Leachate Experiments to Evaluate Weathering of Waste Rock for Backfill Aquifers in Restored Coal Mine Pits, Powder River Basin, USA

Abstract

Restoration of open-pit mines may utilize waste rock for landscape reconstruction, which can include the construction of backfill aquifers. Weathering and contaminant transport may be different in backfill aquifers compared to the surrounding aquifer because of newly available mineral surfaces and transportable nano- to micro-scale particles generated during mining. Waste rock from the Cordero Rojo open-pit coal mine in the Powder River Basin was exposed to benchtop leachate experiments for 20 weeks at temperatures of 5 °C and 20 °C. Collected leachate was analyzed for Eh, pH, specific conductance, alkalinity, and cation and anion concentrations as unfiltered and 0.45-μm and 0.2-μm filtered concentrations. During the experiment, leachate Eh and pH substantially varied during the first 55 days, which corresponds to a period of high specific conductance (>1000 µS/cm) and alkalinity (>200 mg/L). Correspondingly, anion and cation concentrations were the largest during this early weathering stage, and the filter fractions indicated multiple forms of transported elements. After this early weathering stage, column leachate evolved towards a weathering equilibrium of neutral, oxidizing, and low solute conditions indicated by positive Eh values, pH near 7, and specific conductance <500 μS/cm. This evolution was reflected in the decline and stabilization or non-detection of metal(loid) concentrations reflective of a shift to primarily bulk aluminosilicate weathering when coal- and salt-associated elements, such as arsenic, cadmium, and selenium, were not detected or at minimal concentrations. Over the course of the experiment, the solute trend of certain elements indicated particular weathering processes—cadmium and nanoparticle transport, selenium and salt dissolution, and arsenic and pyrite oxidation. The mining of overburden formations and use of the waste rock for backfill aquifers as part of landscape reconstruction will create newly available mineral surfaces and nanoparticles that will weather to produce solute concentrations not typically found in groundwater associated with the original overburden.

Kinetic Model Evaluation of Arsenic and Selenium Sources and Mobility in Waste Rock of the Powder River Basin, USA

“…The release of As and Se from the waste rock in the leach column were part of the release of the larger solute body (Martin and Langman 2024) that produced a typical weathering trend as indicated by a quick and early peak in speci ic conductance followed by a slower decrease until Day 40 and a inal stage of apparent equilibrated weathering (Fig. 9).…”

“…2003; American Society for Testing and Materials 2016) and screened to < 6.3 mm in the ield (standard sample size discrimination for kinetic/humidity columns (Lapakko and White 2000;Lapakko 2003; American Society for Testing and Materials 2018). The 300 kg of screened waste rock was sealed in 0.02 m 3 buckets and transported to the University of Idaho where the waste rock was temporarily stored at 5 °C until dried at 125 °C for 48 hours (Martin and Langman 2024).…”

“…Duplicate samples were randomly collected during each leachate collection to assess analysis accuracy during the 20-week period. Greater detail of the leach column experiment is presented in Martin and Langman (2024), which describes an evaluation of leachate trends for ield parameters and high-concentration solutes released from the waste rock.…”