The distribution and transport of solubilized Depleted Uranium (DU) from testing tank ammunition at Aberdeen Proving Ground (APG), MD, and the ability of indigenous microorganisms to reduce U(VI) to U(IV) were examined in field and laboratory experiments. Environmental monitoring 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. Results show limited transport in soils, surface water, and surface water sediments within the impact area and no transport at the installation boundary. Infiltration to groundwater was not indicated. The oxidation, mobilization, and transport of DU fragments were measured in four field study sites to determine the effect of soil conditions on the fate of DU. Fragments were placed on and 10-cm below the soil surface to evaluate transport of oxidation products of DU ammunition. Transfer of DU oxidation products from fragments placed on the soil surface was limited to soils directly below the fragments from 0-10 cm and activities ranged from 100 to 6200 Bq/kg. 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 2.3 to 3.4 g to oxidation in one-year and transferred up to 16,000 Bq/kg to soils. In contrast, fragments in wetlands (under water 8-10 months/year) lost only 0.5 g to oxidation products in the 1 to 2 years of exposure and only 2000 Bq/kg were transferred to soils. Reduction of U(VI) to U(IV) occurred in microcosms containing ground and surface water and surface water sediments, however reduction occurred only in ground water microcosms when amended with phosphate. The results indicate that oxidation, mobilization, and transport of DU are limited under the site reducing conditions. Furthermore, biological reduction of U(VI) to U(IV) is an important process involved in limiting the transport of DU at APG.