Single-particle crushing strength under different relative humidity conditions

This paper presents a fundamental study on the effect of the relative humidity on the rockfill crushing strength. This aspect plays an important role in the mechanical behaviour of rockfill, and it is known that certain characteristics of the granular materials, such as compressibility and shear strength, depend on the confining stress, which is a function of the particles crushing. An increased interest has been observed regarding the effect of the relative humidity in the mechanical behaviour of rockfill. Unfortunately, limited research has been conducted until now regarding the study of individual particle crushing. Therefore, this paper thoroughly investigated particle crushing, by performing single-particle crushing tests on rockfill particles divided into four size ranges, under different relative humidity conditions. The experimental results reveal a considerable influence of the relative humidity in the studied rockfill particles, whose strength of the particles with the greatest dimensions in saturated conditions was reduced by half. Consistent macro-mechanical evidence demonstrates that particle’s size and relative humidity conditions depict the most important factors that influence particle crushing strength.

     

Macrofaunal density and biomass in the Campeche Canyon, Southwestern Gulf of Mexico

The composition, density and community structure of the benthic macrofauna were investigated in sediments of the Campeche Canyon in the SW Gulf of Mexico. Total macrofaunal density ranged from 9466±2736 ind m⁻² at the continental shelf station to 1550±195 ind m⁻² in the canyon. Density values significantly diminished with distance from the coast and depth; only a few stations in the center of the canyon displayed larger density values (E-37 with 4666±1530 ind m⁻², E-36 with 5791±642 ind m⁻² and E-26 with 6925±2258 ind m⁻²). Densities were positively correlated to organic nitrogen in the sediment (r=0.82) and coarse silt (r=0.43), and negatively with depth (r=−0.74) and distance from the coast (r=−0.68). At all stations, the polychaete worms contributed most to the multi-species community structure. The nematodes and Foraminifera displayed their highest densities in the center of the canyon. The biomass values declined significantly with depth. We conclude that the macrofauna density and biomass changed in response to organic matter contents in the sediment, both with distance from the coast and with depth.     

Trace element and molecular markers of organic carbon dynamics along a shelf–basin continuum in sediments of the western Arctic Ocean

Molecular organic biomarkers together with trace element composition were investigated in sediments east of Barrow Canyon in the western Arctic Ocean to determine sources and recycling of organic carbon in a continuum from the shelf to the basin. Algal biomarkers (polyunsaturated and short-chain saturated fatty acids, 24-methylcholesta-5,24(28)-dien-3β-ol, dinosterol) highlight the substantial contribution of organic matter from water column and sea-ice primary productivity in shelf environments, while redox markers such as acid volatile sulfide (AVS), Mn, and Re indicate intense metabolism of this material leading to sediment anoxia. Shelf sediments also receive considerable inputs from terrestrial organic carbon, with biomarker composition suggesting the presence of multiple pools of terrestrial organic matter segregated by age/lability or hydrodynamic sorting. Sedimentary metabolism was not as intense in slope sediments as on the shelf; however, sufficient labile organic matter is present to create suboxic and anoxic conditions, at least intermittently, as organic matter is focused towards the slope. Basin sediments also showed evidence for episodic delivery of labile organic carbon inputs despite the strong physical controls of water depth and sea-ice cover. Principal components analysis of the lipid biomarker data was used to estimate fractions of preserved recalcitrant (of terrestrial origin) and labile (of marine origin) organic matter in the sediments, with ranges of 12–79%, 14–45%, and 37–66% found for the shelf, slope, and basin cores, respectively. On average, the relative preserved terrestrial organic matter in basin sediments was 56%, suggesting exchange of organic carbon between nearshore and basin environments in the western Arctic.     

