SpiritLevel

Level Zero

Pallanza Bay, Italy. 2010. This is where this not-quite-a-detour-and-more-like-a-redirecting started: how do you talk about science with people who aren’t trained in or maybe even focused on science? [This place – Pallanza Bay – also happens to be where Alessandro Volta discovered methane in the reed beds way back in the 1770s and where plumbing still goes whang-o now and again when his discovery bubbles up in the wrong places]. I was there for a chlor-alkali site characterization and the question of who did what when and how that might make different parties more or less liable in the apportioning of remediation costs. There were many parts of that site characterization that were interesting, but what caught my attention up there along the shoreline was this: The Museum of the Art of Hat-Making. Yeah. Mercury. And hat making. Which means there is the seed of a story there and that story contains, at least in a literary sense, a rabbit hole. But I am first, and foremost, an engineer, and a Southern girl, which means this story has to start with that run what you brung – meaning, with that sense of starting from what you know and building from there.

Level One

Over the span of its operation, a typical mercury cell chlor-alkali facility would have released somewhere in the neighborhood of 10 tons of elemental mercury into the aquatic environment. There are (or were) an unknown number of these facilities around the world, but a reasonable estimate might be 250 and includes locations everywhere: in the U.S., in Canada, in Latin America, in many of the countries of the European Union. To this day, nobody knows how many the Soviets built. Or where. They were built on rivers, lakes, fjords, in estuaries, in lagoons, and on harbors; by 2019 the vast majority have been abandoned, closed or converted to other technologies. The process itself begins with NaCl brine and ends with caustic soda and chlorine. Both caustic soda and chlorine have many uses; the chlorine, as example, to make pesticides, bleach for the pulp and paper industry, and PVC. 

In the period from roughly the middle of the 20th century until its end, chlor-alkali production relied globally on mercury and electricity to split that Na from the Cl. This process wasn’t a closed loop and elemental mercury was cheap: if the process was run efficiently, production might release ~10 lbs of mercury into wastewater a year. Depending on when you were on that 20th century arc, that wastewater could have been discharged into the river, lake, fjord, estuary, lagoon, or harbor. Or it could have been discharged into a treatment pond. Depending on when you were on that 20th century arc, the bottom of that pond might have been sealed with fine clay or a membrane liner. If the process was run inefficiently – although not necessarily in violation of any discharge regulations and as appeared to have happened in many locations in the initial years of facility operation – then over the life of the facility, production of chlor-alkali likely released somewhere in the neighborhood of 10 tons of elemental mercury into the aquatic environment. 

By volume, this much elemental mercury would overflow 3 × 55 gallon drums. Maybe this seems like a lot of mercury and maybe it doesn’t. The spectrum of ecological and human health impacts that result from this overflow depends on a long list of factors. Fundamentally, the type of ecosystem that that mercury was released into matters significantly. Even more fundamentally, the extent to which people living in the vicinity of that release have had to rely on that aquatic ecosystem for food and livelihood matters even more significantly. Critically, it is the type of environment – the river, lake, fjord, estuary, lagoon or harbor – that determines how far from the facility you have to be living for ‘in the vicinity’ to not include your community. 

Level Two

Energy baseball.

Think of a chlor-alkali cell like you might very generally think of a car battery. But in reverse.

In a typical car battery, there are plates and there is fluid. The plates are made of lead and are stacked or slotted into cells. In each cell, the plates alternate: solid lead (Pbs) and lead dioxide (PbO2). In a typical lead-acid battery, the fluid that circulates within and between these cells is sulfuric acid (H2SO4). Batteries work like a criss-cross game of baseball: pitchers and catchers and every time the ball is thrown energy changes hands. At the beginning of the game – in a fully charged battery – the plates alternate in cells and are different: solid lead and lead dioxide and the fluid is sulfuric acid. At the end of the game – in a fully discharged battery – the plates in each cell are the same: lead sulfate (PbSO4) – and the fluid is essentially water (H2O). Pitchers and catchers and the ball is made of electrons: to move the world, the ball goes pitcher to catcher and energy is made. Recharging the battery – the catcher there in their crouch throwing balls back to the pitcher – converting those plates from lead sulfate back to solid lead and lead dioxide and that fluid back from water to sulfuric energy – costs energy.

