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TECHNICAL REPORT
Heavy Metals in the Environment

Sources of Salinity Near a Coal Mine Spoil Pile, North-Central Colorado

^a U.S. Geological Survey, Mail Stop 973, Denver Federal Center, Denver, CO 80225
^b USGS, Mail Stop 939, Denver Federal Center, Denver, CO 80225
^c USGS, Mail Stop 963, Denver Federal Center, Denver, CO 80225

* Corresponding author (rzielinski{at}usgs.gov)

Received for publication June 14, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION SITE DESCRIPTION SAMPLE COLLECTION AND ANALYTICAL... RESULTS AND DISCUSSION ADDITIONAL APPLICATIONS CONCLUSIONS REFERENCES

 
A small (1 km²) salt-affected stream drainage on the High Plains north of Denver, Colorado was sampled to determine the near-surface dispersion of soluble salts and metals from low-sulfur coal mining waste (spoil). Surface waters collected along the 0.8-km stream reach, and aqueous leachates of spoil and naturally saline local soil, were analyzed for chemical constituents and sulfur isotopes. In this semiarid setting with abundant carbonate-bearing surficial sediments, the limited, mildly acidic drainage from the spoil pile is quickly neutralized, restricting the mobility of many elements. However, some spoil-derived constituents were clearly traceable within the upper 0.4 km of the stream reach. Spoil leachates and surface water near the spoil pile have distinctive compositions of major anions and cations, and elevated levels of dissolved nitrate compared with downstream waters. Spoil-derived sulfate was traceable because it has generally positive values of ³⁴S that contrasted with generally negative values of ³⁴S in soil leachates and evaporite salts from the surrounding area. Spatial–chemical sampling of surface water showed an abrupt increase in dissolved U, Se, B, Li, and Mn in the lower 0.4 km of the stream reach where shallow ground water from surrounding irrigated fields contributed to surface flow. The downstream evolution of surface water chemistry and sulfur isotopic composition is consistent with mixing between spoil-affected upstream water and irrigation-return water. The methods described should be applicable at other sites in similar settings where the environmental effect of low-sulfur coal mining waste must be assessed and where access to samples of shallow ground water is limited.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SITE DESCRIPTION SAMPLE COLLECTION AND ANALYTICAL... RESULTS AND DISCUSSION ADDITIONAL APPLICATIONS CONCLUSIONS REFERENCES

 
COAL mining produces surficial deposits of waste rock (mine spoil) that can be sulfide bearing and a source of acidity, metals, and salinity. The well-known problem of acid drainage from mined lands is a complex function of lithology, climate, hydrology, and the local inventory of acid-generating sulfides and acid-neutralizing carbonates (Brady et al., 1998; Rose and Cravotta, 1998). Generally speaking, coal mines in arid and semiarid western North America are not significant sources of acid drainage. Contributing factors include low pyrite content of mined coal and associated overburden, and ample amounts of acid-neutralizing carbonate in soils and near-surface sediments (Boon and Smith, 1985). Modest amounts of precipitation and the common occurrence of poorly drained clayey soils also limit the movement of dissolved metals and salts from these coal mine sites. Despite these mitigating factors, mine spoil at western U.S. coal mines can be a source of dissolved Mn, Se, Na, Ca, Mg, NO₃, and SO₄ (Skogerboe et al., 1979; Trouart and Knight, 1984; Naftz and Rice, 1989; Martin et al., 1988; Reiten and Wheaton, 1992). These mobile constituents can adversely affect the quality of surface water and shallow ground water near mines and can contribute to salinization of local soils.

At coal mine sites where acid drainage is minimal and where naturally saline soils and water are present, natural sources of metals and salinity can be difficult to distinguish from sources related to coal mining. Further complications arise in areas of multiple land use where coal-mining activities may coexist with oil and gas development, urbanization, or agricultural activities (irrigation, fertilization). Effective assessment and monitoring of such coal mine sites requires the use of chemical and isotopic tracers that can be specific to coal mine sources.

This report presents a case study in which chemical and sulfur isotope data are used to identify constituents originating from a coal mine spoil pile and to determine the extent of their dispersion into a nearby surface drainage. Analyses of local surface salts, aqueous leachates of spoil pile material, and aqueous leachates of local saline soils are used to characterize the composition of salinity sources. Synoptic sampling of surface water originating near the spoil pile and flowing through naturally saline cultivated lands illustrates the progressive mixing of the two salinity sources and the attenuation of the signal from the spoil pile. The methodology of this study should be applicable in similar geologic and climatic settings where surficial accumulations of low-sulfur, coal mine waste rock are exposed to weathering.

