Water Quality Trends in a Flooded 35 Year Old Mine-Pool

Abstract

Thirty five years of water quality data from a pumped, mostly flooded, mine-pool were examined for trends in mine drainage parameters. At the start of pumping in 1970, the Lancashire 15 mine-pool discharged acidic water with average iron (Fe) concentration exceeding 900 mg/L. Average sulfate (SO 4) concentration was about 3700 mg/L. After 14 years of pumping about 21 minepool volumes, Fe and SO 4 were about 20% of their initial concentrations. Alkalinity had increased from less than 50 to about 120 mg/L, and pH was about 6.0. In 1986, an overlying mine complex closed and flooded. Its' waters have low concentrations of Fe and SO 4 , and are hydraulically connected to the Lancashire 15 mine-pool. The combined mine-pool waters reduced Fe by about 50% in Lancashire 15. Since 1986, Fe and SO 4 concentrations have continued a slow, irregular decline at the rate of 1 to 2 mg/L/yr for Fe and about 10 to 15 mg/L/yr for SO 4. Short term fluctuations due to seasonal and pump rate variations occur, but long term concentration trends can be described with curvilinear models. The Lancashire 15 discharge is sodium-sulfate (Na-SO 4) type water. Geochemical calculations show that cation exchange of calcium (Ca) for Na is a feasible explanation for the observed water composition. Mixing calculations show that mine-pool composition can be explained by cation exchange; continuing dissolution of iron and sulfur bearing minerals, iron oxyhydroxide formation and about 80% of recharge as leakage from adjacent and overlying mines, and 20% recharge from unmined strata. The Lancashire 15 mine-pool quality has improved significantly since closure and flooding. After leaching an estimated 55 pool volumes, Fe concentrations are about 5% of original values, and the waters are net alkaline. Continued mineral dissolution, and inefficient leaching due to dispersion and short circuiting, are likely responsible for current water quality conditions.

Water Quality Evolution in Flooded and Unflooded Coal Mine-Pools

“…The declining trends have continued in more recent monitoring data for Lancashire 15. Metals and acidity are now about 5% of initial conditions, and SO 4 -2 about 10% (Perry et al, 2005). The T&T 2 mine, which is only about 20% flooded, has shown little change in pH after 25 pool volumes and 10 years of monitoring.…”

Hydrologic Characterization of a Large Underground Mine Pool in Central Pennsylvania

“…Secondly, if the mine is dewatered to a great extent, a large portion of the Lower Kittanning (B) seam mine works will be subaerially exposed. This will cause water quality to deteriorate significantly from the present conditions due to additional pyrite oxidation and subsequent mine drainage production (Perry et al, 2005). This rapid and significant water quality degradation will cause the treatment facilities, designed for present conditions, to fail.…”

“…The water quality issues are addressed by a companion paper in these proceedings (See Perry et al, 2005). The well water loss and receiving stream issues are addressed in the project final report and are not discussed here.…”