Development of a Long-Term Monitoring System to Monitor Cover System Conditions

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1 Development of a Long-Term Monitoring System to Monitor Cover System Conditions Uday Kumthekar, J. D. Chiou, Martin Prochaska Fluor Fernald, Inc. P.O. Box Cincinnati, Ohio Craig H. Benson Geo Engineering Program, University of Wisconsin Madison 1415 Engineering Drive Madison, Wisconsin Abstract - Environmental remediation at the Fernald Environmental Management Project is nearing completion, but longterm technology needs continue to emerge at the site. Remote, real-time, autonomous monitoring technologies are needed to ensure the integrity of the site and its remedy systems once cleanup is complete. The Fernald Post Closure Stewardship Technology Project (PCSTP) has selected technologies to address initial site needs. This paper will explore the monitoring requirements of the Fernald On-Site Disposal Facility (OSDF) final cover system, the parameters selected as critical for comprehensive long-term monitoring, and the process by which technologies were chosen to monitor those parameters. I. INTRODUCTION The Fernald On-Site Disposal Facility (OSDF) is an engineered, above-grade waste-disposal facility being constructed to permanently store low-level radioactive waste at the Fernald Environmental Management Project (FEMP), located 29 km (18 mi) northwest of Cincinnati, Ohio. Long-term monitoring of the OSDF is a priority of the Department of Energy, regulators, stakeholders, and the managing contractor for the site, Fluor Fernald, Inc. The Fernald Post Closure Stewardship Technology Project (PCSTP) was formed at FEMP to identify, demonstrate, and implement technologies capable of monitoring the OSDF after site remediation is complete. The goals of monitoring are to: 1) consistently evaluate OSDF performance (i.e., the satisfaction of functional requirements and design criteria); 2) provide early warning alarms in case of failure; 3) identify and analyze the failure mechanism; and 4) provide data for corrective action. To achieve these goals, the project team needed to go beyond the traditional approach of monitoring leak detection in the liner and installing groundwater monitoring wells down-gradient of the facility. A team of experts, regulators, and stakeholders was formed in November 2000 to review drivers and long-term monitoring needs not included in the original OSDF design; to establish monitoring parameters for each monitoring need; and to develop, evaluate, and recommend technologies for each monitoring parameter. The team was also charged with designing and engineering an integrated monitoring, data collection, and reporting system and developing a long-term maintenance schedule. The first focus of the team was to review drivers for long-term monitoring of the OSDF cover system. The main drivers at the FEMP include: regulatory requirements, functional requirements, OSDF design criteria, the OSDF Post-Closure Care and Inspection Plan, and the Fernald Environmental Management Project Final Land Use Plan. Monitoring needs identified for final cover system performance were grouped into three areas: (i) ecological system associated with the vegetative cover and in the OSDF buffer area, (ii) physical changes in the cover system and buffer area, and (iii) effectiveness of institutional controls. Recommendations made by the team of experts were used to design a system for monitoring the physical condition of the final cover for the OSDF. This paper describes the process for selecting a cover performance monitoring system for the first cell of the OSDF, which was completed in late fall of II. BACKGROUND The OSDF will ultimately be filled with 1.9 million m 3 (2.5 million yd 3 ) of impacted material (lowlevel radioactive soil and construction debris) derived from remediation activities at the FEMP. Presently, the OSDF is estimated to have seven cells and an areal extent of 28 ha (70 acres). Three cells have been constructed as of May 2002, and the final cover system has been constructed over Cell 1. Cells 2 and 3 are active and accepting impacted material, and cells 4 & 5 are under construction.

The OSDF is intended to isolate impacted material from the environment for at least 200 years, with continual effectiveness for up to 1000 years to the extent practicable and reasonably achievable.

These controls are incorporated into the engineering and construction of the OSDF multi-layer liner system and final cover system. Each cell in the OSDF is lined with a 1.

