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D. Mallants, D. Jacques, J. Šimůnek, and M.Th. van Genuchten

HP1: A coupled numerical code for variably saturated water flow, solute transport and biogeochemical reactions in soils and sediments. D. Mallants, D. Jacques, J. Šimůnek, and M.Th. van Genuchten. Outline. HP1: HYDRUS1D-PHREEQC Possibilities of the code Benchmarking PCE-dissolution

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D. Mallants, D. Jacques, J. Šimůnek, and M.Th. van Genuchten

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  1. HP1: A coupled numerical code for variably saturated water flow, solute transport and biogeochemical reactions in soils and sediments D. Mallants, D. Jacques, J. Šimůnek, and M.Th. van Genuchten

  2. Outline • HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution • Migration of decay chain of adsorbing contaminants during precipitation/evaporation • Illustration of ‘coupled’ effects • TNT degradation under steady state flow • Cd leaching in an acid podzol: lysimeter experiments • Long-term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  3. HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution • Migration of decay chain of adsorbing contaminants during precipitation/evapotranspiration • Illustration of ‘coupled’ effects • TNT degradation under steady state flow • Cd leaching in an acid podzol: lysimeter experiments • Long-term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  4. Simulation Tool A Coupled Numerical Code for Variably Saturated Water Flow, Solute Transport and Biogeochemistry in Soil Systems Simulating water flow, transport and bio-geochemical reactions in environmental soil quality problems HP1 HP1 Biogeochemical model PHREEQC-2.4 Flow and transport model HYDRUS-1D 2.0

  5. HP1 Coupling procedure • Coupling method: non-iterative sequential approach (weak coupling) • Within a single time step: • First solve water flow equation (HYDRUS) • Second: solve heat transport equation • Then solve convection-dispersion equation for solute transport for element master/primary species (inert transport) (HYDRUS) • Finally solve for each element, calculate speciations, equilibrium reactions, kinetic reactions, … (PHREEQC)

  6. HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution • Migration of decay chain of adsorbing contaminants during precipitation/evapotranspiration • Illustration of ‘coupled’ effects • TNT degradation under steady state flow • Cd leaching in an acid podzol: lysimeter experiments • Long term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  7. HP1 – model features • 1D FE water flow in variably-saturated media • 1D FE transport of multiple solutes by CDE • 1D heat transport • Mixed equilibrium / kinetic biogeochemical reactions • Aqueous speciation (reactions in pore-water) • Cation exchange (on clay, organic matter, …) • Surface complexation (e.g. iron oxyhydroxides) • Mineral dissolution / precipitation • Any kinetic reactions (oxidation/reduction, (bio)degradation, dissolution/precipitation)

  8. HP1 examples • Transport of heavy metals (Zn2+, Pb2+, and Cd2+) subject to multiple cation exchange • Transport with mineral dissolution of amorphous SiO2 and gibbsite (Al(OH)3) • Heavy metal transport in a medium with a pH-dependent cation exchange complex • Infiltration of a hyperalkaline solution in a clay sample (kinetic precipitation-dissolution of kaolinite, illite, quartz, calcite, dolomite, gypsum, …) • Long-term transient flow and transport of major cations (Na+, K+, Ca2+, and Mg2+) and heavy metals (Cd2+, Zn2+, and Pb2+) in a soil profile. • Kinetic biodegradation of TNT (multiple degradation pathways)

  9. Typical application and processes involved • Cycling of radionuclides/metals in soil-plant systems • Heterogeneous physical/chemical properties • Water flow under rainfall - evapotranspiration conditions • Root growth and water uptake • Microbiological growth • Degradation of organic matter with radionuclide/metal release • Transport/adsorption/decay • Uptake of radionuclides/metals by plants

  10. HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution under steady-state flow conditions • Migration of decay chain of adsorbing contaminants during precipitation/evapotranspiration • Illustration of ‘coupled’ effects • TNT degradation under steady state flow • Cd leaching in an acid podzol: lysimeter experiments • Long-term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  11. Test I: PCE degradationPCE degradation pathway(Schaerlaekens et al., Hydrological Processes, 1999) • PCE,TCE: organic contaminant • Solvent, degreasing agent, dry-cleaning • VC: vinylchloride: carcinogenic Perchloroethylene Trichloroethylene

