Risk Based Corrective Action (RBCA) and Risk-based Concentrations (RBC)

The term "Corrective Action" implies risk management. It implies that you have assessed the risk, and now will do something about it. Remember the scenario from Module 2: A worker/neighbor/ member of the public shows up with a Ziploc bag filled with something, ..."this is from the area behind the storage shed...where all the vegetation is dead." Your next action is really risk management. You will tell the visitor to get lost, or take the bag down the hall to a different department, or you will go to your boss and start trying to get time and budget to do whatever it is you will do next. This is risk management, prior to risk assessment. Or you could say you made a quick mental risk assessment. From this simple beginning to a multi-million dollar cleanup, there are many management decisions that must be made, each based upon the information gathered from earlier phases. In the case of Superfund, this decision-making process has been formalized, as we discussed in Module 2.

As a practical matter, most environmental contamination is not Superfund, or never gets to the Superfund process for one reason or the other. Also, petroleum contamination is excluded from Superfund. So what decision-making process do we follow? Of course if there is an applicable state or federal regulation, we have to follow that. Often those regulations are not that specific, and even if they are, they often specify the very process that is outlined below.


The notion of a risk-based concentration starts with the global assumption that there is a level of contamination at which the risk of adverse effects is so low that there is no need to proceed with a detailed (and expensive) risk assessment. These are called risk-based concentrations or risk-based screening levels. In 1986 or thereabouts, I got an 81/2 by 11 sheet from the New Jersey Environmental Agency that has RBC's for 20 or so of the most common soil contaminants. (New Jersey is my home state, but I avoid going there; I was given that sheet at a conference in Washington D.C.) For each chemical it had the chemical's name and a concentration. End of story. About that time the EPA Region 3, which if you look on a map is the tier of states just below New Jersey and almost as polluted, started to develop a similar RBC table. Unlike the New Jersey table, which I believe was developed by real estate brokers who wanted to sell land, the EPA table was developed by a toxicologist. It started out simple and un-bureaucratic, it made sense and was easy to read. The RBC has been under assault ever since. Today it starts with a 10 page disclaimer and is no longer "simple." It is still valuable, however. It is not meant to cover air pollution and leaching to groundwater but that is for bureaucratic reasons rather than science.

Here is a cartoon of the RBC process, from the EPA's website:

(It seem recently the EPA has chance RBC to "Regional" etc., so we have to go through a few more layer to get what we want. So we are looking for Regional Screening Levels (RSLs) and read the Welcome Page, then click on the Users Guide and read the top few paragraphs. That will give you some insight into the complexities of generating such a table, if you are very resilient, you can glance at the equitations further down, many of which you have seen before, but should not need to use again for this course - just to persuade you there was some logic and method in coming up with the numbers you'll see soon. Then back to the Welcome page and next Generic Tables where you can find the tables discussed below. Note there is a link to a Calculator. I have not tried the calculator, but I suggest you work with tables in Excel to get a feel for what all is there. Then go to the RBC (RSL) table itself, for the following example we'll use the "Summary Table" and Excel (Color) Excel file. Here I've used the "XL Color version" Here's what the first items in the table looks like

So what do the column heading mean? Go to Users Guide and then at the bottom, Table 1 will have all the definitions of the abbreviations and their default values. The top of that Users Guide there are some useful definitions that by now you should be familiar with, but would not hurt to review. The "key" refers to the top row of the RSL table that tells what is the source of that tox number. "VOC" refers to the designation as Volatile Organic Contaminate, which puts it in a different class for some analyses.

The next two rows are just the chemical name and the CAS number

Now the RBC/RSLs. What this says is that for Acetochlor (the fourth line down) if you had an industrial site with 1.2 x 104 mg of acetochlor per kg of soil (1.2 g / kg), the risk is very small. Further that the risk is from non-cancer effects.

The last section shows some bureaucratic situation. Region III can set its own limits for air and water, but not for other regions. Hence they do publish their own:

(Dear 2017 students, EPA is changing lots of little things besides the designation RSL or RBC, anyhow, I did not try to rewrite all the following, since I believe you can figure it out about as fast as I can explain it. )

Where SSL stands for soil screening level. Formerly the DAF, which stands for Dilution/Attenuation Factor, was given. Now a DAF is 1.0 in the table, but you can factor yourself, if you believe the DAF should be more. As contaminants in soil leachate move through soil and ground water, they are subjected to physical, chemical, and biological processes that tend to reduce the eventual contaminant concentration at the receptor point (i.e.,drinking water well). These processes include adsorption onto soil and aquifer media, chemical transformation (e.g., hydrolysis, precipitation), biological degradation, and dilution due to mixing of the leachate with ambient ground water. The reduction in concentration can be expressed succinctly by a DAF, which is defined as the ratio of contaminant concentration in soil leachate to the concentration in ground water at the receptor point. When calculating SSLs, a DAF is used to backcalculate the target soil leachate concentration from an acceptable ground water concentration (e.g., MCLG). For example, if the acceptable ground water concentration is 0.05 mg/L and the DAF is 10, the target leachate concentration would be 0.5 mg/L.

So how are RBC's calculated? I was hoping you would not ask. In the site you were at, Users Guide, if you click the link on the small Table of Contents towards the right, where it says "Equations," and scroll down, you will see lots of equations, most ever worse than the ones I subjected you to a few weeks ago. Uggg. You do need to know that EPA Region III made very conservative assumptions. The word "conservative" means that the assumption is probably very protective of human health. In fact they make many assumptions, all of which are conservative, so that the final number is very conservative. For example, you learned earlier that a HQ of less than 1 was safe. Here they use of a HQ of 0.1 to get the RBC. However there are many more assumptions besides that in the process. So why does one bother with a risk assessment if you have the RBCs? The answer is that the since the RBC's are 10 times or 100 times more conservative that necessary, if you were do to do a cleanup to the RBC standard, you might be cleaning 10-fold or 100-fold more than you need to. If your site is below the RBC, you are home free and should not have to do anything. (Although note I used the word "should.") If your site is over the RBC, you will probably want to do a real risk assessment. But there is a logical approach to this, and that is called RBCA, on the NEXT page.

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