Reading laboratory data reports and interpreting their results can be confusing. We hope this fact sheet will make it more understandable. If you still have questions, contact your DEC project manager or the laboratory that analyzed the samples. Most labs are very helpful in answering questions about their data.

Sections Found in a Lab Report

Lab reports typically have several sections. These sections may include: 1) the transmittal or cover letter, 2) the case narrative, 3) the analytical results, and 4) the quality assurance/quality control (QA/QC) sample results. QA/QC is performed to give a much greater confidence in the data. A lab report may also include a printout of the analytical instrument readings and evidence that the testing equipment was properly calibrated.

The transmittal letter and case narrative are sometimes combined into one section. The narrative explains any problems encountered in the analysis of the samples. The problems may or may not have affected the quality of the results. The analytical results section will show the analysis performed by the lab, the test result, and notes that indicate any problems encountered. These notes are called “data qualifiers.” Most labs use a standard set of codes for data qualifiers.

What Do the Results Mean?

When you look at the report’s analytical results section, you will see columns indicating the sample matrix (soil, water, etc), the contaminants tested for, and the test results. The results usually are reported in units of parts per million (ppm, mg/L or mg/kg), parts per billion (ppb, µg/L or µg/kg), or percentages. If the contaminant was not found, the column will show “ND” for not detected.

The abbreviation ND is often followed by a number, such as ND (0.5). This does not necessarily mean that the chemical was actually there. It means simply that anything below 0.5 ppm or ppb (whichever units were used for that analysis) would not have been found because it was below the test equipment’s detection limit.

When dealing with drinking water, another column that may be shown is the “maximum contaminant level (MCL).” This number is the highest level of contamination considered acceptable in drinking water. If the number on your result is less than the MCL, your drinking water is considered acceptable. If the MCL column is not present, ask your local DEC office about a regulatory standard for the contaminant. The detection limit of the equipment should be lower than the MCL. If not, you have nothing to compare your results to, and you should ask for a new test or evaluate other labs and/or methods. 

Trip Blank: This is a sample container, often filled with distilled water. The trip blank travels unopened to the site with the empty sample containers and returns unopened to the laboratory with the samples. This is done to confirm that no contamination has been picked up during the “trip” to and from the sample location. If contamination is found in the trip blank, it means that the samples also may have been contaminated from a source outside the sample area. The trip blank should be non-detect (ND). If it isn’t, your sample results may not be accurate, and a new test should be run.

Duplicate: A duplicate is a second sample, as identical as possible to the first one. The sampler will not usually tell the lab about a duplicate sample in order to check on how well the lab reproduces an analytical result. The test results for both samples should be the almost the same.

Surrogate: A lab will monitor its analytical system by “spiking” a sample with another chemical similar to the contaminant to be tested for. If a good percentage (usually 70-120%) of the surrogate chemical is recovered, it shows that the lab’s test can accurately measure the contaminant sought. If you see “surrogate” results on your analysis report, it does not mean that particular chemical that was really in your sample.

Matrix Spike: Occasionally something in the sample itself — its soil, water, or sediment — interferes with the test for the contaminant. To check for that interference, a lab may conduct a “matrix spike” (and a matrix spike duplicate). A known amount of the same chemical being tested for is added to two extra samples before the analysis. If your report shows “matrix spike” data, check to see that the amount of chemical recovered is a good percentage (70-120%). If the percent recovery is low, it means that something in the matrix is interfering with the test, more work may need to be done to overcome the interference. The percent recovery corresponds to the chemical added, not to what was in the original sample. A non-spiked sample also is analyzed for that contaminant, so look for the result which does not include the spike.