PNNL

Abstract

The U.S. Department of Energy’s Office of River Protection has constructed interim surface barriers over a portion of the T and TY tank farms as part of the Interim Surface Barrier Demonstration Project. The interim surface barriers (hereafter referred to as the surface barriers or barriers) are designed to minimize the infiltration of precipitation into the soil zones containing radioactive contaminants and minimize the movement of the contaminants. As part of the demonstration effort, vadose zone moisture is being monitored to assess the effectiveness of the barriers at reducing soil moisture. Solar-powered systems were installed to continuously monitor soil water conditions at four locations in the T (i.e., instrument Nests TA, TB, TC, and TD) and the TY (i.e., instrument Nests TYA and TYB) Farms beneath the barriers and outside the barrier footprint as well as site meteorological conditions. Nests TA and TYA are placed in the area outside the barrier footprint and serve as controls, providing subsurface conditions outside the influence of the surface barriers. Nest TB provides subsurface measurements to assess surface-barrier edge effects. Nests TC, TD, and TYB are used to assess changes in soil-moisture conditions beneath the interim surface barriers. Except for occasional times for TC and TD and planned dates for TYB, during FY10, the battery voltage at the TMS and instrument Nests in both T and TY tank farms remained above 12.0 V, denoting that the battery voltages were sufficient for the stations to remain functional. All the HDUs were functioning normally, but some pressure-head values were greater than the upper measurement limit. The values that exceeded the upper limit may indicate wet soil conditions and/or measurement error, but they do not imply a malfunction of the sensors. Similar to FY07 through FY09, in FY10, the soil under natural conditions in the T Farm (Nest TA) was generally recharged during the winter period (October–March), and they discharged during the summer period (April–September). Soil water conditions above about 1.5-m to 2-m depth from all three types of measurements (i.e., CP, NP, and HDU) showed relatively large variation during the seasonal wetting-drying cycle. For the soil below 2-m depth, the seasonal variation of soil water content was relatively small. The construction of the TISB was completed in April 2008. In the soil below the TISB (Nests TC and TD), the CP-measured water content showed that ? at the soil between 0.6-m and 2.3-m depths was stable, indicating no climatic impacts on soil water conditions beneath the barrier. The NP-measured water content in the soil between about 3.4 m (11 ft) and 12.2 m (40 ft) since the completion of the barrier decreased by 0.007 to 0.014 m3 m-3. The HDU-measured soil-water pressure at 1-m, 2-m, and 5-m depths decreased by 0.7 to 2.4 m, indicating soil water drainage at these depths of the soil. In the soil below the edge of the TISB (Nest TB), the CP-measured water content was relatively stable through the year; the NP-measured water content showed that soil water drainage was occurring in the soil between about 3.4 m (11 ft) and 12.2 m (40 ft) but at a slightly smaller magnitude than in Nests TC and TD; the HDU-measurements show that the pressure head changes at Nest TB since the completion of the barrier were generally less than those at TC and TD, but more than those at TA. These results indicate that the TISB is performing as expected by intercepting the meteoric water from infiltrating into the soil, and the soil is becoming drier gradually. The barrier also had some effects on the soil below the barrier edge, but at a reduced magnitude. There was no significant difference in soil-water regime between the two nests in the TY tank farm because the barrier at the TY Farm was just completed one month before the end of the FY.