By Scott N. Hamlin and Charles N. Alpers
Water-Resources Investigations Report 96-4257
WATER QUALITY
Results of Sampling
Water-quality data for April and December 1992 and January and July 1993 are summarized in table 1. The USGS wells were sampled in April and December 1992; Mine Run Reservoir and two Utility District wells were sampled in December 1992. A seep and one artesian well were sampled in January 1993, and another seep was sampled in July 1993. Summarized in table 2 are exsolved-gas data for well GS-18, which penetrates the mine workings near shaft 3 (fig. 3).
As part of the quality-control program, field equipment blanks were run through the sampling process, including filtration and preservation, in April and December 1992. The concentrations of most constituents determined in the field equipment blanks were less than detection limits. Calcium, magnesium, sulfate, silica, aluminum, iron, and zinc were present at concentrations near their detection limits in one of the two field blank samples and below their detection limits in the other; chloride was present at concentrations near its detection limit in both field blanks (table 1). Properties measured in the field included specific conductance, pH, temperature, dissolved oxygen, and redox potential. A depth profile for dissolved oxygen and temperature was compiled for Mine Run Reservoir at 1- and 6- foot depths using a 3-L (liter) van Dorn grab sampler.
During April 1992, values for specific conductance ranged from 1,810 µS/cm (microseimens per centimeter) at well GS-2 to 18,000 µS/cm at well GS-6. The pH of sampled water ranged from 3.7 at well GS-8A to 7.8 at well GS-2. Water temperatures ranged from 17.5°C at well GS-5 to 19.5°C at well GS-3 (table 1).
Samples for analysis of water quality were collected in December 1992 from wells installed in 1991 and 1992 in the slag area and the mine workings (areas of shafts 3 and 4) and from Mine Run Reservoir (fig. 3; table 1). Specific conductance ranged from 1,710 microsiemens per centimeter at well GS-19 to 9,300 µS/cm at Mine Run Reservoir at the 6-foot depth. The pH of the water sampled in December 1992 ranged from 2.8 (GS-20 and Mine Run Reservoir at 1- and 6-foot depths) to 6.2 at GS-4B. A pH value of about 11 for well GS-12 in the slate unit was assumed to result from contamination from the cement plug above the bentonite seal at the bottom of the 6-inch PVC casing. Therefore, water-quality data from well GS-12 were not included in table 1 and were not used to evaluate the ground-water chemistry. Temperatures ranged from 13.5°C in Mine Run Reservoir at the 6-foot depth to 19.5°C in wells GS-3 and -18 (table 1).
Comparison of water-quality data for April and December 1992 (table 1) for five wells sampled on both occasions indicates transient behavior consistent with a contaminant plume in a dynamic hydraulic setting. All wells showed increases in specific conductance with time; four of five wells showed increases in copper, zinc, cadmium, iron, and aluminum, and three of five showed increases in sulfate. During the same period, some wells showed decreases in pH. Rising specific conductance, metal, and sulfate concentration and falling pH reflect a higher proportion of acid drainage in ground water, consistent with the interpretation that acid drainage in ground water was migrating toward Camanche Reservoir (fig. 4) in a prograding plume during this period.
Distribution of Acid Mine Drainage
Dissolved constituents at the Penn Mine site are derived from oxidation of sulfide minerals and from dissolution of associated aluminosilicate and oxide-hydroxide minerals in waste-rock piles exposed at several locations on the site. Percolation and infiltration of acid mine drainage to ground water will lower pH and raise concentrations of metals and sulfate in the ground water. Another possible source of these constituents could be the dissolution of soluble sulfate minerals in bedrock intersected by the underground mine workings. The generation of acid mine drainage was discussed in greater detail by Hamlin and Alpers (1995).
The slag materials exposed near Camanche Reservoir contain greater than 6 percent zinc by weight (Wiebelt and Ricker, 1948). No published data are available for the cadmium content of the slag; however, seven grab samples of massive slag analyzed by inductively coupled plasma after HF/HNO3/HClO4 digestion were found to have cadmium concentrations ranging from 14 to 1,700 ppm (parts per million) (mean = 280 ppm); the same seven samples showed a range in zinc concentration from 3.2 to 25 weight percent (mean = 7.8 weight percent) and a range in copper of 0.2 to 2.2 weight percent (mean = 0.8 weight percent) (M. Parsons, Stanford Univ., written commun., 1996). Thus, the slag could be a source of zinc, copper, and cadmium to ground water, especially if leached under acidic conditions.
