Approximately 130,000 kg of depleted uranium (DU) from ammunition testing have been deposited in soils since 1974 and remain in the environment at Aberdeen Proving Ground (APG), MD. Removal of the DU and DU contaminated soils is complicated by the presence of dense vegetation and unexploded ordnance (UXO). Excavation of contaminated soils would require denuding parts of APG, thereby damaging the wildlife habitat and increasing erosion, which could possibly degrade water quality in streams at APG and in the Chesapeake Bay. Personnel exposure to UXO during the remediation activities would also pose an immediate threat of injury. Environmental radiation monitoring (ERM) data showed limited transport of DU within the impact area and no transport to the Bay. Previous studies of the environmental behavior of DU munitions at APG indicated that sorption reactions and biological processes attenuate DU transport. The present work investigated chemical, physical, and biological processes influencing DU transport at APG by examining the following: (1) the effects of rainfall and the water table on the presence of DU in ground and surface waters, (2) the ability of indigenous bacteria to reduce U(VI) and precipitate U(IV) from solution, and (3) the atmospheric and aqueous oxidation rates under oxidizing and reduced conditions.
ERM data of ground and surface water, soils, and surface water sediments collected from 1992 to 2004 were examined to determine if DU had been mobilized and transported to natural waters. Infiltration to groundwater was not found; however, large counting uncertainties made it difficult to identify the source of U as concentrations were near background. Additional ground and surface water samples were collected and analyzed using extended counting periods, which provided the detection capability needed to identify the source of U measured in water samples. The presence of DU correlated with rainfall and high water tables, which suggests that the DU measured in samples resulted from leaching and/or surface runoff of DU in the saturated zone rather than from infiltration.
Laboratory batch experiments were performed with ground and surface water and surface water sediments to determine if indigenous bacteria are capable of reducing U(VI). Reduction in ground and surface water samples was observed only in samples with nitrogen and phosphate amendments and that did not contain high concentrations of nitrate (< 1 mg/L). In contrast, indigenous bacteria in surface water sediments reduced uranium with or without nutrient amendments.
Field and column studies provided an effective means of evaluating the oxidation, mobilization, and transport of DU in APG soils. Transfer of DU oxidation products from DU fragments placed on the soil surface was limited to soils from 0 to 10 cm below the fragments. Subsurface fragments were oxidized uniformly on all surfaces; however, the amount lost to oxidation varied under different soil conditions. Subsurface fragments in the vadose zone lost up to 5.0 wt.% to oxidation in one-year, whereas fragments in wetlands lost only 0.6 wt.% to oxidation products in the one to two years of exposure.
Column studies showed strong retention of U(VI) under all conditions tested; essentially all of the U in the column feed was retained in the columns over the period of the experiments, except for sandy loam soils, which had effluent concentrations measuring 3 to 4% of influent values. Most of the U mass was retained in the first 1.0-cm depth in soils. U retention included both U(IV) precipitation and binding of U(VI) to soil constituents, such as organic matter and clay. Most of the U(VI) was sorbed reversibly, such that it was readily extracted from the soils by aqueous solutions containing carbonate buffer. The addition of acetate slightly enhanced the bioreduction of U(VI) to U(IV). The presence of U (3 to 4% of the influent concentration) in the effluent of the sandy loam soils indicates that U complexed with colloidal humic substances contained in local surface water.
The results show that the transport of DU is limited under the reducing conditions present at these sites. Sorption and/or complexation with natural organic matter and biological reduction of U(VI) to U (IV) are important processes involved in limiting the transport of DU at APG. However, dynamic conditions, such as drought conditions that may change a wetland from a reduced environment to an oxidizing environment or infiltration from heavy rainfall, potentially can lead to re-mobilization and transport of DU. Therefore, continued long-term vigilance and monitoring of conditions and periodic sampling and analysis for DU at likely locations of possible contamination are recommended policies for implementation.
|Advisor:||Bouwer, Edward J.|
|School:||The Johns Hopkins University|
|School Location:||United States -- Maryland|
|Source:||DAI-B 68/04, Dissertation Abstracts International|
|Keywords:||Aberdeen Proving Ground, Depleted uranium, Maryland, Subsurface|
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