Lecture No. 20. Pit Lakes

Birnessite and As(III) Oxidation (Continued)

Investigations, previous to the Moore, Walker, and Hayes paper, of As (III) oxidation to As (V), had identified birnessite as an electron-acceptor but had not addressed the role played by its crystal structure. For example, Oscarson et al. showed it as an MnO₂ ¨ Mn²⁺ reaction. They saw the scant release of Mn²⁺ as evidence of complexation or possibly a precipitate of Mn₃(AsO₄)₂.

Moore et al. carried out experiments with synthetic potassium birnessite that was prepared by reaction of KMnO₄ and HCl. (Different reaction conditions made birnessites containing 1.7% K and 8.2% K.) They then measured the kinetics of reactions between sodium arsenite solutions and birnessite at various temperatures (5¡ , 25¡ , and 40¡ C) and pH values (3.5, 7.5, and 11.0). Times of these experiments ran as long as 64 h. Different As to birnessite ratios were also tried. As (III) and As (V) in solution were separated by an ion-exchange method and determined separately by ICP (inductively coupled plasma) atomic emission spectrophotometry. The chemical composition and crystal structures of the birnessite were determined at t = 0 and various stages of reaction. The structure was studied by Cu Ka x-ray powder diffraction patterns.

The formulas they found for the two kinds of K-birnessite were K_0.33Mn₇^3.9+O₁₄ á 7 H₂O (for 1.7% K) and K_1.6Mn₇^3.8+O₁₄ á 4.9 H₂O.

As the reaction with As proceeded, K⁺ ions were gradually released into solution. Some Mn²⁺ was also released, but not as readily as K⁺.

The arsenic oxidation by birnessite appears to be a two-stage process. In the first stage, As is oxidized by Mn (III) or Mn (IV) in the interlayers. In the second, slower, stage, the arsenic reacts with Mn (IV) in the layers themselves. As the reaction proceeded towards completion a gel formed, possibly with the formation of krautite, Mn(H₂O)AsO₃OH.

There is no reason not to think that the birnessite-promoted oxidation of arsenic in nature is very different from what we saw in this laboratory work. Manganese (IV) is a strong enough oxidizing agent, and the layer structure appears to lend catalytic action because of its high surface area.

Lakes

Until now we have mostly looked at metal geochemistry as it applies to streams and their interfaces with groundwater. Now we are going to take a look at lakes.

Limnology: Fundamental Principles and Nomenclature

Lakes account for only 0.01% of the water at the earth's surface, but they are important to humans for several reasons. They are used as sources and sinks for water, as fisheries, and for navigation and recreation.

The chemistry of a freshwater lake depends largely on temperature changes. These lead to density changes and the presence or absence of convection.

Consider a lake in a cold temperate region (e.g., Montana). During the summer, the surface water warms up much faster than the deep water. The warmer surface water is less dense than the cooler, deep water, so it stays on the surface. The wind mixes the surface water a little, but only to a relatively shallow depth. (Wave depth is normally about ¸ the wavelength.) There is not much wave mixing below maximum wave depth. The lake tends to become stratified, with a warmer upper layer, or epilimnion, and a cooler lower layer, or hypolimnion. The boundary between these layers is a fairly narrow layer called the thermocline, where the temperature drops sharply with increasing depth. (See the sketch.)

During the autumn, the surface temperature begins to fall. At some point the epilimnion gets colder and thus denser than the hypolimnion. This brings about an unstable situation. Something, usually the wind, upsets this unstable situation, and the lake overturns. For a time the lake water is mixed from top to bottom. As the air temperature gets even colder, the surface water reaches 4¡ C, where water has its maximum density. As it cools further, the surface water becomes less dense, and the lake is again stratified. This time the epilimnion is colder, but again it is less dense. Eventually the lake surface freezes.

When spring arrives and the surface thaws, the lake remains stratified for a time. As the surface water warms to near 4¡ C, it becomes denser. This brings about the spring overturn, and the lake again is mixed from top to bottom.

A lake that overturns twice a year in this way is called dimictic.

In a milder temperate climate (e.g., a Mediterranean climate) where lakes do not freeze, a lake will stratify during the summer and there will still be a fall overturn. The lake water stays above 4¡ C all winter though, so the lake will tend to stay well-mixed during the winter as the denser, cooler surface water will sink and warmer, less dense water will rise. A lake of this type, which overturns once a year, is called monomictic.

There is another type of monomictic lake: the alpine type. In some lakes at high altitudes or high latitudes the water never gets warmer than 4¡ C. Summer stratification does not happen, so there is no fall overturn. However, there still will be a spring overturn after the ice melts. For example, the water at the bottom may stay around 2¡ C all year. In the winter, the surface water is near 0¡ C and is thus less dense. When the surface water warms above 2¡ (but is still below 4¡ ), it gets denser and the lake overturns.

In tropical zones without pronounced seasons and with fairly constant air temperatures, a shallow to medium-depth lake may stratify for an indefinite period. Eventually a period of cool weather may cool the surface enough to cause overturn. Such a lake, with occasional or episodic rather than periodic overturn, is called oligomictic.

