Modeling of DNAPL-Dissolution,Rate-Limited Sorption and Biodegradation Reactions in Groundwater Systems

This article presents an approach for modeling the dissolution process of single component dense non-aqueous phase liquids (DNAPL), such as tetrachloroethene and trichloroethene, in a biologically reactive porous medium. In the proposed approach, the overall transport processes are conceptualized as three distinct reactions. Firstly, the dissolution (or dissolving) process of a residual DNAPL source zone is conceptualized as a mass-transfer limited reaction. Secondly, the contaminants dissolved from the DNAPL source are allowed to partition between sediment and water phases through a rate-limited sorption reaction. Finally, the contaminants in the solid and liquid phases are allowed to degrade by a set of kinetic-limited biological reactions. Although all of these three reaction processes have been researched in the past, little progress has been made towards understanding the combined effects of these processes. This work provides a rigorous mathematical model for describing the coupled effects of these three fundamental reactive transport mechanisms. The model equations are then solved using the general-purpose reactive transport code RT3D (Clement, 1997).     

Modeling of DNAPL-Dissolution, Rate-Limited Sorption and Biodegradation Reactions in Groundwater Systems

This article presents an approach for modeling the dissolution process of single component dense non-aqueous phase liquids (DNAPL), such as tetrachloroethene and trichloroethene, in a biologically reactive porous medium. In the proposed approach, the overall transport processes are conceptualized as three distinct reactions. Firstly, the dissolution (or dissolving) process of a residual DNAPL source zone is conceptualized as a mass-transfer limited reaction. Secondly, the contaminants dissolved from the DNAPL source are allowed to partition between sediment and water phases through a rate-limited sorption reaction. Finally, the contaminants in the solid and liquid phases are allowed to degrade by a set of kinetic-limited biological reactions. Although all of these three reaction processes have been researched in the past, little progress has been made towards understanding the combined effects of these processes. This work provides a rigorous mathematical model for describing the coupled effects of these three fundamental reactive transport mechanisms. The model equations are then solved using the general-purpose reactive transport code RT3D (Clement, 1997).     

Using deterministic models to assess risk in sediment-impacted estuaries

Terrestrial sediments transported off disturbed watersheds can threaten downstream estuarine and coastal environments. Practitioners in the field of environmental management need to be able to assess how activities that disturb the land are likely to impact on downstream estuarine and coastal receiving waters. Such assessments usually take the form of estimating risk, which has two components: the extent of undesirable consequences and the probability of occurrence of those consequences. The consequence in this case is primarily the ecological damage. In order to estimate risk, a methodology was developed around the use of a deterministic computational model of watershed/estuarine sediment transport. Model simulations are run which cover the physically realistic range of the controlling variables (e.g., wind speed, wind direction, sediment runoff). The model results are related to estuary damage and combined with the probability of occurrence for each scenario, providing a means of quantifying risk. In addition, answers to `long-term average', `most likely' and `worse case' type questions can be made. The procedure has the advantages of being easily implemented, transparent and repeatable. It is also more efficient than a standard Monte-Carlo procedure. The risk associated with changes in landuse can be examined without repeating model simulations. The example used to illustrate the method is that of a developing watershed in which proposed building activities are expected to alter the sediment yield to the downstream estuary during floods, which in turn is expected to smother shellfish beds. By elucidating risk associated with each development scenario, the management debate can be focused on choice of an acceptable risk level.     

土壤保持服务:概念、评估与展望

土壤保持服务作为一项重要的生态系统调节服务,是防止区域土地退化、降低洪涝灾害风险的重要保障。针对在全球范围内影响最大的土壤水蚀,基于土壤侵蚀、运移和输出过程,对土壤保持服务的概念、评估方法进行了梳理总结。土壤保持服务是指生态系统防止土壤流失的侵蚀调控能力及对泥沙的储积保持能力。土壤保持服务的评估往往是基于通用土壤流失方程RUSLE(Revised Universal Soil Loss Equation),以潜在土壤侵蚀量(裸地时土壤侵蚀量)与实际土壤侵蚀量之差,作为指标衡量。由于生态过程具有尺度依赖性,对土壤保持服务的有效评估,需要采用多尺度方法。土壤保持服务与人类需求紧密相关,在未来土壤保持服务研究中应强调连接土壤保持服务与人类福祉,对土壤保持服务产生、流动、使用的全过程及土壤保持服务时空动态与人类福祉变化的关系进行探究。     

Opportunistic use of estuarine habitat by juvenile bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, from the Elwha River before,during, and after dam removal