2 The OSDF is intended to isolate impacted material from the environment for at least 200 years, with continual effectiveness for up to 1000 years to the extent practicable and reasonably achievable. This performance goal will be achieved through internal hydrologic control, external hydrologic control, geotechnical stability, resistance to erosion, and resistance to biointrusion. These controls are incorporated into the engineering and construction of the OSDF multi-layer liner system and final cover system. Each cell in the OSDF is lined with a 1.8-m (5 ft) -thick multi-layer liner system that consists of the following layers (from bottom to top): compacted clay liner, secondary geosynthetic clay liner, secondary geomembrane liner, geotextile cushion, leak detection system (LDS) drainage corridor with LDS sub-drain pipe, primary geosynthetic clay liner, primary geomembrane liner, geotextile cushion, leachate collection system (LCS) drainage corridor with LCS sub-drain pipe, and geotextile filter (Fig. 1a). Leachate from the LDS and LCS is monitored for flow rate and quality at valve houses outside the perimeter of the OSDF. (a) (b) Fig. 1. Schematic of liner (a) and final cover (b) for FEMP s OSDF.

3 After each cell is filled to capacity, a final cover system is installed. The final cover system is 2.65 m (8.75 feet) thick and consists of the following layers (from bottom to top): compacted clay cap, geosynthetic clay cap, geomembrane cap, geotextile cushion, cover drainage layer, biointrusion barrier with choking layer, granular filter, vegetative soil layer, topsoil, an erosion mat, and vegetation (Fig. 1b). The final cover is designed to limit percolation into the underlying impacted material, promote drainage while minimizing erosion, and accommodate settlements without affecting the integrity of the cover. III. MONITORING STRATEGY The issue of long-term stewardship is important both to stakeholders and to experts in the field of waste management. Recently, the National Academy of Sciences published a report citing stewardship activities as a major component of a three-part plan for institutional management of DOE legacy waste sites. a OSDF cover system monitoring plays a major role in the FEMP s longterm stewardship plan. Through a series of working meetings, the project team selected critical monitoring parameters based on the functional requirements and design criteria of the OSDF. Parameters were grouped into three categories geophysical, ecological, and institutional and prioritized according to their impact on cell performance and ability to fit into the OSDF construction schedule. The final list of critical monitoring parameters, including each item s impact upon cover system performance, is detailed below: 1. Pore water pressure in the drainage layer Buildup of water pressure in the drainage layer head must be kept below a critical value to maintain physical stability. 2. Total and differential settlement Settlement must be kept at a minimum so as not to impact barrier performance, hydraulic gradients, and the free flow of moisture throughout the drainage layer. Distortions must be limited to < 10 percent. 3. Soil water content and soil water potential These elements are critical to the health of the root zone within the vegetative layer, which protects all other layers and must remain in place for other barriers to retain effectiveness. 4. Soil temperature above barrier layer To function properly, the barrier system must not freeze. 5. Overall condition of cover This parameter includes institutional controls, such as maintenance of signage within the buffer area, as well as ecological controls, such as the monitoring of biotic intrusion throughout the cover system. Erosion must be prevented through the maintenance of a healthy vegetative layer, which in turn ensures that the bio-intrusion layer remains functional. The technologies selected to monitor these critical parameters are presented in Table I. Table I. Critical Parameters and Selected Monitoring Technologies in the OSDF Final Cover System. Parameter Component Monitoring Technology Monitored Drainage layer Surface & internal cover grades, Barrier layer (distortion) Status of root zone Barrier layer (freezing) Cover system & buffer area Pore water pressure in drainage layer Settlement (total and differential) Soil water content, soil water potential Soil temperature above barrier layer Overall condition of cover Submersible pressure transducers Topographic survey using settlement plates & rods, ground penetrating radar (GPR) targets Dielectric water content sensors, thermal dissipation potential sensors Thermocouples Routine topographic survey Web cam Visual and/or remote sensing Based on the objectives of the PCSTP, the team decided that certain parameters, such as the leakage rate through the cover system and the lateral drainage rate through the drainage layer, would not be monitored at the OSDF. The objective of this project was to design a monitoring system that would provide data describing the physical condition of the cover. This objective was selected because engineering experience with final covers incorporating composite barriers indicates that physical stability is the most important factor affecting long-term performance. That is, a barrier that remains physically stable (i.e., is not damaged by distortion, thermal stress, erosion, or slope failure) will function as intended. The monitoring system deployed on the cover of Cell 1 of the OSDF was designed to monitor each of these key issues. a COMMITTEE ON THE REMEDIATION OF BURIED AND TANK WASTES, BOARD ON RADIOACTIVE WASTE MANAGEMENT, NATIONAL RESEARCH COUNCIL, Long-Term Institutional Management of U.S. Department of Energy Legacy Waste Sites, Chapters 1-2, National Academy Press, Washington, D.C., (2000).