	SITE DESCRIPTION

TOP ABSTRACT INTRODUCTION SITE DESCRIPTION SAMPLE COLLECTION AND ANALYTICAL... RESULTS AND DISCUSSION ADDITIONAL APPLICATIONS CONCLUSIONS REFERENCES

 
The study site is at an elevation of 1530 m in the semiarid High Plains of central Colorado, approximately 40 km north of Denver (Fig. 1). The site is located in southwestern Weld County, an area of particularly intense and diverse land use. Agricultural activities date from the late nineteenth century. A system of irrigation ditches has served the area since 1900. Other land uses included historic coal mining (1860–1974) in the Boulder–Weld coal field (Kirkham and Ladwig, 1980), major production of oil and gas (1950s–present), and recent encroachment of urban development associated with expansion of the Denver metropolitan area. The site was chosen because it contains a prominent coal mine spoil pile, a nearby intermittent stream, naturally saline uplands, and abundant salts of possible mixed origin in the stream drainage (Fig. 2).

View larger version (62K): [in this window] [in a new window]   Fig. 1. Location map of the study area (modified from Kirkham and Ladwig, 1980).

View larger version (40K): [in this window] [in a new window]   Fig. 2. Map of the study site showing locations of water (W) and auger (A) samples.

The coal mine spoil pile consists of a chaotic mixture of variably fragmented carbonaceous shale, sandy shale, claystone, minor sandstone, and coal. This assemblage of lithologies reflects the composition of the lower Laramie Formation bedrock that was excavated to provide ventilation shafts, and access to the mine workings. The spoil pile is devoid of vegetation. Small patches of white efflorescent salts on the surface of the pile are easy to identify against the dark gray- to tan-colored rocks. Thin deposits of mine spoil and coal are mixed with soil over a large area to the southeast of the spoil pile near the mine shaft and the former locations of a railroad spur and assorted buildings (Fig. 2).

Directly north of the spoil pile, at the head of a stream drainage, is a small (150 m²) wetland (Fig. 2) that is dominated by cattail (Typha spp.). At the time of sampling in May 1999, there was no apparent surface flow into the wetland but much of the wetland had several centimeters of standing water. Salt crusts were present on moist ground surrounding the wetland and on wetland vegetation. The wetland is sustained by flow of shallow ground water within local surficial sediments of late Pleistocene age. These relatively permeable mixtures of colluvium and eolian deposits provide a suitable host for a shallow unconfined aquifer developed on underlying weathered bedrock of the Laramie Formation.

Surface runoff from the cattail wetland serves as the source of a meandering stream (0.5 m wide, 4 to 10 cm deep) that heads at the wetland, continues downstream for approximately 0.8 km, and finally terminates at a shallow (<1 m deep) pond (Fig. 2). The presence of a broad, shallow water table beneath the downstream portion of the stream valley is indicated by the greater expanse of grasses, wetland herbs, and saturated ground on both sides of the main channel, and from the increased volume of water in the main channel.

At numerous locations within the stream drainage, salt scars, patches of moist, saline soil, and surface pools develop where erosion has nearly exposed the contact between bedrock and overlying sediments. These contact seeps are located near the break in slope between stream valley walls and the valley bottom. Closer to the center of the stream drainage, salt crusts and salt-coated vegetation are found at the margins of water-saturated valley-fill sediments and at the margins of the pond.

Upland soils in the study area belong to the Nunn loam (fine, smectitic, mesic Aridic Argiustoll) and Weld loam (fine, smectitic, mesic Aridic Argiustoll) series (Crabb, 1980). These soils are well-drained loams and clay loams that are neutral to mildly alkaline and calcareous at depth. The swale formed by the stream drainage is occupied by the Ulm clay loam (fine, smectitic, mesic Ustic Haplargid), a poorly drained clay and clay loam that is mildly to moderately alkaline and calcareous throughout. Locally, the Ulm loam is underlain by shale bedrock at depths of 1 to 1.5 m (Crabb, 1980). Soils are a mixture of colluvium (sheetwash) derived from upland sources, primarily the underlying Laramie Formation, and eolian deposits of more diverse origin (Schwochow, 1972). Areas of saline soils within the 1-km² drainage basin are characterized by poor development of vegetation, presence of salt-tolerant vegetation, and development of surface crusts of white-alkali salts.

