bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the removal of metals from groundwater by a reactive barrier: Experimental results
Authors: Fryar Alan E. , Schwartz Franklin W.
Published: February 2004

The development of reactive barrier systems to remove metals from groundwater requires an improved understanding of the behavior of reaction fronts and the coupling between fluid flow, solute transport, reaction, and hydraulic changes. We report the results of experiments in which a dilute Fe(ClO4)3 solution was pumped through columns packed with mixtures of quartz sand and crushed calcite. The deposition of ferric oxyhydroxide peaked at, but was not restricted to, the visible precipitate front, where the neutralization of acidity generated by hydrolysis was completed. Even beyond this reaction front, calcite dissolution and ferric oxyhydroxide precipitation continued gradually. Upgradient from the front, where all calcite had been removed from the porous medium, ferric oxyhydroxide precipitation continued because of nucleation around older precipitate at a pH less than the influent pH. The velocity of the reaction front varied with the ratio of the initial volume of calcite to the pore water velocity, whereas the width of the zone of pronounced calcite dissolution varied as a function of the Damkohler number. These experiments illustrate how a model barrier can effectively and rapidly remove ferric iron from solution without fouling of the reactive surfaces.
Multiscaling theory of flood peaks: Regional quantile analysis
Authors: Gupta Vijay K. , Mesa Oscar J. , Dawdy David R.
Published: February 2004

We study the spatial random field of peak flows indexed by a channel network. Invariance of the probability distributions of peak flows under translation on this indexing set defines statistical homogeneity in a network. It implies that floods can be indexed by the network magnitude, or equivalently the drainage area, which serves as a scale parameter. This definition generalizes to homogeneity of flows in a geographical region containing several networks which are not necessarily the subnetworks of a single network. The widely used quantile regression method of the United States Geological Survey (USGS) provides one simple criterion to approximately designate homogeneous geographic regions. It is argued that the redefinition of homogeneity via the constancy of the coefficient of variation (CV) of floods implied by the index flood assumption is ad hoc. Invariance of the probability distributions of peak flows under scale change is used to develop the simple scaling and the multiscaling theories of regional floods in terms of their quantiles. The simple scaling theory predicts a constant CV and log‐log linearity between flood quantiles and drainage areas such that the slopes in these equations do not change with the probability of exceedance. Multiscaling theory of floods is developed to exhibit differences in floods between small and large basins. This theory shows that the CV for small basins increases, and for large basins it decreases, as area increases. Moreover, the quantiles do not obey log‐log linearity with respect to drainage areas. However, for large basins an approximate log‐log linearity between quantiles and drainage areas is shown to hold. The slopes in these equations decrease as p decreases; i.e., larger floods have smaller slopes than smaller floods. This approximation provides a theoretical interpretation of the results of the empirical quantile regression method in homogeneous regions where simple scaling or the index flood assumption does not hold. Recent results on physical interpretations of the scaling theories are also summarized here. A simple nonlinear method is developed to estimate the parameters in the multiscaling theory. This method and other features of the theory are illustrated using flood data from central Appalachia in the United States.
Particle‐tracking equations for the solution of the advection‐dispersion equation with variable coefficients
Authors: Kitanidis Peter K.
Published: February 2004

The equations of motion of a particle are derived for the random walk (or discrete parcel particle tracking) method used for the solution of the advection‐dispersion equation with spatially variable coefficients. The derivation is based on integration by parts of the advection‐dispersion equation, rather than on stochastic‐integral calculus used in other derivations. The analysis highlights the fundamental nature of the drift term, which is the term that needs to be added to the mean velocity to account for variability in the dispersion coefficients and thus provide physically reasonable results.
Scale and the Nature of Spatial Variability: Field Examples Having Implications for Hydrologic Modeling
Authors: Seyfried M. S. , Wilcox B. P.
Published: February 2004

In this paper we examine how the nature of spatial variability affects hydrologic response over a range of scales using five field studies as examples. The nature of variability was characterized as either stochastic, when random, or deterministic, when due to known, nonrandom sources. We have emphasized how that characterization may change with the scale of hydrologic model. The five field examples, along with corresponding sources of variability, were (1) infiltration and surface runoff affected by shrub canopy, (2) groundwater recharge affected by soil depth, (3) groundwater recharge and streamflow affected by small‐scale topography, (4) frozen soil runoff affected by elevation, and (5) snowfall distribution affected by large‐scale topography. In each example there was a scale, the deterministic length scale, over which the hydrologic response was strongly dependent upon the specific, location‐dependent ecosystem properties. Smaller‐scale variability may be represented as either stochastic or homogeneous with nonspatial data. In addition, changes in scale or location sometimes resulted in the introduction of larger‐scale sources of variability that subsume smaller‐scale sources. Thus recognition of the nature and sources of variability can reduce data requirements by focusing on important sources of variability and using nonspatial data to characterize variability at scales smaller than the deterministic length scale. All the sources of variability described are present in the same watershed and affect hydrologic response simultaneously. Physically based models should therefore utilize both spatial and stochastic data where scale appropriate. Other implications for physically based modeling are that modeling algorithms should reflect larger‐scale variability which generally has greater impact and that model and measurement grids should be consistent with the nature of variability.
Pore Scale Variation in Retardation Factor as a Cause of Nonideal Reactive Breakthrough Curves: 2. Pore Network Analysis
Authors: Sugita Fumi , Gillham Robert W. , Mase Charles
Published: February 2004

