114-2 Persistent Metal Loading from Dispersed Legacy Mine Waste in a Remediated Mountain Watershed

Session: Environmental Geochemistry and Health (Part I)

Presenting Author:

Jean Morrison

Authors:

Morrison, Jean M.¹, Mills, Christopher T.², Walton-Day, Katie³, Runkel, Robert L.⁴, Croteau, Marie-Noele⁵, McGee, Ben N⁶, Manning, Andrew H.⁷, Campbell, Kate Marie⁸, Kane, Tyler James⁹, Wanty, Richard B¹⁰

(1) United States Geological Survey, Denver, CO, , (2) United States Geological Survey, DENVER, CO, , (3) U.S. Geological Survey, Denver, CO, , (4) US Geological Survey, MS 415, Federal Ctr, Denver, CO, USA, (5) United States Geological Survey, Moffett Field, CA, USA, (6) United States Geological Survey, Denver, CO, USA, (7) United States Geological Survey, Denver, CO, , (8) USGS, G3 Science Center, Boulder, CO, , (9) U.S. Geological Survey, Boulder, CO, , (10) Colorado School of Mines, Golden, CO, ,

Abstract:

Restoring watersheds affected by legacy mining requires consideration of mine waste dispersed in streambed and alluvial sediments. The Akron Mine in the Tomichi Creek Mining District, Colorado (USA), operated from the 1880s to the 1950s, producing Pb, Ag, Zn, Cu, and Au. Reclamation of the mine and mill sites (2015-2016) improved water quality in Tomichi Creek immediately downstream. However, a September 2021 baseflow synoptic study of an approximately 4 km reach of this circumneutral stream showed downstream increases in concentrations (0.45-µm filtered) of Pb (from <1 to 3 µg/L), Cd (from 0.1 to 0.4 µg/L), and Zn (from 10 to 70 µg/L). Much of the increased metal loading occurred just below two separate beaver ponds. The loading increases occurred with negligible increase in discharge suggesting inputs were due to seepage with relatively high metal concentrations and/or stream water interaction with mine wastes dispersed in bed sediments and alluvium.

Follow-up investigations (2023-2025) evaluated stream-water chemistry under different discharge conditions (spring runoff to baseflow) and interactions between stream water and dispersed mine waste in sediments and alluvium. Stream-water concentrations (0.45-µm filtered) of Cd, Zn, and Pb again increased downstream of the mine-mill site but were less than those observed in 2021, including in Oct. 2024 when stream conductivity and major ion concentrations were similar to those observed in 2021 suggesting variability in metal transport. Metal loads were greatest during spring runoff. Filtration through 0.2- and 0.1-µm pore sizes and ultrafiltration showed Pb associated with Fe-rich colloids whereas Cd and Zn were predominantly dissolved.  

Fine-grained streambed sediments (<64 µm) contained elevated Pb (2,100-3,600 ppm), Zn (1,700-3,800 ppm), and Cd (7-21 ppm) throughout the reach, although Cd and Zn were substantially lower near the Akron mine-mill site. Substantially higher metal concentrations were observed in alluvial material previously submerged by beaver ponds during the 2021 study, where metal loads increased. Concentrations of Pb, Zn, and Cd in associated seep and surface waters exceeded stream water concentration by an order of magnitude or more and represent a persistent source of downstream metal loading. Elevated Pb, Zn, and Cd measured in aquatic invertebrates indicate a pathway for ecological exposure and may contribute to impaired trout populations observed at some locations in the stream.

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