Module 03, Sub-module 3D, Closure
*** Q. the computer model, and most others too probably, seems full of constants and other information, including calculations. what happens if one or more of these are in error. are there usualy beta releases of these programs or is the peer reviewing of papers usually sufficient?
A. The professional is always responsible for the output, regardless of the input. Presumably the computer generated data is just one of the many inputs and must be combined with professional judgment to produce an output. You must be satisfied that the program produces accurate results, before you rely upon it.
*** Q. If half-life is the time that it takes for the concentration
to decline by half, shouldn't it be a number representing some
portion of time (such as years, minutes, hours), and is that time
specified by the time associated with the rate constant?
A. The units of the rate constant are always one over time or time to the minus one power, and the half-life is always time. You can convert the units as you wish. The rate constant (k) and half-life are related by t1/2=0.693/k
Q. Related question.
A. If you want to know where the 0.693 comes from click here.
*** Q. Will all spill situations reach equilibrium, or are some considered to be continuously unsteady?
A. In real life the spill never reaches equilibrium. The work we are doing with equilibrium shows us in what direction the chemicals are going, but not how fast.
*** Q. In Mackay's model, what is the difference between "soil" and "sediment" (is it organic vs. mineral? or does sediment imply it is in water? or what?).
A. Soil is dry and on the land, sediment is wet and on the bottom of the water. Mackay assumes the same organic carbon content of both. Again, this is not a bad estimate if you are modeling a state or province. If you were interested in a particular area, you could determine those parameters by sampling and laboratory analysis. Then change the default parameters in the model.
***Q. In the partitioning description, I just want to make sure that I have understood your wording correctly. If the liquid chemical was relatively soluble in water than more chemical would dissolve into the water and maybe just a little might remain un-dissolved as a separate layer. (A. Yes. In this instance, we don't consider the losses to air, as in a jar that was sealed. ) These layers (or phases) then depends upon the volatility (?? in gases) and the solubility in liquids to determine how much will be in each layer.
A. Now if you open the jar to air, or just have a jar with a big headspace over the liquids, you have, initially three phases: water, air, and chemical you added. What happens next depends on how soluble and volatile the chemical is. At equilibrium you might just have two phases: contaminated air and contaminated water. If the chemical were not too volatile or too soluble you would have three phases: 1.) air contaminated with the chemical, 2.) water contaminated with the chemical, and 3.) chemical contaminated with water (and maybe air as well).
*** Q. Toxicology: My understanding of toxicology in the past is that when considering a dose-response relationship it is common to use the "worst case scenario". In other words if we were looking at a contaminant mainly in the soil and there was a chance for trespassers we would have to consider the highest concentration, and we would have to consider that a trespassing child would eat 2kg/day of soil. My question is doesn't this lead to over-estimated risk? And since this is such a perceived-risk society (the perceived risk is sometimes more than the actual risk thanks to scare tactics by "environmentalists") shouldn't we be reasonable with our estimating techniques?
A. We will discuss this in exposure assessment. Briefly, each class of receptor must be considered.
*** Q. Ethics: In regards to our pretend situation I was surprised that there was nothing written about what to do when and if an author plagiarizes or fakes data. Lawyers can be disbanded; doctors lose their license, what happens to scientist and engineers?
A. Professional Engineers can lose their license. Regarding scientific publications, if the plagiarized matter got through the reviewers and was published, someone would notice and quickly notify the editors, who would check it out and demand some sort of explanation form the author which they would publish and/or publish the editors' analysis. It would be very embarrassing and would hurt the scientist towards getting future research. Unless it was a minor mistake, the editor would also notify the institution that the author worked for, which would refer it to an ethics committee that would report to the head of the institution and the committees report would be made public.
**Q. There has been a lot of talk about using a four-phase partitioning model
(instead of three-phase) at contaminate sites. Do you know much about the strengths
and weakness of the four-phase model?
A. The four phases being soil, soil water, soil air, and NAPL. Certainly if you have a NAPL layer, you would need to consider it. When you get into NAPL in pores and do on, it gets very complicated. The oil industry spends a lot of effort on this related to oil recovery. For human risk assessment, you need to consider the movement of the contaminant away from the source towards a receptor, so those phase issues become those of a source, how much is leaching out. The programs we use later make a lot of simplifying assumptions, and in practice, that is what must be done. The issue there is that if you are unsure, you use "conservative" assumptions. These assumptions get pyramided throughout the process, making, in some case, absurd results.