This paper examines the effect of pore scale heterogeneity on the shape of breakthrough curves for a reactive solute through the application of a pore network model. The particular objective was to determine whether pore scale heterogeneity, in the absence of kinetic effects, could contribute to the increased dispersion that is characteristic of measured BTCs for reactive solutes. The pore network was generated stochastically, while flow and solute transport were simulated deterministically. A lognormal pore size distribution was assigned, and reactions were represented by a distribution coefficient that was defined on a unit surface area basis and was assumed to be constant everywhere in the model. Flow and transport within the network were simulated by finite element and particle tracking methods, respectively. The nonreactive solute exhibited almost ideal behavior, while the reactive solute showed a considerable degree of nonideality, with skewed and dispersed shapes. The results of a sensitivity analysis showed greater nonideality in reactive BTCs for networks with broader pore size distribution, shorter travel distance, and greater distribution coefficient. Unlike the predictions of kinetic models, BTCs were found to be independent of average pore water velocity. Although it is a highly idealized representation of a porous medium, the network model gave symmetrical BTCs for nonreactive solutes, lending credibility to its ability to represent solute transport in a qualitatively correct manner. Therefore, the fact that the model produced nonideal BTCs for reactive solutes, in the absence of kinetic process, provides considerable support for pore scale heterogeneity in retardation as a cause of nonideality.
Groundwater discharge by phreatophyte shrubs in the Great Basin as related to depth to groundwater
Authors: Nichols William D.
Published: February 2004

An equation describing the mean daily discharge of groundwater by transpiration from phreatophyte shrubs as a function of plant density, leaf area index, and depth to groundwater was developed using an energy combination model calibrated with energy fluxes calculated from micrometeorological data. The energy combination model partitions the energy budget between the soil and canopy permitting plant transpiration to be separated from evaporation from the soil. The shrubs include greasewood, rabbitbrush, shadscale, and sagebrush. Converting a daily groundwater discharge rate calculated by the equation to an annual rate requires an estimate of the number of days the plants used only groundwater. Rates used during previous studies in the Great Basin range from 0.030 to 0.152 m yr−1; rates calculated with the equation developed during this study range from 0.024 to 0.308 m yr−1 for the reported field conditions. Annual rates estimated in this study differ from the estimated annual rates used in previous studies by factors ranging from 0.8 to 5.0.
Lumped Models of Convective Solute Transport in Heterogeneous Porous Media: 1. One‐Dimensional Media
Authors: de Valk C. F. , Raats P. A. C.
Published: February 2004

Lumped models of convective solute transport in one‐dimensional porous media are derived from the mass balance and the law of mass action. These models predict cumulative leaching from cumulative solute application and cumulative irrigation or rainfall. The solutes can be subject to adsorption or exchange governed by a nonlinear isotherm. The most important feature of the models is that although the properties of the medium and the liquid phase are allowed to be nonuniform, application requires only knowledge of the integrated moisture storage and adsorption isotherm over the medium section under consideration. Sufficient conditions are derived for the validity of lumped models and convenient expressions for evaluation of the model output are given. The results will be applied in part 2 of this paper, forthcoming, which will deal with convective solute transport along fixed streamlines in three‐dimensional porous media.
Effects of Microbial Metabolic Lag in Contaminant Transport and Biodegradation Modeling
Authors: Wood Brian D. , Ginn Timothy R. , Dawson Clint N.
Published: February 2004