  12. Test I: PCE degradationComparison with analytical solution

  13. Test II: Migration of decay chain speciesProblem definition • Three contaminants (Cont_a, Cont_b, Cont_c) • First-order degradation Cont_a  Cont_b  Cont_c  • Linear (Cont_a)/ nonlinear Freundlich (Cont_b, Cont_c) sorption • Homogeneous soil profile • (Soil covered with grass (rooting depth 20 cm)) • Atmospheric boundary conditions (time dependent) • HP1 comparison with HYDRUS-1D µ3= 0.02 d-1 µ2= 0.06 d-1 µ1= 0.005 d-1 nF = 1 nF = 0.9 nF = 0.8

  14. Test II: Migration decay chain speciesWater flow boundary conditions (y)

  15. Test II: Migration decay chain speciesWater content profiles

  16. Test II: Migration decay chain speciesConcentration-depth profiles • BC: Step-functioninput for Cont_a (1 M) & Cont_b (0.1 M) Leaching Breakthrough

  17. Test II: Migration decay chain species Concentration-time profiles • Excellent agreement between HP1 and HYDRUS • Performance criterion for HP1 becomes more strict: Pe×Cr < 0.4

  18. HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution under steady-state flow conditions • Migration of decay chain of adsorbing contaminants during precipitation/evapotranspiration • Illustration of ‘coupled’ effects • TNT degradation under steady-state flow • Cd leaching in an acid podzol: lysimeter experiments • Long-term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  19. Transport of TNT and its Daughter Products • Soil profile: 100 cm, loam, Ks=1 cm/h, 10 days • TNT in top 5 cm of soil: 1 mg/kg (6.61e-6 mol) • TNT dissolution: rate = 4.1 mg/cm2/hour (1.8e-5 mol/cm2/hour) • Solid 2ADNT at equilibrium with solution, 2ADNT solubility = 2,8 g/L • Degradation • TNT -> 66% is transformed in 2ADNT and 34% is to 4ADNT • Transformation constants [1/hour] • TNT 0.01 • 2ADNT 0.006 • 4ADNT 0.04 • Sorption (instantaneous) • Adsorption coefficients Kd [L/kg]: • TNT 3 • 2ADNT 5 • 4ADNT 6 • TAT 0

  20. Transport of TNT and its Daughter Products • This example indicates that ground water may be more vulnerable to leaching of TNT daughter products (notably TAT) than of the parent compound itself, and that monitoring for the daughter products may provide an early warning of possible TNT leaching.

  21. Cd leaching in acid podzol Introduction • Nothern region of Belgium: historical contamination of soils with Cd, Pb, Cu, Zn by atmospheric deposition originated from the non-ferro industry (historical contamination, beginning 20th century) • Risk of flooding with water containing increased salt concentrations

  22. Cd leaching in acid podzol Objectives • To describe the leaching of major cations, Zn and Cd from a lysimeter after application of an increased salt concentration (tracter test) • To assess the effect of increased salt concentrations (CaCl2) on Cd leaching using a new coupled reactive transport model HP1

  23. A E C1 C2 Cd leaching in acid podzol Problem definition (Seuntjens et al., 2000) • Podzol soil (Kempen) contaminated with heavy metals (Cd, Zn, Pb) • Lysimeter (80-cm-diameter, 100-cm-long) • Equipped with TDR probes • Bottom: grid based wick sampler system • Displacement exp.: boundary conditions Time (d)CaCl2 (mol/l) 0-27.9 0.005 27.9-28.9 0.05 (tracer) 28.9-80 0.005 CEC (meq/kg) 24.4 11.7 83.9 62.9 14.4 7.4 Bh1 Bh2

