High Resolution Temporal and Spatial Mapping of Mercury and Methylmercury in Surface Waters of the Sacramento – San Joaquin Delta

Science Center Objects

Mercury (Hg) is a contaminant of significant concern in the San Francisco Bay-Delta estuary and watershed (Bay-Delta). The formation, fate, and transport of methylmercury (MeHg), a particularly toxic organic form of Hg that readily bioaccumulates in wildlife, has been studied extensively throughout the system. However, there is widespread recognition of the need for more comprehensive monitoring of both Hg and MeHg than is practical with traditional field-sampling methods. Now, with the availability of new tools such as remote-sensing (RS) platforms, in-situ optical sensors, and flow-through systems deployed on high-speed watercraft, scientists have the opportunity to gain a better understanding of the spatial and temporal dynamics of Hg forms and their transport and fate in this complex aquatic system. This project combines traditional field-based sampling and continuous fixed-station monitoring with high-speed boat traverses and satellite RS, focused on the Sacramento – San Joaquin Delta. Data gathered from this combination of methods enables researchers to map surface-water total Hg and MeHg forms within the Delta at temporal and spatial resolution never before achievable.

Conceptual model of the proposed work.

Conceptual model of the proposed work. Dissolved and particulate matter and chlorophyll-a (representing phytoplankton) correlate with optically active water-quality constituents. Because of their reflective characteristics, these constituents can be measured from satellites by using optical sensors.

Objective

The USGS California Water Science Center and USGS Earth System Processes Division (Water Mission Area) are working with the University of California, Merced to model surface-water total Hg and MeHg concentrations at high temporal and spatial resolution in surface water of the Delta. The objective of this research is to develop a visualization and concentration estimation tool for these two Hg forms in the Delta. This will be accomplished by developing multi-variable relationships that estimate aqueous total Hg and MeHg concentrations based on their relationship with a number of water-quality parameter proxies that are measured by continuous monitoring, such as turbidity and fluorescent dissolved organic matter (FDOM). Such proxies are easily and rapidly measured using established water-quality monitoring network capabilities.

Total Hg and MeHg concentrations will also be estimated via satellite RS. Because of their reflective characteristics, some proxies can also be measured using optical sensors. To allow greater spatial and temporal resolution using this new method, scientists will use RS to create 30-m grid resolution timeseries maps of surface-water constituents. Such images can then be created for both past and future RS flyover events to generate historic and up-to-date ‘predictive’ maps of concentration gradients across the Delta. In addition to forms of total Hg and MeHg, the project will also produce spatial maps of chlorophyll-a (phytoplankton) and suspended sediment.

Scientific Approach

The development of relationships between Hg and non-Hg proxies (turbidity, FDOM, etc.) will be an ongoing and iterative process during and after the period of sample collection. Both high temporal frequency (15-minute interval) continuous fixed-station monitoring of in-situ optical properties and routine (16-day interval, LANDSAT 8; and 5-day interval, SENTINEL-2A and -2B) RS data will be used to develop high-spatial-resolution (30 m pixels across the whole Delta) maps of Hg and non-Hg surface-water constituents. Developing this visualization tool requires surface-water field sampling for Hg, non-Hg, and associated optical metrics across a wide spatial range and at multiple temporal frequencies (within tidal cycles, monthly, and seasonally) to develop and assess the robustness of the relationships between Hg and non-Hg constituents.

Maps showing concentrations of particulate methylmercury

Maps showing concentrations of particulate methylmercury (pMeHg) based on 3-day mapping transects in the Delta during October 2019 and January 2020, using turbidity and turbidity and latitude as proxies, respectively. Red circles indicate fixed stations where continuous in-situ data are collected, and where total Hg and MeHg forms are sampled over tidal cycles.

Relevance and Benefits

Results from this project will provide valuable insight on the dynamics of Hg and non-Hg water-quality constituents in surface waters of the Delta. This level of detail for both Hg and non-Hg constituents will inform the stakeholder community of scientists, resource managers, regulatory agencies, and the public. This work is also important in addressing integrated watershed assessment, research, and modeling, and in delivering water data and analyses to the nation. Resulting maps will identify high-MeHg areas where additional sampling may be needed to support more detailed fish-consumption advisories. This is important, given the high level of subsistence fishing by disadvantaged communities in the Delta.

This project is directly related the USGS priority to expand and enhance water-resource monitoring networks. Expectations are high for useful results from this next generation of high-resolution data gathering for the Delta. In addition to mapping concentrations of total Hg, MeHg, chlorophyll-a, and suspended sediment at high temporal and spatial resolution, RS technology may also be useful for other applications such as helping to quantify the success of large-scale wetland restoration projects.

Timeline and Results

This project started in summer 2019 and is expected to be completed in spring 2022. The final project report, describing the work and results, will be published at that time. All of the data generated by this project will be made publicly available through the USGS National Water Information System (NWIS) database and as a ScienceBase data release.

Funding

This work is funded through the California Department of Fish and Wildlife from the Water Quality, Supply, and Infrastructure Improvement Act of 2014 (CWC §79708[g]). Additional support for this work is provided by the USGS Water Mission Area ESPD, USGS Cooperative Matching Funds, and UC Merced.