Donald L. Suarez

Modeling of carbon dioxide transport and production in soil: 1. Model development

Knowledge of the CO2 concentration in the unsaturated zone is essential for prediction of solution chemistry in the vadose zone and groundwater recharge as well as for quantifying carbon source/sink terms as part of the global CO2 mass balance. In this paper we present a predictive simulation model, SOILCO2, based on process-oriented relationships. The model includes one-dimensional water flow and multiphase transport of CO2 utilizing the Richards and the convection-dispersion equations, respectively, as well as heat flow and a CO2 production model. The transport of CO2 in the unsaturated zone can occur in both the liquid and gas phases. The gas transport equation accounts for production of CO2 and uptake of CO2 by plant roots associated with root water uptake. The CO2 production model considers both microbial and root respiration which is dependent on water content, temperature, growth, salinity and plant and soil characteristics. Heat flow is included, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.

The use of a surface complexation model to describe the kinetics of ligand-promoted dissolution of anorthite

The dissolution of anorthite (CaAl2Si208) in the presence of fluoride and oxalate was studied under controlled pH and CO2 conditions. The dissolution of anorthite in the absence of complex-forming ligands was nearly pH independent between pH 5 and 9. The presence of complex-forming ligands resulted in a dissolution rate that increased linearly with decreasing solution pH. The dissolution data were modeled using the theory that the reaction rate is proportional to the surface concentration of activated sites. Surface activated sites are formed by the adsorption of protons and complexing ligands. On anorthite, the proton-promoted activated sites are probably Si centered and the ligand-promoted activated sites Al centered. Two surface chemistry models were combined in order to describe the overall reaction. At pH values less than 4.2, the rate of the proton-promoted dissolution was linearly proportional to Γ4, where Γ is the surface concentration of adsorbed protons. The rate of the ligand-promoted dissolution was found to be linearly related to the surface concentration of adsorbed ligand. These findings suggest that for most natural systems, pH and complexing ligands have a synergistic effect on feldspar dissolution. The observation in nature that Ca-rich plagioclase feldspars are less resistant to weathering than Na-rich feldspars can be attributed to the increased reactivity of organic ligands towards Ca-rich feldspars. The increased reactivity is attributed to the higher proportion of Al in the Ca-feldspars.

Coordination of Adsorbed Boron: A FTIR Spectroscopic Study

We studied B adsorption on amorphous aluminum and iron hydroxides, allophane, and kaolinite as a function of pH and initial B concentration. Boron adsorption lowered the point of zero charge of all four adsorbents, implying specific adsorption (inner-sphere complexation) of B. We provided novel information on the coordination of B adsorbed at mineral-water interfaces by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The ATR-FTIR spectra of interfacial B species were influenced by pH and mineral type. Strong trigonal B and weak tetrahedral B bands of the asymmetric stretching mode were observed on the difference spectra at pH 7 for amorphous iron hydroxide, whereas both strong trigonal and tetrahedral B bands were found at pH 10. A strong IR band of asymmetric stretching of tetrahedral B shifted to higher frequencies in am-Fe(OH) 3 paste at both pHs relative to that of boric acid solution at pH 11. Trigonal B asymmetric stretching bands shifted to higher frequencies on the difference spectra for am-Al(OH) 3 and allophane at both pHs compared to that of boric acid solution at pH 7. Polymerization of B on mineral surfaces is shown to be possible. The results provide spectroscopic evidence that both B(OH) 3 and B(OH) 4 - are adsorbed via a ligand exchange mechanism.

TWO-DIMENSIONAL TRANSPORT MODEL FOR VARIABLY SATURATED POROUS MEDIA WITH MAJOR ION CHEMISTRY

We present the development and demonstrate the use of the two-dimensional finite element code UNSATCHEM-2D for modeling major ion equilibrium and kinetic nonequilibrium chemistry in variably saturated porous media. The model is intended for prediction of major ion chemistry and water and solute fluxes for soils under transient conditions. Since the solution chemistry in the unsaturated zone is significantly influenced by variations in water content, temperature, and CO2 concentrations in the soil gas, all these variables are also calculated by the model. The major variables of the chemical system are Ca, Mg, Na, K, SO4, Cl, NO3, alkalinity, and CO2. The model accounts for equilibrium chemical reactions between these components such as complexation, cation exchange, and precipitation-dissolution. For the precipitation-dissolution of calcite and dissolution of dolomite, either equilibrium or multicomponent kinetic expressions are used which include both forward and back reactions. Other dissolution-precipitation reactions considered include gypsum, hydromagnesite, and nesquehonite. Since the ionic strength of soil solutions can often reach high values, both modified Debye-Huckel and Pitzer expressions were incorporated into the model to calculate single ion activities. The need for an iterative coupling procedure between the solute transport and chemical modules is demonstrated with an example which considers root water uptake and irrigation using moderately saline water. The utility of the model is further illustrated with two-dimensional simulations with surface and subsurface irrigation from a line source.

