If a Tree Falls In Liberty Park, Does It Reveal Anything About Lead Levels In the Groundwater? (Part I)
For those who are interested (if any), here’s a summary of my summer research project so far. Watch this space for further updates!
A hundred-year-old tree died in Liberty Park in downtown Salt Lake City a few years ago. As living organisms incorporate trace metals (like lead) from their environment, any change in the level of groundwater contamination by these trace metals should be reflected by the organisms that rely on that water. Since the growth and development of trees can be tracked by the ages of their rings, different rings should have different levels of these metals in different rings.
The specific question my project seeks to address is this: Did the use of leaded gasoline in the mid-20th century result in a significant increase in the level of lead in the groundwater in Salt Lake City?
It’s worth noting that the water here is already rich in minerals--it’s very hard water. All groundwater, especially here, has at least some lead in it anyway. It’s also true that mining in the late 19th and early 20th centuries resulted in increased levels of lead entering the local water system. The strategy, then, is to analyze wood samples from the tree from different time periods and see if there is a rise in the lead concentration that coincides with the use of lead additives in gasoline--and if so, how much.
This project was actually started by another researcher who used microwave digestion to process a small number of wood samples. What is microwave digestion? Well, you take a small sample (half a gram or less) and add it to a mixture of concentrated nitric acid and concentrated hydrogen peroxide. (The hydrogen peroxide used in this process is ten times as strong as what you buy at the drugstore.) Place this sample in a plastic container that releases gases at very high pressure, and cook it in an industrial-strength microwave oven for half an hour or so. This process converts all the organic matter--proteins, cellulose, etc.--into water and carbon dioxide. What remains is a solution with the minerals (like lead) dissolved in it.
Samples of this solution are then analyzed for the element in question. The lab I’m working in uses an instrument called an inductively-coupled plasma mass spectrometer (ICP-MS). It’s a big, complex, $400,000 instrument, but here’s the quick oversimplified version: The sample is injected into a stream of argon gas that travels through an ionized plasma at around 4000°C. This knocks an electron off an atom of the sample, giving it a charge. It then travels through an electromagnetic field that bends its path; only atoms with the right mass:charge ratio can get through. The particles that get through are then counted by a detector of some kind. This instrument can separate the particles we care about--lead atoms, in this case--from everything else in very, very small quantities.
How small? Well, you know what the word ‘percent’ means, right? It means one in a hundred--parts per hundred, you might say. One in a thousand, by the same reasoning, would be called parts per thousand. With this instrument, we routinely measure parts per trillion.
How much is one part per trillion? Consider an Olympic-size swimming pool. It holds approximately a million liters of water, with a mass of a million kilograms or a billion grams. How much is a gram? It’s about the mass of a paper clip. One paper clip in an Olympic swimming pool is one part per billion. So, what is one part per trillion? It’s one paper clip in a thousand Olympic swimming pools.
Yeah, the ICP-MS is that sensitive. That’s why we use it for this kind of research. Unfortunately, that’s also part of the problem.
More on this next time.
A hundred-year-old tree died in Liberty Park in downtown Salt Lake City a few years ago. As living organisms incorporate trace metals (like lead) from their environment, any change in the level of groundwater contamination by these trace metals should be reflected by the organisms that rely on that water. Since the growth and development of trees can be tracked by the ages of their rings, different rings should have different levels of these metals in different rings.
The specific question my project seeks to address is this: Did the use of leaded gasoline in the mid-20th century result in a significant increase in the level of lead in the groundwater in Salt Lake City?
It’s worth noting that the water here is already rich in minerals--it’s very hard water. All groundwater, especially here, has at least some lead in it anyway. It’s also true that mining in the late 19th and early 20th centuries resulted in increased levels of lead entering the local water system. The strategy, then, is to analyze wood samples from the tree from different time periods and see if there is a rise in the lead concentration that coincides with the use of lead additives in gasoline--and if so, how much.
This project was actually started by another researcher who used microwave digestion to process a small number of wood samples. What is microwave digestion? Well, you take a small sample (half a gram or less) and add it to a mixture of concentrated nitric acid and concentrated hydrogen peroxide. (The hydrogen peroxide used in this process is ten times as strong as what you buy at the drugstore.) Place this sample in a plastic container that releases gases at very high pressure, and cook it in an industrial-strength microwave oven for half an hour or so. This process converts all the organic matter--proteins, cellulose, etc.--into water and carbon dioxide. What remains is a solution with the minerals (like lead) dissolved in it.
Samples of this solution are then analyzed for the element in question. The lab I’m working in uses an instrument called an inductively-coupled plasma mass spectrometer (ICP-MS). It’s a big, complex, $400,000 instrument, but here’s the quick oversimplified version: The sample is injected into a stream of argon gas that travels through an ionized plasma at around 4000°C. This knocks an electron off an atom of the sample, giving it a charge. It then travels through an electromagnetic field that bends its path; only atoms with the right mass:charge ratio can get through. The particles that get through are then counted by a detector of some kind. This instrument can separate the particles we care about--lead atoms, in this case--from everything else in very, very small quantities.
How small? Well, you know what the word ‘percent’ means, right? It means one in a hundred--parts per hundred, you might say. One in a thousand, by the same reasoning, would be called parts per thousand. With this instrument, we routinely measure parts per trillion.
How much is one part per trillion? Consider an Olympic-size swimming pool. It holds approximately a million liters of water, with a mass of a million kilograms or a billion grams. How much is a gram? It’s about the mass of a paper clip. One paper clip in an Olympic swimming pool is one part per billion. So, what is one part per trillion? It’s one paper clip in a thousand Olympic swimming pools.
Yeah, the ICP-MS is that sensitive. That’s why we use it for this kind of research. Unfortunately, that’s also part of the problem.
More on this next time.
posted by Michael at
9:26 AM
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