Geology topics

Charles A. Cravotta

Publications and source records attributed to Charles A. Cravotta.

70 records · Page 4Linked to original sources

USGS2004

Optimization of limestone drains for long- term treatment of acidic mine drainage, Swatara Creek Basin, Schuylkill County, PA

Limestone drains were constructed in 1995, 1997, and 2000 to treat acidic mine drainage (AMD) from the Orchard, Buck Mtn., and Hegins discharges, respectively, in the Swatara Creek Basin, Southern Anthracite Coalfield, east-central Pennsylvania. This report summarizes the construction characteristics and performance of each of the limestone drains on the basis of influent and effluent quality and laboratory tests of variables affecting limestone dissolution rates. Data for influent and effluent indicate substantial alkalinity production by the Orchard and Buck Mtn. limestone drains and only marginal benefits from the Hegins drain. Nevertheless, the annual alkalinity loading rates have progressively declined with age of all three systems. Collapsible-container (cubitainer) testing was conducted to evaluate current scenarios and possible options for reconstruction and maintenance of the limestone drains to optimize their long-term performance. The cubitainer tests indicated dissolution rates for the current configurations that were in agreement with field flux data (net loading) for alkalinity and dissolved calcium. The dissolution rates in cubitainers were larger for closed conditions than open conditions, but the rates were comparable for coated and uncoated limestone for a given condition. Models developed on the basis of the cubitainer testing indicate (1) exponential declines in limestone mass and corresponding alkalinity loading rates with increased age of limestone drains and (2) potential for improved performance with enlargement, complete burial, and/or regular flushing of the systems.

Pennsylvania↗

USGS2004

Effects of Abandoned Coal-Mine Drainage on Streamflow and Water Quality in the Mahanoy Creek Basin, Schuylkill, Columbia, and Northumberland Counties, Pennsylvania, 2001

This report assesses the contaminant loading, effects to receiving streams, and possible remedial alternatives for abandoned mine drainage (AMD) within the Mahanoy Creek Basin in east-central Pennsylvania. The Mahanoy Creek Basin encompasses an area of 157 square miles (407 square kilometers) including approximately 42 square miles (109 square kilometers) underlain by the Western Middle Anthracite Field. As a result of more than 150 years of anthracite mining in the basin, ground water, surface water, and streambed sediments have been adversely affected. Leakage from streams to underground mines and elevated concentrations (above background levels) of acidity, metals, and sulfate in the AMD from flooded underground mines and (or) unreclaimed culm (waste rock) degrade the aquatic ecosystem and impair uses of the main stem of Mahanoy Creek from its headwaters to its mouth on the Susquehanna River. Various tributaries also are affected, including North Mahanoy Creek, Waste House Run, Shenandoah Creek, Zerbe Run, and two unnamed tributaries locally called Big Mine Run and Big Run. The Little Mahanoy Creek and Schwaben Creek are the only major tributaries not affected by mining. To assess the current hydrological and chemical characteristics of the AMD and its effect on receiving streams, and to identify possible remedial alternatives, the U.S. Geological Survey (USGS) began a study in 2001, in cooperation with the Pennsylvania Department of Environmental Protection and the Schuylkill Conservation District. Aquatic ecological surveys were conducted by the USGS at five stream sites during low base-flow conditions in October 2001. Twenty species of fish were identified in Schwaben Creek near Red Cross, which drains an unmined area of 22.7 square miles (58.8 square kilometers) in the lower part of the Mahanoy Creek Basin. In contrast, 14 species of fish were identified in Mahanoy Creek near its mouth at Kneass, below Schwaben Creek. The diversity and abundance of fish species in Mahanoy Creek decreased progressively upstream from 13 species at Gowen City to only 2 species each at Ashland and Girardville. White sucker (Catostomus commersoni), a pollution-tolerant species, was present at each of the surveyed reaches. The presence of fish at Girardville was unexpected because of the poor water quality and iron-encrusted streambed at this location. Generally, macroinvertebrate diversity and abundance at these sites were diminished compared to Schwaben Creek and other tributaries draining unmined basins, consistent with the observed quality of streamwater and streambed sediment. Data on the flow rate and chemistry for 35 AMD sources and 31 stream sites throughout the Mahanoy Creek Basin were collected by the USGS during high base-flow conditions in March 2001 and low base-flow conditions in August 2001. A majority of the base-flow streamwater samples met water-quality standards for pH (6.0 to 9.0); however, few samples downstream from AMD sources met criteria for acidity less than alkalinity (net alkalinity = 20 milligrams per liter as CaCO3) and concentrations of dissolved iron (0.3 milligram per liter) and total manganese (1.0 milligram per liter). Iron, aluminum, and various trace elements including cobalt, copper, lead, nickel, and zinc, were present in many streamwater samples at concentrations at which continuous exposure can not be tolerated by aquatic organisms without an unacceptable effect. Furthermore, concentrations of sulfate, iron, manganese, aluminum, and (or) beryllium in some samples exceeded drinking-water standards. Other trace elements, including antimony, arsenic, barium, cadmium, chromium, selenium, silver, and thallium, did not exceed water-quality criteria for protection of aquatic organisms or human health. Nevertheless, when considered together, concentrations of iron, manganese, arsenic, cadmium, chromium, copper, lead, nickel, and zinc in a majority of the streambed sediment samples from Mahanoy Creek and

