Vertical Solute Transport Research Articles

Biotic interactions between benthic infauna and aerobic methanotrophs mediate methane fluxes from coastal sediments.

Coastal ecosystems dominate oceanic methane (CH4) emissions. However, there is limited knowledge about how biotic interactions between infauna and aerobic methanotrophs (i.e. CH4 oxidizing bacteria) drive the spatial-temporal dynamics of these emissions. Here, we investigated the role of meio- and macrofauna in mediating CH4 sediment-water fluxes and aerobic methanotrophic activity that can oxidize significant portions of CH4. We show that macrofauna increases CH4 fluxes by enhancing vertical solute transport through bioturbation, but this effect is somewhat offset by high meiofauna abundance. The increase in CH4 flux reduces CH4 pore-water availability, resulting in lower abundance and activity of aerobic methanotrophs, an effect that counterbalances the potential stimulation of these bacteria by higher oxygen flux to the sediment via bioturbation. These findings indicate that a larger than previously thought portion of CH4 emissions from coastal ecosystems is due to faunal activity and multiple complex interactions with methanotrophs.

Modeling the vertical distribution of soil organic carbon in temperate forest soils on the basis of solute transport

Temperate forests are of pivotal importance in global carbon cycle, as they currently act as a carbon sink. Moreover, the continued expansion of the forest provides significant benefits in terms of mitigating climate change. Soil organic carbon (SOC) constitutes a vital component of the carbon inventory harbored within forest soils. Thus, understanding the dynamics and distribution of SOC in temperate forest soils can be essential to better predict the forest SOC inventories, and can help to provide theoretical basis for further studies in soil carbon management technologies. Spatial variability of SOC has been studied extensively, but the mechanism that regulates the vertical pattern of SOC is still not clear. In the present study, we hypothesized that the vertical pattern of SOC in temperate forest soils is dominated by the vertical transport of solute in soil, and a theoretical vertical scaling of SOC was proposed based on percolation theory. Theoretical range of SOC in the national forests in northeastern China and the United States were also predicted. The agreement between the observed SOC profiles and the theoretical scaling supported the hypothesis and suggested that percolation theory can be applied to depict the vertical distribution of SOC, while the application could be limited if vegetation cover and soil texture alter the dominant controlling factor of SOC distribution. The concordance between empirical data and the predicted range also showed significant potential of integrating percolation theory into comprehensive models for carbon stock estimation.

Cycles‐L: A Coupled, 3‐D, Land Surface, Hydrologic, and Agroecosystem Landscape Model

AbstractManaging landscapes to increase agricultural productivity and environmental stewardship can be informed by spatially‐distributed models that operate at spatial and temporal scales that are intervention‐relevant. This paper presents Cycles‐L, a landscape‐scale agroecosystem and hydrologic modeling system, using as a test case a watershed in Pennsylvania. Cycles‐L emerges from melding the landscape and hydrology structure of Flux‐PIHM, a 3‐D land surface hydrologic model, with the agroecosystem processes in the Cycles model. Consequently, Cycles‐L can simulate processes affected by topography, soil heterogeneity, and management practices, owing to its physically‐based hydrology that can simulate horizontal and vertical transport of solutes with water. The model was tested at a 730‐ha experimental watershed within the Mahantango Creek watershed. Cycles‐L simulated well stream water and mineral nitrogen discharge (Nash‐Sutcliffe coefficient 0.55 and 0.60, respectively) and grain yield (root mean square error 1.2 Mg ha−1). Cycles‐L outputs are as good or better than those obtained with the uncoupled Flux‐PIHM (water discharge) and Cycles (grain yield) models. Modeled spatial patterns of nitrogen fluxes like denitrification illustrate the combined control of crop management and topography. For example, denitrification is almost twice as high when simulated with Cycles‐L than when simulated with Cycles 1‐D. Due to its spatial and temporal resolution, Cycles‐L fills a gap in the availability of models that operate at a scale relevant to evaluate interventions in the landscape. Cycles‐L can become a central component in tools for climate change scenario analysis, precision agriculture, precision conservation, and artificial intelligence‐based decision support systems.

