities. The lack of elevated concentrations of mercury correlating with the locations of coal power plants in the Grand Canyon study area is interpreted to mean that the power plants are not a significant source of mercury in this region, since coal power plants are the major source of mercury pollution today. Similarly, the lack of elevated concentrations of uranium, cesium and strontium correlating with the Palo Verde Nuclear Generating Station in Maricopa County are interpreted to mean that the reactors are not a significant source for those elements in this region. The major inputs to the lichens in the region were determined to be from soil sources, with construction activities in Maricopa County contributing to a higher elemental load for all elements except cadmium as compared to the Grand Canyon region. Alternately, the effect of climatic factors on lichen growth rates could be the cause of these elevated levels. Since the milder and wetter climate of northern Arizona should encourage faster lichen growth, these lichens should have had less time to accumulate elements as compared to lichens in Maricopa County.