Feasibility Study of Using Centrifuge for Investigating LNAPL Migration in Unsaturated Soils

Centrifuge modeling appears potentially useful for studying geo-environmental problems such as pollutant migration in subsurface systems. In this study, the “modeling of models” technique was used to validate the feasibility of using a geotechnical centrifuge to model the transport behavior of light non-aqueous phase liquids (LNAPLs) in unsaturated soils. All the experiments were conducted to simulate a gasoline spill from a leaking underground storage tank (UST) and the subsequent subsurface migration of the gasoline. When the gravity in the centrifuge reached the desired g-level, the gasoline was released from the UST and then it migrated in the unsaturated soil corresponding to a prototype time equivalent of one year. After the centrifuge tests, soil samples were collected using sampling tubes and the concentrations of individual constituent in the LNAPL were directly measured by means of gas chromatograph analysis. Results obtained from the centrifuge tests at different g-levels show that similar migration patterns are found for LNAPL transport in unsaturated porous media. The location of the peak concentration and the behavior of lateral spreading can be adequately described. In addition, centrifuge test data show that the migration pattern of LNAPLs is related to the soil type and the physical properties of individual constituents in the LNAPLs.     

Effect of Soil Fabric on Transport of a LNAPL through Unsaturated Fine-Grained Soils: A Centrifugal Model Study

Centrifugal model tests were performed to study the impact of the fabric of a fine-grained soil on transport of a light non-aqueous phase liquid (LNAPL). An image processing technique was developed to extract contaminant transport and fate data from the centrifugal model. Two unconsolidated sites with different moisture contents and a saturated site consolidated due to self-weight were simulated using the centrifuge. The LNAPL migrated in the vertical direction as a narrow plume and formed a free product pool above the saturated zone in unsaturated and unconsolidated soils. However, the LNAPL migrated in the horizontal direction before moving in the vertical direction as a broad plume in the consolidated site. The test results showed that the final width of the plume in the unsaturated zone of the consolidated site was nearly two times as large as that for the unconsolidated sites. In addition, the rate of leak from the underground storage tanks (USTs) on consolidated soils was substantially higher when compared with those on the unconsolidated state. The comparison of LNAPL saturation profiles at the centerline of the centrifugal models during leakage showed that, depending on the soil fabric at a given time and depth, the LNAPL phase would be different; i.e., mobile or immobile (residual) in the same soil type. The test results provided additional insight into contribution of soil fabric on transport and fate of contaminants. The soil fabric controls the geological and hydro-geological properties of fine-grained soils and hence the contamination plume.     

A pan‐Arctic synthesis of CH4 and CO2 production from anoxic soil incubations

Permafrost thaw can alter the soil environment through changes in soil moisture, frequently resulting in soil saturation, a shift to anaerobic decomposition, and changes in the plant community. These changes, along with thawing of previously frozen organic material, can alter the form and magnitude of greenhouse gas production from permafrost ecosystems. We synthesized existing methane (CH₄) and carbon dioxide (CO₂) production measurements from anaerobic incubations of boreal and tundra soils from the geographic permafrost region to evaluate large‐scale controls of anaerobic CO₂ and CH₄ production and compare the relative importance of landscape‐level factors (e.g., vegetation type and landscape position), soil properties (e.g., pH, depth, and soil type), and soil environmental conditions (e.g., temperature and relative water table position). We found fivefold higher maximum CH₄ production per gram soil carbon from organic soils than mineral soils. Maximum CH₄ production from soils in the active layer (ground that thaws and refreezes annually) was nearly four times that of permafrost per gram soil carbon, and CH₄ production per gram soil carbon was two times greater from sites without permafrost than sites with permafrost. Maximum CH₄ and median anaerobic CO₂ production decreased with depth, while CO₂:CH₄ production increased with depth. Maximum CH₄ production was highest in soils with herbaceous vegetation and soils that were either consistently or periodically inundated. This synthesis identifies the need to consider biome, landscape position, and vascular/moss vegetation types when modeling CH₄ production in permafrost ecosystems and suggests the need for longer‐term anaerobic incubations to fully capture CH₄ dynamics. Our results demonstrate that as climate warms in arctic and boreal regions, rates of anaerobic CO₂ and CH₄ production will increase, not only as a result of increased temperature, but also from shifts in vegetation and increased ground saturation that will accompany permafrost thaw.     

