Radionuclide transport in fractured chalk under abrupt changes in salinity.

Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.

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

Colloid 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.

Effect of atmospheric temperature on underground radon: A laboratory experiment.

The effect of atmospheric temperature on underground radon flow was investigated in a customized climate-controlled laboratory (CCL) system, which enabled us to isolate the impact of ambient atmospheric temperature variations on underground radon transport. The soil thermal gradients that developed, following atmospheric warming, acted as the driving force for the diffusive radon flow, resulting in a decrease in the radon concentration along the experimental column setup at a rate of ∼70 Bq∙m-3 per oC∙m-1 (∼0.4% of the radon concentration). When the ambient temperature decreased, compared to the soil temperature, an air-soil temperature difference developed along the column, which acted as a driving force for radon to flow along the column and promptly increased the radon concentration at a rate of ∼140 Bq∙m-3 per oC (∼0.8% of the radon concentration). The overall radon concentration changes under the experimental conditions were up to 30%. The changes in the molecular diffusion coefficient in the experimental temperature range were ∼7%, with thermal diffusion as a possible additional mechanism contributing to radon transport due to temperature. The cyclic changes in ambient temperature in the forced conditions experiments were found to be directly correlated with underground radon oscillations. The same frequency for ambient temperature and radon concentration, along the experimental column in low frequency warming-cooling cycles (i.e., 4-8 days), was found. This good correlation was lost at higher frequencies (two days or more), due to the asymmetrical response of radon to atmospheric warming and cooling. The results of this study explain some of the field observations in underground radon monitoring.

Monte Carlo modeling of scintillation detectors for continuous underground radon monitoring.

Nuclear simulation methods were applied to two systems that investigate radon transport within geological porous media: a) a laboratory system built to test, under controlled climate conditions, the effect of temperature on radon transport, and b) a field monitoring system comprising gamma and alpha detectors in an abandoned water well. The use of Monte Carlo simulations of NaI and BGO scintillation detectors in continuous underground radon measurements by gamma counting, to estimate the photon flux in the detector volume, is presented. The advantages of shielding side-view NaI detectors were demonstrated for a laboratory system containing ground phosphate rock, including avoiding high counting rates and reducing the effective source volume in radon transport studies. The gross gamma counting procedure was shown to result in a lower uncertainty than spectrometric measurement, by at least a factor of two, despite it being a simpler and more suitable procedure for field measurements. The calculation of simulated source volumes for a BGO detector in a borehole and the measurements in the field support the assumption that the gamma signal comes primarily from radon flowing in the bedrock's air-filled pores. As a practical outcome of this study, positioning the detector a few cm off-center from the borehole's axis increased the gamma counting efficiency; however, measurements in groundwater taken too close to the iron casing had a lower detection efficiency. The conversion factor from the scintillator signal to the radon activity concentration, for the laboratory system, was calculated. Monte Carlo simulations demonstrated the advantages of the gross counting procedures using gamma scintillation detectors in field underground high-frequency radon monitoring.

eGreenhouse: Robotically positioned, low-cost, open-source CO2 analyzer and sensor device for greenhouse applications.

Advances in gas sensors and open-source hardware are enabling new options for low-cost and light-weight gas sampling devices that are also robust and easy to use and construct. Although the number of studies investigating these sensors has been increasing in the last few years, they are still scarce with respect to agricultural applications. Here, we present a complete system for high-accuracy measurements of temperature, relative humidity, luminosity, and CO2 concentrations. The sensors suite is integrated on the previously developed HyperRail device (Lopez Alcala et al., 2019) - a reliable, accurate, and affordable linear motion control system. All measurements are logged with a location and time-stamp. The system was assembled from only off-the-shelf or 3D printable products. We deployed the system in an agricultural greenhouse to demonstrate the system capabilities.

Colloid-facilitated transport of 238Pu, 233U and 137Cs through fractured chalk: Laboratory experiments, modelling, and implications for nuclear waste disposal.

The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1-5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. The results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.