Dissolved metals found at elevated concentrations in acidic water at the Penn Mine include aluminum, cadmium, copper, iron, and zinc. Water with pH values less than 4.5 can contain several hundred to several thousand milligrams of dissolved aluminum per liter (Hem, 1985; Nordstrom and Ball, 1986; Alpers and Nordstrom, 1991). Dissolved aluminum in ground water at the Penn Mine ranged from less than 10 to 720,000 µg/L (micrograms per liter) (table 1). Copper may be present in concentrations as high as a few hundred milligrams per liter in acid mine drainage (Hem, 1985). Values for dissolved copper in ground water from the Penn Mine ranged from less than 10 to 130,000 µg/L (table 1). Concentrations of dissolved iron in ground water at the Penn Mine ranged from less than 10 to 660,000 µg/L (table 1). Dissolved zinc in ground water from the Penn Mine ranged from 20 to 630,000 µg/L (table 1). Cadmium is found with zinc, including the sulfide mineral sphalerite and the sulfosalt mineral series tetrahedrite-tennantite, in most crustal environments; these minerals are present at Penn Mine. Dissolved concentrations of cadmium in ground-water samples from Penn Mine ranged from less than 10 to 3,700 µg/L (table 1).
The areal distribution of selected water-quality properties and constituents was plotted. The distribution map for pH in December 1992 is shown in figure 9, below. The other major constituents of acid mine drainage (iron, aluminum, and sulfate) and associated trace elements (copper, zinc, cadmium) show distributions similar to that of pH. Acid mine drainage is characterized in ground water by values of pH less than 5. Low values of pH are associated with high concentrations of sulfate at Penn Mine (fig. 10, below).
Concentrations of sodium and chloride at Penn Mine tend to increase with depth and were highest in the slate unit (Hamlin and Alpers, 1995). In the 200-foot boreholes, where the rock units were separated by inflatable packers in the vicinity of the metavolcanic-slate contact (GS-1, -4, and -8), chloride concentration ranged from 290 to 570 mg/L in the slate intervals and from 8.1 to 97 mg/L in the metavolcanic rock intervals; sodium concentration ranged from 360 to 810 mg/L in the slate intervals and from 29 to 280 mg/L in the metavolcanic rock intervals (table 1). The highest concentrations of chloride (5,000 mg/L) and sodium (3,200 mg/L) were found in the 400-foot borehole (GS-6), which penetrated more than 350 feet of the slate unit. These data strongly suggest that the source of sodium and chloride is associated with the slate unit.
Another way to delineate the plume of acid mine drainage in ground water downgradient from Mine Run Dam is by plotting the distribution of the stable isotopes deuterium [D or hydrogen-2 (2H)] and 18O. During the evaporation process, water molecules containing heavy isotopes tend to become concentrated in the residual water, and molecules containing lighter isotopes become enriched in the water vapor. Hence, isotopic values for bodies of surface water exposed to evaporation tend to be enriched in heavy isotopes relative to most ground water. The process of isotopic evaporative enrichment produces delta deuterium and delta 18-oxygen values that lie on a line with a slope between 3 and 6 to the right of the meteoric line (Craig and others, 1963). This phenomenon is observed in a plot of isotope data from ground water and Mine Run Reservoir at the Penn Mine (fig. 11, below). The data can be fit by least-squares linear regression (r2=0.985), which gives a line described by the equation
Geochemical Correlations
Correlations among dissolved constituents indicate the effects of acid mine drainage on ground-water quality at the Penn Mine. Dissolved iron correlates positively with sulfate in ground water, consistent with oxidation of pyrite as the probable source of these constituents (Hamlin and Alpers, 1995). Other constituents, such as aluminum and other dissolved metals, show a similar correlation with pH.
Concentrations of dissolved metals in natural waters can be limited by solubility equilibrium with solid phases. The concentrations of ferric iron and aluminum in ground water at the Penn Mine also are strongly dependent on pH (Hamlin and Alpers, 1995). Geochemical modeling with the WATEQ4F code (Ball and Nordstrom, 1991) suggests that dissolved ferric iron is probably in equilibrium with a form of hydrous ferric oxide or ferrihydrite [nominally Fe(OH)3] (Hamlin and Alpers, 1995). Sulfate concentrations are highest in low-pH waters, whereas iron concentrations can be limited by solubility of a sulfate mineral such as hydronium-bearing jarosite [(K,Na,H3O) Fe3 (SO4)2(OH)6] (Alpers and others, 1989) or schwertmannite [Fe8O8(OH)6SO4] (Bigham, 1994). Similarly, aluminum concentrations appear to be in equilibrium with gibbsite [Al(OH)3], except in ground water with pH values less than 5 (Hamlin and Alpers, 1995). Again, sulfate becomes dominant at low pH and aluminum can be in equilibrium with an aluminum-sulfate mineral such as jurbanite [AlSO4(OH)·5H2O].
