In mid-November 2022, a contamination issue was discovered with the MDN sample collection bottles that had been in circulation since the end of August 2022. Once identified, the lab immediately stopped sending out bottles from the contaminated bottle lot. It should be noted that this bottle lot was appropriately screened with a robust lot validation protocol and no contamination was identified. With further extensive testing, it was determined that the contamination was not uniform across the bottle lot and only present in a subset of the lot, meaning some of the bottles were contaminated and some were not.
The bottle contamination potentially impacted samples collected from September 2022 through January 2023. To determine whether a specific sample was likely affected by the bottle contamination a robust method involving comparisons to historical data was implemented. Samples collected during this time period were compared to historic mercury concentrations on a site and seasonal specific basis. In addition, this analysis addressed and compensated for the dependence of mercury concentration on precipitation volume. This analysis was performed for all samples that could have been affected by the suspect bottle lot. Once we have completed this review, we will bring our analysis and recommendations to the NADP Spring Meeting (May 2023) for further discussion and a final plan for these samples, data, and resulting annual maps and averages.
A qualifier will be added to all of the potentially contaminated sample concentrations and associated data will not be shared on the website until further discussion at the Spring meeting.
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