Solute Transport Patterns Research Articles

Microbial mediated carbon and nitrogen cycling in the spatially heterogeneous vadose zone: A modeling study

AbstractSpatially distributed properties of the subsurface result in varying water saturation and preferential flow paths, which lead to heterogeneous solute transport patterns and heterogeneous microbial environments. This, in turn, influences the distribution of nutrients and energy gradients, microbial biomass, and activity thereof. By their very nature, current field sampling techniques do not resolve subsampling scale heterogeneities in microbial biomass and activity, resulting in inaccurate estimates of microbially mediated carbon and nitrogen turnover in the heterogeneous subsurface. Thus, in this study, we undertook a numerical modeling approach to study the impact of spatial heterogeneity on microbially mediated carbon and nitrogen turnover in the vadose zone. We adapted an established biogeochemical process network that captures a variety of respiration pathways, carbon decomposition strategies, and microbial life processes to simulate microbially mediated carbon and nitrogen turnover in variably saturated spatially heterogeneous settings, using an established numerical tool (OGS#BRNS). The fractionation of microbial communities into active and inactive states, as well as immobile and mobile states followed could be linked to the bulk average saturation. Lastly, we identified three reactive systems, distinguished by the rate ratio of aerobic respiration and transfer of oxygen from the air to the water phase, to evaluate the impact of spatial heterogeneity on carbon and nitrogen removal in subsurface heterogeneous domains. Specifically, when this ratio is approximately 1, there is no impact on carbon removal, while when this ratio is very high, then carbon removal decreases as the domain tends to be oxygen limited.

Centennial‐Scale Shifts in Hydrophysical Properties of Peat Induced by Drainage

AbstractHydrophysical properties of peat influence the partitioning of rainfall into infiltration versus runoff, determine water flow and solute transport patterns, and regulate the carbon and nitrogen cycles in peatlands. Compared with mineral soils, our understanding of hydraulic properties of peat soils is limited, especially of the temporal dynamics of peat properties. A data set of peat subsidence as well as the bulk density (BD) change rate following artificial drainage was assembled from the literature. The collected data cover a time period of up to 272 years of land drainage for forestry and agriculture in boreal and temperate climate zones. The results show that the subsidence rate and BD change rate, and hydrophysical properties of peat can be estimated based on land drainage duration and land use. The most severe shift in pore structure of peat occurs within the first 20 years of land drainage. Peatland drainage reduces macroporosity with pore diameter greater than 50 μm, but increases the volume of pores <5 μm. In the long term, peat thickness loss is responsible for more than 80% of water storage loss. In conclusion, the derived functions between subsidence rate, BD change rate, and drainage duration provide a new approach to estimate the hydrophysical properties of peat (pore structure, saturated hydraulic conductivity, specific yield, and soil water storage) on a centennial scale. The derived hydrophysical parameter values can be used for long‐term hydrological modeling, especially if measured hydraulic parameters of peat are not available.

Density-dependent solute transport in a layered hyporheic zone

Hyporheic exchange is affected by bedform geometry, which induces complex flow paths within the bedform. Additional factors that influence flow and solute transport in the hyporheic zone are layered profile sediments and density-driven flow. This study explored the combined effects of these factors on hyporheic exchange through laboratory experiments and numerical simulations involving infiltrating solute displacing less-dense resident water in a layered bedform with a low permeability layer (LPL). The bedform consisted of three horizontal layers, in which the hydraulic conductivity of the middle layer (LPL) was less than that of the top (TL) and bottom layers (BL). The results demonstrated that a previously unexplored combination of mechanisms (density effects and layered bedform) produces irregular spatial patterns of solute transport in the hyporheic zone. For instance, the width of solute plume within the bottom layers becomes narrowed compared with tracer transport. With increasing density contrast between infiltrating solute and resident water, the solute plume becomes much narrower, forming fingers. Numerical modeling further shows that the hydraulic conductivity contrast (HCC) and relative thickness (RT) of the hyporheic zone layers also affect the spatial solute transport patterns. As the hydraulic conductivity contrast or relative thickness increases, the plume becomes much narrower. Horizontal ambient flow (HAF) dominated in the bottom layers, and lateral solute spreading and mixing intensified with a higher hydraulic conductivity contrast and relative thickness. Furthermore, the vertical solute plume was detached by the horizontal ambient flow in the bottom layers with a discontinuous low permeability layer, forming a discontinuous zone of vertical solute transport.

