Improving stormwater infiltration and retention in compacted urban soils at impervious/pervious surface disconnections with biochar.

Urban development often results in compacted soils, impairing soil structure and reducing the infiltration and retention of stormwater runoff from impervious features. Biochar is a promising organic soil amendment to improve infiltration and retention of stormwater runoff. Soil at the disconnection between impervious and pervious surfaces represents a critical biochar application point for stormwater management from urban impervious features. This study tested the hypothesis that biochar would significantly improve water retention and transmission at four sites, where varying percentages (0%, 2%, and 4% w/w) of biochar were amended to soils between impervious pavement, and pervious grassed slopes. Field-saturated hydraulic conductivity (Ksat) and easily drainable water storage capacity were monitored at these sites for five months (two sites) and 15 months (two sites). At the end of the monitoring periods, the physical, chemical, and biological properties of each site's soil were assessed to understand the impact of biochar on soil aggregation, which is critical for improved soil structure and water infiltration. Results indicated that the field Ksat, drainable water storage capacity, and plant available water content (AWC) were 7.1 ± 3.6 SE, 2.0 ± 0.3 SE, and 2.1 ± 0.3 SE times higher in soils amended with 4% biochar, respectively, compared to the undisturbed soil. Factor analysis elucidated that biochar amendment increased the organic matter content, aggregate mean weight diameter, organo-mineral content, and fungal hyphal length while decreasing the bulk density. Across the 12 biochar/soil combinations, the multiple linear regression models derived from factor analysis described the changes in Ksat and AWC reasonably well with R2 values of 0.51 and 0.71, respectively. Using soil and biochar properties measured before biochar addition, two recent models, developed from laboratory investigations, were found helpful as screening tools to predict biochar's effect on Ksat and AWC at the four field sites. Overall, the findings illustrate that biochar amendment to compacted urban soils can significantly improve soil structure and hydraulic function at impervious/pervious surface disconnections, and screening models help to predict biochar's effectiveness in this context.

Understanding a wood-derived biochar's impact on stormwater quality, plant growth, and survivability in bioretention soil mixtures.

Bioretention systems are planted media filters used in stormwater infrastructure. Maintaining plant growth and survival is challenging because most designs require significant sand. Conventional bioretention soil media (BSM) might be augmented with biochar to make the BSM more favorable to plants, to improve nutrient removal efficiency, and enhance plant survivability during drought while replacing compost/mulch components that have been linked to excess nutrient export. Pots with BSMs representing high and moderate sand content were amended with wood biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. After 20 weeks, 4% biochar amendment significantly increased stormwater infiltration (67%) and plant available water (52%) in the high sand content BSM (NC mix, which meets requirements for the state of North Carolina (US) and contains no compost/mulch), and these favorable hydraulic properties were not statistically different from a moderate sand content, biochar-free BSM with compost/mulch (DE mix, which meets requirements for state of Delaware (US)). While biochar amendment improved plant height (25%), the number of shoots (89%), and total biomass (70%) in the NC mix, these parameters were still less than those in the biochar-free DE mix containing compost/mulch. TN and NO3-1 removal were also improved (28-35%) by biochar amendment to NC mix, and the resulting TN and TP loadings to groundwater were 10 and 7 times less, respectively than biochar-free DE mix with compost/mulch. During the drought period, biochar amendment increased the time to switchgrass wilting by ∼8 days in the NC mix but remained 40% less than the biochar-free DE mix. A recalcitrant carbon-like biochar mitigates some of the deleterious effects of high sand content BSM on plants, and where nutrient pollution is a concern, replacement of compost/mulch with wood biochar in BSM may be desired.

Impact of wood-derived biochar on the hydrologic performance of bioretention media: Effects on aggregation, root growth, and water retention.

