Steady-state Mass Balance Research Articles

Accounting for nitrogen fixation in simple models of lake nitrogen loading/export.

Coastal eutrophication, an important global environmental problem, is primarily caused by excess nitrogen and management efforts consequently focus on lowering watershed N export (e.g., by reducing fertilizer use). Simple quantitative models are needed to evaluate alternative scenarios at the watershed scale. Existing models generally assume that, for a specific lake/reservoir, a constant fraction of N loading is exported downstream. However, N fixation by cyanobacteria may increase when the N loading is reduced, which may change the (effective) fraction of N exported. Here we present a model that incorporates this process. The model (Fixation and Export of Nitrogen from Lakes, FENL) is based on a steady-state mass balance with loading, output, loss/retention, and N fixation, where the amount fixed is a function of the N/P ratio of the loading (i.e., when N/P is less than a threshold value, N is fixed). Three approaches are used to parametrize and evaluate the model, including microcosm lab experiments, lake field observations/budgets and lake ecosystem model applications. Our results suggest that N export will not be reduced proportionally with N loading, which needs to be considered when evaluating management scenarios.

Modeling energy consumption in membrane bioreactors for wastewater treatment in north Africa.

Two pilot-scale membrane bioreactors were operated alongside a full-sized activated sludge plant in Tunisia in order to compare specific energy demand and treated water quality. Energy consumption rates were measured for the complete membrane bioreactor systems and for their different components. Specific energy demand was measured for the systems and compared with the activated sludge plant, which operated at around 3 kWh m(-3). A model was developed for each membrane bioreactor based on both dynamic and steady-state mass balances, microbial kinetics and stoichiometry, and energy balance. Energy consumption was evaluated as a function of mixed-liquor suspended solids concentration, net permeate fluxes, and the resultant treated water quality. This work demonstrates the potential for using membrane bioreactors in decentralised domestic water treatment in North Africa, at energy consumption levels similar or lower than conventional activated sludge systems, with the added benefit of producing treated water suitable for unrestricted crop irrigation.

Element Cycling in the Critical Zone as Viewed by New Isotope Tools

Isotope Geochemists are dedicating substantial effort at developing isotope ratio-based tools that are now beginning to provide detailed insights into the processes that cycle elements between the compartments of the weathering zone. The first group makes use of the metal and metalloid elements of which the stable isotope ratios shift mainly when a) secondary minerals are formed; b) elements are cycled through higher plants. Distinct stable isotope ratios emerge for a given cycled element as a function of the isotope fractionation factor of the process involved and of the elements’ relative fluxes through these compartments (e.g. soil water, secondary minerals, plants, remaining primary minerals). A second tool provides absolute fluxes from the weathering zone: The ratio of the meteoric cosmogenic nuclide 10Be (T1/2 = 1.39 My) to the stable isotope 9Be combines a tracer of roughly known flux to the Earth's surface (over time scales characteristic of weathering) with one that depends on its release rate from rock by weathering. The resulting 10Be/9Be isotope ratio discloses both the Earth surfaces’ denudation rate and its weathering intensity. For both tools, a set of steady state mass balance equations describe these systems from the scale of a soil column over the hillslope scale to an entire river basin.

Kinetics of inactivation and dilution effects on the mass balance of fungal phytopathogens in anaerobic digesters

Knowledge of fate and behavior of plant pathogens in the biogas production chain is limited and hampers the estimation and evaluation of the potential phytosanitary risk if digestate is spread on arable land as a fertilizer. Therefore, simulation is an appropriate tool to demonstrate the effects which influence the steady state of pathogen infected plant material in both digesters and digestate. Simple approaches of kinetics of inactivation and mass balances of infected material were carried out considering single-step as well as two-step digestion. The simulation revealed a very fast to fast reduction of infected material after a singular feeding, reaching a cutback to less than 1% of input within 4 days even for D90-values of 68 h. Steady state mass balances below input rate could be calculated with D90-values of less than 2 h at a continuous hourly feeding. At higher D90-values steady state mass balances exceed the input rate but are still clearly below the sum of input mass. Dilution further decreases mass balances to values 10−5 to 10−6 Mg m−3 for first-step digestion and 10−8 to 10−9 for second-step.

