Nutrient and Mercury Concentrations and Loads in Tahoe Basin Snowpack

Approximately seventy percent of Lake Tahoe Basin precipitation falls as snow during the winter and spring. During snowpack storage, chemicals that accumulate throughout the season through wet and dry deposition are subject to transformations and emissions that affect the end-of-season chemical load in runoff and infiltrating groundwater. This study describes dynamics of nitrogen (N), phosphorus (P), and mercury (Hg) concentrations and loads in Tahoe Basin snowpack to fill a gap in the watershed's nutrient and pollutant mass balance. Bi-weekly snowpack cores and storm-based surface samples were collected at seven sites along two elevation gradients in the Tahoe Basin during the 2012 and 2013 snow years. Snowpack N content is controlled largely by deposition of nitrate (NO3-) and total ammonia nitrogen (TAN: NH3 + NH4+). NO3- deposition is linked with snow accumulation and snowpack concentrations are consistent throughout the sampling seasons. NO3- snowpack concentrations have no discernible spatial pattern and are likely driven by NOx emissions from out-of-basin sources. Unlike NO3-, TAN deposition is associated with dry deposition and concentrations increase towards the end of winter. This late season influx of TAN is likely connected with increased vertical mixing of the boundary layer and the onset of agricultural activity in the San Joaquin Valley. P deposition is strongly correlated with both longitude and elevation. These spatial patterns of P loading are consistent with particulate-bound dry deposition, originating mainly from in-basin urban sources. Lastly, Hg deposition shows little spatial or temporal variability throughout the Basin. This pattern is consistent with out-of-basin sourcing, likely from global background atmospheric concentrations. Hg speciation shows a post-depositional shift from dissolved to particulate phase as the dominant form. This shift is consistent photochemical induced gaseous emission of dissolved Hg and preferential retention of particulate Hg within the snowpack. Along with this change, snowpack Hg concentrations increase with elevation, possibly due to decreased light penetration into deeper snowpack and reduced photochemical reemission. Study results reflect the highly dynamic nature of snowpack chemical storage and its role in chemical loading to alpine watersheds.