Franco M. Francisca

Thermal conductivity of hydrate‐bearing sediments

[1] A thorough understanding of the thermal conductivity of hydrate-bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone. The coexistence of multiple phases (gas hydrate, liquid and gas pore fill, and solid sediment grains) and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate-bearing sediments. Previous studies have been unable to capture the full parameter space covered by variations in grain size, specific surface, degree of saturation, nature of pore filling material, and effective stress for hydrate-bearing samples. Here we report on systematic measurements of the thermal conductivity of air dry, water- and tetrohydrofuran (THF)-saturated, and THF hydrate-saturated sand and clay samples at vertical effective stress of 0.05 to 1 MPa (corresponding to depths as great as 100 m below seafloor). Results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size, effective stress, porosity, and fluid-versus-hydrate filled pore spaces. The thermal conductivity of THF hydrate-bearing soils increases upon hydrate formation although the thermal conductivities of THF solution and THF hydrate are almost the same. Several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment, increased mean effective stress due to hydrate formation under zero lateral strain conditions, and decreased interface thermal impedance as grain-liquid interfaces are transformed into grain-hydrate interfaces.

Parametric study of the physical properties of hydrate‐bearing sand, silt, and clay sediments: 2. Small‐strain mechanical properties

v and sediment specific surface. The S velocity increases with hydrate saturation owing to an increase in skeletal shear stiffness, particularly when hydrate saturation exceeds Shyd≈ 0.4. At very high hydrate saturations, the small strain shear stiffness is determined by the presence of hydrates and becomes insensitive to changes in effective stress. The P velocity increases with hydrate saturation due to the increases in both the shear modulus of the skeleton and the bulk modulus of pore‐filling phases during fluid‐to‐hydrate conversion. Small‐strain Poisson’s ratio varies from 0.5 in soft sediments lacking hydrates to 0.25 in stiff sediments (i.e., subject to high vertical effective stress or having high Shyd). At Shyd ≥ 0.5, hydrate hinders expansion and the loss of sediment stiffness during reduction of vertical effective stress, meaning that hydrate‐rich natural sediments obtained through pressure coring should retain their in situ fabric for some time after core retrieval if the cores are maintained within the hydrate stability field.

Cancer incidence and pattern of arsenic concentration in drinking water wells in Córdoba, Argentina

Cancer occurrence is associated with Arsenic (As) in drinking water. In Argentina, there are high As concentrations in groundwater but there is no published evidence yet of an association between geographic patterns of cancer incidence and the distribution of As in groundwater supplies. The purpose of this study is to assess the association between cancer incidence patterns and As in Córdoba provinces aquifers. Age standardized incidence rates (ASIRs) were obtained from Córdoba Cancer Registry (CCR), and As data from official reports of monitoring wells. A multilevel model was applied. Total ASIRs by aquifers for males/females were 191.01/249.22 (Rioja plain); 215.03/225.37 (Pampa hills); and 239.42/188.93 (Chaco-Pampa plain). As was associated with increased risk of colon cancer in women, and lung and bladder cancers in both sexes. It had no association with breast cancer. ASIRs were related to As, controlling for unobserved heterogeneity. An overlapping pattern of higher As and higher risks was evident for lung, bladder and female colon cancers.

Assessment of natural arsenic in groundwater in Cordoba Province, Argentina

Groundwater in the central part of Argentina contains arsenic concentrations that, in most cases, exceed the value suggested by international regulations. In this region, Quaternary loessical sediments with a very high volcanic glass fraction lixiviate arsenic and fluoride after weathering. The objectives of this study are to analyze the spatial distribution of arsenic in different hydrogeological regions, to define the naturally expected concentration in an aquifer by means of hydrogeochemistry studies, and to identify emergent health evidences related to cancer mortality in the study area. The correlation between arsenic and fluoride concentrations in groundwater is analyzed at each county in the Cordoba Province. Two dimensionless geoindicators are proposed to identify risk zones and to rapidly visualize the groundwater quality related to the presence of arsenic and fluoride. A surface-mapping system is used to identify the spatial variability of concentrations and for suggesting geoindicators. The results show that the Chaco-Pampean plain hydrogeologic region is the most affected area, with arsenic and fluoride concentrations in groundwater being generally higher than the values suggested by the World Health Organization (WHO) for drinking water. Mortality related to kidney, lung, liver, and skin cancer in this area could be associated to the ingestion of arsenic-contaminated water. Generated maps provide a base for the assessment of the risk associated to the natural occurrence of arsenic and fluoride in the region.

Hydraulic conductivity of compacted soils controlled by microbial activity.

The hydraulic conductivity defines the displacement of liquids inside porous media and affects the fate and transport of contaminants in the environment. In this research the influence of microbial growth and decay inside soil pores on hydraulic conductivity is analysed. Long-term tests performed in silt–bentonite mixtures permeated with distilled water and a nutrients solution demonstrated that hydraulic conductivity of compacted silt–bentonite samples decreases with time of permeation as a bioclogging mechanism develops. The injection of antibiotics and antifungals in the specimens produces a rebound in the hydraulic conductivity associated with the decay of microbial activity. These results show that biomediated reactions can be used to control the flow rate through compacted soil liners.

