James R. Kercher

Mineralogical heterogeneity in fractured, porous media and its representation in reactive transport models

Abstract Reactive transport models that simulate processes in porous media have, generally, required abstracted representation of porosity, permeability, and mineralogy. This study compares abstracted, homogeneous representations of porosity and permeability, mineral surface areas and distributions, to discrete distribution representation of these same properties. Discretization was accomplished by high-resolution (ca. 1 μm2) characterization of fractured tuffaceous rock from Yucca Mountain, Nevada, using optical microscopy and X-ray fluorescence spectroscopy. A sample area of 106 μm2 was mapped in detail, and the resulting element and porosity maps were digitized. The domain was decomposed into 12,208 cells that were 8.77 × 10−6 m on a side. Simulations were conducted in which a dilute fluid enters the discretized porous medium at modest flow rates. Simulation results using a discrete mineral distribution point to the conclusion that slow flow rates, in which fluid residence times are on the order of days, provide fluid composition results that are very similar to those obtained from the homogeneous mineral distribution representation. At higher flow rates, where fluid residence times are on the order of hours, contrasts in fluid composition persist throughout the flow domain. The results demonstrate that the fluid composition characteristics in the homogeneous and discrete mineral representations will be similar only when the bulk average contact times for the individual mineral phases along the flow paths are approximately equivalent (within a few percent) for the two cases.

Analysis of silva: A model for forecasting the effects of SO2 pollution and fire on western coniferous forests☆

Abstract A forest succession simulator, SILVA, has been developed for the mixed-conifer forest type of the Sierra Nevada, California, to simulate the effects of SO 2 and fire on forest dynamics. SILVA was developed by extensively modifying a northeastern U.S. simulator. The state variables of the model are the diameters at breast height (dbh) of each tree on a forest stand. Ponderosa pine is found to be a relatively stable dominant for the site used in the simulations. White fir and sugar pine are relatively stable subordinate species. Incense-cedar shows a slowly fluctuating time-series. Sensitivity analyses suggest that parameters determining growth rates are of major importance and changes in such parameters can often produce a relative effect on basal area larger than their relative change. Factor affecting fire induced mortality are of lesser importance. The effects of competition change the relative magnitude of the calculated sensitivities during the time course of the simulation. Relative rankings of parameters according to their sensitivities also change during the time course of the simulation. Those parameters that exhibit large changes in sensitivity are also important in determining the outcome of competition. We investigated the convergence of the means of the time series of each species. The dominant ponderosa pine converged relatively rapidly with the number of runs. The subordinate species such as white fir converged much more slowly.

Parameter estimation for a global model of terrestrial biogeochemical cycling by an iterative method

Abstract We have implemented a geographically distributed ecosystem model TERRA for the carbon, nitrogen, and water dynamics of the global terrestrial biosphere. The ecosystem model in each grid cell has state variables of soil water; vegetation carbon, soil carbon, vegetation nitrogen, soil organic nitrogen, soil inorganic nitrogen, and a variable for allocation. Eight parameters associated with eight carbon or nitrogen fluxes are determined during model calibration at specific sites for each of 17 vegetation types that cover the globe. Calibration is performed by an iterative method that brings calculated fluxes into agreement with observed fluxes over successive iterations. For the 17 vegetation types, calibration required a geometric mean number of 123 iterations for convergence with a geometric S.D. of 2.25. The minimum and maximum iterations required was 52 and 1060 for xeromorphic woodland and tropical evergreen forest, respectively. The parameter controlling gross primary productivity Cmax was found to be correlated with maximum projected leaf area index (plus cover of nonvascular plants) with a correlation coefficient of 0.90 (r2=0.81). Correlation of parameters with the input fluxes and average standing crops corresponding to the iteration equation was high in those cases in which temperature and precipitation were not explicit factors in the iteration equation. The parameters Kfall, Lnc, and Nloss had correlation coefficients of 1.0 each with the appropriate ratio of the matching observed transfer flux and standing crop. In iteration equations with explicit dependence on temperature, soil moisture, and other factors, the correlation coefficients were less than 1.0 with these ratios; the parameters Cmax, Kr, Kd, Nmax, and Nup had correlation coefficients of 0.83, 0.66, 0.60, 0.61, and 0.87, respectively. These correlation coefficients indicate the importance of environmental factors such as temperature and soil moisture in the calibration process and the robustness of the iteration method in determining parameters in systems with time-varying coefficients.

