ENVIRONMENTAL FACTORS THAT AFFECT THE RELEASE OF METALS FROM MINING-CONTAMINATED SEDIMENTS

Surface waters contaminated by historical metal mining practices typically have low pH values and high metal concentrations, which may be toxic to aquatic organisms. In this study, we investigated environmental factors that affect the release of metals from mining-contaminated sediments. Metal-contaminated lake sediments were collected downstream from the Silver Valley Mining District in northern Idaho, dried in an oven, and then dry sieved using the following mesh sizes: 0.105, 0.150, 0.180 and <2 mm. To determine the total recoverable metals, the sediments were digested with concentrated nitric acid and hydrogen peroxide while under heat. To examine the environmental conditions which affect leaching (in this case, grain size and flowing versus stagnant water), we placed ~10 g of each sediment size fraction in separate glass burettes and seeped 0.1 M NaCl to simulate average stream water through the sediments. Sediment digests and solutions from the columns were analyzed for Ca, Na, and trace metals (Cd, Pb, and Zn) using an ICP-OES.

Preliminary data indicate initial leaches of the columns remove larger amounts of Ca, Cd, Pb and Zn, and concentrations level off as more solution moves through the column. For example, 70.6 μg Zn/g soil was leached from the 0.180-2.0 mm sediment fraction by the first 40 mL solution, which equated to 8% of the total Zn in the sediments, and decreased to ~1 μg/g soil after 500 mL was passed through. Initial leaches removed 6% and 0.12% of total Cd and Pb, respectively (1.08 and 1.97 μg/g soil). However, after the sediments sat in a stagnant solution overnight, the amount of leached Ca, Cd, Pb and Zn increased again. For example, the amount of Pb and Zn released after saturation was 86% and 32% of the initial amount leached.

Smaller concentrations of Ca, Pb, Zn, and larger concentrations of Cd were released from the sediments in the 0.105-0.150 mm fraction column when compared to the larger fraction size. In comparison, the total percent leached from the 0.180-2.0 mm and 0.105-0.150 mm fractions are 0.2% and 0.05% Pb, 24% and 2.4% Ca, and 7.6% and 8.2% Cd, accordingly. The results of this study demonstrate great potential for simulating environmental saturation conditions and their effects on releasing metals into aquatic ecosystems.