EVALUATING TEMPORAL TRENDS AND SOURCES OF LEAD IN THE COLUMBIA RIVER GORGE USING EPIPHYTIC LICHENS AND LEAD ISOTOPES

The Columbia River Gorge (CRG) is a region between Washington and Oregon with complex topography and sparse instrumented air monitoring sites. Lead (Pb) pollution in the CRG originates from both natural and anthropogenic sources. Anthropogenic Pb sources include vehicular exhaust, diesel exhaust from trains and marine vessels, power plant emissions, and pollution from the city of Portland. After this Pb is released into the atmosphere, it can be transported and accumulate in the environment. Ephiphytic lichens accumulate metals in their tissue, absorbing them directly from the atmosphere. Their tissue concentrations reflect environmental metal levels and can be used to assess air quality over a large geographical range. This study uses lichens to evaluate decadal changes in atmospheric Pb deposition in the CRG and to identify Pb sources and relative contribution using Pb isotope ratios.

The USDA Forest Service Air Program collected lichen samples from the CRG between 1993 and 2014. Lichen Pb concentrations reveal a steep decline in the atmospheric deposition of Pb over the past 20 years. The maximum observed Pb concentration decreased by a factor of ~10 between 1993-1994 and 2013–2014 for each of the three target species collected in the highest abundance: from 94 to 9.2 ppm for Platismatia glauca (87 samples), 50 to 3.8 ppm for Evernia prunastri (112 samples), and 45 to 5.6 ppm for Hypogymnia inactiva (47 samples). Pb isotope ratios will be used to evaluate the relative importance of pollution from the city of Portland to the west, a coal-fired power plant to the east, vehicular and train traffic, coal being transported in open cars, and historical leaded gasoline emissions.