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EXECUTIVE SUMMARY

pumping rates or annual pumping volumes than those simulated by the Regional Model.
Consequently, before a new well is sited in that area or a significant increase in pumping
occurs from an existing wellfield in that area, it is recommended that an analysis first be
conducted of the potential effects of the contemplated change on the perchlorate
containment program.

ES.6 Sentinel Monitoring Program

DHS Policy Memo 97-005 requires the implementation of sentinel monitoring in
groundwater upgradient of impacted wells to provide early warning of any unanticipated
changes in groundwater quality. Based on this policy, the sentinel monitoring plan for the
impacted Saugus Formation production wells is intended to provide advanced warning of
concentration changes or the presence of additional contaminants in groundwater that
might affect the perchlorate treatment processes. Additionally, groundwater elevation and
pumping data will be collected under the sentinel monitoring plan to evaluate the
effectiveness of the perchlorate containment plan that is described in this report.

As shown on Figure ES-8 and in Table ES-3, the monitoring well network for the sentinel
monitoring program will monitor both the Alluvial Aquifer and the Saugus Formation
upgradient of each production well. Monitoring will occur at eleven wells, seven of which
do not yet exist. Well locations were selected according to the following considerations:

1. Locating sentinel wells sufficient distances from the production well to allow adequate
time to respond to significant concentration changes

2. Using existing monitoring wells, to the degree possible

3. Locating new monitoring wells in areas where site access will not cause undue
restrictions on drilling, installing, and monitoring new sentinel monitoring wells

Table ES-4 lists the chemical constituents to be monitored, and the frequency at which
monitoring will occur as the operational plan for the impacted wells is implemented. The
program will focus primarily on monitoring for perchlorate, volatile organic compounds,
nitrate, and sulfate, which are the constituents most likely to affect the treatment system if
present at concentrations greater than those observed to date. General minerals (anions and
cations) will be sampled on a biannual basis to provide geochemical information that may
be helpful for evaluating groundwater migration in the vicinity of each impacted
production well. However, nitrate and sulfate will be analyzed annually because of their
potential influence on the ion-exchange treatment system, which is the system likely to be
selected for perchlorate treatment at SCWC-Saugus1, SCWC-Saugus2, and NCWD-11.
Performance monitoring of the pumping plan’s ability to meet the containment objectives
will be accomplished by monitoring groundwater levels and pumping rates during system
startup and analyzing these data with the Regional Model. Water level monitoring will be
conducted at each sentinel well that is completed in the Saugus Formation and at multi-port
monitoring well MP-5, which is also completed in the Saugus Formation and is located
downgradient of SCWC-Saugus1 and SCWC-Saugus2. Water levels will be measured at
these wells during the start-up period for the containment system, as well as immediately
prior to startup. The water level trends will then be compared with water level trends that

RDD/041840005 (CLR2612.DOC) ES-7