The iron gets into the water when pyrite crystals (FeS2) in the coal dissolves. While the iron precipitates, the sulfur remains in the water, making it more acidic. Dealing with the acid can be a huge problem in large coal and metal mines.
The pool of water that collects at the base of the quarry, is probably fairly acidic.
Not all the pyrite is dissolved however, and since this particular coal seam has a lot of pyrite, it is not economical to burn since the burnt sulfur (as sulfur dioxide gas) would have to be captured — otherwise it produces acid rain.
The rich black coal seam sits on top of blocky limestone rock. Above the limestone is a red, weathered soil.
We discussed quite a variety of topics just based on the visit to the landfill/quarry.
A single, half-day, visit to the landfill and quarry brought up quite the variety of topics, ranging from the quarry itself, to the reason for the red colors of the cliff walls, to the uses of the gases that come out of the landfill. I still have not gotten to the details about the landfill itself, but I’ve put together a page that links all my posts about the quarry and landfill so far.
The water cycle, at the quarry.
The water cycle is intricately tied to all the other topics that came up on our visit to the quarry/landfill. For some things, the tie to water is direct and inextricable.
It’s groundwater that dissolves the pyrite in the coal seam and then precipitates an orange iron stain on the quarry cliffs.
Rainwater seeping into the landfill leaches out chemicals that have to be prevented from getting into the groundwater, rivers or lakes.
Gases like hydrogen sulfide can react with water (and oxygen) in the air to produce acid rain. Not to mention that water is needed for the decaying processes that produce the hydrogen sulfide, and other landfill gases like methane, to begin with.
For other things the link to water is not necessarily so obvious:
The sediment that was compressed into the limestone that is being quarried, was formed beneath the shallow seas that once covered this region in the geologic past. Limestone is also dissolved by rainwater to create caverns, underground rivers and spaces for geodes.
Methane gas not only requires water for it to be released via decomposition of garbage, but also produces carbon dioxide when burned. Carbon dioxide is a greenhouse gas, so it affects the global temperature and contributes to the melting glaciers, rising sea levels and changes in climatic patterns such as the amount of rain we’re going to receive in the midwest.
The water cycle picture starts simply, but gets complicated very quickly.
A bigger, fuller picture of the water cycle as it interacts with the quarry.
The quarry's primary purpose is to extract limestone for construction.
The landfill/quarry we visited was originally a limestone quarry; once they had the hole in the ground they needed to fill it with something so why not trash (and why not get paid to fill it).
Shoveling boulders. The rock pieces look small but only because the shovel is so big.
The limestone bedrock is blasted daily to create some massive boulders. The boulders are then loaded on some equally massive dumptrucks. There are scarce few minutes between trucks, so a lot of rocks are being moved.
Dumptruck moving rocks. Massive boulders in the foreground. Unloading dumptruck.
The trucks then dump their load into a large building where the rocks are crushed. Our guide made us stop the bus to watch the process. While watching a dumptruck unloading might seem mundane, the enormous size of the truck and its boulder load did seem to captivate the students.
Once the rocks are crushed, the resulting sediment is sorted by size (sand, pebbles and gravel, I think) and piled up. The piles are massive. I’ve been wanting a good picture that shows the angle of repose; I got several.
The angle of repose of a pile of sediment. Also notice the greenish color of the water in the pond to the bottom left. Water with lots of fine limestone particles (silt) and dissolved limestone, tends to have that color.
The pebbles and gravel are used for road construction and provide a matrix for concrete.
Since limestone dissolves fairly easily in rainwater, the sand-sized and smaller particles (< 2mm diameter) aren't used for construction -- hard, insoluble quartz sand is preferred.
Limestone: calcium carbonate (CaCO3)
However, the limestone sediment piles sit out in the open and some the finer grains (silt sized particularly), and any dissolve calcium carbonate, get washed into the nearby ponds, which turn a beautiful, bright, milky green.
Finally, in addition to the limestone sediment piles, there is also one enormous pile of broken up concrete. One of the things that stuck with the students was that fact that you can recycle concrete.
The freshwater lens beneath small islands is fed by rainwater and maintained because the freshwater is less dense than the surrounding saltwater.
One of the questions that came up when we were talking about dealing with the highly contaminated leachate that drains out of landfills, is what would happen to it if it was just put into a lake or the ocean. Would the liquid just mix into the water, or would it stay separate.
Rainwater seeping through a landfill picks up a lot of nasty chemicals, and by the time it gets to the bottom it is a highly contaminated leachate.
