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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 the MDEQ Remediation 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 Result Report

A line of information is given for each result reported under the test method information section. The items of information in each line (from left to right) correspond to the following definitions:


Sample number - The computer ID number for each analyte under a given test code.


Analyte - Name of the reported analyte/substance.


"RPT LIMITS" Reporting Limits are given as two values separated by a dash. The first value is the Reporting Quantitation Limit (RQL). Results below the RQL can't be reported with sufficient accuracy, and a qualitative result will be given. The second value is the estimated Method Detection Limit (MDL). Results found to be below the MDL will be reported as "ND" (Not Detected). A report of ND means there is no scientific evidence that any of the tested substance in the sample.


"MCL" The "Maximum Contaminant Limit" is a legal limit established under state and federal law. Public supplies should not exceed this limit for the given contaminant under federal/state regulation. While a "safety factor" is usually built into the limit setting process, safety can not be assured for any supply which exceeds the limit.


Commonly used references for interpreting testing information


"Environmental Health Ready Reference" Michigan Environmental Health Association c/o MDEQ, Laboratory Services Section, P.O. Box 30270, Lansing, MI 48909-7770


"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