SEASONAL CHANGES IN DIEL CYCLING OF DISSOLVED IRON CONCENTRATIONS IN A COAL MINE DRAINAGE-IMPACTED WETLAND

Diel fluctuations of pH, specific conductance, and iron species have been documented in Dillan Creek, a small coal mine-impacted tributary to Deckers Creek, Masontown WV. Samples were collected hourly over 16-24 hour periods and analyzed for dissolved ferrous and dissolved total (ferrous plus ferric) iron using a Hach DR2800 spectrophotometer. Fe(II) concentrations increased during the day and decreased at night during winter sampling. Total Fe concentration showed an overall slight increase of 0.15 mg/L during the winter sampling period. Sampling conducted in the summer shows an opposite trend with Fe(II) increasing at night and decreasing during the day. Total Fe concentration cycles during the summer mimicked Fe(II) with a maximum concentration of 4.61 mg/L at 5 AM and a minimum concentration of 3.39 mg/L at 5 PM. One possible explanation is evapotranspiration-controlled pumping that alters the mixing ratio between the creek water and Fe-rich ground water from the riparian zone. In order to evaluate the pumping mechanism, a transect of piezometers was installed at variable depths in the creek and in the riparian zone. Depth-to-water was measured in all piezometers during diel sampling events and continuous dataloggers were used to measure pressure changes in some piezometers. Fe(II) concentrations in Dillan Creek are typically much lower (0.6–4.5 mg/L) than in the surrounding ground water (~30 mg/L). During the winter diel sampling period in March, the water table in the riparian zone bordering Dillan Creek remained approximately constant over the sampling interval. However, during summer sampling in May and June, the water table elevation varied by 0.1 cm over an 18-hour period. A possible explanation for the seasonal change in diel cycles is the increased pumping, and thereby alteration of stream-ground water mixing, during the summer.