Stratigraphy and Chemistry of Sulfidic Flood-Plain Sediments in the

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Chapter 19

Stratigraphy and Chemistry of Sulfidic Flood-Plain Sediments in the Upper Clark Fork Valley, Montana 1

2

David A.Nimick and Johnnie N. Moore 1

Water Resources Division, U.S. Geological Survey, Helena, MT 59626 Department of Geology, University of Montana, Missoula, MT 59812 2

Sulfide-rich tailings deposited by historic floods have contaminated large areas of the upper Clark Fork flood plain in western Montana. About 704,000 m of mine wastes are spread over 274 ha of the flood plain along a 10-km reach near the river's headwaters. The tailings deposits primarily are fine-grained overbank deposits and point-bar deposits containing reworked mixtures of tailings and other sediment. Analyses of total and weak-acid extracts of the sediments show that As, Cu, Fe, Mn, Pb, and Zn are released into solution by oxidation of the sulfides contained in the tailings. These constituents either move upward to the land surface and precipitate as hydrated metal sulfates or move downward to be concentrated in weak-acid-extractable phases in reduced tailings or pre-mining flood-plain deposits. Erodible flood-plain tailings are a major source of trace elements to the river. 3

Large quantities of sulfidic, flood-deposited tailings rich in As, Cd, Cu, Fe, Mn, Pb, and Zn are spread over the upper Clark Fork flood plain in western Montana (1-4). The Clark Fork is the head of the Pend Oreille River in the Columbia River basin. The tailings originated primarily from 1864 to about 1915 from the uncontrolled disposal of wastes produced from mining and smelting sulfide ores into Clark Fork tributaries draining Butte and Anaconda (5). Copper was the principal metal mined from ores containing chalcocite (Q12S), bornite (Cu5FeS4), enargite (CU3ASS4), and chalcopyrite (CuFeS2). Other associated sulfide minerals were pyrite (FeS2), sphalerite (ZnS), galena (PbS), greenockite (CdS), arsenopyrite (FeAsS), and acanthite (Ag2S) (6-8\ Floods carried tailings to the Clark Fork and caused widespread deposition (8) of tailings on the Clark Fork flood plain downstream to Milltown (2,9). The Clark Forkfloodplain between Warm Springs and Milltown has been designated as a Superfund site by the U.S. Environmental Protection Agency. Several studies have examined trace-element concentrations in streambank deposits along the river channel (1,2,4). Those studies found considerable variability in trace-element concentrations and only little evidence of downstream decreases in trace-element concentrations in flood-plain tailings. In the one previous study of the chemistry and stratigraphy of flood-plain tailings (10), concentrations of trace

0097-6156/94/0550-0276$06.00/0 © 1994 American Chemical Society In Environmental Geochemistry of Sulfide Oxidation; Alpers, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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Stratigraphy of Sulfidic Flood-Plain Sediments 111

elements in pore water, ground water, and sediments in a small area near Racetrack (Figure 1) were used to identify the movement of trace elements within and from tailings. Although ground water is a potential receptor of trace elements released from flood-plain tailings, only limited contamination attributable to flood-plain tailings has been documented in the coarse-grained alluvium that underlies floodplain tailings Qfl; Nimick, D. Α., unpublished data). Contaminant plumes have been identified downgradient from the thick tailings deposits found in impoundments near Warm Springs (I) and Milltown (11,12). As the flood-plain sediments are eroded into theriverwithtime,potentially toxic trace elements are released to the aquatic environment. Biological effects attributed to the elevated concentrations of dissolved and particulate-bound metals include reduced diversity of benthic invertebrates, limited trout populations, and fishkills (12i 17). Although little is known currently about the distribution and thickness of the flood-plain tailings and the concentrations and partitioning of arsenic and heavy metals in the tailings, an understanding of these factors is critical for assessing the extent of contamination and for planning effective remedial activities. This paper describes the thickness and distribution as well as the chemical data for a 10-km reach of the Clark Fork (Figure 1) that characterize the flood-plain contamination. The data add to a limited but improving understanding of the sources and quantity of trace elements in large contaminatedriversystems. Sampling and Analytical Methods Tailings thickness was mapped from about 680 measurements in soil pits and streambanks. Flood-plain deposits were sampled at 20 of these sites. Acid extracts of sediments were made by combining 0.6 g of sample with 20 ml 5% HC1. The mixture was shaken for 2 hours and filtered through a 0.45-μπι filter. Total extracts used a HF-HNO3-HCI microwave digestion (18.19). All extracts were analyzed for As, Cd, Cu, Fe, Mn, Ni, Pb, and Zn by inductively coupled argon plasma emission spectrometry. All analyses are reported in p,g/g, dry weight basis. Sampling, analytical, and quality-control methods are described more fully elsewhere (20). Although partial extractions frequently are used to determine partitioning of trace elements in sediments, their use is problematic (21-23). The weak-acid extract for this study was not used to determine partitioning, but rather was designed to determine which trace elements might be released to theriverand biota. In particular, it was used to provide a crude approximation of concentrations of bioavailable trace elements (24). The cold dilute HC1 used in this extract can dissolve Fe and Mn oxide coatings and associated trace elements, dissolve carbonates, replace trace-element ions adsorbed on organic and inorganic materials, and dissolve amorphous or diagenetic sulfides in reduced sediment (25,26). Stratigraphy The Clark Fork has been subjected to increased loads of metal-contaminated sediment since mining began in Butte in 1864 and the construction of smelters in Anaconda in 1884. Prior to the early 1900's, no sediment-control structures existed on Silver Bow or Warm Springs Creeks, which are the Clark Fork tributaries into which mine wastes were dumped. Wastes were transported to the Clark Fork by at least four major floods in the 1890's and the largest flood on record in 1908 (27). Stratigraphie evidence indicates that these few floods deposited considerable quantities of essentially pure tailings as they built a "mining terrace" (28) over the pre-existing flood plain and low terraces in the Clark Fork valley. Although the flood plain aggraded during this period, analysis of the relative position of present (1988) and pre-mining flood-plain deposits indicates that the present channel is near its pre-