All of the above types of lakes, which mix from top to bottom at some time or another, are called holomictic.

All other lakes, which do not mix from top to bottom, are called meromictic. These lakes include

• Deep tropical lakes, such as the ones in the East African Rift, which remain warm at the surface and cold at depth, and which never overturn.
• Exceptionally deep lakes at all latitudes, which mix at the top but which have a permanent hypolimnion. (But see exceptions to this rule.)
• Some lakes with hot springs at their bottoms. If the spring water is high in dissolved matter, the spring water will be denser than the fresh water near the surface, in spite of being hot. Lakes of this type may remain permanently stratified.

Note, however, that the hot springs may also promote mixing if their water is relatively fresh. Crater Lake is well-mixed the year round primarily because of the many hot springs on its floor.

Exceptionally deep lakes in cool-temperate climates may be oligomictic. One example is Lake Baikal in Siberia. Lake Baikal is 1623 m deep. The phase diagram for water shows a pressure dependence, and at great depths the temperature of maximum density decreases as pressure increases. This temperature is about 4¡ C under surface conditions, 3.5¡ at 250 m depth, and 3¡ at 500 m. Normally the water at depth is at a temperature of about 3.5¡ C. During the winter, if strong winds push colder surface water to a depth of about 250 m, that water suddenly becomes denser than the deep water, leading to a condition of instability. The denser water sinks, the bottom water is displaced, and the entire lake overturns. Lake Baikal overturns in this way about every 8 years. Lake Tahoe is another oligomictic temperate lake; it overturns only during winters with violent storm winds.

Stratified lakes may become anoxic at depth. Photosynthesis near the surface is partially balanced by respiration in the lake, but in most lakes photosynthesis produces slightly more organic matter than is removed by respiration. This excess organic matter eventually finds its way to the bottom. In a stratified lake, aerobic bacteria metabolizing this organic matter exhaust the oxygen, and the hypolimnion becomes anoxic. If the bottom of a holomictic lake becomes anoxic, it eventually becomes oxic when the lake overturns. On the other hand, a meromictic lake almost always has a permanently anoxic hypolimnion.

(This also happens in some marine settings. The Black Sea is anoxic at the bottom. Five major European rivers empty into the sea. This influx of fresh water produces a top layer that is less saline than the deeper waters, and vertical mixing is prevented.)

A lake with an anoxic hypolimnion has an oxycline that coincides or nearly coincides with the thermocline (if it is thermally stratified) or with the halocline (if it is stratified by salinity). There is often a lot of redox chemistry happening right at the oxycline. For example, if the deeper water contains a lot of dissolved iron (II) or manganese (II), there will be an oxidation front at the oxycline, and there will commonly be a layer of precipitated oxyhydroxides at that level. Even if sunlight penetrates to the oxycline, the suspended solids cut off most light from the hypolimnion.

Chemical and Hydrological Balance in a Lake

Water is added to a lake by inflow from streams, by precipitation into the lake, and by groundwater discharge. Water is lost to evaporation, to groundwater recharge, and to streams draining the lake.

Most lakes have inlets and outlets. With a few exceptions (the Great Lakes constitute an important exception), precipitation directly into a lake is small compared to precipitation into the drainage basins of streams that feed the lake. Groundwater inputs are also minor. For lakes that have outlets (most lakes), we can neglect evaporation and groundwater recharge. This allows us to write a simple equation for chemical balance in a lake. Consider a lake with surface inlets and outlets:

F_i = rate of surface flow of water into the lake

F_o = rate of surface flow out of the lake

M = total mass of a dissolved substance (e.g., phosphorus) in the lake

C_i = concentration of the substance in the inflow

C = concentration of the substance in the lake

R_p = rate of removal of the substance by precipitation and sedimentation

R_d = rate of addition of the substance to the lake by dissolution

t = time

Then

Let R_b = rate of burial of the substance = R_p–R_d

Consider a contaminant in lake water. Suppose a lake is at or reasonably near a steady volume (not rising or falling significantly) and at a steady state in its chemical concentrations. Assume we know the discharges of streams entering and leaving the lake. Also suppose that the burial rate is insignificant or that we know the burial rate. Then we can calculate the maximum concentration of the contaminant in the waters entering the lake that will not cause the lake to exceed a permitted level.

Example: Phosphorus (from Berner and Berner)

Suppose we must maintain dissolved phosphorus under 5 m g/L (or 5 mg/m³) in the lake. Also suppose that all of the phosphorus input is being carried in one stream that receives treated wastewater. The discharge of the phosphorus-rich stream is 100 m³/s. The discharge of the stream draining the lake is 150 m³/s. The burial rate is 250 mg P/s.

Since the lake is at steady state, dM/dt = 0. Then

or

Therefore

The contaminated stream cannot contain more than 10 m g/L phosphorus if the lake is not to exceed 5 m g/L.

