EXPLANATION OF THE REPORTING CONVENTION FOR LONG-TERM METHOD DETECTION LEVELS FOR THE STATE ANNUAL DATA REPORTS
The following paragraphs should be added to the introduction of each State annual data report as an explanation of the reporting convention used by the NWQL.
The USGS National Water Quality Laboratory collects quality-control data on a continuing basis to evaluate selected analytical methods to determine long-term method detection levels (LT–MDL’s) and laboratory reporting levels (LRL’s). These values are re-evaluated each year on the basis of the most recent quality-control data and, consequently, may change from year to year.
This reporting procedure limits the occurrence of false positive error. The chance of falsely reporting a concentration greater than the LT–MDL for a sample in which the analyte is not present is 1 percent or less. Application of the LRL limits the occurrence of false negative error. The chance of falsely reporting a non-detection for a sample in which the analyte is present at a concentration equal to or greater than the LRL is 1 percent or less.
Accordingly, concentrations are reported as <LRL for samples in which the analyte was either not detected or did not pass identification. Analytes that are detected at concentrations between the LT–MDL and LRL and that pass identification criteria are estimated. Estimated concentrations will be noted with a remark code of “E.” These data should be used with the understanding that their uncertainty is greater than that of data reported without the “E” remark code.