The upland areas within the drainage basin are mostly cultivated with rotated crops of barley (Hordeum vulgare L.) and corn (Zea mays L.). Corn, in particular, requires ample nitrogen, which is supplied by applications of anhydrous ammonia and manure during the early growing season (G. Cleland, local farmer, personal communication, 1999). Water is provided by precipitation (300–380 mm/yr) and by diversion from irrigation ditches. The irrigation season extends from April to October. Irrigation practices within the study area enhance the natural mobility of soluble salts within the soil profile. Some irrigation water probably percolates through surface soils to recharge the shallow unconfined aquifer. Transport of soluble salts in irrigation runoff and ground water return is indicated by the common occurrence of saline soils at the topographically low end of graded fields (Fig. 2). Where irrigated fields and areas of saline soils closely adjoin the stream drainage, they provide a potential source of natural and agricultural salts to the stream.

Several oil wells, an injection well, and a few storage tanks are present in the drainage basin (Fig. 2). Oil wells are part of the Spindle oil field that produces principally from the Upper Cretaceous Hygiene Sandstone and Terry Sandstone Members of the Pierre Shale at depths of 1370 to 1550 m (Porter, 1989). Saline formation waters coproduced with the oil typically range from 10000 to 25000 mg/L total dissolved solids and are of the Na–Cl–HCO₃ compositional type (G. Breit, USGS, personal communication, 1999). No evidence of salt crusts, dead vegetation, or significant oil spills was observed in the vicinity of these facilities.

	SAMPLE COLLECTION AND ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION SITE DESCRIPTION SAMPLE COLLECTION AND ANALYTICAL... RESULTS AND DISCUSSION ADDITIONAL APPLICATIONS CONCLUSIONS REFERENCES

 
Surface Water
Ten surface water samples were collected on 13 May 1999: one sample (W1) from the surface of the cattail wetland, several samples (W2–W7) at increasing downstream distances from the cattail wetland, and one sample (W9) from the pond. An additional sample (W8) representing a possible mixture of surface water and shallow ground water, was collected at the western margin of the stream valley near a site of active seeps that contributed water to the valley bottom and to a tributary of the main channel (Fig. 2). Surface water sample W10 was collected from a site upgradient from the spoil pile and represents surface runoff and ground water return of irrigation water supplied by a local irrigation ditch.

The specific conductance, temperature, and pH were measured in situ. Samples for laboratory analysis (250 mL) were collected in screw-cap polyethylene bottles to minimize incorporation of sediment. Refrigerated water samples were filtered upon return to the lab at the end of day, usually within 8 h of collection, but subsampling and alkalinity titrations were not performed until the following day. This procedure was followed in order to (i) permit timely synoptic sampling, (ii) simulate sampling that could be practically applied at sites far removed from vehicles and water processing equipment, and (iii) take advantage of sample processing under controlled laboratory conditions. Rapid, postcollection changes in pH and chemistry caused by oxidation of ferrous iron or changes in dissolved CO₂ are unlikely because surface water samples have ample opportunity to equilibrate with the atmosphere prior to sampling. If accurate determination of dissolved iron and alkalinity is an overriding concern, samples should be fully processed in the field.

Surface water samples were filtered through 0.45-µm pore-size cellulose acetate membranes and separated into acidified and unacidified aliquots. Alkalinity (as HCO^-₃) was determined by titration to pH = 4.5 with standard acid (0.0082 M H₂SO₄). Aliquots (25 mL) were acidified with ultrex-grade nitric acid to pH < 2.0 for major cation analysis by atomic absorption spectrometry (AA) and trace element analysis by inductively coupled plasma–mass spectrometry (ICP–MS). The remaining unacidified, filtered sample was submitted for major anion analysis by ion chromatography (IC) and for sulfur isotope determinations (see below). Analytical precision and accuracy were estimated based on replicate analysis of artificial standards and are better than 10% (relative standard deviation, RSD) for all chemical determinations. All surface water and soil leachates were checked for charge balance of major dissolved cations and anions (Hem, 1985), and were within 8% of charge balance.

Coal Mine Spoil and Saline Soil
Six samples from different portions of the mine spoil pile (A1–A6, Fig. 2) and five samples from a single profile in salinized upland soil (A7, Fig. 2) were collected with a 5-cm-diameter stainless steel soil auger. Samples were collected in depth intervals of 20 to 35 cm for mine spoil and 10 to 20 cm for saline soil. Mine spoil materials were chosen to represent within-site variability in coloration and lithology. Surface accumulations of efflorescent white salts were typically present near sampling sites. A shovel was used to remove approximately 30 cm of surficial material prior to sampling of mine spoil to minimize contamination with windblown dust and to sample below the zone of most intensive weathering. Saline soil was sampled at the lower end of a cultivated field and adjacent to the drainage. Highly saline soil was indicated by lack of crop growth and by obvious concentration of evaporite salts at the soil surface and as coatings on peds.

