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Fact Sheet

October 2009

What You Should Know About Groundwater and Soil Vapor Sampling, Sedgwick County Carbon Dioxide Site, Sedgwick County, Kansas


Groundwater is the water that flows in natural storage areas below the earth's surface. Rain water and surface water soak into the ground surface and build up in the soil and cracked rock providing a source of water for human use. These water-soaked zones are called aquifers.

Groundwater in aquifers can resurface as a spring, or it can release into a water body, such as a stream, river, lake, pond or ocean. It can also resurface when it is withdrawn from the ground by way of a well and pump.

Soil vapor is the gaseous elements and compounds that occur in the small spaces between particles of soil. Such gases can move through or leave the soil or rock, depending upon the changes in pressure.

Chemical testing of groundwater samples in a laboratory is often the only way to find out detailed information on the quality of the groundwater. During groundwater sampling, a probe is driven into the ground, and groundwater is drawn to the surface for analysis.

Soil vapor sampling is a method used to detect contaminants in the shallow area beneath the surface of the soil. This process is especially useful in the analysis of soils with a high sand content and of shallow groundwater. A probe is driven into the ground, and samples of vapors in the soil are drawn to the surface for analysis.

The planned groundwater and soil vapor sampling is offered free of charge to residents near areas where EPA is concerned about soil vapor intrusion into homes. EPA needs your permission to collect groundwater and soil vapor samples from your property.

EPA will contact you to set a time that is convenient to obtain the sample, or obtain your permission to sample your yard when you are away or at work. Allowing EPA to sample your yard for groundwater and soil vapors does not commit you to taking any action.

Sampling your yard for groundwater and soil vapors will normally take a few hours, depending on the testing that is conducted. Samples are collected and taken to a laboratory for examination. The average turn-around time for examination of samples tested, depending on the type of samples and contaminants, ranges from 30-60 days.

When the sample results are received by EPA from the laboratory, they are reviewed for accuracy of data. This review process typically takes one to two weeks. Afterwards, individual reports with cover letters are prepared and sent to each property owner where samples were collected. The overall time frame from sample collection to the property owner's receipt of results can range from 60-90 days.

(Matrix, Units, Analyte, Letters)
At the top of the results, there are codes used to uniquely identify the particular sample. The name of the laboratory doing the testing might also appear on the page, as well as the time and date the sample was collected from your property. Here are some answers to frequently asked questions about your laboratory results.

What is a matrix?  The matrix refers to the type of material that makes up the sample. A matrix might include soil, water, or air. For example, since your yard may be sampled for groundwater or soil vapors, your matrix entry would read “water” or “air”.

What are units?  Units are the measurement used for your analysis. For example, your height is measured in units of feet and inches. Water samples are measured in units of mass per volume. You may see symbols such as ug/l (micrograms per liter) or ppb (parts per billion). Ug/l is a measure of the mass of the substance per liter of water. To put this in perspective, one part per billion is roughly the same as one drop of chlorine in an Olympic-sized swimming pool. Vapor or air samples are measured in units of mass per volume. You may see symbols such as ug/m3 (micrograms per cubic meter) ug/m3 is a measure of the mass of the substance per cubic meter of air.

What is an analyte?  The word analyte is a general term for the substance we are looking for in the sample. The laboratory does testing to find specific analytes, or substances in the soil vapor sample. For instance the laboratory may be looking for solvents or pesticides. The lab report calls these substances analytes.

The report should list each analyte for which the lab was testing and what amounts were found.

What are all these letters?  As part of the quality check, EPA may mark the measurement of certain analytes (the substance in the water) with a flag or qualifier to give readers additional information about the measurement.

The flag is usually in the form of a capital letter appearing next to the numerical result. Examples of Flags:

U - Not detected. The analyte is not present at or above the number next to the U. This number is the amount of analyte that would have to be present to be detected by the lab.

B- To ensure that the laboratory is not adding contamination of its own to the results, the lab always tests a clean water sample with the real samples.

R - Unreliable result. A re-test is needed for a correct measurement.

J- The analyte was detected, but the value of the result is an estimate.

L - The analyte was detected, but the lab report may show an amount lower than what is actually there. The real result may be higher.

U J - The U before the J means that the analyte was not detected in the sample, but the detection limit (number prior to the U J) is an estimated amount.

K- The analyte was detected, but the level is probably lower.

O - No result was obtained.


For additional information concerning soil vapor testing, contact:

Dianna Whitaker
U.S. Environmental Protection Agency Office of Public Affairs
901 N. Fifth St.
Kansas City, KS  66101

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