A model is introduced for microbial kinetics in porous media that includes effects of transients in the metabolic activity of subsurface microorganisms. The model represents the microbial metabolic activity as a functional of the history of aqueous phase substrates; this dependence is represented as a temporally nonlocal convolution integral. Conceptually, this convolution represents the activity of a microbial component as a fraction of its maximum activity, and it is conventionally known as the metabolic potential. The metabolic potential is used to scale the kinetic expressions to account for the metabolic state of the organisms and allows the representation of delayed response in the microbial kinetic equations. Calculation of the convolution requires the definition of a memory (or kernel) function that upon integration over the substrate history represents the microbial metabolic response. A simple piecewise‐linear metabolic potential functional is developed here; however, the approach can be generalized to fit the observed behavior of specific systems of interest. The convolution that results from the general form of this model is nonlinear; these nonlinearities are handled by using two separate memory functions and by scaling the domains of the convolution integrals. The model is applied to describe the aerobic degradation of benzene in saturated porous media. Comparative simulations show that metabolic lag can be used to consistently describe observations and that a convolution form can effectively represent microbial lag for this system. Simulations also show that disregarding metabolic lag when it exists can lead to overestimation of the amount of substrate degraded.
Field‐Scale Solute Transport in a Heavy Clay Soil
Authors: Bronswijk J. J. B. , Hamminga W. , Oostindie K.
Published: February 2004

The transport of a bromide tracer was studied in a heavy clay soil. After applying the tracer on day 0, water and bromide content profiles in the unsaturated zone were determined on days 6, 46, 209, 335, and 572. Furthermore, water samples were taken frequently from the groundwater and drain discharge and analyzed for bromide content. In the clay soil studied, solute transport occurred in three domains: large continuous macropores, smaller more tortuous mesopores, and the pores inside the soil aggregates. Bromide transport through the first domain, i.e., preferential flow through large macropores, amounted only to a few percent. This resulted, however, in maximum bromide concentrations in groundwater and drain discharge. Quantitatively, the most important solute transport occurred through the mesopores. This transport was characterized by high spatial variability, significant lateral transport, and low mobile water volumes (6%). At the field scale this resulted in the rapid leaching of solute in comparison with earlier experiments on sand and loam soils. After only 6 days and 34 mm net precipitation, the average bromide concentration peak had reached a depth of 55 cm. In spite of the heterogeneous transport and the presence of preferential flow, field‐average solute concentration profiles exhibited regular and smooth shapes. Solute transport inside the soil aggregates played only an indirect role, i.e., via solute retardation as a result of convection and diffusion into the aggregates.
Modeling the Effects of Variable Groundwater Chemistry on Adsorption of Molybdate
Authors: Stollenwerk Kenneth G.
Published: February 2004

Laboratory experiments were used to identify and quantify processes having a significant effect on molybdate (MoO42−) adsorption in a shallow alluvial aquifer on Cape Cod, assachusetts. Aqueous chemistry in the aquifer changes as a result of treated sewage effluent mixing with groundwater. Molybdate adsorption decreased as pH, ionic strength, and the concentration of competing anions increased. A diffuse‐layer surface complexation model was used to simulate adsorption of MoO42−, phosphate (PO43−), and sulfate (SO42−) on aquifer sediment. Equilibrium constants for the model were calculated by calibration to data from batch experiments. The model was then used in a one‐dimensional solute transport program to successfully simulate initial breakthrough of MoO42− from column experiments. A shortcoming of the solute transport program was the inability to account for kinetics of physical and chemical processes. This resulted in a failure of the model to predict the slow rate of desorption of MoO42− from the columns. The mobility of MoO42− ncreased with ionic strength and with the formation of aqueous complexes with calcium, magnesium, and sodium. Failure to account for MoO42− speciation and ionic strength in the model resulted in overpredicting MoO42− adsorption. Qualitatively, the laboratory data predicted the observed behavior of MoO42− in the aquifer, where retardation of MoO42− was greatest in uncontaminated roundwater having low pH, low ionic strength, and low concentrations of PO43− and SO42−.
Remarks on a Min‐Max Optimization Technique for the Management of a Single Multiannual Reservoir Aimed at Hydroelectric Generation and Water Supply
Authors: Nardini A. , Montoya D.
Published: February 2004

A risk averse (min‐max) technique for the management of a multipurpose reservoir was first proposed by Orlovski et al. (1983) in a “strict uncertainty” framework. In this paper an extension of such a type of min‐max technique to the case of a single multiannual reservoir aimed at hydroelectric generation and water supply is presented. A new algorithm is proposed which was developed in the context of a real‐world system existing in central Chile.
Groundwater Remediation Design Using a Three‐Dimensional Simulation Model and Mixed‐Integer Programming
Authors: Sawyer Charles S. , Ahlfeld David P. , King Alan J.
Published: February 2004

A three‐dimensional groundwater flow management model for making decisions on the design of hydrodynamic control of a groundwater flow system using a combination of extraction and/or injection wells is developed. The model takes into account constraints imposed on the system to stop the horizontal spread of contaminants and to ensure a net upward flow in areas where downward vertical gradients exist. The mathematical formulation of the groundwater remediation problem as a mixed‐integer model and the strategy for solving the model are presented. Numerical results are presented for the Toms River Plant site, which is modeled as a five‐layer aquifer system with interconnecting aquitards. A sensitivity analysis on the relative magnitude of the continuous operating costs and the fixed‐charge costs is also presented.
A Synthesis of Approaches to Upscaling of Hydraulic Conductivities
Authors: Sánchez‐Vila Xavier , Girardi Jorge P. , Carrera Jesús
Published: February 2004