**Q. In sub-module 3A it states that the baseline risk assessment evaluate
the exposure to the receptors. Once the evaluation is made, the budget for the
clean up of the site may be acquired years later. In the case of a contaminant
that continues to disperse in the soil, I would imagine that the cost of remediation
would change with time. How does one accurately project the rate of contaminant
dispersion in the soil? Soil may have pockets with absorptive and/or non-absorptive
soils. Without spending money on core samples and other investigative measures,
one would not have a clue as to how much money is required for remediation.
My question, in part, is how does one accurately predict the amount of money
required for soil remediation, and are investigative measures included in the
total remediation budget?
A. Yes. All of what you are describing are part of a proper rise assessment. But, the funding for a true risk assessment is a risk management decision. But risk management needs to be based on risk assessment??? I believe philosophers call that a conundrum. There is no good answer, except that, as a practical matter, some sort of screening risk assessment must be done, and after that the work must proceed in stages. We'll look at one standard method of doing that toward the end of the course.
**Q. I have a question about modeling in general. It appears that there are
many assumptions, variables, and contingencies involved. For example, the lake
is well-mixed, the soil contamination is rather homogenous, the air above the
city is defined volumetrically and is not highly mobile, etc. etc. etc.; it
makes me question how accurate modeling is in general. Is there any evidence
where predictions for a site were made beforehand via modeling, and then compared
say 5 or so years down the road to come up with maybe a regression analysis
of how modeling compares to actual events? An r-squared value would be great.
Does one exist?
Also along these lines, does the modeling become less accurate as a function of time? In other words, I would expect modeling to be pretty good for short-term predictions, but I would expect larger discrepancies later. Does any data exist for this scenario? I guess I am looking for a contingency factor for these things.
A. There are many types of models. For teaching purposes I try to pick something simple, usually just to illustrate one point. A model can never tell you if your assumptions are "right," although it can tell you if they are wrong. One tests a model with "ground truth" or "confirmation." If your model results do not agree with reality, you can go back and adjust the model parameters until it does agree. That's OK. But the model is used to predict the future, so you go back in a year and check again. If the model with the new parameters does not agree with reality, you know the model is wrong. If it does agree, you know it agreed, but that is not the same as "the model is right," because the following year it may not agree.
Q. On totally different note, every time I see the word fugacity, I think of the italian word Fuga. Fuga is a psychological term meaning someone is in a pathological amnesiac condition or state caused by severe stress where the person knows what they are doing but cannot remember afterwards.
A. Sounds like a college professor.
Q. In the Rose experiment it is stated that after a certain
time MEK distributes itself so that it's chemical potential is
same in all media or phases.
I couldn't make out the meaning. Does that mean that all the media are same in their respective manner in order to receive MEK?
Doesn't the type of media affect the chem. Potential of MEK?
A. The term "chemical potential" is related to Gibbs free energy, which is related to entropy, enthalpy, and temperature. You would have to review a chemistry or physical chemistry text to explore that. But intuitively, media such as air and water, are very different in their capacity to hold a chemical. Methane is almost insoluble in water, but freely mixable in air. If you wanted to keep methane in water, you would have to apply pressure to the methane, as you keep carbon dioxide in a soft drink. When you released the pressure the methane would immediately form bubbles and escape into the air. So the methane in water has potential energy. The same is true with a more liquid chemical that is poorly soluble and is volatile. The molecules of the chemical want to leave the water and enter the air, likewise they have potential energy. However the reverse is true, if you start with the entire amount of chemical in the air and none in the water, there is a potential for the chemical to leave the air and enter the water. Albeit a small potential if the chemical is poorly soluble in water. At equilibrium, most of the chemical would remain in the air. The system would be in equilibrium and there would not be any more net transfer from the air to the water. At that point the chemical potential of the chemical in the air and the water is the same.
Q. What exactly is fugacity? My Webster's defines fugacious as "lasting a short time" or "disappearing before the usual time."
