Skip to main content

Interpretation of Test Results

Any technical information found in these pages is primarily intended for environmental health and water supply professionals. Others should be aware of the value of consulting professionals for selection and interpretation of the types of testing which will best address individual concerns.

Laboratory testing of drinking water is done to investigate causes of problems with water quality or to assure that sample sources may be used safely for domestic, recreational, or other purposes. Legal standards for many possible drinking water contaminants have been established. These standards are referred to as "Maximum Contaminant Limits" and are regulated under the federal Safe Drinking Water Act for protection of public water supplies.

Many substances have not been sufficiently studied to set a legal standard. Laboratory tests are designed to detect as many substances in water as possible, including many unregulated compounds. Assistance in decision making related to unregulated substances may be provided by the laboratory, county sanitarians, or staff within EGLE Remediation and Redevelopment Division, Laboratory Services Section.

In many cases, it is not possible to use only testing data to judge current, and especially future, protection of a water sample source from harmful contamination. Overall evaluation of water supply safety should be based on a combination of information including field observations, system construction and operation, local hydrology, historical land use, and "related" testing information.

Laboratory Report

The laboratory report provides both Testing Information and Regulatory Information. The first three columns are the testing information:

Analyte Name          Result                        Date tested  

Analyte name - Name of the analyte/substance for which testing was performed.

The next 4 columns provide regulatory information:

RL (Reporting Limit) – This is the lowest concentration that would be reported by the laboratory. Substances not found or found at concentrations less than the RL are reported as Not Detected.

MCL/AL (Maximum Contaminant Level/Action Level) - The Maximum Contaminant Level is set by U.S. EPA and is the highest level which is acceptable under the Safe Drinking Water Act.

Method – The analytical method used.

CAS# - A unique identifier for the analyzed substance developed by Chemical Abstracts Service.

Commonly used references for interpreting testing information

U.S. EPA Website

"Standard Methods for Examination of Water and Wastewater", American Public Health Association, 1015 Fifteenth Street, N.W., Washington, D.C. 20005

"Drinking Water and Health", National Academy Press, 2101 Constitution Avenue, N.W., Washington, D.C. 20418