Translation of field tracer-test results into bounding predictions of matrix diffusion in the shallow subsurface at Idaho National Laboratory, USA

Matrix-diffusion parameters deduced from an infiltration tracer test at Idaho National Laboratory (INL), USA, are combined with other site information in an analysis involving two dimensionless lumped parameters to assess the effects of matrix diffusion on contaminant transport at the INL over longer distance and time scales than were evaluated in the test. Matrix diffusion was interrogated in the test by comparing, in three different observation wells, the breakthrough curves of two simultaneously injected nonsorbing solutes that have different diffusion coefficients. The matrix-diffusion parameters deduced from the different breakthrough curves were in good agreement, suggesting that the parameters may be broadly applicable at the INL. With this in mind, the uncertainties in the individual parameters that make up the two lumped parameters were estimated, and the resulting ranges of parameter values were used to assess matrix diffusion over larger scales. Assessments of the effects of flow transients, spatial heterogeneity in transport parameters, and sorption on solute transport in the shallow subsurface flow system were also conducted. The methods presented here should be generally applicable to other settings for making bounding assessments of the effects of matrix diffusion while honoring the information obtained from tracer tests and other supporting data.     

Electrical conductivity of orthopyroxene and plagioclase in the lower crust

The electrical conductivities of lower crustal orthopyroxene and plagioclase, as well as their dependence on water content, were measured at 6–12 kbar and 300–1,000°C on both natural and pre-annealed samples prepared from fresh mafic xenolith granulites. The complex impedance was determined in an end-loaded piston cylinder apparatus by a Solarton-1260 Impedance/Gain Phase analyzer in the frequency range of 0.1–10⁶ Hz. The spectra usually show an arc over the whole frequency range at low temperature and an arc plus a tail in the high and low frequency range, respectively, at high temperature. The arc is due to conduction in the sample interior, while the tails are probably due to electrode effects. Different conduction mechanisms have been identified under dry and hydrous conditions. For the dry orthopyroxene, the activation enthalpy is ~105 kJ/mol, and the conduction is likely due to small polarons, e.g., electrons hopping between Fe²⁺ and Fe³⁺. For the dry plagioclase, the activation enthalpy is ~161 kJ/mol, and the conduction may be related to the mobility of Na⁺. For the hydrous samples, the activation enthalpy is ~81 kJ/mol for orthopyroxene and ~77 kJ/mol for plagioclase, and the electrical conductivity is markedly enhanced, probably due to proton conduction. For each mineral, the conductivity increases with increasing water content, with an exponent of ~1, and the activation enthalpies are nearly independent of water content. Combining these data with our previous work on the conductivity of lower crustal clinopyroxene, the bulk conductivity of lower crustal granulites is modeled, which is usually >~10⁻⁴ S/m in the range of 600–1,000°C. We suggest that the high electrical conductivity in most regions of the lower crust, especially where it consists mostly of granulites, can be explained by the main constitutive minerals, particularly if they contain some water. Contributions from other highly conducting materials such as hydrous fluids, melts, or graphite films are not strictly necessary to explain the observed conductivities.     

XMCD at Fe L<Subscript>2,3</Subscript> edges,Fe and S K edges on Fe<Subscript>7</Subscript>S<Subscript>8</Subscript>

Pyrrhotite (Fe₇S₈) is a natural iron sulphide that can participate in rock magnetisation. Its electronic structure is not yet surely described. X-ray magnetic circular dichroism (XMCD) at Fe L_2,3 edges on Fe₇S₈, coupled with multiplet calculations, shows that iron is present only as Fe²⁺ in this magnetic iron sulphide. It reveals a strong magnetic orbital moment. XMCD at Fe and S K edges shows the quite strong polarization of both Fe and S in Fe₇S₈.     

Static compression of α-MnS at 298 K to 21 GPa

To investigate the equation of state of -MnS at high pressure and the possibility of a phase transition, the compression curve was measured at 298 K from 0 to 21 GPa using powder x-ray diffraction with a diamond anvil cell. The compression data are fit to a thirdorder Birch-Murnaghan equation of state, with parameters K ₀ = 72(2) GPa and K ₀ = 4.2(13). To compare present results with previous work, the data sets from three previous investigations (Clendenen and Drickamer 1966; Wakabayashi et al. 1968; Kraft and Greuling 1988) are refit to a Birch-Murnaghan equation of state. In the low pressure region (P < 10=" gpa),=" the=" results=" of=" clendenen=" and=" drickamer=" (1966)=" agree=" with=" the=" present=" data;=" however=" the=" results=" of=" wakbayashi=" et=" al.=" (1968)=" differ=" by=" more=" than=" 10%.=" a=" greater=" discrepancy=" between=" the=" present=" and=" previous=" results=" occurs=" above=" 10=" gpa.=" kraft=" and=" greuling=" (1988)=" reported=" a=" structure=" transition=" at=" 7=" gpa,=" and=" clendenen=" and=" drickamer=" (1966)=" observed=" a=" structure=" distortion=" at=" approximately=" 10=" gpa;=" the=" present=" data=" show=" no=" evidence=" of=" either=" transition,=" and=" are=" well=" fit=" by=" a=" single=" equation=" of=" state=" from=" 0=" to=" 21=" gpa.=" nonhydrostatic=" stress=" is=" discussed=" as=" one=" possibility=" for=" the=">     

Quantifying fracture geometry with X-ray tomography: Technique of Iterative Local Thresholding (TILT) for 3D image segmentation