A. You can find all about fugacity in the back chapters of a thermodynamics textbook. It was one of the chapters my slow learner class never got to. Physical chemists likewise glance at fugacity on their way somewhere else. Mackay discovered that, when trying to determine the equilibrium between the many types media, fugacity provided him an orderly calculation method that lent itself to relatively simple computer calculations. The method of "fugacity balance" is similar to a mass balance. His text gives a much better explanation and many good examples. I try to avoid it because it not intuitive to many students and, once you have mastered it, it really is only for steady state, a place we never get. It is an excellent entry into environmental fate, and if you are spending time in these issues, I recommend his book.
Q. Is it possible to publish the names of the students alongside their questions when you answer them so that when looking at the question and answer, if I find that my line of thinking matches with that person then I can communicate with him/her.
A. I'm still pondering that. In the meanwhile, if you have something specific, just email me and I will forward it to the other student.
Q. It is not clear to me in calculating the cost of cleaning
up the MEK in the example given in 3A how the dimensions were
arrived at. I may be overlooking the obvious, but I only see the
depth given in the section view. How do we know the other dimensions?
A. I just stated them.
Q. I was a little confused with the Level I Model. I'm not sure what all the parameters are and what's truly being calculated. Perhaps I just need to go back and review this area again.
A. The only parameter we did not talk about was Kow, and we worked on that in Module 4. We did not discuss some of the others, because they are really just estimates of general parameters used in the "evaluative environment," that is, the model. For example, it assumes fish are about 5% lipid by weight. Which is not true, but "fish" in this case includes all the biological life in the water, plants and microorganisms, 5% is probably not a bad estimate. See next.
Q. Again, from Mackay's model if it is a large area, how would the chemical reach equilibrium without transportation? Does the model just assume the chemical can/ will be transported to all locations?
A. The Level I model assumes the chemicals have equilibrated between the phases. The model is of equilibrium because there is no bulk transport into or out of the system we are evaluating and time is not a factor. Therefore it would not matter how the material is being transported between the phases.
Q. I think Mackay's Level I model is very clever. Is it mostly
for illustrative purposes? I am wondering, given the assumptions
that are made about the environment, how applicable this is to
real systems. What is the sensitivity of model output to changes
in volumes of environmental media? I have worked with groundwater
models that make broad assumptions about boundary conditions,
but with good data for calibration to the points of interest,
can be fairly helpful in quantifying changes in an actual aquifer
A. See above. The Level I model is useful but limited.
Q. Also, the dioxin level in HW Problem #8 was actually calculated
using 2,3,7,8 -TCDD (it was the only option) and not 2,3,6,7 -TCDD
as suggested. Does this make a difference?
A. Yes there is a difference between 2,3,6,7 and 2,3,7,8. The "6,7" was my attempt to remember numbers, "7,8" is correct. I fixed this half way through the week. Here is a brief talk about what those numbers mean, which will be part of sub-module 5A Chemical Terms
Q. Also, can new chemicals be added to this program and what
parameters are required to do this?
A. We get into this in Module 4.
Q. The Tox Tutor was very interesting. I had just heard a
speaker from ATSDR (Agency for Toxic Substances and Disease Registry)
speak about a similar website. I'm not sure if it was this one
she was referring to or there's another one out there!
A. That must be the same one.
Q. This format is a great way to teach this class. I would
have never known there were so many net-based resources out there!
And having them a click away, rather than copying down the URLs
and going back to a computer to find them is very handy.
A. Thanks. I am wrestling with some issues about how you can save the course. But, links change.
Q. I was a little baffled by the flow from the streams equaling 100m^3 per hour in one paragraph and then being 1000m^3 of contaminated in 10 hours (DUH--that part makes sense to me). The part that doesn't make sense is when the 1000m^3/hr was used later in the example as the flow rate. I think I must have missed something in my attempt to read the entire module in one day since I was at a business conference all last week. I do understand how you used the 1000m^3/hr rate to get the final equation just not how you got that particular flow rate.
A. I just restated a different problem. I should have used some different numbers.
Question regarding Author ethic comment #6
Does an author only have one chance to submit an article to one journal? or does that really mean one submittal of each paper at one time to one journal? What if an author submits a manuscript to say Nature who then rejects it so the author than submits it to another maybe less prestigious journal?