Size Fractionation (Dissolved, Colloidal and Particulate) of Trace Metals in the Thur River, Switzerland

The distribution of some trace metals (Cu, Zn, Ni, Co,Fe, Mn) and of DOC over a particulate (> 1 m),a colloidal (size < 0.45 m and molecular weight > 10 kD) and an ultrafiltered fraction (< 10 kD)was determined at several sites on the Thur River,Switzerland, at various times of the year. Thecomplexation of Cu by strong ligands in theultrafiltrate and in the conventional filtrate (<0.45 m) was compared using a ligand-exchange/CSV method.The <0.45 m concentrations of Cu (from anaverage of 7 nM to 24 nM), Zn (<5–23 nM), Ni (5–13 nM),Co (1.5–3 nM) and Mn (7–92 nM)increased downstream. The major part of Cu, Zn, Niand Co usually occurred in the ultrafiltratefraction at all sites, whereas Fe and Mn were mostlyin the particulate fraction, under conditions of lowsuspended matter content (< 10 mg L^-1) in theriver. The percentage of metal in the colloidalfraction, with respect to the 0.45-m filtrate,decreased in the order: Cu (median 11%) > Zn Ni(median 5–6%) > Mn Co (median < 5%). DOCalso consisted mostly of molecules in the < 10 kDrange.Cu was strongly complexed by natural organic ligandsin all filtrate and ultrafiltrate samples. A largepart of the strong Cu binding ligands consisted ofcompounds in the < 10 kD range, but colloidalligands with similar properties also occurred. Cu wasdistributed among the dissolved and the colloidalligands, roughly in proportion to organic carbon.The colloidal fraction (as defined here) did notincrease in its proportional amount downstream and wasonly of limited significance in transporting traceelements in the Thur River under low discharge conditions.     

Experimental investigation of the effects of temperature and ionic strength on Mo isotope fractionation during adsorption to manganese oxides

The Mo stable isotope system is being applied to study changes in ocean redox. Such applications implicitly assume that Mo isotope fractionation in aqueous systems is relatively insensitive to frequently changing environmental variables such as temperature (T) and ionic strength (I). A major driver of fractionation is the adsorption of Mo to Mn oxyhydroxide surfaces [Barling J. and Anbar A. D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett.217(3-4), 315-329]. Here, we report the results of experiments that determine the extent to which Mo isotope fractionation during adsorption of Mo to the Mn oxyhydroxide mineral birnessite is sensitive to T and I. The results are compared to new predictions from quantum chemical computations. We measured fractionation from 1 to 50 °C at I = 0.1 m and found that Δ^97/95Mo_{dissolved-adsorbed} varies from 1.9‰ to 1.6‰ over this temperature range. Experiments were also performed at 25 °C in synthetic seawater (I = 0.7); fractionation at this condition was the same within analytical error as in low ionic strength experiments. These findings confirm that the Mo isotope fractionation during adsorption to Mn oxyhydroxides is relatively insensitive to variations and T and I over environmentally relevant ranges. To relate these findings to potential mechanisms of Mo isotope fractionation, we also report results for density functional theory computations of the fractionation between and various possible structures of molybdic acid as a function of temperature. Because no plausible species fractionates from with a magnitude matching the experiments, we are left with three possibilities to explain the fractionation: (1) solvation effects on the vibrational frequencies of aqueous species considered thus far are significant, such that our calculations in vacuo yield inaccurate fractionations; (2) a trace aqueous species not yet considered fractionates from and then adsorbs to birnessite; or (3) a surface complex not present in solution forms on birnessite in which Mo is not tetrahedrally coordinated. Our findings help validate assumptions underlying paleoceanographic applications of the Mo isotope system and also lead us closer to understanding the mechanism of isotope fractionation during adsorption of Mo to Mn oxyhydroxides.     

Effect of the dilution process on the attenuation of contaminants in aquifers

This paper discusses the effects of the dilution process on levels of aquifer contamination attenuation, developing analytical equations for application in theoretical and practical cases. The key finding is that, in aquifers, the dilution process causes a reduction of the contaminant concentration. Analytical equations for the assessment of the contaminant attenuation capacity, by means of the dilution process in an aquifer, are put forward. In particular, the attenuation of the contaminant concentration in an aquifer, and thus the dilution, is described by the volumetric flow rate per unit width perpendicular to the flow direction (q _u). Moreover, a classification of q _u was developed, considering six classes of attenuation capacity due to dilution. For the equations validation, nitrate contamination attenuation due to dilution was analysed in two sample areas in Piedmont (North-West Italy). The results confirmed the hypothesis, evidencing high nitrate concentrations in areas with low q _u values, connected to low dilution rate of the contaminant in the groundwater; vice versa, low nitrate concentrations were evidenced in areas with high q _u values, due to a high nitrate dilution rate.     

