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Consequences of mixing assumptions for time-variable travel time distributions
Velde, Y. van der; Heidbüchel, I. ; Lyon, S.W. ; Nyberg, L. ; Rodhe, A. ; Bishop, K. ; Troch, P.A. - \ 2015
Hydrological Processes 29 (2015)16. - ISSN 0885-6087 - p. 3460 - 3474.
solute transport - stream chemistry - stable-isotopes - residence time - transit times - water storage - catchment - model - age - dispersion
The current generation of catchment travel time distribution (TTD) research, integrating nearly three decades of work since publication of Water's Journey from Rain to Stream, seeks to represent the full distribution in catchment travel times and its temporal variability. Here, we compare conceptualizations of increasing complexity with regards to mixing of water storages and evaluate how these assumptions influence time-variable TTD estimates for two catchments with contrasting climates: the Gårdsjön catchment in Sweden and the Marshall Gulch catchment in Arizona, USA. Our results highlight that, as long as catchment TTDs cannot be measured directly but need to be inferred from input-output signals of catchments, the inferred catchment TTDs depend strongly on the underlying assumptions of mixing within a catchment. Furthermore, we found that the conceptualization of the evapotranspiration flux strongly influences the inferred travel times of stream discharge. For the wet and forested Gårdsjön catchment in Sweden, we inferred that evapotranspiration most likely resembles a completely mixed sample of the water stored in the catchment; however, for the drier Marshall Gulch catchment in Arizona, evapotranspiration predominantly contained the younger water stored in the catchment. For the Marshall Gulch catchment, this higher probability for young water in evapotranspiration resulted in older water in the stream compared to travel times inferred with assumptions of complete mixing. New observations that focus on the TTD of the evapotranspiration flux and the actual travel time of water through a catchment are necessary to improve identification of mixing and consequently travel times of stream water. Copyright © 2014 John Wiley & Sons, Ltd.
Improved management of winter operations to limit subsurface contamination with degradable deicing chemicals in cold regions
French, H.K. ; Zee, S.E.A.T.M. van der - \ 2014
Environmental Science and Pollution Research 21 (2014)15. - ISSN 0944-1344 - p. 8897 - 8913.
penetrating radar data - solute transport - spatial variability - soil heterogeneity - dc resistivity - porous-media - water - snow - flow - polarization
This paper gives an overview of management considerations required for better control of deicing chemicals in the unsaturated zone at sites with winter maintenance operations in cold regions. Degradable organic deicing chemicals are the main focus. The importance of the heterogeneity of both the infiltration process, due to frozen ground and snow melt including the contact between the melting snow cover and the soil, and unsaturated flow is emphasised. In this paper, the applicability of geophysical methods for characterising soil heterogeneity is considered, aimed at modelling and monitoring changes in contamination. To deal with heterogeneity, a stochastic modelling framework may be appropriate, emphasizing the more robust spatial and temporal moments. Examples of a combination of different field techniques for measuring subsoil properties and monitoring contaminants and integration through transport modelling are provided by the SoilCAM project and previous work. Commonly, the results of flow and contaminant fate modelling are quite detailed and complex and require post-processing before communication and advising stakeholders. The managers’ perspectives with respect to monitoring strategies and challenges still unresolved have been analysed with basis in experience with research collaboration with one of the case study sites, Oslo airport, Gardermoen, Norway. Both scientific challenges of monitoring subsoil contaminants in cold regions and the effective interaction between investigators and management are illustrated.
The impact of aquifer heterogeneity on the performance of aquifer thermal energy storage
Sommer, W.T. ; Valstar, J.R. ; Gaans, P. van; Grotenhuis, J.T.C. ; Rijnaarts, H. - \ 2013
Water Resources Research 49 (2013)12. - ISSN 0043-1397 - p. 8128 - 8138.