I’m afraid I did not go with an easy answer. It depends after all on two things: how different the density is of the leachate from seawater; and how turbulent is the water.
Turbulent water will make the leachate more likely to mix, while a greater density difference would cause them to “want” to remain separate. An extremely dense leachate might just settle to the bottom of a lake and stay there.
Small Islands
One example of two fluids that are in contact but stay separate is in the groundwater beneath small islands. Rain water falls on the island and seeps into the ground. It’s fresh, but the water in the surrounding ocean and the water that’s already underground are both salty. Salty water is more dense than the fresh so the freshwater will float on top of the salty water creating a thin lens.
Dimensions of a freshwater lens. Image via the USGS.
How thick is the lens? For every meter that the fresh groundwater is above sea level, there are 40 meters of fresh water below sea level (1:40). This is because saltwater has density of about 1.025 g/cm3, while freshwater has a density of about 1.000 g/cm3 (note that I use four significant figures in each of these values).
The freshwater lens can be a great source of drinking water on these isolated small islands, but like the islands themselves, they are threatened by rising sea levels due to global warming.
Methane in a landfill. It's produced by decomposing organic material, is extracted via wells, and is then burned to produce heat (for a school and a set of greenhouses) and electricity (soon anyway).
Decomposing waste in landfills produces quite a lot of methane gas (CH4). Perhaps better known as natural gas, methane is one of the simplest hydrocarbons, and a serious atmospheric pollutant (it’s a powerful greenhouse gas). In the past the methane produced was either released into the atmosphere or just burned off.
Greenhouses that are warmed by methane produced by the landfill. It's a cheap, close source of energy.
I remember seeing the offshore oil rigs burning natural gas all night long — multiple miniature sunrises on the horizon — in the days before the oil companies realized they could capture the gas and sell it or burn it to produce energy. The landfill companies have realized the same thing. So now, wells pockmark modern landfills and the methane is captured and used.
Looking down the slope of the landfill to see the Pattonville High School, which uses natural gas from the landfill for heating.
First, of course, the hydrogen sulfide gas (H2S), is separated from the methane — H2S produces acid rain, so it’s emissions are limited by the EPA — then, the gas from the landfill we visited, is piped to:
greenhouses, where it’s burnt to produce heat;
the Pattonville High School, which is right next to the landfill and burns the gas for heating;
and (soon) to a electricity generating power plant that will burn the gas to produce heat which will boil the water that will produce the steam that will turn the turbines that will generate the electricity.
Electric power plant -- still under construction -- that's fueled by methane from the landfill.
You may have noticed the common theme of all these uses of natural gas: it has to be burned to be useful. The combustion reaction is:
CH4 (g) + 2 O2 (g) —-> CO2 (g) + 2 H2O (g)
which produces carbon dioxide (CO2) that is also a greenhouse gas, but is, at least, not nearly as powerful at greenhouse warming as is methane.
The limestone walls of the quarry were stained red with iron precipitate.
The cliffs of the quarry were stained red. Blood, seeping out from between the bedding planes between layers of rocks, might have left similar traces down the sides of the near-vertical cliffs’ faces. But these stains are actually made of iron.
Rain falling on the land above the quarry, seeps into the ground. There it moves downward through the soil, leaching out some of the minerals there, but going ever downward. Downward until it meets a layer of soil or rock that it can’t get through. Clay layers are pretty impermeable, though in this case it’s a layer of coal. The water can’t move through the near-horizontal coal seam very fast, so instead it moves sideways across, and eventually seeps out onto the cliff face.
The red on the walls of the cliffs are an oxidised iron precipitate (rust). The iron most likely was dissolved out of the pyrite in the coal seams.
The seeping water still has those minerals it dissolved in the soil. It also has more dissolved minerals from the coal it encountered too. Coal forms in swamps when trees and other plants fall into the waters and are buried before they can completely decompose. Decomposition is slow in stagnant swampy waters because most of the insects and microorganisms that do the decomposing usually need oxygen to help them with their work. Stagnant water does not circulate air very well and what little oxygen gets to the bottom of the swamp-water is used up pretty fast. You could say that conditions at the bottom of the swamp are anoxic (without oxygen), or reducing.
Coal formation. Image from the National Energy Education Development Project.A shiny pyrite crystal in a lump of coal (happy holidays). Image via USGS.
