Garrison Sposito - US grants

The objective of this project is to quantify and determine the stability of aluminum-organic complexes that can occur in water draining from forested drainage basins that are impacted by acid deposition. Water-soluble organic compounds derived from forest canopy litter can play a critical role in the mobilization of aluminum in forest soils, but relatively little is known concerning their complexes with aluminum. In this project, a systematic investigation of these complexes is being undertaken involving studies of the acid-base chemistry of water-soluble forest leaf litter; studies of aluminum complexes with leaf litter ligands using infrared, electron spin resonance, and fluorescence excitation spectroscopy; comparative, quantitative studies of aluminum-leaf litter complexes with compleximetric and fluorimetric methods, and calculation of pH-independent stability constants for aluminum-leaf litter ligand complexes. The chemical speciation of aluminum in natural waters has emerged as a problem of critical importance because of the effect that acidic deposition from atmospheric pollutants has on solution of aluminum compounds in soil, and the impact of those compounds on water quality. This research may lead to insights into how water may be decontaminated by proper engineering design and management of drainage basins.

This project will continue to research of an interdisciplinary team to establish an accurate theoretical model of the electrical double layer at the surface of 2:1 clay minerals suspended in 1-1 and 2-1 electrolyte solutions. The research proposed has become feasible recently because of important, new results obtained by the Principal Investigators and other researchers. The renewal project objectives are: (I) To perform Monte Carlo simulations of the ion distributions in a 1-1 electrolyte near the surface of a 2:1 clay mineral, including fully molecular-scale water-water, water-ion, and ion-ion interactions. (II) To perform statistical mechanical calculations of the interactions ("swelling pressure") between planar charged surfaces immersed in 1-1, 2-1, and 1-2-1 electrolyte solutions. The results obtained will provide a significantly-improved, quantitative foundation for the surface chemistry of 2:1 clay minerals as it occurs typically in mineral weathering and diagenesis.

This project will bring together an interdisciplinary team in theoretical hydrology to address the problem of defining rigorously the relation between macroscopic, catchment-scale parameters describing runoff generation, net recharge, or groundwater flow through scaling procedures applied to the non-linear mathematical description of the underlying, local-scale physics. The project objectives are: (1) To drive, using rigorous methods in the analysis of spatial data, a catchment-scale, process-based model of streamflow generation in systems that are highly variable at the local scale. (2) To develop improved methods of conditioning stochastic models of catchment-scale spatial variability. (3) To investigate the general consequences of scale-invariance in the local-scale velocity field for predictive models of subsurface water flows in heterogenous media. The results obtained will contribute to understanding the scale dependence and scale invariance of fundamental hydrologic processes as they are resolved from the field plot experiment to the regional or drainage-basin response.//

New laboratory methods developed by the Principal Investigator to characterize the components of surface charge will be applied to benchmark soils from the tropical Americas in order to test their sensitivity and ability to provide reproducible variable-charge measurements that are accurate for highly heterogeneous, structurally fragile mineral mixtures. The project objectives are: (1) to apply a cesium-adsorption method to measure structural surface charge, and (2) to apply new titration and ion adsorption methods to measure variable-charge behavior in highly weathered soils. If these applications prove to be successful, they will provide the basis for a systematic approach to chemical modeling of tropical soils related to their role in continental-scale weathering and the global-scale cycling of chemical elements.

This award is under the International Postdoctoral Fellows Program, which enables U.S. scientists and engineers to conduct three to twelve months of research at foreign centers of proven excellence. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award will support a twelve-month postdoctoral research visit by Dr. Jon D. Chorover of the University of California, Berkeley, to work with Professor Jacques Buffle of the University of Geneva in Switzerland. The transport of chemicals in natural aquatic systems is regulated by the partitioning of solutes between the solid and aqueous phases. Colloids, the smallest of the solid particles, may be readily entrained and transported by the mobile fluid, thereby providing transport for sorbed inorganic and organic adsorptives. Although colloid mediated transport is considered an important mechanism for contaminant transport, the basic processes affecting colloidal behavior are poorly understood. Organic-polymer/inoranic colloid associations are ubiquitous in natural aquatic and soil systems and these aggregates have profoundly different colloidal properties than the inorganic particles alone. The technology for in situ study of colloidal systems has improved dramatically over the last decade, yet the effects of experimental artifacts are not well known. Careful investigations of colloidal structure and stability therefore incorporate several complimentary analyses. In the proposed project, Dr. Chorover and Professor Buffle will study interactions between anionic macropolymers and inorganic microcolloids. The award recommendation provides funds to cover international travel, a stipend for twelve months, travel to meetings, and a dependent allowance for Dr. Chorover's family.