Simulation of flow through heterogeneous media often requires discretizing the flow domain into blocks and assigning an equivalent block conductivity value to each one of them. The process of defining block conductivities from point values is termed upscaling. A number of approaches to upscaling are available, most of which consider the uncertainty associated with any natural property, so that they cast the problem in a stochastic frame. Recently, Indelman and Dagan (1993a, b) provided a general stochastic methodology to upscaling in heterogeneous anisotropic formations by means of the dissipation energy function; unfortunately, they did not provide any “practical” method to compute block values from point ones. The objective of this work is twofold: First, we analyze different practical approaches to compute block conductivities and find that all of them provide very similar results in terms of actual computed values; second, we check that all approaches verify approximately a number of conditions stated by Indelman and Dagan (1993a). Specifically, we show analytically that for regular blocks, the methodologies of both Rubin and Gómez‐Hernández (1990) and Desbarats (1992) (which we call “practical” methodologies) satisfy the condition that the effective conductivity obtained from a field where the elementary conductivities are defined over a certain support (we call this the actual formation) is identical to that obtained from the same field with conductivities defined at a larger support (upscaled formation). The analysis is carried out by working with the logarithm of block conductivities and using a small‐perturbation expansion and thus is strictly valid for small variances. On the other hand, we show numerically that the two methodologies satisfy approximately an important condition stated in terms of the dissipation energy: that block‐averaged dissipation values computed are indeed very close to the true dissipation values in each block. The agreement is even better if we consider statistical moments instead of point values. As an important conclusion we should note that all practical methodologies considered in this work perform equally well and, more important, constitute a simple way to treat an otherwise very complex problem.
Transport of Anions and Herbicides in a Loamy and a Sandy Field Soil
Authors: Flury Markus , Leuenberger Jörg , Studer Bjørn , Flühler Hannes
Published: February 2004

A field experiment was conducted to investigate preferential transport of herbicides and to explore processes that cause rapid movement. Bromide, chloride, and three herbicides (triasulfuron, atrazine, and terbuthylazine) with different mobility characteristics were applied to six 1.4 × 1.4 m field plots on a loamy and a sandy soil. At both sites, three of the plots were covered with a plastic roof 1 month before the beginning of the experiment to achieve different initial water contents between the plots. Two days before the beginning of the tracer experiment, crops were removed, and the soil surface was homogenized with a spade to a depth of 15–20 cm. One day after application of the chemicals the plots were irrigated with a sprinkling apparatus. The cumulative amounts of infiltration until the time of sampling were 30, 60, and 90 mm within 1, 2, and 3 days, respectively. A trench was excavated, and soil cores were taken horizontally from a 1 × 1 m profile in a regular 0.1 × 0.1 m grid. The loamy and the sandy soil showed completely different transport patterns. In the loamy soil the bulk mass of herbicides remained in the top layer; however, considerable amounts of herbicides were transported below the root zone. A few percent for triasulfuron and atrazine and <1% for terbuthylazine were detected below 0.5 m depth after 90‐mm cumulative infiltration. Traces of all herbicides were found down to 0.9 m. The depth distributions for anions and all herbicides were similar. These results show that the herbicides were only partly adsorbed by the soil matrix. A fraction of these chemicals was transported with or without minor adsorption along cracks or fissures. In the sandy soil, chemical movement was confined to the top 0.4 m, and the penetration depth of the herbicides was consistent with their mobility characteristics: triasulfuron showed greatest mobility, atrazine was moderately mobile, and terbuthylazine was the least mobile of all three.
Optimizing Irrigation Management for Pollution Control and Sustainable Crop Yield
Authors: Musharrafieh Ghassan R. , Peralta Richard C. , Dudley Lynn M. , Hanks Ronald J.
Published: February 2004

Irrigation strategies which maximize crop yield while preventing salt from leaching to the groundwater or undesirable salt increases in the root zone are computed by using a one‐dimensional simulation/optimization management model. The included constraint equations maintain a water volume balance and salt transport in the unsaturated zone. Implicit finite difference forms of the unsaturated water flow equation (Richards' equation), the diffusion‐convection solute transport equation, functions describing the hydraulic properties of the medium, a root extraction function, and other constraints are used. The model uses a large discretization in time. A cyclic prediction and correction type of approach is adopted to eliminate the inaccuracy that would otherwise result from the coarse discretization. As a result of the procedure presented, intercell water and mass flux rates in the optimization model have the same accuracy as those in a more finely discretized simulation model. The model is applied to a research farm in Huntington, Utah, where salty water is used for irrigation. In that process detailed soil water and salt profiles are computed and spatially distributed moisture content and concentration constraints are satisfied.
Storage‐Reliability‐Resilience‐Yield Relations for Over‐Year Water Supply Systems
Authors: Vogel Richard M. , Bolognese Ralph A.
Published: February 2004