Factors affecting clay dispersion and aggregate stability of arid-zone soils

We investigated the stability of 34 aridzone toil samples from 15 soil series, using clay dispersion and aggregate stability as structural indexes. The study evaluated clay dispersion and aggregate stability as affected by: pH, electrical conductivity, sodium adsorption ratio, soluble silica, cation exchange capacity, exchangeable sodium percentage, inorganic carbon, organic carbon, free iron oxide, free aluminum oxide, clay, surface area, quartz, kaolinite, illite, chlorite, vermiculite, and montmorillonite. The most significant single-variable linear regressions were obtained for percentage of clay dispersed versus log (montmorillonite) (r2 = 0.52**) and for percentage of aggregate stability versus organic carbon (r2 = 0.27**). Significant variables for multiple linear regression for percentage of clay dispersed were montmorillonite, exchangeable sodium percentage, and electrical conductivity (r2 = 0.67**). For percentage of aggregate stability, significant variables in the multiple linear regression were quartz, montmorillonite, and surface area (r2 = 0.49**). Principal factor analysis results indicated that the structural indexes were related most to the soil variables stabilizing structure by physically binding particles. These binding agents are aluminum and iron oxides and organic matter.

Some factors affecting the dissolution kinetics of anorthite at 25°C

Abstract Batch dissolution experiments were conducted at 25°C to determine the effects of agitation, particle size, suspension density, wetting and drying cycles, drying temperature, sequential rinses, ionic strength, and the addition and removal of products on the rates of anorthite (An93) dissolution. In general, the dissolution kinetics at constant pH were not zero order with respect to products in solution, and this nonlinear release persisted beyond the time when Ca and Si stoichiometric dissolution was reached. The failure to establish zero-order kinetics could not be attributed to the weathering of damaged surfaces or fine, broken particles. Leached layer depths, calculated from solution composition, ranged from 2.6–3.5 nm; but a Ca-depleted surface layer was observed by energy dispersive X-ray analysis only on the reaction fines. Agitation rate had a marked effect on dissolution rate, contrary to expectations based on a surface reaction control mechanism. Anorthite dissolution in the presence of cation- and anion-exchange resins produced zero-order kinetics at sustained high rates. We hypothesize that these linear rates were due to the continuous removal of Al from solution by the resins. Consistent with these results, there was no effect of added Ca or Si on the rate of reaction; but the addition of Al slowed the initial rate of reaction at pH 3.6 and 6.0 but not at pH 3.0. Long-term dissolution studies (up to 4.5 ys) resulted in final reaction rates over two hundred times slower than previously reported for feldspar dissolution. These data are consistent with the idea that the presence of Al in solution and the incorporation of Al into the hydrous silanol surface slow the rate of anorthite dissolution and are important factors affecting the rate over all time periods. The addition of KCl slowed the dissolution rate either through competitive exchange with structural Ca or adsorbed H, or by blocking the polymerization reactions at the surface.

Modeling of carbon dioxide transport and production in soil: 2. Parameter selection, sensitivity analysis, and comparison of model predictions to field data

In paper 1 of this two-paper series (Simůnek and Suarez, this issue) we presented a description of the numerical model, SOILCO2, for CO2 transport and production in the unsaturated zone. In paper 2 the model sensitivity to various parameters is evaluated by both steady state and transient simulations, with a range in the parameter values typically found under field conditions. We also select parameter values for optimal plant and microbial CO2 production and production dependence on temperature, water content, osmotic potential and gas composition for plant and microbial respiration, all based on literature review. The predictive capabilities of the SOILCO2 model are evaluated by comparing model simulations to published field data from Missouri for three different crops and two growing seasons under transient conditions as well as a data set collected in Riverside, California, under relatively constant water content at depth. The model provided good prediction of the CO2 flux to the atmosphere as well as the concentrations in the root zone for all data sets.

