Geochemical Studies of Commercial Low-Level Radioactive Waste Disposal Sites - Topical Report (NUREG/CR-4644, BNL-NUREG-52004 AN, RW)

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Publication Information

Manuscript Completed: December 1985
                    Revised: May 1986
                    Published: June 1986

R. Dayal,* R.F. Pietrzak, and J.H. Clinton

*Present Address: Rock Sciences Section. Civil Research Department.
                              Ontario Hydro. Toronto. Ontario. Canada

Nuclear Waste Management Division
Department Of Nuclear Energy
Brookhaven National Laboratory
Upton, Long Island, Ny 11973

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555
Under Contract No. DE-AC02-76CH00016
NRC FIN A3042

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

Availability Notice

Abstract

The results of source term characterization studies for the commercially operated low-level waste (LLW) disposal sites located in the eastern United States provide an understanding of the importance of hydrological and geochemical factors in controlling the mechanics of leachate formation, evolution of leachate compositions, microbial degradation of organic waste and development of anoxia in the trenches, and the nature and extent of leaching of waste materials. The varying degrees of the intensity of these processes, as determined by the different site characteristics, are clearly reflected in the contrasting leachate geochemistries of Maxey Flats and West Valley trenches, as compared to those of Barnwell and Sheffield trenches. These are important geochemical considerations which not only define LLW source terms but also shed light on the nature and extent of geochemical changes that are likely to occur along a redox gradient outside of the trench environment.

Unsegregated, poorly packaged, and unstabilized wastes, which were buried at the sites, are readily leachable and biodegradable. Especially at the Maxey Flats and West Valley sites, where the infiltrated water accumulates in the trenches leading to a bathtub effect, the relatively long residence time of accumulated water results in continually intense leaching and microbial degradation of waste materials for extended time periods. These leachates exhibit strongly anoxic, reducing conditions, as reflected by negative redox potentials, depletion of dissolved oxygen and sulphate, high alkalinity and ammonia concentrations, as well as high concentrations of dissolved iron and manganese, present primarily as Fe²⁺ and Mn²⁺. Although the Barnwell and Shef field leachates exhibit a fair degree of anoxia, as reflected in depletion of dissolved oxygen and some alkalinity and ammonia enrichments, no significant sulphate depletion relative to ambient groundwaters is observed, indicating that the anaerobic degradation processes are not as advanced as those reflected by the leachate characteristics of Maxey Flats and West Valley trenches. This is also evident from the relatively higher redox potentials measured for the Barnwell and Sheffield leachates.

The enrichments, to varying degrees, of inorganic organic, and radionuclide constituents associated with fuel cycle and non-fuel cycle LLW reflect not only the nature of the leaching process and reactivity and amount of waste materials but also the differences in the site characteristics. Relative to ambient groundwaters, the trench leachates are generally enriched in Na⁺, K⁺, Fe_T, MN_T, Cl^– , dissolved organic and inorganic carbon and several organic compounds, as well as radionuclides such as H-3, Co-60, Cs-134, Cs-137, Sr-90, Pu-238, Pu-239,-240, Am-241, attributable to leaching of waste materials. The Maxey Flats and West Valley leachates exhibit modifications which are generally considerably more pronounced than those observed in the Barnwell and Sheffield leachates. This is also reflected in generally higher values for specific conductance, ionic strength, and total dissolved solids in Maxey Flats and West Valley leachates.

The observations made at the existing sites have important implications for site selection and disposal of low-level radioactive wastes at future shallow land burial sites in that waste package requirements will include waste segregation, improved stabilization, and proper packaging. In accordance with current disposal procedures, stabilized and/or packaged waste not only ensures trench stability but also decreases the. rate and extent of leaching and microbial degradation of waste materials. In addition, the uncertainties in the source term are considerably reduced. Furthermore, to avoid water accumulations in the trenches, future sites will most likely be located in relatively well-drained systems, such as that represented by the Barnwell site, where the residence-of accumulated water in the trenches is relatively short. Many of the current waste package requirements and site selection criteria outlined in 10CFR61 for the shallow land disposal of low level radioactive wastes are based on lessons learned from experiences at existing shallow land burial sites.