From atmospheric winds to fracture ventilation: Cause and effect

Vadose zone fractures and soil cracks exposed to the atmosphere have an impact on gas exchange processes at the Earth-atmosphere interface. In this study we explored and quantified the role of ground-surface winds on fracture ventilation. While the governing physical mechanisms that cause ventilation are relatively well understood, this is the first work to quantify these processes in natural fractures and to determine the net effect on gas exchange. In this study field measurements pointed to a correlation between surface wind velocity and the ventilate rate of surface-exposed fractures. To better explore and quantify this phenomenon, laboratory experiments were carried out using a Hele-Shaw chamber to simulate a natural fracture and the ventilation of smoke, used as a gas tracer, was explored as a function of controlled surface-wind and fracture aperture. It was found that ventilation depth is linearly correlated to wind velocity and nonlinearly with fracture aperture. Results were used to formulate an empirical model for Earth-atmosphere air exchange. This model can be used to estimate by how much the presence of fractures enhances that exchange under windy conditions. Incorporating this venting process into Earth-atmosphere gas exchange simulations is another step toward improving our ability to better predict and quantify soil aeration, soil temperature variation, water vapor loss and processes related to climate change, such as the fate and transport of greenhouse gases.Keywords: Carbon-Dioxide, Porous solids, Vadose zone, Vapor movements, Air movement, Unsaturated Zone, Pressure fluctuations, Diffusion, Gas, Soi

Drying of Porous Media

Drying of porous media is part of our daily experience, yet this common process is central to many environmental and engineering applications ranging from soil evaporation affecting hydrological water balance and climatic processes, to the drying of food and building materials, and driving plant life through transpiration. Drying rates from porous media may exhibit complex dynamics reflecting internal transport mechanisms and motion of phase change fronts that determine rates of drying and critically affect surface energy partitioning. These interactions and resulting drying dynamics present a challenge to the prediction of drying rates and interplay among mass and energy exchange even for fixed boundary conditions

Infrared thermography of evaporative fluxes and dynamics of salt deposition on heterogeneous porous surfaces

Evaporation of saline solutions from porous media, common in arid areas, involves complex interactions between mass transport, energy exchange and phase transitions. We quantified evaporation of saline solutions from heterogeneous sand columns under constant hydraulic boundary conditions to focus on effects of salt precipitation on evaporation dynamics. Mass loss measurements and infrared thermography were used to quantify evaporation rates. The latter method enables quantification of spatial and temporal variability of salt precipitation to identify its dynamic effects on evaporation. Evaporation from columns filled with texturally-contrasting sand using different salt solutions revealed preferential salt precipitation within the fine textured domains. Salt precipitation reduced evaporation rates from the fine textured regions by nearly an order of magnitude. In contrast, low evaporation rates from coarse-textured regions (due to low capillary drive) exhibited less salt precipitation and consequently less evaporation rate suppression. Experiments provided insights into two new phenomena: (1) a distinct increase in evaporation rate at the onset of evaporation; and (2) a vapor pumping mechanism related to the presence of a salt crust over semidry media. Both phenomena are related to local vapor pressure gradients established between pore water and the surface salt crust. Comparison of two salts: NaCl and NaI, which tend to precipitate above the matrix surface and within matrix pores, respectively, shows a much stronger influence of NaCl on evaporation rate suppression. This disparity reflects the limited effect of NaI precipitation on matrix resistivity for solution and vapor flows

Relationships between gas-liquid interfacial surface area, liquid saturation, and light transmission in variably saturated porous media

Liquid saturation and gas-liquid interfacial area are important parameters for evaluating the transport and fate of contaminants in unsaturated subsurface environments. Recent findings indicate that interfacial surface area controls the relative degree of transmitted light in laboratory systems containing translucent porous media. Equations are derived to estimate the specific gas-liquid interfacial area from the area under the primary-drainage branch of the S[subscript eff]-h characteristic curve as parameterized using common water retention functions. The total area under the curve provides the maximum available specific gas-liquid interfacial area available at residual saturation, which can be incorporated into the relationship to determine the gas-liquid interfacial area at intermediate degrees of saturation via light transmission. Experimental results, and analysis of external data sets, support these findings. Closed-form relationships are presented as enhancements to a recent method for determination of liquid saturations above residual using light transmission. A physically based model is developed and tested for the quantification of liquid contents below residual saturation.Keywords: gas-liquid interfacial surface area, light transmission, liquid saturation, residual saturation, characteristic curve, unsaturated porous medi

Direct visualization of colloid transport over natural heterogeneous and artificial smooth rock surfaces

