INVESTIGATION OF HYDROXIDE MINERAL PRECIPITATION AND THE FATE OF TRACE METALS IN AN ACIDIC PIT LAKE, ANTHRACITE DISTRICT, PENNSYLVANIA

The Pond Creek abandoned mine site in Freeland, Pennsylvania, contains a chain of over six of pit lakes that range in pH from 4.33 to 3.65. Preliminary water quality data show elevated levels of Fe (3.21 mg/L), Mn (0.66 mg/L), and Al (2.22 mg/L) within surface and ground water. Concentrations of Fe, Al, Zn, Co, and Ni increase from the lake surface to a depth of 5 m, and decrease from a depth of 5 m to the bottom of the lake (25 m). We hypothesize that the Fe and Al trends are the result of metal-hydroxide precipitation and sedimentation whereas the Zn, Co, and Ni trends are the product of adsorption onto hydroxide surfaces. The geochemical speciation program PHREEQCi shows ferrihydrite, goethite, and gibbsite to be near saturation at the lake bottom with mineral saturation indices ranging from -1 to +1. Semi-quantitative X-ray fluorescence (XRF) analysis on lake sediments collected at a depth of 2 m showed an average composition of Si (58 wt%), Al (17 wt%), Fe (14 wt%), and K (9 wt%), whereas sediments collected at a depth of 25 m showed Si (47 wt%), Fe (25 wt%), Al (20 wt%), and K (7 wt%). The 10 wt% increase in Fe concentration and 3% increase in Al concentration between shallow and deep sediments suggests that Fe- and Al-bearing phases may be precipitating at depth. We used X-ray diffraction techniques to identify the minerals present in lake sediments collected at 2 m and 25 m depth, and quantitative XRF to define metal concentrations in the sediment. Measured increases in Fe and Al concentrations will validate previous semi-quantitative observations, and comparisons of dissolved Zn, Co, and Ni concentrations to sediment concentrations collected at 25 m may indicate the sequestering of these metals through surface adsorption. Paradoxically, hydroxide precipitation reactions contribute to lake acidity and degrade water quality, whereas surface adsorption reactions remove trace metals from solution and thus improve water quality. Determining the mechanisms which control these reactions will be key to identifying strategies to improve pit lake water quality in this region.