Samples were placed in thick-walled sealable plastic bags for transport to the laboratory where they were air-dried at 40°C and passed through a ceramic-plate jaw crusher with a 3-mm opening to uniformly disaggregate clods and peds. The majority of each sample (200 g) was weighed into a plastic beaker and 200 mL of deionized water added. Each 1:1 weight mixture was stirred vigorously, allowed to stand overnight at room temperature, and again stirred vigorously prior to pouring into two 250-mL polycarbonate centrifuge bottles. The soil–water slurries were centrifuged at 8000 rpm for 40 min. Clear supernatent (125–140 mL) was decanted and filtered (0.45 µm). Specific conductance, pH, and alkalinity of each leachate were measured immediately. Leachate samples were analyzed by the same techniques as described for surface water samples. The combined precision of leaching and chemical analysis of duplicate soil samples was estimated in an earlier study to vary between 2 and 14% for major dissolved constituents (Otton and Zielinski, 1998).

Evaporite Salts
Concentrates of efflorescent salts were collected from the undisturbed surface of saline soil and coal spoil by scraping or lifting with a stainless steel spatula or knife blade. Samples were placed in sealable plastic bags and were analyzed within a few days by powder X-ray diffraction (XRD) using CuK radiation generated at 40 kilovolts and 25 milliamperes. Additional sample was used for analysis of sulfur isotopes.

Reference Samples of Coal, Bedrock, and Evaporite Salts
Eight reference samples of unweathered coal from three beds, and seven samples of similar-age shale from the lower Laramie Formation were obtained from a 6.5-cm-diameter core. Core samples were recovered in March and April 1999, in support of a cooperative study of the stratigraphy and paleontology of the Denver Basin by the Denver Museum of Natural History and the U.S. Geological Survey. The core site is located in Elbert County, Colorado, approximately 90 km south of the study site (Fig. 1). Samples (4–6 g) were collected from the interval of 1689 to 1850 m, and analyzed for ash and sulfur content, forms of sulfur (ASTM Method D-2492; American Society for Testing and Materials, 1998), and sulfur isotopic composition.

Eighteen reference samples of natural efflorescent salts were collected from outside the study site in saline soils of the Boulder–Weld coal field (Fig. 1) and areas to the north and south. Bedrock units included the Laramie Formation, Pierre Shale, and the Tertiary Denver and Arapahoe Formations (Tweto, 1979). These evaporite salts are the products of upward capillary movement of soil water and its subsequent evaporation. Evaporite salts accumulate near the margins of ponds, near seeps, and at other locations where the water table is within a few meters of the surface for at least part of the year, typically in late winter and spring. In many cases the local water table is elevated by recharge from a nearby reservoir, pond, drainage ditch, or irrigation ditch. In other cases a shallow perched water table is sustained by infiltration of precipitation or irrigation water. Salts were submitted for X-ray diffraction and sulfur isotope analysis.

Sulfur Isotope Analysis
Soluble sulfate in surface water, leachate, and efflorescent salt samples was precipitated as barium sulfate by adding barium chloride to the filtered sample solutions. Total sulfur in coal and bedrock was extracted from powdered samples by Eschka fusion and recovered as barium sulfate following the procedure of Tuttle et al. (1986). The barium sulfates were combusted in an elemental analyzer to form SO₂ gas. The sulfur isotopic composition of the gas was determined using a Micromass Optima mass spectrometer (Micromass UK Limited, Manchester, UK) by a continuous flow method modified from Giesemann et al. (1994). Sulfur isotopic composition is reported in terms of a ³⁴S value:

where R is the atomic ³⁴S to ³²S ratio and the standard is Cañon Diablo troilite (CDT). Reproducibility was ±0.2. Analysis of the NBS 127 seawater sulfate standard averaged 21.0 (n = 5) compared with the value of 20.99 recommended by Rees et al. (1978) for the modern oceans.

