Regional Sources of Methyl Chloride, Chloroform and Dichloromethane Identified from AGAGE Observations at Cape Grim, Tasmania, 1998–2000

There are large uncertainties in identifying and quantifying the natural and anthropogenic sources of chloromethanes – methyl chloride (CH₃Cl), chloroform (CHCl₃) and dichloromethane (CH₂Cl₂), which are responsible for about 15% of the total chlorine in the stratosphere. We report two years of in situ observations of these species from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41° S, 145° E). The average background levels of CH₃Cl, CHCl₃ and CH₂Cl₂ during 1998–2000 were 551± 8, 6.3± 0.2 and 8.9± 0.2 ppt (dry air mole fractions expressed in parts per 10¹²) respectively, with a two-year average amplitude of the seasonal cycles in background air of 25, 1.1 and 1.5 ppt respectively. The CH₃Cl and CHCl₃ records at Cape Grim show clear episodes of elevated mixing ratios up to 1300 ppt and 55 ppt respectively, which are highly correlated, suggesting common source(s). Trajectory analyses show that the sources of CH₃Cl and CHCl₃ that are responsible for these elevated observations are located in coastal-terrestrial and/or coastal-seawater regions in Tasmania and the south-eastern Australian mainland. Elevated levels of CH₂Cl₂ (up to 70 ppt above background) are associated mainly with emissions from the Melbourne/Port Phillip region, a large urban/industrial complex (population 3.5 million) 300 km north of Cape Grim.Now at the Centre for Atmospheric ChemistryNow at School of Environmental Sciences     

Biocurtain Design Using Reactive Transport Models

Bioremediation is an attractive alternative to traditional remediation methods for a variety of ground water contaminants. However, widespread implementation of bioremediation is currently limited by the complexity of the dynamic chemical and biological processes that need to be understood and incorporated into the design approach. Reactive transport models provide a powerful tool to simulate these complex interactions and, thus, can be used to improve and guide the design of bioremediation systems. We present a remediation design approach for intermittently stimulated biodegradation using multicomponent reactive transport models, parameterized using a series of nondimensional Damkohler numbers. Designs were based on either (1) a target aqueous phase concentration at the exit of the treatment system, or (2) the total contaminant mass fraction removed from a region of interest. The equation set used to develop this design approach is specific to the case of intermittent electron donor addition to drive cometabolic transformations. We illustrate the design procedure for a biocurtain that removes carbon tetrachloride. Our results for this case indicate that intermittent injection is significantly more efficient than strategies based on continuous pumping. Example design parameters include the length of the biologically active zone (i.e., biocurtain), the effective rate of degradation in this zone, and the interval between electron donor injection cycles. The presented dimensionless parametric approach can be used to design bench-scale column studies and should be helpful for scale-up to field-scale remediation systems.     

Air Channel Formation, Size, Spacing, and Tortuosity During Air Sparging

Characterizing mass transfer during in situ air sparging requires knowledge of the size, shape, and interfacial area of air channels. These characteristics were determined by analysis of digital images of air channels passing through submerged glass beads having particle size in the sand range. Pore-scale channeling occurred in all cases. The analysis showed that the air channels were narrower, more tortuous, more closely spaced, and moved nearly vertically through the coarser media. In the finer media, air channels had larger diameter, were spaced further apart, and passed nearly horizontally through the media. The mean diameter of the channels varied between 2.8 and 8.1 mm, and the mean spacing varied between 8.3 and 19.4 mm. Estimates of the area of the air-water interface per unit volume of soil (a₀), computed using data from the digital images and an assumed arrangement of channels, ranged from 0.02 to 0.2 mm²/mm³. Larger a₀ were obtained for coarser media and uniformly graded media. These estimates of a₀ compare well with published values for common packed-column materials and for unsaturated soils.     

The Horizontal Reactive Media Treatment Well (HRX Well®) for Passive In‐Situ Remediation

A new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well.     

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.     

