Latitudinal gradients of snow contamination in the Rocky Mountains associated with anthropogenic pollutant sources

Seasonal snow is an important source of drinking water, recreation, and agriculture in the Rocky Mountain region. Monitoring snow-water quality is important to assess potential impacts to downstream water resources, impacts to the albedo and energy balance of the snowpack, and assessing sources of natural and anthropogenic aerosols and gases.  Here, a suite of metals were measured from seasonal snowpack from water year (WY) 2018. Calcium, lanthanum, and cerium concentrations support the importance of mineral dust to the southern Rocky Mountains. Mercury (Hg), zinc (Zn), and cadmium (Cd) concentrations show a similar spatial pattern to mineral dust, whereas antimony (Sb) concentrations are highest in the northern Rocky Mountains. Using enrichment factors to adjust for the contributions from mineral dust, it was demonstrated the pollution fraction of Hg, Zn, Sb and Cd was greater in the northern Rocky Mountains. These observations were compared to spatial trends of the pollution fraction of Hg from WY2009 to WY2018, regional monitoring networks, and back trajectory analyses. The agreement between these datasets revealed temporally consistent pollution sources and transport processes to the northern Rocky Mountains snowpack. Potential sources include current and historical mining and smelting in the region. Strategies to limit the emissions of these metals to the Northern Rockies would need to focus on remediation of contaminated sites and continued monitoring and mitigation of active mining and smelting

Methods
Samples analyzed for this study consisted of integrated snowpack samples collected during the spring of WY2018. Samples were collected near peak accumulation, but prior to snowmelt conditions (Ingersoll et al., 2009). Snowpits were dug to the ground and one depth-integrated sample was collected from a clean face of the snowpit. For DRI metal analysis, 49 sites were sampled. One depth-integrated sample was collected using plastic scoops that were  pre-cleaned with 1% nitric acid (HNO3) and rinsed with ultrapure water. Samples were stored in Whirl-Pak bags and kept frozen until analysis. 

At DRI, samples were kept frozen until sample preparation, when they were melted in Whirl-Paks and sonicated for 3 minutes. Samples were then poured into pre-cleaned (1% nitric acid washed), metal-free, 15 mL vials that were pre-rinsed with ultrapure water and dried. Snow samples were then acidified to 1% HNO3 and stored for three months to allow for acid leaching of all elements in the sample (Arienzo et al., 2019). After three months, samples were analyzed for Ca, Ce, La, Cd, Sb, and Zn concentration using a Thermo-Finnigan Element2 (Thermo Scientific, Bremen, Germany) High-Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) instrument at DRI. Samples were introduced to the HR-ICP-MS instrument using an autosampler and prior to injection into the HR-ICP-MS, each discrete sample was acidified to 2% HNO3 with the addition of 115In as an internal standard, similar to the technique used in previous studies (Arienzo et al., 2019). Calibration of the instrument was conducted prior to analysis using a suite of standards with known concentrations that bracketed the sample concentrations. Detection limits were calculated as three times the standard deviation of 10 measurements of blank ultrapure 1% HNO3 that ranged from 0.0001 mg/L for Ce and La to 0.12 mg/L for Ca (Table 2).