Simulated groundwater dynamics and solute transport in a coastal phreatic aquifer subjected to different tides

A 2 D cross-shore transect model with variable density and saturation was established in FEFLOW to investigate the groundwater dynamics and solute transport under the influences of three tidal conditions. This study resulted in four major conclusions. The phreatic aquifer acted as a low-pass filter accompanied by a longer period in groundwater fluctuations. More obvious stability was observed in the solute transport at the lower water table of the tide-fixed, in the spring tide of tide-1, and in the valley stage of tide-2, the ranges of the fluctuations in solute concentrations showed a phenomenon of “reverse distance” and the morphologies showed a “bimodal wave” pattern under the variable-amplitude tides. The solute migration rates were hastened by tidal boundaries and flux patterns of solute transport presented fluctuations. Sensitivity analyses indicated that the solute concentration was insensitive to the boundary transformation when the hydraulic conductivity was low or the dispersivity was high. This model provides a reliable analogical approach for the management of pollutant dispersion in coastal areas lacking data.

Impact of short‐rotation coppice with poplar and willow on soil physical properties

AbstractAs the land area of short‐rotation coppice (SRC) increases, their soil physical impacts have to be evaluated. The objective of this study was to detect the effects of long‐term SRC with poplar and willow on the vertical distribution of soil physical properties (bulk density, water retention, penetration resistance) and on solute transport patterns. An 18‐year‐old SRC located in northeastern Germany was compared to an adjacent continuous arable cropping system by means of soil sampling, penetrologger measurements and dye tracer experiments. The topsoil's bulk density was significantly lower under SRC than under cropland. This effect was especially pronounced in the uppermost 10 cm, where also the air capacity and the plant‐available water content were higher under SRC. The penetration resistance in 25–50 cm depth was reduced under SRC compared to the cropland, indicating a loosening of the plough pan. Dye tracer experiments showed that the importance of preferential flow was higher under SRC due to tree root channels and an increasing colonisation with invertebrates. SRC has ecologically advantageous effects on soil physical properties of the topsoil, however, combined with an enhanced risk of preferential solute transport upon application of agrochemicals.

Emergent Macrophyte Root Architecture Controls Subsurface Solute Transport

AbstractEmergent macrophytes (helophytes) grow in the active channel of fluvial ecosystems. Subsurface flow beneath this area (i.e., hyporheic zone) is considered critical for ecological processes. However, little is known about the influence of helophyte roots on subsurface solute transport. We investigated the effect of three helophyte species with different root architecture (Iris pseudacorus L., Phragmites australis L., and Scirpus lacustris L.) on solute transport along subsurface flow paths. We considered both the physical and the biological roles of the roots, expecting that (1) roots will act as structures that create heterogeneities in the sediment (physical role); thus, root architecture will alter subsurface flow paths; (2) roots will remove water via evapotranspiration (biological role), leading to slower flow velocity; and (3) both scenarios will result in longer water residence times. We performed conservative tracer pulse additions in 12 flow‐through flumes subjected to four treatments: absence of helophytes (Control) and presence of helophytes (Iris, Scirpus, and Phragmites). Tracer breakthrough curves were used to compare solute transport patterns between the treatments by fitting a mobile‐immobile model and by applying temporal moment analysis. Results showed that helophyte roots increase subsurface water residence time by creating heterogeneities in the substrate and by removing water. Furthermore, hydraulic retention increased with the percent volume of fine roots but decreased in the presence of thicker roots. Based on these results we suggest that the root architecture of helophytes and their capacity to remove water via evapotranspiration should be considered when planning stream restoration activities aimed to improve water quality.

Mapping the spatiotemporal evolution of solute transport in articular cartilage explants reveals how cartilage recovers fluid within the contact area during sliding

The interstitial fluid within articular cartilage shields the matrix from mechanical stresses, reduces friction and wear, enables biochemical processes, and transports solutes into and out of the avascular extracellular matrix. The balanced competition between fluid exudation and recovery under load is thus critical to the mechanical and biological functions of the tissue. We recently discovered that sliding alone can induce rapid solute transport into buried cartilage contact areas via a phenomenon termed tribological rehydration. In this study, we use in situ confocal microscopy measurements to track the spatiotemporal propagation of a small neutral solute into the buried contact area to clarify the fluid mechanics underlying the tribological rehydration phenomenon. Sliding experiments were interrupted by periodic static loading to enable scanning of the entire contact area. Spatiotemporal patterns of solute transport combined with tribological data suggested pressure driven flow through the extracellular matrix from the contact periphery rather than into the surface via a fluid film. Interestingly, these testing interruptions also revealed dynamic, repeatable and history-independent fluid loss and recovery processes consistent with those observed in vivo. Unlike the migrating contact area, which preserves hydration by moving faster than interstitial fluid can flow, our results demonstrate that the stationary contact area can maintain and actively recover hydration through a dynamic competition between load-induced exudation and sliding-induced recovery. The results demonstrate that sliding contributes to the recovery of fluid and solutes by cartilage within the contact area while clarifying the means by which it occurs.

