Soil Pore Gas Research Articles

Isotopic Characterization of Mercury Atmosphere-Foliage and Atmosphere-Soil Exchange in a Swiss Subalpine Coniferous Forest.

To understand the role of vegetation and soil in regulating atmospheric Hg0, exchange fluxes and isotope signatures of Hg were characterized using a dynamic flux bag/chamber at the atmosphere-foliage/soil interfaces at the Davos-Seehornwald forest, Switzerland. The foliage was a net Hg0 sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27‰) in δ202Hg values. The soil served mostly as net sources of atmospheric Hg0 with higher Hg0 emission from the moss-covered soils than from bare soils. The negative shift of δ202Hg and Δ199Hg values of the efflux air relative to ambient air and the Δ199Hg/Δ201Hg ratio among ambient air, efflux air, and soil pore gas highlight that Hg0 re-emission was strongly constrained by soil pore gas evasion together with microbial reduction. The isotopic mass balance model indicates 8.4 times higher Hg0 emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg0 to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg0 isotope signatures. This study improves our understanding of Hg atmosphere-foliage/soil exchange in subalpine coniferous forests, which is indispensable in the model assessment of forest Hg biogeochemical cycling.

Deposition and Re-Emission of Atmospheric Elemental Mercury over the Tropical Forest Floor.

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg0) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg0 deposition to and evasion from soil in a Chinese tropical rainforest were investigated using Hg stable isotopic techniques. Our results showed a mean air-soil flux as deposition of -4.5 ± 2.1 ng m-2 h-1 in the dry season and as emission of +7.4 ± 1.2 ng m-2 h-1 in the rainy season. Hg re-emission, i.e., soil legacy Hg evasion, induces negative transitions of Δ199Hg and δ202Hg in the evaded Hg0 vapor, while direct atmospheric Hg0 deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg0 deposition to soil was estimated to be 48.6 ± 13.0 μg m-2 year-1. Soil Hg0 re-emission was estimated to be 69.5 ± 10.6 μg m-2 year-1, of which 63.0 ± 9.3 μg m-2 year-1 is from surface soil evasion and 6.5 ± 5.0 μg m-2 year-1 from soil pore gas diffusion. Combined with litterfall Hg deposition (∼34 μg m-2 year-1), we estimated a ∼12.6 μg m-2 year-1 net Hg0 sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg0 re-emission and therefore a relatively weaker atmospheric Hg0 sink.

Development and Application of Fiber-Optic Sensing Technology for Monitoring Soil Moisture Field

Accurate acquisition of the moisture field distribution in in situ soil is of great significance to prevent geological disasters and protect the soil ecological environment. In recent years, rapidly developed fiber-optic sensing technology has shown outstanding advantages, such as distributed measurement, long-distance monitoring, and good durability, which provides a new technical means for soil moisture field monitoring. After several years of technical research, the authors’ group has made a number of new achievements in the development of fiber-optic sensing technology for the soil moisture field, that is, two new fiber-optic sensing technologies for soil moisture content, including the actively heated fiber Bragg grating (AH-FBG) technology and the actively heated distributed temperature sensing (AH-DTS) technology, and a new fiber-optic sensing technology for soil pore gas humidity are developed. This paper systematically summarizes the three fiber-optic sensing technologies for soil moisture field, including sensing principle, sensor development and calibration test. Moreover, the practical application cases of three fiber-optic sensing technologies are introduced. Finally, the development trend of fiber-optic sensing technology for soil moisture field in the future is summarized and prospected.