In Environmental Geochemistry of Sulfide Oxidation; Alpers, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION


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NIMICK& MOORE

Stratigraphy ofSulfidic Flood-Plain Sediments


mining altitude. Bank height of the mining terrace is 1-2 m above the river. Even though migrating channel meanders and channel avulsions have eroded parts of the mining terrace, original tailings deposits still overlie pre-mining flood-plain deposits in large areas of the valley. Sediments on the valley floor are separated into three categories: overbank tailings, reworked tailings, and pre-mining flood-plain deposits (Table I). Most of the tailings are in widespread overbank deposits, whereas reworked tailings have a more limited distribution near present and former locations of the Clark Fork. Table I. Description of Rood-Plain Deposits along the Clark Fork Unit Name Description Overbank Tailings Upper tailings Very fine sandy silt with interbedded planar and ripple cross laminae; tan with orange and gray staining; as much as 35 cm thick. Lower tailings Lenticular deposits of fine to medium silty sand; discontinuous planar and ripple cross laminae; orangish light brown to medium brown with mottled reddish-orange and gray iron staining; as much as 60 cm thick. Reduced tailings Similar to lower tailings except gray to black; less than 40 cm thick. Reworked Tailings Reworked tailings Fluvial deposits containing reworked mixtures of tailings and cleaner sediments found as silty fine sand in overbank deposits, silty sand and sand in channel-accretion deposits, and sand and gravel in point bars and some levees; generally light brown with little iron staining; support growth of grass and willows; overbank deposits commonly rootbound. Pre-Mining Flood-Plain Deposits Buried "A" horizon Dark-brown to black organic-rich soil horizon marking top of pre-mining flood plain; as much as 40 cm thick. Buried subsoil Dark-brown thin-bedded silt and sand overlying sand and gravel. Overbank tailings have been divided into three units. The upper two units lie above the saturated zone and are oxidized, whereas the lower unit is reduced. The top unit, called upper tailings, is essentially pure tailings and contains little, if any, uncontaminated sediment. Iron hydroxide coatings were observed on nearly all grains when upper tailings were viewed with an energy-dispersive X-ray scanning electron microscope (SEM-EDX). Goethite crystals compose a large part of these coatings. Large organic fragments (leaves and sticks) are present on some foreset beds. Smelter slag or glass composes about 15% of the tailings. If exposed at the surface, the upper tailings form barren areas devoid of vegetation. A crust of efflorescent salts forms on these barren areas during warm dry periods (28.). Throughout the study area, the upper and lower tailings are separated by a thin (50 cm) are mostly in a narrow band near the river but some are near the course of the late-1800's channel; these tailings account for two-thirds of the total volume. Thick tailings are extensive at the upstream end of the study area and where the flood plain is narrow (Figure 2). Tailings 10-30 cm thick are extensive where die flood plain is wide. Thin tailings are found away from the river at the margins of the flood plain, mostly west of the river. East of the river, several alluvial fans of intermittent tributaries limited the eastward extent of Clark Fork flooding. Some agricultural lands east of the river may have been contaminated by floods or irrigation water diverted from the river, but any tailings deposited have been reworked and masked by plowing. The volumes of flood-plain tailings measured during this study indicate that previous estimates of the volume may have been conservative. Johnson and Schmidt Q5) estimated that 1,000,000 nw of tailings are present along 24 km of the Clark Fork valley between Warm Springs and Deer Lodge. On the basis of this study and field reconnaissance of the area between the downstream end of the study area and Deer Lodge, the volume of flood-plain tailings between Warm Springs and Deer

