Case Studies: Successful and Unsuccessful Redevelopment of Contaminated Sites - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 484

Former
contaminant
source (sump)

Adjacent contaminated site

West

Case study 5 site

Former chrome plating tanks

East

Treatment
injection points

Area treated using
in situ injection of
a chemical reactant

Water table

Zone of saturation

Direction of groundwater flow

Legend
Silty sand
Silt/clay
Sand
Gravel

Excavated to 0.5 m
Excavated to 1.5 m
Excavated to 7.0 m
Treatment zone

FIGURE 14.21
Geologic cross section and areas selected for soil excavation and in situ treatment.

The selected technology for groundwater remediation was to pump and treat ground-
water within seven discrete zones using calcium polysulfide. The treated groundwater
was reinjected to create a circulation pattern within each of the seven treatment zones.
Each treatment zone was located along the longitudinal axis of the contaminant plume at
approximate intervals ranging from 500 to 1000 m. The spacing was contingent upon con-
taminant concentrations (i.e., higher concentrations earned a tighter spacing) and access
to the subsurface. Care was taken to ensure that the location of aboveground treatment
equipment was placed in areas that created minimal disturbance and maintained security.
This site exhibited a synergistic effect between a geologically vulnerable area and a
chemical with a high groundwater contaminant risk factor. When groundwater is affected,
costs soar; this is borne out by the more than $25 million it took to remediate this site's soil
and groundwater.
The lessons learned from this case study include

• Geologic maps are essential to all remediation efforts. In areas where there is high
geologic vulnerability and highly mobile contaminants, the synergistic effects
produced by their interaction can be better predicted by knowing the aquifer
structure and subsurface flow patterns.
• Application of effective urban investigative techniques such as collecting ground-
water samples using a cone penetrometer (CPT) method in easements and aquifer
profiling assists with the investigation and remedial efforts.
• Contamination occurring at sites where there is vulnerable geology and high-risk
contaminants is likely to exhibit synergistic effects, and be costly to remediate.