Determinants of phosphorus mobilization during hemodialysis

Our recent work proposed a pseudo one-compartment model for describing intradialysis and postdialysis rebound kinetics of phosphorus. In this model, phosphorus is removed directly from a central distribution volume with the rate of phosphorus mobilization from a second, very large compartment proportional to the phosphorus mobilization clearance. Here, we evaluated factors of phosphorus mobilization clearance and postdialysis central distribution volume from 774 patients in the HEMO Study. Phosphorus mobilization clearance and postdialysis central distribution volume were 87 (65, 116) ml/min, median (interquartile range), and 9.4 (7.2, 12.0) liter, respectively. The phosphorus mobilization clearance was significantly higher for male patients than for female patients. Both the phosphorus mobilization clearance and the postdialysis central distribution volume were significantly associated with postdialysis body weight but negatively with the predialysis serum phosphorus concentration. The postdialysis central distribution volume was also significantly associated with age. Overall, the postdialysis central distribution volume was 13.6% of the postdialysis body weight. Thus, the phosphorus mobilization clearance during hemodialysis is higher when predialysis serum phosphorus concentration is low and higher in male patients than in female patients. The central distribution volume of phosphorus is a space approximating the extracellular fluid volume.

Modeling novel stable isotope ratios in the weathering zone

When rock is converted to weathering products, the involved processes can be fingerprinted using the stable isotope ratios of metals (for example Li, Mg, Ca, Fe, Sr) and metalloids (B, Si). Here we construct a framework for interpreting these “novel” stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context. The approach is applicable to any novel stable isotope system and is based on a simple steady-state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents. Our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays, and uptake of nutrients by plants. The model results show that the isotope composition of a given element in the weathering zone compartments depends on (1) the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material, consisting of secondary precipitates and organic matter; (2) the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants. A relationship is established between isotope ratios, isotope fractionation factors, and indexes for chemical weathering [such as chemical depletion fractions (CDF) and elemental mass transfer coefficients (τ)] derived from simple elemental concentration measurements. From this relationship, isotope fractionation factors can be calibrated from chemical and isotope data measured on field material. Furthermore, we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock, water, and sediment. This calculation can be applied to samples from soils, from rivers, and from the sedimentary record, and does not require knowing the isotope fractionation factors involved in the reactions. Finally, we apply the model to the oceanic Li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the Cenozoic.

Application of radon-222 to investigate groundwater discharge into small shallow lakes

The objective of this work was to assess the groundwater discharge component of the hydrological budgets of several small lakes in north and central Florida using 222Rn (radon, t1/2=3.8days) as a quantitative groundwater tracer. Methane gas and conductivity were also used as secondary natural tracers to help locate active sites of seepage. We present results based on a steady-state radon mass balance model developed to assess groundwater discharge into shallow unstratified lakes. Model assumptions include a constant 222Rn input flux over relatively short (days–weeks) periods and a well-mixed water column. Detailed surveys in several Florida lakes using continuous 222Rn measurements supported these assumptions. In five of the seven studied lakes a high to moderate groundwater inflow was detected using this technique, while in two of them the discharge was very low to not detectable. Based on these results and the size of the lakes the calculated water-renewal times based on groundwater inflows were in a range between 3 and 40months. Close 2-year examination of the groundwater dynamics at two of the lakes that showed substantial discharge, did not show great seasonal groundwater discharge variations. These radon-derived groundwater seepage fluxes agreed well with seepage meters and water budget calculations performed independently for some of the lakes. The approach proved to be very efficient, relatively inexpensive, and should be able to be applied as a routine procedure for estimating groundwater discharge for similar lakes elsewhere. The ultimate purpose of our efforts is to use this method in obtaining important information for the hydrological budget of these lakes that will allow evaluating nutrient loadings and TMDLs (Total Maximum Daily Loads) calculations.

