Toshiko Komatsu

PREDICTIVE-DESCRIPTIVE MODELS FOR GAS AND SOLUTE DIFFUSION COEFFICIENTS IN VARIABLY SATURATED POROUS MEDIA COUPLED TO PORE-SIZE DISTRIBUTION: I. GAS DIFFUSIVITY IN REPACKED SOIL

The soil gas and solute diffusion coefficients and their dependency on soil total porosity (&PHgr;), fluid-phase (air or water) contents, and pore-size distribution largely control chemical release, transport, and fate in soil. The diffusion coefficients hereby play a key role in both local and global environmental issues including spreading, biodegradation and volatilization of hazardous chemicals at polluted soil sites, and soil uptake, production, and emission of greenhouse gases. In a series of papers, we present new advances in describing and predicting the gas and solute diffusion coefficients in variably saturated porous media, carefully distinguishing between repacked and undisturbed media. Also, we establish direct links between gas and solute diffusivity and pore-size distribution, with further links to pore continuity and tortuosity. In this first paper, a porosity correction term is added to a recently presented model for predicting gas diffusivity in repacked soil. The obtained POrosity-Enhanced (POE) model assumes that increased &PHgr; creates additional interconnectivity between air-filled pores. The POE model is tested against data for 18 repacked soils ranging from 0 to 54% clay, including new data measured in this study for both noncompacted and compacted, high-porosity soils. The POE model accurately predicts gas diffusivity across a wide &PHgr; range up to 0.75 m3 m−3, whereas the original model is accurate only for &PHgr; up to 0.55 m3 m−3. A unifying, two-parameter function for gaseous phase pore continuity (fg) is suggested. The fg function illustrates developments in gas diffusivity models during the last century, including assumptions behind the increasingly precise prediction models for repacked soil. Last, the POE model is coupled with the widely used van Genuchten (vG) soil-water characteristic model, hereby establishing an accurate and predictive link between soil gas diffusivity and pore-size distribution. The closed-form POE-vG gas diffusivity model is highly useful to evaluate effects of pore-size distribution and soil type on gas diffusivity and gas transport in repacked soil systems.

PREDICTIVE-DESCRIPTIVE MODELS FOR GAS AND SOLUTE DIFFUSION COEFFICIENTS IN VARIABLY SATURATED POROUS MEDIA COUPLED TO PORE-SIZE DISTRIBUTION: III. INACTIVE PORE SPACE INTERPRETATIONS OF GAS DIFFUSIVITY

Accurate description of the soil-gas diffusion coefficient (DP) as a function of air-filled (&egr;) and total (&PHgr;) porosities is required for studies of gas transport and fate processes. After presenting predictive models for DP in repacked and undisturbed soils (Part I and II), this third paper takes a more descriptive approach allowing for the inclusion of inactive air-filled pore space, &egr;in. Three model-based interpretations of &egr;in are presented: (1) a simple power-law model (labeled Millington-Call) with the exponent (V) taken from Millington (1959; Science 130:100-102), and expanded with a constant &egr;in term (= 0.1 m3 m−3), (2) a model (SOLA) based on analogy with solute diffusion and assuming a linear increase in pore continuity from zero at the threshold air-filled porosity where gas diffusion ceases (&egr;th) to a maximum at &egr; = &PHgr;, (3) a power-law model (VIPS) assuming variable &egr;in that linearly decreases from a maximum at &egr; = &egr;th to zero at &egr; = &PHgr;. Assuming &egr;th = 0.1 m3 m−3, all three models satisfactorily predicted DP in 18 repacked soils. The difference between the three models is mainly pronounced for higher-&PHgr; soils, and each model has its own advantage. The SOLA model together with similar models for solute diffusivity allows a direct comparison of pore continuity in the soil gaseous and liquid phases, suggesting large differences in tortuosity and inactive fluid-phase between the two phases. The low-parameter Millington-Call model could account for variability in measured DP along a field transect (Yolo, California) by varying &egr;in with ±0.03 m3 m−3 and is applicable for stochastic gas transport simulations at field scale. The mathematically flexible VIPS model highly accurately fitted DP(&egr;) data for undisturbed soil, illustrating the large possible variations in &egr;th and V. The VIPS model is coupled with the van Genuchten (vG) soil-water characteristic model, yielding a closed-form expression for DP as a function of soil-water matric potential. The VIPS-vG model is useful to illustrate the combined effects of pore size distribution and inactive pore space on soil-gas diffusivity.

