Lecture No. 13. Case Studies II. Milltown and the Upper Clark Fork

Continuing with the Milltown Reservoir situation, recall that sulfides tend to be stable as long as they are in an anoxic environment. That does not always get recognized by people doing cleanup work.

An Illustrative Incident

A few years ago, Milltown Dam was damaged by an ice jam. In repairing the dam, it was necessary to dredge. From a geochemical standpoint, the best thing to do with the dredge spoils would have been to pump them into a deep spot in the lake near the dam. However, partly because of poorly written laws, EPA and other agencies forced Montana Power to dump the spoils on the floodplain. The result is that there is now a toxic waste dump on the Clark Fork floodplain that needs ongoing monitoring.

Note: EPA had no choice, because of the legal definition of "waste" and the legal requirements around it.

What We Know

1. The metals originated upstream as sulfides.
2. The sulfides should oxidize in an oxic environment.
3. Oxidation of the sulfides would release metals and As into solution.
4. As (V) is sorbed to oxyhydroxides in the oxic zone.
5. At least some As and a lot of other things are fixed as sulfides in the anoxic zone.
6. Arsenic and manganese are present in solution in the groundwater …
7. … but copper, zinc, and other transition metals are not.
8. Groundwater flow is strongly downward through the bottom sediments.
9. Most of the arsenic in the groundwater is As (III).

So what is happening here?

1. In the oxic zone, Fe (III) and Mn (IV) precipitate as oxyhydroxides; As (V) sorbs onto them.
2. Oxyhydroxides move downward with the flow.
3. In the anoxic zone, sulfate gets reduced to sulfide.
4. Metal sulfides precipitate (Cu, Zn, Pb, …) but not As.

Reason: As reacts rather slowly compared to most metals. By the time it is ready to react, there may not be enough sulfide left. Another piece of evidence for sulfide limitation is the fact that a lot of the iron stays in solution too.

It is the falling and rising of the reservoir stage that drives this "redox pump." More reduced Fe and As get oxidized and form colloidal oxyhydroxides that move down through the anoxic zone. Then the stage comes back up, reducing some Fe (III) oxyhydroxides and As (III), then it is drawn down again, etc.

This type of redox pump probably exists at many hydroelectric reservoirs. If the reservoir sediments are contaminated, the contaminants may get moved into the groundwater. Question: What about the reservoirs on the Columbia, which are all contaminated to a greater or lesser degree with radioactive elements? If there are silted-up areas in the tailwaters, the redox pump may be operating. Uranium, for instance, may be cycling between U (IV) and U (VI).

An obvious solution: Stabilize the reservoir level as much as possible. (We can tolerate a week or two of low water before the oxidation reactions take off.)

In the last two overheads we see bubble diagrams — diagrams where the vertical axis is depth (or height), the horizontal axis is time (days since drawdown), and the bubble diameter is proportional to quantity (e.g., pH, Fe concentration, etc.). The diagrams are Figures 8 and 9 of the Moore paper.

In Figure 8, we see that sulfate concentrations are fairly low at depth. When the reservoir stage is raised, sulfate goes into solution in the shallower sediments, then begins to drop. Arsenic seems only to be in solution in the deeper, more anoxic sediments. Iron is in solution in the deeper sediments — a good indication that there is little or no sulfide available. This may be because of competition from other metals, or because sulfate reduction is slow. The high manganese concentrations in the shallow sediments after the reservoir fills are from reduction of MnO₄ and other high-valent manganese.

In Figure 9, bicarbonate is higher in the deeper sediments, consistent with the higher alkalinity at depth. The high calcium levels at the top are probably from dissolution of calcium salts — calcite, gypsum, and/or other minerals. The pH and sodium data are shown. There’s not much here. We know pH rises in the anoxic zone. Sodium is conservative, meaning that it does not change significantly in solubility or redox state anywhere in the expected range of pH and E_h. A conservative tracer is good for following dilution, since only dilution will lower Na⁺ concentration. A conservative tracer injected into an aquifer (e.g., bromide) provides us with a way to monitor the spread of a contaminant plume.

Note: See the guidelines for preparing a manuscript to be used with your term paper (handed out today).