A model of phosphorus cycling to explore the role of biomass turnover in submerged aquatic vegetation wetlands for Everglades restoration

Engineered wetlands using submerged aquatic vegetation (SAV) are a cornerstone to the Stormwater Treatment Area (STA) project for stripping phosphorus from agricultural stormwater and lake water before entering protected Everglades marshes in south Florida, USA. However, recent efforts have suggested that the apparent lowest achievable outflow P (C*) in SAV systems (∼16μg/l) may not be low enough for proposed regulatory criteria. Thus, deepened predictive understanding on the functionality of these systems is of critical importance. Here, we develop a steady-state mass balance model of intermediate complexity to investigate C* in SAV systems. The model focuses on the role of SAV biomass turnover and P release to the water column, drawing upon established principles from shallow lake studies. This study introduces several large and unique datasets collected from a single study site (STA-2 Cell 3) over a 10-year period and demonstrates coherence in these data through the modeling approach. The datasets included inflow–outflow values, P storage in accrued sediment at two intervals, annual surveys of SAV species composition, gradients of SAV tissue-P, and gradients of internal water column P concentration (previously published). The model was implemented and calibrated in an uncertainty framework with Monte Carlo methods, threshold screening, and multi-criteria limits of acceptability. Model calibration and validation appeared successful, resulting distributions of model parameters and accepted model simulations were relatively narrow, and results deepened perspectives on the previously identified C*. Rooted SAV species may be mining substantial P from underlying soils via root uptake and thus contributing internal loads. Steady turnover and decomposition of SAV biomass may be accounting for up to about a third of the background C*. These perspectives are relevant to STA optimization; our unique data, usage, and calibration strategy should be of interest to the aquatic ecosystem modeling community in general.

Susceptibility of Forests in the Northeastern USA to Nitrogen and Sulfur Deposition: Critical Load Exceedance and Forest Health

The objectives of this study were to assess susceptibility to acidification and nitrogen (N) saturation caused by atmospheric deposition to northeastern US forests, evaluate the benefits and shortcomings of making critical load assessments using regional data, and assess the relationship between expected risk (exceedance) and forest health. We calculated the critical loads of nutrient N and of sulfur (S) + N using the steady-state mass balance method at >4,000 regional and national vegetation and soil monitoring network plots in the northeastern USA. Regional calculations of critical loads necessitate use of soil maps which provide a range for each soil characteristic resulting in a broad range of critical load of S + N and exceedance values. For the scenario most representative of regional conditions, over 80 % of the critical loads fell into the range of 850–2050 eq ha−1 yr−1; at 45 % of the plots, deposition exceeded the critical load. In contrast, the critical load for nutrient N, 200–300 eq ha−1 yr−1, was lower. Site measurements, especially to estimate soil weathering, would increase the certainty of the critical load. We observed significant negative correlations between critical load exceedance and growth (17 species) and crown density (4 species); we observed significant positive correlations of exceedance with declining vigor (four species), with crown dieback (six species) and crown transparency (seven species). Among the species which demonstrate the most significant detrimental responses to atmospheric deposition are balsam fir, red spruce, quaking aspen, and paper birch. These results indicate that significant detrimental responses to atmospheric deposition are being observed across the northeastern USA.

Steady-state mass balance model for mercury in the St. Lawrence River near Cornwall, Ontario, Canada

We have developed a local mass balance model for the St. Lawrence River near Cornwall, Ontario that describes the fate and transport of mercury in three forms, elemental, divalent, and methylated, in a five compartment environment (air, water, sediments, periphyton, and benthos). Our objective was to construct a steady-state mass balance model to determine the dominant sources and sinks of mercury in this environment. We compiled mercury concentrations, fluxes, and transformation rates from previous studies completed in this section of the river to develop the model. The inflow of mercury was the major source to this system, accounting for 0.42 mol month−1, or 95.5% of all mercury inputs, whereas outflow was 0.28 mol month−1, or 63.6% of all losses, and sediment deposition was 0.12 mol month−1, or 27.3% of all losses. Uncertainty estimates were greatest for advective fluxes in surface water, porewater, periphyton, and benthic invertebrates.

Steady State Phosphorus Mass Balance Model during Hemodialysis Based on a Pseudo One-Compartment Kinetic Model

