Mercury dynamics in Pond A8 and Alviso Slough have been studied in order to adaptively manage Pond A8 gate
operations and concerns regarding mercury remobilization and bioaccumulation. On June 1, 2011, Pond was
opened to muted tidal flows on a seasonal basis. Gates were closed on December 1 of 2011, and then 3 gates were
open on June 1 of 2012. During 2012 and 2013, 3 gates were opened on June 1, and closed again on December 1.
In early March of 2014, 3 gates were opened early for that year, and on September 29, 2014, 2 additional gates
were opened for a total of 5 out of 8 gates being opened to muted tidal flows into Pond A8. The 5 gates have
remained open since then, including for the first time remaining open past December 1. These operational
changes were allowed by working with the National Marine Fisheries Service (NMFS). During 2015 researchers
studied what the effects of leaving the gates open during the winter months had on mercury bioaccumulation and
water quality.

The results of the 2015 study found that bird egg mercury concentrations were about the same levels found at
reference areas collected during the same timeperiod. Similarly, pond and slough fish mercury levels were at
levels consistent with nearby reference areas. Water samples of mercury in the pond and sloughs supported the
conclusions from the fish sampling. Alviso Slough scour results show that erosion is still occurring in Alviso Slough,
still mostly near the A6 breaches as had been observed previously, though more erosion is now occurring in the
rest of the slough. From 2010 to October 2015 about 35kg to 39 kg total Hg remobilized over the entire length of
slough – ~64 % is from the zone including the A6 breaches with about one-third of the total is immediately near A6

Overall, the results show that mercury levels of birds, fish and water have stabilized to what they would be without
restoration efforts. Keeping the gates open through the winter months did not affect mercury levels. Leaving the
gates open did not appreciably increase erosion in the slough; a result that is supported by scour model results
which indicate limited slough erosion would occur in the short term even with all 8 gates open.

Based on the above results, researchers have indicated it would be acceptable to open all 8 gates in Mid-August of
2016. Researchers are continuing to study mercury in biota, scour and remobilization of mercury, and water levels
at the southern levee, in 2016.

Pond Complex
Associated File(s)
Download Document PDF - update_on_mercury_studies_june_2016_final1.pdf (91.21 KB)