Field-scale performance of biochar-amended soil covers for landfill methane oxidation

A field validation of three biochar-amended soil covers (2%, 10%, and 100% biochar-amended soils) along with a soil control cover was conducted within the intermediate cover of an active municipal solid waste (MSW) landfill in conjunction with laboratory studies evaluating the effect of biochar in enhancing methane (CH4) oxidation in cover soils. Baseline CH4 emissions and pre-existing site conditions were characterized prior to installation of test plots simulating three cover designs evaluated in related laboratory studies. Static chamber measurements of surface CH4 fluxes and sampling of soil pore gas at different depths were conducted across the 8-month monitoring period to assess cover performance. Surface fluxes from the test plots exhibited wide spatial variability, with one location emitting fluxes > 1100 g CH4 m−2 day−1 in one survey. Potential rates of CH4 oxidation were determined in batch assays of exhumed soil core subsamples following termination of the field trial and ranged from ~1 to 350 μg CH4 g−1 day−1. The heterogeneity of the waste led to nonuniform CH4 loads in the test plots. The soil control test plot was exposed to higher CH4 loads and biochar-amended test plots were exposed to significantly lower CH4 loads. As a result, the soil control test plot showed higher CH4 oxidation rates (257–289 μg CH4 g−1 day−1) than the biochar-amended test plots. Similarly, the soil control plot also showed higher relative abundance of methanotrophs which was positively correlated with the CH4 oxidation rates. The test plot with 10% biochar-amended soil experienced CH4 loads nearly 25% of that in soil control and still showed CH4 oxidation rates (260 μg CH4 g−1 day−1) comparable to that of soil control which showed the efficacy of biochar amendment in enhancing CH4 oxidation rates. The environmental and CH4 exposure conditions affected the microbial community composition in the test plots and showed the dominance of type I methanotrophic genus such as Methylomonas and Crenothrix spp. Overall, the waste heterogeneity led to nonuniform CH4 exposure conditions at each test plot making it hard to distinctly quantify the effect of biochar amendment on CH4 oxidation rates.

Environmental controls on ecosystem-scale cold-season methane and carbon dioxide fluxes in an Arctic tundra ecosystem

Abstract. Understanding the processes that influence and control carbon cycling in Arctic tundra ecosystems is essential for making accurate predictions about what role these ecosystems will play in potential future climate change scenarios. Particularly, air–surface fluxes of methane and carbon dioxide are of interest as recent observations suggest that the vast stores of soil carbon found in the Arctic tundra are becoming more available to release to the atmosphere in the form of these greenhouse gases. Further, harsh wintertime conditions and complex logistics have limited the number of year-round and cold-season studies and hence too our understanding of carbon cycle processes during these periods. We present here a two-year micrometeorological data set of methane and carbon dioxide fluxes, along with supporting soil pore gas profiles, that provide near-continuous data throughout the active summer and cold winter seasons. Net emission of methane and carbon dioxide in one of the study years totalled 3.7 and 89 g C m−2 a−1 respectively, with cold-season methane emission representing 54 % of the annual total. In the other year, net emission totals of methane and carbon dioxide were 4.9 and 485 g C m−2 a−1 respectively, with cold-season methane emission here representing 82 % of the annual total – a larger proportion than has been previously reported in the Arctic tundra. Regression tree analysis suggests that, due to relatively warmer air temperatures and deeper snow depths, deeper soil horizons – where most microbial methanogenic activity takes place – remained warm enough to maintain efficient methane production whilst surface soil temperatures were simultaneously cold enough to limit microbial methanotrophic activity. These results provide valuable insight into how a changing Arctic climate may impact methane emission, and highlight a need to focus on soil temperatures throughout the entire active soil profile, rather than rely on air temperature as a proxy for modelling temperature–methane flux dynamics.

Mechanism of biochar soil pore–gas–water interaction: gas properties of biochar-amended sandy soil at different degrees of compaction using KNN modeling