Purpose Mathematical models of phosphorus kinetics and mass balance during hemodialysis are in early development. We describe a theoretical phosphorus steady state mass balance model during hemodialysis based on a novel pseudo one-compartment kinetic model. Methods The steady state mass balance model accounted for net intestinal absorption of phosphorus and phosphorus removal by both dialysis and residual kidney function. Analytical mathematical solutions were derived to describe time-dependent intradialytic and interdialytic serum phosphorus concentrations assuming hemodialysis treatments were performed symmetrically throughout a week. Results Results from the steady state phosphorus mass balance model are described for thrice weekly hemodialysis treatment prescriptions only. The analysis predicts 1) a minimal impact of dialyzer phosphorus clearance on predialysis serum phosphorus concentration using modern, conventional hemodialysis technology, 2) variability in the postdialysis-to-predialysis phosphorus concentration ratio due to differences in patient-specific phosphorus mobilization, and 3) the importance of treatment time in determining the predialysis serum phosphorus concentration. Conclusions We conclude that a steady state phosphorus mass balance model can be developed based on a pseudo one-compartment kinetic model and that predictions from this model are consistent with previous clinical observations. The predictions from this mass balance model are theoretical and hypothesis-generating only; additional prospective clinical studies will be required for model confirmation.

Shifting westerlies and precipitation patterns during the Late Pleistocene in southern Africa determined using glacier reconstruction and mass balance modelling

South Africa experiences a range of different climatic regimes and is thus an ideal region to investigate Late Pleistocene environmental and climate change. However, detailed quantifiable palaeoclimate data are sparse in the region. In particular, reliable palaeoclimatic data are essential to resolve ongoing controversies regarding temperature depression and moisture availability during glacial periods in the sub-continent. Small glaciers close to the glaciation threshold are highly sensitive to changes in temperature and precipitation and are therefore ideal indicators of past climatic conditions during their existence. This paper derives some of the first quantitative data on Last Glacial Maximum (LGM) palaeoprecipitation in southern Africa, based on glacier reconstruction and mass balance modelling for the Lesotho Highlands.The reconstruction of former glaciers and their dynamics enables the determination of glacier viability under specific climatic envelopes. Glacier reconstructions at five sites in the Lesotho Highlands yield palaeoglaciers with Equilibrium Line Altitudes (ELAs) ranging from 3095 to 3298 m a.s.l., and reconstructed steady-state mass balance and flow dynamics are comparable with modern analogues. Topoclimatic factors are investigated, with temperature-radiation-index modelling indicating that topographic shading was an important factor determining the existence of small glaciers in this region. The occurrence of glaciation in the Drakensberg during the LGM suggests a potential increase in precipitation and change in its seasonality. Such trends are likely associated with an increased frequency of westerly wave (cold front) disturbances due to the northward shift of pressure belts, which would also increase precipitation as snow at higher altitudes. The application of a high resolution climate model (HadAM3h) to test this, displays a change in the seasonal timing of precipitation during the Last Glacial cycle, with a decrease in precipitation evident during the summer months. This is likely to have had important implications for the mass balance and survival of small niche glaciers in the region, with more precipitation falling during the spring – winter – autumn months as snow.

Earth surface erosion and weathering from the 10Be (meteoric)/9Be ratio

The isotope ratio of the meteoric cosmogenic nuclide 10Be to the mineral-derived stable isotope 9Be discloses both the Earth surface denudation rate and its weathering intensity. We develop a set of steady state mass balance equations that describes this system from a soil column over the hillslope scale to an entire river basin. The prerequisites making this new approach possible are: (1) the 9Be concentration of parent rock (typically 2.5±0.5ppm in granitic and clastic sedimentary lithologies) is known; (2) both Be isotopes equilibrate between the fluids decomposing rock and reactive solids formed during weathering; and (3) a critical spatial scale is exceeded at which the fluxes of both isotopes into and out of the weathering zone are at steady state over the time scale of weathering (typically ∼10kyr). For these cases the isotope ratios can be determined in bulk sediment or soil, on leachates from the reactive (adsorbed and pedogenic mineral-bound) phase in sediment or soil, and even on the dissolved phase in river water. The 10Be/9Be ratio offers substantial advantages over the single-isotope system of meteoric 10Be. The latter system allows to directly determine erosion rates only in the case that 10Be is fully retentive in the weathering zone and that riverine sorting has not introduced grain size-dependent 10Be concentration gradients in sediments. We show the feasibility of the 10Be/9Be tracer approach at the river scale for sediment and water samples in the Amazon basin, where independent estimates of denudation rates from in situ-produced 10Be exist. We furthermore calculate meaningful denudation rates from a set of published 10Be/9Be ratios measured in the dissolved load of globally distributed rivers. We conclude that this isotope ratio can be used to reconstruct global paleo-denudation from sedimentary records.

