Exposure Units Practice

For this practice, we will use a class exercise and the case study exercise from Module 6: Data Screening Analysis of the Public Health Assessment Training (PHAT), where we identified certain contaminants requiring further evaluation.

First, we will practice using the PHAT Module 6’s exercise that focused on carbon tetrachloride detected in discrete samples collected from drinking water at a private residence.
Then, you will practice defining the exposure unit by using the arsenic in soil data of the PHAT Module 6’s case study exercise.
Finally, we will show you an example of a table compiling data on arsenic in soil samples.

Carbon Tetrachloride in Private Well Water Exercise

1. Define exposure units to evaluate.

Review the Gala Lumber Company (GLC) site map.

The Gala Lumber Company site map shows the six offsite private residential well locations.

Review the map description.

Map description:

Here is the map from PHAT Module 6, which shows the GLC site. The map includes six offsite private residential well locations. These locations are within the rectangular box and are PW-1, PW-2, PW-3, PW-4, PW-5, and PW-6. Three rounds of groundwater sampling occurred at these wells in 2015. Based on this sampling, carbon tetrachloride was identified as a potential contaminant of concern in groundwater during our screening analysis.

The map also shows the direction of groundwater flow. At the GLC site, groundwater north of Haynes Creek flows in a northwesterly direction, while south of Haynes Creek groundwater flows in a southwestern direction.

Question: How should you define the exposure units for residents drinking this private well water?

Answer: You would define six separate exposure units, one for each different private well. This means that PW-1, PW-2, PW-3, PW-4, PW-5, and PW-6 would all be individual exposure units. This assumes that residents obtain drinking water from their own private wells.

Remember: You would NOT combine the six private wells into a single exposure unit. This incorrectly assumes that over time, residents obtain their drinking water at random from across the six wells.

2. Compile data for each exposure unit.

After defining the exposure units to evaluate, you should compile the environmental sampling data for each unit. We will do this step for you.

When compiling sampling data, do the following:

Separate data by each exposure unit.
Include data for completed or potential exposure pathways of concern.
Distinguish data for acute, intermediate, and chronic scenarios, if appropriate for your site.
Distinguish discrete sampling from non-discrete sampling data (see definitions in text box).
Always define exposure units before calculating EPCs. We talk about that in the Estimating Exposure Point Concentrations primer.

A discrete sample refers to an individual environmental sample from a given point and time that is independent of other samples.

ATSDR’s guidance on non-discrete sampling focuses on two types of non-discrete sampling: composite sampling and incremental sampling methodology (ISM). Composite sampling involves combining multiple discrete environmental samples for analysis. ISM is a structured form of composite sampling designed to characterize the overall contamination level within an area of interest, where many incremental soil samples (typically 30 or more) are collected and combined to form a single ISM sample for lab processing and analysis.

In the last step, you defined the exposure units—the six drinking water wells—and at real sites you will perform six separate evaluations for each of the exposure units. But for our example, we are just using carbon tetrachloride in PW-3.

You will begin by compiling data for each potential contaminant of concern and exposure pathway. For this example, we will focus on the one contaminant (carbon tetrachloride) that exceeded its comparison value during the screening, and one exposure pathway (drinking water ingestion). The carbon tetrachloride data for PW-3 from PHAT Module 6 are summarized in the table below.

**Carbon Tetrachloride Sampling Results for PW3 (from January 2015 to May 2015)**
	01/15	03/15	05/15
ppb	280	260	275

Note: This example has been simplified for training purposes. We will explore only one medium for each contaminant here, but real sites often involve other pathways that add to the total dose. For example, when VOCs such as carbon tetrachloride are present in water, inhalation (e.g., via vapor intrusion) and dermal exposures can significantly contribute to the total exposure dose.

In PHAT Module 6, we saw the three sampling data points for PW-3 (as shown in the table above), collected on 1/15, 3/15, and 5/15. For this example, let’s say we also learned that additional data were collected monthly for the rest of that year. We compiled the data already shown above from 1/15, 3/15, and 5/15, with the additional follow-on monthly data. Here’s our data compilation for PW-3:

**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

These data will be carried into the next Estimating Exposure Point Concentration primer.

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