Impact of collection container material and holding times on sample integrity for mercury and methylmercury in water

Mercury (Hg) and methylmercury (MeHg) concentrations in streamwater can vary on short timescales (hourly or less) during storm flow and on a diel cycle; the frequency and timing of sampling required to accurately characterize these dynamics may be difficult to accomplish manually. Automated sampling can assist in sample collection; however use has been limited for Hg and MeHg analysis due to stability concerns during extended storage times. We examined the viability of using automated samplers with disposable low‐density polyethylene (LDPE) sample bags to collect streamwater with high levels of Hg (>45 ng L−1) and moderate levels of MeHg (0.2−2.5 ng L−1). Specifically, we investigated the effect of holding times ranging from hours to days on streamwater collected during baseflow and storm flow. Unfiltered and filtered Hg and MeHg concentrations decreased with increases in time before sample processing; holding times of 24 hours or less resulted in concentration changes (mean 11 ± 7% different) similar to variability in duplicates collected manually during analogous field conditions (mean 7 ± 10% different). Comparisons of samples collected with manual and automated techniques (holding times < 24 h) throughout a year for a wide range of stream conditions were also found to be similar to differences observed between duplicate grab samples. These results demonstrate automated sampling into LDPE bags can be effectively used to collect streamwater for Hg and MeHg analysis, and encourage the testing of these materials and methods for implementation in other aqueous systems where high‐frequency sampling is warranted.