Seal integrity and feasibility of CO<Subscript>2</Subscript> sequestration in the Teapot Dome EOR pilot: geomechanical site characterization

This paper reports a preliminary investigation of CO₂ sequestration and seal integrity at Teapot Dome oil field, Wyoming, USA, with the objective of predicting the potential risk of CO₂ leakage along reservoir-bounding faults. CO₂ injection into reservoirs creates anomalously high pore pressure at the top of the reservoir that could potentially hydraulically fracture the caprock or trigger slip on reservoir-bounding faults. The Tensleep Formation, a Pennsylvanian age eolian sandstone is evaluated as the target horizon for a pilot CO₂ EOR-carbon storage experiment, in a three-way closure trap against a bounding fault, termed the S1 fault. A preliminary geomechanical model of the Tensleep Formation has been developed to evaluate the potential for CO₂ injection inducing slip on the S1 fault and thus threatening seal integrity. Uncertainties in the stress tensor and fault geometry have been incorporated into the analysis using Monte Carlo simulation. The authors find that even the most pessimistic risk scenario would require ∼10 MPa of excess pressure to cause the S1 fault to reactivate and provide a potential leakage pathway. This would correspond to a CO₂ column height of ∼1,500 m, whereas the structural closure of the Tensleep Formation in the pilot injection area does not exceed 100 m. It is therefore apparent that CO₂ injection is not likely to compromise the S1 fault stability. Better constraint of the least principal stress is needed to establish a more reliable estimate of the maximum reservoir pressure required to hydrofracture the caprock.     

Groundwater residence times and flow paths in fractured rock determined using environmental tracers in the Mission Tunnel; Santa Barbara County,California, USA

Multiple geochemical tracers [ion chemistry, stable isotopes of water, chlorofluorocarbons (CFC), tritium] and a 25-year-long record of discharge were used to understand residence times and flow paths of groundwater seeps in the fractured rock aquifer surrounding the Mission Tunnel, Santa Barbara, California. Tritium data from individual seeps indicate that seep waters are a mixture of >45-year-old (recharged prior to the nuclear bomb tests) and young groundwater. CFC data support this interpretation, however, a two-end member mixing model cannot completely explain the age tracer data. Microbial degradation and partial re-equilibration complicate the CFC signal. Spectral analysis of precipitation and groundwater seepage records shows that seepage lags precipitation by 3 months. This delay is related to the advancement of the wetting front and increasing the number of active flow paths. Additionally, the amount of seepage produced by precipitation is less during extended periods of drought than during normal or wet periods, suggesting antecedent conditions strongly affect flow through this fractured rock aquifer.     

Assessment of the role of facies heterogeneity at the fine scale by numerical transport experiments and connectivity indicators

The heterogeneity of facies at the scale of individual lithological levels controls, at a macroscopic scale, water flow and contaminant transport in porous sediments. In particular the presence of organized features such as permeable connected levels, has a significant effect on travel times and dispersion. Here, the effects of facies heterogeneity on flow and transport are studied for three blocks, whose volume is of the order of a cubic meter, dug from alluvial sediments from the Ticino valley (Italy). Using the results of numerical tracer experiments on these domains, the longitudinal dispersion coefficient is computed with an Eulerian approach based on the fit of the breakthrough curves with the analytical solution of the convective-dispersive transport equation. Moreover, the dispersion tensor is computed with a Lagrangian approach from the second order moments of particle distributions. Three types of connectivity indicators are tested: (1) connectivity function; (2) flow, transport and statistical connectivity; (3) original (intrinsic, normal and total) indicators of facies connectivity. The connectivity function provides the most complete information. Some of the transport and statistical connectivity indicators are correlated with dispersivity. The simultaneous analysis of the three indicators of facies connectivity emphasizes the fundamental geometrical features that control transport.