In a battery – in baseball – the goal is the game: pitchers to catchers and with the energy you generate maybe your team wins. In a chlor-alkali cell, the process is somewhat akin to recharging the battery: apply energy in the form of electricity to the plates and it’s the fluid between them you’re focused on. In chlor-alkali baseball, the pitcher (the solid lead plate in your car battery) is made of titanium; the catcher (the lead dioxide plate in your car battery) is a thin film of mercury flowing over steel. The fluid that fills the space between them is a brine 5-10x as salty as seawater (NaCl + H2O).There are several steps in this process, but, overall, the chemistry looks like this:

               

This reaction costs energy (there on the left hand side of the equation) but what it generates – caustic soda (NaOH) and chlorine (Cl2) there on the right – has value. Pitchers to catchers. Catchers to pitchers. Energy baseball.

Level Three

And here, maybe, we have to go all the way back to the source for some understanding of context. And so this part begins with a question:

How do you tell the story of something that is no longer there?

Maybe you start with this: It’s a bit less than 5 hours by train from Cadiz to Madrid. There are thirteen trains per day, most days of the year, and you’re on board and you’re thinking about work. Or the weekend. Or the trip you’ve just finished. Or the vacation time you’re about to begin. And ~ 400 km from Cadiz – at a crossing where you might not even see the sign – you’ll pass within 60 km of the Almaden mine. And the name might ring a bell or it might not – we all stare idly out the same window and notice different things – but the train has just passed within 60 km of the single largest deposit of mercury in the world. A deposit that – until mining ceased in 2004 – was in continuous operation for ~ 2000 years and in that time yielded approximately 1/3 of the world’s total mercury ore.

This one location.

And nobody really understands it – the distribution of that element. The geologists don’t understand it. The engineers don’t understand it. Why do elements concentrate like they do? In this case, in Spain. Slovenia. Italy. Peru. California. Why is the mercury where it is? And there at the top of that list – that first location – that one in Spain – that one called Almaden – is the largest of all deposits. At Almaden, the ore body there so rich in cinnabar that elemental mercury – the mercury in thermometers – sweats from the veins in the rock walls.

If you’re a scientist or an engineer and you’re thinking about mercury in the environment you’re thinking of numbers like this: in terms of concentration, mercury is somewhere around 1/20 of a part per million (ppm) as a “global average background” in soil. As it moves around the globe – in mining waste, in air, in water – that concentration enriches and it increases: it is around 1/10 of a part per million in soil on the otherwise uncontaminated downwind edge of a continent where everything carried on wind currents – in this case, specifically the mercury found in coal – ends up eventually sifting down and depositing; it is up to one part per million in that coal itself; it is up to 10 parts per million in contaminated sediment – maybe downstream of a facility that used elemental mercury in those room-sized, not-quite batteries to make bleach or PVC or DDT – downstream where the momentum of the river changes and whatever is in suspension settles to the bottom and is stored; and it is maybe 100 parts per million or higher in the soil in places where mercury is still mined and where environmental controls might not be what they are elsewhere on the planet.

These numbers are context. In the Almaden Mine, the mercury concentration in the ore body reaches 8%. That is, within the rock itself, the concentration of mercury reaches 80,000 parts per million. Working in this environment was – and remains – very hard on the miners.

When mining at Almaden began in earnest in the 1500s – after the discovery of silver in the Americas and the realization that – with the newly invented patio process – access to mercury meant access to silver – the Spanish Crown gave convicts a choice: the galleys or the mine. It is estimated that during the period in which the mines were worked principally by convicts, ~ 25% of those convicts died during their term of labor, mostly and most likely from mercury exposure. An unknown percentage of convicts were released from their term of labor insane. Later, the work force also included slaves and gypsies, arrested, in their case, for anything and everything including the crime of speaking Romany.  For slaves and for gypsies, the terms of their sentence were indefinite: for slaves, because they’d been purchased outright; for gypsies because for their term of sentence to end, the Crown required that they prove evidence of a settled home to return to. And they – the Romany – by confluence of culture, vocation and social caste, could never fulfill this criterion for legal release. For these individuals, working the mine at Almaden was a life sentence.