4 Although the monitoring system will be able to identify surface ponding or drainage layer blockage that may provide potential sources of leakage water through the cover, it was not designed to characterize the hydrology of the OSDF cover or to determine the leakage rate from the cover (as might be done in a research project to validate hydrological models). Hydrological characterization was not the objective of the project. If hydrological characterization were the objective, then a much different system would have been designed and installed. Such a system most likely would include a lysimeter to monitor leakage, a collection system for the drainage layer, diversion structures for surface run-on and run-off, and metering devices to monitor flows for each hydrological process. Hydrological characterization was not an objective because the monitoring system is not a research project being conducted to validate design theories for final covers. Rather, the monitoring system was designed with the intent of providing end users (DOE and the stakeholders) with the data needed to assess whether the cover was stable, and thus would continue to function as intended. IV. INSTRUMENT SELECTION The system to monitor the parameters in Table I was designed to meet four criteria: (i) long-term performance with little maintenance, (ii) deployment in the near term (within 12 mos.), (iii) remote access and control, and (iv) capability to integrate into a data management system. The system also needed to be sufficiently flexible so that other sensing technologies could be incorporated during deployment in subsequent cells should other additional or advanced technologies become available. Because near term deployment was a criterion, commercially available instruments were selected. The need for long-term performance with little maintenance required that instruments with a proven track record be used (i.e., experimental technologies were not an option). The components used to interrogate the instruments and store the data needed to be available with accessories and software so that the system could be accessed and controlled remotely (e.g., collection of data from the system from an off-site location), and downloaded directly into a computer system for automated analysis. The sensors, datalogging equipment, and computer interface selected to meet the criteria are summarized in Table II. Instruments for automated measurements of differential and total settlement meeting the design criteria could not be identified. Thus, these measurements will be made manually until suitable instruments can be identified. The remote visual survey via a web cam also has not yet been implemented. Table II. Monitoring Instruments within OSDF Final Cover Component or Parameter Instrument Data acquisition CSI a CR23X datalogger with fiber optic link to radio transmitter LAN b interface Multiplexing Water content CSI NL100 10BaseT network link CSI AM25T solid state thermocouple multiplexers, and AM 16/32 analog multiplexers CSI CS615 water content reflectometers Soil water potential Barrier layer temperature CSI 229 soil water potential probes and CSI CE8 current source CSI 229 soil water potential probe Pressure and temperature of water in drainage layer Geokon 4500 vibrating wire pressure transducers with embedded thermistor a CSI = Campbell Scientific Incorporated, Logan, Utah, USA; b LAN = local area network. V. DEPLOYMENT The sensors were installed in a series of nests on the final cover of Cell 1, as shown in Fig. 2. Ten soil water status nests were installed for monitoring the root zone, and seven pressure transducer risers were installed for monitoring pressure and temperature in the drainage layer. Settlement plates and rods were co-located with the pressure transducer risers. Eight sets of three Ground Penetrating Radar (GPR) plates were installed at three interfaces within the cover system.

5 Each soil water status nest contains four water content reflectometers and four soil water potential probes placed at equal vertical spacing in the vegetative soil layer (Fig. 3). Three nests are located along the length of the east, north, and west slopes to monitor the root zone along the three primary directions (Fig. 2). A nest was also placed on the top deck to monitor conditions at the point with the highest elevation and the shallowest slope. The nests were designed so that the water content and water potential sensors could be easily replaced by a single person without heavy equipment if a sensor malfunctioned. The design includes watertight quick connects that permit replacement of sensors with minimal effort and no need for re-wiring. = Soil Water Status Nest = Settlement Plate = Press. Transducer = GPR Plate = Cabling = Fiber Optic N cover perimeter Fig. 2. Layout of instrument nests on final cover for Cell 1. Fig. 3. Typical soil water status nest.