Bioretention systems are one example of green stormwater infrastructure that may mitigate the hydrologic impact of stormwater runoff. To improve water retention while maintaining rapid stormwater infiltration, conventional bioretention soil media (BSM) might be augmented with biochar. Biochar may improve the BSM's structure by increasing soil aggregation, which might improve water retention and increase stormwater infiltration while also improving root growth. Pots with BSMs representing high and moderate sand content media were amended with a wood-derived biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. In the high sand content medium (NC mix), biochar amendment increased hydraulic conductivity (Ksat), and this effect increased with time. At 0 weeks, 2% and 4% (w/w) biochar increased Ksat by 4 ± 2% and 10 ± 4%, respectively, while at 30 weeks the increase was 30 ± 10 and 70 ± 20%, respectively, above biochar-free media. Similar improvements were seen in plant available water (PAW) in NC mix. However, minimal improvements in Ksat and PAW from biochar amendment were found in the moderate sand content BSM that contained compost and mulch (DE mix). Where biochar promoted Ksat, this was correlated with increased water-stable aggregate size (r = 0.86), fine root volume (r = 0.88), and below ground biomass (r = 0.83). Important factors affecting Ksat and aggregation in the NC mix were biochar's influence on organo-mineral association, fungal hyphae length, and plant roots. Wood-derived biochar amendment to BSM may obviate the need for compost/mulch since biochar has similar effects on improving BSM hydrology and root growth without the risk of undesired nutrient leaching.

Ferrocyanide enhanced evaporative flux to remediate soils contaminated with produced water brine.

Accidental releases of highly saline produced water (PW) to land can impact soil quality. The release of associated salts can clog soil pores, disperse soil clays, and inhibit plants and other soil biota. This study explores a novel remediation technique using ferrocyanide to enhance the evaporative flux of soil porewater to transport dissolved salts to the soil surface, where crystallization then occurs. The addition of ferrocyanide modifies crystal growth that enhances salt transport, allowing salt efflorescence on the soil surface and physical removal. Release sites were simulated through beaker sand column experiments using two PWs collected from the Permian Basin. PW composition altered efflorescence, with up to ten times as much ferrocyanide required in PWs than comparable concentrations of pure NaCl solutions. The addition of EDTA reduced dissolved cation competition for the ferrocyanide ion, improving PW salt recovery at the soil surface. The speciation model, PHREEQC, was used to predict the onset of salt precipitation as a function of evaporative water loss and model the effect of aqueous ferrocyanide and EDTA speciation on efflorescence. The results highlight the utility of predictive modeling for optimizing additive dosages for a given release of PW.

Diurnal landfill methane flux patterns across different seasons at a landfill in Southeastern US.

Diurnal patterns of methane flux are examined at a landfill in the Southeastern US. Methane fluxes are measured by an eddy covariance (EC) tower during representative one-week periods in three seasons: summer, fall, and winter. Measured methane fluxes are compared with atmospheric pressure, temporal variation of atmospheric pressure, wind shear velocity, and air temperature. Landfill methane flux varies significantly with shear velocity and temporal changes in atmospheric pressure when the atmosphere is neutral. Under unstable atmospheric conditions, air temperature correlates best with methane flux, which is corroborated with an independent dataset of tracer correlation method (TCM) measurements for similar measurement periods. These field data support a mathematical model previously proposed to describe atmospheric effects on methane flux from landfills. The field data also indicate significant diurnal methane flux variations, with daytime fluxes up to 23 times greater than nighttime fluxes. Because the majority of historical TCM measurements of whole landfill methane flux are between 12 pm and 6 pm at this landfill, when daily emissions are highest because of atmospheric effects, average diurnal fluxes might have been overestimated by as much as 73%. Methane emissions are most representative of diurnal average emissions when atmospheric stability is near-neutral, which occurs in the late morning (∼11 am) and in the early evening (∼5 pm) at this site.

Preparing and characterizing repacked columns for experiments in biochar-amended soils.

Laboratory soil column experiments have been frequently performed for investigating various soil-related processes. In recent years, the demand for using biochar as a soil amendment for environmental and agricultural purposes has increased significantly. To assess the beneficial impacts of biochar, laboratory column experiments may be conducted using repacked biochar-amended soil before large-scale biochar application. Biochar is a porous material that might have transient hydrophobicity, and particle density, size, and shape that often differ from native soil. These factors might cause several experimental problems in repacked laboratory columns, including unrealistic hydraulic and solute transport and transformation measurements, spatial variation of biochar content, and error in estimating the repacked biochar-amended soil properties. Therefore, it is necessary to modify standard repacked column packing procedures for biochar-amended soil. In this work, several modifications are described for preparing repacked biochar-amended soils. The modifications are rinsing and oven-drying biochar, determining the optimum moisture content to achieve a homogenous mixture, determining the desired bulk density before column packing, and mixing and packing under wet conditions. In addition, repacked columns should be characterized by their inter, intra, and total porosities and pore volume after column packing.•Steps are recommended prior to packing the repacked biochar-amended soil columns: rinsing biochar and pre-determining optimum moisture content and bulk density.•Columns are wet-packed in subsections at the optimum moisture content to the desired bulk density. Following packing, the inter, intra, and total porosities and pore volume should be determined.•These steps will reduce unrealistic transient results, inhibit nonuniform packing and heterogeneity of biochar content, and provide important information for interpreting the performance of biochar-amended media.

