Chemical Characteristics of Leachate in Low-Level Radioactive Waste Disposal Facilities

Low-level radioactive waste (LLW) comprises all radioactive waste that is not classified as high-level radioactive waste, transuranic waste, spent nuclear fuel, byproduct material [as defined in Section 11e.(2) of the Atomic Energy Act], or naturally occurring radioactive material (DOE 1999). This type of waste includes items that have become contaminated with radioactive material or have become radioactive through exposure to neutron radiation (NRC 2002). The activity of LLW can range from just above natural background levels to high activity in certain cases [e.g., in reactor vessel parts, NRC (2002)]. LLW waste is characterized by radioactive decay, in which an unstable atomic nucleus loses energy by emitting ionizing radiation (NRC 2002). Mixed waste (MW) is LLW that is codisposed with other wastes, such as hazardous waste. Radionuclides of concern in LLW or MW include ${}^{14}\mathrm{C}$, ${}^{242}\mathrm{Cm}$, ${}^{58}\mathrm{Co}$, ${}^{60}\mathrm{Co}$, ${}^{134}\mathrm{Cs}$, ${}^3\mathrm{H}$, ${}^{129}\mathrm{I}$, ${}^{94}\mathrm{Nb}$, ${}^{59}\mathrm{Ni}$, ${}^{63}\mathrm{Ni}$, ${}^{241}\mathrm{Pu}$, ${}^{90}\mathrm{Sr}$, and ${}^{99}\mathrm{Tc}$. Alpha-emitting transuranic nuclides with a half-life $>5\mathrm{years}$ and all nuclides with a half-life $<5\mathrm{years}$ may also be present in LLW (CFR 2001).

LLW is disposed in near-surface facilities designed to limit the dose received by receptors in the surrounding environment (Tian et al. 2016a, b). Facilities for MW are also designed to limit the dose and must also meet applicable regulatory criteria for the codisposed waste stream. Migration of meteoric water through LLW creates leachate as constituents in the waste dissolve through a combination of physical, chemical, and microbial processes in a manner analogous to leachate generation in municipal solid waste (MSW) landfills (Christensen and Kjeldsen 1989; Christensen et al. 1998; Kjeldsen et al. 2002). Multilayer liner systems with composite barriers, leachate collection systems, and leak detection are used for MW facilities and for some LLW facilities to control the release of constituents in leachate (Benson et al. 2003; Powell et al. 2011; Rustick et al. 2013).

Limited information exists regarding concentrations of radionuclides and other chemical components in LLW leachate. Abdelaal and Rowe (2015) provide a brief summary of the characteristics of LLW leachate from six sites in North America, reporting ${}^{238}\mathrm{U}$ ($0.006–15\mathrm{mg}/\mathrm{L}$) and ${}^{226}\mathrm{Ra}$ ($3.9–50\mathrm{Bq}/\mathrm{L}$) as the dominant radionuclides with other radionuclides (e.g., ${}^{210}\mathrm{Pb}$, ${}^{232}\mathrm{Th}$, ${}^{230}\mathrm{Th}$, and ${}^{235}\mathrm{U}$) present at low concentrations. Their LLW leachates were alkaline (pH 8.0–12.3) and oxidizing ($\mathrm{Eh}=-200–476\mathrm{mV}$). Heavy metals (e.g., Al, Ba, Fe, Pb, Ni, Ag, and Zn) had concentrations $<1\mathrm{mg}/\mathrm{L}$. The major anions were nitrite/nitrate measured as total nitrogen ($\sim 30\mathrm{mg}/\mathrm{L}$), ${\mathrm{F}}^{-}$ ($\sim 30\mathrm{mg}/\mathrm{L}$), and ${\mathrm{SO}}_4^{2-}$ ($\le \mathrm{3,150}\mathrm{mg}/\mathrm{L}$).

Leachate data were evaluated in this study from four composite-lined LLW and MW disposal facilities operated by the U.S. DOE to support environmental restoration activities. Composition of the leachates is divided into four categories: inorganic macrocomponents, metals and metalloids, radionuclides, and organic compounds. Temporal trends, temporal variability, and site-to-site variability are described and a typical LLW-MW leachate is suggested.