滇西北高原纳帕海湿地土壤氮矿化特征

采用树脂芯原位培育法,研究了纳帕海沼泽、沼泽化草甸和草甸土壤氮的矿化特征。结果表明,铵态氮（NH₄⁺-N）为沼泽、沼泽化草甸土壤中无机氮的主要存在形式,分别占无机氮含量的96.76%和75.24%,而硝态氮（NO₃^--N）为草甸土壤中无机氮的主要存在形式,占无机氮含量的58.77%。植物生长期内,纳帕海湿地土壤的净氮矿化速率表现为沼泽化草甸 > 草甸 > 沼泽,表明干湿交替的土壤环境更利于土壤氮矿化作用的进行,土壤中氮素有效性和维持植物可利用氮素的能力更强。整个生长季,沼泽和草甸土壤氮矿化为硝化作用,而沼泽化草甸土壤氮矿化为氨化作用。土壤硝态氮含量、有机质含量、碳氮比和含水量均对纳帕海沼泽、沼泽化草甸和草甸土壤的氮矿化产生显著影响。     

Adsorption and movement of metalaxyl in soils under unsaturated flow conditions

Adsorption and mobility of the fungicide metalaxyl was studied in three soil types, viz., Hessaraghatta loamy sand (HLS), Bellary clay (BC), and Ooty clay (OC) soils, under laboratory conditions. The K values obtained from the Freundlich adsorption isotherms were 0.04, 10.65 and 13.32 µg/g for HLS, BC and OC soils respectively. Mobility of metalaxyl in these soils was inversely related to K values (r = –0.97). Although, metalaxyl persisted for over 60 days in all the soils, it did not move beyond a depth of 17.5 cm in any of the soils under unsaturated flow conditions. Mobility of metalaxyl was fastest in HLS and slowest in OC soils.     

Simulated cadmium transport in macroporous soil during heavy rainstorm using dual-permeability approach

Numerical modelling is used to analyze the transport of cadmium in response to an extreme rainfall event. The cadmium transport through the soil profile was simulated by the one-dimensional dual-permeability model, which assumes the existence of two mutually communicating domains: the soil matrix domain and the preferential flow domain. The model is based on Richards’ equation for water flow and advection-dispersion equation for solute transport. A modified batch technique allowed us to consider domain specific sorption, i.e. each of the domains has its own distribution coefficient. The dual-permeability model predicts that the cadmium can be transported substantially below the root zone after the storm. On the other hand, classical single permeability approach predicted that almost all applied cadmium stays retained near the soil surface.     

不饱和土壤CH4的吸收与氧化

不饱和土壤是已知唯一的 CH4 生物壑。综述了不饱和土壤 CH4 的吸收、氧化过程及其影响因素。不饱和土壤中 CH4 氧化的临界浓度低 ,因而甲烷氧化菌可氧化大气 CH4 并将其当作唯一的碳源和能源。土壤 CH4 吸收率与土壤湿度通常呈负相关关系。土壤湿度过高 ,大气 CH4 和 O2 向土壤中扩散受阻 ;或土壤湿度过低引起水分胁迫均导致甲烷氧化菌活性下降。NH 4对土壤中 CH4 氧化的抑制作用可归结为 NH3和 CH4 在甲烷单氧酶水平上的竞争、由氧化作用向硝化作用的转移以及 NH 4氧化生成的 NO- 2 的毒性。NH 4对 CH4 氧化的抑制作用与土壤有效氮含量成正比。各类氮肥对 CH4 氧化抑制作用 :化肥 >有机肥 ;铵态氮肥 >尿素。 NO- 3对 CH4 氧化没有抑制效应。阳离子代换量 (CEC)高的土壤 NH 4对 CH4 氧化的抑制作用轻。 CH4 氧化菌对大气 CH4 的高亲和力及 CH4 氧化所需较低的活化能导致其温度系数 Q1 0 较小。地温较低时 ,土壤氧化 CH4 的能力随温度升高而升高。当地温高于 CH4 氧化的最佳温度时 ,CH4 氧化菌难以与硝化细菌及其它微生物竞争利用土壤空气中的 O2 ,导致其活性降低。甲烷氧化菌对 p H值变化不敏感。团粒结构较好的壤土可保护 CH4 氧化菌免受干扰。未受干扰的森林土壤 CH4 氧化率的峰值一般出现在亚表     