International audienceColloid transport in fractured rock formations is an important process impacting the fate of pollutants in the subsurface. Despite intensive and outstanding research on their transport phenomena, the impact of small-scale surface heterogeneity on colloid behavior at the fracture scale remains difficult to assess. In particular, there is relatively little direct experimental evidence on the impact of natural fracture surface heterogeneity on colloid transport. To investigate this, we developed an experimental setup allowing the direct visualization of fluorescent colloid transport in a flow cell containing a natural chalk rock sample while simultaneously monitoring effluent colloid concentrations. We used samples containing both a natural fracture surface and an artificially made smooth surface from the same chalk core. We characterized the roughness and chemical composition of both surface types and numerically calculated each surface's velocity field. From the experiments, we obtained direct images of colloid transport over the surfaces, from which we calculated their dispersion coefficients and quantified the residual deposition of colloids on the rock surface. We also measured the colloid breakthrough curves by collecting eluent samples from the flow cell outlet. The natural fracture surface exhibited larger physical and chemical heterogeneity than the smooth, artificially generated surface. The aperture variability across the natural surface led to preferential flow and colloid transport which was qualitatively apparent in the fluorescent images. The colloid transport patterns matched the calculated velocity fields well, directly linking the surface topography and aperture variation to colloid transport. Compared to the artificially made surface, the natural surface also showed higher dispersion coefficients, which corresponded to the colloids' earlier breakthrough from the flow cell. While we found differences between the elemental composition of the natural and artificially smooth surfaces, we could not observe their impact on the colloids' surface attachment and retention. The main novelty in this work is the coupling of direct colloid transport imaging, breakthrough curve measurements, and colloid surface deposition analyses, in a flow cell containing a natural carbonate rock sample. Our experimental setup can be used to further investigate the link between surface heterogeneity, both chemical and physical, and colloid transport and deposition in natural rock fractures

Impact of surface heterogeneity on colloid transport over a natural fractured rock

International audienceThe transport of colloids in fractured media has long been identified as a potential source for enhanced pollutant migration. While extensive research has been carried out for systems at different length scales, the understanding of fundamental mechanisms controlling the enhancement or retardation of colloids in fractured media is still incomplete. In particular, the direct observation of colloid transport over natural fractured rock surfaces, and the relation between transport behavior to the heterogeneity of the surface, has rarely been investigated. Here we used a custom designed flow cell to perform colloid transport experiments with natural rock samples taken from a chalk formation in the Negev desert of Israel. We used samples containing a natural fracture surface with varying degrees of heterogeneity, and a sample cut and polished from non-fractured rock cores. We used a transparent glass top cover to allow direct visualization of the synthetic fluorescent colloids. We mounted the flow cell under a fluorescence microscope, and passed a suspension containing the colloids through the cell. We took images of the rock surface periodically to assess the dynamics of colloid flow over the surface, and the deposition of colloids on the surface itself. Images of the advancing colloid front revealed that their breakthrough was strongly influenced by the topography of the rock surface samples, which resulted in a strongly preferential transport pattern for the more heterogeneous rock surface. Natural fracture samples also exhibited slightly earlier arrival of the colloids as measured from their breakthrough curves. The natural fracture surfaces also exhibited higher residual concentration of deposited colloids, after the pulse was washed from the cell. In addition, areas with no colloid deposition on the natural fracture surfaces were related to the deposition of various metal oxides on the rock surface, as indicated by scanning electron microscope analysis. Our results show how both physical and chemical heterogeneity of natural rock surfaces can impact colloid transport processes, and indicate their importance for understanding colloid transport on larger scales

Comparative Study of Methods for Delineating the Wellhead Protection Area in an Unconfined Coastal Aquifer

Various delineation methods, ranging from simple analytical solutions to complex numerical models, have been applied for wellhead protection area (WHPA) delineation. Numerical modeling is usually regarded as the most reliable method, but the uncertainty of input parameters has always been an obstacle. This study aims at examining the results from different WHPA delineation methods and addressing the delineation uncertainty of numerical modeling due to the uncertainty from input parameters. A comparison and uncertainty analysis were performed at two pumping sites—a single well and a wellfield consisting of eight wells in an unconfined coastal aquifer in Israel. By appointing numerical modeling as the reference method, a comparison between different methods showed that a semi-analytical method best fits the reference WHPA, and that analytical solutions produced overestimated WHPAs in unconfined aquifers as regional groundwater flow characteristics were neglected. The results from single well and wellfield indicated that interferences between wells are important for WHPA delineation, and thus, that only semi-analytical and numerical modelling are recommended for WHPA delineation at wellfields. Stochastic modeling was employed to analyze the uncertainty of numerical method, and the probabilistic distribution of WHPAs, rather a deterministic protection area, was generated with considering the uncertain input hydrogeological parameters

Field Scale Mobility and Transport Manipulation of Carbon-Supported Nanoscale Zerovalent Iron in Fractured Media

In field applications, mostly in porous media, transport of stabilized nano zerovalent iron particles (nZVI) has never exceeded a few meters in range. In the present study, the transport of Carbo-Iron Colloids (CIC), a composite material of activated carbon as a carrier for nZVI stabilized by carboxymethyl cellulose (CMC), was tested under field conditions. The field site lies within a fractured chalk aquitard characterized by moderately saline (∼13 mS) groundwater. A forced gradient tracer test was conducted where one borehole was pumped at a rate of 8 L/min and CMC-stabilized CIC was introduced at an injection borehole 47 m up-gradient. Two CIC–CMC field applications were conducted: one used high 100% wt CMC (40 g/L) and a second used lower 9% wt loading (∼2.7 g/L). Iodide was injected as a conservative tracer with the CIC–CMC in both cases. The ratio between the CIC–CMC and iodide recovery was 76% and 45% in the high and low CMC loading experiments, respectively. During the low CMC loading experiment, the pumping rate was increased, leading to an additional CIC recovery of 2.5%. The results demonstrate the potentially high mobility of nZVI in fractured environments and the possibility for transport manipulation through the adjustment of stabilizer concentration and transport velocity