Classification by Level of Productivity

A lake with little organic matter and a low level of primary productivity (photosynthesis) is called oligotrophic. One with a high level of organic matter and a high level of productivity is called eutrophic. The term mesotrophic is used for a lake that lies between these extremes. Oligotrophic lakes are usually clear and have little visible evidence of algae or other microorganisms. They tend to be well-oxygenated. Eutrophic lakes are often cloudy from algal growth. Their lower levels are rich in organic matter, and, if they are stratified, their hypolimnions tend to be anoxic.

There is more information on limnology, for anyone who is interested, in Berner and Berner, Chapter 6. This book is on reserve in the Mansfield Library.

Pit Lakes

When operation of an open-pit mine is discontinued, we are left with a large hole in the ground. The hole usually fills with water by inflow of groundwater, runoff from adjacent drainage basins, or both. During the first half of the 20^th Century, most pit lakes were the results of coal mining. High-powered steam shovels were introduced in 1911, and surface mining quickly became a major source of coal. Surface coal mining left hundreds of pit lakes in the Midwest and Appalachians. However, since the enactment of the Surface Mining Control and Reclamation Act (Public Law 95-87), the creation of coal pit lakes in the United States has virtually stopped. Currently most new pits in this country result from metal mining, and most concern over environmental problems resulting from pit lakes has been related to metal mines.

There are currently 85 open-pit metal mining operations in the United States extracting metals other than iron, aluminum, or uranium. There are 21 in Canada, 84 in Australia, and 30 in Chile. Other countries with substantial numbers of open-pit metal mines include Mexico, Brazil, the Philippines, Peru, Indonesia, and Papua New Guinea. There will probably be a large amount of open-pit metal mining in central Asia in the future; in 1995 the government of Kazakhstan opened up 75 metal-industry projects to foreign investors, and most of these were mining projects. The current heavy gold exploration in West Africa is also likely to lead to many new open-pit mines.

In Nevada alone, more than 30 new pit lakes are expected to form in the next 20 years, and these lakes will contain over 1.2« 10⁹ m³ of water. The quality of that impounded water is critically important, especially in a water-poor State like Nevada. That goes in spades for the many pit lakes expected to form worldwide during the next 50 years.

Solutes are released into the water of a pit lake by weathering of wall rocks and other exposed rock in the surrounding drainage. A common situation is the one found in the Berkeley Pit in Montana, the Liberty Pit in Nevada, and the Spenceville Pit in California. These lakes are acidic (pH < 3), and they also contain high levels of heavy metals and sulfate.

Pit lakes differ physically from most natural lakes in having markedly higher relative depths. Percent relative depth is defined in terms of the lake's maximum depth, z_m, and its width, d. Assuming an approximately circular lake, the width is a function of surface area, A₀:

The percent relative depth, RD, is then defined as

A typical natural lake has a relative depth of less than 2%, although some may exceed 5%. Pit lakes, on the other hand, commonly have relative depths between 10% and 40%. This causes the lake water to stratify in many cases, and the chemistry of the lake water can vary a lot with depth. Here are some analytical data from water samples taken at various depths in the Berkeley Pit. (The lake is about 900 ft deep.)

This situation is fairly common, with solute concentrations becoming much higher with depth.

If the surface mine is an aluminum mine, the pervasive oxidation of the host rock guarantees that there will be little or no pyrite present. Similarly, many iron mines are in highly oxidized rock. Uranium mining may expose some pyrite, and sulfide-ore mining is guaranteed to do so, as is mining of high-sulfur coal.

As a result of pyrite oxidation (which we covered in earlier lectures), many pit lakes contain high levels of acid, sulfate, and dissolved metals. A lake's chemical characteristics depend on the alkalinity of the local ground water, the composition of the wall rocks, the chemistry of the surrounding vadose zone, and the quality and quantity of runoff from the surrounding land. Rock that is exposed to oxidizing conditions during dewatering can be a major source of acid, even though it lies below the water table before mining begins and after the lake has filled.

Controls on Acidity

There are some pit lakes that are neutral or even alkaline. If a lake is in contact with a source of carbonate (such as limestone or dolomite, a carbonate-cemented sedimentary rock, or carbonate veins in an orebody) it may neutralize some or all of the acidity in the water. If there is enough carbonate, it may neutralize all of the acid, and the lake will be neutral or alkaline. It is possible that a lake in host rock containing limited carbonates may stay neutral until the carbonate is exhausted, then become acidic as further pyrite oxidation occurs. On the other hand, there have been cases where initially acidic lakes were neutralized by influx of alkaline groundwater.

In near-neutral pit lakes, bicarbonate is the principal species influencing the water's pH, which will generally be above 6. In some cases, hydrolysis of iron (II), copper, and other transition metals can buffer the pH near 6. In acidic pit lakes, the main buffering system is hydrated aluminum, which maintains the pH below 4.5. In very acidic water, hydrated iron (III) and bisulfate also can function as important buffers around pH 2. Pit lakes with pH between 4.5 and 6.0 are rare, because there is no effective buffer in that range.

To previous lecture: Lecture No 19. Determining Geochemical Background II

To next lecture: Lecture No 21. Pit Lakes II

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