Scientific Investigations Report↗

USGS2003

Effects of abandoned coal-mine drainage on streamflow and water quality in the Shamokin Creek Basin, Northumberland and Columbia Counties, Pennsylvania, 1999-2001

This report assesses the contaminant loading, effects to receiving streams, and possible remedial alternatives for abandoned mine drainage (AMD) within the upper Shamokin Creek Basin in east-central Pennsylvania. The upper Shamokin Creek Basin encompasses an area of 54 square miles (140 square kilometers) within the Western Middle Anthracite Field, including and upstream of the city of Shamokin. Elevated concentrations of acidity, metals, and sulfate in the AMD from flooded underground anthracite coal mines and (or) unreclaimed culm (waste rock) piles degrade the aquatic ecosystem and water quality of Shamokin Creek to its mouth and along many of its tributaries within the upper basin. Despite dilution by unpolluted streams that more than doubles the streamflow of Shamokin Creek in the lower basin, AMD contamination and ecological impairment persist to its mouth on the Susquehanna River at Sunbury, 20 miles (32 kilometers) downstream from the mined area. Aquatic ecological surveys were conducted by the U.S. Geological Survey (USGS) in cooperation with Bucknell University (BU) and the Northumberland County Conservation District (NCCD) at six stream sites in October 1999 and repeated in 2000 and 2001 on Shamokin Creek below Shamokin and at Sunbury. In 1999, fish were absent from Quaker Run and Shamokin Creek upstream of its confluence with Carbon Run; however, creek chub ( Semotilus atromaculatus ) were present within three sampled reaches of Carbon Run. During 1999, 2000, and 2001, six or more species of fish were identified in Shamokin Creek below Shamokin and at Sunbury despite ph as low as 4.2 at Sunbury and elevated concentrations of dissolved iron and iron-encrusted streambeds at these sites. Data on the flow rate and chemistry for 46 AMD sources and 22 stream sites throughout the upper basin plus 1 stream site at Sunbury were collected by the USGS with assistance from BU and the Shamokin Creek Restoration Alliance (SCRA) during low base-flow conditions in August 1999 and high baseflow conditions in March 2000. The water-quality data were used to determine priority ranks of the AMD sources on the basis of loadings of iron, manganese, and aluminum and to identify possible remedial alternatives, including passive-treatment options, for consideration by water-resource managers. The ranking sequence for the top AMD sources based on the high base-flow data generally matched that based on the low base-flow data. The contaminant loadings generally increased with flow, and 10 previously identified intermittent AMD sources were not discharging during the low base-flow sampling period. The top 3 AMD sources (SR19, SR12, and SR49) on the basis of dissolved metals loading in March 2000 accounted for more than 50 percent of the metals loading to Shamokin Creek, whereas the top 15 AMD sources accounted for more than 98 percent of the metals loading. When sampled in March 2000, these AMD sources had flow rates ranging from 0.7 to 19 cubic feet per second (1,138 to 32,285 liters per minute) and pH from 3.5 to 6.4 standard units. Only 1 of the top 15 AMD sources (SR21) was net alkaline (alkalinity > acidity); the others were net acidic and will require additional alkalinity to facilitate metals removal and maintain near-neutral pH. For the top 15 AMD sources, dissolved iron was the principal source of acidity and metals loading; concentrations of iron ranged from 3.7 to 57 milligrams per liter. Dissolved manganese ranged from 1.8 to 7.1 milligrams per liter. Dissolved aluminum exceeded 3.8 milligrams per liter at six of the sites but was less than 0.2 milligram per liter at six others. Alkalinity can be acquired by the dissolution of limestone and (or) bacterial sulfate reduction within various passive-treatment systems including anoxic or oxic limestone drains, limestone- lined channels, or compost wetlands. Subsequently, the gradual oxidation and consequent precipitation of iron and manganese can be accommodated within settling ponds or aerobic wetlands. Assuming an iron removal rate of 180 pounds per acre per day (20 grams per square meter per day), constructed treatment wetlands at the top 15 AMD sites would require a minimum area ranging from 0.1 to 17.8 acres (405 to 71,670 square meters). Implementation of passive treatment would not be feasible at most of the top 15 and many lower priority AMD sites considering the proximity of many discharges to streams, roads, or railroads, and the limited availability or access to land at the discharge location. The reduction of infiltration and removal of culm waste and (or) the relocation of the discharge to nearby areas could decrease the AMD quantities and facilitate treatment at some of the priority AMD sites.