Mobility of Radionuclides in Fractured Carbonate Rocks: Lessons from a Field-Scale Transport Experiment.

Current research on radionuclide disposal is mostly conducted in granite, clay, saltstone, or volcanic tuff formations. These rock types are not always available to host a geological repository in every nuclear waste-generating country, but carbonate rocks may serve as a potential alternative. To assess their feasibility, a forced gradient cross-borehole tracer experiment was conducted in a saturated fractured chalk formation. The mobility of stable Sr and Cs (as analogs for their radioactive counterparts), Ce (an actinide analog), Re (a Tc analog), bentonite particles, and fluorescent dye tracers through the flow path was analyzed. The migration of each of these radionuclide analogs (RAs) was shown to be dependent upon their chemical speciation in solution, their interactions with bentonite, and their sorption potential to the chalk rock matrix. The brackish groundwater resulted in flocculation and immobilization of most particulate RAs. Nevertheless, the high permeability of the fracture system allowed for fast overall transport times of all aqueous RAs investigated. This study suggests that the geochemical properties of carbonate rocks may provide suitable conditions for certain types of radionuclide storage (in particular, brackish, high-porosity, and low-permeability chalks). Nevertheless, careful consideration should be given to high-permeability fracture networks that may result in high radionuclide mobility.

The role of atmospheric conditions in CO2 and radon emissions from an abandoned water well.

Boreholes and wells are complex boundary features at the earth-atmosphere interface, connecting the subsurface hydrosphere, lithosphere, and biosphere to the atmosphere above it. It is important to understand and quantify the air exchange rate of these features and, consequently their contribution as sources for greenhouse gas (GHG) emissions to the atmosphere. Here, we investigate the effect of atmospheric conditions, namely atmospheric pressure and temperature, on air, CO2, and radon transport across the borehole-ambient atmosphere interface and inside a 110-m deep by 1-m diameter borehole in northern Israel. Sensors to measure temperature, relative humidity, CO2, and radon were placed throughout a cased borehole. A standard meteorological station was located above the borehole. Data were logged at a high 0.5-min resolution for 9 months. Results show that climatic driving forces initiated 2 different advective air transport mechanisms. (1) Diurnal and semidiurnal atmospheric pressure cycles controlled daily air transport events (barometric pumping); and (2) There was a correlation between borehole-atmosphere temperature differences and transport on a seasonal scale (thermal-induced convection). Barometric pumping was identified as yielding higher fluxes of vadose zone gases than thermal-induced convection. Air velocities inside the borehole and CO2 emissions to the atmosphere were quantified, fluctuating from zero up to ~6 m/min and ~5 g-CO2/min, respectively. This research revealed the mechanisms involved in the process throughout the year and the potential contribution role played by boreholes to GHG emissions.

Radionuclide transport in brackish water through chalk fractures.

Mobility of radionuclides originating from geological repositories in the subsurface has been shown to be facilitated by clay colloids. In brackish water, however, colloids may flocculate and act to immobilize radionuclides associated with them. Furthermore, little research has been conducted on radionuclide interactions with carbonate rocks. Here, the impact of bentonite colloid presence on the transport of a cocktail of U(VI), Cs, Ce and Re through fractured chalk was investigated. Flow-through experiments were conducted with and without bentonite colloids, present as a mixture of bentonite and Ni-altered montmorillonite colloids. Ce was used as an analogue for reactive actinides in the (III) and (VI) redox states, and Re was considered an analogue for Tc. Filtered brackish groundwater (ionic strength = 170 mM) pumped from a fractured chalk aquitard in the northern Negev Desert of Israel, was used as a solution matrix. Rhenium transport was identical to that of the conservative tracer, uranine. The sorption coefficient (Kd) of U(VI), Cs and Re, calculated from batch experiments with crushed chalk, proved to be a good predictor of mass recovery in transport experiments conducted without bentonite colloids. A meaningful Kd value for Ce could not be calculated due to its precipitation as a Ce-carbonate colloids. Transport of both U(VI) and Cs was indifferent to the presence of bentonite colloids. However, the addition of bentonite in the injection solution effectively immobilized Ce, decreasing its recovery from 17-41% to 0.8-1.4%. This indicates that radionuclides which interact with clay colloids that undergo flocculation and deposition may effectively be immobilized in brackish aquifers. The results of this study have implications for the prediction of potential mobility of radionuclides in safety assessments for future geological repositories to be located in fractured carbonate rocks in general and in brackish groundwater in particular.