An approximate yet general approach for describing the overall behavior of water supply systems dominated by carry‐over storage is introduced. Generalized relationships among reservoir system storage, yield, reliability, and resilience are introduced for water supply systems fed by autoregressive normal and lognormal annual inflows. Relationships for reservoir system resilience are derived which represent the likelihood that a system will recover from a failure, once a failure has occurred. Monte Carlo experiments document that a two‐state Markov model can reproduce the relationships between resilience and reliability for a wide class of water supply systems. A two‐state Markov model combined with some existing analytical relationships among storage, reliability, and yield provides a very general theoretical foundation for understanding the trade‐offs among reservoir system storage, yield, reliability, and resilience.
Grain Size Patchiness as a Cause of Selective Deposition and Downstream Fining
Authors: Paola Chris , Seal Rebecca
Published: February 2004

One facet of the debate about the hypothesis of equal mobility of all grain sizes in a mixture is that equal mobility seems contrary to field evidence for selective deposition as an important mechanism of downstream fining. We argue that regardless of the ultimate outcome of the equal mobility debate, variability in local mean grain size across a reach can give rise to strong selective deposition even if locally equal mobility is satisfied exactly. We consider a system in which the sediment is arranged into locally well mixed zones (“patches”) whose mean grain size varies randomly across the reach. We assume that the shear stress is randomly distributed with a mean value near the critical value for the reach‐averaged median size, that the variance in stress scales with the variance in grain size, and that the development of fine patches in areas of high shear stress is supply limited. The main controls on fining rate are then the ratio of mean stress to critical stress for the section‐averaged mean grain size and the ratio of the patch standard deviation (assumed constant) to the standard deviation of patch means. In addition, in any system in which fining occurs by selective transport and deposition, the fining rate is strongly influenced by the spatial distribution of deposition rate.
Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation
Authors: Nelson David M. , Tréguer Paul , Brzezinski Mark A. , Leynaert Aude , Quéguiner Bernard
Published: February 2004

We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012 mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi‐compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012 mol Si yr−l to the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2 yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid‐ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2 yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid‐ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom‐rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or accelerate its transport to the seafloor.
Ice Age terrestrial carbon changes revisited
Authors: Crowley Thomas J.
Published: February 2004

N. Shackleton (1977) first proposed that changes in the marine δ13C record (Δδ13C) could be used to infer ice age changes in carbon storage on land. The previously published best estimate from the marine record is equivalent to about 490 Gt (0.32 Δδ13C). However, Adams et al. (1990) utilized a pollen database to estimate a 1350 Gt change in carbon storage, which would cause a Δδ13C of about 0.90‰. The nearly trillion ton difference in estimates amounts to almost half of the total carbon stored on land. To address the nature of this discrepancy, I have reexamined the terrestrial carbon record based on a new pollen database compiled by R. Webb and the Cooperative Holocene Mapping Project (COHMAP) group. I estimate about 750–1050 Gt glacial‐interglacial change in terrestrial carbon storage, with the range reflecting uncertainties in carbon storage values for different biomes. Additional uncertainties apply to rainforest and wetland extent and presence of C4 plants, which have a significantly different isotopic signature than C3 plants. Although some scenarios overlap a new estimate of carbon storage based on the oceanic Δδ13C record (revised to 0.40‰ and 610 Gt), most estimates seem to fall outside the envelope of uncertainty in the marine record. Several possible explanations for this gap involve: (1) a missing sink may be involved in land‐sea carbon exchange (e.g., continental slopes); (2) the geochemistry of the exchange process is not understood; (3) carbon storage by biome may have changed under ice age boundary conditions; or (4) the standard interpretation of whole ocean changes in the marine δ13C record requires reevaluation. This latter conclusion receives some support from comparison of the δ13C records for δ18O Stages 2 and 6. For the Stage 6 glacial, the δ13C changes are 50–60% larger than for the Stage 2 glacial. Yet implications of increased aridity are not supported by longterm trends in atmospheric dust loading. To summarize, the above analysis implies that, despite the uncertainties remaining in estimates of terrestrial carbon storage changes, a case can be made that our understanding of the transfer process is incomplete and that the eventual explanation may require clarification of factors affecting the marine δ13C record.
Seasonal variation in chemical distributions in the Amazon (Solimões) River: A multiyear time series
Authors: Devol Allan H. , Forsberg Bruce R. , Richey Jeffrey E. , Pimentel Tania P.
Published: February 2004

