Using Real-Time Vadose Zone Monitoring for Long Term Performance Assessment of a Corrective Action Management Unit Containment Cell, Sandia National Laboratories, New Mexico

Abstract

Sandia National Laboratories in Albuquerque, New Mexico (SNL/NM), operates a Corrective Action Management Unit (CAMU) for the United States Department of Energy (DOE). In 1997 SNL/NM was granted a permit modification that allowed construction and operation of a CAMU. The CAMU follows regulatory guidance that facilitates expedient and cost-effective cleanup and management of hazardous remediation wastes. Treatment operations were completed in January 2003 in conjunction with containment of 845,000 cubic feet (23,930 cubic meters) of treated soil. The containment cell is situated approximately 485 feet (148 meters) above groundwater in a semiarid region marked by low rainfall and high evapotransporation. These site conditions required a unique approach to monitoring the containment cell performance and ultimately protecting groundwater. To satisfy Resource Conservation and Recovery Act groundwater monitoring requirements, a Vadose Zone Monitoring System (VZMS) for rapidly detecting leaks was incorporated into the containment cell design. One component of the VZMS, the Primary Subliner (PSL) monitoring system, utilizes the containment cell subliner to focus potential leakage into five longitudinal trenches. Each trench contains a wicking material and a vitrified clay pipe used to provide access for a neutron probe to measure soil moisture content directly under the containment cell. The other component of the VZMS, the Vertical Sensor Array (VSA), consists of 22 time-domain reflectometers that provide a backup to the PSL. Environmental Protection Agency regulators accepted vadose zone monitoring of the CAMU containment cell as a substitution for groundwater monitoring wells because of its high probability for early detection of leakage if it were to occur. This monitoring approach would also enable timely implementation of a corrective action to mitigate the possibility of any impacts to groundwater. The CAMU VZMS provides a superior methodology for the detection and subsequent characterization of any potential leaks emanating from waste contained in the cell versus the use of groundwater monitoring wells. One of the main advantages offered by the VZMS is its ability to provide real-time data on containment cell performance. Because of the layout, aerial coverage, and the multiple monitoring parameters incorporated into the VZMS, the specific location of a leak from the cell can be defined as well as the nature of the contaminant liquid (volatile organic versus inorganic compounds). The SNL/NM CAMU is the only facility within the DOE complex that implements this innovative approach to environmental restoration waste management and monitoring. A significant cost savings to taxpayers for on-site waste treatment and containment versus off-site disposal was achieved. A cost saving of approximately $200 million was realized by utilization of the CAMU versus off-site waste disposition. The VZMS monitoring system will be utilized during the 30 year post-closure care period for the containment cell.