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Degassing of 3H/3He, CFCs and SF6 by denitrification: Measurements and two-phase transport simulations

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Author: Visser, A. · Schaap, J.D. · Broers, H.P. · Bierkens, M.F.P.
Type:article
Date:2009
Institution: TNO Bouw en Ondergrond
Source:Journal of Contaminant Hydrology, 3-4, 103, 206-218
Identifier: 241359
Keywords: Geosciences · 3H/3He · CFCs · Degassing · Denitrification · Groundwater dating · SF6 · Two-phase transport modeling · Aquifers · Chlorofluorocarbons · Degassing · Denitrification · Fluid dynamics · Gas dynamics · Gases · Groundwater · Hydrogeology · Multiphase flow · Nitrification · Sulfur hexafluoride · Two phase flow · Underground reservoirs · H/He · CFCs · Groundwater dating · SF · Two-phase transport modeling · Helium · CFC · dating method · degassing · denitrification · hydrological modeling · measurement method · modeling · tracer · transport process · water table · chlorofluorocarbon · helium · nitrogen · sulfur hexafluoride · tritium · water · article · calibration · chemical model · chemistry · computer simulation · gas · isolation and purification · pressure · soil · Calibration · Chlorofluorocarbons · Computer Simulation · Gases · Helium · Models, Chemical · Nitrogen · Pressure · Soil · Sulfur Hexafluoride · Tritium · Water

Abstract

The production of N2 gas by denitrification may lead to the appearance of a gas phase below the water table prohibiting the conservative transport of tracer gases required for groundwater dating. We used a two-phase flow and transport model (STOMP) to study the reliability of 3H/3He, CFCs and SF6 as groundwater age tracers under agricultural land where denitrification causes degassing. We were able to reproduce the amount of degassing (R2 = 69%), as well as the 3H (R2 = 79%) and 3He* (R2 = 76%) concentrations observed in a 3H/3He data set using simple 2D models. We found that the TDG correction of the 3H/3He age overestimated the control 3He/3He age by 2.1 years, due to the accumulation of 3He* in the gas phase. The total uncertainty of degassed 3H/3He ages of 6 years (± 2 σ) is due to the correction of degassed 3He* using the TDG method, but also due to the travel time in the unsaturated zone and the diffusion of bomb peak 3He*. CFCs appear to be subject to significant degradation in anoxic groundwater and SF6 is highly susceptible to degassing. We conclude that 3H/3He is the most reliable method to date degassed groundwater and that two-phase flow models such as STOMP are useful tools to assist in the interpretation of degassed groundwater age tracer data. © 2008 Elsevier B.V. All rights reserved.