II. Mining-Derived Metals Contamination in Surface and Groundwaters

Via natural processes, such as hydrothermal activity, metals get concentrated into ores. Sulfide ores are no exception.

The metals from ores get remobilized by two sets of processes:

1. Human activities such as mining and processing, operate on a short time scale.
2. Weathering and erosion are long-term, slow processes.

Remobilization puts the metals into the surface waters and groundwaters. It is necessary to distinguish between the natural and anthropogenic processes.

As shown in the diagram, metals move from compartment to compartment by various natural and anthropogenic processes.

Key:

1= natural ore-forming processes

2 = mining

3 = smelting

4 = weathering

5 = natural transport and redox processes

The main routes by which biota pick up the contaminants are from surface water and from the atmosphere.

What Metal Mining Does

1. It increases the natural erosion and weathering rate.

1. This changes the geochemical environment dramatically. E.g., sulfides convert quickly to oxides and other forms as the environment flips from anoxic to oxic.
2. The surface water and ground water become increasingly connected through tunnels, pits, and other excavations.
3. Oxygen has increased access to sulfides by these same mechanisms.
4. Surface area increases by orders of magnitude; compare the physical properties of undisturbed ore with those of tailings.
5. Changes in landforms can alter drainage.

2. A very large amount of waste (waste rock, tailings, smelter flue dust, slag, etc.) is associated with metal extraction. Example: In the upper Clark Fork basin, about 1500 km² is covered by waste from both historic and modern mining.

1. Extraction and concentration steps (mining, milling) commonly produce 9 tons of waste for each ton of ore concentrate that goes to the smelter.
2. Smelting produces metal, slag, flue dust, and waste gases.

Wastes can have impacts as primary, secondary, and tertiary pollutants. See examples on overhead:

1. Primary contaminants include waste rock, tailings, and slag; their initial impacts are localized.
2. Secondary forms of contamination include pollution in ground water at open pits, ground water beneath tailings ponds, sediment in river channels, sediments on floodplains, reservoir sediments, and smelter flue dust and gases, which get carried long distances and contaminate soils, water, and air.
3. Tertiary forms include river sediment reworked from a contaminated floodplain and groundwater contaminated by reworked sediments.

Tailings

They have the highest metal concentrations of any waste type.

Here are some figures for tailings at locations around the world:

The tailings are sulfides and can have very small grain sizes (silt on down). Some of the 19^th-Century tailings had larger grain sizes.

Here are some examples of sulfide minerals from Butte:

1. Pyrite (FeS₂) is very abundant in the wall rock and ore, about 4% overall.

2. Copper ores

• Chalcocite (Cu₂S)
• Bornite (Cu₅FeS₄)
• Chalcopyrite (CuFeS₂)
• Enargite (Cu₃AsS₄)

3. Other sulfides

• Arsenopyrite (FeAsS)
• Sphalerite (ZnS)
• Galena (PbS)
• Acanthite (Ag₂S)

And there can be 50 or 60 other sulfide minerals present.

The signatures of exposed sulfides are

• low pH
• high sulfate
• maybe high metals

Problems in the Clark Fork River

1. Fish Kills

Note that most of these fish kills were during the summer months, especially July and August.

A FW&P employee witnessed one of these kills. A thunderstorm had moved through the area a few minutes earlier. Suddenly fish came to the surface gasping, and some jumped ashore. The fish suffocated; they were unable to get air through their gills.

Brown trout from the 1989 fish kill were analyzed and compared with hatchlings. The metal found in the gills was very striking:

There are many mill tailings deposits along Silver Bow Creek. There is a bypass (the Mill-Willow Bypass) around the Warm Springs Ponds. The ponds are settling ponds. Silver Bow Creek normally runs into the ponds, while Mill Creek and Willow Creek, which are relatively clean, run down the bypass. At the lower end of the settling ponds, Silver Bow Creek runs into the bypass, and then Warm Springs Creek joins the other creeks. This point is the official beginning of the Clark Fork River.

When it rains hard, Silver Bow Creek overflows a weir and mostly flows down the Mill-Willow Bypass. It was immediately after such rains that fish kills happened at the Warm Springs juncture and downstream from there.

