Transport of elemental mercury in the unsaturated zone from a waste disposal site in an arid region

Mercury contained in buried landfill waste may be released via upward emission to the atmosphere or downward leaching to groundwater. Data from the US Geological Survey’s Amargosa Desert Research Site (ADRS) in arid southwestern Nevada reveal another potential pathway of Hg release: long-distance (10² m) lateral migration of elemental Hg (Hg⁰) through the unsaturated zone. Gas collected from multiple depths from two instrumented boreholes that sample the entire 110-m unsaturated zone thickness and are located 100 and 160 m away from the closest waste burial trench exhibit gaseous Hg concentrations of up to 33 and 11 ng m⁻³, respectively. The vertical distribution of gaseous Hg in the borehole closest to the disposal site shows distinct subsurface peaks in concentration at depths of 1.5 and 24 m that cannot be explained by radial diffusive transport through a heterogeneous layered unsaturated zone. The inability of current models to explain gaseous Hg distribution at the ADRS highlights the need to advance the understanding of gas-phase contaminant transport in unsaturated zones to attain a comprehensive model of landfill Hg release.     

Eolian controlled modification of the martian south polar layered deposits

Data from the Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) mission and the Thermal Emission Imaging System (THEMIS) aboard the Mars Odyssey mission have revealed unique surface features in a particular region of the South Polar Layered Deposits (SPLD). The dominant morphology is large-scale quasi-parallel grooves that extend for hundreds of kilometers with only tens of meters of vertical relief, that we have termed here the “Wire Brush” terrain. The grooves are also transected by disjointed, yet roughly continuous, low-relief sinuous ridges that cross roughly perpendicular to the trend-direction of the large-scale grooves and show only tens of meters of relief. We interpret these ridges to be eroded remnants of folded layers. At the northern end of the large-scale grooves there are non-symmetric mounds. They are frequently preceded by a significant depression and/or trailing grooves that are parallel to the Wire Brush trend. We find that a two-stage process involving winds that intermittently remove a low-density crust exposing the underlying ice to ablation is the interpretation that best explains the multitude of features observed here. These features appear to be currently inactive indicating higher winds in previous epochs.     

A new hematite formation mechanism for Mars

Abstract— The origin of hematite detected in Martian surface materials is commonly attributed to weathering processes or aqueous precipitation. Here, we present a new hematite formation mechanism that requires neither water nor weathering. Glass‐rich basalts with Martian meteorite‐like chemistry (high FeO, low Al₂O₃) oxidized at high (700 and 900 °C) temperatures in air and CO₂, respectively, form thin (<1 μm) hematite coatings on their outermost surfaces. Hematite is manifested macroscopically by development of magnetism and a gray, metallic sheen on the glass surface and microscopically by Fe enrichment at the glass surface observed in element maps. Visible and near‐infrared, thermal infrared, and Raman spectroscopy confirm that the Fe enrichment at the oxidized glass surfaces corresponds to hematite mineralization. Hematite formation on basaltic glass is enabled by a mechanism that induces migration of Fe²⁺ to the surface of an oxidizing glass and subsequent oxidation to form hematite. A natural example of the hematite formation mechanism is provided by a Hawaiian basalt hosting a gray, metallic sheen that corresponds to a thin hematite coating. Hematite coating development on the Hawaiian basalt demonstrates that Martian meteorite‐like FeO contents are not required for hematite coating formation on basalt glass and that such coatings form during initial extrusion of the glassy basalt flows. If gray hematite originating as coatings on glassy basalt flows is an important source of Martian hematite, which is feasible given the predominance of igneous features on Mars, then the requirement of water as an agent of hematite formation is eliminated.     

