Barium and High Explosives in a Semiarid Alluvial System, Cañon de Valle, New Mexico

Concentrations and distributions of Ba, a component of the high explosive baritol, and RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), a high explosive, were evaluated from 1996 to 2002 within a semiarid alluvial system in Cañon de Valle, New Mexico. A high explosive machining facility discharged effluent containing these chemicals to the canyon from 1951 to 1996. The connectivity between alluvial groundwater, surface water, and sediment was specifically addressed to understand the distributions and dynamics of Ba and RDX in the alluvial system. Surface water, groundwater, and sediment were characterized by conducting hydrologic measurements, geomorphic mapping, and collecting samples. Barium and RDX in sediment preferentially reside in fine-grained deposits that represent the suspended load redeposited on floodplains following channel scour. However, RDX and Ba show markedly different behaviors in surface water and alluvial groundwater because of contrasting geochemical characteristics and transport mechanisms. Barium precipitates in sediments as barite and witherite and readily sorbs to sediment minerals. Therefore, sediment transport is an important control on its distribution in the canyon. In contrast, RDX appears to occur predominantly in the dissolved phase, behaves conservatively, and is most significant in groundwater. There is a strong correlation between RDX concentrations in water and the saturated thickness of the alluvial aquifer. During prolonged wet periods, the alluvial aquifer enlarges, causing more RDX to be mobilized within the alluvial system. Subsurface processes in the alluvial aquifer are therefore most important in controlling present RDX transport, whereas surface processes associated with floods are most important in controlling Ba transport.