Practice

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Let’s use what you learned to calculate an EPC for our classroom case study data.

We talked about how you will calculate EPCs differently, depending on the type of samples you have.

For this practice, we will use our class exercises from PHAT, where we identified certain contaminants requiring further evaluation.

  • This practice section will focus on the exercise where carbon tetrachloride was detected in discrete samples collected from drinking water at a private residence.
  • In addition to this practice, you can also do a case study exercise, where you calculate an EPC on your own, using the data on arsenic detected in discrete soil samples collected from the daycare center.
As a reminder, the steps conducted before this Step 3 included the following:
  • Step 1: Define exposure units to evaluate. You defined the exposure units—the six drinking water wells (PW-1 through PW-6). For our example, we are using PW-3.
  • Step 2: Compile data for each exposure unit. For the purposes of this example, we are focusing on the one potential contaminant of concern (COC) that exceeded its CV during the screening, carbon tetrachloride, and one exposure pathway, drinking water ingestion. The carbon tetrachloride data for PW-3 are summarized in the table below.
Carbon tetrachloride sampling results in parts per billion (ppb) for PW-3 by sampling date
1/15 3/15 5/15 6/15 7/15 8/15 9/15 10/15 11/15 12/15
280 260 275 190 210 180 220 290 265 230
Now, you’re ready to estimate the EPC.

First, you need to know about your exposure groups and their duration of exposure. In this example, we will focus on two groups:

  • Residents with chronic exposure
  • A house guest with intermediate exposure

So, based on the EPC guidance we talked about and the information on our example site, what do we know that will help us calculate the EPC?

  • These are discrete samples, collected at a single exposure unit.
  • There are more than eight samples for the exposure unit.
  • There are no non-detect data points.
  • The 95UCL should be used as the EPC for chronic and intermediate exposures.

Note: Example data sets were simplified for this training and do not include data with non-detects. For real sites with non-detect data, refer to the EPC Guidance documents for information on how to handle them.

To calculate the 95UCL for the EPC, you will need to use the ATSDR EPC Tool.

Start by entering the data into the EPC template.

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Upload the template into the EPC Tool.

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Click Calculate EPC and explore the results.

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Next, go to the last section to calculate an EPC for the case study exercise.

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