Friday, December 28, 2012

The Village of DePue: By how much and how many - Part 9

Tis the day after Christmas, and Excel sorting I will go....

Okay, it took me a few more days to get through all this....

In my last post I looked at the Illinois requirement for residential use which told me to meet Appendix B, Table A objectives.

Based on the data in the Excel files I downloaded from the Cleanup DePue website, six contaminants were found in OU-4 - the area where people have access to, that were above the Illinois cleanup objective concentrations in Appendix B, Table A.

I now need to sort the data to see by how many of them are over the objectives.  This, along with how much the exceedance is, will give me a good indication of risk.  I am only concerned with OU-4 because that is where exposure can take place.

Right now, I have a ton of data from soil samples collected in OU-4.  What I don't know is what the plan addresses.  What I do know is that the plan is much more complicated  in evaluating risk than what I am doing here.  What I am trying to do in these posts is make an assessment as to the claim of  "way-above-normal concentrations of pollutants at hundreds of contaminated sites" so don't go telling people "Bowman says you only need to clean it up to this level."

With that in mind, lets do some sorting...

Ahh, but first we need to get some things out of the way first.
  1. csv_post_date is 1996 and 2009
  2. Thallium is identified as less then "<" a value.  Therefore less than 10 will mean it is below TACO Appendix B, Table A objectives.  I am removing thallium from the CoC list.
  3. Other values identified with "<" will be assumed to be the number below it.
  4. Only one sample exceeded the mercury objective of 23 mg/kg.  Since the analytical value is only 24.4 I am going to remove it from the CoC list.
Now that I have it sorted, of the 125 soil samples:
  • 102 exceed the objective for one of the chemicals, arsenic, barium, cadmium, or lead. 
  • Of these 102 OU-4 samples, all but five exceed the objective for arsenic, which is set at background.
  • Only two samples show exceedance for all four of the CoCs.
How, then, do I make sense out of this data?  That's what's missing from the Cleanup DePue's web site.  Tons of data and no context.  This is where I need to be careful in how I describe what I see.  I don't have access to the plan so I cannot speak on the validity of the risk calculations.  I also do not know what the remediation plan is for OU-4.  If they are going to leave the soil in place, well maybe these folks in DePue have something to beef about.  If these "hot spots" are going to be removed, then what I say from this point on is moot.

Oh how I wish I knew what Cleanup DePue and Nancy Loeb, director of the Environmental Advocacy Clinic at Northwestern University School of Law’s Bluhm Legal Clinic and pro-bono counsel for the Village of DePue want.  All I know is they think the plan insufficient to protect them.

The other thing missing from their site is what cleanup levels do they think are health protective and what areas do they want these levels met?  I have been writing these previous posts trying to answer my own curiosity about "way-above-normal concentrations of pollutants at hundreds of contaminated sites"

My problem is that I don't know what they construe as "way-above-normal concentrations."  I am missing some important pieces here.  Since I don't now how the remediation plan is addressing the soil in OU-4, I can only look at what I see from the sample data that I have.

Right now, arsenic is where my focus is because of the number of OU-4 samples where it exceeds the threshold - or cleanup objective in Section 742 Appendix A, Table G.

What I see when I sort the OU-4 soil sample concentrations is that arsenic shows up like this:
  • 27 of the samples are more than twice background
  • Two samples are three times background (3.0 and 3.3)
  • One sample is 4.2 times above back ground
  • The average exceedance is 1.7 times background with a median exceedance of 1.6.
Depending on how one looks at it, twice the amount could be seen as "way-above-normal concentrations of pollutants at hundreds of contaminated sites."  Unfortunately, that's not how it works out in this case with arsenic.

Arsenic is set at background because natural background concentrations of arsenic are often well above the health-based, direct-exposure goals in soil.  That's a bit confusing.  On one hand you tell me the "safe" concentration in soil for arsenic is 0.39 mg/kg and then you tell me you only need to clean it up to background.

Yeah...that's what we are telling you.  Our health-based cleanup goals (objectives) are theoretical erring on the conservative side.  Because we suspect arsenic to be a human carcinogen, we set the acceptable risk for an adverse health affect to no more than one additional cancer out of one million cancers.