Gas transport parameters along field transects of a volcanic ash soil

Variations in gas transport parameters at the field scale govern the transport, fate, and emission of greenhouse gases and volatile organic chemicals in soil. In this study, we evaluated predictive models for soil-gas diffusivity (Dp/Do) and air permeability (ka) based on measurements along a 117-m transect and a parallel 33-m transect of a humic volcanic ash soil (Andisol) in Nishi-Tokyo, Japan. Measurements were done on 100-cm3 undisturbed soil samples, with 3-m spacing between sampling points, and included water retention, soil-gas diffusion coefficient (Dp), ka at different soil-water matric potentials, and saturated hydraulic conductivity. Traditionally used predictive gas diffusivity models underestimated Dp/Do in wet soil and largely overestimated Dp/Do under dry conditions because of soil aggregation effects. A linear model for Dp/Do as a function of air-filled porosity (ϵ), taking into account inactive/remote air-filled pore space, accurately described Dp(ϵ)/Do from wet to oven-dry conditions and well captured the spatial variations in Dp/Do along the transects. The ka exhibited a nonlinear relation with ϵ, and ka(ϵ) was best predicted from a recently presented power-law model, with measured ka at −100 cm H2O of soil-water matric potential (ka,100) as a reference point. Trends of decreasing soil-water retention and increasing ϵ along transects were observed. Similar trends in ka and saturated hydraulic conductivity were not observed because the convective fluid transport parameters were mainly governed by soil structure and not by fluid phase contents. Autocorrelograms suggested a spatial correlation range of 10 to 20 m for gas transport parameters (Dp/Do and ka). Measurements of ϵ and ka at conditions close to −100 cm H2O of soil-water matric potential are suggested for rapid assessment of the magnitude and spatial variations in gas transport properties at the field scale.

Extreme Compaction Effects on Gas Transport Parameters and Estimated Climate Gas Exchange for a Landfill Final Cover Soil

Landfill sites have been implicated in greenhouse warming scenarios as a significant source of atmospheric methane. In this study, the effects of extreme compaction on the two main soil-gas transport parameters, the gas diffusion coefficient (Dp) and the intrinsic air permeability (ka), and the cumulative methane oxidation rate in a landfill cover soil were investigated. Extremely compacted landfill cover soil exhibited negligible inactive soil-air contents for both Dp and ka. In addition, greater Dp and ka were observed as compared with normal compacted soils at the same soil-air content (e), likely because of reduced water-blockage effects under extreme compaction. These phenomena are not included in existing predictive models for Dp(e) and ka(e). On the basis of the measured data, new predictive models for Dp(e) and ka(e) were developed with model parameters (representing air-filled pore connectivity and water-blockage effects) expressed as functions of dry density (ρb). The developed Dp(e) and ka(e) m...

Correlating Gas Transport Parameters and X-Ray Computed Tomography Measurements in Porous Media

Abstract Gas transport parameters and X-ray computed tomography (CT) measurements in porous medium under controlled and identical conditions provide a useful methodology for studying the relationships among them, ultimately leading to a better understanding of subsurface gaseous transport and other soil physical processes. The objective of this study was to characterize the relationships between gas transport parameters and soil-pore geometry revealed by X-ray CT. Sands of different shapes with a mean particle diameter (d50) ranging from 0.19 to 1.51 mm were used as porous media under both air-dried and partially saturated conditions. Gas transport parameters including gas dispersivity (&agr;), diffusivity (DP/D0), and permeability (ka) were measured using a unified measurement system (UMS). The 3DMA-Rock computational package was used for analysis of three-dimensional CT data. A strong linear relationship was found between &agr; and tortuosity calculated from gas transport parameters ( ), indicating that gas dispersivity has a linear and inverse relationship with gas diffusivity. A linear relationship was also found between ka and d50/TUMS2, indicating a strong dependency of ka on mean particle size and direct correlation with gas diffusivity. Tortuosity (TMFX) and equivalent pore diameter (deq.MFX) analyzed from microfocus X-ray CT increased linearly with increasing d50 for both Granusil and Accusand and further showing no effect of particle shape. The TUMS values showed reasonably good agreement with TMFX values. The ka showed a strong relationship when plotted against deq.MFX/TMFX2, indicating its strong dependency on pore size distribution and tortuosity of pore space.

