WATER QUALITY TRENDS IN A FLOODED 35 YEAR OLD MINE-POOL
Journal American Society of Mining and Reclamation
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
... ess 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. Abstract https://doi.