  24. Cd leaching in acid podzol Leaching experiment set-up TDR probes Cable tester Leachate collectors

  25. Cd leaching in acid podzol Leaching experiment modelling (1) • Components in solution: H, Ca, Na, K, Mg, Al, Cl, Br, Cd, Zn • Speciation reactions in soil solution • Complexation reactions of Zn, Cd with OH-, Cl-: • Cd(OH)+, Cd(OH)2, Cd(OH)3-, Cd(OH)32- • Cd(Cl)+, Cd(Cl)2, Cd(Cl)3-, Cd(Cl)32-

  26. Cd leaching in acid podzol Leaching experiment modelling (2) • Ion exchange reactions (solid phase interaction) • Half reactions (X-: exchange complex): H+ + X- = HX Ca2+ + 2 X- = CaX2 • H, Ca, Na, K, Mg, Cd, Zn • Equilibrium constants are adapted to fit the measurements (site-specific Log_K values) • Equilibrium with gibbsite (Al(OH)3)

  27. Cd leaching in acid podzol Multi-component modelling results (1)

  28. Cd leaching in acid podzol Multi-component modelling results (2)

  29. Cd leaching in acid podzol Multi-component modelling results (3)

  30. Cd leaching in acid podzol Multi-component modelling results (4)

  31. Cd leaching in acid podzol Cd remobilisation due to complex formation CdCln2-n Complexation or competition? Complexation!

  32. Cd leaching in acid podzol Conclusion • Increased Cd mobilization due to • exchange Ca-Cd • complexation with Cl- (most important) • Geochemical speciation models required (instead of e.g. Kd approach) • HP1: allows for transient flow conditions

  33. HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution • Migration of decay chain of adsorbing contaminants during precipitation/evapotranspiration • Illustration of ‘coupled’ effects • TNT degradation under steady state flow • Cd leaching in an acid podzol: lysimeter experiments • Long term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  34. Geochemical transport under transient variably-saturated flow • Cycling of metals in soil-plant systems • Heterogeneous physical/chemical properties • Water flow under rainfall - evaporation conditions • Root growth and water uptake • Metal transport/adsorption/speciation • Uptake of metals by plants • Degradation of organic matter with metal release

  35. steady-state actual surface flux = P-ETact A E Bh2 C1 potential surface flux = P-ETpot C2 Long-term transient flow and transportTransient infiltration at surface Bh1

  36. Geochemical Reactions: Multisite cation exchange Long-term transient flow and transportEffect of transient infiltration on Cd migration • Podzol soil • Multi-site exchange complex • CEC: organic matter • CEC=f(pH) • Complex formation: Cl-metals • Variable infiltration

  37. pH Water Content 0 0 4.4 4.2 - 2 - 2 ) ) m 4 m c c ( ( h h t 3.8 t p - 4 - 4 p e e D D 3.6 0.26 3.4 - 6 - 6 0.22 3.2 1975 1976 1977 1978 1975 1976 1977 1978 Time (year) Time (year) 3 0.18 Log(Cl) (mmol/kg soil) Log(Aqueous Cd) (mmol/kg soil) -3.8 0 0 0.14 -4.2 0.1 -4.6 - 2 - 2 ) ) m m c c -5 ( ( 0.06 h h t t - 4 p - 4 p -5.4 e e D D 0.02 -5.8 - 6 - 6 -6.2 1975 1976 1977 1978 1975 1976 1977 1978 -6.6 Time (year) -0.8 Time (year) -1.1 -1.4 -1.7 -2 -2.3 Long-term transient flow and transportCd mobility and bio-availability as function of , pH, Cl- (1)

  38. 0.4 5 t n p H 0.36 e 4.5 t n o 0.32 H c A 4 p r e 0.28 E t a Bh1 3.5 W 0.24 Bh2 W a t e r C o n t e n t C1 0.2 3 d C l C - 4 1 0 s C2 u l o - 2 C 1 0 e u C d - 5 1 0 q A - 6 - 3 1 0 1 0 1972 1974 1976 1978 1980 1982 T i m e ( y e a r ) Long-term transient flow and transportCd mobility and bio-availability as function of , pH, Cl- (2)