USEFUL SOIL-WATER REPELLENCY INDICES : LINEAR CORRELATIONS

Water repellency (WR) has been classically characterized at fixed (usually oven-dry) soil water content (&thgr;g) in terms of the soil water contact angle (CA), &agr;. However, &agr; has been previously reported to depend upon &thgr;g in a nonlinear fashion, such that WR increases from a wettable state close to saturation (&thgr;g-min) up to a maximum, &agr;max, decreasing afterward either monotonically or rising again to a second local or absolute &agr; maximum nearby the dried soil state. Hence, a CA versus water content (&agr;-&thgr;g) curve may be described in terms of different WR parameters, such as &thgr;g-min, &thgr;g-max, &agr;max, or the integrated area below the &agr;-&thgr;g curve, S. Based on previous &agr;-&thgr;g measurements carried out with the molarity of an ethanol droplet (MED) test, both in mineral and volcanic soils from different world regions, including cultivated and natural forest soils, and textures ranging from clay-loam to sandy, we confirm here the usefulness of the integrated area below the &agr;-&thgr;g curve (S) as a WR describing index for a large variety of &agr;-&thgr;g curve shapes. We found a simple relationship between S and the soil water content at which WR is triggered, &thgr;g-min, such that S = 16.903 &thgr;g-min (R2 = 0.946), which provides an easy method for the rapid characterization of the overall WR degree of soils. S was also linearly correlated with the soil organic matter (SOM) content (R2 = 0.817) for 1 g (100 g)−1 < SOM < 88 g (100 g)−1, such that the best estimate of S was that obtained by combining linearly both &thgr;g-min and the SOM content (R2 = 0.990). Linear correlations were also found between &thgr;g-max, that is, the soil water content at which &agr; is maximum, and S (R2 = 0.834) or the SOM content (R2 = 0.705), and consequently between &thgr;g-max and &thgr;g-min (R2 = 0.830). In addition, both &thgr;g-min and &thgr;g-max were found to depend linearly upon the soil water content at −33 kPa and −1500 kPa, respectively. Finally, a mean soil WR may be defined as the ratio S/&thgr;g-min. We found that the maximum CA, &agr;max, and the mean soil WR S/&thgr;g-min were positively correlated (R2 = 0.780), such that a particular soil with high (low) values of maximum CA is expected to exhibit a high (low) WR degree on average across the whole water regimen from −33 kPa down to oven-dry moisture. Such an estimate of the mean WR index S/&thgr;g-min was further improved if both &agr;max and the SOM content were available (R2 = 0.825).ABBREVIATIONS CA: contact angle; IRDI: Integrative Repellency Dynamic Index; MED: molarity of an ethanol droplet; SOM: soil organic matter; WDPT: water drop penetration time; WR: water repellency.

Photodegradation of the synthetic fragrance OTNE and the bactericide triclosan adsorbed on dried loamy sand--results from models and experiments.

Fragrances such as OTNE (marketed as Iso-E-Super®) and bactericides such as triclosan (marketed as Igrasan) are present in waste water and thus finally sorbed to sewage sludge. With that sludge they can reach agricultural fields where they potentially can undergo photodegradation processes. In this study the photodegradation of OTNE and triclosan on dried loamy sand was measured under artificial sunlight conditions in laboratory experiments. These compounds were artificially added with concentrations of 1 μg g(-1) on pre-rinsed dried loamy sand. The decrease in concentration with light irradiation was measured for 32d in comparison to soil samples without light irradiation. The estimated light source intensity was 27 W m(-2). Within the experiment, the apparent half-life was 7 and 17d for OTNE and triclosan respectively. The decrease did not simply follow first-order kinetics. The apparent rate constant decreased in the latter stage of reaction, suggesting that part of the chemicals were inaccessible for degradation. Two models, i.e., a diffusion-limited model, and a light penetration-limited model, were used in comparison to the measured data to explain the observed degradation limitations in the latter stages of the experiments. Comparing the hereby obtained model parameters with estimated physico-chemical parameters for the soil and the two chemical compounds, the light penetration-limited model, in which the degradation in the soil surface layer is assumed to be limited due to the shading effect of light in the upper thin soil layer, showed to be the most realistic in describing the photodegradation.