Transport of Tracers and Pesticides Through Fractured Clayey Till: Large Undisturbed Column Experiments and Model‐Based Interpretation

AbstractLeaching of contaminants through fractured aquitards such as clayey tills may occur due to typically slow matrix advection and diffusion, or it can be dramatically enhanced in the presence of preferential flow through fractures and macropores. Sorption also plays a crucial role since it can significantly impact contaminant leaching and residence times. These different mass‐transfer mechanisms imply a distinct transport behavior and very different time scales for contaminant leaching toward underlying aquifer systems. However, the prevalent controls on contaminant transport and their effects are difficult to assess. This paper shows a detailed characterization of flow and transport processes under water‐saturated conditions in two large undisturbed columns (LUCs) collected from two visually similar fractured/macroporous clayey tills typical for the Northern hemisphere. Flow‐through tracer and pesticide experiments revealed a contrasting transport behavior. In one column, transport through fractures/macropores was dominant, whereas matrix advection and diffusion had a distinct influence on solute transport in the other column. Detailed 3D discrete‐fracture‐matrix models were developed to illuminate prevalent controls on contaminant transport and to quantitatively interpret the flow‐through experiments with a minimal number of fitting parameters. Nonequilibrium sorption kinetics were included to reproduce the transport behavior of the considered pesticide. The parameters determined from the two LUC experiments were integrated in a vertical cross‐section model to investigate the influence of varying fracture properties on vertical solute transport through surficial clayey till aquitards. The analysis showed that small fracture apertures in deeper parts of the aquitard could substantially prolong solute migration times and control solute fluxes.

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.

Depth-dependent groundwater response to coastal hydrodynamics in the tropical, Ganges river mega-delta front (the Sundarbans): Impact of hydraulic connectivity on drinking water vulnerability

With increasing population and urbanization in the deltas across global mega-river basins, groundwater stress is increasing due to changing ocean-land disposition, hydroclimatic extremes, over-exploitation and natural/anthropogenic contamination. Such change in coastal hydrodynamic process makes an impact in to the resilience of groundwater-sourced drinking water vulnerability. The groundwater responses studied in this work in the ecologically-vulnerable, UNESCO World Heritage site of the Sundarbans delta-front aquifers at the river Ganges-Brahmaputra mega delta, demonstrate a unique example of such intricacies and complexities. High-resolution, temporal, depth-dependent hydraulic connectivity and hydrochemistry was studied for this complex, climate-driven, tidal-induced hydrological interactions between surface water (seawater of Bay of Bengal [BoB] and Ganges river water) with shallow and deep groundwater by stable isotopic (δ18O and δD) signatures and piezometry. A wide range of salinity content and δ18O composition of groundwater samples for 14–25 (salinity: 3–45 ppT, δ18O: −3.63 to −1.15‰) and 30–50 meter below ground level (mbgl) (salinity: 5–25 ppT, δ18O: −3.41 to −1.34‰) was recorded, while, comparatively, a narrow range of temporal variability was observed for the depths of 115 (salinity: 1–4 ppT, δ18O: −5.04 to −2.15‰) and 333mbgl (salinity: 1 to 3 ppT, δ18O: −4.43 to −2.81‰). Study results revealed that vertical transport of solute has been induced by coastal hydrodynamics. However, human interferences, in terms of groundwater abstraction and land use-land cover change (e.g. brackish aquaculture) do have substantial impact on evolving groundwater response, thereby impacting resilience to drinking water option in this densely populated area. Fluctuations of groundwater level in relation with δ18Ogw composition showed shallow depths (14–50 mbgl) are hydraulically connected by local flow and conducive of chemical exchanges. The deeper groundwater (115–330 mbgl depths) flow suggests regional-scale hydraulic connections along flow paths. In-spite of sub-meter scale seasonal groundwater level changes, salinity variability is observed between 1 and 4 ppT at >100 mbgl depths, suggest mixing between chemically-distinct water reservoirs, conserving hydraulic mass-balance. However, such observation suggests severe implications for groundwater vulnerability, which might aggravate with future intensification of irrigational abstraction, as well as with changing hydroclimatological regime and extreme events. Thus, future planning and management strategies of sustainable supply of safe water in such ecologically-sensitive groundwater systems need to incorporate such dynamic chemical evolution.

