Evaporation Effects on Oxygen and Hydrogen Isotopes in Deep Vadose Zone Pore Fluids at Hanford, Washington

Stable isotopes of oxygen and hydrogen were measured in pore fluid extracted from a sediment core located in a relatively undisturbed area in the Hanford site near the S-SX Tank Farms. Pore fluids from most of the 70-meter thick vadose zone section have ?18O values that are shifted to higher values than those for winter precipitation (and Columbia River water) by 3 to 4?. The shift of ?18O, and the ?18O-?D slope of about 4 in the deep vadose zone pore fluids is attributed to partial evaporation during residence in the upper meter of the soil section. A model relating the isotopic shift to recharge and soil properties suggests that the shift should be inversely proportional to recharge, and larger for coarser soils with lower water retention. When applied to Hanford soils and precipitation patterns, the model predicts that vadose zone pore fluids at Hanford should typically be shifted by +2? to +6? in ?18O relative to the values in wet season precipitation, even for relatively high values of annual net infiltration (up to 100 mm/yr or 60% of annual precipitation). The model has implications for groundwater as well as vadose zone ?18O. The effects of vegetation are not included,more » so only upper limit values for the net infiltration flux can be inferred from vadose zone ?18O. The shifted ?18O of natural pore fluids allows identification of the presence of subsurface water that comes from industrial discharges at the Hanford site. An example is provided by a low-?18O, high water content horizon at a depth of 44m in the core, which is interpreted as industrial water that was transported laterally above a capillary barrier as a result of a nearby, near-surface point discharge that happened within the past 50 years.« less

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