Sposito 9505629 Simulation methodologies developed recently by the Principal Investigator and his coworkers will be applied to perform Monte Carlo (MC) and molecular dynamics (MD) calculations of molecular configurations at the interface between 2:1 clay minerals and aqueous solution. Quantum chemical calculations of clay mineral molecular structure also will be performed to examine the accuracy of the potential functions used in the simulations. The project objectives are: (1) to perform and evaluate MC and MD simulations of hydrated Li+, Na+, K+, Cs2+, and Mg2+-saturated 2:1 clay minerals to deduce the configurations, mobilities, and reactivities of adsorbed water molecules and counter-ions; (2) to perform ab initio quantum chemical calculations of the counter -ion-clay mineral potential energy surface for monovalent and bivalent cation-saturated 2:1 clay minerals to improve the accuracy of the MC/MD simulations. The results will be compared with a variety of experimental data, and will provide general comparative insights as to the structure of the interface between 2:1 clay minerals and aqueous solutions that are relevant to low-temperature geochemical weathering processes.

9704630 Sposito This project entails the application of dynamical systems theory to the problem of groundwater flow and transport in an isotropic aquifer of arbitrary spatial heterogeneity. The overall objective of the project is to provide new physical concepts that permit greater advantage to be taken of recent breakthroughs in experimental methodology and computer simulation of field-scale groundwater movement. The specific objectives of the project are: To calculate and interpret the vorticity field for steady groundwater flow in a stratified aquifer, based on field measurements of the hydraulic conductivity, and to develop and "extended phase" representation of this flow. To establish general conditions under which tracer-plume spatial concentration moments modeled as averages over an ensemble of advective flow trajectories can be equated to plume concentration moments determined from a single field experiment. To investigate, using the methods of dynamical systems theory, conditions under which chaotic motions of a tracer following advective flowpaths can be induced, leading to more efficient tracer spreading in a heterogeneous aquifer. These three objectives, if met successfully, will result in the following technical advances: (1) a confirmation of the hypothesis that the flow topology of an arbitrary, steady groundwater flow can always be transformed mathematically to that of steady flow in a perfectly-stratified aquifer; (2) precise criteria as to when stochastic transport models can be applied accurately to solute transport data from a single field experiment; and (3) a strategy for using chaotic advection to improve contaminant solute spreading in groundwater. One postdoctoral researcher, Dr. Scott Weeks (Ph.D. in Applied Mathematics), will be supported by project funds.

DEB 0543558
W. L. Silver, M. K. Firestone, and G. Sposito

Iron Redox Biogeochemistry: Controls on Carbon and Phosphorus Cycling in Humid Tropical Forests

Carbon plays a central role in global climate change, and the growth, death, and decomposition of biota in humid tropical forests have a large impact on the global carbon cycle. Tropical forests dominantly occur on highly weathered soils rich in iron and low in phosphorus. Phosphorus is considered the nutrient most commonly limiting to net primary productivity in the tropics. Modeling studies, however, suggest that the vegetation uses considerably more phosphorus than is thought to be available in soils. Iron minerals can sequester phosphorus under aerobic conditions, and release it via iron reduction under anaerobic conditions. Iron reduction may also contribute significantly to the release of carbon dioxide, a greenhouse gas, during anaerobic periods. Fluctuating redox conditions are common in these soils, allowing iron to be rapidly regenerated during oxic periods and drive phosphorus and carbon dioxide release during the anoxic phases. The overarching hypothesis of the proposed research is that the redox biogeochemistry and microbiology of iron provide a key to understanding the phosphorus bioavailability paradox and the cycling of phosphorus and carbon in highly weathered soils.

This project will train Puerto Rican high school students in field-based scientific research, and help fund summer research apprenticeships as part of the Geballe Research Opportunities for Undergraduates Program. The proposed research will also train two undergraduate students, two graduate students, and two post doctoral scientists.