Estuaries are used by anadromous fishes, either as the definitive marine habitat or as transition habitat as they move to fully marine waters, and extent of estuary use may vary with habitat conditions and fish attributes. Bull trout (Salvelinus confluentus) are commonly fluvial or adfluvial, though anadromous populations also exist. However, little is known about estuary use, especially by juveniles of this threatened species. We sampled the estuaries of the Elwha River, where a spawning population exists, and the nearby Salt Creek, where none exists, to reveal seasonal timing of estuarine use by juvenile bull trout, size of those using the estuary, and possible use of the non-natal estuary. We captured juvenile bull trout (all ≥100 mm FL, most <300 mm) in the Elwha River estuary in all months except August, but primarily December through May. None was captured in Salt Creek’s estuary despite comparable sampling effort. We also evaluated how dam removal on the Elwha River influenced bull trout estuarine occupancy by sampling before, during, and after dam removal, because this process enlarged the estuary but also increased turbidity and sediment transport in the lower river. Catches were low before dam removal, increased during and immediately after removal, and returned to low levels in recent years, suggesting that juveniles temporarily sought refuge from conditions associated with dam removal. Our findings indicate juvenile bull trout occupy estuarine habitat opportunistically; this information may aid conservation efforts as anadromous populations occur elsewhere in rivers with estuaries altered by human development.     

TRACE METAL CYCLING IN THE U.S. COASTAL ZONE: A SYNTHESIS

This synthesis of trace element research in estuarine communities of the U.S. coastline and the Caribbean provides a summary applicable to the shoreline of the tropical and temperate regions of the world which have mangroves, kelp beds, riverine marshes and seagrass communities. An inventory of sediments and leaf tissues shows Mn and Fe to be the most highly concentrated elements with Hg and Cd present in lowest concentrations. Generally, trace element concentrations in roots are much higher than in leaves and other tissues above the sediment. Tissue to sediment concentration ratios show that Cd is most likely to be bioamplified and that Cu, Hg, Sr and Zn may have relatively high concentration ratios which can exceed unity. A conceptual model was constructed to integrate the forcing functions, compartmental couplings, and dynamics common to these estuarine systems. Seasonality is important for changes in some trace element concentrations in plants and litter. Trace element additions to water or sediment increased certain trace element concentrations in plants and dead organic matter. It is clear that estuarine plant communities serve as living filters of estuarine trace elements. However, increased knowledge of trace element cycling in estuarine systems and relationships between trace element concentrations in plants and the estuarine food chain is needed, particularly food chains to man. There is a need for structured long-term estuarine research to allow direct comparison of results among estuarine study sites, to identify the similarity of population and system processes among estuaries and to define the geographical scale over which estuarine research results may be generalized.     

Does Channel Incision Affect In‐stream Habitat? Examining the Effects of Multiple Geomorphic Variables on Fish Habitat

Incised river channels are dynamic components of fluvial systems, represent geomorphic degradation, and are encountered worldwide. Ecological effects of incision can be far‐reaching, affecting habitat availability and channel processes. Although incision can reflect habitat degradation, some studies suggest that important in‐stream habitats do not differ with the degree of incision. Therefore, we tested whether in‐stream habitat variables that are important to imperiled fishes differ in river reaches with varying degrees of incision. Because incision (measured using entrenchment ratio) had no discernable effect on in‐stream habitat characteristics (i.e., proportion fines, gravel, cobble, and macrophyte occurrence and length), we expanded our analysis to assess the effects of 29 additional geomorphic variables on in‐stream habitat. These analyses indicated that bank height, bed mobility, D₈₄, cross‐sectional area, bankfull width, and wetted perimeter accounted for 42% of macrophyte occurrence and 64% of macrophyte length variance. Postflood surveys indicated that macrophyte occurrence on cobble declined as bank height and bed mobility increased, and sediment size decreased, suggesting that sediment size and bed mobility have a stronger influence on in‐stream habitat than incision. Although channel incision often indicates environmental degradation, important aspects of habitat are not described by this measurement. Strategies that depend on incision to identify restoration sites may have limited habitat benefits in Southeastern Piedmont streams and rivers. Instead, landscape or shoal‐scale restoration approaches that increase coarse sediment proportions may increase macrophyte occurrence, length, and persistence. Sediment budgets that identify coarse and fine sediment sources and transport may be useful to prioritize restoration approaches.     