Predicting the impact of biochar on the saturated hydraulic conductivity of natural and engineered media.

If biochar is applied to soil or stormwater treatment media, the saturated hydraulic conductivity (K) may be altered, which is a critical property affecting media performance. While a significant number of studies document biochar's effect on a porous medium's K, predictive models are lacking. Herein models are advanced for predicting K for repacked natural soil and engineered media when amended with biochar of various particle sizes and application rates. Experiments were conducted using three repacked natural soils, two uniform sands, and a bioretention medium amended with a wood biochar sieved to seven different biochar particle size distributions and applied at three rates. Experimental measurements showed a strong positive correlation between the interporosity of each medium and K. Across all media, the classic Kozeny-Carman (K-C) model predicted K and the relative change in K because of biochar amendment for each medium best. For soils alone, a recently developed model based on existing pedotransfer functions was optimal. The K-C model error was improved if the particle specific surface area was increased for large biochar particles, which indicates the importance of biochar particle shape on pore structure and K. X-ray Computed Tomography was coupled with pore network modeling to explain the unexpected decrease in K for sands amended with medium and large biochar. While biochar increased interporosity, mean pore radii decreased by ~25% which reduced K. The X-ray measurements and pore network modeling help to explain anomalous results reported for biochar-amended sands in other studies.

Spatial heterogeneity of biochar (segregation) in biochar-amended media: An overlooked phenomenon, and its impact on saturated hydraulic conductivity.

While the use of biochar as a soil amendment is gaining popularity for environmental and agricultural purposes, spatial heterogeneity of biochar (segregation) in biochar-amended media and its underlying causes have been overlooked. In this study, for the first time particle segregation in biochar-amended media and its impact on the media's saturated hydraulic conductivity (Ksat) were investigated. Two uniformly graded media were amended with different sizes of a wood-based biochar under dry and wet conditions. While the intended biochar volume fraction (bf) was 17.5%, in dry-packed columns biochar was often segregated and the measured bf ranged from 7.5 ± 0.8 SE% (SE = standard error) to 23.6 ± 1.8 SE% across all spatial locations. If, however, 20% water (volume of water/bulk volume of packed media) was added to the mixtures during mixing, homogeneous packings were achieved. In dry-packing, segregation was governed by the difference in the physical properties of the media and the biochar: particle size, density, and shape. In wet-packing, segregation was prevented due to the inter-particle adhesion forces associated with water. Although X-ray computed tomography images showed that the presence of segregation altered particle distributions and pore morphologies, the Ksat for wet-packed and dry-packed columns were statistically identical. The results of this study suggest that laboratory methods for packing biochar-amended media should include moisturizing the mixture to inhibit particle segregation. Mixing under wet conditions is recommended for any type of soil and biochar and for any scale of application, in both the laboratory and field.

Drying of fecal sludge in 3D laminate enclosures for urban waste management.

Laminated hydrophobic membranes have been proposed as liners for container-based sanitation systems in developing countries. The laminate allows drying of fecal sludge, which might significantly reduce the frequency of container emptying, while containing liquids and solids. While previous laboratory tests demonstrated rapid drying of fecal sludge or water retained in laminates, experiments did not assess the effects of system dimension or scale on performance. In this study fecal sludge drying and water evaporation were evaluated in 3D laminate boxes (decimeter scale) or 3D laminate-lined 40 L and 55 gallon drums (meter scale) that are prototypes of toilet containers for field application. A stagnant film model described fecal sludge drying and water evaporation in the laminate boxes and laminate-lined drums well. The effective diffusion length (λ) for the laminate was fitted in all systems and increased with system dimension and scale: λ increased by a factor of 1.4 from 1D decimeter-scale envelopes to 3D decimeter-scale boxes, and by a factor of 1.3-1.7 from 3D decimeter-scale boxes to 3D meter-scale drums. The longer λ with increasing dimension and scale is likely due to nonuniform temperature and relative humidity in the air outside the laminate and nonuniform temperature within the laminate. Using best-fit λ for the laminate-lined 40 L and 55 gallon drum experiments conducted in a controlled laboratory, drying was predicted for an 11-day field experiment. Although the air temperature and relative humidity varied significantly in the field tests from -1 °C to 26 °C and 35% to 97%, respectively, the stagnant film model predicted drying over the 11-day period reasonably well with total error ≤ 13% using 24-h average air temperature and relative humidity. Drying of fecal sludge in laminate-lined drums in the field might be adequately described with a stagnant film model using daily-average weather conditions, if wind speeds are low.