Water-Resources Investigations Report↗

USGS2002

Use of stable isotopes of carbon, nitrogen, and sulfur to identify sources of nitrogen in surface waters in the Lower Susquehanna River Basin, Pennsylvania

Samples of nitrogen-source material, soil, and water were collected from several small, primarily single-source subbasins in the Lower Susquehanna River Basin during 1988–90 to determine the feasibility of using measurements of stable isotope ratios of carbon ( 13 C), nitrogen ( 15 N), and sulfur ( 34 S) to identify sources of nitrogen (N) in stream water. Chemical and isotopic compositions were measured for six N-source types consisting of rain water, forest leaf litter, synthetic fertilizer, farm-animal manure, municipal-sewage effluent and sludge, and septictank effluent and sludge. Compositions of associated, nearby samples of topsoil, subsoil, runoff water, and stream water were measured to evaluate changes in compositions of transported N-containing materials near the N source. Animal manure, human waste (sewage plus septic), and forest-leaf litter can be distinguished on the basis of 13 C; however, most N-sources cannot be distinguished on the basis of 15 N and 34 S, owing to wide ranges and overlap of compositions among different N-source types. Although values of 15 N for soil and runoff-water samples are qualitatively similar to those of the applied N source, values of 13 C and 34 S for runoff-water and stream-water samples appear to reflect the compositions of relatively large reservoirs of the elements in soil organic matter and minerals, respectively, and not the composition of the applied N source. The ratio of organic carbon to total nitrogen (C-org:N), combined with 13 C, is useful for distinguishing agricultural soils, which have characteristically high 13 C and low C-org:N, from forested soils. The C-org:N values of suspended particulates in runoff or stream waters generally are lower than those of nearby soils, however, and indicate that chemical transformations and resultant isotopic fractionation can be important controls on the compositions of N-containing compounds in the soil and water. In aqueous samples including surface water and liquid N-sources, isotopic ratios commonly differ between coexisting dissolved fractions of NO 3- N and NH 3- N and between dissolved and particulate fractions of N or S, probably because of isotopic fractionation during transport or N-source processing. Isotopic measurements provide qualitative information about important reactions that can affect N concentrations in surface waters. However, mass-balance computations generally are not sufficiently accurate to estimate the proportions of multiple sources contributing to the N load in the streams studied because of (1) variations in source chemical and isotopic compositions and (2) nonconservative behavior and fractionation during transport over short distances (hundreds of meters). Uncertainties in mass-balance computations are complicated by the propagation of errors associated with measurements of discharge, chemical concentrations,and isotopic compositions of relatively dilute, small streams.

