Horizontal and Vertical Transport of Uranium in an Arid Weapon-Tested Ecosystem

Abstract

Armor-penetrating projectiles and fragments of depleted uranium (DU) have been deposited in soils at weapon-tested sites. Soil samples from these military facilities were analyzed by inductively coupled plasma-optical emission spectroscopy and X-ray diffraction to determine U concentrations and transport across an arid ecosystem. Under arid conditions, both vertical transport driven by evaporation (upward) and leaching (downward) and horizontal transport of U driven by surface runoff in the summer were observed. Upward vertical transport was simulated and confirmed under laboratory-controlled conditions, to be leading to the surface due to capillary action via evaporation during alternating wetting and drying conditions. In the field, the 92.8% of U from DU penetrators and fragments remained in the top 5 cm of soil and decreased to background concentrations in less than 20 cm. In locations prone to high amounts of water runoff, U concentrations were reduced significantly after 20 m from the source due to high surface runoff. Uranium was also transported throughout the ecosystem via plant uptake and wild animal consumption between trophic levels, but with limited accumulation in edible portions in plants and animals.

Keywords: Yuma; bioconcentration; depleted uranium; mobility; penetrator; proving ground; soil.

Figure 1.

The landscape and the sampling area at Gun Position 20 (GP 20) at Yuma Proving Ground in Arizona. The sampling area included the flat field with impact areas from ballistics (shown as disturbed terrain) and large drainage ditch with a high abundance of vegetation. Open field soil samples, red dots; soil samples beneath exposed penetrators, blue dots; soil samples along drainage ditch, yellow dots.

Figure 2.

X-ray powder diffraction (XRD) analysis of soil samples from the field and ditch at GP 20 and from penetrators that were previously fired and unfired from the DU weathering site.

Figure 3.

Uranium distribution at GP 20 for soil profiles directly beneath an oxidized DU rod. Samples were collected at 0–2, 2–5, and 5–10 cm intervals. Error bars represent standard deviations.

Figure 4.

Uranium distribution at the DU weathering site for soil profiles directly over and below both fired and unfired DU penetrators. Samples were collected at 5 cm intervals. Error bars represent standard deviations.

Figure 5.

Uranium distribution at GP 20 at various distances from the oxidized DU rod. Bars represent standard deviation.

Figure 6.

Uranium concentrations from unimpacted soils at GP 20 at various distances leading to the ditch. Bars represent standard deviations.

Figure 7.

Uranium concentrations within the ditch at GP 20. Samples were collected at 10 m intervals. After Sample 4, a physical barrier was present.

Figure 8.

Depth profiles in centimeters (cm) of U upward transport above the U source in laboratory simulations using Yuma soil. U concentrations (mg/kg) were detected in the soil column of various U sources (A: uranyl, UO₂, and UO₃ and B: schoepite) at different depths above the U source at the bottom of the column due to upward transport.