Quantifying biochar content in a field soil with varying organic matter content using a two-temperature loss on ignition method.

While the use of biochar as a soil amendment for agronomic and environmental management is gaining popularity, quantification of biochar in soil is still challenging. The objective of this work was to develop a fast, simple and inexpensive method to quantify biochar content in field soil with varying organic matter content - the two-temperature loss on ignition (LOI) method. In this approach, biochar mass fraction in a biochar-amended soil is computed by measuring the dry mass of biochar/soil mixture after heating sequentially at two temperatures: low temperature (LT), and high temperature (HT). This method requires the LOI profile for pure soil and pure biochar that are representative of soil and biochar in the field. Although the soil LOI profile may vary due to spatial variation in soil organic matter (SOM) content, the method only requires that the relative soil LOI at LT with respect to LOI at HT is uniform because of similarity in SOM chemical composition. In this method, LT and HT are selected such that the maximum difference in LOI exists at these temperatures between pure soil and biochar. The method was tested by quantifying the biochar content in roadway filter strips with and without a wood biochar pyrolyzed at high temperature (550 °C). The estimates of biochar content from the method matched independent measurements for soils with low (-0.23 ±â€¯0.09 CI%, CI = 95% confidence interval, versus actual 0%) and high (3.9 ±â€¯0.3 CI% versus actual 4.0 ±â€¯1.1 CI%) biochar mass fraction. The method is applicable when SOM content is low to moderate (e.g. <15%) and mostly composed of labile organic compounds, and when biochars are pyrolyzed at moderate to high temperatures (i.e. >400 °C) and composed of relatively low ash content (e.g. <30%).

A pilot-scale, bi-layer bioretention system with biochar and zero-valent iron for enhanced nitrate removal from stormwater.

Nitrogen (N) removal in conventional bioretention systems is highly variable owing to the low nitrate (NO3-) elimination efficiency. We hypothesized that amending bioretention cells with biochar and zero-valent iron (ZVI) could improve the NO3- removal performance. A well-instrumented, bi-layer pilot-scale bioretention cell was developed to test the hypothesis by investigating its hydrologic performance and NO3- removal efficacy as affected by biochar and ZVI amendments. The cell containing 18% (v/v) wood biochar in the vadose zone and 10% (v/v) ZVI in the saturation zone was monitored for 18 months of field infiltration tests using synthetic stormwater amended with bromide (tracer) and NO3-. Compared to the Control cell without amendments, the Biochar/ZVI cell increased water retention by 11-27% and mean residence time by 0.7-3.8 h. The vadose zone of the Biochar/ZVI cell removed 30.6-95.7% (0.6-12.7 g) of NO3-N from the influent, as compared with -6.1-89.6% (-0.1-2.9 g) by that of the Control cell. While the performance varied with synthetic storm events and seasons, in all cases the Biochar/ZVI cell resulted in greater NO3- removal than the Control cell. This improvement was presumably due to biochar's ability to improve water retention, facilitate anoxic conditions, increase residence time, and provide electrons for microbial denitrification. The saturation zone with ZVI amendment further promoted NO3- removal: removal was 1.8 times greater relative to the control in the first infiltration test, but was minimal in following tests. The reduction in performance of the ZVI amendment in subsequent tests might be due to diminished NO3-N input to the saturation zone after treatment by the biochar-amended vadose zone. The redox potential and dissolved oxygen content at the vadose/saturation zone interface also indicated more favorable denitrification conditions in the Biochar/ZVI cell. Biochar amendment demonstrated significant promise for increasing nitrate removal in bioretention systems.

