Recharge Data Package for the 2005 Integrated Disposal Facility Performance Assessment

Abstract

CH2M HILL Hanford Group, Inc., (CH2M HILL) is designing and assessing the performance of a near-surface disposal facility at Hanford for radioactive and hazardous waste. The waste includes immobilized low-activity waste (ILAW), which consists of vitrified low-level radioactive waste that will be retrieved from Hanford's single-and double-shell tanks, unvitrified low-level radioactive waste, mixed low-level waste, and vitrification melters. The CH2M HILL effort to assess the performance of this disposal facility is known as the Integrated Disposal Facility (IDF) Performance Assessment (PA) activity. The goal of this activity is to provide a reasonable expectation that the disposal of waste will be protective of the general public, groundwater resources, air resources, surface-water resources, and inadvertent intruders. Achieving this goal will require predictions of contaminant migration from the facility. To make such predictions will require estimates of the fluxes of water moving through the sediment within the vadose zone around and beneath the disposal facility. These fluxes, loosely called recharge rates, are the primary mechanism for transporting contaminants to the groundwater. v evaporation. The barrier still performed as expected, but only if the shrub-steppe plant community remained. In essence, the dune sand makes the barrier performance sensitive to vegetation conditions such as fire removal and species replacement. Under the climate change condition most likely to promote recharge (i.e., increased precipitation and decreased temperature), recharge through the barrier remained <0.1 mm/yr in contrast to recharge in Rupert sand, which increased from 2.2 to 27 mm/yr. Land use restrictions are expected to preclude farming at the IDF. To understand the consequences of farming, a simulation was conducted of irrigated potatoes. The results showed that irrigation on the surface barrier significantly increased recharge. Remaining issues concern assumptions about climate change, bioturbation, dune sand deposition, unstable and preferential flow, variability of the properties of the barrier materials and surrounding soil, longevity of the barrier, flaws in the barrier, possible facility deposition of chloride, and the importance of temperature and water vapor flow when recharge rates are lower than 1 mm/yr. The recharge estimates provided in this report were based on a pre-conceptual design of the surface barrier. The final barrier design and the materials that will be used to construct it have not yet been identified. When they are, the final design should be re-evaluated to confirm that its performance is acceptable. In the same vein, the properties of the soil that will surround the final barrier will depend on the plan for reclamation following construction. Once identified, the proposed reclaimed soil should be re-evaluated to confirm that its performance is acceptable. Lastly, the recharge estimates provided in this report were based on a set of assumptions regarding future climate, vegetation, and land us...

Compendium of Data for the Hanford Site (Fiscal Years 2004 to 2008) Applicable to Estimation of Recharge Rates

“…Several tracers are available that enable estimates of recharge rates: the ones used at Hanford have included chloride and chlorine-36 (Fayer et al 1999, Fayer andSzecsody 2004) and the stable isotopes deuterium and oxygen-18 (DePaolo et al 2004, Fayer and Szecsody 2004, Singleton et al 2006. Isotopes of strontium were also used (Singleton et al 2006).…”

“…Site-specific recharge measurements for areas of special interest were undertaken such as at the prototype Hanford Barrier to measure the effectiveness of proposed infiltration barriers . Data packages were prepared to support compliance assessments including the Solid Waste Landfill (SWL) (Gee et al 2005), the Integrated Disposal Facility (IDF) (Rockhold et al 1995, Fayer et al 1999, Fayer and Szecsody 2004 and the recharge data package for the RCRA Facility Investigation (Fayer and Keller 2007).…”

“…By the late 1990s tracer-based methods began to be employed in addition to water balance measurement based methods to estimate recharge (Murphy et al 1996, Prych 1998, Fayer et al 1999, Maher et al 2003, DePaolo et al 2004, Fayer and Szecsody 2004, Gee et al 2005, Singleton et al 2006, Keller et al 2007). …”

“…For the immobilized lowactivity waste (ILAW) 2001 Performance Assessment, Fayer et al (1999) used the chloride and chlorine-36 tracer techniques to estimate recharge rates. For the 2005 IDF PA, two tracer techniques were used: chloride mass balance (CMB) and deuterium and oxygen-18 (Fayer and Szecsody 2004). A description of these two techniques can be found in Appendix B of Fayer and Szecsody (2004).…”

Hanford Site National Environmental Policy Act (NEPA) Characterization Report, Revision 17

“…Recharge also varies temporally with the majority occurring in the winter and spring. There is some evidence that the most significant recharge events are associated with rapid melting of relatively large snowpacks, which may only occur a few times in a decade (Fayer and Szecsody 2004).…”