And you can’t really talk about the metal without also talking about the mineral: cinnabar. Or more correctly, the minerals: cinnabar and metacinnabar. The red and the black. Or in the scheme of the philosophers and the alchemists, the light and the heavy.  Even more correctly (though with no greater clarity), there are actually three forms of the mineral: cinnabar, metacinnabar and hypercinnabar. The red and the black and the other black. The transition between these minerals – if it can be thought of as a transition – if one form can actually change into the next – is still a conundrum. Temperature seems important: heat metacinnabar to > 400 C and it can be transformed to cinnabar; continue heating to > 500 C and hypercinnabar may appear. That you can create these transitions in the laboratory does not mean they occur in nature though. And while it is generally agreed that the three forms of HgS are distinct minerals with distinct crystal shapes, what happens outside of the laboratory when high temperature magmas cool and minerals begin to precipitate is anyone’s guess.

In all cases though, the mineral is HgS. And cinnabar – the red form – has been used by people for over 10,000 years – for as long as we’ve had that thing that we call culture, really – as a pigment. Vermillion. It has appeared throughout the world’s history in cosmetics, lacquers, wall paints, ceramics, and medicines. Yeah – I know – in medicines. And if you want to convert HgS to elemental mercury (Hg0 ) you roast the ore. The sulfur will be oxidized and the mercury will be liberated. And if you’ve roasted in a retort or some other vessel with a lid, the volatilized mercury will be captured and will cool and condense and dribble down as Hg0. The stuff of thermometers.

And here is where I go back to Almaden. And the richness of the cinnabar deposit there. And the fact that Hg0 occurs at ambient temperatures in the ore body itself. As a geologist or as an engineer, THAT is wild.

Level 4

Five graphs. Six curves. Or maybe 7, depending on how you arrange them. They don’t tell you how to solve the problem, but they show you where the problem might be. And why. 

Start by making a graph – draw a horizontal line and a vertical line; let these lines intersect at the left end of the horizontal and bottom end of the vertical. The horizontal is the x-axis – this is what is called the ‘independent variable’ axis – what you put on the x-axis drives the relationship you’re looking at. The y-axis is the ‘dependent variable’ axis- what you put on the y-axis is what you think might be changing in response to how what you’ve put on the horizontal line changes. One line is an axis. Two lines intersecting are axes. These two lines that you’ve drawn are the axes of a graph. The idea you are testing in your graph is whether the ‘x’ that you’ve chosen drives the ‘y’ that you think is a reasonable process or variable or change to have been created in response. In general, for graphs of the sorts coming below, the smallest values of whatever variable you’re looking at: the lowest concentration, the slowest rate, the shortest time, the shallowest depth lie closest to the intersection of the axes. For either axis and whatever variable, the values in question increase moving away from that intersection. Unless specified otherwise, it is reasonable to assume that the values of x and y (written {x,y}) at the intersection of the axes are {0,0}.

When you look at a graph, you are paying attention to both axes: change and response; change and response. If you can generate enough relevant data points for the relationship you are interested in, you can draw a line of some shape through these points. A line is a curve without a bend. If your data arrange themselves in a line you have a relationship that trucks out predictably toward infinity in however way you choose to define the boundaries of infinity. A curve is a line that bends in response to some force or factor that alters the predictability of the change and response: There is a dam on a stream; there is tree that they’ve paved the road around; there are only so many parking spaces in the parking lot and at some point they are full regardless of how many people continue to want to come shopping. Sometimes you can see the obstruction that bent your line into a curve; sometimes you can’t see the obstruction but you can measure its impact; sometimes in the data you can simply sense that the obstruction is there. To help explain a graph to yourself – to explain what the relationship in the data might be telling you, the first thing you can do is to try to make three statements – tell three stories – about what you see in the data you’re observing. 

Graph 1 / Curve 1: Landscape-scale control on methyl mercury production

Why is this important to know? Because for methyl mercury to be made in the landscape, inorganic mercury has to be present. If there is a general relationship between the amount of inorganic mercury that is present and either the amount of methyl mercury that is made OR the speed at which it is made, it would be good to know these things.  

