Geologic Vulnerability - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 218

operations, and low contaminant mass released to the environment, the cost to remediate
a kilogram of contaminant at the site of high vulnerability (Site 2) is 75 times greater than
to remediate a kilogram of contaminant at the low vulnerability site (Site 1). This cost dif-
ferential is solely due to the geology present at each site.
The high vulnerability site is located in a geologic area (a sand) that is environmentally
vulnerable to contamination because (1) the highly permeable soil allows contaminants to
infiltrate readily and migrate to groundwater and (2) contaminant plumes are transported
at relatively high seepage velocities in the sand aquifer to potentially sensitive receptors
that include potable water wells and surface water. The costs for the high vulnerabil-
ity site would have been much higher had the contaminants migrated along the bottom
portion of the aquifer; this is sometimes the case with the type of contaminants present
(DNAPLs) since they are denser than water. Luckily, the decreased hydraulic conductivity
within the lower portion of the aquifer prevented this from happening at the higher vul-
nerability site. In addition, the costs would have been significantly higher had there been
a completed human pathway represented by the ingestion of contaminated groundwater.
Table 6.7 summarizes the major differences between these two sites of environmental
contamination. Please note that the vulnerability map ranking accurately predicted the
relative costs of remediation.

TABLE 6.7
Site Comparison Table

Parameter Site 1—Low Vulnerability Site 2—High Vulnerability

Predominant geology Clay Sand

Presence of shallow groundwater No Yes

Size of site 6.5 ha (16 ac) 3.2 ha (8 ac)

Length of operation 70 years 30 years

Land use Heavy industry Heavy industry

Types of contaminants VOCs, PNAs, PCBs, and

heavy metals

VOCs and PCBs

Number of contaminants remediated 27 9

Contaminant mass remediated 227 kg 59 kg

Cleanup criteria Same as Site 2 for

overlapping compounds

Same as Site 1 for
overlapping compounds

Cleanup cost a $400,000 $7,800,000

Remedial methods (soil) Excavation Excavation

Remedial methods (groundwater) Remediation not required Air sparging and soil vapor

Extraction

Other remedial control measures None, unrestricted closure Restricted closure included:

• Deed restriction
• Institutional controls
• Industrial land use only

Timeframe 18 months 14.5 years

Cost per kilogram of contaminant $362 $27,180

Vulnerability ranking using Table 6.2 13 65

a Cleanup costs include costs for investigation and remediation.