Hydrological Responses and Flow Pathways in an Acrisol on a Forested Hillslope with a Monsoonal Subtropical Climate

The nature of subsurface flow depends largely on hydraulic conductivity of the vadoze zone, permeability of the underlying bedrock, existence of soil layers differing in hydraulic properties and macropore content, soil depth, and slope angle. Quantification of flow pathways on forested hillslopes is essential to understand hydrological dynamics and solute transport patterns. Acrisols, with their argic Bt horizons, are challenging in this respect. To further elucidate flow pathways of water and short-term variability of soil moisture patterns in Acrisols, a field study was conducted on a forested hillslope in a sub-catchment of the Tie Shan Ping (TSP) watershed, 25 km northeast of Chongqing City, China. This catchment is covered by a mixed secondary forest dominated by Masson pine (Pinus massoniana). Soil saturated hydraulic conductivity (Ksat) was significantly reduced at the interface between the AB and Bt horizons (2.6 × 10−5 vs. 1.2 × 10−6 m s−1), which led to that the flow volume generated in the Bt horizon was of little quantitative importance compared to that in the AB horizon. There was a marked decrease in porosity between the OA and AB horizons, with a further decrease deeper in the mineral subsoil. Especially, the content of soil pores > 300 μm was higher in the AB horizon (14.3%) than in the Bt horizon (6.5%). This explained the difference in soil Ksat values. This study showed that Bt horizon had limited water transport capability, forcing part of the infiltrated rainwater as interflow through the OA and AB horizons. Thus, the topsoil responded quickly to rainfall events, causing frequent cycles of saturation and aeration of soil pores.

Methods for characterizing the co-development of biofilm and habitat heterogeneity.

Biofilms are surface-attached microbial communities that have complex structures and produce significant spatial heterogeneities. Biofilm development is strongly regulated by the surrounding flow and nutritional environment. Biofilm growth also increases the heterogeneity of the local microenvironment by generating complex flow fields and solute transport patterns. To investigate the development of heterogeneity in biofilms and interactions between biofilms and their local micro-habitat, we grew mono-species biofilms of Pseudomonas aeruginosa and dual-species biofilms of P. aeruginosa and Escherichia coli under nutritional gradients in a microfluidic flow cell. We provide detailed protocols for creating nutrient gradients within the flow cell and for growing and visualizing biofilm development under these conditions. We also present protocols for a series of optical methods to quantify spatial patterns in biofilm structure, flow distributions over biofilms, and mass transport around and within biofilm colonies. These methods support comprehensive investigations of the co-development of biofilm and habitat heterogeneity.

Monitoring and modeling the effects of groundwater flow on arsenic transport in Datong Basin

Although arsenic-contaminated groundwater in the Datong Basin has been studied for more than 10 years, little has been known about the complex patterns of solute transport in the aquifer systems. Field monitoring and transient 3D unsaturated groundwater flow modeling studies were carried out on the riparian zone of the Sanggan River at the Datong Basin, northern China, to better understand the effects of groundwater flow on As mobilization and transport. The results indicate that irrigation is the primary factor in determining the groundwater flow paths. Irrigation can not only increase groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations, but also change the HS− concentration, redox conditions of the shallow groundwater. Results of net groundwater flux estimation suggest that vertical infiltration is likely the primary control of As transport in the vadose zone, while horizontal water exchange is dominant in controlling As migration within the sand aquifers. Recharge water, including irrigation return water and flushed saltwater, travels downward from the ground surface to the aquifer and then nearly horizontally across the sand aquifer. The maximum value of As enriched in the riparian zone is roughly estimated to be 1 706.2 mg·d−1 for a horizontal water exchange of 8.98 m3·d−1 close to the river and an As concentration of 190 μg·L−1

Classification and delineation of groundwater–lake interactions in the Nebraska Sand Hills (USA) using electrical resistivity patterns

