Deep Vadose Zone Contaminant Flux Evaluation at the Hanford BY-Cribs Site using Forward and Imposed Concentration Modeling Approaches

Journal Article · Fri Dec 01 00:00:00 EST 2017 · Environmental Processes

DOI:https://doi.org/10.1007/s40710-017-0272-5· OSTI ID:1559114

Oostrom, Martinus ^[1]; Truex, Michael J. ^[2]; Rockhold, Mark L. ^[2]; Johnson, Timothy C.  ^[2]

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1. INTERA, INC.
2. BATTELLE (PACIFIC NW LAB)

For sites with vadose zone contamination, the nature and extent of groundwater contaminant plumes are related to the flux into groundwater. In thick vadose zones at arid sites, transport is likely relative slow, complicating direct measurement of contaminant flux. A predictive numerical analysis was applied to estimate contaminant flux to groundwater at the Hanford BY Cribs site. The site received large amounts of waste water in the 1950’s, containing contaminants such as technicium-99 and nitrate. Recently, large increases in groundwater concentrations have been observed in site groundwater wells and, for potential remediation purposes, the assumption that the changes are the result of increased vadose zone fluxes into groundwater needed to be evaluated. The analysis approach uses forward simulations, tracking contaminants during and after disposal, and imposed concentration (IC) simulations, where concentrations, derived from electrical conductivity data obtained in 2007, are imposed on the flow field. A total of three IC methods, relating electrical conductivity to nitrate concentrations, are developed and tested. The numerical analysis shows that the forward simulations predict high fluxes during and directly after disposal, with decreasing trends starting not later than 1970. Although the IC simulations show differences in the nitrate vadose zone mass imposed in 2007 compared to forward simulations, the IC simulations also show declining nitrate fluxes over time. The predicted fluxes of the forward and IC simulations after 2007 are in reasonable agreement. Given the declining nature of the contaminant fluxes, it can be concluded that the observed concentration increases in the groundwater below the BY Cribs site cannot be explained as resulting from an increase in contaminant flux from the vadose zone. An alternative explanation, based on the observed decrease in aquifer thickness below the site, is presented.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1559114
• Report Number(s): PNNL-SA-127119
• Journal Information: Environmental Processes, Vol. 4, Issue 4
• Country of Publication: United States
• Language: English