A. That is no problem, once it is rejected, the author can resubmitted elsewhere.
Question regarding Reviewer obligations #2
If you say a paper takes so many months to review, how do you know how long is too long or how can you judge that you may be able to give reasonable consideration?
A. It always seems too long. You can pester the editors to find the status of your submitted paper, but it is probably in the in-box of some overworked reviewer.
Q. How would a reviewer know if any other manuscripts (except of course if the reviewer was reviewing multiple manuscripts) of substantial similarity were submitted to another publication if they are supposed to treat manuscripts with confidentiality? What constitutes substantial similarities if new knowledge is often "of minutia"?
A. They wouldn't, but if you submitted to two or more journals, once one accepted, you would want to withdraw from the second. When the editors and reviewers of the second journal saw your work in the first journal, they would be angry about the time and effort they spent reviewing you paper.
Q. I suppose the selective toxicity would also make the transition
from animal studies to human applicability even more confounding
and potentially difficult to support.
A. Yup. We'll spend quite a while on this topic.
Q. The tox tutor reads that the detoxification process converts
lipid soluble compounds to polar compounds. Would this mean making
lipophilic substances more hydrophilic (water-soluble)? This
would then aid in the excretion of that particular substance.
A. That's exactly correct.
Q. So metabolism may make a substance more toxic or less toxic than the parent substance? Note: Some chemicals actually increase the amount and activity of microsomal enzymes thus resulting in the increase of the chemicals own metabolism.
A. Yes. An important issue if the chemical is being activated - made more toxic - by the enzymes.
Q. How accurate can we assume the program "Level 1" to be, in particularly in biological processes? For example, during the 2,3,7,8-TCDD process, the percentage of chemical in fish was extremely high. I have somewhat of a problem with that number. What parameters are set to obtain numbers such as it does? What is the percentage error? To tell me that 99% of the chemical is in fish at the equalibrium state makes me question whether or not it is true. There are just too many variables within the biological title "fish". What about mammals such as whales, crustasions such as crabs or serpents such as sea snakes? I just have a tough time believing a computer program even though it is just a tool to help me find the most potential pathways of chemicals. I tend to believe that a person could act more narrow-minded with the help of a computer program such as "Level 1" than other sources.
A. Remember Level 1 address equilibrium, not how long it takes. TCDD is a heavy non-volatile, non-water soluble compound, so it would take a very long time to reach equilibrium. It is also a very persistent compound. The term "fish" includes all aquatic life, plants and microorganisms. This is accounted for the percentage of "fish" that is considered lipid, 5%. So it is not too bad an approximation. The point is, that you would look for and be concerned about the TCDD in animal tissue. You would not be consoled if you checked the water and did not find any TCDD, or very little.
Q. I am copying each lesson to eventually burn onto a cd when the class is over to have as a future reference (having a cd rather than 500 pages of paper is much nicer). I am having a problem with saving some of the pages though. Pages with images in them do not save with the images in them. Is it possible to create a page in a html format with the image created within the html format rather than in an image format such as a jpg or gif? I am capable of saving all of the pages and images, but when I go back to look at a page with an image in it, I have to search through all of the images within that lesson to find the correct image to go with the correct page.
A. I am working on this.
Q. Some people can develop in themselves an immunity to, say, poisons, if they take a small doses of it over a period of time. Now, if two adults, one of whom has an immunity, of approximately the same weight, take a dosage of a poisonous substance, and one suffers, another don't, would the substance be considered poisonous?
A. We will spend some time on this. If we were testing humans, an outbred species subject to a varied environment, we would find there were individuals that were very tolerant and some that were susceptible to toxic effects. As a practical matter we cannot test people, so we test laboratory animals that are inbred and subject to a nearly uniform environment. So how do we draw conclusions about people from the animal studies? That an important matter we will spend quite some time on.
Question What are the similarities and differences (function and reliability) between the "Primary Literature" Peer-Reviewed Journal Articles and the "Secondary Literature" Peer-Reviewed Review Articles?
A. The former is more reliable on the specific points regarding
the research that is being reported. The latter is usually more
usable and perhaps reliable because it surveys the whole field.
An individual article might be contradicted by all the other
research in the field. A good review will tell you this.
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