Pennsylvania↗

USGS2002

Design and performance of limestone drains to increase pH and remove metals from acidic mine drainage, Chapter 2

Data on the construction characteristics and the composition of influent and effluent at 13 underground, limestone-filled drains in Pennsylvania and Maryland are reported to evaluate the design and performance of limestone drains for the attenuation of acidity and dissolved metals in acidic mine drainage. On the basis of the initial mass of limestone, dimensions of the drains, and average flow rates, the initial porosity and average detention time for each drain were computed. Calculated porosity ranged from 0.12 to 0.50 with corresponding detention times at average flow from 1.3 to 33 h. The effectiveness of treatment was dependent on influent chemistry, detention time, and limestone purity. At two sites where influent contained elevated dissolved Al (>5 mg/liter), drain performance declined rapidly; elsewhere the drains consistently produced near-neutral effluent, even when influent contained small concentrations of dissolved Fe^+ ( <5 mg/liter). Rates of limestone dissolution computed on the basis of average long-term Ca ion flux normalized by initial mass and purity of limestone at each of the drains ranged from 0.008 to 0.079 year -1 . Data for alkalinity concentration and flux during 11-day closed-container tests using an initial mass of 4kg crushed limestone and a solution volume of 2.3 liter yielded dissolution rate constants that were comparable to these long-term field rates. An analytical method is proposed using closed-container test data to evaluate long-term performance (longevity) or to estimate the mass of limestone needed for a limestone treatment. This method condisers flow rate, influent alkalinity, steady-state alkalinity of effluent, and desired effluent alkalinity or detention time at a future time(s) and aplies first-order rate laws for limestone dissolution (continuous) and production of alkalinity (bounded).

Book chapter↗

USGS2000

Geochemical investigations by the U.S. Geological Survey on uranium mining, milling, and environmental restoration

Recent research by the U.S. Geological Survey has characterized contaminant sources and identified important geochemical processes that influence transport of radionuclides from uranium mining and milling wastes. 1) Selective extraction studies indicated that alkaline earth sulfates and hydrous ferric oxides are important hosts of 226 Ra in uranium mill tailings. The action of sulfate-reducing and ironreducing bacteria on these phases was shown to enhance release of radium, and this adverse result may temper decisions to dispose of uranium mill tailings in anaerobic environments. 2) Field studies have shown that although surface-applied sewage sludge/wood chip amendments aid in revegetating pyritic spoil, the nitrogen in sludge leachate can enhance pyrite oxidation, acidification of groundwater, and the consequent mobilization of metals and radionuclides. 3) In a U.S. Environmental Protection Agencyfunded study, three permeable reactive barriers consisting of phosphate-rich material, zero-valent iron, or amorphous ferric oxyhydroxide have been installed at an abandoned uranium upgrader facility near Fry Canyon, UT. Preliminary results indicate that each of the permeable reactive barriers is removing the majority of the uranium from the groundwater. 4) Studies on the geochemistry of rare earth elements as analogues for actinides such as uranium and thorium in acid mine drainage environments indicate high mobility under acid-weathering conditions but measurable attenuation associated with iron and aluminum colloid formation. Mass balances from field and laboratory studies are being used to quantify the amount of attenuation. 5) A field study in Colorado demonstrated the use of 234 U/ 238 U isotopic ratio measurements to evaluate contamination of shallow groundwater with uranium mill effluent.

Utah↗

USGS2000

Detecting change in water quality from implementation of limestone treatment systems in a coal-minded watershed

During 1996-97, a variety of limestone-based treatment systems were implemented to neutralize acidic mine drainage and reduce the transport of dissolved metals in the northern part of the Swatara Creek watershed, which drains a 43-mi2 (112-km2) area in the Southern Anthracite Field upstream from Ravine, Pa. Since 1996, the current project has monitored water quality upstream and downstream of each treatment and at integrator sites on lower reaches of Swatara Creek. Continuous measurements of pH and specific conductance and periodic sampling for alkalinity, acidity, sulfate, and metals upstream and downstream of each treatment system show that (1) open limestone channels and limestone-sand dosing generally had negligible effects on water quality and (2) limestone diversion wells and limestone drains generally were effective at producing near-neutral pH and attenuating dissolved metals during baseflow but were less effective during stormflow conditions. Storm runoff in this area commonly is acidic, and, as streamflow volume increases during stormflow conditions, a smaller fraction of total flow is treated and (or) residence time in the treatment system is reduced. Monitoring on the mainstem of Swatara Creek indicates watershed-scale effects owing primarily to changes in mining practices and secondarily to watershed-wide implementation of treatment systems. Most underground mines in the Swatara Creek Basin were abandoned before 1960 and are presently flooded. Drainage from these mines contributes substantially to baseflow in Swatara Creek. For Swatara Creek at Ravine, Pa., which is immediately downstream of the mined area, long-term data collected since 1959 indicate sulfate concentration declined from about 150 mg/L in 1959 to 75 mg/L in 1999; pH increased sharply from 3.5-4.4 (median ~4) to 4.6-7.0 (median ~6) after 1975. These trends resulted from a decline in pyrite oxidation and the onset of carbonate buffering. Because these long-term attenuation processes have had such a pronounced effect on water quality in Swatara Creek, the effects of recent implementation of limestone treatments are difficult to detect at a watershed scale. Nevertheless, during ecological surveys prior to 1991, no fish were found in Swatara Creek at Ravine. Only six species of fish were found in 1994 and 1996. However, increasing numbers of fish have been found annually since 1996. In 1999, 21 species of fish were documented. The recent monitoring on the mainstem of Swatara Creek indicates the limestone treatments mitigate extreme fluctuations in pH during storm events; however, additional buffering capacity is needed to maintain near-neutral pH of Swatara Creek during large storm events. Concentration 2 of sulfate, specific conductance, and pH are inversely related to streamflow at Ravine, indicating dilution and acidification during stormflow. Declines in stream-water pH to values approaching 5.0 could result in the remobilization of adsorbed or precipitated metals associated with sediments; declines in pH below 5.0 could cause injury to aquatic organisms. Generally, to maintain stream pH during storms, additional or larger limestone diversion wells could be constructed to begin or increase alkalinity production as the stream stage rises and/or additional or larger limestone drains could be constructed to produce greater amounts of alkalinity and enhance the buffering capacity of baseflow.

