USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Refining estimates of atmospheric deposition for sediment particles and particulate nutrients in the Lake Tahoe Basin

Principal Investigators:
Antony Chen, John Watson, and Xiaoliang Wang, Desert Research Institute
Wen-Ming Chien, University of Nevada-Reno

Proposal [pdf]

Final Report [pdf]

Project Summary

Lake Tahoe, an oligotrophic lake appreciated for its fresh water and geographic setting, has been reported of its declining water clarity over the past few decades due to nutrient and sediment particle inputs. Contributions from atmospheric deposition of particulate matter (PM) have been suggested to be substantial, yet inadequately quantified. This study established three long-term monitoring sites (July 2013 – August 2014) to measure 24-hr, size-resolved dry and wet PM deposition in near-shore, offshore-background, and upper watershed conditions in the Lake Tahoe Basin. The objectives are to: 1) investigate spatiotemporal variations of PM deposition flux, 2)obtain dry deposition velocity using mass deposition flux and PM concentration measurements,and 3) provide estimated annual number and mass deposition flux (NDF and MDF, respectively)via dry and wet processes. Dry deposition was quantified on passive substrates using a scanningelectron microscope (SEM), while wet deposition was based on particles suspended in rain and/orsnow water analyzed by laser diffraction spectroscopy. MDF were derived from NDF assumingspherical particles and size-specific particle densities.

The seasonal and annual NDFdry flux at the three monitoring sites showed log-normal size distributions where particles of 0.5 – 1 μm and 4 – 10 μm diameter were the most abundant among five size bins (0.5 – 1 μm, 1 – 2.5 μm, 2.5 – 4 μm, 4 – 10 μm, 10 – 20 μm, 20 – 32 μm, and 32 – 64 μm). For the size range of 0.5 – 20 μm, all seasonal NDFwet exceed NDFdry though its size distribution is more skewed towards fine particles < 2.5 μm which contribute little to the particle mass. Dry deposition velocity appears to increase rapidly with particle size, while wet deposition velocity is more uniform across all size ranges. Higher NDFdry and greater monthly variability are found at the near-shore than the offshore and upper-watershed sites, suggesting substantial impacts from nearby beach, traffic, and construction activities. Higher NDFwet in spring across all three sites is consistent with increased precipitation.

The annual NDFdry+wet of "sediment only" particles (0.5 – 20 μm) is estimated to be between 6.18×1010 and 1.12×1011 # m-2 year-1, pretty consistent with a previous estimate of 1.01×1011 #m-2 year-1 (0.5–16 μm) by the Lake Tahoe Total Maximum Daily Load (TMDL) using ambient concentration and modeled deposition velocity. However, the annual MDFdry+wet of 14.1 to 19.2 MT km-2 year-1 from this study is several times even the upper TMDL estimate of 2.35 MT km-2 year-1, despite that the partitions between dry and wet deposition do not differ very much. The discrepancy is mainly attributed to different size distributions in NDF, particularly NDFdry, between the two studies. Overall, dry deposition accounts for only 13 ‒ 24% of deposited particle number but 67 – 84% of particle mass. Accounting for all potential uncertainties, TMDL likely underestimates MDFs and the impact of atmospheric deposition on lake clarity.

This study proves passive particle collection using an automated sampler, coupled with microscopic counting of individual particles, to be efficient for quantifying daily, size-resolved particle deposition fluxes. NDFdry by this approach was verified with results of a larger-footprint eddy correlation method through a two-week collocated campaign. Moreover, the passive samples allowed analysis using a computer controlled SEM (CCSEM) technique to yield elemental composition of single particles, based on which particles could be classified. It was found that 74 –87% of coarse and large particles > 4 μm diameter are mineral dust. Sulfur-contained andphosphorus-contained particles, though minor, were found at all three sites, suggesting anappreciable loading of pollutant and nutrient into the lake through atmospheric deposition.