TY - JOUR

T1 - How Dynamic Boundary Conditions Induce Solute Trapping and Quasi-stagnant Zones in Laboratory Experiments Comprising Unsaturated Heterogeneous Porous Media

AU - Cremer, C. J.M.

AU - Neuweiler, I.

N1 - Funding information: This work was conducted in the context of the DFG Project FOR 1083 (NE 824/6?2): MUSIS (Multiscale Interfaces in Unsaturated Soil). The authors wish to thank the Deutsche Forschungsgesellschaft (DFG) for funding. We furthermore thank John Koestel and two further anonymous reviewers for a constructive discussion. Data statement: Data utilized in this manuscript is made publicly available online ( http://www.hydroshare.org/resource/40344e265cef4dae989dca91e545d286 ).

PY - 2020/1/28

Y1 - 2020/1/28

N2 - The vadose zone is subject to dynamic boundary conditions in the form of infiltration and evaporation. A better understanding of implications for flow and solute transport, arising from these dynamic boundary conditions in combination with heterogeneous structure, will help to improve the prediction of the fate of solutes. We present laboratory experiments and numerical simulations of heterogeneous porous media under unsaturated conditions where controlled, temporally varying precipitation and evaporation are applied to study the effect of dynamic boundary conditions on solute transport in the presence of material interfaces. Dye tracers Eosine Y and Brilliant Blue FCF are utilized to visualize solute transport and analyze redistribution processes in a flow cell. Water and solute fluxes in and out of the flow cell are quantified. While in dynamic experiments application of small infiltration rates (significantly below the saturated hydraulic conductivities of the materials) led to a reversal of transport paths between infiltration and succeeding evaporation, larger infiltration rates altered downward transport such that flow and transport paths differed from those observed during evaporation. Differences in transport paths ultimately led to a redistribution and trapping of solute in one material which manifested as pronounced tailing in breakthrough curves. Trapping was induced not by the formation of a stagnant zone as result of large parameter contrast but by an interplay of dynamic boundary conditions and material heterogeneity. This study thereby highlights the importance to consider dynamic boundary conditions in predictions of solute leaching.

AB - The vadose zone is subject to dynamic boundary conditions in the form of infiltration and evaporation. A better understanding of implications for flow and solute transport, arising from these dynamic boundary conditions in combination with heterogeneous structure, will help to improve the prediction of the fate of solutes. We present laboratory experiments and numerical simulations of heterogeneous porous media under unsaturated conditions where controlled, temporally varying precipitation and evaporation are applied to study the effect of dynamic boundary conditions on solute transport in the presence of material interfaces. Dye tracers Eosine Y and Brilliant Blue FCF are utilized to visualize solute transport and analyze redistribution processes in a flow cell. Water and solute fluxes in and out of the flow cell are quantified. While in dynamic experiments application of small infiltration rates (significantly below the saturated hydraulic conductivities of the materials) led to a reversal of transport paths between infiltration and succeeding evaporation, larger infiltration rates altered downward transport such that flow and transport paths differed from those observed during evaporation. Differences in transport paths ultimately led to a redistribution and trapping of solute in one material which manifested as pronounced tailing in breakthrough curves. Trapping was induced not by the formation of a stagnant zone as result of large parameter contrast but by an interplay of dynamic boundary conditions and material heterogeneity. This study thereby highlights the importance to consider dynamic boundary conditions in predictions of solute leaching.

KW - conservative solute transport

KW - dynamic boundary conditions

KW - laboratory experiment

KW - unsaturated porous media

UR - http://www.scopus.com/inward/record.url?scp=85076932609&partnerID=8YFLogxK

U2 - 10.1029/2018WR024470

DO - 10.1029/2018WR024470

M3 - Article

AN - SCOPUS:85076932609

VL - 55

SP - 10765

EP - 10780

JO - Water resources research

JF - Water resources research

SN - 0043-1397

IS - 12

ER -