heat-transport - solute transport - porous-media - hydraulic conductivity - geothermal systems - field - simulation - flow - macrodispersion - dispersion
Heterogeneity in hydraulic properties of the subsurface is not accounted for in current design calculations of aquifer thermal energy storage (ATES). However, the subsurface is heterogeneous and thus affects the heat distribution around ATES wells. In this paper, the influence of heterogeneity on the performance of a doublet well system is quantified using stochastic heat transport modeling. The results show that on average, thermal recovery decreases with increasing heterogeneity, expressed as the lognormal standard deviation of the hydraulic conductivity field around the doublet. Furthermore, heterogeneity at the scale of a doublet ATES system introduces an uncertainty in the amount of expected thermal interference between the warm and cold storage. This results in an uncertainty in thermal recovery that also increases with heterogeneity and decreases with increasing distance between ATES wells. The uncertainty in thermal balance due to heterogeneity can reach values near 50 percent points in case of regional groundwater flow in excess of 200 m/yr. To account for heterogeneity whilst using homogeneous models, an attempt was made to express the effect of heterogeneity by an apparent macrodispersivity. As expected, apparent macrodispersivity increases with increasing heterogeneity. However, it also depends on well-to-well distance and regional groundwater velocity. Again, the uncertainty in thermal recovery is reflected in a range in the apparent macrodispersivity values. Considering the increasing density of ATES systems, we conclude that thermal interference limits the number of ATES systems that can be implemented in a specific area, and the uncertainty in the hydraulic conductivity field related to heterogeneity should be accounted for when optimizing well-to-well distances.
A spatially distributed model of pesticide movement in Dutch macroporous soils
Tiktak, A. ; Hendriks, R.F.A. ; Boesten, J.J.T.I. ; Linden, A.M.A. van der - \ 2012
Journal of Hydrology 470-471 (2012)november. - ISSN 0022-1694 - p. 316 - 327.
clay soil - preferential flow - solute transport - hydraulic conductivity - unsaturated soils - water-flow - infiltration - simulation - rainfall - bromide
In the Netherlands, a spatially distributed version of the pesticide fate model PEARL is routinely used to assess the leaching potential of pesticides to groundwater. Recently, the model was modified to simulate the movement of pesticides to surface water. The peak concentration is considered to be the most important exposure endpoint for the ecotoxicological effect assessment for aquatic organisms. Macropore flow is an important driver for the peak concentration, so the leaching model PEARL was extended with a macropore module. Macropore parameters were derived from generally available soil data such as organic matter content and clay content using newly developed pedotransfer functions. These pedotransfer functions were constructed using a wide range of Dutch clayey soils. Results indicate a good correlation between these variables and soil structural parameters, which is due to the homogeneous mineralogical composition of Dutch clayey soils. Results of the spatially distributed modelling indicate that due to rapid transport through macropores, the concentration of pesticides in drainage water is generally higher in clayey soils than in light textured soils. In clayey soils, the boundary hydraulic conductivity and organic matter content were the most important drivers for the concentration in drainage water. Results further indicate that the concentration of pesticide in drainage water increases with increasing half-life and decreases with increasing sorption coefficient. However, the effect of these parameters is much less than obtained with the convection–dispersion equation because due to preferential flow most of the reactive part of the soil profile is bypassed.
Soil as a filter for groundwater quality
Keesstra, S.D. ; Geissen, V. ; Mosse, K. ; Piiranen, S. ; Scudiero, E. ; Leistra, M. ; Schaik, L. van - \ 2012
Current Opinion in Environmental Sustainability 4 (2012)5. - ISSN 1877-3435 - p. 507 - 516.
municipal waste-water - preferential flow - solute transport - mitigation strategies - lumbricus-terrestris - earthworm burrows - structured soils - land application - site preparation - vadose zone
The filtering function of soil is an important ecosystem service for groundwater and surface water protection. The efficiency of soils as a filter depends on the behaviour of pollutants in the soil and the hydrological transport processes. This paper aims to identify knowledge gaps in processes influencing pollutant behaviour in soils and their potential transport to groundwater. Currently most soil-filter function research is approached from two disciplines, one originating from agronomical/environmental sciences; one from more fundamental hydrological process research. Combining insights and approaches from both disciplines through collaboration could lead to better understanding of this complex system and enhance assessments of management strategy changes, both over the long term as well as in different climatic settings.
Quantifying heterogeneous transport of a tracer and a degradable contaminant in the field, with snowmelt and irrigation
Schotanus, D. ; Ploeg, M.J. van der; Zee, S.E.A.T.M. van der - \ 2012
Hydrology and Earth System Sciences 16 (2012). - ISSN 1027-5606 - p. 2871 - 2882.
