Remediation: Techniques and Cost - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 396

Another factor to consider when examining the remediation of groundwater is the
time required to complete the remediation. Remediating groundwater may require more
than 20-30 years or more to complete. Factors influencing the time required and also
likely to impact the cost to remediate groundwater include (USEPA 1999b; Rogers et al.
2006):
• Geology and hydrogeology: Various types of geology influence the technology
selected and also dictate the rate of contaminant recovery. For example, low per-
meability aquifers may require longer remediation efforts.
• Contaminant concentration and distribution: Extensive and mature groundwater
plumes are likely to have contaminants diffused into the lower permeable zones
within the affected aquifer. In these situations, remediation time may be extended.
• Cleanup goals: Achieving lower residual contaminant concentrations with cleanup
can require more time.
• Technology selected: PRBs often take longer because they are more passive reme-
dial systems relying heavily on natural groundwater flow to deliver contaminants
to the remediation area.
• Remedial-system design and maintenance: The rate of cleanup is one important
factor here, but other factors including structural impediments such as buildings
and roads must also be considered. In addition, the spacing of extraction or injec-
tion wells may not be optimized due to engineering issues and other uncontrol-
lable circumstances, such as underground infrastructure.
• Whether there are multiple types of contaminants present: Contaminant plumes
with multiple phases and types of contaminant (e.g., dissolved phase and free
phase and DNAPS VOCs and metals), may have to be remediated in steps or
phases and require additional time.
• Location: Urban areas often dictate or limit the technologies available for shorten-
ing the time required for groundwater remediation. For instance, buildings, roads,
and other structures may lengthen the time required to investigate and conduct
source control measures. As a result, groundwater remediation is initiated later
and takes longer to complete.
• Investigative errors and delays: Failure to fully characterize the nature and extent
of contamination can be disastrous and will result in an ineffective remediation
attempt.
• Source control: If all the sources of contamination have not been identified and
abated, remediation of groundwater may be ineffective and fail.
Selected groundwater remedies at Superfund sites are presented in Figure 11.22.
Groundwater pump and treat is the most common remedial method selected for the
treatment of groundwater at Superfund sites (70% frequency). Other in situ methods, how-
ever, such as air sparging, PRBs, and chemical treatment are now being selected more
frequently (USEPA 2007). This trend is due to an increase in the efficiency of more innova-
tive technologies and their acceptance by remediation professionals. In addition, many
groundwater pump and treat systems are expensive and have not achieved cleanup goals
(USEPA 2007). Figure 11.23 shows a breakdown of other groundwater remediation tech-
nologies that are now being either selected or tested.