Time-dependency of naphthalene sorption in soil: Simple rate- diffusion- and isotherm-parameter-based models

SORPTION mechanisms and kinetics govern contaminant fate and bioavailability in soil to a great extent. In this study, temporal development of naphthalene sorption was measured on four soils and showed continuously increasing sorption with time. Two mechanistically based analytical models were fitted to the sorption kinetics data: (i) a T wo-R ate S orption K inetics model (TRSK) and (ii) an I ntraparticle D iffusion model (ID) with an initial sorption capacity, Xi. Both models described measured data well, with the TRSK model providing the best fit (lowest root mean square error). Although the two models are mathematically different, the parameters describing the slow sorption rate in the two models (in ID the effective diffusion coefficient, Da/l2, and in TRSK the slow rate coefficient, rs) exhibited comparable values. Both Da/l2 and rs showed a minor concentration dependency. Sorption isotherms measured at different time scales were described well by the Freundlich equation and showed that the Freundlich coefficient, K′F, increased with time. The Freundlich exponent, n′a, decreased at short sorption times (t < 4 days) but was reasonably constant at longer sorption times (t > 4 days). An empirical, two-term K′F(t) model, used together with a constant value of the Freundlich exponent, n′a (defined as n′a measured at t > 4 days), gave good predictions of the measured sorption at different time scales and represented a simple alternative to the ID and TRSK models. Calculating the naphthalene retardation factor as a function of time, R(t), using the two-term K′F(t) model implied that sorption non-linearity is highly important when evaluating both short- and longer-term naphthalene mobility in soil and will often overshadow the effects of sorption time-dependency.

PAHs concentration and toxicity in organic solvent extracts of atmospheric particulate matter and sea sediments

The concentration of polycyclic aromatic hydrocarbons (PAHs) and the toxicity to marine bacteria (Vibrio fischeri) were measured for the organic solvent extracts of sea sediments collected from an urban watershed area (Hiroshima Bay) of Japan and compared with the concentrations and toxicity of atmospheric particulate matter (PM). In atmospheric PM, the PAHs concentration was highest in fine particulate matter (FPM) collected during cold seasons. The concentrations of sea sediments were 0.01-0.001 times those of atmospheric PM. 1/EC50 was 1-10 L g(-1) PM for atmospheric PM and 0.1-1 L g(-1) dry solids for sea sediments. These results imply that toxic substances from atmospheric PM are diluted several tens or hundreds of times in sea sediments. The ratio of the 1/EC50 to PAHs concentration ((1/EC50)/16PAHs) was stable for all sea sediments (0.1-1 L μg(-1) 16PAHs) and was the same order of magnitude as that of FPM and coarse particulate matter (CPM). The ratio of sediments collected from the west was more similar to that of CPM while that from the east was more similar to FPM, possibly because of hydraulic differences among water bodies. The PAHs concentration pattern analyses (principal component analysis and isomer ratio analysis) were conducted and the results showed that the PAHs pattern in sea sediments was quite different to that of FPM and CPM. Comparison with previously conducted PAHs analyses suggested that biomass burning residues comprised a major portion of these other sources.

Temperature change affected groundwater quality in a confined marine aquifer during long-term heating and cooling

Global warming and urbanization together with development of subsurface infrastructures (e.g. subways, shopping complexes, sewage systems, and Ground Source Heat Pump (GSHP) systems) will likely cause a rapid increase in the temperature of relatively shallow groundwater reservoirs (subsurface thermal pollution). However, potential effects of a subsurface temperature change on groundwater quality due to changed physical, chemical, and microbial processes have received little attention. We therefore investigated changes in 34 groundwater quality parameters during a 13-month enhanced-heating period, followed by 14 months of natural or enhanced cooling in a confined marine aquifer at around 17 m depth on the Saitama University campus, Japan. A full-scale GSHP test facility consisting of a 50 m deep U-tube for circulating the heat-carrying fluid and four monitoring wells at 1, 2, 5, and 10 m from the U-tube were installed, and groundwater quality was monitored every 1-2 weeks. Rapid changes in the groundwater level in the area, especially during the summer, prevented accurate analyses of temperature effects using a single-well time series. Instead, Dual-Well Analysis (DWA) was applied, comparing variations in subsurface temperature and groundwater chemical concentrations between the thermally-disturbed well and a non-affected reference well. Using the 1 m distant well (temperature increase up to 7 °C) and the 10 m distant well (non-temperature-affected), the DWA showed an approximately linear relationships for eight components (B, Si, Li, dissolved organic carbon (DOC), Mg(2+), NH4(+), Na(+), and K(+)) during the combined 27 months of heating and cooling, suggesting changes in concentration between 4% and 31% for a temperature change of 7 °C.