well-structured soil - solute transport - water-flow - preferential flow - spatial variability - propylene-glycol - unsaturated zone - saturated soil - steady-state - nonequilibrium
To examine the persistence of preferential flow paths in a field soil, and to compare the leaching of a degradable contaminant with the leaching of a tracer, two field experiments were performed using a multi-compartment sampler (MCS). The first experiment was carried out during the snowmelt period in early spring, characterized by high infiltration fluxes from snowmelt. The second experiment was carried out in early summer with irrigation to mimic homogeneous rainfall. During the second experiment, the soil was warmer and degradation of the degradable contaminant was observed. For both experiments, the highest tracer concentrations were found in the same area of the sampler, but the leached tracer masses of the individual locations were not highly correlated. Thus, the preferential flow paths were stable between the two experiments. With a lower infiltration rate, in the second experiment, more isolated peaks in the drainage and the leached masses were found than in the first experiment. Therefore, it is concluded that the soil heterogeneity is mainly caused by local differences in the soil hydraulic properties, and not by macropores. With higher infiltration rates, the high and low leaching cells were more clustered. The leached masses of the degradable contaminant were lower than the leached masses of the tracer, but the masses were highly correlated. The first-order degradation rate and the dispersivity were fitted with CXTFIT; the first-order degradation rate was 0.02 d-1, and the dispersivity varied between 1.9 and 7.1 cm. The persistence of the flow paths during the experiments suggests soil heterogeneity as the driver for heterogeneous flow and solute transport in this soil. At the MCS scale, heterogeneous snowmelt did not seem to have much influence on the flow and solute paths
Quantifying catchment-scale mixing and its effect on time-varying travel time distributions
Velde, Y. van der; Torfs, P.J.J.F. ; Zee, S.E.A.T.M. van der; Uijlenhoet, R. - \ 2012
Water Resources Research 48 (2012)6. - ISSN 0043-1397 - 13 p.
flow route contributions - solute transport - transit-time - lowland catchment - residence time - steady-state - hydrology - model - soil - discharge
Travel time distributions are often used to characterize catchment discharge behavior, catchment vulnerability to pollution and pollutant loads from catchments to downstream waters. However, these distributions vary with time because they are a function of rainfall and evapotranspiration. It is important to account for these variations when the time scale of interest is smaller than the typical time-scale over which average travel time distributions can be derived. Recent studies have suggested that subsurface mixing controls how rainfall and evapotranspiration affect the variability in travel time distributions of discharge. To quantify this relation between subsurface mixing and dynamics of travel time distributions, we propose a new transformation of travel time that yields transformed travel time distributions, which we call Storage Outflow Probability (STOP) functions. STOP functions quantify the probability for water parcels in storage to leave a catchment via discharge or evapotranspiration. We show that this is equal to quantifying mixing within a catchment. Compared to the similar Age function introduced by Botter et al. (2011), we show that STOP functions are more constant in time, have a clearer physical meaning and are easier to parameterize. Catchment-scale STOP functions can be approximated by a two-parameter beta distribution. One parameter quantifies the catchment preference for discharging young water; the other parameter quantifies the preference for discharging old water from storage. Because of this simple parameterization, the STOP function is an innovative tool to explore the effects of catchment mixing behavior, seasonality and climate change on travel time distributions and the related catchment vulnerability to pollution spreading.
Simulation of movement of pesticides towards drains with a preferential flow version of PEARL
Tiktak, A. ; Hendriks, R.F.A. ; Boesten, J.J.T.I. - \ 2012
Pest Management Science 68 (2012)2. - ISSN 1526-498X - p. 290 - 302.
dutch clay soil - unsaturated soils - solute transport - macropore flow - water-flow - sandy soil - model - infiltration - rainfall - bromide
Background: As part of the Dutch authorisation procedure for pesticides, an assessment of the effects on aquatic organisms in surface waters adjacent to agricultural fields is required. The peak concentration is considered to be the most important exposure endpoint for the ecotoxicological effect assessment. Macropore flow is an important driver for the peak concentration, so the leaching model PEARL was extended with a macropore module. The new model has two macropore domains: a bypass domain and an internal catchment domain. The model was tested against data from a field leaching study on a cracking clay soil in the Netherlands.Results: Most parameters of the model could be obtained from site-specific measurements, pedotransfer functions and general soil structural knowledge; only three macropore-flow-related parameters needed calibration. The flow-related macropore parameters could not be calibrated without using the concentration in drain water. Sequential calibration strategies, in which firstly the water flow model and then the pesticide fate model are calibrated, may therefore be less suitable for preferential flow models.Conclusion: After calibration, PEARL could simulate well the observed rapid movement towards drains of two pesticides with contrasting sorption and degradation rate properties. The calibrated value for the fraction of the internal catchment domain was high (90%). This means that a large fraction of water entering the macropores infiltrates into the soil matrix, thus reducing the fraction of rapid flow.