Make a graph:

The x-axis variable (the independent variable) =  the concentration of inorganic mercury present in the sediment or soil

The y-axis variable (the dependent variable) = the concentration of methyl mercury present in that same sample of dirt

Take these data from 1400+ inorganic mercury/methyl mercury data pairs from sites all over the world and plot them on one graph and what you see is this: the relationship between these two variables is roughly a straight diagonal line up to about 10 milligrams per kilogram (mg/kg) or parts per million (ppm) inorganic mercury. Above this concentration of inorganic mercury on the x-axis, the data curve becomes a flat horizontal line.  

To help explain a graph to yourself – to explain what the relationship in the data might be showing you, the first thing you can do is to make three statements – three stories – about what you see. In this graph, these stories could be:

  1. At a concentration of less than 10 ppm inorganic mercury, as the concentration of inorganic mercury increases, the concentration of methyl mercury increases too. The relationship is a diagonal line so if we know what the concentration of inorganic mercury is, we can predict what the concentration of methyl mercury is likely to be.
  2. At a concentration of 10 ppm inorganic mercury or higher, even as the concentration of inorganic mercury continues to increase, the concentration of methyl mercury does not change.
  3. If it is bacteria (which it is – more about this later in a different graph with a different set of curves) that turn inorganic mercury into methyl mercury, is there something happening above 10 ppm inorganic mercury that is affecting the bacteria?  

Even if you can’t see an obstruction, sometimes you can sense that it’s there.

Why is the information in this graph worth knowing? If you are thinking about how to clean up a location where total mercury in sediment is around 10 ppm or lower, the relationship in this graph suggests that IF you can decrease the concentration of inorganic mercury in surface sediment (for example, by dredging it out or putting a stable cap of clean material over it), THEN you will decrease the concentration of methyl mercury. IF you are decreasing the concentration of methyl mercury THEN you are also very likely decreasing the speed at which bacteria are changing inorganic mercury into methyl mercury. This decrease in the speed of methyl mercury production has meaning for ecological recovery.

IF, however, you are thinking about how to clean up a site where total mercury in sediment is 10 ppm or higher, the relationship in this graph suggests that you could significantly clean up the mercury that’s there (say, decreasing an average concentration from 100 ppm to 10 ppm) and that clean up might have little to no impact on the methyl mercury question. That clean up might still have significant value for other reasons: keeping high concentrations of mercury from eroding and being transported into other areas like wetlands is important (more about that later in a different graph with a different set of curves) –  but if the goal of the clean up is to protect the local food web from methyl mercury exposure, a high concentration sediment clean up might result in little change to what you see in fish tissue in the general area of the clean-up site. What this statement implies is important: the concentration of total mercury in sediment is not always a good indicator of the extent to which the food web is at risk. The concentration of total mercury in sediment is easy to measure though, and if you have a small(-ish) budget or a large(-ish) site or complications like very deep water in which you are sampling (or some combination of all three challenges), measures of total mercury in sediment may be the best or most cost effective or most widely distributed set of data you have to work with. So you work with what you’ve got.

Overall, then, it is possible to say that in thinking about what you intend to accomplish at your site, it is good to know (1) where your site might lie on this graph in terms of the concentrations of mercury present; and (2) what a reasonable definition of ‘success’ could be for everybody associated with or impacted by the site.

Graph 2 / Curve 2: Landscape-scale control on methyl mercury transport

Why is this important to know? Because if there is mercury present in the landscape and bacteria have enough of what they need to  convert inorganic mercury to methyl mercury, then your landscape may need monitoring to understand whether the food web is at risk.

Make a graph:

the x-axis variable = the percentage of your landscape that is wetlands

the y-axis variable = the strength of the relationship between dissolved organic carbon and dissolved methyl mercury in surface water. 

The relationship between these two variables is roughly a straight diagonal line: What you can do with graphs to help explain them to yourself is to make three stories from what you see. In this graph, these stories could be:

  1. The higher the percentage of wetlands in your landscape, the stronger the relationship between dissolved organic carbon and dissolved methyl mercury in water in your landscape.
  2. If your landscape has a low percentage of wetlands, the relationship between the amount of carbon in the water and the amount of methyl mercury in the water won’t be very strong; in this case, knowing something about one thing (carbon transport) won’t tell you much about a second thing (methyl mercury transport).
  3. If your landscape has a high percentage of wetlands, the relationship between the amount of carbon in water and the amount of methyl mercury in water may be strong enough that you can predict something about one thing (the concentration of methyl mercury in water) from a second thing (the concentration of carbon in water).  