Lab-Scale Biodegradation Study of BTEX under the Unsaturated Condition and Its Effect on Soil Matric Potential

Benzene, toluene, ethylbenzene, and xylene are collectively known as BTEX which contributes to volatile environmental contaminants. This present study investigates the microbial degradation of BTEX in batch and continuous soil column experiments and its effects on soil matric potential. Batch degradation experiments were performed with different initial concentrations of BTEX using the BTEX tolerant culture isolated from petroleum-contaminated soil. In batch study, the degradation pattern for single substrate showed that xylene was degraded much faster than other compounds followed by ethylbenzene, toluene, and benzene with the highest μ_max = 0.140 h^?1 during initial substrate concentration of 100 mg L^?1. Continuous degradation experiments were performed in a soil column with an inlet concentration of BTEX of about 2000 mg L^?1 under unsaturated flow in anaerobic condition. BTEX degradation pattern was studied with time and the matric potential of the soil at different parts along the length of the column were determined at the end of the experiment. In continuous degradation study, BTEX compounds were degraded with different degradation pattern and an increase in soil matric potential was observed with an increase in depth from top to bottom in the column with applied suction head. It was found that column biodegradation contributed to 69.5% of BTEX reduction and the bacterial growth increased the soil matric potential of about 34% on an average along the column height. Therefore, this study proves that it is significant to consider soil matric potential in modeling fate and transport of BTEX in unsaturated soils.     

The impact of different soil parameters on the community structure of dominant bacteria from nine different soils located on Livingston Island, South Shetland Archipelago, Antarctica

Microorganisms inhabit very different soil habitats in the ice-free areas of Antarctica, playing a major role in nutrient cycling in cold environments. We studied the soil characteristics and the dominant bacterial composition from nine different soil profiles located on Livingston Island (maritime Antarctica). The total carbon (TC) and total nitrogen (TN) values were high for the vegetated soils, decreasing with depth, whereas the values for the mineral soils were generally low. Soil pH was more acidic for moss-covered soils and neutral to alkaline for mineral soils. Numbers of culturable heterotrophic bacteria were higher at vegetated sites, but significant numbers were also detectable in carbon-depleted soils. Patterns of denaturing gradient gel electrophoresis (DGGE) revealed a highly heterogeneous picture throughout the soil profiles. Subsequent sequencing of DGGE bands revealed in total 252 sequences that could be assigned to 114 operational taxonomic units, showing the dominance of members of the Bacteroidetes and Acidobacteria. The results of phospholipid fatty acid analysis showed a lack of unsaturated fatty acids for most of the samples. Samples with a prevalence of unsaturated over saturated fatty acids were restricted to several surface samples. Statistical analysis showed that the dominant soil bacterial community composition is most affected by TC and TN contents and soil physical factors such as grain size and moisture, but not pH.     

Water Content Mediated Microaerophilic Toluene Biodegradation in Arid Vadose Zone Materials

We investigated the conditions promoting toluene biodegradation for gasoline-contaminated near-surface (0.6 m depth) and subsurface (4.7 to 5.0 m depth) vadose zone soils sampled from an arid environment. At both depths, water addition was required for toluene biodegradation to occur. In near-surface samples, no inorganic nutrient addition was necessary and (i) biodegradation was fastest at 0.0 MPa, (ii) biodegradation rates decreased with decreasing water potential down to ?1.0 MPa, and (iii) biodegradation was undetectable at ?1.5 MPa. For subsurface material, toluene depletion was stimulated either by slurrying with a nutrient solution or by adjusting the moisture content to 20% (0.0 MPa) with nutrient solution and lowering the oxygen concentration (to effectively 1 mg L-1 in the aqueous phase). Thus, in the subsurface material, toluene depletion was microaerobic and nutrient-limited, occurring only under low oxygen and with inorganic nutrient addition. Our studies implicate microaerophily as an important characteristic of the toluene-degrading communities in these dry soils, with soil water as a primary controller of oxygen availability.     