Stochastic modeling of salt accumulation in the root zone due to capillary flux from brackish groundwater
Shah, S.H.H. ; Vervoort, R.W. ; Suweis, S. ; Guswa, A.J. ; Rinaldo, A. ; Zee, S.E.A.T.M. van der - \ 2011
Water Resources Research 47 (2011)9. - ISSN 0043-1397 - 17
bodemchemie - verzilting - bodem-plant relaties - wortelzonestroom - grondwater - modellen - soil chemistry - salinization - soil plant relationships - root zone flux - groundwater - models - water-controlled ecosystems - soil-moisture dynamics - hydraulic redistribution - solute transport - eucalyptus-camaldulensis - hydrologic processes - active-role - irrigation - vegetation - salinity
Groundwater can be a source of both water and salts in semiarid areas, and therefore, capillary pressure–induced upward water flow may cause root zone salinization. To identify which conditions result in hazardous salt concentrations in the root zone, we combined the mass balance equations for salt and water, further assuming a Poisson-distributed daily rainfall and brackish groundwater quality. For the water fluxes (leaching, capillary upflow, and evapotranspiration), we account for osmotic effects of the dissolved salt mass using Van‘t Hoff's law. Root zone salinity depends on salt transport via capillary flux and on evapotranspiration, which concentrates salt in the root zone. Both a wet climate and shallow groundwater lead to wetter root zone conditions, which in combination with periodic rainfall enhances salt removal by leaching. For wet climates, root zone salinity (concentrations) increases as groundwater is more shallow (larger groundwater influence). For dry climates, salinity increases as groundwater is deeper because of a drier root zone and less leaching. For intermediate climates, opposing effects can push the salt balance either way. Root zone salinity increases almost linearly with groundwater salinity. With a simple analytical approximation, maximum concentrations can be related to the mean capillary flow rate, leaching rate, water saturation, and groundwater salinity for different soils, climates, and groundwater depths.
Parameterization of Macropore Flow Using Dye-Tracer Infiltration Patterns in the SWAP Model
Schaik, N.L.M.B. ; Hendriks, R.F.A. ; Dam, J.C. van - \ 2010
Vadose Zone Journal 9 (2010)1. - ISSN 1539-1663 - p. 95 - 106.
preferential flow - solute transport - system extremadura - scale variation - soil profiles - water - spain - methodology - hillslopes - catchment
Preferential flow is known to influence infiltration, soil moisture content distribution, groundwater response, and runoff generation. Various model concepts are used to simulate preferential flow. Preferential flow parameters are often determined by indirect optimization using outflow or discharge measurements, thereby providing limited insight into model performance concerning soil moisture distribution. In this study, we used a physically based macropore concept, embedded in the SWAP model, in combination with dye infiltration patterns to parameterize macropore infiltration for three locations in a catchment: hilltop, hillslope, and valley bottom. The model with the calibrated macropore parameters was applied and validated under natural field conditions, using detailed data on soil moisture content, rainfall, and discharge. The results show that the macropore model parameters can be optimized well to reproduce the dye-tracer infiltration patterns. The simulations of the dye patterns show much better results when macropore flow is included. Using the tracer infiltration patterns, however, the optimized maximum depth of macropores depends completely on the maximum depth of the stained area, while the macropores are known to extend deeper into the soil. Therefore, for long-term simulations, the wetting of deeper layers is too slow for the simulations both with and without macropores. Runoff production was better simulated with macropores. For the simulations without macropores, a higher lumped saturated conductivity was used; despite the resulting increased infiltration into the soil matrix, runoff generation remained far too high
Leaching surfaces to characterize transport in a heterogeneous aquifer: Comparison between flux concentrations and flux concentrations estimated from temporal moment analysis
Bloem, E. ; Vanderborght, J. ; Rooij, G.H. de - \ 2008
Water Resources Research 44 (2008). - ISSN 0043-1397 - 13 p.
natural gradient experiment - lake tracer tests - solute transport - cape-cod - hydraulic conductivity - spatial variability - sand aquifer - porous aquifer - dispersion - field
For subsurface solute transport, flux concentrations are key, while usually resident concentrations are measured. Flux concentrations are frequently estimated from resident concentrations by temporal moment analysis. We tested this approach by simulating transport of an injected tracer during steady flow in an aquifer with a heterogeneous saturated hydraulic conductivity. We constructed grid cell¿scale breakthrough curves (BTCs) from flux concentrations and approximated BTCs from resident concentrations and estimated flux concentrations. We assembled these BTCs into spatiotemporal leaching surfaces at various aquifer cross sections for subsequent analysis. Resident concentrations were unsuitable to assess solute movement in the aquifer. Temporal moment analysis worked well when the entire aquifer cross section was considered but performed poorer at the grid cell scale because it approximates the local velocity by the trajectory average. The leaching surfaces served as valuable tools to demonstrate and quantify the limitations of temporal moment analysis
Simulation of pesticide leaching in the field and in zero-tension lysimeters
Boesten, J.J.T.I. - \ 2007
Vadose Zone Journal 6 (2007)4. - ISSN 1539-1663 - p. 793 - 804.