Why is this information worth knowing? Because dissolved organic carbon can be inexpensive and simple to monitor; dissolved methyl mercury is neither. IF it is possible to monitor dissolved organic carbon as a ‘stand in’ for methyl mercury and IF you have a landscape rich in wetlands, THEN you can likely generate the kind of data that let you ask useful questions: do concentrations change in response to water temperature? or the amount of oxygen in the water? do they change in response to season? to flood patterns? to tidal cycles? If you have some understanding of the why they change, you can start thinking about solutions in terms of the how to make them change differently. 

Another way to say this same thing is that with the particular linear relationship in Graph 2: (1) you have an overview of whether there might be a problem with food web transfer of methyl mercury in your landscape (i.e., is there mercury present [and where is your site on the x-axis of Graph 1?] and what is your rough percentage of wetlands?) and, subsequently, (2) if you do have a problem, you might have a relatively inexpensive way to generate a data set to assess whether you can possibly move the needle on either how fast or how much methyl mercury is being made in that landscape and how it is moving through that wetland. These are the types of questions that are useful for understanding what influences biological exposure and uptake of methyl mercury in the environment.

Graph 3 / Curves 3 & 4: Fine-scale/small-scale control of methyl mercury production/loss

Why is this information worth knowing? This is the crux of the process that results in methyl mercury accumulation in food webs. This is centimeter-scale control and this graph is drawn slightly differently. In this case, when you draw your lines, let them intersect at the left end of the horizontal and top end of the vertical. On your first pass through this graph, let the processes hang in space. Once the processes make sense, they can be anchored in different environments in different ways.

Make a graph:

The x-axis variable = the rate at which bacteria are either making or taking apart methyl mercury. 

The y-axis variable = depth. 

Depth is generally considered the ‘z-axis’ and can be thought of in sediment or in the water column; when depth is in sediment, z = 0 is called the ‘sediment-water interface’. When depth is in the water column, z = 0 can be the actual water surface or it can be an surface within the water where something is varying above and below that interface. An example could be salt content in the water of an estuary: the fresh water flowing out of the estuary from the river has a low salt content while the salt water flowing into the estuary from the ocean has a higher salt content. These two types of water may not mix as they flow past each other in opposite directions and may create a moving interface with its own z = 0. Or a lake in which a combination of depth, stillness and temperature result in the lower portion of the lake water not mixing well with the water closer to the lake surface.

Data for this graph are two curves:

  1. The curve with the bend is the rate at which methyl mercury is made
  2. The curve without the bend is the rate at which methyl mercury is taken apart.

Both curves are driven by bacteria and not all bacteria work in the same ways. Tell yourself three stories from the overlap of these two curves:

  1. At some shallow depth near the top of the graph, the rate at which methyl mercury is taken apart (straight line) is faster than the rate at which it is made (curved line).
  2. At some intermediate depth in the middle of the graph, the rate at which methyl mercury is made is faster than the rate at which it is taken apart.
  3. At some deeper depth near the bottom of the graph, the rate at which methyl mercury is taken apart is again faster than the rate at which it is made.

One idea that these three stories suggest is this: there are many kinds of bacteria that can take methyl mercury apart and they are found at all depths in sediment. There are, however, only a few kinds of bacteria that can make methyl mercury and they are most active in a narrow band of sediment depth. It is this narrow band in which making is greater than taking apart that creates the biggest ecological problems associated with mercury.

The implication of this graph that is important to consider is that the critical depth at which production exceeds consumption – the depth at which the making is faster than the taking apart – is controlled by the environment. In a freshwater lake without much algae and lake water rich in oxygen, this critical depth might occur 12 inches deep in sediment. In a wetland, with some  bacteria and plant and animal life and water that moves slowly but fast enough to keep some oxygen in circulation, this critical depth might occur within the top 1 inch of the sediment. In a salt marsh with a lot of bacteria and plant and animal life and water that may stand in stagnant ponds until all the oxygen in the pond water is gone, this critical depth might rise right out of the sediment and into the water column itself.

