There is significant random and systematic variability in As concentrations in numerous public water system wells in Minnesota. Arsenic concentrations fluctuate above and below the USA's As drinking water Maximum Contaminant Level (MCL) of 10 μg/L. The average As concentration is commonly within one standard deviation of the MCL. Results of intensive sampling conducted over the course of approximately 1 year at 3 public water system wells is consistent with the analysis of historic As measurements. In some cases, significant As concentration variability was noted over a short period of time. In these wells, the As concentration was less than 10 μg/L shortly after pumping started, but the As concentration increased over time to a level exceeding 10 μg/L. In these wells, the As concentration decreased to below 10 μg/L again when pumping was briefly restarted after being stopped for 4 h. The As concentration variability is likely due to As adsorption reactions between Fe oxides in the well borehole and in the aquifer near the well borehole during periods when the pump is not in operation. When it is crucial to accurately determine true average As concentrations - for example, at one of the many wells that fluctuate above and below the regulatory As limit of 10 μg/L - it is worthwhile collecting samples frequently during pumping to more accurately determine the average As concentration. Determining a reliable average depends on the standard deviation (SD) of the measurements, with more measurements required if the SD is larger.
Bibliographical noteFunding Information:
This research was jointly funded by the Center for Urban and Regional Affairs and the Water Resources Center at the University of Minnesota – Minneapolis, and the Minnesota Department of Health. Water geochemical analyses were performed by Rick Knurr in the University of Minnesota, Department of Geology and Geophysics aqueous geochemical laboratory. We thank Dr. Bernhardt Saini-Eidukat (North Dakota State University), Dr. Bill Arnold (University of Minnesota), and Applied Geochemistry’s two reviewers and its associate editor, Dr. Richard Wanty (US Geological Survey), for reviewing the manuscript and providing detailed, insightful comments, which improved the manuscript.
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