Geology topics

Research about Clarion County, Pennsylvania

Source-linked reports with geographic coverage including Clarion County, Pennsylvania.

3 recordsLinked to original sources

USGS1994

Geochemical and geohydrological characteristics of bedrock and spol from two methods of mining at a reclaimed surface coal mine, Clarion County, PA, USA

Two methods of mining caused subtle differences in geochemical and geohydrological characteristics of spoil at a reclaimed surface coal mine in western Pennsylvania. A dragline was used in the southern area of the mine, and bulldozers and front-end loaders were used in the northern area. Mining methods used in the intervening, middle area are uncertain. In general, overburden at the mine consisted of sideritic gray shale and siltstone. Calcareous zones were laterally discontinuous. However, a 1.2-m thick stratum of pyritic shale above the mined coal was laterally continuous and had total sulfur (S) concentrations >2.5 weight percent (wt %). Regardless of mining methods, pyritic material in backfill is inverted relative to its stratigraphic sequence in bedrock. Where bulldozers and front-end loaders were used, the pyritic shale was selectively handled and buried in compacted layers above the water table, and only low-S ( <0.2 wt%) material was buried near the pit floor. Where the dragline was used, high-S ( ≥.5 wt %) material was placed near the surface, but above intermediate-S material. In the middle area, where middle mining methods are uncertain, high-S material was randomly distributed, ear the surface and on the pit floor, within the zone of water-table fluctuation. In the northern and middle areas, mass-weighted average S in spoil was comparable to that in premining bedrock. In contrast, average neutralization potential of spoil was about one-third of that of premining bedrock, possibly because of preferential weathering of carbonates in shallow bedrock (premining) or spoil. Despite differences in mining methods, hydraulic conductivities for spoil were similar among the northern, middle, and southern areas, ranging from 10 -8.2 to 10 -3.0 meters per second (m/s), with median hydraulic conductivities from 10 -3.8 to 10 -3.6 m/s. Hydraulic conductivities for spoil were not always greater than those for underlying bedrock.

Pennsylvania↗

USGS1994

Effects of selective handling of pyritic, acid-forming materials on the chemistry of pore gas and ground water at a reclaimed surface coal mine in Clarion County, PA, USA

A change from dragline to “selective handling” mining methods at a reclaimed surface coal mine in western Pennsylvania did not significantly affect concentrations of metals in ground water because oxidation of pyrite and dissolution of siderite were not abated. Throughout the mine, placement of pyritic material near the land surface facilitated the oxidation of pyrite, causing the consumption of oxygen (O 2 ) and release of acid, iron, and sulfate ions. Locally in the unsaturated zone, water sampled within or near pyritic zones was acidic, with concentrations of sulfate exceeding 3,000 milligrams per liter (mg/L). However, acidic conditions generally did not persist below the water table because of neutralization by carbonate minerals. Dissolution of calcite, dolomite, and siderite in unsaturated and saturated zones produced elevated concentrations of carbon dioxide (CO 2 ), alkalinity, calcium, magnesium, iron, and manganese. Alkalinity concentrations of 600 to 800 mg/L as CaCO 3 were common in water samples from the unsaturated zone in spoil, and alkalinities of 100 to 400 mg/L as CaCO 3 were common in ground-water samples from the underlying saturated zone in spoil and bedrock. Saturation indices indicated that siderite could dissolve in water throughout the spoil, but that calcite dissolution or precipitation could occur locally. Calcite dissolution could be promoted as a result of pyrite oxidation, gypsum precipitation, and calcium ion exchange for sodium. Calcite precipitation could be promoted by evapotranspiration and siderite dissolution, and corresponding increases in concentrations of alkalinity and other solutes. Partial pressures of O 2 (Po 2 ) and CO 2 (Pco 2 ) in spoil pore gas indicated that oxidation of pyrite and precipitation of ferric hydroxide, coupled with dissolution of calcite, dolomite, and siderite were the primary reactions affecting water quality. Highest vertical gradients in Po 2 , particularly in the near-surface zone (0-1 m), did not correlate with concentrations of total sulfur in spoil. This lack of correlation could indicate that total sulfur concentrations in spoil do not reflect the amount of reactive pyrite or that oxidation rates can be controlled more by rates of O 2 diffusion than the amount of pyrite. Hence, if placed in O 2 -rich zones near the land surface, even small amounts of disseminated pyritic material can be relatively significant sources of acid and mineralized water.

Pennsylvania↗

USGS1991

Geochemical evolution of acidic ground water at a reclaimed surface coal mine in western Pennsylvania

Concentrations of dissolved sulfate and acidity in ground water increase downflow in mine spoil and underlying bedrock at a reclaimed surface coal mine in the bituminous field of western Pennsylvania. Elevated dissolved sulfate and negligible oxygen in ground water from bedrock about 100 feet below the water table suggest that pyritic sulfur is oxidized below the water table, in a system closed to oxygen. Geochemical models for the oxidation of pyrite (FeS 2 ) and production of sulfate (SO 4 2- ) and acid (H + ) are presented to explain the potential role of oxygen (O 2 ) and ferric iron (Fe 3+ ) as oxidants. Oxidation of pyrite by O 2 and Fe 3+ can occur under oxic conditions above the water table, whereas oxidation by Fe 3+ also can occur under anoxic conditions below the water table. The hydrated ferric-sulfate minerals roemerite [Fe 2+ Fe 4 3+ (SO 4 ) 4 ·14H 2 O], copiapite [Fe 2+ Fe 4 3+ (SO 4 ) 6 (OH) 2 ·20H 2 0], and coquimbite [Fe 2 (SO 4 ) 3 · 9H 2 O] were identified with FeS 2 in coal samples, and form on the oxidizing surface of pyrite in an oxic system above the water table. These soluble ferric-sulfate 11 salts11 can dissolve with recharge waters or a rising water table releasing Fe 3+ , SO 4 2- . and H + , which can be transported along closed-system ground-water flow paths to pyrite reaction sites where O 2 may be absent. The Fe 3+ transported to these sites can oxidize pyritic sulfur. The computer programs WATEQ4F and NEWBAL were used to compute chemical speciation and mass transfer, respectively, considering mineral dissolution and precipitation reactions plus mixing of waters from different upflow zones. Alternative mass-balance models indicate that (a) extremely large quantities of O 2 , over 100 times its aqueous solubility, can generate the observed concentrations of dissolved SO 4 2- from FeS 2 , or (b) under anoxic conditions, Fe 3+ from dissolved ferric-sulfate minerals can oxidize FeS 2 along closed-system ground-water flow paths. In a system open to O 2 , such as in the unsaturated zone, the aqueous solubility of O 2 is not limiting, and oxidation of pyrite by O 2 and Fe 3+ accounts for most SO 4 2- and Fe 2+ observed in acidic ground water. However, in a system closed to O 2 , such as in the saturated zone, O 2 solubility is limiting; hence, ferric oxidation of pyrite is a reasonable explanation for the observed elevated SO 4 2- with increasing depth below the water table.

Pennsylvania↗