	RESULTS AND DISCUSSION

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Mineralogy of Evaporite Salts
Efflorescent salt crusts on the mine spoil pile and on saline soils of the area are dominated by sulfate salts of sodium, magnesium, and calcium (Table 1), in agreement with previous studies of evaporite salts in the northern Great Plains (Keller et al., 1986; Skarie et al., 1987; Mermut and Arshad, 1987; Kohut and Dudas, 1993). The ephemeral nature of these salts and their sensitivity to temporal variations in temperature and humidity complicate attempts to find consistent relationships between mineral assemblage and geologic setting or geographic location. For example, dehydration of samples prior to analysis probably causes underestimation of the hydrated minerals mirabilite and konyaite compared with their respective less-hydrated forms, thenardite and bloedite (Kohut and Dudas, 1993). Some additional generalizations that can be made include the absence of carbonate minerals in the evaporites, the absence of halite from the spoil pile, and the presence of nitratine (NaNO₃) on the spoil pile (Table 1). Nitratine was also detected at another spoil pile in the area during limited reconnaissance sampling.

Comparison of leachate compositions for mine spoil and saline soil (Table 2) confirms a moderately acid soil pH (3.8–6.0) for mine spoil and alkaline pH (7.3–7.9) for local soil. Acidity of the mine spoil leachate is probably the result of oxidation of pyrite, precipitation of ferric oxyhydroxides, and dissolution of iron-bearing hydroxy-sulfate phases such as jarosite and natrojarosite (Table 1) (Hossner and Raven, 1993; Cravotta, 1994). Total dissolved solids and specific conductance were greater for leachates of saline soil than for mine spoil, in agreement with the visibly high abundance of evaporative salts at the soil sample site compared with the mine spoil sites.

Sodium is the major cation in both types of leachates and is particularly dominant in the saline soil leach. Greater proportions of soluble calcium and magnesium in the mine spoil reflect a weathering history that includes acid dissolution of calcite and dolomite (Clark and Williams, 1991). Low concentrations of dissolved iron in leachates (<0.3–0.5 mg/L) are consistent with solubility control by secondary iron oxides under oxidizing conditions. The leaching results may underestimate the solubility of iron in deeper portions of the spoil pile where more reducing conditions are probable.

As expected, most trace elements measured by inductively coupled plasma–mass spectrometry are more abundant in the acidic leachates of mine spoil than in the soil leachates (Table 2). In contrast, a few trace elements that form oxyanions in alkaline solution (As, Mo, Se, and U) are more abundant in the alkaline leachates of soil. Considering measured trace element concentrations in both types of leachates, maximum concentrations exceed USEPA drinking water standards for Mn (50 µg/L), and Se (50 µg/L) (Van der Leeden et al., 1990), and also exceed a proposed drinking water standard for U (20 µg/L) (USEPA, 1991). Maximum concentrations of dissolved B exceed the 5000 µg/L concentrations that are considered harmful to plants (Gough and Severson, 1995).

Surface Water Chemistry
Surface waters at the site are circumneutral to alkaline (pH = 6.75–8.50), slightly to moderately saline (1900–9000 µS/cm), and of the sodium-sulfate chemical type (Table 3). This chemical character is reported in surface water from a nearby intermittent stream (Gaggiani et al., 1987). The evolution of surface waters to sodium and sulfate-rich compositions is illustrated in triangular diagrams that plot normalized concentrations of major anions or cations, expressed on a milliequivalent per liter basis (Fig. 3). These plots also illustrate compositional differences between the leachates of mine spoil and saline soils, and show the extent to which surface water compositions trend toward compositions of soil leachates.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Triangular plots showing the relative proportions of the major cations and anions in surface waters, mine spoil leachates, and saline soil leachates.

View larger version (26K): [in this window] [in a new window]   Fig. 4. Calculated saturation indices of selected mineral phases in nine samples of surface water (W1–W9) collected at progressive distances downstream from the cattail wetland below the coal spoil pile. Positive values of saturation index (log of ion activity product/equilibrium constant) indicate oversaturation with respect to a particular phase.

Some measured concentrations in surface water samples exceed USEPA drinking water standards for dissolved SO₄ (500 mg/L), Cl (250 mg/L), total dissolved solids (500 mg/L), NO₃ (44 mg/L), Mn (50 µg/L), and Se (50 µg/L) (Van der Leeden et al., 1990). Some concentrations also exceed recommended maximum concentrations in irrigation water of SO₄ (1000 mg/L), Cl (350 mg/L), total dissolved solids (2000 mg/L), Mo (10 µg/L), and Se (20 µg/L). Dissolved B and Na concentrations are in a range that restricts use on some crops (Van der Leeden et al., 1990). Dissolved Se in the pond (41 µg/L) exceeds the USEPA recommended water quality criteria of 5 µg/L for possible harmful effects to aquatic life (USEPA, 1999).