Biomass accumulation and potential nutritive value of some forages irrigated with saline-sodic drainage water

A controlled study using a sand-tank system was conducted to evaluate 10 forage species (bermudagrass, ‘Salado’ and ‘SW 9720’ alfalfa, ‘Duncan’ and ‘Polo’ Paspalum, ‘big’ and ‘narrow leaf’ trefoil, kikuyugrass, Jose tall wheatgrass, and alkali sacaton). Forages were irrigated with sodium-sulfate dominated synthetic drainage waters with an electrical conductivity of either 15 or 25 dS/m. Forage yield was significantly reduced by the higher (25 dS/m) salinity level of irrigation water compared to the lower (15 dS/m) level. There was wide variation in the sensitivity of forage species to levels of salinity in irrigation water as reflected by biomass accumulation. With the exception of bermudagrass, which increased accumulation at the higher level of salinity, and big trefoil, which failed to establish at the higher level of salinity, ranking of forages according to the percent reduction in biomass accumulation due to the higher level of salinity of irrigation water was: Salado alfalfa (54%) = SW 9720 alfalfa (52%) > Duncan Paspalum (41%) > narrow leaf trefoil (30%) > alkali sacaton (24%) > Polo Paspalum (16%) > Jose tall wheatgrass (11%) = kikuyugrass (11%). Bermudagrass and Duncan Paspalum were judged to be the best species in terms of forage yield and nutritive quality. Kikuyugrass, which had the third highest biomass accumulation, was judged to be unacceptable due to its poor nutritional quality. Although narrow leaf trefoil had a relatively high nutritional quality, its biomass accumulation potential was judged to be unacceptably low. Alfalfa cultivar’s biomass accumulations were the most sensitive to the higher level of salinity, among forages that survived at the higher salinity level, although actual accumulations at the higher salinity were high relative to other forages. Increased salinity influenced several forage quality parameters, including organic matter (OM), crude protein (CP), neutral detergent fibre (NDF), and in vitro gas production, generally leading to higher nutritional quality at the higher salinity level, although their significance varied amongst species and cuttings.

Effect of high boron application on boron content and growth of melons

Management options for reducing drainage water volumes on the west side of the San Joaquin Valley of California, such as reuse of saline drainage water and water table control, have the potential to adversely impact crop yields due to a build up in soil solution boron concentration. An earlier experiment had shown that extrapolation of B soil tests to field conditions provided poor predictability of B content of melons despite statistically significant relationships. Consequently, three tests for extractable soil B were evaluated for their ability to predict conditions of potential B toxicity in melons grown under controlled conditions. Melons were grown for 95 days in two consecutive years in containers of Lillis soil (very-fine, smectitic, thermic Halic Haploxerert) that had been pretreated with solutions containing B concentrations as great as 5.3 mmol L−1. Extractable soil B was determined using ammonium acetate, DTPA-sorbitol, and a 1:1 aqueous soil extract at the beginning and end of the experiment. The B treatments caused various deleterious effects on melon growth and development. Fresh and dry plant matter decreased significantly with increasing B concentrations, while B concentration of plant leaves, stems, and fruits increased significantly with increasing B. The number of days to first flowering was significantly delayed from 35 days at B treatments < 2 mmol L−1 to 51 days at B treatments > 3 mmol L−1. Fruit set was completely inhibited at the highest B treatment of 5.3 mmol L−1. Plant analysis revealed a highly significant relationship between soil extract B obtained with all three extractants and leaf, stem, and fruit B content. Correlation coefficients for plant stems and fruits were much higher than for plant leaves. Correlation coefficients for all soil tests were almost equivalent, although the highest values were obtained for the DTPA-sorbitol extract indicating the greatest predictive capability. The soil tests were well able to predict B damage to melons in a container experiment.

Saturated hydraulic conductivity prediction from microscopic pore geometry measurements and neural network analysis

Traditional models to describe hydraulic properties in soils are constrained by the assumption of cylindrical capillarity to account for the geometry of the pore space. This study was conducted to develop a new methodology to directly measure the porosity and its microscopic characteristics. The methodology is based on the analysis of binary images collected with a backscattered electron detector from thin sections of soils. Pore surface area, perimeter, roughness, circularity, and maximum and average diameter were quantified in 36 thin sections prepared from undisturbed soils. Saturated hydraulic conductivity Ksat, particle size distribution, particle density, bulk density, and chemical properties were determined on the same cores. We used the Kozeny-Carman equation and neural network and bootstrap analysis to predict a formation factor from microscopic, macroscopic, and chemical data. The predicted Ksat was in excellent agreement with the measured Ksat (R2=0.91) when a hydraulic radius rH defined as pore area divided by pore perimeter and the formation factor were included in the Kozeny-Carman equation.