Iron in air will rust as it reacts with water and oxygen — rust is the red mineral hematite (Fe2O3) that you see on the walls of the quarry. Iron in a reducing environment, on the other hand, will form minerals like pyrite (FeS2). According to our guide, the thin coal seam in the quarry has a fair bit of pyrite. In fact, because of the pyrite, the coal has too much sulfur for it to be economical to burn. Like the landfill gas, hydrogen sulfide, burned sulfur turns into sulfur dioxide, which reacts with water droplets in the air to create acid rain so sulfur emissions are regulated.
The water that seeps along and through the coal seam will dissolve some of the pyrite, putting iron into solution. However, the iron will only stay dissolved as long as the water remains anoxic. As soon as the high-iron water is exposed to air, the iron will react with oxygen to create rust. Thus the long stains of rust on the cliff walls show where the water emerges from underground and drips down the cliff face.
Diagram showing the coal seam, and the seeping water that creates the iron (rust) staining.
Iron precipitate in other environments
On our Natchez Trace hike we found it quite easy to stick fingers into the red precipitate at the bottom of the stream.
We’ve seen the precipitation of iron (rust) as a result of changes in redox (oxidizing vs reducing) conditions before: on the sandbar on Deer Island in the Gulf of Mexico; in the slow streams along the Natchez Trace Park‘s hiking trails in Tennessee. Iron precipitation is an extremely common process in natural environments, and it’s easily noticeable. Just look for the red.
The rich black of decaying organic matter, sits just beneath the rusty-orange surface sediment. The red is from hematite (rust) and shows that the surface is oxidizing, while the black shows that just a few centimeters beneath the surface, there is no oxygen to decompose the organic matter (a reducing environment). This image was taken on Deer Island on the Gulf Coast.
Diagram of the hydrogen sulfide system in a landfill.
Although it makes up less than 1% of the gases produced by landfills, hydrogen sulfide (H2S) is the major reason landfills smell as bad as they do. H2S is produced by decomposition in the landfill, and if it’s not captured it not only produces a terrible, rotten-egg smell, but also produces acid rain, and, in high enough concentrations, it can be harmful to your health (OSHA, 2005; Ohio Dept. Health, 2010).
Decomposition
A wall partially covered with drywall. Image via FEMA via Wikimedia Commons (Nauman, 2007).
Some hydrogen sulfide is produced when organic matter decays, but for big landfills like the one we visited, construction materials, especially gypsum wallboard (drywall), are probably the biggest source.
Gypsum is a calcium sulphate mineral, that’s made into sheets of drywall that are used cover the walls in most houses because it’s easy to work with and retards fire. The U.S. used 17 million tons of gypsum for drywall in 2010 according to the USGS’s Mineral Commodity Summary (USGS, 2011 (pdf)).
Gypsum:
CaSO4•2(H2O)
As you can see from the chemical formula, each gypsum molecule has two water molecules attached. In a fire, the heat required to evaporate the water keeps the temperature of the walls down to only 100 degrees Celcius until the water has evaporated out of the gypsum board.
A number of landfills have banned drywall because it produces so much hydrogen sulfide, but the one we visited still takes it. It’s big enough that they capture the landfill gasses, including the hydrogen sulfide, and then separate it from the other, more useful gasses, like methane, which can be burned to produce heat energy. H2S can also be burned, but they you end up contributing to acid rain.
H2S and Acid Rain
When hydrogen sulfide reacts with oxygen in the atmosphere it produces sulfur dioxide.
2 H2S (g) + 3 O2 (g) —-> 2 SO2 (g) + 2 H2O (g)
Sulfur dioxide, in turn, reacts with water droplets in clouds to create sulfuric acid.
SO2 (g) + H2O (g) —-> H2SO4 (aq)
Acid rain accelerates the dissolution of statues. (Image by Daniele Muscetta)
When those droplets eventually coalesce into raindrops, they will be what we call acid rain.
Acid rain damages ecosystems and dissolves statues. It used to be a major problem in the midwestern and eastern United States, but in 1995 the EPA started a cap and trade program for sulfur dioxide emissions (remember sulfur dioxide is produced by burning hydrogen sulfide) that has made a huge difference.
The head (top) of a well (vertical metal pipe) that captures the gas from inside the landfill.
Capturing H2S
Probably because of the EPA’s restrictions, the landfill company pipes all the gases it collects through scrubbers to extract the hydrogen sulfide. There are a few ways to capture H2S, they all involve running the gas through a tank of some sort of scavenging system that holds a chemical that will react with hydrogen sulfide and not the other landfill gases. At the landfill we visited the remaining landfill gas, which consisted of mostly methane, was used for its energy.