solute transport - soil - groundwater - models - flow - degradation - parameters - movement - aldicarb - drainage
Zero-tension lysimeters play an important role in groundwater risk assessments for pesticides in the European Union. In these assessments, measured lysimeter leachate concentrations are usually used directly for decision making. When doing so, one assumes (i) that the lysimeter bottom boundary condition itself did not lead to underestimating field leaching concentrations, (ii) that the number of application years and the duration of the lysimeter study were adequate to measure the maximum concentration in time, and (iii) that the pesticide-lysimeter system considered was sufficiently vulnerable with respect to leaching. These assumptions were tested using simulations with a Darcian water flow model combined with a chromatographic pesticide leaching model. The scenario consisted of a layered light-textured soil cropped with cereals and of multiyear weather data. The groundwater level in the field usually fluctuated between 0.7 and 2.5 m depth. The lysimeter bottom boundary condition resulted in pesticide leaching concentrations lower than those calculated for the field system. Simulations showed that a lysimeter study of 2 yr was too short to measure the maximum leaching concentration for pesticides with organic-matter/water distribution coefficient values exceeding 40 L kg¿1. The probability that a lysimeter study results in a leaching concentration below 0.1 µg L¿1 by a coincidental favorable combination of pesticide¿soil parameters was assessed by Monte Carlo simulations. This probability exceeded 20% for pesticides that would give leaching concentrations of 1 µg L¿1 under field conditions. Therefore, it is advisable that for each lysimeter study, modeling be used to assess the likelihood that the leaching concentration would be below 0.1 µg L¿1, considering all relevant systematic and random factors.
Testing MACRO (version 5.1) for pesticide leaching in a Dutch clay soil
Scorza Júnior, R.P. ; Jarvis, N.J. ; Boesten, J.J.T.I. ; Zee, S.E.A.T.M. van der; Roulier, S. - \ 2007
Pest Management Science 63 (2007). - ISSN 1526-498X - p. 1011 - 1025.
solute transport - field soils - hydraulic conductivity - model description - porous-media - water - flow - simulation - bentazon - scale
Testing of pesticide leaching models against comprehensive field-scale measurements is necessary to increase confidence in their predictive ability when used as regulatory tools. Version 5.1 of the MACRO model was tested against measurements of water flow and the behaviour of bromide, bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] and imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] in a cracked clay soil. In keeping with EU (FOCUS) procedures, the model was first calibrated against the measured moisture profiles and bromide concentrations in soil and in drain water. Uncalibrated pesticide simulations based on laboratory measurements of sorption and degradation were then compared with field data on the leaching of bentazone and imidacloprid. Calibrated parameter values indicated that a high degree of physical non-equilibrium (i.e. strong macropore flow) was necessary to describe solute transport in this soil. Comparison of measured and simulated bentazone concentration profiles revealed that the bulk of the bentazone movement in this soil was underestimated by MACRO. Nevertheless, the model simulated the dynamics of the bentazone breakthrough in drain water rather well and, in particular, accurately simulated the timing and the concentration level of the early bentazone breakthrough in drain water. The imidacloprid concentration profiles and its persistence in soil were simulated well. Moreover, the timing of the early imidacloprid breakthrough in the drain water was simulated well, although the simulated concentrations were about 2-3 times larger than measured. Deep groundwater concentrations for all substances were underestimated by MACRO, although it simulated concentrations in the shallow groundwater reasonably well. It is concluded that, in the context of ecotoxicological risk assessments for surface water, MACRO can give reasonably good simulations of pesticide concentrations in water draining from cracking clay soils, but that prior calibration against hydrologic and tracer data is desirable to reduce uncertainty and improve accuracy.