In thinking about mercury being released from sediment into overlying water, the sediment-water interface (that z = 0 for sediment) can function like a trap-door: in the sediment of freshwater lakes, when that trap door opens, what is released into the water is inorganic mercury – the methylation that occurred deeper in the sediment having been ‘undone’ near the sediment surface. In the wetland, when the trap door opens and the critical depth is 1 inch deep or shallower, what is released into the water is likely methyl mercury. In the salt marsh – and in particular in those places on the marsh where there are pools or pannes of standing water – the trap door is simply propped open. If you are a small fish, whether you are in the freshwater lake, the wetland or the salt marsh can significantly influence how much methyl mercury you are exposed to. If you are a larger fish, what the food web looks like around that small fish can significantly influence the rate and extent to which you are exposed to methyl mercury from eating that small fish. If you are a person who eats the larger fish, you are also in that food web and the overall exposure passes on to you.  

Why is this information useful to know? Because it is helpful to think about the overlap of these “make and take apart” curves in a location you’re assessing for remedy and, specifically: (1) what is the ‘z’ at which the making outstrips the taking apart; and (2) where is that ‘z’ relative to the sediment-water interface in this location? There are many ways to solve problems and sometimes the solution isn’t what you initially thought it might be. Sometimes the problem you’ve identified doesn’t even need solving.

Graph 4 / Curve 5: Waterbody scale – fish exposure (rivers more than lakes)

This one isn’t always true, but the implications of the problem become clear when you read the graph backwards. Or when you flip it upside down.

Make a graph:

The x-axis variable = the concentration of total mercury in stream water. 

The y-axis variable = the concentration of total mercury in fish swimming in that stream.  

The shape of the graph is the shape of the problem. Tell yourself three stories from the data:

  1. At a very low concentration of total mercury in streams, it is possible to find a large range of total mercury concentrations in the fish swimming in that stream.
  2. At a wide range of concentrations of total mercury in streams, it is possible to find little or no change in the concentration of mercury in fish swimming in that stream.
  3. If you have a high concentration of total mercury in a stream, that concentration could be reduced by a significant amount without seeing any measurable impact on the concentration of total mercury in the fish swimming in that stream.

In telling yourself that third story, you have introduced a concept of time. Flip the graph upside down and think about what might be (or might have been) driving the change in the x-axis variable. IF the concentration of total mercury in water had been declining over time because a facility had been decreasing the amount of mercury that they were releasing and then effectively stopped that release, this shape that looks like a curve might well be two straight lines: one (dotted blue here and mostly vertical) reflecting the decrease in fish exposure as the pipe is slowly being capped and a second (dotted blue and mostly horizontal) reflecting ongoing lower dose exposure from the creation and transport processes sketched out in Graphs 1-3. The second line in this flipped graph – the one that may appear almost horizontal – is the long tail of the hoped for ecological recovery from historical mercury releases into the environment. 

Another way to present this same scenario in a way that also matches what you see in field data might look like this:

Graph 5 / Curves 6 & 7 – Landscape and site scale – the time disconnect in recovery

Make a graph:

The x-axis variable = time 

The y-axis variable = the concentration of total mercury 

Data for this graph are two curves:

  1. The curve that starts high and arcs downward is the concentration of mercury in river water during and after the process of a facility closure.
  2. The curve that starts low and rises slowly is the concentration of mercury in fish within that landscape after the process of facility closure.

The reason for the slow rise over time in fish tissue mercury even while overall mercury exposure is going down is Graph 3: methyl mercury production in the environment. While the majority of the historical release has likely found its way into riverbank soil and sediment where it is mostly buried from all but the most significant erosion and not particularly interacting with the environment, the processes in Graph 3 continue to make methyl mercury and that methyl mercury continues to diffuse up and out of the sediment trap door. That methyl mercury diffuses into the algae who are eaten by the tiny creatures (zooplankton) who are eaten by the smaller fish who are eaten by the larger fish and on up. This is the crux of the challenge with mercury-impacted sediment sites: where you are in space and time and ecosystem type matter. What matters too is the extent to which people living in the vicinity of historical releases have had to rely on that aquatic ecosystem for food and livelihood. It is the relative balance of the general processes described in these 5 graphs that determine how far from a historical release you have to be living for that release to not be currently – and continuously – impacting your community.