A Particle Tracking Algorithm for Arbitrary Unstructured Grids

A new semi-analytical algorithm for generating the local groundwater velocity field within irregular three-dimensional grid cells (such as a Voronoi cell) is presented. The method supports particle tracking in the MODFLOW-USG software package for a variety of supported unstructured grid formats. It is shown to replicate the results from the fully analytical Pollock's method when used on a rectilinear grid, and is demonstrated for difficult test cases where there is high spatial variability in the flow field within individual cells. The method is now available as an option in the mod-PATH3DU software package and may be used with most MODFLOW-USG packages without additional intervention.     

Effects of Intraborehole Flow on Purging and Sampling Long-Screened or Open Wells

Hydraulic head differences across the screened or open interval of a well significantly influence the sampled water mixture. Sample bias can occur due to an insufficient pumping rate and/or due to native groundwater displacement by intraborehole flow (IBF). Proper understanding of the sampled water mixture is crucial for accurate interpretation of environmental tracers and groundwater chemistry data, and hence groundwater characterization. This paper uses numerical modeling to quantify sample bias caused by IBF in an un-pumped high-yield well, and the influence of pumping rate and heterogeneity on the volume of pumpage required to purge an IBF plume. The results show that (1) the pumping rate must be at least an order of magnitude greater than the IBF rate to achieve permeability-weighted yield, (2) purge volume was 2.2 to 20.6 times larger than the IBF plume volume, with the ratio depending on plume location relative to hydraulic conductivity and head distributions, and (3) after an example 1000-day un-pumped period, purging required removal of at least three orders of magnitude more water than the common practice of three to five well volumes. These results highlight the importance of knowing the borehole flow regime to identify IBF inflow and outflow zones, estimate IBF rates, and to develop a strategic sampling approach.     

Tracking denudation and sediment production and transport with cosmogenic 10Be in arid,high-altitude Himalayan half-grabens,Zanskar, northern India

Understanding the extent to which local factors, including bedrock and structure, govern catchment denudation in mountainous environments as opposed to broader climate or tectonic patterns provides insight into how landscapes evolve as sediment is generated and transported through them, and whether they have approached steady-state equilibrium. We measured beryllium-10 (¹⁰Be) concentrations in 21 sediment samples from glaciated footwall and hanging wall catchments, including a set of nested catchments, and 12 bedrock samples in the Puga and Tso Morari half-grabens located in the high-elevation, arid Zanskar region of northern India. In the Puga half-graben where catchments are underlain by quartzo-feldspathic gneissic bedrock, bedrock along catchment divides is eroding very slowly, about 5 m/Ma, due to extreme aridity and ¹⁰Be concentrations in catchment sediments are the highest (~60–90 × 10⁵ atoms/g SiO₂) as colluvium accumulates on hillslopes, decoupled from their ephemeral streams. At Puga, ¹⁰Be concentrations and the average erosion rates of a set of six nested catchments demonstrate that catchment denudation is transport-limited as sediment stagnates on lower slopes before reaching the catchment outlet. In the Tso Morari half-graben, gneissic bedrock is also eroding very slowly but ¹⁰Be concentrations in sediments in catchments underlain by low grade meta-sedimentary rocks, are significantly lower (~10–35 × 10⁵ atoms/g SiO₂). In these arid, high-elevation environments, ¹⁰Be concentrations in catchment sediments have more to do with bedrock weathering and transport times than steady-state denudation rates. © 2020 John Wiley & Sons, Ltd.     

Predicting seasonal ocean variability around New Zealand using a coupled ocean-atmosphere model

We present a first assessment of the Sea Surface Temperature (SST) and 300m Heat Content (HC) ensemble mean skill of the Australian Community Climate and Earth-System Simulator-Seasonal v1.0 (ACCESS-S1) around New Zealand on seasonal timescales, using a set of retrospective ensemble forecasts for 1990-2012. This was verified against Reynolds AVHRR analysis and Bluelink ReANalysis 3.5 (BRAN3.5). For inshore areas with depth <300m, the model shows skill forecasting for summer at a lead time of 0 months, with 66% and 65% of model grid cells having correlation coefficients exceeding 0.6 for SST and 300m HC respectively. Regions of lower skill seem to be associated with the model representation of the complex and variable frontal systems around New Zealand, and an overly-strong response to the El Niño Southern-Oscillation (ENSO). Nevertheless, the skill of ACCESS-S1 around New Zealand suggests there is potential for the development of useful seasonal forecast products for the region.