Browsing by Subject "SOLUTE TRANSPORT"

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  • Li, Xiaodong; Meng, Shuo; Puhakka, Eini; Ikonen, Jussi; Liu, Longcheng; Siitari-Kauppi, Marja (2020)
    To determine the diffusion and sorption properties of radionuclides in intact crystalline rocks, a new electromigration device was built and tested by running with I- and Se(IV) ions. By introducing a potentiostat to impose a constant voltage over the studied rock sample, the electromigration device can give more stable and accurate experimental results than those from the traditional electromigration devices. In addition, the variation in the pH of the background electrolytes was minimised by adding a small amount of NaHCO3 as buffers. To interpret the experimental results with more confidence, an advection-dispersion model was also developed in this study, which accounts for the most important mechanisms governing ionic transport in the electromigration experiments. Data analysis of the breakthrough curves by the advection-dispersion model, instead of the traditional ideal plug-flow model, suggest that the effective diffusivities of I- and Se(IV) are (1.15 +/- 0.06) x 10(-13) m(2)/s and (3.50 +/- 0.86) x 10(-14) m(2)/s, respectively. The results also show that I- is more mobile than Se(IV) ions when migrating through the same intact rock sample and that their sorption properties are almost identical.
  • Bonnet, M.; Sardini, P.; Billon, S.; Siitari-Kauppi, M.; Kuva, J.; Fonteneau, L.; Caner, L. (2020)
    Because cracks control the global mechanical and transport properties of crystalline rocks, it is of a crucial importance to suitably determine their aperture distribution, which evolves through alteration processes and rock weathering. Due to the high variability of crack networks in rocks, a multiscale approach is needed. The C-14-PMMA (polymethylmethacrylate) method was developed to determine crack apertures using a set of artificial crack samples with different controlled apertures and tilt angles and also using Monte Carlo simulations. The experiments and simulations show the same result: the estimation of apparent aperture w(A) was successful regardless of tilt angle, even if the estimates are less accurate for low tilt angles (
  • Haggblom, Olle; Salo, Heidi; Turunen, Mika; Nurminen, Jyrki; Alakukku, Laura; Myllys, Merja; Koivusalo, Harri (2019)
    In northern clay soils, field drainage is needed to ensure suitable moisture conditions for crop growth and farming operations. Supplementary drainage installations improve the efficiency of old drainage systems, but the hydrological impacts of the drainage procedures are not comprehensively understood or quantified. The objective was to simulate the hydrological behavior of a clay field section and to quantify the effects of a supplementary drainage on the water balance of the field section. The study site, in southern Finland, was originally subsurface drained in 1952 with 32 m drain spacing. Supplementary drainage was installed in 2014, decreasing the drain spacing to 10.7 m. Simulations were carried out with a dual-permeability hydrological model and two model parametrizations describing the field hydrology (1) before and (2) after the supplementary drainage installation. The parameterizations were used for simulations of a nine-year period to quantify the hydrological impacts of the supplementary drainage. For the periods without snow on the ground, the modified Nash-Sutcliffe efficiency for daily drain discharge and topsoil layer runoff values ranged from 0.43 to 0.53 and from 0.44 to 0.53, respectively. During the original drainage setup scenario, the average annual drain discharge was lower (7.8% of precipitation) than topsoil layer runoff and groundwater outflow (10.3% and 26.4%, respectively). For the supplementary drainage scenario, most of the water outflow was through drain discharge and groundwater outflow (20.4% and 21.9%, respectively). The supplementary drainage installation increased the average annual drain discharge by a factor of 2.6, while the annual average topsoil layer runoff and groundwater outflow decreased by 75% and 18%, respectively. The supplementary drainage setup was found to expedite the drying of the field section in spring by 8 days on average compared to the original drainage setup.
  • Li, Xiaodong; Puhakka, Eini; Liu, Longcheng; Zhang, Wenzhong; Ikonen, Jussi; Lindberg, Antero; Siitari-Kauppi, Marja (2020)
    A surface complexation model of Se(IV) sorption on biotite with one type of strong sorption sites and two types of weak sorption sites were developed based on experimental data obtained from titration, sorption edge and sorption isotherm experiments. Titration data was collected using a batch-wise manner together with back-titration to calibrate the effect of mineral dissolution in 0.01 M KClO4 background electrolytes from pH 3 to 11 in an inert atmosphere glovebox. Further calibrations of the titration curve include proton exchange and cation exchange in which the calculations of cation occupancies on biotite surfaces were taken into account. The sorption edge measurements were determined by measuring the sorption of 10(-9) M total Se with a radioactive Se-75 tracer on converted biotite in 0.01 M KClO4 solution from pH 3 to 11. Se sorption was observed to be strongly dependent on pH. Surface complexation modelling was performed by deriving a set of optimized parameters that can fit titration, sorption edge and sorption isotherm (at pH similar to 7.7) experimental data. A CASTEP code implemented into Materials Studio was used to calculate the site densities and site types on the biotite surfaces. Weak sorption sites with site densities of 3.2 sites/nm(2) and 1.4 sites/nm(2) were derived from the codes and used in the sorption model. A computer code that coupled PHREEQC with Python was developed for the fitting and optimizing processes. The model was validated by sorption data at pH similar to 9.5. The results show that the model can provide quantitative predicts of Se(IV) sorption in groundwater conditions of a deep geological repository and help improve the performance assessments by giving more convincing estimates of the release of radionuclides towards aquifers and biosphere.
  • Salo, Heidi; Warsta, Lassi; Turunen, Mika; Nurminen, Jyrki; Myllys, Merja; Paasonen-Kivekäs, Maija; Alakukku, Laura; Koivusalo, Harri (2017)
    Subsurface drain trenches are important pathways for water movement from the field surface to subsurface drains in low permeability clayey soils. The hydrological effects of trenches installed with well conducting backfill material and gravel inlet patches are difficult to study with only experimental methods. Computational three-dimensional soil water models provide additional tools to assess spatial processes of such drainage system. The objective was to simulate water flow pathways with 3-D FLUSH model in drain spacing and trench depth scale with two model configurations: (1) the total pore space of soil was treated as a single continuous pore system and (2) the total pore space was divided into mobile soil matrix and macropore systems. Both model configurations were parameterized almost solely with field data without calibration. Data on soil hydraulic properties and drain discharge measurements were available from a clayey subsurface drained agricultural field in southern Finland. The effect of soil hydraulic variability on water flow pathways was assessed by generating computational grids in which the hydraulic properties were sampled randomly from five measured soil sets. Both model configurations were suitable to describe the recorded drain discharge, when model was parameterized in finer scale than drain spacing and the parameterization described highly conductive subdomains such as macropores in a dual-permeability model or the trench in a single pore system model. Models produced similar hourly discharge and water balance results with randomly sampled soil hydraulic properties. The results provide a new view on consequences of soil heterogeneity on subsurface drainage. The practical implication of the results from different drainage scenarios is that gravel trench appears to be important only in soils with a poorly conductive subsoil layers without direct macropore connections to subsurface drains. Solely drain discharge data was not sufficient to determine the differences in water flow pathways between the two model configurations and more output variables, such as groundwater level, should be taken into account in making assessments on the effects of different drainage practices on field drainage capacity. (C) 2016 Elsevier B.V. All rights reserved.