Assessing clogging of laminated hydrophobic membrane during fecal sludge drying.

A new sanitation technology has been proposed in which a laminated hydrophobic membrane contains and enhances drying of fecal sludge in a toilet, with particular focus on application to urban regions of low-income countries. The proposed technology uses a laminated hydrophobic membrane liner as an integral component of container-based sanitation systems. The focus of this study is to quantitatively evaluate the laminate's clogging after repeated use, which will affect replacement interval and might limit the laminate's application in container-based toilets. The membrane of the laminated hydrophobic membrane used in this process is hydrophobic and only allows vapor transport. Drying of water vapor using the laminated hydrophobic membrane occurs due to moderate temperature or humidity gradients, while other constituents such as aqueous dissolved solutes of fecal sludge are retained. Controlled laboratory experiments evaluated repeated use of a laminated hydrophobic membrane for fecal sludge drying, with mild brushing/rinsing of the laminate between each application. Drying occurred at a constant rate as long as the fecal sludge moisture content exceeded 11.6 (g/g), below which water activity <1. Over five drying cycles, at a significance level of α = 0.05 the dimensionless drying rate in the constant-rate period was not reduced. While scanning electron microscopy and energy dispersive X-ray analyses of used laminated hydrophobic membrane showed deposition of fecal sludge on the inner fabric of the laminate, particulate accumulation was never sufficient to alter the fecal sludge drying rate. Experiments with only water indicated that the fecal sludge increased the effective diffusion length through the laminate by 10-30%. These data demonstrate that clogging of the laminated hydrophobic membrane is minor over five cycles of fecal sludge drying with mild rinsing between cycles, indicating that use of the laminate may be feasible in many field applications.

Atmospheric modeling to assess wind dependence in tracer dilution method measurements of landfill methane emissions.

The short-term temporal variability of landfill methane emissions is not well understood due to uncertainty in measurement methods. Significant variability is seen over short-term measurement campaigns with the tracer dilution method (TDM), but this variability may be due in part to measurement error rather than fluctuations in the actual landfill emissions. In this study, landfill methane emissions and TDM-measured emissions are simulated over a real landfill in Delaware, USA using the Weather Research and Forecasting model (WRF) for two emissions scenarios. In the steady emissions scenario, a constant landfill emissions rate is prescribed at each model grid point on the surface of the landfill. In the unsteady emissions scenario, emissions are calculated at each time step as a function of the local surface wind speed, resulting in variable emissions over each 1.5-h measurement period. The simulation output is used to assess the standard deviation and percent error of the TDM-measured emissions. Eight measurement periods are simulated over two different days to look at different conditions. Results show that standard deviation of the TDM- measured emissions does not increase significantly from the steady emissions simulations to the unsteady emissions scenarios, indicating that the TDM may have inherent errors in its prediction of emissions fluctuations. Results also show that TDM error does not increase significantly from the steady to the unsteady emissions simulations. This indicates that introducing variability to the landfill emissions does not increase errors in the TDM at this site. Across all simulations, TDM errors range from -15% to 43%, consistent with the range of errors seen in previous TDM studies. Simulations indicate diurnal variations of methane emissions when wind effects are significant, which may be important when developing daily and annual emissions estimates from limited field data.

Modeling biosolids drying through a laminated hydrophobic membrane.

The adaptation of the membrane distillation process as a low-cost and sustainable approach to biosolids drying and stabilization is investigated, which may have application in container-based sanitation systems proposed in low-income urban regions. Three-layer laminated, breathable, hydrophobic membranes enclose the biosolids, facilitating drying but preventing transport of contaminants. The membranes used in this process are non-wetting with pore spaces that only allow vapor transport. Water vapor can be expelled due to a moderate vapor pressure gradient. Other constituents, including both particulate and dissolved matter are retained. The permeate, therefore, is expected to be of high purity and pathogen-free. This study presents experimental results showing usable rates of moisture transfer through the laminated hydrophobic membranes with temperature gradients, ΔT = -2 °C, corresponding to the condition that biosolids do not receive external heating in which laminate-enclosed biosolids are 2 °C cooler than outside, as well as conditions that samples are 2 °C and 10 °C warmer than the ambient temperature (ΔT = 2 and 10 °C, respectively). The conditions result in reduction in the moisture content of the laminate-enclosed biosolids from about 97% to 12-30% and the permeate is observed to be free of fecal coliforms, indicator organisms for pathogens. The initial constant-rate drying period is described well with a stagnant film model that accounts for different temperature gradients, laminate surface area, and ambient relative humidity. The proposed model may be used to assess the feasibility of incorporating laminated hydrophobic membranes to enhance biosolids drying in container-based sanitation systems as well as other applications.