Soil compaction has contrasting effect on soil strength (i.e., positive) and vegetation growth (i.e., negative), respectively. Biochar has been utilized mostly in combination with soils in both agricultural fields (i.e., loose soils) and geo-structures (i.e., dense soil slopes, landfill cover) for improving water retention due to its microporous structure. Biochar is also found to be useful to reduce gas permeability in compacted soil recently. However, the efficiency of biochar in reducing gas permeability in loose and dense soils is rarely understood. The objective of this study is to analyze effects of compaction on gas permeability in soil at different degrees of compaction (i.e., 65%, 80% and 95%) and also different biochar amendment contents (0%, 5% and 10%). Another aim is to identify relative significance of parameters (soil suction, water content, biochar content and compaction) in affecting gas permeability. Experiments were conducted before applying k-nearest neighbor (KNN) modeling technique for identifying relative significance of parameters. Biochar was synthesized from a coastal invasive species (water hyacinth), which has relatively no influence on food chain (as unlike in biochar produced from biomass such as rice husk, straw, peanut shell). Based on measurements and KNN modeling, it was found that gas permeability of biochar-amended soil is relatively lower than that of soil without amendment. It was found from KNN model that for denser soils, higher amount of soil suction is mobilized for a significant increase in gas permeability as compared to loose soils. Among all parameters, soil suction is found to be most influential in affecting gas permeability followed by water content and compaction.

Process factors driving dynamic exchange of elemental mercury vapor over soil in broadleaf forest ecosystems

A data gap in the evergreen broadleaf (EB) forest ecosystems has resulted in large uncertainties in estimating the quantity of global air–soil exchange of elemental mercury vapor (Hg0). In this study, we systematically measured the soil pore gas Hg0 concentration, air–soil Hg0 exchange flux and associated environmental parameters to elucidate the process factors driving the air–soil Hg0 exchange in the EB forest ecosystem. The observed air–soil Hg0 exchange flux shows evasion during summer and deposition during winter and indicates that the forest floor is a net atmospheric Hg0 source with an annual flux of +6.7 ± 20.5 μg m−2 yr−1. Structural equation modeling infers that temperature is the most important driver causing the air–soil Hg0 exchange, followed by atmospheric Hg0 concentration. Combined with air–foliage exchange data reported in Yuan et al. (2019) (https://doi.org/10.1021/acs.est.8b04865), the EB forest ecosystem emerges as an atmospheric Hg0 sink with a net flux of −20.1 ± 24.1 μg m−2 yr−1. Using data documenting global air–soil Hg0 exchange flux in forest ecosystems, we estimate Hg0 emission from the EB forest floor to be 347 ± 384 Mg yr−1, nearly 2 times greater than that from the boreal/temperate forest floor, highlighting the importance of EB forest ecosystems in the global Hg biogeochemical cycle.

Re-emission of legacy mercury from soil adjacent to closed point sources of Hg emission

Mercury (Hg) emissions from point sources to air may disperse over long distance depending on Hg speciation in the plume. A significant fraction of Hg, particularly in its divalent forms, deposits locally and causes pollution to surrounding biomes. The objective of this study was to investigate (1) the historic Hg deposition to the immediate vicinity of an industrial complex that had intentional use of Hg (i.e., chlor-alkali and polyvinyl chloride production) for 5 decades until 2011, and (2) the Hg0 re-emission from soil to air soon after the closure of the facility. The spatial distribution of near-ground Hg0 vapor in air, soil Hg concentration and stable isotope ratio, air-soil Hg0 flux and Hg0 concentration in soil pore-gas were measured. It was found that the surrounding soils are severely contaminated with Hg due to the Hg release of the industrial complex, displaying soil Hg content up to 4.8 μg g−1. A spatial trend of Hg mass dependent isotope fractionation signature (δ202Hg = −2.11‰ to 0.72‰) with respect to the distance from the closed facility was identified, representing a mixing between regional background and industrial Hg sources. Hg release from the industrial operation enhanced surface soil Hg content within a 6.5-km radius from the facility. Inside the facility, residual Hg wastes (i.e., electrolysis sludge and consumed HgCl2 catalyst) represent a strong localized emission source of atmospheric Hg0. Near-ground atmospheric Hg0 concentration and soil Hg0 efflux progressively elevated toward the facility with an increase by 2–3 orders of magnitude compared to the values observed in the off-site background. These results suggest that the natural soil surfaces surrounding the closed industrial facility act as a large nonpoint source emitting legacy deposited Hg as much as the release from naturally enriched mines.