Post-combustion CO2 capture from a natural gas combined cycle by CaO/CaCO3 looping

CaO/CaCO3 looping has been evaluated as a potential alternative to MEA for post-combustion CO2 capture from natural gas fired combined cycles (NGCC). CaO/CaCO3 looping has the advantage that there are no emissions of potentially toxic chemical residues and by-products to the environment. A steady-state heat and mass balance model was set up and solved using model parameters from the literature. Post-combustion capture with CaO/CaCO3 looping requires heat supply at high temperature in the calciner and is provided through internal combustion of natural gas with oxygen. Process simulations of six different configurations for the entire NGCC with post-combustion capture resulted in net thermal efficiency in the range 45.6–48.1%, given the process assumptions made and a CO2 capture rate in the range of 90.3–91.3%. These figures should be compared to 49.5% for the NGCC reference case with MEA-based capture. The resulting purity of captured CO2 is lower for CaO/CaCO3 looping due to presence of excess oxygen and inert nitrogen and argon. Increased CO2 purity can, however, be obtained through higher O2 purity, however at the cost of higher air separation unit energy penalty. It is believed that further process integration and optimisation may enable further increase of the process efficiency for CO2 capture with CaO/CaCO3 looping. Moreover, another benchmarking approach of interest would be to perform a comparative life-cycle assessment (LCA) study to benchmark the environmental impact of CaO/CaCO3 looping with that of MEA scrubbing.

An online tool for calculation of free-energy balance for the renal inner medulla

Concentrating models of the renal inner medulla can be classified according to external free-energy balance into passive models (positive values) and models that require an external energy source (negative values). Here we introduce an online computational tool that implements the equations of Stephenson and colleagues (Stephenson JL, Tewarson RP, Mejia R. Proc Natl Acad Sci USA 71: 1618-1622, 1974) to calculate external free-energy balance at steady state for the inner medulla (http://helixweb.nih.gov/ESBL/FreeEnergy). Here "external free-energy balance" means the sum of free-energy flows in all streams entering and leaving the inner medulla. The program first assures steady-state mass balance for all components and then tallies net external free-energy balance for the selected flow conditions. Its use is illustrated by calculating external free-energy balance for an example of the passive concentrating model taken from the original paper by Kokko and Rector (Kokko JP, Rector FC Jr. Kidney Int 2: 214-223, 1972).

Analyzing a micro-solid oxide fuel cell system by global energy balances

A simple method to predict the thermal characteristics of a micro-SOFC system is presented in this study. The basic design requirements for a thermally self-sustaining operation at a stack temperature of about 550 °C are assessed. Based on steady-state global energy and mass balances, the influence of the electrical efficiency, the overall air-to-fuel ratio and the heat losses on the operating temperature is discussed. It was found that at high electrical efficiencies and, hence, low heat release rates, a recuperator is needed to achieve the desired operating temperature. At lower electrical efficiencies, in contrast, disposing the released heat becomes an issue and an efficient cooling of the stack is required. Whether a recuperator or additional cooling components are necessary also depends on the electrical power output, the stack size and the thermal insulation specifications. The threshold between cooling and recuperator mode is of special interest, since this operating point allows a simple design of the thermal system without recuperator and only a minimum of air to be supplied to the system. This threshold efficiency, which is the maximal electrical efficiency that allows a thermally self-sustaining operation without recuperator, is above 50% under adiabatic operating conditions. In a non-adiabatic system, the threshold efficiency is reduced to about 45% in a 20 Wel and to about 20% in a 2.5 Wel system even with a state-of-the art thermal insulation. The considered thermal insulations dominate the system volume. Therefore, the ability of heat loss minimization is highly dependent on the targeted volumetric power density of the system.