The results of a 10‐year time series study of the chemistry of the Amazon River mainstem near Manaus, Brazil, are presented. All variables measured showed distinct seasonal patterns linked to the discharge hydrograph except respiration rate and PO4−3 concentration. Stepwise multiple regression analysis showed that alkalinity, calcium, fine suspended sediment, and sulfate were correlated, primarily, with the percentage of the total water discharge that was derived from Andean drainages. Silicate, potassium, and the weight percentages of carbon and nitrogen in the coarse suspended sediment were correlated with the percentage of water attributable to local lowland drainages. These correlations suggest that seasonal cycles of these variables were controlled by their source strength. Coarse suspended sediment and the concentration all of the particulate carbon, nitrogen, and phosphorus species were correlated with river surface slope, suggesting that the seasonal cycles of these variables were controlled by river turbulence. The biogeochemically active elements O2, CO2, NO3− and SO4=, along with pH, Na, and Cl were all highly correlated with river discharge. The shapes of the seasonal cycles of O2 and CO2 and much of their amplitude could be reproduced by a quasi steady state model in which respiration was balanced by air‐water gas exchange. In the model, increases in river depth during the annual cycle result in increased depth‐integrated respiration rates. This produces a drawdown of O2 concentration, which increases air‐water gas exchange, until the two processes are in balance. Thus the model produces seasonal cycles in which minimum dissolved O2 and maximum dissolved CO2 coincide with high water and the converse at low water, in agreement with the observations. The remainder of the amplitude signal was probably either advected in from upstream or produced by lateral exchange with the fringing floodplain.
Effects of global change on carbon storage in tropical forests of South America
Authors: McKane Robert B. , Rastetter Edward B. , Melillo Jerry M. , Shaver Gaius R. , Hopkinson Charles S. , Fernandes David N. , Skole David L. , Chomentowski Walter H.
Published: February 2004

We used a process‐based model of ecosystem biogeochemistry (MBL‐GEM) to evaluate the effects of global change on carbon (C) storage in mature tropical forest ecosystems in the Amazon Basin of Brazil. We first derived a single parameterization of the model that was consistent with all the C stock and turnover data from three intensively studied sites within the Amazon Basin that differed in temperature, rainfall, and cloudiness. The range in temperature, soil moisture, and photosynthetically active radiation (PAR) among these sites is about as large as the anticipated changes in these variables in the tropics under CO2‐induced climate change. We then tested the parameterized model by predicting C stocks along a 2400‐km transect in the Amazon Basin. Comparison of predicted and measured vegetation and soil C stocks along this transect suggests that the model provides a reasonable approximation of how climatic and hydrologic factors regulate present‐day C stocks within the Amazon Basin. Finally, we used the model to predict and analyze changes in ecosystem C stocks under projected changes in atmospheric CO2 and climate. The central hypothesis of this exercise is that changes in ecosystem C storage in response to climate and CO2 will interact strongly with changes in other element cycles, particularly the nitrogen (N) and phosphorus (P) cycles. We conclude that C storage will increase in Amazonian forests as a result of (1) redistribution of nutrients from soil (with low C:nutrient ratios) to vegetation (with high C:nutrient ratios), (2) increases in the C:nutrient ratio of vegetation and soil, and (3) increased sequestration of external nutrient inputs by the ecosystem. Our analyses suggest that C:nutrient interactions will constrain increases in C storage to a maximum of 63 Mg/ha during the next 200 years, or about 16% above present‐day stocks. However, it is impossible to predict how much smaller the actual increase in C storage will be until more is known about the controls on soil P availability. On the basis of these analyses, we identify several topics for further research in the moist tropics that must be addressed to resolve these uncertainties.
The magnitude and persistence of soil NO, N2O, CH4, and CO2fluxes from burned tropical savanna in Brazil
Authors: Poth Mark , Anderson Iris Cofman , Miranda Heloisa Sinatora , Miranda Antonio Carlos , Riggan Philip J.
Published: February 2004

Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2O, CH4, and CO2from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N‐trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2O and a sink for atmospheric CH4. Burning also elevated CO2fluxes, which remained detectably elevated 1 year later.
On the contribution of CO2fertilization to the missing biospheric sink
Authors: Friedlingstein P. , Fung I. , Holland E. , John J. , Brasseur G. , Erickson D. , Schimel D.
Published: February 2004

