Mountaintop Removal Coal Mining Contaminates Snowpack across a Broad Region

Mountaintop removal coal mining is a source of downstream pollution. Here, we show that mountaintop removal coal mining also pollutes ecosystems downwind. We sampled regional snowpack near the end of winter along a transect of sites located 3–60 km downwind of coal mining in the Elk River valley of British Columbia, Canada. Vast quantities of polycyclic aromatic compounds (PACs), a toxic class of organic contaminants, are emitted and transported atmospherically far from emission sources. Summed PAC (ΣPAC) snowpack concentrations ranged from 29–94,866 ng/L. Snowpack ΣPAC loads, which account for variable snowpack depth, ranged from <10 μg/m2 at sites >50 km southeast of the mines to >1000 μg/m2 at sites in the Elk River valley near mining operations, with one site >15,000 μg/m2. Outside of the Elk River valley, snowpack ΣPAC loads exhibited a clear spatial pattern decreasing away from the mines. The compositional fingerprint of this PAC pollution matches closely with Elk River valley coal. Beyond our study region, modeling results suggest a depositional footprint extending across western Canada and the northwestern United States. These findings carry important implications for receiving ecosystems and for communities located close to mountaintop removal coal mines exposed to air pollution elevated in PACs.

The rapid increase in coal production in the 1970s reflects the onset of mountaintop removal coal mining.Together, the Fording River and Greenhills mines have accounted for roughly half of total coal production over the past three years.(unfiltered) fractions in g/L.Not all snowpack samples were analyzed for trace elements.Also shown is the reporting limit (RL, also known as the method detection limit) and concentrations in the procedural blank.Many elements were below the RL in virtually every sample analyzed (e.g., total and dissolved Se, dissolved Pb, etc.)   Table S1.Snowpack sampling sites and measured snow water equivalents (SWE).Site names list the nearest mine (GH = Greenhills; EV = Elk Valley; FR = Fording River; LC = Line Creek), the cardinal direction to the sampling site from the mine, and the distance to the nearest mine in km.For example, site GH-SE-8 is located approximately 8 km southeast of the Greenhills Mine.Sites marked with an * indicate a duplicate snow pit dug at the same general sampling location.

Figure S1 .
Figure S1.Total coal production through time in the Elk River valley (A) and over the past three years at the four Elk River valley coal mines (B) in million tons of clean coal.1 The rapid increase in coal production in the 1970s reflects the onset of mountaintop removal coal mining.Together, the Fording River and Greenhills mines have accounted for roughly half of total coal production over the past three years.

Figure S2 .
Figure S2.Daily snow water equivalent (SWE) for 2021-2022 and 2022-2023 along with the range of historical quartiles for each station.Snow pillow SWE data are from: https://rivers.alberta.ca/.

Figure S3 .Figure S4 .
Figure S3.(A) Summed polycyclic aromatic compound (ΣPAC) concentrations for all 2022 and 2023 snowpack samples on a log scale.(B) The percent alkylated of the ƩPAC concentration on a linear scale.

Figure S5 .
Figure S5.Snowpack trace element concentrations in both (A) dissolved (filtered) and (B) total (unfiltered) fractions in g/L.Not all snowpack samples were analyzed for trace elements.Also shown is the reporting limit (RL, also known as the method detection limit) and concentrations in the procedural blank.Many elements were below the RL in virtually every sample analyzed (e.g., total and dissolved Se, dissolved Pb, etc.)

Figure S6 .
Figure S6.Modeled HYSPLIT forward air mass trajectories over three days at twice daily timesteps using North American Mesoscale Forecast System archived forecast data.In contrast with Figure 4, in this projection only grid cells with residence time density (normalized) values >0.4 are shown.Using the center of each grid cell (black dots) reveals that one of twelve grid cells lie within BC -the remainder are located within Alberta.

Figure S7 .
Figure S7.The sum of the same 17 PACs measured by Usenko et al. 2 in their study of western United States National Parks.Usenko et al. reported ƩPAC loads of 310 g/m 2 at Snyder Lake (west of the Continental Divide) and 40 g/m 2 at Oldman Lake (east of the Continental Divide).However, Usenko et al. measured 16 parent (unsubstituted) PACs plus retene.They did not measure alkylated homologs, which are the dominant species in our samples and in coal (see Figure 3 in the main text).

Table S2 .
List of the various unsubstituted (parent) and alkylated PAC homologs measured as part of this study and average percent recovery of spike (surrogates).

Table S3 .
Snowpack ΣPAC concentrations in the various field, laboratory, and procedural blanks collected as part of this study.The procedural blank included pouring laboratory blank water over our sampling shovel and into our sampling container.