Client: City of Tacoma Project: Tacoma Landfill: Hydrogeologic Characterization, Modeling, & Remedial Investigations Location: Tacoma, Washington

Background
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).

Challenges
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.

Approach
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.

Outcome
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.

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