  39. Long-term transient flow and transportConclusions • Temporal variability of physical soil variables (θ) results in temporal variability in geochemical variables (pH, Cl-,…) • Applied to heavy metal mobility and bio-availability: • Water content variations linearly related to pH and inversely to Cl- variations • pH inversely related to dissolved metal concentration (multi-site cation exchange f(pH)) • Cl- concentration linearly related to dissolved metal concentration (complex formation)

  40. HP1: HYDRUS1D-PHREEQC • Possibilities of the code • Benchmarking • PCE-dissolution under steady-state flow conditions • Migration of decay chain of adsorbing contaminants during precipitation/evapotranspiration • Illustration of ‘coupled’ effects • TNT degradation under steady state flow • Cd leaching in an acid podzol • Long term transient flow and transport of major cations and heavy metals in a soil profile • U-transport in agricultural field soils

  41. Introduction / objectives (1) • Motivation: assessment of post-closure safety for surface repository • Inherent uncertainties, especially for the long-term • Use of multiple lines of reasoning • Complementary safety indicators for evaluating and confirming safety: e.g., RN fluxes, U-concentration • Objective: estimate long-term U-leaching from agricultural soils, compare with U-fluxes from planned surface repository

  42. Introduction / objectives (2) Multiple lines of reasoning Individual dose  Dose limit, dose constraint U-flux from NF  U-flux from soil, host formation [U]radwaste  [U]concrete, mine waste

  43. Introduction • A new biogeochemical transport code:HP1 • Problem statement: soil, geochemical reactions, BC/IC • Simulation results • U-fluxes from soil vs. surface repository • Conclusions

  44. Simulation depth: 1 m Problem statement (1)Multilayered soil profile • Dry Podzol,7 horizons • All horizons characterized • Thickness • Unsaturated hydraulic properties • pH • Organic matter content • Fe2O3 content Source: Seuntjens et al., 2001. J. Contam. Hydrol.

  45. Problem statement (2)Geochemical equilibrium reactions • Aqueous speciation reactions • Chemical components: C, Ca, Cl, F, H, K, Mg, N(5), Na, O(0), O(-2), P, S(6), U(6) • Multi-site cation exchange reactions • Related to amount of organic matter • Increases with increasing pH • Surface complexation reactions • Specific binding to charged surfaces (FeOH) • Related to amount of Fe-oxides

  46. Log_K1 (HY) Log_K2 (HY) ... Log_K6 (HY) Problem statement (3)Multi-site cation exchange reactions UO2OH+ UO22+ UO22+ UO2Cl+ • Because more groups of humic and fulvic acids dissociate as pH ↑ • proton selectivity decreases when pH ↑ • negative charge of organic matter ↑

  47. Problem statement (4)pH-dependent negative charge Based on Appelo et al., 1998. Appl. Geoch. • U-species accounted for: • UO22+, UO2OH+, UO2Cl+, UO2F+, UO2H3PO42+, ... adsorbs

  48. Problem statement (5)Surface complexation • Surface complexation model • 0.875 reactive sites/mol Fe (Waite et al., 1994. G.C. Acta) • Surface complex: FeOUO2+ (Dzombak & Morel, 1990) • Changing processes in U adsorption with increasing pH U-species replaced by other cations Increased deprotonation Increased U-sorption

  49. Problem statement (6)Initial and Boundary conditions • Initial condition • No U initially present in soil profile (<> few 10 Bq/kg) • Boundary condition • 200-year time series of synthetic meteorological data to calculate preciptiation and potential evaporation • Composition rain water from measurements • P-fertilizer (Ca(H2PO4)2): ~3000 Bq 238U/kg • Applied each year on May 1 (1 g P/m2) • 1.610-1 mol Ca(H2PO4)2 /m² in 1 cm of rain • =>3.810-6 mol U /m2 in 1 cm of rain (~105 Bq/ha)

  50. Introduction • A new biogeochemical transport code:HP1 • Problem statement: soil, geochemical reactions, BC/IC • Simulation results • U-fluxes from soil vs. surface repository • Conclusions

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