Integrating observations and models to determine the effect of seasonally frozen ground on hydrologic partitioning in alpine hillslopes in the Colorado Rocky Mountains,USA

This study integrated spatially distributed field observations and soil thermal models to constrain the impact of frozen ground on snowmelt partitioning and streamflow generation in an alpine catchment within the Niwot Ridge Long-Term Ecological Research site, Colorado, USA. The study area was comprised of two contrasting hillslopes with notable differences in topography, snow depth and plant community composition. Time-lapse electrical resistivity surveys and soil thermal models enabled extension of discrete soil moisture and temperature measurements to incorporate landscape variability at scales and depths not possible with point measurements alone. Specifically, heterogenous snowpack thickness (~0–4 m) and soil volumetric water content between hillslopes (~0.1–0.45) strongly influenced the depths of seasonal frost, and the antecedent soil moisture available to form pore ice prior to freezing. Variable frost depths and antecedent soil moisture conditions were expected to create a patchwork of differing snowmelt infiltration rates and flowpaths. However, spikes in soil temperature and volumetric water content, as well as decreases in subsurface electrical resistivity revealed snowmelt infiltration across both hillslopes that coincided with initial decreases in snow water equivalent and early increases in streamflow. Soil temperature, soil moisture and electrical resistivity data from both wet and dry hillslopes showed that initial increases in streamflow occurred prior to deep soil water flux. Temporal lags between snowmelt infiltration and deeper percolation suggested that the lateral movement of water through the unsaturated zone was an important driver of early streamflow generation. These findings provide the type of process-based information needed to bridge gaps in scale and populate physically based cryohydrologic models to investigate subsurface hydrology and biogeochemical transport in soils that freeze seasonally.     

Laboratory simulations of prebiotic molecule stability in the jarosite mineral group; end member evaluation of detection and decomposition behavior related to Mars sample return

Recently, the prebiotic amino acid glycine has been found associated with natural jarosite samples from locations around the world. Since the discovery of jarosite on Mars, extensive research focuses on linking this mineral group with possible detection of biosignatures in the geologic record on Earth and Mars. Multiple analytical methods, including extraction and mass spectrometry techniques, have identified glycine and other biomolecules in jarosite samples. The jarosite end members jarosite (sensu stricto-potassium jarosite), natrojarosite (sodium jarosite), and ammoniojarosite (ammonium jarosite) have different thermodynamic stabilities, decompose at different rates, and have potentially different susceptibilities to substitution. The relationship between the thermodynamic stability of the jarosite end members and the effect that glycine has on the thermal decomposition behavior of each end member was investigated using thermal gravimetric analysis. Introducing glycine into the synthesis procedure (75 ppm) of the potassium, sodium, and ammonium jarosite end member has elucidated the effects that glycine has on the thermal stability of the mineral group. Potassium jarosite appears to be the least susceptible to the effects of glycine, with the sodium and ammonium end members showing marked changes in thermal decomposition behavior and decomposition rates. These results suggest that the sodium and ammonium jarosites are more suitable targets for identifying signs of prebiotic or biotic activity on Mars and Earth than the potassium jarosites. These results have implications for current in situ investigations of the martian surface and future sample return missions.     

Modeling effects of multinode wells on solute transport

Long-screen wells or long open boreholes with intraborehole flow potentially provide pathways for contaminants to move from one location to another in a ground water flow system. Such wells also can perturb a flow field so that the well will not provide water samples that are representative of ground water quality a short distance away from the well. A methodology is presented to accurately and efficiently simulate solute transport in ground water systems that include wells longer than the grid spacing used in a simulation model of the system and hence are connected to multiple nodes of the grid. The methods are implemented in a MODFLOW-compatible solute-transport model and use MODFLOW's Multi-Node Well Package but are generic and can be readily implemented in other solute-transport models. For nonpumping multinode wells (used to simulate open boreholes or observation wells, for example) and for low-rate pumping wells (in which the flow between the well and the ground water system is not unidirectional), a simple routing and local mixing model was developed to calculate nodal concentrations within the borehole. For high-rate pumping multinode wells (either withdrawal or injection, in which flow between the well and the ground water system is in the same direction at all well nodes), complete and instantaneous mixing in the wellbore of all inflows is assumed.     