Immobilization and phytoavailability of cadmium in variable charge soils. I. Effect of phosphate addition

The effect of phosphate on the surface charge and cadmium (Cd) adsorption was examined in seven soils that varied in their variable-charge components. The effect of phosphate on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg^–1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. Addition of phosphate, as KH₂PO₄, increased the pH, negative charge and Cd adsorption by the soils. Of the seven soils examined, the three allophanic soils (i.e., Egmont, Patua and Ramiha) exhibited greater increases in phosphate-induced pH, negative charge and Cd²⁺ adsorption over the other four non-allophanic soils (i.e., Ballantrae, Foxton, Manawatu ad Tokomaru). Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Addition of phosphate effectively reduced the phytotoxicity of Cd. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of phosphate decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fraction in soil. The phosphate-induced alleviation of Cd phytotoxicity can be attributed primarily to Cd immobilization due to increases in pH and surface charge.     

施肥对板栗林土壤CO2通量的影响

2011年6月-2012年6月,在浙江省临安市典型板栗林样地布置施肥试验,研究板栗林土壤CO₂通量与环境因子的关系.试验设置不施肥(对照)、施无机肥、有机肥及有机无机混合肥(1/2无机肥 + 1/2有机肥)4个处理.利用静态箱法测定土壤CO₂排放速率,以及土壤温度、含水量和水溶性有机碳(WSOC)含量.结果表明: 板栗林中土壤CO₂排放呈现显著的季节性变化特征,最小值均出现在2月,最大值均出现在7、8月.施用无机肥、有机肥和有机无机混合肥的土壤年累积CO₂通量比对照分别增加29.5%、47.0% 和50.7%.施用无机肥的土壤WSOC含量(105.1 mg·kg^-1)显著高于对照(76.6 mg·kg^-1),但明显低于有机肥(133.0 mg·kg^-1)和混合肥处理(121.17 mg·kg^-1).无机肥、有机肥和混合肥处理的土壤呼吸Q₁₀值(1.75、1.49和1.57)均高于对照(1.47).土壤CO₂排放速率与土壤5 cm温度、WSOC含量之间呈极显著正相关,但与土壤含水量没有明显的相关性.施肥导致土壤WSOC含量增加可能是板栗林地土壤CO₂排放速率增加的原因之一.     

天山雪岭云杉林土壤CNP化学计量特征随水热梯度的变化

生态化学计量学是研究有机体所需碳(C)、氮(N)、磷(P)等营养元素之间多重平衡关系的科学。本研究以天山雪岭云杉林为研究对象,分析土壤C、N、P化学计量的特征及其与温度和降水的关系,并利用冗余分析(Redundancy analysis,RDA)技术分析了土壤C、N、P化学计量特征与理化因子的关系。结果表明,0—10 cm层C、N、P含量的变化范围分别是44.6—143.4、0.190—0.940、0.086—0.286 g/kg,10—30 m层C、N、P含量分别介于23.0—131.0、0.122—0.589、0.032—0.178 g/kg之间,30—80cm层三者的变化范围分别为14.5—67.0、0.149—0.397、0.062—0.169 g/kg。0—10 cm层C、N、P含量与年均温度存在显著的相关关系,0—10 cm和10—30 cm层P含量与年降水量之间的相关性显著; 0—10 cm层C∶P比与年均温度之间存在显著线性关系,0—10 cm与10—30 cm层N∶P比均随年均温度和年降水量的增大而增大。冗余分析结果表明,在0—30 cm层中,土壤含水量和容重是土壤C、N、P化学计量特征的主要驱动因子,土壤含水量与C∶P和N∶P值成正相关关系,容重与N∶P值成负相关关系;在30—80 cm层中,土壤含水量和土壤粘粒含量是土壤C、N、P化学计量特征的主要驱动因子,土壤含水量与N和P值成正比,容重与C、C∶N、C∶P和N∶P成正比,与N和P成反比;电导率和pH对土壤C、N、P化学计量特征的影响并未达到显著程度,可能是研究区土壤的电导率和pH的差异较小。     