Modeling and uncertainty analysis of seawater intrusion in coastal aquifers using a surrogate model: a case study in Longkou, China

Seawater intrusion is a complex problem involving groundwater with variable density. It has an important impact on the development and utilization of groundwater resources in coastal areas. In previous studies, numerical simulations of seawater intrusion were performed by assigning fixed values to model parameters, thereby not taking into account the influence of stochastic variability of hydrogeological parameters on model predictions. In this study, we developed a three-dimensional mathematical model of seawater intrusion by modeling vertical solute transport due to density changes resulting from seawater intrusion. We then applied a Monte Carlo–based method to analyze uncertainty in model simulations of seawater intrusion. Because the Monte Carlo method requires the simulation model to be repeatedly run, we used the Kriging method to build a surrogate model that significantly reduced the computational load compared to the simulation model while ensuring the desired accuracy. Then we used a local sensitivity analysis method to select the two parameters with the greatest influence on model output. We treated the two selected parameters as random variables, and results show that (1) the three-dimensional, variable-density seawater intrusion model can effectively simulate and predict the distribution of seawater intrusion in the study area; (2) the local sensitivity analysis can accurately identify the hydrogeological parameters that most influence model output; (3) the uncertainty analysis based on the surrogate model reduces computing time substantially and provides a realistic assessment of the effect of hydrogeological parameter variability on seawater intrusion numerical simulation results.

A numerical model for the simulation of snowpack solute dynamics to capture runoff ionic pulses during snowmelt: The PULSE model

Early ionic pulse during spring snowmelt can account for a significant portion of the total annual nutrient load in seasonally snow-covered areas. Ionic pulses are a consequence of snow grain core to surface ion segregation during metamorphism, a process commonly referred to as ion exclusion. While numerous studies have provided quantitative measurements of this phenomenon, very few process-based mathematical models have been proposed for diagnostic and prognostic investigations. A few early modelling attempts have been successful in capturing this process assuming transport through porous media with variable porosity. However, this process is represented in models in ways that misalign with the mechanistic view of the process described in the literature. In this research, a process-based model is proposed that can simulated ionic pulses in runoff by emulating solute leaching from snow grains during melt and the subsequent vertical solute transport by meltwater through the snowpack. To facilitate its use without the need for snow-physics’ models, simplified alternative methods are proposed to estimate some of the variables required by the model. The model was applied to two regions, and a total of 4 study sites, that are subject to significantly different winter climatic and hydrological conditions. Comparison between observations and simulation results suggest that the model can capture well the overall snow melt runoff concentration pattern, including both the timing and magnitude of the early melt ionic pulse. The model enables the prediction of concentration profiles of the dry (snow) and liquid (wet) fractions within the snow matrix for the first time. Although there is a computational cost associated with the proposed modelling framework, this study demonstrates that it can provide more detailed information about the reallocation and transport of ions through snowpacks, which can ultimately be used to improve nutrient transport predictions during snowmelt.

Climatic Drivers for Multi-Decadal Shifts in Solute Transport and Methane Production Zones within a Large Peat Basin.

Northern peatlands are an important source for greenhouse gases but their capacity to produce methane remains uncertain under changing climatic conditions. We therefore analyzed a 43-year time series of pore-water chemistry to determine if long-term shifts in precipitation altered the vertical transport of solutes within a large peat basin in northern Minnesota. These data suggest that rates of methane production can be finely tuned to multi-decadal shifts in precipitation that drive the vertical penetration of labile carbon substrates within the Glacial Lake Agassiz Peatlands. Tritium and cation profiles demonstrate that only the upper meter of these peat deposits was flushed by downwardly moving recharge from 1965 through 1983 during a Transitional Dry-to-Moist Period. However, a shift to a moister climate after 1984 drove surface waters much deeper, largely flushing the pore waters of all bogs and fens to depths of 2 m. Labile carbon compounds were transported downward from the rhizosphere to the basal peat at this time producing a substantial enrichment of methane in Δ14C with respect to the solid-phase peat from 1991 to 2008. These data indicate that labile carbon substrates can fuel deep production zones of methanogenesis that more than doubled in thickness across this large peat basin after 1984. Moreover, the entire peat profile apparently has the capacity to produce methane from labile carbon substrates depending on climate-driven modes of solute transport. Future changes in precipitation may therefore play a central role in determining the source strength of peatlands in the global methane cycle.