Quantifying Burial,the Transport of Matter from the Lake Biosphere to the Geosphere

Burial is one of the most fundamental processes in contexts of massbalance calculations for substances (such as nutrients, organics, metals and radionuclides) in lakes. Substances can leave a lake by two processes, outflow, i.e., the transport to a downstream system, and burial, i.e., the transport by sedimentation from the lake biosphere to the geosphere. This work gives for the first time, to there best of the author's knowledge, a review on the factors and processes regulating burial and presents a general model for burial. This approach accounts for bottom dynamic conditions (i.e., where areas of fine sediment erosion, transport and accumulation prevail), sedimentation, bioturbation, mineralisation, and the depth and age of the bioactive sediment layer. This approach has been critically tested with very good results for radiocesium, radiostrontium, many metals, calcium from liming and phosphorus, but it has not been presented before in a comprehensive way. This model for burial is meant to be used in massbalance models based on ordinary differential equations (i.e., box models) in contexts where burial is not a target y‐variable but a necessary model variable (an x‐variable). This means that there are also specific demands on this approach, e.g., it must be based on readily accessible driving variables so that it is not too difficult to use the model in practice within the context of an overall lake model. The factors influencing burial, e.g., the deposition of materials and the depth of the bioactive sediment layer, are also needed in calculations of sediment concentrations and to determine amounts of substances or pollutants in sediments. To carry out such calculations, one also needs information on sediment bulk density, water content and organic content. This paper also presents new empirical models for such calculations to be used in the new model for burial.     

New developments in solid-state fermentation: II. Rational approaches to the design, operation and scale-up of bioreactors

Over the last decade there has been a significant improvement in understanding how to design, operate and scale-up solid-state fermentation bioreactors. The key to these advances has been the application of mathematical modeling techniques to describe the biological and transport phenomena within the system. This review focuses on the advances in understanding that have come from this modeling work, and the insights it has given us into bioreactor design, operation and scale-up. It also highlights two promising bioreactor designs that have emerged over the last decade or so. For processes in which the substrate bed must remain static throughout the fermentation, the most promising design is the Zymotis design of ORSTOM at Montpellier, France, which involves closely spaced internal heat transfer plates within a packed-bed bioreactor. For those processes in which mixing can be tolerated, the stirred bioreactor developed at INRA, in Dijon, France, has been successfully demonstrated at scales of 1–25 t of substrate. Theoretical work suggests that mathematical models will be useful tools in the scale-up process, however, there are no reports that they have been used in the development of any current large-scale process. Rather, the models have been validated against data obtained from laboratory-scale bioreactors. There is an urgent need to test the accuracy and robustness of the models by applying them within real process development.     

Large wood and fluvial processes

1. Large wood forms an important component of woodland river ecosystems. The relationship between large wood and the physical characteristics of river systems varies greatly with changes in the tree species of the marginal woodland, the climatic and hydrological regime, the fluvial geomorphological setting and the river and woodland management context. 2. Research on large wood and fluvial processes over the last 25 years has focussed on three main themes: the effects of wood on flow hydraulics; on the transfer of mineral and organic sediment; and on the geomorphology of river channels. 3. Analogies between wood and mineral sediment transfer processes (supply, mobility and river characteristics that affect retention) are found useful as a framework for synthesising current knowledge on large wood in rivers. 4. An important property of wood is its size when scaled to the size of the river channel. ′Small′ channels are defined as those whose width is less than the majority of wood pieces (e.g. width < median wood piece length). `Medium' channels have widths greater than the size of most wood pieces (e.g. width < upper quartile wood piece length), and `Large' channels are wider than the length of all of the wood pieces delivered to them. 5. A conceptual framework defined here for evaluating the storage and dynamics of wood in rivers ranks the relative importance of hydrological characteristics (flow regime, sediment transport regime), wood characteristics (piece size, buoyancy, morphological complexity) and geomorphological characteristics (channel width, geomorphological style) in `Small', `Medium' and `Large' rivers. 6. Wood pieces are large in comparison with river size in `small' rivers, therefore they tend to remain close to where they are delivered to the river and provide important structures in the stream, controlling rather than responding to the hydrological and sediment transfer characteristics of the river. 7. For `Medium' rivers, the combination of wood length and form becomes critical to the stability of wood within the channel. Wood accumulations form as a result of smaller or more mobile wood pieces accumulating behind key pieces. Wood transport is governed mainly by the flow regime and the buoyancy of the wood. Even quite large wood pieces may require partial burial to give them stability, so enhancing the importance of the sediment transport regime. 8. Wood dynamics in `Large' rivers vary with the geometry of the channel (slope and channel pattern), which controls the delivery, mobility and breakage of wood, and also the characteristics of the riparian zone, from where the greatest volume of wood is introduced. Wood retention depends on the channel pattern and the distribution of flow velocity. A large amount is stored at the channel margins. The greater the contact between the active channel and the forested floodplain and islands, the greater the quantity of wood that is stored.     