This paper presents a new method—the Technique of Iterative Local Thresholding (TILT)—for processing 3D X-ray computed tomography (xCT) images for visualization and quantification of rock fractures. The TILT method includes the following advancements. First, custom masks are generated by a fracture-dilation procedure, which significantly amplifies the fracture signal on the intensity histogram used for local thresholding. Second, TILT is particularly well suited for fracture characterization in granular rocks because the multi-scale Hessian fracture (MHF) filter has been incorporated to distinguish fractures from pores in the rock matrix. Third, TILT wraps the thresholding and fracture isolation steps in an optimized iterative routine for binary segmentation, minimizing human intervention and enabling automated processing of large 3D datasets. As an illustrative example, we applied TILT to 3D xCT images of reacted and unreacted fractured limestone cores. Other segmentation methods were also applied to provide insights regarding variability in image processing. The results show that TILT significantly enhanced separability of grayscale intensities, outperformed the other methods in automation, and was successful in isolating fractures from the porous rock matrix. Because the other methods are more likely to misclassify fracture edges as void and/or have limited capacity in distinguishing fractures from pores, those methods estimated larger fracture volumes (up to 80 %), surface areas (up to 60 %), and roughness (up to a factor of 2). These differences in fracture geometry would lead to significant disparities in hydraulic permeability predictions, as determined by 2D flow simulations.     

Effect of Decadal Changes in Freshwater Flows and Temperature on the Larvae of two Forage Fish Species in Coastal Nurseries of the St. Lawrence Estuary

Since 2002, the abundance of larvae of rainbow smelt (Osmerus mordax) and Atlantic herring (Clupea harengus) has been monitored in July in coastal nurseries (Anse Ste.-Anne (ASA) and Banc de Rivière-du-Loup (BRL)) in the Middle St. Lawrence Estuary (MSLE), Canada. The two species are spatiotemporally segregated, with smelt larvae being more abundant at the upstream, less saline site (ASA) and having an earlier hatching date. Despite these differences, the abundances of both species from 2002 to 2013 were strongly and positively related to the early spring flow of tributaries at the time of larval smelt emergence and weakly and negatively related to sea-surface temperatures in the nurseries. Larval herring abundance was weakly associated with the upstream bottom residual transport flow of the MSLE’s estuarine circulation in June, at the time of their emergence. Larval herring lengths in BRL were positively related to the sum of degree days (SDD) from hatching to sampling, with the greatest length but lowest condition in 2012. The relationship between body lengths of ASA smelt larvae and SDD was dome-shaped, suggesting lower growth than expected in the warmest years, 2006 and 2012. The highest larval abundances were observed in 2008 and 2011, both years with late tributary freshets, high tributary flows in the early spring and moderately warm summer temperatures. In contrast, low abundances occurred in 2006 and 2012, which were years with low spring tributary flows and high summer temperatures. These results suggest that the dynamics of local tributary freshets is a key driver of larval recruitment success for two key forage species in coastal nurseries until summer and support the use of fish larvae as indicators of environmental changes in the MSLE.     

Cave air and hydrological controls on prior calcite precipitation and stalagmite growth rates: Implications for palaeoclimate reconstructions using speleothems

Hourly resolved cave air P_CO2 and cave drip water hydrochemical data illustrate that calcite deposition on stalagmites can be modulated by prior calcite precipitation (PCP) on extremely short timescales. A very clear second-order covariation between cave air P_CO2 and drip water Ca²⁺ concentrations during the winter months demonstrates the effects of degassing-induced PCP on drip water chemistry. Estimating the strength of the cave air P_CO2 control on PCP is possible because the PCP signal is so clear; at our drip site a one ppm shift in Ca²⁺ concentrations requires a P_CO2 shift of between 333 and 667 ppm. This value will undoubtedly vary from site to site, depending on drip water flow rate, residence time, drip water-cave air P_CO2 differential, and availability of low P_CO2 void spaces in the vadose zone above the cave. High-resolution cave environmental measurements were used to model calcite deposition on one stalagmite in Crag Cave, SW Ireland, and modelled growth over the study period (222 μm over 171 days) is extremely similar to the amount of actual calcite growth (240 μm) over the same time interval, strongly suggesting that equations used to estimate stalagmite growth rates are valid. Although cave air P_CO2 appears to control drip water hydrochemistry in the winter, drip water dilution caused by rain events may have played a larger role during the summer, as evidenced by a series of sudden drops in Ca²⁺ concentrations (dilution) followed by much more gradual increases in drip water Ca²⁺ concentrations (slow addition of diffuse water). This research demonstrates that PCP on stalactites, cave ceilings, and void spaces within the karst above the cave partially controls drip water chemistry, and that thorough characterisation of this process at individual caves is necessary to most accurately interpret climate records from those sites.     

Deciphering late Quaternary land snail shell δO and δC from Franchthi Cave (Argolid,Greece)

This paper investigates the stable isotopic composition from late Pleistocene–Holocene (~ 13 to ~ 10.5 cal ka BP) shells of the land snail Helix figulina, from Franchthi Cave (Greece). It explores the palaeoclimatic and palaeoenvironmental implications of the isotope palaeoecology of archaeological shells at the time of human occupation of the cave. Modern shells from around the cave were also analysed and their isotopic signatures compared with those of the archaeological shells. The carbon isotope composition of modern shells depicts the consumption of C₃ vegetation. Shell oxygen isotopic values are consistent with other Mediterranean snail shells from coastal areas. Combining empirical linear regression and an evaporative model, the δ¹⁸Os suggest that modern snails in the study area are active during periods of higher relative humidity and lower rainfall δ¹⁸O, probably at night. Late glacial and early Holocene δ¹⁸Os show lower values compared to modern ones. Early Holocene δ¹⁸Os values likely track enhanced moisture and isotopic changes in the precipitation source. By contrast, lower late glacial δ¹⁸O could reflect lower temperatures and δ¹⁸Op, compared to the present day. Shell carbon isotope values indicate the presence of C₃ vegetation as main source of carbon to late glacial and early Holocene snails.