he UC-1 Central Mud Pit (CMP) was closed with a vegetated monolayer cover - (Diagram) designed to isolate hydrocarbon-impacted drilling muds from casual contact by humans and animals. The constructed cover system consists of a geogrid (Fornit 30 Geogrid by Huesker Inc.) placed directly over the drilling mud to support heavy equipment, covered by a vegetated monolayer cap. The design uses the weight of the cover to force the pore fluids from the muds while limiting moisture infiltration to the waste beneath. Diversion channels to route precipitation run-on and run-off way from the cover were excavated to the west and east of the cover. A barbed wire and mesh fence was constructed to limit access to the cover. Twelve permanent subsidence monuments were installed on the CMP cover to provide an elevation control reference that allows monitoring of the settlement of the cover over time. The Closure Plan - (Large document! 28.5 Meg pdf file) for the CNTA surface requires biannual subsidence surveys. The CMP cover was designed in such a way that infiltrating precipitation and entrained water within the mud itself is removed through evapotranspiration. Ninety percent (90%) settling (consolidation) is expected to occur over the first 3 to 13.5 years following the completion of the cover. Not unexpectedly, some differential settling has occurred causing several cracks to form in the CMP cover. In order to heal the cracks and seal them from moisture penetration, the cracks were filled with Bentonite chips. Time Domain Reflectometry (TDR) probes - (Diagram) and recording instrumentation are used to measure a profile of the water content in the cover. Two locations in the CMP cover are continuously monitored based on a data transfer via a satellite uplink. In addition to the soil moisture information, local precipitation is also transmitted via the satellite uplink for real-time monitoring of conditions at the CNTA. Transplanted native shrubs and grasses were used to minimize moisture infiltration through the cover via plant transpiration processes. Vegetation was established on the cover by first preparing the cover for seed, seeding the cover, applying mulch to encourage seed germination and provide protection from the wind, and finally transplanting year-old native plant species on the cover. The cover is assuming a more natural appearance and the disturbed area is blending in with the surrounding landscape. Petroleum hydrocarbons in the mud pit should continue to break down over time through natural attenuation. Completion of surface remediation activities occurred in 2001. Post-closure monitoring of the CNTA sites is conducted on a quarterly basis to determine if repairs or maintenance is necessary for any of the facilities and to determine if the engineered mud pit covers are performing as designed.