Fate and Transport of Trichloroethylene in a Chamber with Alfalfa Plants

Experiments in a laboratory chamber were used to investigate the influence of alfalfa plants on the fate and transport of trichloroethylene (TCE) fed at a concentration of 200 μl/L (~290 mg/L) in the entering groundwater. The dimensions of the chamber were 180 cm in axial length, 35 cm in depth and 10 cm in width. Concentrations of TCE were monitored in the aqueous and gas phases. Evapotranspirational fluxes of TCE from the soil to the headspace of the chamber were also measured. TCE concentration in the solid phase was measured as a function of depth. Mathematical modeling of the fate of TCE was developed assuming rate-independent physical equilibrium partitioning between solid, aqueous, and gas phases. The model included volatilization across a thin atmospheric boundary layer near soil surface. Numerical results were first validated with analytical results for simple cases and then compared with experimental data in the chamber. Results indicated that the water content and air content distributions significantly impact the transport and concentration of TCE in soils.     

汉江上游金水河流域土壤常量元素迁移模式

本研究旨在通过研究汉江上游金水河流域土壤无机物风化与有机物分解代谢相互关系,初步揭示研究流域尺度范围内常量元素的生物地球化学循环和空间分异的主驱动因子。通过野外调查与取样、实验室样品检测和空间模拟分析,得到以下的研究结果：一、金水河流域的土壤风化已基本完成早期阶段的去Ca,Na风化阶段,进入K风化阶段;二、土壤风化内外因素（如,土壤母质、矿物结构、温度、降水、风和重力等）作用下形成了明显的空间差异,流域内属于典型的林下有机质积聚过程,土壤矿物中的硅酸盐矿物风化分解和淋溶作用较强;三、土地利用方式改变了土壤有机质分布模式,不合理的人类活动造成农田耕作层和森林枯枝落叶层的有机质减少,土壤腐殖酸的减少影响土壤矿物风化和元素地球化学行为,对农业持续发展形成负面的影响。流域作为南水北调中线水源地,保护森林植被及枯枝落叶层对土壤涵养水分有着重要意义。     

Phytochelatins as biomarkers for heavy metal stress in maize (Zea mays L.) and wheat (Triticum aestivum L.): combined effects of copper and cadmium

Heavy metal contaminated soils often show increased levels of more than one metal, e.g. copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb) or nickel (Ni). In case such soils are used for crop production, prediction of yield reduction or quality decline due to heavy metals in the soil is inadequate when based only on chemical soil analysis. The use of biomarkers such as phytochelatins (PC), non-protein thiols specifically induced in plants upon exposure to heavy metals, may be an additional tool or diagnostic criterion in heavy metal research and in practice. In the present work, Cu and Cd uptake and induction of PC synthesis are studied with hydroponically grown maize and wheat plants exposed to mixtures of the two metals. We observed a close positive relationship between the concentrations of Cd and PC in the plant shoot material. A decreased shoot concentration of Cd after addition of Cu, due to metal competition at common root absorption sites, coincided with lower shoot PC levels. Also differences in metal uptake and xylary metal transport among the two plant species were reflected in corresponding differences in PC concentration. The observed direct relationship between shoot PC concentration and the degree of metal-induced growth inhibition makes the use of PC promising for the purpose tested for.     

Passive volatilization of gasoline from soil

Gasoline spills were simulated in the laboratory with three air‐dried soils, using a synthetic gasoline and unsaturated soil to quantify passive volatilization over a period of up to 16 d. The total and individual gasoline components were monitored as a function of time and depth in the soil.

The time required to deplete the overall gasoline concentration in the soil to 40% of the initial concentration ranged from 0.25 to 10 d for the three soils. Sand was the fastest, followed by loamy sand and silt loam. The volatilization rate of gasoline from soil was found to be dependent on soil, chemical type, and depth. Observation of individual components indicated that a wicking mechanism contributed to the gasoline flux toward the surface.     

