Determining a Constituent Release Index From Overburden Material Using Laboratory Weathering Experiments.

The Appalachian coal industry has been successful in developing technologies to identify, handle, treat, and isolate potentially acid-forming overburden materials during coal mining to reduce or eliminate acid mine drainage. However, the techniques used to predict acid mine drainage potential have not been tested for total dissolved solids (TDS) prediction; therefore, new techniques may need to be devised. Current recommendations to blend acidic and alkaline overburden can lead to handling plans that actually increase TDS release, which may be toxic to benthic macroinvertebrates and fish at levels exceeding the maximum contaminant level of 500 mg L^-1. Our objective was to develop quick laboratory methods for predicting TDS release from overburden materials in the Appalachian region by comparing two extracting solutions and one digestion technique to Acid-Base Accounting (ABA) parameters, Virginia Tech leaching columns, and University of Kentucky field studies. Forty-one overburden samples were collected from surface mines in West Virginia, Virginia, and Kentucky. Ground samples were mixed with a dilute nitric acid (HNO₃ 0.016 M) solution or Ethylenediaminetetraacetic acid (EDTA 0.5 M) solution, and shaken for up to one week to simulate weathering. Microwave digestion (USEPA Method 3051) using nitric acid and hydrochloric acid was performed to provide the upper limit on TDS release. Supernatants were extracted and filtered to be analyzed for pH, electrical conductivity (EC), cations (Al^3+, Fe³⁺, Mn²⁺, Mg²⁺, Ca²⁺, P, Na⁺ and K⁺) by Inductively Coupled Plasma- Optical Emission Spectroscopy, as well as anions (Br^1-, Cl^1-,NO₃^1-, PO₄^3-, and SO₄^2- ) by Ion Chromatography. ABA parameters, which measure percent sulfur (% S) and neutralization potential (NP), were compared to results from our weathering tests. Results showed that predictions of TDS from the ABA were similar to the actual release of constituents in our weathering study. Samples highest in percent sulfur and NP released the highest concentrations of elements, including Fe, Al, and Mn, and Ca, which are primary elements of concern in acid mine drainage. Samples with lower sulfur and NP contents released lower quantities of TDS. Samples mixed with dilute HNO₃ released two to six times more constituents than samples shaken with EDTA. Of the 41 samples, 60% were significantly correlated to the leaching columns at Virginia Tech, with Ca²⁺ being the most highly correlated constituent.