6 The pressure transducer risers were designed to permit free flow of water into and out of the riser (Fig. 4). Geotextile was used to prevent particles from plugging the holes in the riser pipe and fouling of the transducer well. Skirts made with geotextile were used to prevent fines from migrating between soil layers along the interface between the soil and riser pipe. A geotextile cushion was placed on the bottom of the riser to prevent damage to the underlying geomembrane barrier. The risers were constructed from Schedule 120 PVC pipe to prevent damage during construction and for long design life. The pressure transducer risers are located along the two longest slopes (oriented in the northwest and northeast directions, Fig. 2), and along the main northern slope. Risers were located along the longest slopes because pore water pressures are likely to be highest on these slopes. One of the risers along each orientation is located near the toe of the drainage layer, where high pore water pressures could become appreciable if the outlet to the drainage layer becomes fouled. The other locations along each orientation are near the top of the slope and the middle of the slope (Fig. 2). Measurements at these points will be used to define the distribution of pressure along the slope. As with the soil water status nests, a quick connect was provided in each riser so that the transducer could be replaced with minimal effort and without re-cabling if a malfunction occurred. Fig. 4. Typical Pressure transducer riser and settlement plate with rod. Settlement plates were installed on the surface of the drainage layer, with a rod extending to the ground surface (Fig. 4). A manhole provides access to the rod. The riser pipe includes a slip fitting (not shown in Fig. 4) to permit the plate and rod to move independently of the soil and rock surrounding the riser. Settlements will be measured using a survey rod and transit. The datalogger and multiplexers are located on the top deck of Cell 1 in sealed enclosures located in subterranean vaults. Humidity sensors have been

7 incorporated into the enclosures so that the atmosphere surrounding the datalogger and multiplexers can be continuously monitored. All of the enclosures are mounted on aluminum sliders so they can be raised above ground surface for maintenance. A fiber optic cable connects the datalogger to a radio transmitter mounted on a valve house adjacent to the OSDF (Fig. 2). The transmitter interacts with a base station connected to FEMP s local area network (LAN). Data are currently being collected and downloaded via the LAN. However, the base station can be accessed via TCP/IP from any location with access to the internet. Thus, the system is ready for off-site remote monitoring in the future. VI. DATA COLLECTION AND MANAGEMENT Data are currently being collected by the datalogger at 1-hr. intervals, and are downloaded to a computer at the FEMP every twelve hours. This sampling interval is temporary, and will be optimized in the near future after time series analyses are conducted. A data management system is also under development. This system is being designed for automatic processing and evaluation of the data to support the stewardship needs of DOE and to ally the concerns of FEMP s stakeholders. VII. LESSONS LEARNED TO DATE Probably the single most important lesson learned to date relates to the strategy of incorporating the installation of the cell 1 monitoring system with the concurrent construction of the cell 1 cover. Components of the monitoring system were installed as the different elements of the cover were being constructed (e.g., the nests for the pressure transducer risers were installed when the drainage layer was being constructed, and the settlement plate and rods were placed on top of the drainage layer after construction, and so forth). This logical strategy worked well until the final placement of the conduits, junction boxes, etc. were required in and on the various soil layers of the OSDF. The vegetative and topsoil layers were placed and all seeding and erosion control matting was finished except for the corridors where the conduit and soil water status (SW) nests would be placed. The strategy was to leave this area un-seeded until the conduit and SW nests were installed, which would have normally worked fine, except that inclement weather delays forced the conduit and SW installation to be pushed back into the winter months, effectively delaying the final grading and seeding of these corridors to spring of the following year. In hindsight, it would have been prudent to seed and place erosion control matting over the entire OSDF and return at a later data to install the conduit and SW nests where the impact to the construction schedule would be significantly reduced. For future cells, the approach will be to install those components of the monitoring system (i.e., pressure transducer risers, settlement plate and rods, and GPR plates) that extend below the bio-intrusion barrier and come back at a later date, after establishment of vegetation, to install those components that go in the soil layers of the cover system above the biointrusion barrier. VIII. ACKNOWLEDGMENTS The PCSTP wishes to acknowledge the following individuals and organizations for their vision, guidance and funding that have made this project a successful reality: Dr. Susan Brechbill former director of the DOE Ohio Field Office; Skip Chamberlain DOE Sub-Surface Contaminants Focus Area Headquarters lead; Jim Wright Director, DOE Sub-Surface Contaminants Focus Area; Scott McMullin DOE Sub-Surface Contaminants Focus Area Product Line Manager for Containment; and to all of the ISTT members who have contributed their time, talent, and wisdom into making this a successful effort. IX. REFERENCES COMMITTEE ON THE REMEDIATION OF BURIED AND TANK WASTES, BOARD ON RADIOACTIVE WASTE MANAGEMENT, NATIONAL RESEARCH COUNCIL, Long- Term Institutional Management of U.S. Department of Energy Legacy Waste Sites, Chapters 1-2, National Academy Press, Washington, D.C., (2000).

Strategies for Long-Term Monitoring and Stewardship

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