INFLUENCE OF INCREASED HYDROLOGIC INPUT ON THE WATER CHEMISTRY OF AN ACID MINE DRAINAGE SYSTEM

To evaluate fluctuations in water chemistry due to increased hydrologic input, acid mine drainage (AMD) discharging from the abandoned Green Valley coal mine site, western Indiana, and water in a nearby natural stream receiving AMD discharge were monitored weekly from May through September, 2003. During the sampling period, the acidic effluent ranged from pH 2.65 during dry periods to 4.15 subsequent to increase hydrologic input due to rainfall exceeding 5 cm. Similarly, pH of the impacted stream measured as low as 4.06 during dry periods and increased to 6.60 after rainfall. AMD elemental concentrations decreased by up to three orders of magnitude in association with increased discharge with Fe ranging from 18,700 to 641 ppm, Al from 2,900 to 86 ppm, Zn from 4.3 to 0.28 ppm, Li from 2.8 to 0.21 ppm, B from 1.9 to 0.3 ppm, Ni from 0.65 to 0.036 ppm, V from 0.47 to 0.11 ppm, Cr from 0.18 to 0.01 ppm, and Co from 0.05 to 0.001 ppm; higher values represent concentrations during dry periods and lower values represent concentrations during increased discharge. Rainfall similarly impacted water chemistry in the stream with Fe varying from 310 to <10 ppm, Al from 84 to <1 ppm, B from 0.7 to 0.1 ppm, Li from 0.3 to <0.003 ppm, Zn from 0.22 to 0.009 ppm, Ni from 0.18 to 0.004 ppm, Co from 0.03 to 0.0001 ppm, and Cr from 0.007 to 0.002 ppm. These variations are likely due to dilution associated with rainfall and corresponding adsorption of trace elements on Fe and Al colloids that commonly form in AMD systems with increasing pH. Vanadium in the impacted stream was the only element showing a dissimilar trend, measuring up to 0.3 ppm during increased discharge and dropping to 0.075 during the dry period. The data also indicate that within 36 hours after a rainfall event water chemistry returned to within 80% of the concentrations prior to rainfall, suggesting increased hydrologic discharge has a short-lived effect on the AMD system.