Influence of pH on cadmium, copper, and lead removal from wastewater by steel slag

AbstractSteel slag is a byproduct of steel manufacturing which raises an important environmental concern due to the high volume of generation and, specially, its basicity. The aim of this work was to determine its recycling potential and use for the removal of Cd, Cu, and Pb given its ability to increase the pH of a solution to values close to 13.5. Remediation of cadmium-, copper-, and lead-contaminated water was studied by means of induced precipitation/sorption reactions. Precipitation of these metals was evaluated by kinetic and batch tests that were analyzed by commonly used sorption models. The results confirmed that Cd, Cu, and Pb can be removed from solutions by the addition of steel slag particles, while the removal efficiency depends on metal type and on the ion interaction, concentration, and pH of precipitation of each metal.

Digital analysis technique for uncertainty reduction in colorimetric arsenic detection method

This article proposes an alternative to increase the reliability and reproducibility of a colorimetric method to measure arsenic (As) concentrations. The method of analysis developed incorporates a digital analysis technique to eliminate the operator dependence of results, and As concentrations are quantitatively determined from digital levels computed from photographs of the colorimetric reaction that emerges during the test. This technique allows the sensitivity of the detection to be increased at low concentration ranges, which is of fundamental importance for the detection of As given the current acceptable limit for drinking water. The results obtained show a very good correlation between As concentrations determined by means of analytical laboratory techniques and the method proposed in this research.

Potential use of calcareous mudstones in low hydraulic conductivity earthen barriers for environmental applications

Earthen layers play a significant role in isolating contaminants in the subsurface, controlling the migration of contaminant plumes, and as landfill liners and covers. The physical, chemical and mineralogical properties of three calcareous mudstones from the Jagüel and Roca formations in North Patagonia, Argentina, are evaluated to determine their potential for the construction of liners. These mudstones were deposited in a marine environment in the Upper Cretaceous–Paleocene. The tested specimens mainly comprise silt and clay-sized particles, and their mineralogy is dominated by a smectite/illite mixed layer (70–90% Sm) and calcite in smaller proportion. Powdered mudstone samples have little viscosity and swelling potential when suspended in water. The hydraulic conductivity of compacted mudstones and sand–mudstone mixtures is very low (around 1–3×10−10 m/s) and in good agreement with the expected hydraulic behaviour of compacted earthen layers. This behaviour can be attributed to the large amount of fine particles, high specific surface and the close packing of particles as confirmed by scanning electron microscope analysis. The tested materials also show a high cation exchange capacity (50–70 cmol/kg), indicating a high contaminant retardation capability. The calcareous mudstones show satisfactory mineralogical and chemical properties as well as an adequate hydraulic behaviour, demonstrating the potential use of these materials for the construction of compacted liners for the containment of leachate or as covers in landfills. These findings confirm the potential usage of marine calcareous mudstones as a low-cost geomaterial in environmental engineering projects.

Arsenic Entrapment in Reactive Columns of Residual Soils

AbstractThe purpose of this research is to evaluate the aptitude of two residual soils for the remediation of groundwater contaminated by arsenic (As). Batch and soil column tests were performed to evaluate removal of As from water. Permeable soil columns were prepared with mixtures of sand (90%) and two lateritic soils (10%). Results showed that removal of As in batch tests ranged from 95–99% when using a solid-liquid ratio equal to 1∶10. Experimental results obtained in the soil column tests and calibrated simulation models showed that the amount of water that can be decontaminated depends on the initial As concentration and the flow rate. Maximum adsorption capacities in bath test and removal efficiency in soil columns tests clearly relates with the amount of iron compounds in the solid phase. The quality of residual soil samples are related to a high iron content to favor As removal and low clay-size particle content to achieve acceptable flow rates for water purification in low-cost permeable filters...

Digital Image Analysis of Distribution of Immiscible Fluids in Natural Porous Media

This article proposes a method of digital image analysis to monitor immiscible flow in soil samples. Four different natural soil samples were studied: coarse, medium, fine, and silty sand. Paraffin oil and a sodium fluorescein solution were used as the displaced and displacing phase, respectively. Immiscible flow tests were performed in a Plexiglas cell and monitored by means of digital image analysis and direct volumetric measurement. The images were processed to obtain gray levels at different stages during the tests. Saturation of paraffin oil was then computed from the gray level and a linear regression equation was developed for each soil tested. The proposed method enabled valuable information to be obtained of the processes that take place at the face of the samples during liquid displacement and in the middle of the samples at the end of the tests. This procedure was very effective for analyzing different flow patterns, isolating areas with trapped paraffin oil, and to determining the ganglia size distribution at different stages of the immiscible flow tests. The results indicated that particle size and microstructure heterogeneities determine the flow pattern features observed during the immiscible displacement of paraffin oil by water.