T Tank Farm Interim Surface Barrier Demonstration -- Vadose Zone Monitoring FY07 Report
CH2M HILL Hanford Group, Inc. is currently in the process of constructing a temporary surface barrier over a portion of the T Tank Farm as part of the T farm Interim Surface Barrier Demonstration Project. The surface barrier is designed to prevent the infiltration of precipitation into the contaminated soil zone created by the Tank T-106 leak and minimize movement of the contamination. As part of the demonstration effort, vadose zone moisture monitoring is being performed to assess the effectiveness of the barrier at reducing soil moisture. A solar-powered and remotely-controlled system was installed to continuously monitor soil water conditions in four instrument nests (i.e., A, B, C, and D) and the site meteorological condition. Each instrument nest was composed of a capacitance probe with multiple sensors, multiple heat-dissipation units, a neutron probe access tube and a datalogger. Nests A and B also contained a drain gauge each. The principle variables monitored for this purpose are soil-water content, soil-water pressure, and soil-water flux. In addition to these, soil temperature, precipitation, and air temperature are measured. Data from each of the dataloggers were transmitted remotely to the receiving computer. The neutron probe access tube was used to perform quarterly manual measurements of soil-water content using a neutron probe. This monitoring system was used to assess the soil water conditions in the soil outside and within the footprint of the surface barrier to be emplaced in the Hanford T Tank Farm. Data to date is baseline under the condition without the interim surface barrier in place. All the instruments except the two drain gauges were functional in FY07. The capacitance-probe measurements showed that the soil-moisture content at relatively shallow depths (e.g., 0.6 and 0.9 m) was increasing since October 2006 and reached the highest in early January 2007 followed by a slight decrease. Soil-moisture contents at the depths of 1.3 m and deeper were relatively stable during the whole monitoring period. The soil-water content variation was between 0.012 and 0.043 m3m-3, with smaller variation in deeper soil. The neutron probe measurements show that, the normalized neutron counts had relatively large variation in the soil above 5 ft (1.52 m) bgs and were relatively stable in the deeper soil. The peak values of the soil-water-pressure head at 1-m depth appeared in early January 2007 followed by a decrease. The heat-dissipation unit at the 2.0-m depth showed increasing soil-water pressures from January to April or May 2007 before they started decreasing. The soil-water-pressure heads at 5- and 10-m depths were relatively stable. During FY07, the soil-water-pressure-head variation was between 0.48 and 3.49 m H2O-height, with smaller variation in deeper soil. The soil water was moving downward from late December 2006 to mid-April 2007 and upward at other time in the soil from 1 to 2 m bgs of Nest A. The soil-water movement was always downward below 2 m bgs for Nest A and below 1 m bgs for Nest B. This dominant downward moisture movement indicates that, under the condition without a surface barrier, the soil was gaining water from precipitation in FY07.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 927979
- Report Number(s):
- PNNL-17306; 830403000; TRN: US0804575
- Country of Publication:
- United States
- Language:
- English
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