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Abstract:The effectiveness of Acid-Base Accounting (ABA) for predicting surface coal mine drainage quality in Pennsylvania was evaluated. Comparisons between ABA and mine drainage alkalinity, acidity, and sulfate were made for 38 mines in the bituminous coalfield. Neutralization Potential (NP), Maximum Potential Acidity (MPA), and Net Neutralization Potential (NNP) were evaluated with and without "thresholds."Calculations using "thresholds" counted only those values for NP greater than 30 mt CaC03/l,OOO mt and percent sulfur greater than o. 5%. "Without thresholds" computations included all values. Stoichiometric equivalence factors of 31.25 and 62.5 were used to compute MPA. NP and NNP are the best predictors of postmining drainage quality. Alkaline or acid drainage quality is controlled by as little as 1% to 3% carbonate in the overburden. NNP less than 1% generally results in acidic drainage, and NNP greater than 3% yields alkaline drainage. An empirical relation exists between alkalinity and NP. Postmining alkalinity can be estimated as 4 to 6 times NP (without thresholds) .MPA is not a reliable predictor of drainage quality, except in the absence of calcareous strata, where a positive relation exists between acidity and MPA.

Inverse geochemical modelling was used to investigate rock–water interactions in flooded underground coal mines in northern Appalachia, USA. In early flooding, Pittsburgh seam mine waters are usually acidic ( c.  pH 3), with dissolved metals Fe and Al ranging from 10 to >100 mg l −1 . Within a few decades, however, waters in fully flooded mines usually have pH of about 7 S.U., and alkalinity >300 mg l −1 CaCO 3 Eq. Eh shifts from oxidizing ( c . 500 to 700 mv) to reduced (−100 to −200 mv) conditions. Sodium concentrations may increase an order of magnitude; sulphate and iron concentrations may also increase. Water samples were collected from several mine-pools in West Virginia and Pennsylvania. A conceptual model was developed based on quantitative hydrology, mine-pool chemistry, mining conditions and mineralogy. The model was tested with the geochemical code PHREEQC. Simulations included mixing recharge and acid mine waters, precipitation–dissolution reactions involving carbonates, sulphates, oxy-hydroxides and sulphides, and ion adsorption and exchange. Na exchange was a dominant process in all models. Carbonates are orders of magnitude undersaturated in the juvenile mine-pool, but near saturation in the mature mine-pool, suggesting they are a primary source of acid neutralization and alkalinity. The mature mine-pool is simultaneously near equilibrium with iron sulphide, iron carbonate and iron oxy-hydroxide mineral phases. The rapid change in mine-pool water quality has substantial implications for management of these systems. Corresponding author eperry@osmre.gov

Major proposed changes in drainage control from a large underground mine pool required a detailed assessment of the hydrology, geochemistry, and impacts on the receiving streams. Proposed changes entail relocation of the withdrawal and treatment facilities to an adjoining, but separate watershed and increasing the pumping rate by 35%. Fifteen major and numerous smaller mines in two coal seams comprise the mine pool system. Degree of interconnection between mines ranges from horizontal and vertical seepage through natural fractures, subsidence-induced fractures, and coal cleat to open pass-throughs, slopes, and shafts. Water levels of several mines rise and fall in a mirrored fashion with only a few meters of head difference. An adjacent mine pool with 59.7 meters of head and an intact barrier ranging from 9.4 to 457 meters thick contributes at least 28% to the discharge rate. Mine storage capacity (5.38 billion liters) equates to a porosity of about 11%, a significant reduction from the original extraction volume of 63%. Mean ground water yield of the complex is 28 million liters per day. Recharge rate of 0.41 liters per minute per hectare is less than expected due to the thick overburden over much of the mine complex. The mine complex responds sharply to large precipitation events and large-volume pumping due to the relatively low storage volume and large aerial extent of the mine complex. Water levels respond to large precipitation events within three days and rises exceeding one meter have been recorded. Conversely, current maximum pumping at 35.6 million liters per day will draw the pool down an average of 0.09 meters per day. The proposed pumping of 37.9 million liters per day will, over the long term, exceed groundwater recharge and will dewater large portions of the mine complex. This will adversely impact the water quality, may induce additional subsidence, and could dewater some domestic water wells. Addition of water from an adjacent mine would allow the discharge of 37.9 million liters per day.

Water quantity and quality conditions are described for a mine-pool aquifer in a fully flooded complex of underground mines in northern West Virginia. Abandoned mines in the Pittsburgh coal bed are contiguous, and separated by coal barrier pillars ranging from as little as 9 to about 60 meters thick. Barrier pillars are transmissive enough to circulate significant quantities of water between mines, yet they control head distribution and flow direction within the aquifer. The mine-pool acts as a partly confined to confined aquifer, and recharge is approximately balanced by withdrawal of about 5700 Liters/minute, leakage to adjacent mines, and unquantified outflow to unmined areas. Resulting drawdown prevents the mine-pool from discharging directly into overlying streams. A centrally located subgroup of mines within the aquifer currently acts as a groundwater sink, but water levels are slowly increasing in the sink, and in some outflow areas. Mine waters are highly reduced, with circumneutral pH, and variable Fe concentrations from 5 to over 100 mg/L. Total alkalinity averages about 200 mg/L with a mixed Ca-Na-HCO 3-SO 4 composition in recharge areas. End of flow path waters contain up to 600 mg/L alkalinity, and are Na-SO 4 type waters with higher dissolved solids and metals concentrations. The shift from Ca to Na dominated waters is attributed mainly to cation exchange. Potentiometric head is increasing in the aquifer, and mine-pool withdrawal may have to be increased to prevent discharge to the surface. Mine-pool quality remains poor, and has shown slow improvement in 6 years of monitoring.