Using artificial burrows to evaluate inhalation risks to burrowing mammals

Since the onset of plutonium production at the U.S. Department of Energy's Hanford Site, several hundred cubic meters of carbon tetrachloride (CCl4) has been discharged to the soil column, resulting in a dispersed CCI4 vapor plume in the subsurface. Inhalation of volatile organic chemicals could be an important exposure pathway for burrowing animals there. Historical levels of CCl4 in soil pore gas exceeded the inhalation ecological screening level for CCl4. Thus, the inhalation exposure pathway was evaluated with the use of artificial burrows deployed at locations that had elevated levels of CCl4 in soil gas. Artificial burrows were designed on the basis of information available for Hanford Site fossorial wildlife. After installation, the artificial burrow atmosphere was sampled and analyzed for CCl4 and its degradation products: chloroform, methylene chloride, and chloromethane. Sampling was repeated on 3 occasions to capture varying atmospheric conditions affecting exposure concentrations. CCl4 was detected in the artificial burrows, and maximum exposures were observed during relatively low barometric conditions. The highest CCl4 detections were still well below the inhalation-based ecological screening level and CCl4 degradation products were never detected. This study shows that artificial burrows are an efficient method for obtaining relevant exposure data and illustrates the utility of directly measuring the medium for exposure under ecologically realistic conditions.

Infiltration Rate vs. Gas Composition and Pressure in Soil Columns

Infiltration is influenced by soil gases. Studies have found that CO2 has a large impact on infiltration. Since CO2 is highly soluble, soil flooded with it before infiltration has greater saturation and a greater infiltration rate. Our objective was to compare infiltration rates when soil pore gas consisted of (i) CO2 at atmospheric pressure, (ii) air at atmospheric pressure, and (iii) air at various vacuum levels. A vacuum should, similar to CO2, reduce encapsulated gas and increase the infiltration rate. Results were that infiltration under 1‐ to 2‐mm constant‐head conditions tended to be more rapid at low air pressures. Falling‐head infiltration, from 67.8‐ to 13.8‐cm water head, was fastest with CO2, intermediate with a vacuum that removed all dry air gases, and slowest with air at atmospheric pressure. The final falling‐head infiltration rates with CO2 and vacuum were 1.9 and 1.6 times faster, respectively, than with atmospheric air.

Design of Dry Barriers for Containment of Contaminants in Unsaturated Soils

AbstractA dry barrier consists of a laterally continuous soil layer that is dried by air flow. Dry atmospheric air is passed through a soil layer (preferably of a coarse texture). accumulates water vapor, then emerges from the formation as moist air. Removal of soil moisture limits or prevent downward water movement. Drying a soil layer with air is possible when atmospheric air contains less water vapor than soil pore gas. The design of an effective dry barrier is highly site dependent and requires knowledge of the site geology, of the extent of the contaminant plume, and climatic conditions. Climatic conditions at the site are important in two ways: (1) the efficiency of atmospheric air in drying: soil depends on its absolute humidity: and (2) the amount of infiltration from precipitation influences how much water has to be removed from the soil to maintain a dry barrier. The size of the barrier, combined with the climatic conditions, determines the air flow required to maintain the system. while the hydraulic characteristics of the soils determine the location and design of air injection and extraction wells. The electric power rate structure must be known to calculate operating costs. The principal factors which determine the feasibility of a dry barrier include: (1) the climate at the site and, most importantly, low‐absolute atmospheric humidity during much of the year; (2) a substantial unsaturated zone; and (3) the ability to circulate air through the unsaturated zone. The amount of initial soil moisture in the layer through which air is circulated may be an important factor in the feasibility of a dry barrier. A design study is presented that illustrates the trade‐offs between high capital costs of constructing a large number of air circulation wells vs. the high operating costs required to pump large volume of air over long distances through fewer wells. This study found that, under conditions similar to those in many arid regions, a dry barrier may be a technically and economically viable method of providing temporary waste containment.