Project: Focused Feasibility Study for Arsenic
Location: Port of Tacoma, Washington
Our clients predecessors manufactured sodium hydroxide and an arsenic
pesticide, disposing of wastes in two ponds—highly alkaline, sodium
hydroxide waste in one, and arsenic-laden waste in another. The alkaline
wastes increased the pH of groundwater underlying the site drastically,
leaching arsenic from the second pond and silica from the native sandy
aquifer. These substances, now dissolved in groundwater, migrated toward
the Hylebos waterway, where a 1000-foot sheet-pile wall had been constructed
through two aquifers to slow arsenic movement. PGG was hired to assess
the potential for arsenic to contaminate marine sediments under a range
of conditions and to design measures for preventing such contamination.
Arsenic fate and transport processes are extremely sensitive to pH; at
this site, pH values ranged from nearly neutral (about 7) to over 12.
In addition, the marine waters, which become increasingly saline near
the Hylebos, were suspected of causing changes in arsenic speciation (and
thus mobility) along the flow path. Upon encountering neutral marine waters,
the dissolved silica carried in the high-pH groundwater precipitates out
of solution, leaving behind deposits of opal and brusite. Adding to the
site’s complexity was its tidally influenced hydraulics.
We began by evaluating the long history of exploration and monitoring
data. We then implemented a field exploration program to fill gaps in
data needed to predict the behavior of the arsenic as it approached the
marine sediments. This work, which included using direct-push technology
and data loggers in key areas, greatly enhanced our understanding of geologic,
hydraulic, and chemical processes. We developed two groundwater flow models:
one to execute site-wide analyses such as water balances and one to simulate
the highly variable tidal conditions near the waterway. The models helped
us calculate aquifer and tidal mixing factors and allowed us to assess
the risk to sediments.
Our focused feasibility study demonstrated that our client’s proposed
remedial plans would protect shallow sediments but not deeper ones, which
required additional source control. EPA used the work as a basis to approve
plans for the critical construction window of summer 2004.