Numerical simulations to assess the tracer dilution method for measurement of landfill methane emissions.

Landfills are a significant contributor to anthropogenic methane emissions, but measuring these emissions can be challenging. This work uses numerical simulations to assess the accuracy of the tracer dilution method, which is used to estimate landfill emissions. Atmospheric dispersion simulations with the Weather Research and Forecast model (WRF) are run over Sandtown Landfill in Delaware, USA, using observation data to validate the meteorological model output. A steady landfill methane emissions rate is used in the model, and methane and tracer gas concentrations are collected along various transects downwind from the landfill for use in the tracer dilution method. The calculated methane emissions are compared to the methane emissions rate used in the model to find the percent error of the tracer dilution method for each simulation. The roles of different factors are examined: measurement distance from the landfill, transect angle relative to the wind direction, speed of the transect vehicle, tracer placement relative to the hot spot of methane emissions, complexity of topography, and wind direction. Results show that percent error generally decreases with distance from the landfill, where the tracer and methane plumes become well mixed. Tracer placement has the largest effect on percent error, and topography and wind direction both have significant effects, with measurement errors ranging from -12% to 42% over all simulations. Transect angle and transect speed have small to negligible effects on the accuracy of the tracer dilution method. These tracer dilution method simulations provide insight into measurement errors that might occur in the field, enhance understanding of the method's limitations, and aid interpretation of field data.

Nutrient release and ammonium sorption by poultry litter and wood biochars in stormwater treatment.

The feasibility of using biochar as a filter medium in stormwater treatment facilities was evaluated with a focus on ammonium retention. Successive batch extractions and batch ammonium sorption experiments were conducted in both deionized (DI) water and artificial stormwater using poultry litter (PL) and hardwood (HW) biochars pyrolyzed at 400°C and 500°C. No measureable nitrogen leached from HW biochars except 0.07 µmol/g of org-N from 400°C HW biochar. PL biochar pyrolyzed at 400°C leached 120-127 µmol/g of nitrogen but only 7.1-8.6 µmol/g of nitrogen when pyrolyzed at 500°C. Ammonium sorption was significant for all biochars. At a typical ammonium concentration of 2mg/L in stormwater, the maximum sorption was 150 mg/kg for PL biochar pryolyzed at 400°C. In stormwater, ion competition (e.g. Ca(2+)) suppressed ammonium sorption compared to DI water. Surprisingly, ammonium sorption was negatively correlated to the BET surface area of the tested biochars, but increased linearly with cation exchange capacity. Cation exchange capacity was the primary mechanism controlling ammonium sorption and was enhanced by pyrolysis at 400°C, while BET surface area was enhanced by pyrolysis at 500°C. The optimal properties (BET surface area, CEC, etc.) of biochar as a sorbent are not fixed but depend on the target pollutant. Stormwater infiltration column experiments in sand with 10% biochar removed over 90% of ammonium with influent ammonium concentration of 2mg/L, compared to only 1.7% removal in a sand-only column, indicating that kinetic limitations on sorption were minor for the storm conditions studied. Hardwood and poultry litter biochar pyrolyzed at 500°C and presumably higher temperature may be viable filter media for stormwater treatment facilities, as they showed limited release of organic and inorganic nutrients and acceptable ammonium sorption.

Short-term landfill methane emissions dependency on wind.