The rayed crater Zunil and interpretations of small impact craters on Mars

A 10-km diameter crater named Zunil in the Cerberus Plains of Mars created ∼¹⁰⁷ secondary craters 10 to 200 m in diameter. Many of these secondary craters are concentrated in radial streaks that extend up to 1600 km from the primary crater, identical to lunar rays. Most of the larger Zunil secondaries are distinctive in both visible and thermal infrared imaging. MOC images of the secondary craters show sharp rims and bright ejecta and rays, but the craters are shallow and often noncircular, as expected for relatively low-velocity impacts. About 80% of the impact craters superimposed over the youngest surfaces in the Cerberus Plains, such as Athabasca Valles, have the distinctive characteristics of Zunil secondaries. We have not identified any other large (?10 km diameter) impact crater on Mars with such distinctive rays of young secondary craters, so the age of the crater may be less than a few Ma. Zunil formed in the apparently youngest (least cratered) large-scale lava plains on Mars, and may be an excellent example of how spallation of a competent surface layer can produce high-velocity ejecta (Melosh, 1984, Impact ejection, spallation, and the origin of meteorites, Icarus 59, 234-260). It could be the source crater for some of the basaltic shergottites, consistent with their crystallization and ejection ages, composition, and the fact that Zunil produced abundant high-velocity ejecta fragments. A 3D hydrodynamic simulation of the impact event produced 10¹⁰ rock fragments ?10 cm diameter, leading to up to 10⁹ secondary craters ?10 m diameter. Nearly all of the simulated secondary craters larger than 50 m are within 800 km of the impact site but the more abundant smaller (10-50 m) craters extend out to 3500 km. If Zunil is representative of large impact events on Mars, then secondaries should be more abundant than primaries at diameters a factor of ∼1000 smaller than that of the largest primary crater that contributed secondaries. As a result, most small craters on Mars could be secondaries. Depth/diameter ratios of 1300 small craters (10-500 m diameter) in Isidis Planitia and Gusev crater have a mean value of 0.08; the freshest of these craters give a ratio of 0.11, identical to that of fresh secondary craters on the Moon (Pike and Wilhelms, 1978, Secondary-impact craters on the Moon: topographic form and geologic process, Lunar Planet. Sci. IX, 907-909) and significantly less than the value of ∼0.2 or more expected for fresh primary craters of this size range. Several observations suggest that the production functions of Hartmann and Neukum (2001, Cratering chronology and the evolution of Mars, Space Sci. Rev. 96, 165-194) predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications.     

ISCO for Groundwater Remediation: Analysis of Field Applications and Performance

A critical analysis of in situ chemical oxidation (ISCO) projects was performed to characterize situations in which ISCO is being implemented, how design and operating parameters are typically employed, and to determine the performance results being achieved. This research involved design of a database, acquisition and review of ISCO project information, population of the database, and analyses of the database using statistical methods. Based on 242 ISCO projects included in the database, ISCO has been used to treat a variety of contaminants; however, chlorinated solvents are by far the most common. ISCO has been implemented at sites with varied subsurface conditions with vertical injection wells and direct push probes being the most common delivery methods. ISCO has met and maintained concentrations below maximum contaminant levels (MCLs), although not at any sites where dense nonaqueous phase liquids (DNAPL) were presumed to be present. Alternative cleanup levels and mass reduction goals have also been attempted, and these less stringent goals are met with greater frequency than MCLs. The use of pilot testing is beneficial in heterogeneous geologic media, but not so in homogeneous media. ISCO projects cost $220,000 on average, and cost on average $94/yd³ of target treatment zone. ISCO costs vary widely based on the size of the treatment zone, the presence of DNAPL, and the oxidant delivery method. No case studies were encountered in which ISCO resulted in permanent reductions to microbial populations or sustained increases in metal concentrations in groundwater at the ISCO-treated site.