A gridded biospheric carbon model is used to investigate the impact of the atmospheric CO2increase on terrestrial carbon storage. The analysis shows that the calculated CO2fertilization sink is dependent not just on the mathematical formulation of the “β factor” but also on the relative controls of net primary productivity (NPP), carbon residence times, and resource availability. The modeled evolution of the biosphere for the period 1850–1990 shows an increasing lag between NPP and the heterotrophic respiration. The time evolution of the modeled biospheric sink (i.e., difference between enhanced NPP and enhanced respiration) does not match that obtained by deconvolution of the ice core CO2time series. Agreement between the two is reasonable for the first half of the period, but during the recent decades the deconvoluted CO2increase is much too fast to be explained by the CO2fertilization effect only. Therefore other mechanisms than CO2fertilization should also contribute to the missing sink. Our results suggest that about two thirds to three fourths of the 1850–1990 integrated missing sink is due to the CO2greening of the biosphere. The remainder may be due to the increased level of nitrogen deposition starting around 1950.
Toward a global estimate of black carbon in residues of vegetation fires representing a sink of atmospheric CO2and a source of O2
Authors: Kuhlbusch T. A. J. , Crutzen P. J.
Published: February 2004

Vegetation fire residues of various kinds of plant materials were analyzed for black carbon (BC) and the corresponding hydrogen. These data clearly show that BC can be defined as the fire produced carbon fraction with a molar H/C ratio of ≤ 0.2 which is resistant to heating to 340°C in pure oxygen. Relationships of black carbon production to fire characteristics such as fuel type, gaseous emissions, etc., were studied demonstrating that black carbon is mainly a product of the hot flaming combustion and that more than 80% of the black carbon produced remains on ground after the fire with the rest being emitted with the smoke. This black carbon mainly in the fine residue may significantly contribute to the background concentrations of black carbon in the atmosphere. From the ratios of black carbon in the residue to carbon exposed to the fire, and to carbon emitted as CO2we estimate the global black carbon formation to be 50–270 Tg yr−l(teragram = 1012g). As this carbon is very resistant to microbial decay, it may well represent a sink for biospheric carbon. This sink may be significantly greater, for example, up to a factor of 2 when reducing the pretreatment temperature to 300°C, if some of the less resistant carbon with higher H/C ratios which is removed by our method would also show significant resistance to microbial breakdown. The fire‐induced sequestration of carbon from the short‐term biospheric to the long‐term geological cycle due to the formation of black carbon may represent a significant sink of atmospheric CO2and source of O2and possibly has influenced today's atmospheric oxygen content.
On the relations between the oceanic uptake of CO2 and its carbon isotopes
Authors: Heimann Martin , Maier‐Reimer Ernst
Published: February 2004

The recent proposals to estimate the oceanic uptake of CO2 by monitoring the oceanic change in 13C/12C isotope ratio [Quay et al., 1992] or the air‐sea 13C/12C isotopic disequilibrium [Tans et al., 1993] is reviewed. Because the history of atmospheric CO2 and 13CO2 since preindustrial times is almost the same and increasing in an almost exponential fashion, the oceanic penetration depth of both tracers must be the same. This dynamic constraint permits the establishment of yet a third method to estimate the global ocean uptake of CO2 from 13C measurements. Using available observations in conjunction with canonical values for global carbon cycle parameters, the three methods yield inconsistent oceanic CO2 uptake rates for the time period 1970–1990, ranging from 0.6 to 3.1 GtC yr−1. However, uncertainties in the available carbon cycle data must be taken into account. Using a nonlinear estimation procedure, a consistent scenario with an oceanic CO2 uptake rate of 2.1±0.9 GtC yr−1 can be established. The method also permits an investigation of the sensitivities of the different approaches. An analysis of the results of two three‐dimensional simulations with the Hamburg model of the oceanic carbon cycle shows that the 13C isotope indeed tracks the oceanic penetration of anthropogenic CO2. Because of its different time history, bomb produced radiocarbon, as measured at the time of the Geochemical Ocean Sections Study (GEOSECS), correlates not as well to excess carbon.
Variability in the O2/N2 ratio of southern hemisphere air, 1991–1994: Implications for the carbon cycle
Authors: Bender Michael , Ellis Taylor , Tans Pieter , Francey Roger , Lowe David
Published: February 2004