Several elements (Mn, Co, Ni, Pb, Zn, Li, Rb, B) that are soluble in the acidic leachates of the spoil pile (Table 2) have markedly low concentrations in nearby surface water W1 (Table 3). Slow rates of water infiltration and discharge from the spoil pile and limited surface runoff may explain the present absence of spoil-derived metals in W1. Over the long term, some metal transport may be retarded under the alkaline–oxidizing conditions present in the local soils and sediments. Such conditions can promote metal sorption onto soil particles (iron oxides, clays) and the formation of insoluble hydroxides, carbonates, or phosphates. The cattail wetland adjacent to the spoil pile may also retain some dissolved metals and nutrients through sorptive uptake on organic matter and/or chemical reduction. Similar processes may influence the long-term mobility and transport of metals from the spoil pile via ground water, but description of subsurface transport of contaminants is beyond the scope of this study.

Synoptic sampling of the stream channel (W1–W7) indicated dramatic changes in the chemical composition of surface water as a function of downstream distance from the mine spoil pile. Within a distance of 0.5 km the most dramatic increases in dissolved major constituents were for Na (9x), SO₄ (7x), Mg (6x), and Cl (5x) (Table 3). Trace elements that showed dramatic increases included Mn (>30x), U (30x), Li (8x), Se (5x), Sr (3x), and B (2x). Plots of these elemental concentrations against downstream distance (Fig. 5) indicate a gradual increase over the first 0.4 km (W1–W5) followed by a more abrupt increase over the next 0.1 km (W5–W7). The abrupt increase in concentration at W5 occurs where broad areas of saturated ground begin to flank the main stream channel, and where an increased ground water contribution to stream flow and dissolved solids is postulated.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Dissolved concentrations of selected elements in surface waters W1–W9, plotted as a function of distance downstream from the cattail wetland below the coal spoil pile.

The spatial–chemical information provided by synoptic sampling strongly suggests that irrigated fields bordering the lower part of the drainage and discharges of shallow ground water stored beneath these fields are major sources of salinity and trace elements (U, Se, Li, B, Sr, Mn) to the lower half of the drainage. Irrigation-return waters in areas of southeastern Colorado underlain by Cretaceous shales also contain elevated concentrations of these same trace elements (Zielinski et al., 1995). In contrast to saline water in the lower half of the drainage, surface runoff from upland irrigated fields (W10) can be comparatively fresh (Table 3) because salts are locally displaced to deeper soil horizons or the shallow water table.

The same spatial–chemical analysis can be used to evaluate the role of the spoil pile as a point source of salinity and trace elements. Nitrate is the only measured chemical constituent that shows a clear trend of decreasing concentration (82–7.2 mg/L) as a function of downstream distance. This decrease occurs over the upper 0.4 km of the stream reach (Table 3, Fig. 5). Consistently low values of dissolved nitrate in the lower portion of the drainage are probably influenced by the uptake of nitrate by abundant green filamentous and blue-green algae. Algal blooms at active contact seeps (W8) and soluble nitrate in soil leachates (Table 2) suggest that irrigation return also supplies nitrate to the lower portion of the drainage, but active uptake of dissolved nitrate prevents its accumulation in stream water.

Several trace elements that have elevated concentrations in the mine spoil leachate have only marginally elevated concentrations in surface water W1 when compared with the average of other surface waters W2–W9. Enrichment factors in W1 water are marginal (1.5–2.0x) for Zn, Co (Table 3), and Pb, and modest (5–10x) for Cd (Table 3) and Ni. Other trace elements that are enriched in mine spoil leachate are very low or below detection in W1 (Rb, Mn) or show no obvious trend of concentration decrease immediately downstream of W1 (Li, B).

Sulfur Isotopes
Positive ³⁴S values, ranging from +4 to +10, are typical for organic sulfur in low-sulfur (<1.0% [w/w] total S) coals (Smith and Batts, 1974; Price and Shieh, 1979; Hackley and Anderson, 1986). This rather restricted range reflects the isotopic composition of dissolved sulfate supplied to the freshwater mire in which the coals accumulated, and also indicates minimal fractionation by bacterially mediated sulfate reduction, which can produce extreme fractionation (up to 50) of sulfur isotopes (Faure, 1986). In many freshwater mires the incorporation of sulfur in the sediment as pyrite may be limited by inadequate supplies of dissolved sulfate. Sulfur isotope fractionation is also minimized if a limited reservoir of pore-water sulfate undergoes nearly complete reduction under closed-system conditions.

Fresh reference coal (ash <30%, S <0.6%, pyrite <20% of total S) from the core samples of lower Laramie Formation have ³⁴S values for total sulfur that are generally positive and fall within a narrow range of -1 to +5 (Fig. 6). Core samples of enclosing shale and mixtures of coal and shale (S = 0.02 to 0.43%) span essentially the same ³⁴S range with two outlying samples extending as low as -3.9.