I'll let TACO's explain that:
For carcinogens, risks are estimated as the probability of an individual developing cancer over a lifetime as a result of exposure to a contaminant.
What we are looking at is a probability of cancer, in the case of soil, we set that probability of a "safe" level at a concentration that we expect to see no more than one additional cancer out of one million cancers over a 70 year lifetime.  We do this with a calculation involving our good friend the slope factor:
This value is known as a slope factor (SF), and it converts daily intakes of a carcinogen averaged over a lifetime directly to the upper bound risk of an individual developing cancer.  That is, risk is equal to chronic daily intake (CDI) averaged over 70 years (lifetime) multiplied by the SF. (page 4 of TACOs)
Figuring out the dose that will give you no more than one additional cancer in one million is quite simple:

Calculating how low a concentration of a contaminant needs to be in soil to bring about that dose is a bit more complicated.  For risk, we are assuming that ingestion, dermal contact, and inhalation of dust for getting the chemical into to human receptor.  We know that as little Suzy grows from a toddler to an adult she will come in contact with that contaminated soil.  We assume that little Suzy will be in contact with that soil for some period of time each year for 70 years (default is 350 days/year).  We know, through a bunch of different studies, that people like little Suzy and adult Suzy will ingest a certain amount of soil in a day.  The question becomes how much contaminant in that ingested soil needs to be there to bring about a probability of one additional cancer in a million.

Remember this bad boy calculation?

That's how it is calculated.  When you see "IFS" it is "age-adjusted soil ingestion factor." DFS is the "age-adjusted soil dermal factor."  These formulas take into account the differences in body weight and uptake as little Suzy goes from a toddler to an adult.  They are very, very conservative and very, very protective.

So for arsenic, the amount in soil that will bring about no more than a one in one million probability of an additional cancer is 0.39 mg of arsenic per one kilogram of soil.  Why show y'all this?  Because that's how we come up with a "safe" threshold for a contaminant we suspect to be a carcinogen.  You need to see that in order to understand the next part of the calculation.

If little Suzy is exposed to 0.39 mg of arsenic in one kilogram of soil for 350 days a year for 70 years, we expect her to have a one in one million chance of developing cancer.  This is a probability based on assumptions which are all worst case.  The biggest assumption is the Slope Factor (SF or CSF):
The SF is derived through the plotting of a curve that compares dose to response. Statistical procedures usually calculate the SF as the upper 95th percent confidence limit of the slope of the dose-response curve (i.e., there is only a 5% chance that the cancer risk could be greater). Because this is the upper bound risk, the actual risk is between that value and zero. The SF is roughly equivalent to the risk per unit dose, expressed as (mg/kg/d)-1. As with the RfD, the SF is provided by the U.S. EPA. (page 4 of TACOs)
And what is the SF provided by the EPA for arsenic?  1.5 as it stands today.  With that, we can calculate the chronic daily intake (CDI) for arsenic to get us a one in one million risk probability.  We can use the method described in the California equation above:
  • We know the risk (R) we want is one in one million or 1.0 x 10-6 or 0.000001.
  • We know the slope factor (qhuman) is 1.5 (mg/kg-day)-1
  • We know the average weight of the human is 70 kg
So, a little math...and we can calculate the intake level (I) or CDI
I = (0.000001 x 70 kg) / 1.5 mg/kg-day = 0.000047 mg/day or 0.05 μg/day.
0.05 μg of arsenic consumed for 365 days a year for 70 years should see no more than one additional cancer per one million cancers.  What does that mean?  Here is how Illinois describes it in the TACO's Fact Sheet:
The risk of cancer due to exposure to a contaminant is commonly expressed in exponential terms, e.g., 10-6 and 10-4. These terms equate to a risk of 1 in 1,000,000 and 1 in 10,000 respectively. In the benzene example, the risk estimate of 1.5 x 10-5 means that 1.5 additional cases of cancer above background might occur among 100,000 exposed persons (or 15 cases in 1,000,000 persons) as a result of benzene exposure. The background cancer rate is 1 in 3, meaning that over a lifetime, an American’s probability of getting cancer is 0.333333. Adding a 10-6 risk would increase the probability of an individual getting cancer to 0.333334. With the addition of a 10-4 risk, the probability of an individual getting cancer would be 0.333433.
Confused?  Focus on the numbers here:

The slope factor is a value that is a very conservative number.  It is very, very protective of public health in and by itself.
  • The probability of cancer risk is based on a CDI of that concentration of arsenic each day for 70 years.
  • The probability of consuming that amount over 70 years and getting cancer is one in one million.
  • The chance of getting cancer in your lifetime is one in three - 33.3333%.
  • If we expose you to soil with 0.39 mg/arsenic per kg of soil for 350 days a year for 70 years, the chance of getting cancer increases to 33.3334%
Now if you have stayed with me to this point, you might be asking "what is the risk of cancer at the background concentration of arsenic?"

Good question.

Next post: The Village of DePue:  Theoretical Cancer Risk at Background  - Part 10

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