Development of a Distributed Watershed Contaminant Transport,Transformation, and Fate (CTT&F) Sub-model

CTT&F is a physically based, spatially distributed watershed contaminant transport, transformation, and fate sub-model designed for use within existing hydrological modeling systems. To describe the fate of contaminants through landscape media as well as spatial variations of contaminant distributions, physical transport and transformation processes in CTT&F are simulated for each cell in the model and routed to the watershed outlet. CTT&F simulates contaminant erosion from soil and transport across the land surface by overland flow. The model also simulates contaminant erosion from stream bed sediment and transport through channels in addition to transport of contaminants inputs by overland flow. CTT&F can simulate solid (granular) contaminant transport and transformation, including partitioning between freely dissolved, dissolved organic carbon (DOC) bound, and particle-sorbed phases. To demonstrate model capabilities, CTT&F was coupled with an existing distributed hydrologic model and was tested and validated to simulate RDX and TNT transport using two experimental plots. These experiments examined dissolution of solid contaminants into the dissolved phase and their subsequent transport to the plot outlet. Model results were in close agreement with measured data. Such a model provides information for decision makers to make rational decisions relevant to the fate of toxic compounds.     

Entrainment,deposition, and transport of fine-grained sediments in lakes

Recent work on the settling, diffusion, entrainment, and deposition of fine-grained sediments in fresh water is reviewed and synthesized. Particular attention is given to the dependence of these processes on sediment properties such as particle size. The application of this knowledge to the analysis and numerical modeling of sediment transport is also discussed. Much of the work is concerned with the Great Lakes and, more specifically, with the Western Basin of Lake Erie.     

A cohesive sediment balance for the Scheldt estuary

This article present a method to generate a cohesive sediment balance for the Scheldt estuary (The Netherlands and Belgium). In this balance the net transports have been determined on a regional scale. The complicated circulation patterns on a smaller scale are considered as a black box. The cohesive sediments in the balance are defined as the inorganic sediment with a particle size smaller than 63 μm. In the balance a distinction is made between the relatively ‘clean’ marine mud fraction and the more polluted fluvial fraction. The calculations are made on a basis of field data. The field data of the morphological changes in the estuary are based on the period of 1975–1985. The net natural transport of fluvial mud is estimated to reach beyond the mouth of the estuary. The net marine import of mud is estimated to reach beyond the brackish part of the estuary, located in Belgium. The sediment balance also provides the time scale for the response of an estuary. The time scale related to the reduction in pollution proves to be quite large. Calculations with a simple analytic model with typical values for the Scheldt estuary indicate that several decades will pass until a significant improvement in bottom sediment quality is reached even after a 100% reduction of the loading of pollutants. The presented balance is not the final answer. Improvements can be achieved by more accurate field data, smaller morphological units and verification by other methods (cf. tracer methods, mathematical models).     

考古动物群中的偏移现象——埋藏学的视角

考古动物群是由古代生物群经过死亡群、埋藏群、化石群的逐次"过滤"而形成的一个"采集群"[1]。在这一漫长而复杂的地质历史过程中,不同来源、不同规模、不同作用方式的多种埋藏学因素都在其中打上了自己的深刻"烙印",并在各自的层面上改变了考古动物群的最终面貌,从而导致了化石记录与原始生物群之间的客观偏移。因此,在对考古遗址发掘出土的大量动物化石进行合理解释之前,有必要了解和研究那些可能影响考古动物群最终组成的埋藏学过程或动因。本文以东亚地区的几项埋藏学研究为例,重点阐释了一些可能导致考古动物群偏移的埋藏学过程。     

Considerations in modeling the sediment-water exchange of phosphorus

The potential to release accumulated phosphorus from sediments has been the major motive to study and to model the fate of this nutrient in sediments. For the dynamics of the sediment-water interaction the sizes of the pools involved and the rates of conversion/transport from one pool to another are of primary interest. As the sediment pools for phosphate are generally much larger than the pools in the water column, a rather slow adjustment of the sediment to management measures will occur. For the analysis of management measures it is obvious that the gradual change in sediment composition must be taken into account. Only for rather short periods the sediment composition can be assumed to be constant; this may be appropriate for studies of e.g. the annual cycle. The sediment-water interaction is a complex resultant of physical, chemical and biological processes, including:

physical processes: advection due to seepage or consolidation, pore-water diffusion, transport and mixing of solids by resuspension, sedimentation and bioturbation.

chemical processes: adsorption and desorption, dissolution and (co)precipitation, inclusion.

biological processes: mineralization of a wide range of organic compounds by various (micro)organisms, each with their own nutrient requirements and electron acceptors.