Quantfication of longitudinal dispersion by upscaling Brownian motion of tracer displacement in a 3D pore-scale network model
Acharya, R.C. ; Dijke, M.I.J. van; Sorbie, K.S. ; Zee, S.E.A.T.M. van der; Leijnse, A. - \ 2007
Advances in Water Resources 30 (2007)2. - ISSN 0309-1708 - p. 199 - 213.
porous-media - hydrodynamic dispersion - molecular-diffusion - solute transport - flow - transition - advection - aquifers - length - beds
We present a 3D network model with particle tracking to upscale 3D Brownian motion of non-reactive tracer particles subjected to a velocity field in the network bonds, representing both local diffusion and convection. At the intersections of the bonds (nodes) various jump conditions are implemented. Within the bonds, two different velocity profiles are used. At the network scale the longitudinal dispersion of the particles is quantified through the coefficient DL, for which we evaluate a number of methods already known in the literature. Additionally, we introduce a new method for derivation of DL based on the first-arrival times distribution (FTD). To validate our particle tracking method, we simulate Taylor¿s classical experiments in a single tube. Subsequently, we carry out network simulations for a wide range of the characteristic Péclet number Pe¿ to assess the various methods for obtaining DL. Using the new method, additional simulations have been carried out to evaluate the choice of nodal jump conditions and velocity profile, in combination with varying network heterogeneity. In general, we conclude that the presented network model with particle tracking is a robust tool to obtain the macroscopic longitudinal dispersion coefficient. The new method to determine DL from the FTD statistics works for the full range of Pe¿, provided that for large Pe¿ a sufficiently large number of particles is used. Nodal jump conditions should include molecular diffusion and allow jumps in the upstream direction, and a parabolic velocity profile in the tubes must be implemented. Then, good agreement with experimental evidence is found for the full range of Pe¿, including increased DL for increased porous medium heterogeneity
Effects of cold-girdling on flows in the transport phloem in Ricinus communis: is mass flow ihibited?
Peuke, A.D. ; Windt, C.W. ; As, H. van - \ 2006
Plant, Cell & Environment 29 (2006)1. - ISSN 0140-7791 - p. 15 - 25.
nuclear-magnetic-resonance - water-flow - sieve tubes - noninvasive measurement - assimilate transport - phaseolus-vulgaris - solute transport - plants - translocation - nmr
The effects of cold girdling of the transport phloem at the hypocotyl of Ricinus communis on solute and water transport were investigated. Effects on the chemical composition of saps of phloem and xylem as well as of stem tissue were studied by conventional techniques and the water flow in the phloem was investigated by NMR imaging. Cold girdling reduced the concentration of sucrose but not that of inorganic solutes or amino acids in phloem saps. The possibility that cold treatment inhibited the retrieval of sucrose into the phloem, following leaching from the sieve tubes along a chemical gradient is discussed. Leaching of other solutes did not occur, as a result of missing promoting gradients in stem tissue. Following 3 d of cold girdling, sugar concentration increased and starch was synthesized and accumulated in stem tissue above the cold girdling region and along the cold-treated phloem pathway due to leaching of sugars from the phloem. Only in the very first period of cold girdling (<15-30 min) was mass flow inhibited, but recovered in the rest of cold treatment period to values similar to the control period before and the recovery period after the cold treatment. It is concluded that cold treatment affected phloem transport through two independent and reversible processes: (1) a permanent leaching of sucrose from the phloem stem without normal retrieval during cold treatment, and (2) a short-term inhibition of mass flow at the beginning of cold treatment, possibly involving P proteins. Possible further mechanisms for reversible inhibition of water flow are discussed
Competitive sorption and diffusion of chromate and sulphate in a flow system with goethite in gel beads
Beinum, G.W. van; Meeussen, J.C.L. ; Riemsdijk, W.H. van - \ 2006
Journal of Contaminant Hydrology 86 (2006)3-4. - ISSN 0169-7722 - p. 262 - 278.
silica sand system - ion adsorption - porous-media - physical nonequilibrium - solute transport - ferric-oxide - variable ph - arsenate - model - (hydr)oxides
Column experiments and model simulations were employed to evaluate the processes involved in multicomponent solute transport in a system with heterogeneous flow. Column experiments were performed with goethite embedded in polyacrylamide gel beads. The gel forms an immobile water region that can be accessed by diffusion. A two-region transport model with diffusion into spheres was combined with a surface complexation model to predict reactive transport in the goethite¿gel bead system. Chromate and sulphate breakthrough curves were measured in a set of transport experiments, along with corresponding changes in the pH of the effluent. Sorption and transport of sulphate and chromate in separate columns were predicted from independently measured sorption parameters. The model overestimated the pH changes in the effluent, possibly because of proton buffering by the polyacrylamide gel. The effect of competitive sorption on transport was examined in experiments with both anions present. The model predicted the effect of competition very well in a system initially equilibrated with sulphate, followed by infiltration with chromate. However, when sulphate was infiltrated after equilibration with chromate, chromate desorption and sulphate adsorption were clearly overestimated by the transport model. The exchange between the more strongly bound chromate and the sulphate added subsequently may be too slow to cause a substantial chromate peak in the effluent. This suggests that the local equilibrium assumption was not applicable in this case.
