EXPERIMENTAL EVIDENCE FOR GENERATION OF NET ALKALINE MINE DRAINAGE VIA CATION EXCHANGE-ENHANCED LIMESTONE DISSOLUTION, IRWIN COAL BASIN, PENNSYLVANIA

In Appalachian coal fields, the chemistry of some waters in flooded coal mines transitions from net-acid to net-alkaline during flow from the recharge areas to deeper, low O₂ minepools. The high Na content and low Cl values observed in deep alkaline minepools, as well as the positive correlation between Na and HCO₃ concentrations, suggest that cation exchange is involved in the generation of net-alkaline coal mine drainage. Fluid-rock interactions that contribute to natural alkalinity production in coal mine drainage inform predictive remediation models for the treatment of long-term metal release.

The Irwin syncline in southwestern Pennsylvania contains the Pittsburgh Coal; a century of mining resulted in a series of structurally confined minepools with depths from 30 to 90 m and discharges that range in pH from 3.3 to 6.5. The net-alkaline drainages are Na/HCO₃-SO₄ waters with Na up to 463 mg/L. To investigate cation exchange as a mechanism for generation of alkalinity via limestone dissolution, we determined the cation exchange capacity (CEC) of five composite core samples representative of lithologies likely to be in contact with coal mine drainage in the Irwin Coal Basin. The unbuffered salt extraction method with minor modification was used, with a solid to liquid ratio of 2.5g:100mL. For comparison, four clay standards (kaolinite, montmorillonite, illite, and bentonite) were also analyzed. Exchangeable cations (Na, Mg, Ca, K, and Al) were extracted using a 0.2 M NH₄Cl solution, with a standard deviation <28% based on replicate samples. Composite samples of underclay and argillaceous limestone had the highest CEC (14 and 11 meq/100g, respectively), comparable to the bentonite and kaolinite standards, but had significantly more exchangeable Na (1.13 and 1.82 meq/100g, respectively). The data support the potential for significant Na release from exchangeable sites while interacting with Ca-rich fluids, driving carbonate mineral dissolution. These results will allow cation exchange reactions to be quantified (along pyrite oxidation, calcite dissolution, etc.) to inform ongoing inverse and forward models being generated to simulate the evolution of acidic, Ca/SO₄ minewaters to alkaline Na/HCO₃ + SO₄ type minewaters.