Variation in nutrient characteristics of surface soils from the Luquillo Experimental Forest of Puerto Rico: A multivariate perspective

We assessed the effects of landscape features (vegetation type and topography), season, and spatial hierarchy on the nutrient content of surface soils in the Luquillo Experimental Forest (LEF) of Puerto Rico. Considerable spatial variation characterized the soils of the LEF, and differences between replicate sites within each combination of vegetation type (tabonuco vs. palo colorado vs. dwarf vs. pasture) and topographic position (ridge vs valley) accounted for 11–60% of the total variation in soil properties. Nevertheless, mean soil properties differed significantly among vegetation types, between topographic positions, and between seasons (wet vs dry). Differences among vegetation types reflected soil properties (e.g., bulk density, soil moisture, Na, P, C, N, S) that typically are related to biological processes and inputs of water. In forests, differences between topographic positions reflected elements (e.g., Ca, Mg, K, and Al) that typically are associated with geochemical processes; however, the nutrients and elements responsible for topographic differences in dwarf forest were different from those in other forest types. In pastures, differences between topographic positions were associated with the same soil properties responsible for differences among the other vegetation types. Pastures also had reduced N levels and different soil characteristics compared to undisturbed tabonuco forest. The only soil parameter that differed significantly between seasons was soil moisture. Soils of the LEF do not support the contention that N becomes limiting with an increase in elevation, and suggest that absolute pool sizes of N and P are not responsible for the reduction in productivity with elevation.     

Thaw depth determines reaction and transport of inorganic nitrogen in valley bottom permafrost soils

Nitrate (NO₃^–) export coupled with high inorganic nitrogen (N) concentrations in Alaskan streams suggests that N cycles of permafrost‐influenced ecosystems are more open than expected for N‐limited ecosystems. We tested the hypothesis that soil thaw depth governs inorganic N retention and removal in soils due to vertical patterns in the dominant N transformation pathways. Using an in situ, push–pull method, we estimated rates of inorganic N uptake and denitrification during snow melt, summer, and autumn, as depth of soil–stream flowpaths increased in the valley bottom of an arctic and a boreal catchment. Net NO₃^– uptake declined sharply from snow melt to summer and decreased as a nonlinear function of thaw depth. Peak denitrification rate occurred during snow melt at the arctic site, in summer at the boreal site, and declined as a nonlinear function of thaw depth across both sites. Seasonal patterns in ammonium (NH₄⁺) uptake were not significant, but low rates during the peak growing season suggest uptake that is balanced by mineralization. Despite rapid rates of hydrologic transport during snow melt runoff, rates of uptake and removal of inorganic N tended to exceed water residence time during snow melt, indicating potential for retention of N in valley bottom soils when flowpaths are shallow. Decreased reaction rates relative to water residence time in subsequent seasons suggest greater export of inorganic N as the soil–stream flowpath deepens due to thawing soils. Using seasonal thaw as a proxy for longer term deepening of the thaw layer caused by climate warming and permafrost degradation, these results suggest increasing potential for export of inorganic N from permafrost‐influenced soils to streams.     

Depth of pupation of the wild olive fruit fly, Bactrocera (Dacus) oleae (Gmel.) (Dipt., Tephritidae), as affected by soil abiotic factors

Abstract:  The influence of four abiotic factors (temperature, soil type, compaction, moisture) on the pupation depth of the wild Bactrocera ( Dacus ) oleae (Gmel.) larvae was studied using soils sampled in the field. Two temperatures (25 and 12°C), three different soil types (alluvial deposits, soil from decaying of limestone, soil from decaying of flysch), two compaction levels (low and high) and two moisture levels (10 and 50% field capacity) were tested in a factorial experiment with a total of 96 experimental units. Five larvae were placed on the soil surface of each test container and when burrowing was completed pupae were retrieved and pupation depth was recorded. The majority of larvae pupated in the top 3 cm and the mean depth of all units was 1.16 cm. The means differed significantly depending on soil type, moisture, the temperature–soil type interaction and the soil type–moisture interaction. Larvae pupated at a greater depth in limestone than in the other two soils. Depths were greater in soils at 50% field capacity than in those at 10% field capacity. In limestone and flysch the depth was greater at 25°C whereas no differences were found in alluvial soil. Different moisture levels had diverse effects in the three soil types; in alluvial soil and in flysch the increased moisture resulted in greater values but in limestone these were slightly lower. These results can be used in developing non chemical control measures and designing efficient sampling techniques for the insect in the ground.