Fish, Wildlife, and Parks happened to be doing water quality measurements during the July 12, 1989 fish kill and recorded major changes in surface water.

So the pH dropped by 3 units (1000-fold increase in H⁺ activity) in ¸ hour and stayed low for over 1 ¸ hours.

They also collected water samples and measured Al, Cu, Zn, and Fe in them. These metals rose at storm time, peaked around 17:30, and stayed high for over 2 hours.

And trout don’t even like pH 6!

The upper Clark Fork and its tributaries run through a pretty ordinary flood plain that contains big deposits of tailings that were dumped in the old days. On top of the old pre-mining flood deposits are sulfide-rich tailings deposits, almost 2 m thick in places.

These tailings deposits are sometimes called slickens.

These tailings get rained on fairly often, and they become weathered. Oxygen-rich water seeps in. There is an oxidation front somewhere below ground — oxic above, anoxic below.

When the rain stops, as during periods in the summer, water evaporates, and evaporite mineral deposits form on the slicken surface. These minerals include

• gypsum and/or anhydrite (depending on conditions)
• mixed Fe sulfates such as melanterite and jarosite
• aluminum sulfates (alunite, jurbanite, basaluminite, others)
• mixed copper sulfates such as chalcanthite (CuSO₄ • 5 H₂O), brochantite (Cu₄SO₄(OH)₆), etc.
• zinc sulfates, etc.

The next time a good rain comes along, all these evaporite minerals dissolve. A concentrated liquor of metals and acid almost immediately runs into the creek or the river. Fish die.

In laboratory experiments, the sediments dropped deionized water to pH 3 or 4 in about 1 minute. Concentrations of Cu²⁺, SO₄^2–, Ca²⁺, Zn²⁺, Al³⁺, and Fe³⁺ rose fast. Following a storm, there are puddles of red water — probably containing dissolved Fe³⁺ — on the slickens. One puddle was sampled. Its pH was 2.7, and it contained high dissolved metal concentrations.

One conclusion we may draw is that there would not be any trout alive in the upper Clark Fork if it weren’t for the tributaries. If the fish escape up a tributary soon enough, they survive. Populations are replenished after a major storm by fish swimming in from the tribs.

Controls on Acid

1. Residual acid may exist in the evaporite crust of the slickens.

• H₃O⁺ is present in several of the acid sulfate minerals like jarosite and alunite.
• There can be H₂SO₄ inclusions, coatings, etc.

2. Metal concentrations are high enough so that hydrolysis reactions are significant sources and sinks for acid. For example,

Fe²⁺ + H₂O ¨ FeOH⁺ + H⁺

Zn²⁺ + H₂O ¨ ZnOH⁺ + H⁺

Mn²⁺ + H₂O ¨ MnOH⁺ + H⁺

Cu²⁺ + H₂O ¨ CuOH⁺ + H⁺

ZnOH⁺ + H₂O ¨ Zn(OH)₂ ø + H⁺

CuOH⁺ + H₂O ¨ Cu(OH)₂ ø + H⁺

2 Cu²⁺ + 2 H₂O ¨ Cu₂(OH)₂²⁺ + 2 H⁺

These metal hydrolysis reactions do not drop pH very far below 6. They usually act as buffers around pH 6; copper can drop the pH below 5. But there are other metals that can drop the pH down below 4 (Al³⁺) or even 2.5 (Fe³⁺):

Fe³⁺ + 3 H₂O ¨ Fe(OH)_3ø + 3 H⁺

Al³⁺ + 3 H₂O ¨ Fe(OH)_3ø + 3 H⁺

Here are some data extracted from the Nimick and Moore paper. The pH values were calculated using a geochemical modeling program called PHREEQE.

So the culprits are iron, aluminum, and copper. Copper figures even more prominently than these data would suggest, because its concentration in the evaporites is relatively high and the copper sulfate minerals contain a fair amount of residual acid and/or H₃O⁺.

To previous lecture: Lecture No 12. Carbonic acid. Milltown Case Study.

To next lecture: Lecture No. 14. Mining-Derived Metal Contamination (Continued)

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