Lake–groundwater interactions exhibit a complex three-dimensional (3D) structure that is seldom studied. The utility of waterborne electrical resistivity (ER) surveys is explored for characterization of 3D groundwater flow and solute transport patterns for three lakes in the Nebraska Sand Hills, USA. Waterborne ER surveys, using contrasts between lake and groundwater solutes as natural tracers, are useful for inferring 3D patterns of groundwater flow and solute transport as well as classifying groundwater–lake interactions. Three unique groundwater flow systems are interpreted under each lake from dense networks of two-dimensional (2D) waterborne ER surveys. A lateral transition from high to low ER values beneath the saline Wilson Lake expresses its flow-through regime, where groundwater salinity indicates changes from groundwater inflow to outflow. Alkali Lake ER profiles reveal a prevalent ER increase with depth over the lakebed area that is characteristic of groundwater discharge lakes. ER profiles beneath Gimlet Lake are the most resistive and indicate pockets of high ER related to fresh groundwater discharge into the lake, supporting a flow-through regime with a short flushing time. These ER patterns correctly classify groundwater–lake interactions and provide high spatial resolution of mixing patterns for systems with varying water salinity.

Climate, soil, and vegetation controls on the temporal variability of vadose zone transport

Temporal patterns of solute transport and transformation through the vadose zone are driven by the stochastic variability of water fluxes. This is determined by the hydrologic filtering of precipitation variability into infiltration, storage, drainage, and evapotranspiration. In this work we develop a framework for examining the role of the hydrologic filtering and, in particular, the effect of evapotranspiration in determining the travel time and delivery of sorbing, reacting solutes transported through the vadose zone by stochastic rainfall events. We describe a 1‐D vertical model in which solute pulses are tracked as point loads transported to depth by a series of discrete infiltration events. Numerical solutions of this model compare well to the Richards equation–based HYDRUS model for some typical cases. We then utilize existing theory of the stochastic dynamics of soil water to derive analytical and semianalytical expressions for the probability density functions (pdf's) of solute travel time and delivery. The moments of these pdf's directly relate the mean and variance of expected travel times to the water balance and show how evapotranspiration tends to reduce (and make more uncertain) the mass of a degrading solute delivered to the base of the vadose zone. The framework suggests a classification of different modes hydrologic filtering depending on hydroclimatic and landscape controls. Results suggest that variability in travel times decreases with soil depth in wet climates but increases with soil depth in dry climates. In dry climates, rare large storms can be an important mechanism for leaching to groundwater.

Lacunarity and Fractal Analyses of Soil Macropores and Preferential Transport Using Micro‐X‐Ray Computed Tomography

Quantification of soil macropore networks and solute transport patterns is important to enhance our understanding of preferential flow in structured soils. We investigated soil macropore structure and solute transport dynamics in an intact soil column of 76‐mm diameter and 265‐mm length. Five positions (each with a thickness of 10.7 mm) in the soil column were scanned with a voxel resolution of 78.1 by 78.1 by 86.7 μm using micro‐x‐ray computed tomography. The scanning was done at three stages after the soil column was satiated: (i) before tracer (KI) introduction, (ii) 6 min after tracer introduction, and (iii) 78 min after tracer introduction. The macropore network and tracer distribution were reconstructed at the five scanned positions. Relative lacunarity functions (RLFs) and pore fractal dimensions, in both two and three dimensions, were calculated. Distinct macropore characteristics and flow patterns were observed at the five positions. The biopores were active in solute transport because of their high continuity and low tortuosity. Positive logarithmic trends were found between the fractal dimension and the volume percentage of the macropores and tracer distribution. Generally, the lacunarity function reflected the size distribution of macropores and the spatial pattern of flow and transport. The RLFs indicated that the tracer distributions exhibited more self‐similarity than the macropore networks and that the representative elementary volume had not been reached for the three‐dimensional macropore networks within the size range investigated (cube size ≤63). Lacunarity has diagnostic value in characterizing soil macropore structure and flow pattern and may be coupled with the fractal dimension to better describe and model soil structural properties.

Hydraulic characteristics of a small Coastal Plain stream of the southeastern United States: effects of hydrology and season

AbstractVarious physical and biological properties affect solute transport patterns in streams. We measured hydraulic characteristics of Payne Creek, a low‐gradient upper Coastal Plain stream, using tracer experiments and parameter estimation with OTIS‐P (one‐dimensional transport with inflow and storage with parameter optimization). The primary objective of this study was to estimate the effects of varying discharge, season, and litter accumulation on hydraulic parameters. Channel area A ranged from 0·081 to 0·371 m2 and transient storage area As ranged from 0·027 to 0·111 m2. Dispersion D ranged from 1·5 to 11·1 m2 min−1 and exchange coefficient α ranged from 0·009 to 0·038 min−1. Channel area and dispersion were positively correlated to discharge Q, whereas storage area and exchange coefficient were not. Relative storage size As/A ranged from 0·17 to 0·59, and was higher during fall than other seasons under a similar Q. The fraction of median travel time due to transient storage $F_{\rm med}^{200}$ ranged from 8·8 to 34·5% and was significantly correlated with Q through a negative power function. Both metrics indicated that transient storage was a significant component affecting solute transport in Payne Creek, especially during the fall. Comparison between the measured channel area Ac and A suggested that surface storage was dominant in Payne Creek. During fall, accumulation of leaf litter resulted in larger A and As and lower velocity and D than during other seasons with similar discharge. Seasonal changes in discharge and organic matter accumulation, and dynamic channel morphology affected the magnitude of transient storage and overall hydraulic characteristics of Payne Creek. Copyright © 2005 John Wiley & Sons, Ltd.