Temporal moment analysis for stochastic-advective vertical solute transport in heterogeneous unsaturated soils

Summary Temporal moment analysis of solute transport in unsaturated soils subjected to rainfall events is typically achieved by the numerical solution of the flow field from the Richards equation followed by a numerical solution of the advection–dispersion equation before computing moments. These numerical solutions are computationally very intensive, and may not provide the insights that are possible from simpler analytical representations. In this study, temporal moments of solute transport for unsteady unsaturated flows under rainfall conditions at the soil surface are presented for the first time. A local-scale model for water movement is derived from a sharp front approximation and is combined with a model for transport of solute particles along the main characteristics of the flow field. Expressions for travel times from the local-scale model are first presented for pre- and post-ponding conditions. These local solutions are upscaled to field-scale solute transport by adopting a log-normally distributed spatial hydraulic conductivity field. Semi-analytical expressions are developed for temporal moments of travel times. These expressions are compared to 1-D Monte Carlo simulation results, and to 3-D numerical results for model corroboration. The model is used to investigate the behavior of macroscopic Eulerian effective velocities and dispersion at the field-scale. Expressions for asymptotic effective properties show that effective velocity achieves a constant value while effective dispersion increases linearly with depth. The roles of pre-ponding and post-ponding conditions in determining field-scale dispersion are described.

Local‐ and field‐scale stochastic‐advective vertical solute transport in horizontally heterogeneous unsaturated soils

AbstractDescription of field‐scale solute transport in unsaturated soils is essential for assessing the degree of contamination, estimating fluxes past a control plane and for designing remedial measures. The flow field is usually described by numerical solution of the Richards equation followed by numerical solution of the advection‐dispersion equation to describe contaminant movement. These numerical solutions are highly complex, and do not provide the insights that are possible from simpler analytical representations. In this study, analytical solutions at the local scale are developed to describe purely advective vertical transport of a conservative solute along the principle characteristic of the flow field. Local‐scale model development is simplified by using a sharp‐front approximation for water movement. These local solutions are then upscaled to field‐scale solute transport by adopting a lognormally distributed horizontal hydraulic conductivity field to represent the natural heterogeneity observed in field soils. Analytical expressions are developed for the mean behavior of solute transport at the field scale. Comparisons with experimental observations find that trends of field‐scale solute behavior are reasonably reproduced by the model. The accuracy of the proposed solution improves with increasing spatial variability in the hydraulic conductivity as revealed by further comparisons with numerical results of the Richards equation‐based field‐scale solute movement. In some cases, the sharp‐front approximation may lead to anomalous field‐scale behavior depending on the role of pre and postponded conditions in the field, and this limitation is discussed. The proposed method shows promise for describing field‐scale solute movement in loamy sand and sandy loam soils.

Reactive transport controls on sandy acid sulfate soils and impacts on shallow groundwater quality

AbstractDisturbance or drainage of potential acid sulfate soils (PASS) can result in the release of acidity and degradation of infrastructure, water resources, and the environment. Soil processes affecting shallow groundwater quality have been investigated using a numerical code that integrates (bio)geochemical processes with water, solute, and gas transport. The patterns of severe and persistent acidification (pH < 4) in the sandy, carbonate‐depleted podzols of a coastal plain could be reproduced without calibration, based on oxidation of microcrystalline pyrite after groundwater level decrease and/or residual groundwater acidity, due to slow vertical solute transport rates. The rate of acidification was limited by gas phase diffusion of oxygen and hence was sensitive to soil water retention properties and in some cases also to oxygen consumption by organic matter mineralization. Despite diffusion limitation, the rate of oxidation in sandy soils was rapid once pyrite‐bearing horizons were exposed, even to a depth of 7.5 m. Groundwater level movement was thus identified as an important control on acidification, as well as the initial pyrite content. Increase in the rate of Fe(II) oxidation lead to slightly lower pH and greater accumulation of Fe(III) phases, but had little effect on the overall amount of pyrite oxidized. Aluminosilicate (kaolinite) dissolution had a small pH‐buffering effect but lead to the release of Al and associated acidity. Simulated dewatering scenarios highlighted the potential of the model for risk assessment of (bio)geochemical impacts on soil and groundwater over a range of temporal and spatial scales.

Effective Vertical Solute Transport in Soils by Artificial Macropore System

AbstractSolute transport is an important factor governing soil environmental processes, such as effective fertilizer application or dispersion areas of remediation chemicals for contaminated soils. Macropores are ubiquitous in soils. In unsaturated conditions, they enhance air intrusion into soils, thus reducing the chances of clogging and surface ponding. However, their structure is hard to maintain, and they tend to collapse during long-term infiltration. In this experiment, macropore fillings were introduced into the pores to maintain their structure. Solute transport experiments were conducted for four soils with no macropores, empty macropores, macropores with paper towel fillings, and macropores with glass fiber fillings. The macropores with fillings worked as water pathways that conducted solutions to the deeper profile without saturation at the surface, thus avoiding clogging. When bio-remediation experiments were conducted using these four soil columns, soil columns with glass fiber fillings main...