Leachate data from four LLW disposal facilities operated by DOE were analyzed: the Environmental Restoration Disposal Facility (ERDF) in Hanford, Washington (operated from 1996, leachate data from 1996 to 2010); the On-Site Disposal Facility (OSDF) at the former Fernald Feed Materials Production Center in Crosby, Ohio (operated from 1989, leachate data from 2005 to 2010); the Idaho Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) Disposal Facility (ICDF) at the Idaho National Laboratory in Idaho Falls, Idaho (operated from 2003, leachate data from 2003 to 2010); and the Environmental Management Waste Management Facility (EMWMF) at the Oak Ridge National Laboratory in Oak Ridge, Tennessee (operated from 2002, leachate data from 2003 to 2010). These DOE facilities engaged in production of materials for nuclear weapons during World War II and the Cold War, resulting in contamination of infrastructure and the surrounding environment. DOE’s Division of Environmental Management is responsible for decommissioning the contaminated infrastructure and restoring the environment at these sites. On-site disposal facilities for LLW and MW are used to manage waste streams derived from decommissioning and restoration activities. Except for Fernald, which is fully decommissioned, each of these DOE disposal facilities is operating. A final cover with a composite barrier system has been placed at Fernald’s OSDF (Benson et al. 2003) with an anticipated average percolation rate of $2.3\mathrm{mm}/\mathrm{year}$ (Powell et al. 2011). Waste at the other sites is covered with interim soil cover.

ERDF is located in the semiarid shrub-steppe of the Pasco Basin in the Columbia Plateau in southeastern Washington State with average annual precipitation of $170\mathrm{mm}/\mathrm{year}$. ICDF is located in the high desert of eastern Idaho with average annual precipitation of $210\mathrm{mm}/\mathrm{year}$. Both ERDF and ICDF are in semiarid and seasonal climates, with intermittent snowfall and freezing conditions (SRNL 2014). OSDF is in southwestern Ohio in a humid and seasonal climate with annual precipitation of $570\mathrm{mm}/\mathrm{year}$ with intermittent snowfall and freezing conditions (Powell et al. 2011). EMWMF is in a very humid climate in eastern Tennessee, with infrequent snowfall and freezing conditions and the highest annual precipitation of the four sites ($\mathrm{1,300}\mathrm{mm}/\mathrm{year}$) (Williams et al. 2000; SRNL 2014).

The wastes disposed at ERDF, ICDF, OSDF, and EMWMF consist primarily of contaminated soil and debris generated from building demolition (Benson et al. 2007a, b; Powell et al. 2011; Rustick et al. 2013, 2015; DOE 2016a, b). Sludges, contaminated protective clothing, and contaminated refuse (paper, packing material, glassware, tubing, resins, activated metals, rags) are also disposed in these locations. Portland cement–based grouts are used to fill voids in containers containing miscellaneous wastes and larger items with cavities. Wastes from decontamination and decommissioning that are regulated as hazardous and nonhazardous wastes under the Resource Conservation and Recovery Act (RCRA) and wastes regulated under the Toxic Substances Control Act (TSCA) (e.g., asbestos) are codisposed with LLW (DOE 2009).

Each of these DOE facilities employs a double liner system along with a leachate collection and leachate detection system, as illustrated by the liner system profiles shown in Fig. 1 (Benson et al. 2007a, 2008a, b; Powell et al. 2011; Rustick et al. 2013, 2015; DOE 2016b). The leachate collection system is covered with a 305-mm-thick soil protective layer. Leachate at each site is collected in sumps in the leachate collection system, stored temporarily in tanks, and treated prior to discharge to the environment. The tanks are set to automatically pump when reaching 80% capacity (DOE 2015). Samples of the leachate are collected from the tanks and analyzed quarterly or semiannually at each site as part of regulatory reporting (DOE 2008, 2011, 2014, 2015). Data from these regulatory reporting activities (all in the public domain) were used for the analyses in this study. The data were provided by site managers or their designees.

