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Understanding Lab Reports

While sampling may show that contamination exists, it will not always tell you where the contamination came from. This is because some contaminants originate from multiple sources that all can contribute to a measured concentration in soil or water.

When reviewing sample results, you must compare the measured amount to the applicable numerical standard. For example, if the samples taken were soil samples, you must compare the results to the soil sample standards. There are also different standards based on the route of exposure (inhalation, ingestion, etc.). If the sample taken is for drinking water, you should compare the results to the water ingestion standard.

It is also important to make sure the standard you are comparing the results to are in the same unit, for example parts per millions or parts per billion.

When reviewing sample results, you must compare the results to the applicable standards.

The Reporting Limit and Method Detection Limit do not determine whether the results are considered safe or above any relevant standard.

1ppm or 1mg/L= equivalent to adding one drop of water from an eyedropper into 10 gallons of water.

1ppb or 1μg/L= equivalent to adding one drop of water into a 10,000-gallon swimming pool.

Common Terms Used in Lab Reports

Analyte- The chemical being tested for.

Background- The concentration of a chemical found naturally in the environment or otherwise typical of area-wide levels of that contaminant.

Data Qualifiers- All laboratory information is reviewed to ensure it meets specific quality criteria. Sometimes “qualifiers” are applied to a sample result to note problems or irregularities that may have occurred during analysis. Most labs use a standard set of these codes.

Check with the lab that tested the sample to see what their qualifiers represent. Most labs include a key at the bottom of the report.

Duplicate- A duplicate is a second sample, collected, at the same time, and handled as identically as possible to the first. For quality assurance, a duplicate sample may be submitted to check how well the lab reproduces an analytical result. The lab does not know the sample is a duplicate. The test results for both samples should be the almost the same.

Method Detection Limit (MDL)- The method detection limit is the minimum concentration of a chemical that can be measured and reported with 99% confidence that the analyte concentration is greater than zero. If the result is above the MDL, we know with 99% confidence that the analyte is present in the sample.

Non-Detect (ND)- The lab test did not find an amount of the analyte above the method detection limit.

Reporting Limit (RL)- The reporting limit is the smallest concentration of an analyte that the lab can accurately report in a specific sample.

Sample Blanks- Sample containers filled with a clean media from outside the test area. A blank should be non-detect for all analytes, but because some chemicals are commonly found in the environment, detections can occur in the blanks. If the analyte is detected in both the sample and the blank, it is impossible to determine if the concentration reported is from the test area or some other source.

Screening Level (SL)- Screening Levels are used when a site is initially investigated to determine if potentially harmful levels of contamination are present to warrant further investigation. Screening levels can be used as a conservative clean up objective.

Surrogate- Compounds that have similar chemical composition to the analyte of interest. They are "spiked" into the environmental samples prior to analysis. A lab will monitor its analytical system by “spiking” a sample with a surrogate. If a good percentage of the surrogate chemical is recovered, it shows the lab’s test is accurately measuring the substance sought. These are compounds added to the sample at the lab for quality assurance/quality control purposes. If you see “surrogate” results on your analysis report, it does not mean that particular chemical was really in your sample.

Tiered Approach to Corrective Action Objectives (TACO)- Illinois regulations that define the methods for developing soil and groundwater cleanup objectives that consider site conditions and future land use. Illinois EPA uses TACO to determine screening levels that are protective of human health.

Contact

For more information, please contact the Illinois EPA Office of Community Relations. Additional documents not available on this webpage may be obtained through a Freedom of Information Act request or through IEPA Document Explorer.