Metal ratios can be useful indicators of anomalous chemical analyses resulting from either analytical error or geochemical processes. The ratio of zinc to cadmium (Zn/Cd) is commonly consistent within a single mineral deposit. The Zn/Cd ratio for all but three of the water samples (table 3) falls within the range of 11 to 420, which is consistent with Zn/Cd ratios in ores and mine waters from other massive sulfide deposits (Nordstrom and Ball, 1985). Zn/Cd ratios are considerably higher for samples taken from wells GS-18 (8,000), GS-20 (2,100 during artesian flow on January 28, 1993), and the GS-20 seep (2,400). Values of the ratio of zinc to copper (Zn/Cu) fall between 1 and 200 for all water samples (table 3), except again for GS-18 (3,100), GS-20 (680 during artesian flow), and the GS-20 seep (800). Water samples collected from well GS-20 in December 1992 did not have similar characteristics because artesian flow at well GS-20 had not yet begun. Subsequent artesian flow from well GS-20 and related seeps reflected discharge from the underground mine workings (characterized by water samples from GS-18) in response to rising hydraulic head.
The most likely explanation for the anomalously high values of the Zn/Cd and Zn/Cu ratios is that cadmium and copper have been preferentially scavenged by aqueous hydrogen sulfide (H2S) in the underground mine workings. DiToro and others (1990) indicated the following sequence for metal sulfide solubility from most soluble to least soluble:
MnS > FeS > NiS ZnS > CdS PbS > CuS >HgS.
Therefore, cadmium and copper sulfides are less soluble then zinc sulfide and would be preferentially scavenged by any H2S produced by sulfate reduction.
Heavy stable isotopes (delta Deuterium and delta 18-oxygen) were enriched in partially evaporated water from Mine Run Reservoir and in ground water affected by acid drainage from the reservoir (fig. 11). In contrast, ground water from the mine workings (GS-18 and GS-20 seep) falls on or near the global meteoric water line (fig. 11), showing no enrichment of these heavy isotopes.
Another distinctive characteristic of the water from wells GS-18, GS-20, and the GS-20 seep is the presence of exsolving gas. Chemical analysis of the exsolved gas from well GS-18 (table 2) indicates somewhat reducing conditions; the gas is dominated by carbon dioxide and nitrogen, with very little oxygen, and considerable methane and traces of hydrogen. Exsolution of dissolved gas is probably caused by the reduction in ambient pressure as water is brought to the surface from the mine workings. This occurred during pumping at well GS-18 (160 ft below the ground-water table) and at well GS-20 during periods of artesian flow from shaft 4 of the mine workings.
The relatively reducing environment required for reduction of sulfate to H2S is consistent with the exsolved-gas chemistry for well GS-18 (table 2), which includes considerable methane (CH4 and traces of hydrogen (H2). The presence of detectable oxygen (O2) in the exsolved gas, at concentrations of 0.15 to 0.31 volume percent (table 2), and the relatively oxidizing value of Eh in water from well GS-18 (table 1) indicate redox disequilibrium among dissolved gases and between the dissolved gases and the aqueous phase. The redox disequilibrium could be caused by mixing of two or more waters with different dissolved gasses and dissolved metals in the underground mine workings. Stable-isotope data for sulfur and oxygen in dissolved sulfate are also consistent with the hypothesis of sulfate reduction and possible fluid mixing in the mine workings.
The stable isotopic composition of dissolved sulfate (table 1) also can be used to distinguish water samples from wells GS-18 and -20 and associated seeps from water samples taken elsewhere at the Penn Mine. The contrast in isotopic composition for the dissolved sulfate from the underground mine workings is shown in figure 12, below; values of delta 34-sulfur in sulfate in three samples derived from the underground mine workings (GS-18, GS-20, while artesian, and GS-20-Seep) range from +4.1 to +4.8 per mil, about 3 per mil heavier than samples from Mine Run Reservoir and the wells downgradient from Mine Run Dam, which have delta 34-sulfur values ranging from +1.0 to +2.0 per mil. Water samples from wells GS-19 and GS-20 (non-artesian) have delta 34-sulfur values intermediate between the samples from the underground mine workings and those from the Mine Run Dam area, indicating probable mixing. Values of delta 18-oxygen in sulfate range from -1.0 to +2.5 per mil, with the heaviest values in the sulfate from the mine workings.
