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Backscattered electron micrograph of a granitic lithic thin section isolated from U(VI)-contaminated sediment at 120 ft below ground surface in a Hanford Site waste tank farm. U(VI) preferentially precipitates as sodium boltwoodite (electron dense, white 1-μm laths) in intragrain fractures. ENLARGE

Silt-textured stringers containing phyllosilicates with high cation exchange capacity and water retention properties are common features of Hanford subsurface sediments. These are often associated with discontinuous zones of calcium carbonate (caliche) precipitation because of local effects on water migration. ENLARGE

Redox boundary in Hanford Site 300 Area Ringold sediments about 2.5 m below the Hanford-Ringold contact. Oxygen and other terminal electron acceptors have been consumed by microbiologic respiration. Residual organic matter in the sediments is believed to be the electron donor for this process. This reducing zone appears to collect contaminant U(VI) that migrates from above under conditions of high groundwater U. ENLARGE

Microenvironments & Transition Zones

Microenvironments are defined here as submicron- to meter-scale domains that exert disproportionate influence on the water chemistry of larger vadose or aquifer zones because of integrated microbial, geochemical, and hydrophysical processes that occur at dissimilar or accelerated rates within them. If the observation scale is on the order of a millimeter, then included microenvironments may exist on the scale of nanometers to microns. In contrast, if the observation scale is tens of meters, then a microenvironment could be on the scale of meters, and so on.

Transition zones are field-scale features where chemical, physical, or microbiologic properties change dramatically over relatively short distances (e.g., ≤1 m). They exhibit steep, transport-controlled gradients of system-controlling chemical species such as O2, H+, or organic carbon.

Microenvironments and transition zones frequently dominate subsurface contaminant reactivity, with strong effects resulting from the coupling of chemical reaction, physical transport (advection, diffusion), and microbiologic processes. Past EMSP and NABIR research has documented the importance of these zones at the Hanford Site.

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