Steady state phosphorus mass balance model during hemodialysis based on a pseudo one-compartment kinetic model

Mathematical models of phosphorus kinetics and mass balance during hemodialysis are in early development. We describe a theoretical phosphorus steady state mass balance model during hemodialysis based on a novel pseudo one-compartment kinetic model. The steady state mass balance model accounted for net intestinal absorption of phosphorus and phosphorus removal by both dialysis and residual kidney function. Analytical mathematical solutions were derived to describe time-dependent intradialytic and interdialytic serum phosphorus concentrations assuming hemodialysis treatments were performed symmetrically throughout a week. Results from the steady state phosphorus mass balance model are described for thrice weekly hemodialysis treatment prescriptions only. The analysis predicts 1) a minimal impact of dialyzer phosphorus clearance on predialysis serum phosphorus concentration using modern, conventional hemodialysis technology, 2) variability in the postdialysis-to-predialysis phosphorus concentration ratio due to differences in patient-specific phosphorus mobilization, and 3) the importance of treatment time in determining the predialysis serum phosphorus concentration. We conclude that a steady state phosphorus mass balance model can be developed based on a pseudo one-compartment kinetic model and that predictions from this model are consistent with previous clinical observations. The predictions from this mass balance model are theoretical and hypothesis-generating only; additional prospective clinical studies will be required for model confirmation.

A kinetic analysis and experimental validation of an integrated system of anaerobic filter and biological aerated filter

An anaerobic/aerobic filter (AF/BAF) system was developed treating dairy wastewater. The influent was blended with recirculated effluent to allow for pre-denitrification in the AF followed by nitrification in the BAF. The recirculation ratio ranged 100–300%. The average chemical oxygen demand (COD) removal efficiency was 79.8–86.8% in the AF and the average total nitrogen removal efficiency was 50.5–80.8% in the AF/BAF system. Steady-state mass balances on the AF were used to analyze removal kinetics in the AF. The kinetic model values for effluent COD in the AF were overestimated as compared with experimental data. The integrated suspended and attached biomass growth rates in the AF were estimated. The specific growth rate of the integrated biomass at each recirculation ratios was 0.6213, 0.6647, and 1.20831/day, respectively. The increase in specific growth rate corresponded to increases in biomass sloughing as the recirculation ratio increased.

Impact Loads of Air Pollutants on Paintings: Performance Evaluation by Modeling For Microclimate Frames

Modeling of the transport (fluxes) of gaseous air pollutants to the surfaces of paintings installed inside six different microclimate frames (mc-frames) was performed. Mc-frames are used to protect paintings against the external environment and to provide climate buffering. However, some can trap potentially harmful gases, especially acetic acid, which may be emitted inside the frames. A steady-state mass balance model was used to evaluate how changes in three factors (the ventilation rate, the volume of the mc-frame, and the inclusion of a pollutant absorber) would affect the concentrations of gaseous air pollutants transported to the paintings in the frames. The modeled impact to the paintings was determined to be higher inside than outside for two of six mc-frames when exposed to a 50/50 mixture of acidic (acetic plus formic acid at 100 μg/ m 3 ) and oxidative gases (nitrogen dioxide plus ozone at 2 μg/m 3 ). The concentrations of the pollutants were selected to provide clear degradative effects on the paintings. The modeling indicated the most effective way to reduce the potential pollutant impact on paintings in mc-frames is to reduce both ventilation and volume, and to also install a pollution absorber.

In situ metabolic flux analysis to quantify the liver metabolic response to experimental burn injury

Trauma such as burns induces a hypermetabolic response associated with altered central carbon and nitrogen metabolism. The liver plays a key role in these metabolic changes; however, studies to date have evaluated the metabolic state of liver using ex vivo perfusions or isotope labeling techniques targeted to specific pathways. Herein, we developed a unique mass balance approach to characterize the metabolic state of the liver in situ, and used it to quantify the metabolic changes to experimental burn injury in rats. Rats received a sham (control uninjured), 20% or 40% total body surface area (TBSA) scald burn, and were allowed to develop a hypermetabolic response. One day prior to evaluation, all animals were fasted to deplete glycogen stores. Four days post-burn, blood flow rates in major vessels of the liver were measured, and blood samples harvested. We combined measurements of metabolite concentrations and flow rates in the major vessels entering and leaving the liver with a steady-state mass balance model to generate a quantitative picture of the metabolic state of liver. The main findings were: (1) Sham-burned animals exhibited a gluconeogenic pattern, consistent with the fasted state; (2) the 20% TBSA burn inhibited gluconeogenesis and exhibited glycolytic-like features with very few other significant changes; (3) the 40% TBSA burn, by contrast, further enhanced gluconeogenesis and also increased amino acid extraction, urea cycle reactions, and several reactions involved in oxidative phosphorylation. These results suggest that increasing the severity of injury does not lead to a simple dose-dependent metabolic response, but rather leads to qualitatively different responses.