Pennsylvania↗

USGS2000

Relations among sulfate, metals, sediment, and streamflow data for a stream draining a coal-mined watershed in East-Central Pennsylvania

Streamflow and water-quality data for the northern part of Swatara Creek, which drains a 112-km 2 area in the Southern Anthracite Field of east-central Pennsylvania, indicate iron, copper, and lead, and to a lesser extent manganese and zinc, are transported with suspended particles during stormflow conditions. During stormflow conditions, concentrations of these constituents typically peak prior to peak stream stage as a result of scour and transport of streambed deposits. In contrast, sulfate is transported mostly as a dissolved ion. Concentrations of sulfate, specific conductance, and pH are inversely related to streamflow, indicating dilution and acidification during stormflow. This effect is attributed to the mixing of poorly buffered stream water having pH 6.0-6.5 with weakly acidic storm runoff having pH 4.0-4.5 and low dissolved solids.

Pennsylvania↗

USGS1998

Oxic limestone drains for treatment of dilute, acidic mine drainage

Limestone treatment systems can be effective for remediation of acidic mine drainage (AMD) that contains moderate concentrations of dissolved O 2 , Fe 3+ , or A1 3+ (1‐5 mg‐L ‐1 ). Samples of water and limestone were collected periodically for 1 year at inflow, outflow, and intermediate points within underground, oxic limestone drains (OLDs) in Pennsylvania to evaluate the transport of dissolved metals and the effect of pH and Fe‐ and Al‐hydrolysis products on the rate of limestone dissolution. The influent was acidic and relatively dilute (pH <4; acidity < 90 mg‐L ‐1 ) but contained 1‐4 mg‐L ‐1 Of O 2 , Fe 3+ , A1 3+ , and Mn 2+ . The total retention time in the OLDs ranged from 1.0 to 3.1 hours. Effluent remained oxic (02 >1 mg‐L ‐1 ) but was near neutral (pH = 6.2‐7.0); Fe and Al decreased to less than 5% of influent concentrations. As pH increased near the inflow, hydrous Fe and Al oxides precipitated in the OLDs. The hydrous oxides, nominally Fe(OH)3 and AI(OH)3, were visible as loosely bound, orange‐yellow coatings on limestone near the inflow. As time elapsed, Fe(OH)3 and AI(OH)3 particles were transported downflow. During the first 6 months of the experiment, Mn 2+ was transported conservatively through the OLDs; however, during the second 6 months, concentrations of Mn in effluent decreased by about 50% relative to influent. The accumulation of hydrous oxides and elevated pH (>5) in the downflow part of the OLDs promoted sorption and coprecipitation of Mn as indicated by its enrichment relative to Fe in hydrous‐oxide particles and coatings on limestone. Despite thick (~1 mm) hydrous‐oxide coatings on limestone near the inflow, CaCO 3 dissolution was more rapid near the inflow than at downflow points within the OLD where the limestone was not coated. The rate of limestone dissolution decreased with increased residence time, pH, and concentrations of Ca 2+ and HCO 3‐ and decreased P CO2 . The following overall reaction shows alkalinity as an ultimate product of the iron hydrolysis reaction in an OLD: Fe 2+ + 0.25 O 2 +CaCO 3 + 2.5 H 2 O --> Fe(OH) 3 + 2 Ca 2+ + 2 HCO 3 - where 2 moles of CaCO 3 dissolve for each mole of Fe(OH) 3 produced. Hence, in an OLD, rapidly dissolving limestone surfaces are not stable substrates for Fe(OH) 3 attachment and armoring. Because overall efficiency is increased by combining neutralization and hydrolysis reactions, an OLD followed by a settling pond requires less land area than needed for a two‐stage treatment system consisting of an anoxic limestone drain an oxidation‐settling pond or wetland. To facilitate removal of hydrous‐oxide sludge, a perforated‐pipe subdrain can be installed within an OLD.