View larger version (19K): [in this window] [in a new window]   Fig. 6. Sulfur isotopic composition of total sulfur in core samples and of soluble sulfate in the spoil pile, nearby saline soil, regional soils, and surface waters W1–W9.

In contrast to spoil pile sulfate, sulfate leached from saline soil samples from site A7 has negative ³⁴S values of -5.4 to -6.3 (Fig. 6). Evaporite salt collected from a nearby contact seep has a similarly negative value of -5.0. Soluble sulfate in salt crusts collected on a regional basis also has predominantly negative values of ³⁴S (Fig. 6). This contrasting isotopic composition was initially surprising because the spoil pile and local soils are both derived from the lower Laramie Formation. A possible explanation is that local soils also contain soluble sulfate transported by wind or water from more distant sources. Upper Cretaceous Pierre Shale of marine origin underlies large areas to the northwest, within 10 km of the study site (Tweto, 1979), and is a proposed contributor of clay-size material to Late Quaternary loess of eastern Colorado (Muhs et al., 1999). The sulfur isotopic composition of marine shale is likely to be more variable and generally more negative in ³⁴S than the composition of freshwater shale because sulfur isotopic fractionation is enhanced as abundant seawater sulfate undergoes incomplete reduction. Limited sulfur isotopic data for the Pierre Shale and other Cretaceous shales of western interior North America indicate wide-ranging, but generally negative values of ³⁴S in pyrite (Gautier, 1986).

Sulfate in surface water samples has ³⁴S values that range from -4.3 to +2.8 (Table 3). The most positive value is recorded in sample W1, and values become progressively more negative with distance downstream (W1–W9). A positive value of ³⁴S in water near the spoil pile strongly suggests that sulfate enriched in ³⁴S from the spoil is entering the cattail wetland. Dissolved sulfate carried across the wetland surface retains this positive ³⁴S signature because any partial reduction of dissolved sulfate to other S species leaves remaining sulfate that is even more enriched in ³⁴S (Krouse, 1980; Dowuona et al., 1993). Reduced sulfur species (H₂S or HS^-) are presently fixed in the sediments of the cattail wetland as visually distinctive, black, iron monosulfide minerals. Diffusion of remaining porewater sulfate to the overlying water column is probably limited, because this diffusion is against the concentration gradient for total sulfate.

A plot of ³⁴S against 1/SO₄ in surface waters yields a sloped linear trend that, in this type of representation, defines an apparent mixing relationship between end member compositions W1 and W9 (Fig. 7). Simple evaporative concentration of W1 prior to gypsum saturation produces no fractionation of sulfur isotopes and would define a horizontal trend on this diagram. The position of a 1:1 mix on the trend line is calculated using the concentrations of dissolved sulfate in end members W1 and W9. This 1:1 mixture falls between the plotted positions of samples W5 and W6, which were located approximately 0.4 km downstream from the cattail wetland. The isotopic contribution of spoil-pile sulfate becomes difficult to detect at the location of W6. Sample W6 is located where the chemical signal from spoil-pile nitrate is also lost and where other dissolved constituents such as Se and U show abrupt increases caused by increased discharge of shallow ground water in the lower drainage (Fig. 5).

View larger version (16K): [in this window] [in a new window]   Fig. 7. Sulfur isotopic composition plotted versus 1/SO4 for surface waters W1–W9. Dissolved sulfate increases with distance downstream. Tic marks on the apparent mixing line connecting mixing endmembers W1 and W9 indicate the calculated percentage of W9 in the mixture, based on sulfate concentrations. Estimated precision of 34S values (±0.2) is approximated by the diameter of the dots.

	ADDITIONAL APPLICATIONS

TOP ABSTRACT INTRODUCTION SITE DESCRIPTION SAMPLE COLLECTION AND ANALYTICAL... RESULTS AND DISCUSSION ADDITIONAL APPLICATIONS CONCLUSIONS REFERENCES

Synoptic sampling of surface water provided the opportunity to monitor a preferred pathway for rapid, near-surface dispersion of constituents away from the spoil pile. Surface water sampling along the entire stream is limited to times when a high water table provides excess water to the cattail wetland (spring) or when runoff from storm events produces ephemeral flow in the upper part of the stream drainage (spring and summer). Reconnaissance sampling during dry periods could utilize surface salts in the dry streambed and aqueous leachates of streambed sediments. Salts in the dry streambed are brought to the surface by evaporation and should record contributions of mine spoil constituents to the shallow water table beneath the stream channel. Sulfur isotope compositions should be similarly controlled by mixing of salinity sources, provided that conditions in the shallow subsurface are sufficiently oxidizing to limit bacterial sulfate reduction.

