The University of Maine Maine In-situ Sound & Color Lab
School of Marine Sciences
Measuring Hg and MeHg Fluxes from Dynamic Systems Using High Resolution In Situ Monitoring - Case Study: The Sacramento-San Joaquin Delta

Quantifying net loads in tidal systems is difficult, time consuming, and often very expensive. Owing to the relatively rapid nature of tidal exchange, numerous measurements are required in a brief amount of time to accurately quantify constituent fluxes between a tidal wetland and its surrounding waters. Further complicating matters, the differences in chemical concentrations of a constituent between the flood and ebb tides are often small, so that the net export of the constituent is orders of magnitude smaller than the bulk exchange in either direction over the tidal cycle. Thus, high-resolution sampling coupled with high-sensitivity instruments over an adequate amount of time is required to accurately determine a net flux. These complications are exacerbated for mercury species because of the difficulties related to clean sampling and trace-level analysis. The USGS currently is collecting data to determine the fluxes of total mercury (Hg) and methyl-Hg (MeHg) in dissolved and particulate phases at Browns Island in the San Francisco Bay-Delta, a tidally influenced estuarine system. Our field deployment package consists of an upward-looking current profiler to quantify water flux, and an array of other instruments measuring the following parameters: UV absorption, DO, pH, salinity, temperature, water depth, optical backscatter, fluorescence, and spectral attenuation. Measurements are collected at 30-minute intervals for seasonal, month-long deployments in the main slough of Brown's Island. We infer Hg and MeHg concentrations by using multivariate analysis of spectral absorbance and fluorescence properties of the continuous measurements, and comparing them to those of discrete samples taken hourly over a 25-hour tidal cycle for each deployment. Preliminary results indicate that in situ measurements can be used to predict MeHg concentrations in a tidal wetland slough in both the filtered (r2=0.96) and unfiltered (r2=0.95) fractions. Despite seasonal differences in both constituent concentrations and the character of the optical properties, these correlations remain robust throughout both the spring and fall seasons. These robust correlations are used to generate high-resolution time series for each Hg species, which are then used to calculate net tidal fluxes.

Fleck, J.A., B.A. Bergamaschi, B.D. Downing, M.A. Lionberger, D. Schoellhamer, E. Boss, W. Heim, and M. Stephenson, 2006. Measuring Hg and MeHg Fluxes from Dynamic Systems Using High Resolution In Situ Monitoring - Case Study: The Sacramento-San Joaquin Delta. 2006 Fall Ocean Sciences Meeting.

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