City of Tacoma
Project: Tacoma Landfill: Hydrogeologic Characterization,
Modeling, & Remedial Investigations
Location: Tacoma, Washington
This facility is the City of Tacoma’s primary landfill. During the
early 1980s, contamination was discovered in several domestic water wells
west of the landfill, prompting a remedial investigation under CERCLA
(more commonly known as Superfund).
This landfill is located near several well fields that produce transient
conditions in the underlying groundwater system. One of these well fields
is operated by Tacoma Public Utilities during the summer, when demands
are at their peak; seasonal pumping from these wells effectively reverses
the direction of groundwater flow beneath the landfill. The other well
field is operated by the City of Fircrest. Although our analyses showed
little likelihood of the landfill affecting drinking water supplies from
the Fircrest wells, the public expressed concerns about the facility.
We directed a comprehensive hydrogeologic investigation that involved
installing, sampling, and testing over 70 wells. After characterizing
the site’s hydrogeology, we developed a three-dimensional groundwater
flow model that incorporated aquifer properties, surface water features,
and incident recharge. The model simulated the seasonal flow reversal
and provided data for delineating capture zones and designing a system
of extraction wells to remove the contaminated groundwater. It also helped
us quantify how pumping these extraction wells would impact flows in a
nearby creek. This information was integral to developing methods for
augmenting flows using groundwater from a deeper aquifer.
The extraction system has been effective in cleaning up the aquifer. Not
only have contaminant concentrations decreased significantly since the
early phases of the project, but the extent of contamination is much smaller.
Areas east of the landfill are now contaminant-free, as our model predicted.
Although Fircrest’s supply wells were not in danger, the City later
retained us to “relocate” Well No. 5 to alleviate public concerns.
The new well, which we designed, is now Fircrest’s most productive.