Flow and transport simulation models for prediction of chlorine contact tank flow-through curves.

Turbulent flow, solute transport, and chemical and biological decay are some of the basic processes encountered in water treatment plants. This paper presents recent developments in the numerical simulation of turbulent flow and disinfection processes in disinfection contact tanks. Simulation runs have been conducted for various tank design alternatives and in different grid resolutions. The accuracy of simulated contact tank flow and the disinfection process depends largely on calculations of the hydrodynamic and solute transport characteristics in the tanks. A key factor of this is the accuracy of advection and shear stress term computations, which can be affected by the use of different hydrodynamic submodels and numerical schemes. The performance of a simulation model relies to a great extent on the right combination of such submodels and numerical schemes. In this study, a number of simulation models were tested against realistic tank configurations and measurements to evaluate the various combinations of turbulence models and difference schemes by analyzing predicted flow and solute transport patterns, as well as the corresponding flow-through curves. Models for disinfection tank simulations are recommended based on comparisons of simulation results with measurements. These models may also be applied to other water treatment processes such as wastewater treatment.

Quantifying the impact of preferential flow on solute transport to tile drains in a sandy field soil

The objective of this study was to quantify the impact of preferential finger flow on solute leaching to tile drains and shallow groundwater in a water repellent sandy soil. Measurements of site hydrology and bromide movement were made during a six-month period following application in autumn 1994 to a 900m2 plot. A dye tracing study was also conducted and this confirmed the presence of finger flow in the water repellent topsoil. Water flow and bromide transport was simulated using a mobile-immobile concept, in which a fraction of the soil volume is assumed not to participate in transport. With calibration of ‘difficult’ parameters (e.g. the fractional mobile volume), and accounting for significant water inflows to the plot from the surroundings, there was a good agreement between simulated and observed hydrology and solute transport patterns at the site. Mass balance calculations showed that ca. 46% of the bromide application was lost to the tile drains in the winter period following application, while 16% was lost in shallow lateral groundwater flow. Comparisons between one- and two-domain simulations showed that preferential flow reduced soil water contents and increased drainflow in the early autumn, leading to a more rapid bromide transport to depth. Nevertheless, the impact of preferential finger flow on leaching was minor for the non-reactive tracer bromide in this experiment, presumably because it was applied to relatively wet soil. However, scenario simulations for autumn application of a short half-life pesticide showed more significant effects, with leaching increased by ca. 80% because of preferential flow (from 1.2% to 2.2% of the applied amount).

Uncertainties in Leaching Risk Assessments Due to Field Averaged Transfer Function Parameters

The transfer function model is widely used to estimate solute transport patterns at the field scale. Leaching risk assessments with the transfer function model may be influenced by spatial variability in the net applied water (NAW) distribution, but few researchers have investigated this possibility. The objective of this study was to evaluate the impact of spatial variability in the NAW distribution on leaching risk assessment and identification of leaching risk categories at the field scale. Bromide concentration profiles, irrigation depths, bulk densities, and soil moisture contents were measured in 40 plots across a 57-ha potato (Solanum tuberosum L.) farm, along with field-scale evapotranspiration estimates, to estimate solute transport parameters. Values for field- and plot-scale means and standard deviations for the NAW distribution were estimated using the stochastic convective lognormal transfer function (CLT) model. The probability of NO 3 - leaching below a depth of 2 m was then estimated using field-averaged versus plot-scale estimates for the mean and standard deviation of the NAW distribution. For 30-cm NAW, NO 3 - leaching risks estimated with the CLT model and plot-scale means and standard deviations were very high in 0.4 ha of the field, high in 1.8 ha, moderate in 8.7 ha, low in 23.0 ha, and none in 23.1 ha. In contrast, when field-scale average estimates of NAW were used, there was a low risk of NO 3 - leaching for the entire field. Thus, when estimating leaching risks using the CLT, information about spatial variability of the NAW distribution is important.