TM24-02 Revision of prep dates and analysis dates reported for chlorophyll, plant pigments, MBAS, and pesticides, for 33,381 results from samples collected between May 7, 2020, and August 3, 2022.

This memorandum alerts users of the U.S. Geological Survey National Water Quality Laboratory (NWQL) of revisions to results provided through the Water Data for the Nation portal (WDFN, https://waterdata.usgs.gov/download-samples/#dataProfile=site).

The laboratory preparation (prep) dates and laboratory analysis (analysis) dates reported for the results in scope of this revision were revised in the National Water Quality Laboratory (NWQL) Laboratory Information Management System (LIMS) on July 14, 2025. Each of the methods impacted has an allowable hold time, which is defined as the maximum time(s) that a sample may be held prior to preparation or analysis and its analytical results still be considered valid. Results that are produced outside of the allowable hold time may be less accurate than results 2 produced within the allowable hold time due to degradation of the target compounds between sample collection and laboratory preparation. For some organic analysis, holding times can be reset from the date of sample extraction to the date of sample analysis. If no extraction or sample preparation is required, the analysis holding time is based on the date of sample collection. Revisions to preparation dates and analysis dates can affect the computation of actual time elapsed relative to allowable holding times. When the actual time elapsed exceeds the allowable holding time for the method, the NWQL releases the data with a qualifier of “[@ - holding time exceeded]” in the “LabInfo_LaboratoryComment” field. This revision impacts the application of the holding time exceedance qualifier as it corrects the preparation and analysis dates of the samples.