Climatic and soil-mineralogical controls on the mobility of trace metal contamination released by informal electronic waste (e-waste) processing.

Informal e-waste processing is a growing global problem. Local climate and mineralogical factors strongly control the chemical lability and dispersal of trace metals from informal e-waste processing. Previous work on e-waste contamination primarily focused on well-known sites in similar climates. Our exploratory analysis of a long-term (since 2008) e-waste incineration site in East Jerusalem demonstrated the ways in which local factors combined to uniquely control trace metal contaminant mobility. Our results suggest that the combination of e-waste processing methods, climate, and mineralogy at this site generated a geopolymer-like material combining ash from e-waste incineration and mountain rendzina soil. This material strongly sorbs trace metal contaminants. We measured the concentrations of: Cu, Fe, Mn, Pb, and Zn at 29 locations around and within the burn site. Samples collected less than 10 m from the edge of the incineration area had trace metal concentrations below the United States Environmental Protection Agency (U.S. E.P.A.) screening levels for residential soil. Sequential extraction showed that ∼50-80% of the total mobilized Pb was released from the residual solid fraction, suggesting strong sorption or incorporation into soil components. Large differences in the measured average specific surface areas (SSA) of uncontaminated (26.18 m2/g) and contaminated (4.48 m2/g) samples, despite comparable mineralogy by XRD, suggested the production of a geopolymer-like material. This was supported by close similarities between the SSA values of contaminated samples and those measured for geopolymer materials synthesized in the lab using kaolinite clay and fly ash (e.g., 4.9 m2/g).

Uranium and Cesium sorption to bentonite colloids under carbonate-rich environments: Implications for radionuclide transport.

In the context of geological disposal of radioactive waste, one of the controlling mechanisms for radionuclide migration through subsurface strata is sorption to mobile colloidal bentonite particles. Such particles may erode from the repository backfill or bentonite buffer and yield measurable (0.01-0.1 g/L) concentrations in natural groundwater. The extent of sorption is influenced by colloid concentration, ionic strength, radionuclide concentration, and the presence of competing metals. Uranium (VI) and cesium sorption to bentonite colloids was investigated both separately and together in low ionic strength (2.20 mM) artificial rainwater (ARW) and high ionic strength (169 mM) artificial groundwater (AGW; representative of a fractured carbonate rock aquitard). Sorption experiments were conducted as a factor of colloid concentration, initial metal concentration and opposing metal presence. It was shown that both U(VI) and Cs sorption were significantly reduced in AGW in comparison to ARW. Additionally, the sorption coefficient Kd of both metals was found to decrease with increasing colloid concentration. Competitive sorption experiments indicated that at high colloid concentration (1-2 g/L), Cs sorption was reduced in the presence of U(VI), and at low colloid concentration (0.01-0.5 g/L), both Cs and U(VI) Kds were reduced when they were present together due to competition for similar sorption sites. The results from this study imply that in brackish carbonate rock aquifers, typical of the Israeli northern Negev Desert, both U(VI) and Cs are more likely to be mobile as dissolved species rather than as colloid-associated solids.

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.

Transport of iron nanoparticles through natural discrete fractures.