We present a record of variations in the O2/N2 ratio of air at 41°S latitude from 1991–1994 based on the mass spectrometric analysis of flask samples from Cape Grim, Tasmania, and Baring Head, New Zealand. Results for Cape Grim for the period from June 1991 to February 1992 are in good agreement with previously published data of Keeling and Shertz [1992]. Plotted versus time, O2/N2 ratios show the expected annual cycles. O2/N2 increases in austral spring and summer (caused mainly by net oceanic production) and decreases in fall and winter (caused by ventilation of the seasonal and main thermoclines). The average amplitude of the seasonal cycle implies net oceanic production of about 5 mol C m−2 yr−1 with considerable interannual variability. The O2/N2 ratio of air decreased at the rate of 12±4 per meg/yr (0.012 ‰/yr) between winter 1991 and winter 1993. This value is considerably less than the O2 consumption rate associated with fossil fuel burning (about 20 per meg/yr), suggesting that the land biosphere was an O2 source and an important CO2 sink during this period. Alternatively, the oceans may have been a transient O2 sink during 1991–1993, most likely caused by an enhanced rate of thermocline ventilation with respect to the steady‐state value.
The global ocean flux of particulate organic carbon: Areal distribution and magnitude
Authors: Jahnke Richard A.
Published: February 2004

The magnitude and distribution of the particulate organic carbon (POC) rain rate to the seafloor in the Atlantic, Pacific and Indian Ocean basins between 61°N and 61°S has been estimated from benthic oxygen flux estimates (for water depths ≥ 1000 m only). The calculation utilizes the extensive data sets of sedimentary organic carbon, CaCO3, and accumulation rate to extrapolate between individual benthic flux measurement sites using an empirically‐derived correlation between the seafloor oxygen flux and these parameters. The POC flux through the 1000 m depth horizon was then estimated from published correlations between sediment trap‐determined fluxes and water depth. Total oxygen utilization in the deep ocean is estimated to be 1.2×1014 mol O2 yr−1, a value that agrees well with previous estimates which were based on surface water primary productivity, sediment trap, and depth relationships and with deep water respiration rates estimated from apparant oxygen utilization (AOU)‐14C relationships. On the basis of the derived global ocean flux distribution, it is concluded that (1) dissolved organic carbon (DOC) inputs are not required to account for estimated deep water respiration rates; (2) the majority of the POC input to the deep ocean occurs within 30° of the equator; (3) the proportion of primary production that reaches the deep sea does not vary greatly with latitude; (4) gyre and continental margin regions contribute roughly equally to the deep POC flux with a relatively minor contribution from the equatorial divergence region; (5) of the estimated 7.2×1013 mol C yr−1 of POC that sinks below the 1000 m depth horizon, 45% (3.3×1013 mol C yr−1) reaches the seafloor where it is oxidized; (6) when normalized to basin area, average deep flux rates in the Atlantic and Pacific are similar while highest rates are observed in the Indian Ocean; and (7) the results can be fully reconciled only if the benthic flux of DOC is significantly less than the benthic O2 flux.
Manner of magma ascent at Unzen Volcano (Japan)
Authors: Nakada Setsuya , Motomura Yoshinobu , Shimizu Hiroshi
Published: February 2004

Juvenile materials were found among products of phreatomagmatic eruptions that preceded dacite dome growth at Unzen Volcano in 1991. They give evidence showing that the hydrous magma started degassing with the resultant crystallization around 100 MPa, and was quenched soon thereafter. Ascending at a rate as low as 13 m/d while degassing, however, the still‐molten part inside reacted with water. Phreatomagmatic eruptions started when the magma reached about 1.2 km in depth, and strong ones started at about 0.6 km. Volcanic tremors had occurred at these depths, where the sources of vulcanian explosions and the ground deformation were also located, implying the existence of a possible gas pocket.
Heating, ionization and upward discharges in the mesosphere, due to intense quasi‐electrostatic thundercloud fields
Authors: Pasko Victor P. , Inan Umran S. , Taranenko Yuri N. , Bell Timothy F.
Published: February 2004

Quasi‐electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes are found to significantly heat mesospheric electrons and produce ionization and light. The intensity, spatial extent, duration and spectra of optical emissions produced are consistent with the observed features of the Red Sprite type of upward discharges.
Lightning driven EMP in the upper atmosphere
Authors: Rowland H. L. , Fernsler R. F. , Huba J. D. , Bernhardt P. A.
Published: February 2004

Large lightning discharges can drive electromagnetic pulses that cause breakdown of the neutral atmosphere between 80 and 95 km leading to order of magnitude increases in the plasma density. The increase in the plasma density leads to increased reflection and absorption, and limits the pulse strength that propagates higher into the ionosphere.
Mass and energy budgets of explosive volcanic eruptions
Authors: Pyle David M.
Published: February 2004

Analysis of the frequency‐size relationship of explosive volcanic eruptions for the past 2000 years reveals that the time‐averaged eruptive mass flux of tephra from subaerial explosive eruptions is 3×104 kg s−1. This mass flux corresponds to a thermal energy release of 4×1010 W. Half of the long‐term (>10³ year) mass and thermal energy release from explosive volcanic eruptions is accounted for by eruptions of >10 km³ of tephra.