Nutrient and mercury deposition and storage in an alpine snowpack of the Sierra Nevada, USA

Abstract

Abstract. Bi-weekly snowpack core samples were collected at seven sites along two elevation gradients in the Tahoe Basin during two consecutive snow years to evaluate total wintertime snowpack accumulation of nutrients and pollutants in a high elevation watershed of the Sierra Nevada. Additional sampling of wet deposition and detailed snow pit profiles was conducted the following year to compare wet deposition to snowpack storage and assess the vertical dynamics of snowpack chemicals. Results show that on average organic N comprised 48% of all snowpack N, while nitrate (NO3--N) and TAN (total ammonia nitrogen) made up 25 and 27%, respectively. Snowpack NO3--N concentrations were relatively uniform across sampling sites over the sampling seasons and showed little difference between seasonal wet deposition and integrated snow pit concentrations in agreement with previous studies that identify wet deposition as the dominant source of wintertime NO3--N deposition. However, vertical snow pit profiles showed highly variable concentrations of NO3--N within the snowpack indicative of additional deposition and in snowpack dynamics. Unlike NO3--N, snowpack TAN doubled towards the end of winter and in addition to wet deposition, had a strong dry deposition component. Organic N concentrations in snowpack were highly variable (from 35 to 70%) and showed no clear temporal or spatial dependence throughout the season. Integrated snowpack organic N concentrations were up to 2.5 times higher than seasonal wet deposition, likely due to microbial immobilization of inorganic N as evident by coinciding increases of organic N and decreases of inorganic N, in deeper, aged snowpack. Spatial and temporal deposition patterns of snowpack P were consistent with particulate-bound dry deposition inputs and strong impacts from in-basin sources causing up to 6 times enrichment at urban locations compared to remote sites. Snowpack Hg showed little temporal variability and was dominated by particulate-bound forms (78% on average). Dissolved Hg concentrations were consistently lower in snowpack than in wet deposition which we attribute to photochemical-driven gaseous remission. In agreement with this pattern is a significant positive relationship between snowpack Hg and elevation, attributed to a combination of increased snow accumulation at higher elevations causing limited light penetration and lower photochemical re-emission losses in deeper, higher elevation snowpack. Finally, estimates of basin-wide loading based on spatially extrapolated concentrations and a satellite-based snow water equivalent reconstruction model identify snowpack chemical loading from atmospheric deposition as a substantial source of nutrients and pollutants to the Lake Tahoe basin, accounting for 113 t of N, 9.3 t of P, and 1.2 kg of Hg each year.

On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition

“…These relationships were particularly evident at the Valhalla site, nearest to the city of South Lake Tahoe. However, some evidence suggests that long-range transport of NH 3 from the San Joaquin Valley may also contribute to N deposition in the Tahoe Basin (Dolislager et al, 2012;Pearson et al, 2015). The study of Rao et al (2014) along a Boston to Harvard Forest urbanization gradient illustrates the principal tenet of our study-that near urban centers or road networks, on-road NH 3 emissions have a considerable effect on N deposition.…”

A synthesis of terrestrial mercury in the western United States: Spatial distribution defined by land cover and plant productivity

“…Across relatively short distances in these regions, there are strong gradients in geographic, climatic, and ecosystem variablesincluding pronounced precipitation gradients such as from the western to eastern slopes of the Sierra Nevada where annual precipitation changes from >150 cm yr -1 to <25 cm yr -1 across a distance of just a few dozen kilometers (WRCC, 2014). The broad range of climate and ecosystem types also leads to shifts in wet versus dry deposition of nutrients and pollutants (Pearson et al, 2015) and is associated with distinct patterns of soil development, plant productivity, as well as carbon and nutrient dynamics, aspects with known implications for the biogeochemistry of Hg. Further, the western U.S. is impacted by long-range Hg transport from global sources including east Asia (Obrist et al, 2008;Strode et al, 2008;Weiss-Penzias et al, 2006), while specific local and regional sources also exist in the West such as from natural emissions, precious metal mining, coal-fired power plants, and other urban and industrial activities (e.g., EPA, 2015;Zehner and Gustin, 2002).…”