Spatial variations in the salinity of pore waters in northern deep water Gulf of Mexico sediments: implications for pathways and mechanisms of solute transport

Geofluids (2010) 10, 83–93AbstractSpatial variations in the salinity of pore waters in sedimentary basins can provide important insight into basin‐scale hydrogeologic processes. Although there have been numerous studies of brine seeps in the deep water Gulf of Mexico, much less is known about porewater salinities in the vast areas between seeps. A study has been made of spatial variation in pore water salinities in sediments in an approximately 500 km by 200 km area of the northern deep water (water depth >500 m) Gulf of Mexico sedimentary basin (GOM) to provide insight into pathways and mechanisms of solute transport in this portion of the basin. A second objective was to document salinities in the upper 500 m of the sedimentary section, the approximate depth to which methane hydrates, a potential future energy resource, may be stable. Elevated salinities would reduce the P–T stability range of hydrates. Salinities were calculated from borehole logs using a dual electrical conductivity model. Even though much of the northern GOM is underlain by allochthonous salt most of the undisturbed shallow sedimentary section has not been permeated by hypersaline waters. These waters are limited to areas near brine seeps. Hypersaline waters having salinities in excess of 100 g l−1 become more common at subseafloor depths of 2 km and greater. A field study at Green Canyon 65 and published numerical simulations of fluid flow above tabular salt bodies suggest that brines produced by salt dissolution migrate laterally and pond above salt and/or within minibasins and that the dominant mechanism of vertical solute transport is a combination of compaction‐driven advection and diffusion, not large‐scale thermohaline overturn. Superimposed on this diffuse upward flux of dissolved salt is the more focused and localized expulsion of saline fluids up faults.

Scaling analyses of high-resolution dye tracer experiments / Analyses scalaires d'expériences de traçage coloré à haute résolution

Four unsaturated solute transport experiments with different water fluxes were conducted in a Hele-Shaw cell filled with uniform sand. The transport of the dye tracer used was recorded with a camera and the dye concentration was calculated using image analysis. The concentrations fields were analysed in terms of time moments and converted into vertical solute transport velocity V. Both mean value and standard deviation of V increased with water flux. The autocorrelation function exhibited a linear decrease for short lags. The pronounced variability of V suggested a description in terms of scaling properties, and a scaling regime was indeed found from the resolution 1.8 mm up to almost 0.1 m. The upper limit corresponds roughly to a characteristic scale of fingering structures seen in the dye concentration images. Indications of a second scaling regime at larger scales were found. In the small-scale scaling regime, the power spectrum exponent β was generally slightly below 1 and the intermittency parameter C 1 was on average 0.00025. The moment scaling K q functions were convex, implying a multiscaling process.

Field characterization of vertical bromide transport in a fractured glacial till

A study of the fracture distribution, hydraulic properties, groundwater levels and the transport of bromide was conducted to characterize vertical transport in the oxidized and reduced zones of a fractured glacial till. Detailed vertical profiles of groundwater levels and solute concentrations were obtained over a 4.5-year period. Vertical migration occurred at several time scales, as a low concentration front was rapidly transported at rates of 100–500 m/year ahead of a slower moving main plume, which advanced at rates of 0.2–0.8 m/year. Concentrations in the leading edge of the plume displayed a high degree of spatial variability over short vertical distances through day 1,000. Late in the test, the influence of matrix diffusion became apparent as concentration patterns developed from being irregular to more uniform distributions. Calculations show that the mass within the low concentration plume front accounts for less than 1% of the total solute mass. Simulation of the breakthrough curves using a simple one-dimensional advection-dispersion model of transport in porous media indicates that vertical transport is dominated by advection. Furthermore, the results indicate that vertical transport of solutes in oxidized and reduced zones of the till can be adequately simulated using an equivalent porous media.

Evaluating hypolimnetic diffusion parameters in thermally stratified lakes

The vertical transport of solutes in the anoxic hypolimnion of thermally stratified lakes can be described by a one‐dimensional diffusion model. Using ammonium and dissolved methane as inert tracers, the model was solved, both analytically and numerically, for Lake Kinneret while taking into account sedimentary release, hypolimnetic transport, and the deepening of the thermocline depth. An average seasonal turbulent diffusion coefficient in the hypolymnion of 1.5 ± 0.5 m2 d−1 was confirmed independently by both tracers. In all cases, the modeled hypolimnetic distribution pattern was highly correlated with the actual measurements.