Aspects which are discussed and need to be considered in application of a model in research or management are the level of aggregation and detail that is required and may still be practical, the spatial and temporal scales which are applicable for the processes mentioned and their influence upon the numerical dispersion and model stability, the availability of data for calibration/validation and the resolution of the analytical techniques. These aspects are not independent however. Frequently models are not functional because they contain details which are either unnecessary or suggest a feigned accuracy which is not justified by analytical and experimental resolution of system characteristics.     

Modeling nitrogen cycling in a coastal fresh water sediment

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (N_org) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the N_org deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 μmol N cm⁻² yr⁻¹. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO ₃ ⁻ reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 μmol N cm⁻² yr⁻¹ if DNRA is the dominant pathway. The response to salinization includes a short-term release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO ₃ ⁻ concentrations and greater SO ₄ ²⁻ concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition.     

Stabilization of carbon in mineral soils from mangroves of the Sinú river delta,Colombia

Mangrove forests of the Sinú river delta in Cispatá bay, Colombia, show large differences in soil carbon storage between fringe (oceanic) and basin (estuarine) mangroves. We were interested in testing whether these differences in soil carbon are associated with sediment transport processes or whether most of the carbon is produced in situ within the mangrove system. Given past sedimentation dynamics of the Sinú river, we hypothesized that a large portion of soil carbon in basin mangroves is due to sedimentation. We determined total organic carbon content (TOC) as 660.93 ± 259.18 MgC ha^?1 for basin soils up to a sampling depth of 1 m, and as 259 ± 42.61 MgC ha^?1 for fringe soils up to 80 cm depth (maximum soil depth for fringe soils). Using analyses of mineralogy (Al- and Fe-oxides, clay minerals) as well as isotopic analyses of carbon (δ¹³C), the origin of the sediments and their carbon was determined. We found that basin soils in Cispatá bay show similar mineralogical composition than those of fluvial sediments, but the carbon concentration of river sediments was close to zero. Given the large capacity of the Fe and Al oxides in clay minerals to store dissolved carbon, and that the isotopic composition of the carbon is mostly of plant origin, we concluded contrary to our initial hypothesis that the carbon stored in basin mangrove soils are produced in situ. The deposited fluvial sediments do play an important role for carbon storage, but mostly in providing binding surfaces for the stabilization of organic carbon.     

GIS-based distributed model for simulating runoff and sediment load in the Malian River Basin

GIS-based modeling is an effective approach for reflecting spatially-varied complexities in watershed systems. In this study, a GIS-aided distributive hydrological model was developed to simulate runoff and sediment transport in a loess-plateau context with semi-arid climate, sparse vegetation, and serious soil erosion. The model was then applied to a case study in the Malian River Basin, which is one of the largest catchments in the middle reach of the Yellow River. Based on the GIS technique, the DEM of the study basin was successfully used to delineate the stream network and extract information of catchment characteristics. The results show satisfactory accuracy for runoff simulation. Especially, the integrated soil-erosion and hydrological models can be used for simulating both runoff and sediment loads, which provide a useful decision-support tool for water resources management and pollution control. This study is an attempt to develop a distributed rainfall-runoff model for river basins in the loess plateau region. The developed model can also be extended to larger basins. Further works of field survey and investigation would be helpful for better calibrating critical parameters and thus improving performance of the developed model.     

3D-numerical modelling of cohesive suspended sediment in the Western Scheldt estuary (The Netherlands)

A cohesive sediment transport model considering the effects of flocculation, deposition and erosion is used in an attempt to simulate the suspended sediment distribution in a mesotidal estuary. The numerical model solves the three-dimensional (3D) advection-diffusion equation using a two-time level scheme, and a semi-implicit finite difference approach. The transport model is coupled to a 3D-barotropic hydrodynamic model for the simulation of the major tidal components reproducing the non-linear effects. An application of these models in the Western Scheldt estuary is described. The results of the different tests show that the adopted approach provides a useful basis for a good understanding of the physical processes involved in sediment transport and for the study of practical problems. The sensitivity of the model to key parameters controlling the simulation of bed sediment/water exchanges, shows the importance of a good definition of bottom sediment characteristics and the importance of further development of a consolidation algorithm.