Reconstructing the history of mining and remediation in the Coeur d'Alene, Idaho Mining District using lake sediments

- Lake Coeur d'Alene sediments are a record of mining contamination and remediation.
- Metal(loid) loading decreased after installation of tailings ponds.
- Arsenic concentrations show evidence of diagenesis.
- Sedimentary Cr and Pb concentrations remain at mining era levels.
- Improved management of tailings containing toxic Pb concentrations is required.

Mining that began in the late 1800s intensified during World War II contaminating Lake Coeur d'Alene sediments with potentially toxic elements. We used 80 y of the sediment record to reconstruct metal(loid) loadings to the lake and quantitatively evaluate the effectiveness of tailings management. Sediment core analysis for pollen, chronological markers, and metal(loid)s permitted stratigraphic reconstruction showing that contaminant loading decreased after tailings pond construction, but that most metal(loid) concentrations exceed recommended limits. Arsenic concentrations (250-450 mg kg−1) at the sediment-water interface are potentially toxic; however, low P concentrations in recent sediments (1.0-1.4 mg kg−1) inhibit eutrophication and the concomitant release of soluble As. Zinc (3 g kg−1), Cd (10 mg kg−1), Ag (10 mg kg−1), and Cu (90 mg kg−1) concentrations are now lower than in sediments deposited during active mining, but remain an environmental concern. Sedimentary Cr and Pb concentrations have not changed in the last 50 y, because tailings continue to enter the lake. Although modern Cr concentrations (40 mg kg−1) are unlikely to cause toxicity, current Pb concentrations (4 g kg−1) exceed acceptable limits, creating challenges for remediation. Strategies to manage other mining-contaminated watersheds should include consideration of elemental differences when evaluating remediation effectiveness.