The spatial extent of contaminants and the landscape scale: An analysis of the wildlife, conservation biology, and population modeling literature

Many contaminant releases to the terrestrial environment are of small areal extent. Thus, rather than evaluating the ecological impact on species in the immediate vicinity of the release, it may be more ecologically meaningful to determine if population impacts occur at the landscape level. In order to do this, the cumulative impact of all releases in the landscape under consideration must be evaluated. If the release sites are viewed as localized areas that are no longer available for use by ecological receptors (i.e., no longer part of the habitat), this can be thought of as a form of habitat fragmentation. Habitat fragmentation is typically viewed as the loss of large areas of habitat within a landscape, leaving small isolated patches of intact habitat within a hostile matrix. Small-scale contaminant releases, on the other hand, result in small uninhabitable areas within a primarily intact habitat. With this consideration in mind, we analyzed the wildlife and conservation biology literature to determine if information on habitat size requirements such as home-range or critical patch size could inform us about the potential for impact at the landscape level from release sites based on the size of the release alone. We determined that evaluating the impact of release size had to be conducted within a contextual basis (considering the existing state of the landscape). Therefore, we also reviewed the population modeling literature to determine if models could be developed to further evaluate the impact of the spatial extent of chemical releases on the landscape. We identified individual-based models linked to geographic information systems to have the greatest potential in investigating the role of release size with respect to population impacts at the landscape level.

The impact of climate change on vadose zone pore waters and its implication for long-term monitoring

Protecting groundwater is of growing interest as pressure on these resources grows. Recharge of groundwater takes place through the vadose zone, where complex interactions between thermal-hydrological-geochemical processes affect water quality. Monitoring processes in the vadose zone is an important means of evaluating the long-term health of aquifer systems, and has become an integral part of many subsurface engineering efforts. Monitoring such systems, however, may be affected by changes in climate that slowly propagate through vadose zone systems. We describe in this paper the use of NUFT-C, a reactive transport simulator designed to run on a high performance, massively parallel computer, to compare quantitatively the evolution of a deep vadose zone with changes expected from an engineered high-level nuclear waste repository. The results suggest that the impacts from waste emplacement are, in some instances, similar to those that would be observed as a result of climate change, whereas others are distinguishable from evolution of the natural system. Such simulations facilitate design of long-term monitoring programs that take account of these complex effects. The results emphasize the importance of developing long-term baseline measurements and control sites, in order to enhance confidence in interpretations of complexly evolving data sets that will be obtained from multidecade monitoring efforts.

Closed-form solutions to sensitivity equations in the frequency and time domains for linear models of ecosystems

Abstract We derive closed-form solutions for the properties of linear, donor-controlled, constant-coefficient models of ecosystems. The properties solved for are the frequency response, frequency response under feedback, and sensitivities of the frequency response and the time-domain solution to changes in model parameters. The last is known as transient sensitivity. All solutions are in expansions in the eigenvectors, with the eigenvalues determining the analytic properties in the frequency domain and the transient behavior in the time domain. The computer implementation produces fast code, because the eigenvalues and eigenvectors need only be found once to calculate the results for many different times or frequencies. All solutions are valid for degenerate eigenvalues, provided that linearly independent eigenvectors can be found by the subroutines that solve the eigenvalue equation.

Toward an Ecological Framework for Assessing Risk to Vertebrate Populations from Brine and Petroleum Spills at Exploration and Production Sites

REFERENCE: Efroymson, R. A., Carlsen, T. M., Jager, H. I., Kostova, T., Carr, E. A., Hargrove, W. W., Kercher, J., and Ashwood, T. L., “Toward a Framework for Assessing Risk to Vertebrate Populations from Brine and Petroleum Spills at Exploration and Production Sites,” Landscape Ecology and Wildlife Habitat Evaluation: Critical Information for Ecological Risk Assessment, Land-Use Management Activities, and Biodiversity Enhancement Practices, ASTM STP 1458, L. Kapustka, H. Galbraith, M. Luxon, and G. R. Biddinger, Eds., ASTM International, West Conshohocken, PA, 2004. ABSTRACT: Brine and petroleum spills may affect terrestrial vertebrates through loss of reproductive habitat or reduced food availability rather than direct toxicity. A proposed ecological framework for evaluating impacts of these spills includes individual-based population models, a site conceptual trophic model, habitat suitability maps, and a stochastic brine spill generator. Simulation results for mammal populations in the Tallgrass Prairie Preserve petroleum exploration and production (EP above this threshold the time to extinction decreased with increasing spill area. Vole density was sensitive to the interaction of predation and fragmentation, with fragmentation causing population extinction in the presence of predation, yet stabilizing the population in the absence of predation. We anticipate that our results will aid in future development of “exclusion criteria” for leaving unrestored habitat at E&P sites.