Guadalupe River mercury concentrations and loads during the large rare January 2017 storm

The Guadalupe River watershed has a long history of urbanization and is the location of a historic mining district that produced 40 million kg of mercury (Hg) during operation. The river system is listed as impaired and is a large pathway of Hg to San Francisco Bay. The TMDL calls for stakeholders to reduce load to an average of 9.4 kg per year over 20 years by removing H-laden wastes in the mining and urban areas and implementing other treatment practices. To demonstrate success, the stakeholders are documenting management efforts and have been conducting water sampling to show that average loads are decreasing or that Hg concentrations on suspended particles are trending towards the TMDL target of 0.2 mg/kg (dry wt., annual median). The Guadalupe River is capable of flows >10,000 cfs, and a flood flow of >6,000 cfs has a return interval of approximately 1:5 years yet most sampling has occurred at flows <4,000 cfs. Since most Hg is transported in the system during large storm events when Hg can be mobilized from the historic mining district, it is essential to measure the Hg load during these rare events. The main objective of this study was to sample and estimate loads during such a rare events.