Short-term (2-10h) variations of whole-landfill methane emissions have been observed in recent field studies using the tracer dilution method for emissions measurement. To investigate the cause of these variations, the tracer dilution method is applied using 1-min emissions measurements at Sandtown Landfill (Delaware, USA) for a 2-h measurement period. An atmospheric dispersion model is developed for this field test site, which is the first application of such modeling to evaluate atmospheric effects on gas plume transport from landfills. The model is used to examine three possible causes of observed temporal emissions variability: temporal variability of surface wind speed affecting whole landfill emissions, spatial variability of emissions due to local wind speed variations, and misaligned tracer gas release and methane emissions locations. At this site, atmospheric modeling indicates that variation in tracer dilution method emissions measurements may be caused by whole-landfill emissions variation with wind speed. Field data collected over the time period of the atmospheric model simulations corroborate this result: methane emissions are correlated with wind speed on the landfill surface with R(2)=0.51 for data 2.5m above ground, or R(2)=0.55 using data 85m above ground, with emissions increasing by up to a factor of 2 for an approximately 30% increase in wind speed. Although the atmospheric modeling and field test are conducted at a single landfill, the results suggest that wind-induced emissions may affect tracer dilution method emissions measurements at other landfills.

Quantifying capture efficiency of gas collection wells with gas tracers.

A new in situ method for directly measuring the gas collection efficiency in the region around a gas extraction well was developed. Thirteen tests were conducted by injecting a small volume of gas tracer sequentially at different locations in the landfill cell, and the gas tracer mass collected from each test was used to assess the collection efficiency at each injection point. For 11 tests the gas collection was excellent, always exceeding 70% with seven tests showing a collection efficiency exceeding 90%. For one test the gas collection efficiency was 8±6%. Here, the poor efficiency was associated with a water-laden refuse or remnant daily cover soil located between the point of tracer injection and the extraction well. The utility of in situ gas tracer tests for quantifying landfill gas capture at particular locations within a landfill cell was demonstrated. While there are certainly limitations to this technology, this method may be a valuable tool to help answer questions related to landfill gas collection efficiency and gas flow within landfills. Quantitative data from tracer tests may help assess the utility and cost-effectiveness of alternative cover systems, well designs and landfill gas collection management practices.

The origin and reversible nature of poultry litter biochar hydrophobicity.

Transient changes in wettability complicate the prediction of biochar's hydrologic effects. Biochar wetting properties were characterized from poultry litter biochar (PLBC) produced from slow pyrolysis at temperatures between 300 and 600°C with water drop penetration time (persistence of hydrophobicity) and contact angle (CA; severity of hydrophobicity) measurements. Hydrophobicity was associated with semivolatile organic compounds coating PLBC surfaces, which resulted in 24.4 carbon layers and CAs of 101.1 ± 2.9° at a pyrolysis temperature of 300°C but only 0.4 layers of surface coverage and CAs of 20.6 ± 1.3° when pyrolyzed at 600°C. Mixing PLBC with water removed organic coatings, and storage in water for 72 h decreased CA as much as 81° for the most hydrophobic PLBCs. When mixed with quartz sand of the same particle size, CAs of PLBC-sand mixtures increased from 6.6 ± 1.4° at 0% PLBC mass fraction to 48.3 ± 2.0° at 15% mass fraction. Hydrophobic and hydrophilic PLBCs increased CA by nearly identical amounts at 2 and 5% mass fractions, which was explained by the influence of PLBC particle topology on macroscopic surface roughness of PLBC-sand mixtures. For environmentally relevant situations, PLBC-sand mixtures at mass fractions ≤15% remained water wetting. However, all PLBC additions increased CA, which may alter infiltration rates and induce preferential water flow.

Performance of green waste biocovers for enhancing methane oxidation.

Green waste aged 2 and 24months, labeled "fresh" and "aged" green waste, respectively, were placed in biocover test cells and evaluated for their ability to oxidize methane (CH4) under high landfill gas loading over a 15-month testing period. These materials are less costly to produce than green waste compost, yet satisfied recommended respiration requirements for landfill compost covers. In field tests employing a novel gas tracer to correct for leakage, both green wastes oxidized CH4 at high rates during the first few months of operation - 140 and 200g/m(2)/day for aged and fresh green waste, respectively. Biocover performance degraded during the winter and spring, with significant CH4 generated from anaerobic regions in the 60-80cm thick biocovers. Concurrently, CH4 oxidation rates decreased. Two previously developed empirical models for moisture and temperature dependency of CH4 oxidation in soils were used to test their applicability to green waste. Models accounted for 68% and 79% of the observed seasonal variations in CH4 oxidation rates for aged green waste. Neither model could describe similar seasonal changes for the less stable fresh green waste. This is the first field application and evaluation of these empirical models using media with high organic matter. Given the difficulty of preventing undesired CH4 generation, green waste may not be a viable biocover material for many climates and landfill conditions.