the environment. CERCLA requirements include performance evaluations of the remedy, including monitoring systems, at 5-year intervals. The postclosure monitoring and evaluation periods can be modified if the closed CERCLA site is redeveloped for beneficial use following closure (e.g., development of a contaminated, or brownfield, site). In some cases, disposal cells are being built at these sites that comply with both CERCLA and RCRA regulations.

Closed low-level radioactive waste sites are subject to a 30-year observation and maintenance period, which may be shortened or lengthened, based on site-specific conditions, as described in 40 CFR §61.29. The disposal site must be “designed, used, operated, and closed to achieve long-term stability” (10 CFR §61.44). When closed, the licensee is responsible for postoperational surveillance and must maintain a monitoring system based on the operating history of the site.

UMTRA monitoring requirements appear to be tied to final ownership of the land, either the state or the Department of Energy (DOE). A remediated site may be transferred to a state subject to access by federal authorities or to tribes, or it may remain under DOE ownership. The Secretary of Interior takes ownership of removed radiation waste (42 USC §7914-7916).

No RCRA facilities have reached a 30-year lifetime, so performance can only be judged on short and medium timescales. It is unknown if the regulatory requirements will be reduced or extended at the majority of sites, although indefinite maintenance and monitoring periods are anticipated for many sites. Given the discretion written into the regulations, these types of decisions will likely vary by state.


Monitoring systems at waste containment sites may target a variety of media, including soil, groundwater, surface water, and air. Monitoring systems are often designed for the sole purpose of meeting the statutory requirements discussed above and are rarely designed to directly monitor barrier performance. Ideally, a monitoring system would do both. A well-conceived monitoring system is configured

  • to provide information needed to assess barrier system performance and physical state (e.g., degradation),

  • to provide information to assess the state of the waste mass to understand the progress of waste decomposition and stabilization,

  • to monitor places where model scenarios predict contaminants are most likely to be released,

  • to detect contaminant migration along unanticipated pathways,

  • to provide early warning of a contaminant release and thus facilitate corrective action before migrating contaminants adversely impact human health and/or the environment, and

  • to provide information to determine facility maintenance and rehabilitation needs.

Methods for Monitoring

Monitoring system measurements may be made in situ or on samples recovered from monitoring wells or probes. Monitoring devices may take point, area, or volume measurements. Well points for groundwater, subsurface gas sampling probes, and piezometers for measuring hydraulic head are examples of point measurements. Area measurements include blanket drainage layers behind barriers (e.g., leak detection layers in double-liner systems, pan lysimeters beneath sumps) and some geophysical measurements (e.g., ground-penetrating radar, vertical seismic profiling, electrical resistivity tomography). Volume measurements, such as the electrical measurements of resistivity and conductivity (e.g., capacitance probes, time domain reflectometry) and other types of geophysical measurements (e.g., gamma and neutron probes), gauge the properties of a characteristic volume of soil. Table 3.1 identifies common monitoring methods for contaminant migration at waste containment sites. Geophysical techniques are included in the table and subsequent discussion, although their use in monitoring engineered barriers has been limited for reasons discussed below. Appendix B provides a more detailed list of typical metrics used in monitoring, how they are measured, and their use in monitoring containment system performance.

Saturated Zone Monitoring Systems

Saturated zone (groundwater) monitoring systems are the most commonly employed method to evaluate barrier performance. Both the hydraulic potential (phreatic surface, hydraulic head) and the groundwater chemical composition in the pore water recovered from saturated soil beneath the phreatic surface are measured. Fixed groundwater monitoring systems include direct measurements made with wells, piezometers, or plate lysimeters, and indirect measurements made with electrical and other geophysical measurements. Groundwater monitoring sometimes includes one-time measurements made on samples recovered from push-in probes (e.g., cone penetrometers, hydropunch).

Geophysical methods can be used to monitor groundwater, but they are rarely used in regulatory compliance monitoring systems because techniques have not yet been developed that provide sufficiently quantitative and reliable data. They may, however, be employed in evaluation monitoring programs or in investigations for the development of corrective action programs (e.g., Meju, 2006; Slater and Binley, 2006). Measurements of electrical conductivity or resistivity and electromagnetic potential are sometimes used to establish the extent of the saturated zone and they

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