From Field- to Landscape-Scale Vadose Zone Processes: Scale Issues, Modeling, and Monitoring
Corwin, D.L. ; Hopmans, J. ; Rooij, G.H. de - \ 2006
Vadose Zone Journal 5 (2006)1. - ISSN 1539-1663 - p. 129 - 139.
soil electrical-conductivity - nonpoint-source pollutants - stream tube model - solute transport - temporal variability - unsaturated soil - spatial variability - physical-properties - loam soil - parameters
Modeling and monitoring vadose zone processes across multiple scales is a fundamental component of many environmental and natural resource issues including nonpoint source (NPS) pollution, watershed management, and nutrient management, to mention just a few. In this special section in Vadose Zone Journal we present a collection of papers reflecting current trends in modeling and monitoring vadose zone processes from field to landscape scales. The objectives of this introductory paper are to set the stage for the special issue by providing background information, by showing the interrelationship of the papers, and by identifying the significant contribution(s) of each paper. The spectrum of topics covered includes (i) issues of scale, (ii) spatial analysis of model error, (iii) modeling of NPS pollutants and hillslope stability, (iv) the use of estimation and conditioning tools such as upscaling, pedotransfer functions, and generalized likelihood uncertainty estimation, (v) data assimilation in conjunction with flow modeling and passive microwave remote sensing to estimate moisture distribution, (vi) effective hydraulic parameters across spatial scales, (vii) spatiotemporal stability of soil properties (e.g., Cl¿, B, and NO3¿N transport; salinity; and soil physical and hydraulic properties), and (viii) nested sampling to determine spatial patterns. A commonality among the papers, whether for modeling or monitoring vadose zone processes, is the question of how to address complex issues of spatial and/or temporal variability at the scale of interest. Future research will likely involve inverse modeling, the use of multiple sensors to monitor at various scales, and continued applications of pedotransfer functions, upscaling and downscaling, and hierarchy of scales
Stochastic analysis of nonlinear biodegradation in regimes controlled by both chromatographic and dispersive mixing
Janssen, G.M.C.M. ; Cirpka, O.A. ; Zee, S.E.A.T.M. van der - \ 2006
Water Resources Research 42 (2006). - ISSN 0043-1397 - 12 p.
3-dimensionally heterogeneous aquifers - oxygen-limited biodegradation - miralleswilhelm,f. et-al - pore-scale dispersion - concentration fluctuations - reactive transport - solute transport - porous-media - analytical approximation - convective transport
Nonlinear biodegradation in natural porous media is affected by the heterogeneity of the formation and dispersive mixing processes. We analyze these coupled effects by combining recent advances in analytical one-dimensional modeling of bioreactive transport with stochastic concepts of dispersive mixing in heterogeneous domains. Specifically, we model bioremediation of a sorbing contaminant undergoing nonlinear biodegradation in heterogeneous aquifers applying the stochastic-convective and the advective-dispersive stream tube approaches, in which we use a semianalytical traveling wave solution for one-dimensional reactive transport. The results of numerical simulations agree excellently with both models, which establishes that the traveling wave solution is an efficient and accurate way to evaluate the development of intrastream tube concentration distributions and that the advective-dispersive stream tube approach is suitable to describe nonlinear bioreactive transport in systems controlled by local-scale dispersion. In contrast with conservative transport the mean contaminant flux is shown to be significantly influenced by transverse dispersion, even for realistic Peclet values. Furthermore, asymptotic front shapes are shown to be neither Fickian nor constant (traveling wave behavior), which raises questions about the current practice of upscaling bioreactive transport. The error caused by neglecting local dispersion was found to increase with time and to remain significant even for large retardation differences between electron acceptor and contaminant. This implies that, even if reaction rates are dominated by chromatographic mixing, the dispersive mixing process cannot be disregarded when predicting bioreactive transport.