Salts contributed from nearby oil production sites were considered minor because of the absence of salt-scarred soils near the sites and because sulfate-dominant salts in the drainage contrast markedly with chloride-dominant compositions of local produced waters. In areas where chloride-rich produced water is a suspected contributor of salts, the analytical focus should shift to measurement and interpretation of Cl to conductance, Cl to SO₄, and Cl to Br ratios in water and soil leachates (Whittemore, 1995; Otton and Zielinski, 1998).

The sulfur isotopic composition of low-sulfur feed coal should be retained in fly ash combustion products (Grinenko and Grinenko, 1974). By analogy with this study, positive values of ³⁴S in fly ash and fly ash leachate may contrast with ³⁴S values in surface and shallow ground water at ash disposal sites that are distant from the site of coal mining. This isotopic contrast could facilitate monitoring of the dispersion of soluble sulfate from fly ash disposal sites, particularly in areas with modest background concentrations of dissolved sulfate.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION SITE DESCRIPTION SAMPLE COLLECTION AND ANALYTICAL... RESULTS AND DISCUSSION ADDITIONAL APPLICATIONS CONCLUSIONS REFERENCES

 
Under water-saturated, oxygenated conditions, the coal spoil pile of this study generates slightly acidic pore water that is capable of dissolving a wide variety of elements. At the present time, off-site transport of most of these elements is probably restricted by limited availability of water, geochemical conditions in the local alkaline–oxidizing environment, and potential uptake in a small adjoining wetland. Dispersion of some of the more mobile constituents such as Cl, SO₄, Li, B, Se, or U in surface water draining the site is difficult to trace over significant distances because local shallow ground water that discharges into the surface drainage has comparatively high background concentrations of these elements. Given this combination of factors, the spoil pile is not currently a significant point source of salinity and trace elements to the drainage. Nitrate is supplied by the spoil pile but is rapidly attenuated in surface water, probably by plant uptake. The spoil pile may become a more important source of salinity and trace elements in the future, considering the high concentrations of water-soluble constituents in shallow spoils.

Spatial–chemical sampling along the stream reach and analysis of soil leachates indicate that the major sources of salinity and trace elements to the drainage are local irrigated fields and shallow ground water that drains from beneath the fields. Despite the present minor contribution of dissolved species from the spoil pile, the chemical influence of the spoil pile on local surface water chemistry can still be traced using ratios of major dissolved anions and cations, dissolved nitrate concentrations, and sulfur isotopic compositions of dissolved sulfate. These measurements should be considered at other coal mine sites in arid lands where (i) the environmental effect of spoil-derived salts, nutrients, and trace elements may be more significant, or, (ii) land management decisions require estimates of the pathways and extent of dispersion of mobile constituents from abandoned or remediated coal mine sites.

Sulfur isotope measurements in surface water and shallow ground water can be used to track sulfate derived from weathering of low-sulfur coal and associated overburden in coal mine spoil piles. The utility of these measurements will depend on how strongly the sulfur isotopic composition of sulfate in mine spoil contrasts with that of soluble sulfate in the local surficial environment. Near-surface dispersion of spoil-derived sulfate is more readily detected if local waters have relatively low background concentrations of dissolved sulfate.

Sampling of surface water, salinized soils, and streambed sediments described in this study can be performed rapidly and with minimal equipment. Such sampling provides a practical means for initial assessment of salinity sources at one or more sites, particularly in remote areas or where access to other sampling media such as ground water is limited. More detailed follow-up work or investigations of possible contamination of deeper ground water should employ multipoint sampling of local ground water.

	ACKNOWLEDGMENTS

 
Special thanks to S.B. Roberts of the U.S. Geological Survey and J.L. Hynes of the Colorado Geological Survey for providing helpful discussions and background information on historic coal mining activities at the study site and the Boulder–Weld coal field. Mr. G. Cleland, a local farmer, provided helpful information regarding agricultural practices. Permission for core sampling was graciously provided by K.R. Johnson and R. Raynolds of the Denver Museum of Natural History. Personnel from the USGS who provided critical analytical support included A.L. Meier (inductively coupled plasma–mass spectrometry), C.A. Rice (ion chromatography, atomic absorption spectrometry), and S.B. Sutley (X-ray diffraction). The authors additionally benefited from informal discussions with G.N. Breit and M.L. Tuttle of the USGS.

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