Conference Paper↗

USGS1996

Oxygen transport and pyrite oxidation in unsaturated coal-mine spoil

An understanding of the mechanisms of oxygen (0 2 ) transport in unsaturated mine spoil is necessary to design and implement effective measures to exclude 0 2 from pyritic materials and to control the formation of acidic mine drainage. Partial pressure of oxygen (Po 2 ) in pore gas, chemistry of pore water, and temperature were measured at different depths in unsaturated spoil at two reclaimed surface coal mines in Pennsylvania. At mine 1, where spoil was loose, blocky sandstone, Po 2 changed little with depth, decreasing from 21 volume percent (vol%) at the ground surface to a minimum of about 18 vol% at 10 m depth. At mine 2, where spoil was compacted, friable shale, Po 2 decreased to less than 2 vol% at depth of about 10 m. Although pore-water chemistry and temperature data indicate that acid-forming reactions were active at both mines, the pore-gas data indicate that mechanisms for 0 2 transport were different at each mine. A numerical model was developed to simulate 0 2 transport and pyrite oxidation in unsaturated mine spoil. The results of the numerical simulations indicate that differences in 0 2 transport at the two mines can be explained by differences in the air permeability of spoil. Po 2 changes little with depth if advective transport of 0 2 dominates as at mine 1, but decreases greatly with depth if diffusive transport of 0 2 dominates, as in mine 2. Model results also indicate that advective transport becomes significant if the air permeability of spoil is greater than 10 -9 m 2 , which is expected for blocky sandstone spoil. In the advective-dominant system, thermally-induced convective air flow, as a consequence of the exothermic oxidation of pyrite, supplies the 0 2 to maintain high Po 2 within the deep unsaturated zone.

Conference Paper↗

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↗

USGS1992

Acid base accounting--An improved method of interpreting overburden chemistry to predict the quality of coal-mine drainage

Acidic mine drainage (AMD), which results from the accelerated oxidation of pyrite (FeS 2 ) in mined coal and overburden, has contaminated thousands of miles of streams in the Appalachian region of the United States. Acid‐base accounting (ABA), which simplifies the complex hydrogeochemical system through use of a limited number of variables, commonly is used to predict the post‐mining occurrence of AMD. ABA involves the measurement of sulfur (S) and carbonate (CO 3 ) concentrations in coal‐bearing rocks and the computation of overburden net‐neutralization potential (NNP) in units of tons of calcium carbonate per thousand tons of rock (tons CaCO3/1,000 ton) (Sobek and others, 1978). ABA was developed on the assumption that the stoichiometry of the following overall reaction of FeS 2 and CaCO 3 can be used to convert acid (H + ) into units of CaCO 3 : FeS 2 + CaCO 3 + 3.75 O 2 + 1.5 H 2 O --> Fe(OH) 3 + 2 SO 4 -2 + 2 Ca +2 + 2 CO 2 (g), (1) where the H + from 1 mol (mole) of FeS2 [64 g (gram) of S] is neutralized by 2 mol of CaCO3 (200 g). This method presumes that gaseous carbon dioxide (CO2 ) will exsolve. Thus 3.125 g CaCO3 will neutralize the acid from 1 g S; or 31.25 tons of CaCO 3 will neutralize the acid from 1,000 tons of rock that contains 1.0 percent pyritic S. The total S concentration, in percent, is multiplied by 31.25 and is assumed to be pyritic and acid‐producing in order to compute maximum potential acidity (MPA) for comparison with neutralization potential (NP), in units of tons CaCO 3 /1,000 ton (Sobek and others, 1978). NNP is computed by subtracting mass‐ weighted MPA from NP (Smith and Brady, 1990). if the value of NNP is less than zero, the acid‐producing potential of the rock exceeds its neutralization potential and if mined, therefore, would be expected to produce AMD.

