The Measurement of Water Column Methylmercury Production Potential Rates in Four Oxygenated Reservoirs of the San Francisco Bay Watershed

Valley Water (formerly Santa Clara Valley Water District) provides stream stewardship, wholesale water supply, and flood protection for Santa Clara County, California, in the southern San Francisco Bay Area. A number of the reservoirs owned by the agency sit in the eastern foothills of the Santa Cruz mountains above the city of San Jose and in the watershed below the former New Almaden Mining District, which was historically the largest mercury (Hg) mining district in North America. Aquatic biota in a number of these reservoirs have elevated levels of Hg, presumably as a result of their downstream location relative to the former mining district. The form of Hg that most readily accumulates in aquatic food webs is methylmercury (MeHg), which is produced from inorganic Hg in aquatic systems under oxygen limited and anoxic conditions by specific microbial groups. Reservoirs have been shown to be particularly prone to elevated levels of MeHg and subsequent bioaccumulation due to their propensity for water column stratification and low oxygen conditions in bottom waters and surface sediment. Water column aeration/oxygenation has been used in a number of reservoirs to mitigate in situ MeHg formation by changing the oxidation-reduction status in both bottom waters and surface sediment from conditions that are more conducive to microbial MeHg production (reducing conditions) to those that are less conducive (less reducing to oxidized conditions). As such, Valley Water installed hypolimnetic oxygenation systems in the downstream portions of three reservoirs (Almaden, Calero, and Guadalupe) contaminated with Hg associated with the New Almaden Mining District and in one reservoir (Steven's Creek) that is located in a watershed with no historical Hg mining. Most studies of MeHg production over the last 45 years have focused on the shallow benthic zone, where this process was first identified, has been studied intensively, and where rates of MeHg formation are often readily measurable. More recently however, a few studies have suggested, both by indirect evidence and direct measurement, that under certain conditions the formation of MeHg can also occur within the water column. Compared to the direct measurement of MeHg production in surface sediment (e.g. using radiotracer or stable isotope approaches), MeHg formation in the water column is generally far more technically challenging to directly measure due to the limited concentrations of both organic particulates and bacteria involved in the process, relative to their concentrations in surface sediment. In coordination with Valley Water, the U.S. Geological Survey investigated the potential for water column MeHg production in the four reservoirs noted above. Measurements were made at five depths (ranging from just below the water surface to just above the sediment surface) using a 24-hour stable isotope (200Hg(II)) amendment / bottle incubation approach with freshly collected water samples. Precise sampling depths were determined on site after profiling the complete water column with a water quality sonde and considering the profiles and inflection points associated with temperature, pH, specific conductance and dissolved oxygen. Two sets of experiments were conducted. The first was during May 2019, just prior to the initiation of the seasonal water column aeration process. The second was during August 2019 after the aeration units had been in operation for approximately 3.5 months (May through August). In each case, two experimental treatments were applied to each set of water samples. The first treatment involved raw (unfiltered) water samples. The second treatment involved the same water, but amended with additional suspended particulate material (SPM) collected from the same water collection site using either a 10 micron or 64 micron plankton net (composited vertical tows from the full water column). This second treatment was included to increase the abundance in the incubation bottles of naturally occurring, particulate associated, bacteria that are likely involved in water column MeHg production, and thus increase the signal for detecting this microbial process. In addition to the above experiments designed to measure water column MeHg production, parallel experiments designed to measure water column MeHg degradation were also included during the August 2019 sampling event, using an additional amendment of isotopically enriched Me201Hg to the incubation bottles. Further methodological details associated with the field collection of water and SPM, the MeHg production and degradation incubations, and subsequent analyses can be found in the metadata sections of this data release. This product summarizes all of the data associated with this study, including: analytical characterization of the surface water and SPM used for the incubations, field-collected water quality sonde depth profile data, and the MeHg production and MeHg degradation potential rates. The characterization of the depth-specific unfiltered surface water collected and used for the incubations included the following parameters: ambient total mercury, methylmercury, inorganic divalent 'reactive' mercury, and total suspended solids. Filter-passing (operationally dissolved) surface water constituents included: sulfate, chloride, iron, manganese, nitrate, nitrite, ammonium and phosphate. The characterization of the SPM concentrated slurry used for the incubations included: total mercury, inorganic reactive mercury, and concentration (dry weight per volume). The data results provided in this release includes six data tables, given both as machine-readable 'comma-separated values' format (*.csv) and Microsoft Excel (*.xlxs) format: 1) 'T1_Data.Dictionary.csv (*.xlxs)', the Data Dictionary, which provides definitions and details related to the other five data tables and includes analytical methods citations; 2) 'T2_SW.csv (*.xlxs)', surface water characterization; 3) 'T3_SPM.csv (*.xlxs)', suspended particulate material characterization; 4) 'T4_MPP_MDP.csv (*.xlxs)', MeHg production potential and MeHg degradation potential incubation results; 5) 'T5_Profile.csv (*.xlxs)', water column profile data collected on-site via the Hydrolab water quality sonde, 6) 'T6_QA.csv (*.xlxs)', quality assurance data associated with overlying water and suspended particulate material characterization analyses.