The transport of nano scale iron particles (NIP) in fractures is of concern for remediation of both fractured aquifers and porous aquifers when hydro-fracking and flow in preferential pathways takes place. In this study the transport of various NIP in a natural discrete fractured chalk core was investigated and their mass recoveries calculated. Four different types of NIP were tested and characterized in two ionic strength (IS) solutions at a particle concentration of 100-200 mg/l. The effect of IS, stability (sedimentation rate), particle size, solution viscosity and stabilizer were studied. NIP stability ranged from 1 to 100% following 120 min of stability tests and recoveries ranged from about 6 to 69%. The stabilizer type and concentration were shown to have significant role in NIP recoveries, especially at increased IS. It was evident that gravitational stability is the most crucial factor dominating transport of NIP. Accordingly, stability tests were shown to be a reliable indicator of NIP mobility. The high recoveries of some NIP tested, combined with the lack of clogging effect illustrates the enhanced mobility of NIP in fractures. The wide range of recoveries indicates NIP transport manipulation potential in such media. We therefore suggest that application of NIP in contaminated fractures has considerable potential as a remediation measure. In order to achieve NIP distribution in the aquifer while avoiding leakage to the environment, NIP stabilizer concentration should be adjusted according to the site-specific hydrogeochemical properties of the contaminated media.

Detailed effects of particle size and surface area on 222Rn emanation of a phosphate rock.

The dependency of radon emanation on soil texture was investigated using the closed chamber method. Ground phosphate rock with a large specific surface area was analyzed, and the presence of inner pores, as well as a high degree of roughness and heterogeneity in the phosphate particles, was found. The average radon emanation of the dry phosphate was 0.145 ± 0.016. The emanation coefficient was highest (0.169 ± 0.019) for the smallest particles (<25 µm), decreasing to a constant value (0.091 ± 0.014) for the larger particles (>210 µm). The reduction rate followed an inverse power law. As expected, a linear dependence between the emanation coefficient and the specific surface area was found, being lower than predicted for the large specific surface area. This was most likely due to an increase in the embedding effect of radon atoms in adjacent grains separated by micropores. Results indicate that knowledge of grain radium distribution is crucial to making accurate emanation predictions.

Influence of heteroaggregation processes between intrinsic colloids and carrier colloids on cerium(III) mobility through fractured carbonate rocks.

Colloid facilitated transport of radionuclides has been implicated as a major transport vector for leaked nuclear waste in the subsurface. Sorption of radionuclides onto mobile carrier colloids such as bentonite and humic acid often accelerates their transport through saturated rock fractures. Here, we employ column studies to investigate the impact of intrinsic, bentonite and humic acid colloids on the transport and recovery of Ce(III) through a fractured chalk core. Ce(III) recovery where either bentonite or humic colloids were added was 7.7-26.9% Ce for all experiments. Greater Ce(III) recovery was observed when both types of carrier colloids were present (25.4-37.4%). When only bentonite colloids were present, Ce(III) appeared to be fractionated between chemical sorption to the bentonite colloid surfaces and heteroaggregation of bentonite colloids with intrinsic carbonate colloids, precipitated naturally in solution. However, scanning electron microscope (SEM) images and colloid stability experiments reveal that in suspensions of humic acid colloids, colloid-facilitated Ce(III) migration results only from the latter attachment mechanism rather than from chemical sorption. This observed heteroaggregation of different colloid types may be an important factor to consider when predicting potential mobility of leaked radionuclides from geological repositories for spent fuel located in carbonate rocks.

Influence of Intrinsic Colloid Formation on Migration of Cerium through Fractured Carbonate Rock.

Migration of colloids may facilitate the transport of radionuclides leaked from near surface waste sites and geological repositories. Intrinsic colloids are favorably formed by precipitation with carbonates in bicarbonate-rich environments, and their migration may be enhanced through fractured bedrock. The mobility of Ce(III) as an intrinsic colloid was studied in an artificial rainwater solution through a natural discrete chalk fracture. The results indicate that at variable injection concentrations (between 1 and 30 mg/L), nearly all of the recovered Ce takes the form of an intrinsic colloid of >0.45 µm diameter, including in those experiments in which the inlet solution was first filtered via 0.45 µm. In all experiments, these intrinsic colloids reached their maximum relative concentrations prior to that of the Br conservative tracer. Total Ce recovery from experiments using 0.45 µm filtered inlet solutions was only about 0.1%, and colloids of >0.45 µm constituted the majority of recovered Ce. About 1% of Ce was recovered when colloids of >0.45 µm were injected, indicating the enhanced mobility and recovery of Ce in the presence of bicarbonate.