Map Unit (cm) Ô-1Ô 10-30 30-50 >50 Total

Average Area Thickness Covered (cm) (ha) 5 94.6 20 90.6 40 49.4 70 39,7 274.3

Volume (m*) 47.3ÔÔ 181,200 197,600 277,900 704,000



Figure 2. Map showing thickness of flood-deposited tailings on the Clark Fork flood plain. The locations of the four reaches (labeled A, B, C, and D) are shown in Figure 1.


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ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION

Lodge probably is about 1,500,000 m^. The estimate by Moore and Luoma (2) of 2,000,000 m? of flood-plain tailings between Warm Springs and Milltown also could be small.


Sediment Chemistry Flood-plain tailings contain high concentrations of arsenic and base metals. Ranges of concentrations for each stratigraphie unit are shown in boxplots (Figure 3). The bed-sediment chemical-concentration data (29) in Figure 3 are for fine-grained samples from a 25-km reach of the river that includes the study area. Median concentrations of total Cu, Mn, and Zn are greater than 1,000 μg/g in most of the tailings deposits and more than 2,000 in many of the units. Total Cu concentrations are very high-more than 10,000 μg/g in more than half of the reduced tailings samples and as much as 49,300 μς/ξ in the green sand of the lower tailings. Median concentrations of total As and Pb in tailings deposits are slightly less and generally range from 100 to 2,000 μg/g. Although most total Cd concentrations are less than 10 μ^/g in samples of all sediment types, generalization is difficult because Cd concentrations in many of the samples are less than the detection limit. The ranges of trace-element concentrations reported by Brooks and Moore (IQ) for floodplain tailings at a site near Racetrack are similar to those determined during this study. Ni concentrations are consistently low in all sediment types. Total Ni concentrations generally range from 10 to 30 μg/g uniformly in all sediment types. Brook (22) found similar total Ni concentrations in bed sediments of both the Clark Fork and Clark Fork tributaries and concluded that sediments in the Clark Fork system are not enriched in Ni. This conclusion is consistent with the lack of Ni in ores mined at Butte (£). Fe concentrations vary little between contaminated and uncontaminated sediments or within the various types of tailings. Most total concentrations range from about 20,000 to 50,000 μg/g in tailings and buried "A" horizons and 9,000 to 28,000 μg/g in buried subsoil. None of these values is much higher than the average concentration of 13,700 \ig/g reported for flood-plain sediments from tributary valleys (4). Therefore, flood-plain sediments are not nearly as enriched with Fe as with other metals. The enrichment that does exist probably reflects the original abundance of iron sulfides in the tailings. The lack of anomalous Fe concentrations and the variability through all the sediment types are consistent with the results of other Clark Fork studies (4.10V Total concentrations of As, Cu, Fe, Mn, Pb, and Zn in tailings deposits and in underlying pre-mining sediments differ considerably but the concentration patterns in vertical profiles generally are consistent. Profiles of trace-element concentrations at two representative sites are shown in Figure 4. The highest concentrations generally are found either at the ground surface or at depth near the bottom of the tailings deposits. Concentrations generally are less in the upper tailings and pre-mining sediments. The high concentrations of Cu, Mn, and Zn at the surface are caused by hydrated metal sulfates that precipitate during warm weather (29). These concentrations, which were measured by Nimick (20) in surficial (0-2 cm) samples, represent minimum values because they are water-soluble rather than total concentrations. Trace-element concentrations tend to be lowest in the tailings just below the surface, whether the tailings are part of the upper tailings (eg., lower part of Figure 4) or lower tailings (upper part of Figure 4). Concentrations generally increase with depth below the near-surface tailings, and the highest subsurface concentrations are found within the lower tailings, reduced tailings, or buried "A" horizon. Locally,


19.

Stratigraphy of Sulfidic Flood-Plain Sediments

N i m C K & MOORE

TOTAL

ACID

ACID

TOTAL

Î Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 13, 2015 | http://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0550.ch019

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1

As TOTAL

Cd

Cd ACID

ACID

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TOTAL

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Cu

Cu

TOTAL

Fe

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Mn

Mn

TOTAL

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Stratigraphy and Chemistry of Sulfidic Flood-Plain Sediments in the

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