Evaluation of cover materials for a large scale test facility at Key Lake
Lee, Nan H.
Engineered soil cover systems have gained popularity in recent years as a preferred method of decommissioning and reclaiming waste management facilities. The main functions of a soil cover system are to minimize water infiltration, limit gas migration, resist weathering and erosion, and provide support for vegetation. In 1992, Cameco Corporation constructed a large scale non-vegetated prototype soil cover at Key Lake in north-central Saskatchewan. Their main objective was to evaluate the suitability of using local tills and sands for cover materials during future decommissioning of various waste management facilities at the site. An instrumentation and monitoring program was initiated in 1993 to verify the field performance of the soil cover system. The prototype soil cover was constructed over leached cobble ore that was enclosed within a double lined containment system. The soil cover consisted of a 60 cm layer of outwash glacial sand overlain by a 60 cm layer of compacted till. The test facility is essentially a large scale lysimeter whereby net infiltration is determined by monitoring the change in water table depth, the quantity of water removed from the facility, and the soil moisture profile. The instrumentation and monitoring program included a weather station, thermal conductivity sensors and neutron probe access holes, a Bowen Ratio Instrumentation, and a runoff collection and monitoring system. A laboratory program was undertaken to define pertinent soil parameters such as the soil-water characteristic curve and hydraulic conductivity. Laboratory calibration of the neutron probe was also carried out. A field soil testing program was completed to determine in situ density and hydraulic conductivity. The weather monitoring program yielded reliable precipitation, air temperature, and wind speed data. Instrumentation error was noted for relative humidity, net radiation, and pan evaporation parameters. The surface runoff monitoring system provided reliable runoff data on a real time basis. The thermal conductivity sensors were found to underestimate the soil moisture content, while the neutron probe was found to overestimate. The laboratory testing indicated that the outwash sand and the compacted till possessed similar soil water characteristic curves. The similarities in their water storage and release characteristics preclude the ability of these soils to form an effective capillary barrier when the till is overlain by the outwash sand. The field investigation has revealed that the soil cover system was underlain by a layer of extraneous sand and till material, most likely used to grade the facility prior to constructing the soil cover. The field performance of an engineered soil cover system is determined by the net infiltration through the system. The net infiltration for the 1996-1997 monitoring year was estimated to be 52% (287 mm) of the total annual precipitation(555 mm of precipitation from October 1996 to April 1997, inclusively). There was an insignificant change in the subsurface soil moisture storage. The surface runoff was recorded to be 6% (35 mm) of the total annual precipitation. The actual evaporation was estimated to be 30% (167 mm). The evaluation of the soil cover design has suggested that the Key Lake outwash sand overlain by the Key Lake till will not form a capillary barrier. Furthermore, the thickness of the till layer far exceeded the evaporative zone depth of the material, and therefore, infiltrated water could not have been extracted through evaporation even if the two soils had formed a capillary barrier. The net water storage capacity of the cover system was found to be inadequate (12 mm) to store larger rainfall events, thus further contributing to net infiltration. Incorporating surface vegetation has the potential to increase the total storage capacity to 42 mm; vegetation will also increase total evapotranspiration. A detailed analysis of the surface runoff and infiltration characteristics has indicated that runoff was governed by rainfall intensity and antecedent soil moisture conditions. Surface runoff was generated regardless of soil moisture conditions when the 15-minute rainfall intensity exceeded 1.4 mm. This intensity corresponds to a surface hydraulic conductivity (till) of 1.5 x 10-6 m/s; the laboratory determined hydraulic conductivity was 2.0 x 10-7 m/s.