Simulation of pesticide leaching in a cracking clay soil with the PEARL model
Scorza, R.P. ; Boesten, J.J.T.I. - \ 2005
Pest Management Science 61 (2005)5. - ISSN 1526-498X - p. 432 - 448.
uitspoelen - grondwaterverontreiniging - pesticiden - modellen - zware kleigronden - bentazon - imidacloprid - bodemwaterbeweging - leaching - groundwater pollution - pesticides - models - clay soils - bentazone - soil water movement - spatial variability - solute transport - sandy soil - field - persistence - water - translocation - validation - herbicide
Testing of pesticide leaching models is important to increase confidence in their use in pesticide registration procedures world-wide. The chromatographic PEARL model was tested against the results of a field leaching study on a cracking clay soil with a tracer (bromide), a mobile pesticide (bentazone) and a moderately sorbing, persistent pesticide (imidacloprid). Input parameters for water flow and solute transport were obtained from site-specific measurements and from literature. The model was tested using a stepwise approach in which each sub-model was sequentially and separately tested. Uncalibrated simulations for the water flow resulted in moisture profiles that were too wet. Calibration of the hydraulic relationships resulted in a good description of the moisture profiles. Calibration of the dispersion length was necessary to obtain a good description of bromide leaching. The calibrated dispersion length was 61 cm, which is very long and indicates a large non-uniformity of solute transport. The half-life of bentazone had to be calibrated to obtain a good description of its field persistence. The calibrated half-life was 2.5 times shorter than the half-life derived from the laboratory studies. Concentrations of bentazone in drain water and groundwater were described reasonably well by PEARL. Although measured and simulated persistence of imidacloprid in soil corresponded well, the bulk of the imidacloprid movement was overestimated by PEARL. However, imidacloprid concentrations in drain water were underestimated. In spite of the extensive calibration of water flow and tracer movement, the behaviour of the moderately sorbing pesticide imidacloprid could not be simulated. This indicates that the convection-dispersion equation cannot be used for accurate simulation of pesticide transport in cracking clay soils (even if extremely long dispersion length is used). Comparison of the model results from a poorly sorbed chemical (bentazone) and a moderately sorbed chemical (imidacloprid) were useful in defining the limitations of using a chromatographic flow model to simulate the effects of preferential flow.
Advances in hydropedology
Lin, H. ; Bouma, J. ; Wilding, L.P. ; Richardson, J.L. ; Kutilek, M. ; Nielsen, D.R. - \ 2005
Advances in Agronomy 85 (2005). - ISSN 0065-2113 - p. 1 - 89.
saturated hydraulic conductivity - soil-water content - rudimentary mechanistic model - land-use history - pedotransfer functions - solute transport - surface soil - landscape development - profile development - effective porosity
Hydropedology is an intertwined branch of soil science and hydrology that encompasses multiscale basic and applied research of interactive pedological and hydrological processes and their properties in the unsaturated zone. The synergistic integration of classical pedology with soil physics, hydrology, and other related bio- and geosciences into hydropedology suggests a renewed perspective and a more integrated approach to studying landscape-soil-water dynamics across scales. Pedality, layering of soil horizons, and soil-landscape relationships are three essential characteristics of soils as occurring on the landscape. Fundamental issues of hydropedology include (1) soil structure and layering as indicators of flow and transport characteristics in field soils; (2) soil morphology as signatures of soil hydrology; (3) water movement over the landscape; and (4) hydrology as a factor of soil formation and a driving force of dynamic soil system. Hydrology affects and is affected by all of the five natural soil-forming factors and the four general soil-forming processes. Hence, hydropedology offers potential opportunities for quantifying soil-forming processes. Future needs in advancing hydropedology are encapsulated in the philosophy of bridging disciplines, scales, data, and education. These include (1) systems approaches to understanding and communicating landscape-soil-water dynamics; (2) addressing variability using patterns at various scales; (3) enhancing pedotransfer functions and developing soil inference systems and hydropedoinformatics; and (4) education of the next generation of soil scientists and hydrologists. Hydropedology calls for adequate attention to soil morphology (including soil structure) in the field and soil patterns over the landscape to guide optimal soil physical and hydrological measurements, field monitoring and experimental designs, and understanding and modeling of flow and transport in the critical zone. Identification and prediction of patterns (spatial-temporal organizations) across multiple scales are coming to the forefront in soil science and hydrology, which offer rich and comprehensive insights regarding variability and the underlying processes. We suggest various hydropedological approaches to address diverse knowledge gaps. Given its links to a wide array of environmental, ecological, geological, agricultural, and natural resource issues of societal importance, hydropedology is emerging as a promising field that could contribute significantly to the study of the pedosphere, the hydrological cycle, the earth's critical zone, and the earth system.