Conference Paper↗

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↗

USGS1990

Effectiveness of the addition of alkaline materials at surface coal mines in preventing or abating acid mine drainage--Part 2. Mine site case studies

The effectiveness of preventing or ameliorating acid mine drainage (AMD) through the application of alkaline additives is evaluated for eight surface coal mines in Pennsylvania. Many of the mine sites had overburden characteristics that made prediction of post‐mining water quality uncertain. Alkaline materials were applied at rates ranging from 42 to greater than 1,000 tons as calcium carbonate per acre. In addition, two sites that were mined and reclaimed without alkaline additives are included for comparative purposes. Overburden sulfur concentration and "neutralization potential" (NP) data for multiple strata at each mine site were used to compute the cumulative, mass‐weighted "maximum potential acidity" (MPA) and "net neutralization potential" (NNP = NP ‐ MPA) by using three different calculation methods. Post‐reclamation water‐quality data were used to compute the net alkalinity (= alkalinity ‐ acidity). The most conservative determination of NNP, whereby MPA is calculated by multiplying the total sulfur concentration, in weight percent, by 62.5 instead of 31.25, yielded the best agreement with net alkalinity (patching signs on NNP and net alkalinity). The error in prediction using each method was that the reclaimed overburden was computed to be alkaline overall (NNP > 0), but the post‐reclamation water was acid (net alkalinity < 0). In general, alkaline addition rates were probably insufficient to neutralize, or too late to prevent, acid production in the mine spoil. At six of the seven mine sites that had overburden with insufficient NP relative to MPA (NNP < 0), the addition of alkaline materials failed to create alkaline mine drainage; AMD was formed or persisted. A control site which also had insufficient alkaline material, but did not incorporate alkaline additives, generated severe AMD. Two sites that had substantial, natural alkaline overburden produced alkaline drainage. Although the addition rates appear to be inadequate, other factors, such as unequal distribution and exposure of the acid‐forming or neutralizing materials and hydrogeological variability, complicate the evaluation of relative effectiveness of using different alkaline materials and placement of the acid‐ or alkaline‐producing materials.

Pennsylvania↗

USGS1990

Effectiveness of the addition of alkaline materials at surface coal mines in preventing or abating acid mine drainage--Part 1. Geochemical considerations

The addition of alkaline materials to supplement deficient "neutralization potential" (NP) of mine spoil, and thus to prevent or abate acid mine drainage, has riot been successful at most surface coal mines in Pennsylvania. A basic problem may have been improper accounting for acid‐production potential and thus inadequate addition rates of calcium carbonate (CaCO 3 ), calcium oxide (CaO) , or calcium hydroxide [ Ca (OH) 2 ] at many mines. The commonly used acid‐base accounting method is based on the following overall reaction: FeS 2 + 2 CaCO 3 + 3.75 O 2 + 1.5 H 2 O --> Fe(OH) 3 + 2 SO 4 -2 + 2 Ca +2 +2 CO 2 (g), where the acidity from 1 mole of pyrite (FeS2) is neutralized by 2 moles of CaCO 3 . This method presumes that gaseous carbon dioxide (CO 2 ) will exsolve, and therefore may underestimate by up to a factor of 2 the quantity of CaCO 3 required to neutralize the "maximum potential acidity" (MPA) in the mine spoil. This paper reviews some geochemical reactions involving FeS 2 and various alkaline additives that support the argument that the acid‐base accounting method for computing MPA from overburden analyses should be revised. Considering the stoichiometry of the following overall reaction: FeS 2 + 4 CaCO 3 + 3.75 O 2 + 3.5 H 2 O --> Fe(OH) 3 + 2 SO 4 -1 + 4 Ca +2 + 4 HCO 3 - , 4 moles of CaCO 3 are required to neutralize the maximum potential acidity produced by the oxidation of 1 mole of FeS 2 . Therefore, the multiplication factor for computing MPA from the overburden sulfur concentration, in weight percent, should be increased from 31.25 to 62.5.

Conference Paper↗