All leachate samples were collected using the procedures described in “Environmental Monitoring and Management,” ENV-1-2.20 (DOE 2008), following site-specific quality assurance/quality control (QA/QC) procedures (DOE 2008, 2014, 2015). Sampling QA/QC for ERDF is established in ENV-1-2.36, “River Corridor Quality Assurance Program Plans” (DOE 2008), and for OSDF in “Fernald Preserve Quality Assurance Project Plan” (DOE 2015). The QA/QC protocols at ICDF follow “The Quality Assurance Project Plan for Waste Groups 1, 2, 3, 4, 5, 6, 7, 10 and Inactive Sites” (DOE 2014).

A summary of the analytical methods used to analyze the leachate samples is in Table 1. Total concentrations of major cations and heavy metals are determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) per U.S. EPA Method 6010B and concentrations of major anions are determined by ion chromatography using EPA Methods 9060, 300, 325.2, 353.1, 353.2, 375.2, 4500D, and 4500E (DOE 2008, 2014, 2015). Total organic carbon is measured using EPA Methods 9060 and 415.1. Radionuclides are analyzed using alpha spectroscopy, alpha-gas proportional counting, liquid scintillation, beta-gas proportional counting, and gamma spectroscopy (DOE 2008, 2009, 2011, 2014). Uranium is determined using phosphorescence analysis and by U.S. EPA Method 6010B. Volatile organic compounds (VOCs), semi-VOCs, and pesticides were also analyzed by U.S. EPA methods (Table 1), but were not evaluated systematically in this study (see subsequent discussion).

Bulk chemical characteristics of the leachates (pH, Eh, and TOC) are summarized in Table 2. Radionuclides analyzed at each facility are summarized in Table 3. Concentrations of the constituents in the leachates are summarized in Tables 4–6. The constituents are grouped into four categories: inorganic macrocomponents, including major cations (${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{Na}}^{+}$, and ${\mathrm{K}}^{+}$) and major anions (${\mathrm{Cl}}^{-}$, ${\mathrm{SO}}_4^{2-}$, ${\mathrm{HCO}}_3^{-}$, and ${\mathrm{NO}}_3^{-}/{\mathrm{NO}}_2^{-}$) (Table 4); metals and metalloids such as Al, As, Ba, Cu, Fe, Li, Mn, Ni, Sr, and Zn (Table 5); and radionuclides (Table 6). The radionuclides that are analyzed vary between sites depending on the waste stream being disposed and regulatory agreements.

A typical synthetic LLW leachate (Table 7) was created to represent LLW leachates observed at the DOE LLW facilities. The leachate contains major cations (${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{K}}^{+}$, and ${\mathrm{Na}}^{+}$), major anions (e.g., ${\mathrm{Cl}}^{-}$ and ${\mathrm{SO}}_4^{2-}$), trace metals (e.g., Fe, Al, Cu, and Ni), and radionuclides (e.g., uranium and ${}^{99}\mathrm{Tc}$). Average concentrations were assigned to each component, except for the radionuclides, which were set at upper bound concentrations to represent worst-case conditions (Tian 2012, 2015). The pH ($\sim 7.2$) and Eh ($\sim 120\mathrm{mV}$) of the LLW leachate were also set at the averages in the LLW leachate database. This typical leachate can be used to investigate and predict the long-term performance of components in composite liners, similar to those shown in Fig. 1. For example, Tian et al. (2016a) used this leachate recipe to evaluate the hydraulic conductivity of geosynthetic clay liners to LLW and MW leachate. Similarly, Tian et al. (2016b) used the leachate recipe to evaluate the service life of geomembranes in DOE facilities.

Leachate data from four LLW and MW disposal facilities operated by U.S. DOE were reviewed and characterized in this study. Each of the sites is operated by DOE to support decommissioning and environmental restoration activities at facilities used for producing nuclear weapons during World War II and the Cold War. The leachate composition at each site was divided into four categories: inorganic macrocomponents (major cations and anions), trace elements (metals and metalloids), radionuclides, and organic compounds. The following describes the major findings of the study:

This study was sponsored by the U.S. DOE under Cooperative Agreement No. DE-FC01-06EW07053 entitled The Consortium for Risk Evaluation with Stakeholder Participation III. The opinions, findings, conclusions, and recommendations expressed herein are solely those of the authors and do not necessarily represent the views of DOE.