Solidification and compositional convection of a ternary alloy

We present the results of an experimental study on the solidification of aqueous solutions of potassium nitrate and sodium nitrate cooled from below. Upon cooling, two distinct mushy layers form, primary and cotectic, separated by an approximately planar horizontal interface. A density reversal between the two mushes causes the residual liquid in the upper, primary mush to be more buoyant than the melt overlying it, while the cotectic mush is compositionally stable. The unstable concentration gradient between the melt and primary mush causes convection that keeps the melt well-mixed and reduces the concentration gradient to zero after a finite time. At this point, the cotectic mush overtakes the primary mush and a transition from a convective regime to a diffusive regime occurs. Our measurements show that this transition is rapid and alters the growth rate of the single (cotectic) mush layer that remains. Concentration measurements taken from within the melt during convection and from within the mush during the diffusive regime show good agreement with the concentration evolution predicted by use of the equilibrium ternary phase diagram. We describe a global conservation model for solidification of a ternary alloy in this regime. Predictions from our model forced with empirical data for the heat and solute fluxes are in good agreement with the measured data for the interface positions of the two mushy layers. We also discuss how solid fractions vary with different melt concentrations in a non-convecting alloy and examine the influence of vertical solute transport in the convecting case. The identification of a density reversal in the solidification of a ternary alloy begins to address the complexities in solidification processes of multi-component alloys.

Spatial variability in Sediment Oxygen Consumption under winter conditions in a lagoonal system in New Caledonia (South Pacific)

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere) cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling stations distributed along two coast to reef transects. Three different methods of flux measurements were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen microprofiles determinations. SOC values varied between 450 and 2250 μmol O 2 m −2 h −1. The level of agreement between the three techniques strongly varied as a function of sediment type. Most of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms of the bays did not show significant differences. A central station presenting a dense seagrass bed gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher than SOC measured by incubation techniques. This discrepancy could be explained by physical disturbance of the sediments, macroscale variability in benthic communities or technical efficiency of the sediment sampling device and probably by a combination of all three processes. Nevertheless, for the other sediment types that represented 85% of the lagoon bottoms, the results from the three techniques used for SOC determination were strongly convergent. Based on this assumption, it could be stated that the oxygen fluxes were essentially driven by microbial activity compared to biologically mediated vertical transport of solutes. The SOC values determined during this study were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef area, spatial variability in SOC can be explained by the organic matter content of sediments which clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter near the coast compared to more central stations in the lagoon.

Heuristic numerical and analytical models of the hydrologic controls over vertical solute transport in a domed peat bog, Jura Mountains, Switzerland

AbstractWe report the results of numerical and analytical simulations to test the hypothesis that downward vertical flow of porewater from the crests of domed alpine and kettle bogs controls vertical porewater distributions of major solutes such as Ca and Mg. The domed Etang de la Gruère bog (EGr), Switzerland, characterized by a vertical downward gradient of 0·04 and stratified layers of peat, is chosen as a field site for the model calibration and evaluation. The middle 4‐m section of the 6·5 m thick bog peat is heavily humified and has a hydraulic conductivity of ∼10−5·6 cm s−1. Above and below, peat is less humified with a hydraulic conductivity of ∼10−3 cm s−1. Heuristic finite difference simulations, using Visual MODFLOW, of the bog hydraulics show that the higher conductivity peat at the bog base is critical to create the observed deep, local flow cells that substantively recharge porewater.Model results and Peclet number calculations show that before ∼7000 14C yr BP diffusion of solutes from underlying mineral soils controlled the vertical distribution of porewater chemistry. From 7000 to ∼1250 14C BP the porewater chemistry was probably controlled by both upward diffusion and downward advection, and after ∼1250 14C yr BP porewater chemistry was probably controlled by downward advection. Concentrations of conservative major solutes in the porewaters of alpine, ombrotrophic bogs are the net effect of both downward vertical porewater movement and upward vertical diffusion, the magnitudes of which are delicately poised to the configuration of the bog water table over time and subsurface peat stratigraphy. Copyright © 2002 John Wiley & Sons, Ltd.