SUBSURFACE DISCHARGE OF MERCURY AND METHYLMERCURY FROM A SUBTERRANEAN ESTUARY TO SINCLAIR INLET, PUGET SOUND, WASHINGTON, USA

The USGS is investigating the sources and sinks of mercury and the occurrence and production of methylmercury in Sinclair Inlet, an embayment of Puget Sound in Washington State. One potential source is a nearshore area where total mercury concentrations as high as 5,240 ng/L were measured in a shallow monitoring well at a U.S. Navy facility in Bremerton. To determine if detectable quantities of mercury were discharging into Sinclair Inlet in groundwater at this site, we intensively monitored nearshore groundwater in the vicinity during tidal cycles with a range of 5 m in May and June 2008. The highest concentrations of dissolved mercury measured in the nearshore monitoring well (800 ng/L in May and 2,400 ng/L in June) were detected near the water table in the most saline groundwater several hours after the groundwater flow direction reversed from landward to seaward following a high tide. Additionally, dissolved mercury concentrations ranging from less than 0.04 to 700 ng/L were measured in five piezometers in the lower intertidal zone during June. The concentrations of methylmercury in filtered water samples were 0.1 ng/L from the monitoring well, and 1.5 ng/L or less in the intertidal piezometers. Seawater that may be intruding into the saturated zone upgradient from the well during the rising tide may be interacting with mercury-contaminated fill and releasing mercury into the saline groundwater. The release mechanism may be due to a combination of the displacement of sorbed mercury compounds from sorption sites on the aquifer materials and increases in Hg solubility resulting from its complexation with soluable ligands such as chloride The movement of water and solutes within this subterranean estuary probably is facilitated by an extensive storm drain system that fills with seawater during most tidal cycles. Because of the elevated concentrations of dissolved mercury in the groundwater, as well as the extent and daily frequency of seawater intrusion into the aquifer, the mercury load to Sinclair Inlet from this site may be significant relative to that from other sources. Additional data collection activities are planned to estimate annual mercury loads to the Inlet from this site.