Puddles - A trigger for heterogeneous chemical influx into the unsaturated zone.

Spatial heterogeneity in the chemical concentration of interstitial water in the vadose zone was previously observed under apparently homogeneous surface conditions on two leveled fields sprinkler irrigated with treated sewage effluents on the phreatic Coastal Plain aquifer of Israel. This phenomenon greatly hampers the monitoring of groundwater quality. In this study we report on the presence of puddles of different size and shape that were sporadically observed in these fields. Temporal variability noted in the concentration of treated sewage effluents components in the puddles were considered to be related to evapotranspiration and degradation. For example: increases in the electrical conductivity (up to 1.32 mS/cm), and in the concentrations of chloride (up to 521 mg/L), dissolved organic carbon (up to 28.4 mg/L), and carbamazepine (up to 780 ng/L) and decreases in the concentrations of nitrate (up to 20.1mg/L) and caffeine (3,396 ng/L). Variable trends in concentration were observed for sulfamethoxazole, venlafaxine, 10-hydroxy-10,11-dihydrocarbamazepine and o-desmethylvenlafaxine. The presence of puddles was not necessarily related to areas with high irrigation water input. It is postulated that the continuous chemical variability in the puddles, whose location and size are also variable, determine a heterogeneous influx of solutes into the soil and subsequently into the vadose zone.

Adapting enzyme-based microbial water quality analysis to remote areas in low-income countries.

Enzyme-substrate microbial water tests, originally developed for efficiency gains in laboratory settings, are potentially useful for on-site analysis in remote settings. This is especially relevant in developing countries where water quality is a pressing concern and qualified laboratories are rare. We investigated one such method, Colisure, first for sensitivity to incubation temperatures in order to explore alternative incubation techniques appropriate for remote areas, and then in a remote community of Zambia for detection of total coliforms and Escherichia coli in drinking-water samples. We sampled and analyzed 352 water samples from source, transport containers and point-of-use from 164 random households. Both internal validity (96-100%) and laboratory trials (zero false negatives or positives at incubation between 30 and 40 °C) established reliability under field conditions. We therefore recommend the use of this and other enzyme-based methods for remote applications. We also found that most water samples from wells accessing groundwater were free of E. coli whereas most samples from surface sources were fecally contaminated. We further found very low awareness among the population of the high levels of recontamination in household storage containers, suggesting the need for monitoring and treatment beyond the water source itself.

Virus transport in a discrete fracture.

Tracer experiments were carried out in a naturally discrete-fractured chalk core with solute tracers Li(+) and Br(-), and colloidal tracers of two origins-bacteriophages (MS2, ϕX174 and T4) and fluorescent latex microspheres. The colloidal tracers were either ∼20 nm (MS2, ϕX174 and microspheres) or ∼200 nm (T4 and microspheres) in size. Both solute and colloidal tracers were injected at a constant flux at the fracture inlet and collected at the outlet to evaluate the form of their breakthrough curves (BTCs). The BTCs of all tracers were compared and analyzed. The BTC analysis displayed significant differences in recovery as a function of tracer size and type. Even within the same colloid size, transport of the microspheres and bacteriophages was dissimilar, likely due to minor differences in density, surface chemistry and shape. More pronounced peaks and recoveries were observed with ∼200 nm compared to ∼20 nm microspheres and phages. Arrival time at the outlet was also size-dependent, with larger microspheres and phages having longer residence times than smaller ones, and solutes being 5-15 times slower than colloids of both sizes. The observed differences were explained by a combination of size and electrostatic interactions that facilitates entrance and transport within the pores in the chalk matrix. Overall, our results clearly demonstrate that fractures are favorable carriers for viruses of different sizes with different surface properties. The viruses' properties were also shown to govern their transport through the fractures.

Enhanced transport of colloidal oil droplets in saturated and unsaturated sand columns.

Colloidal-sized triacylglycerol droplets demonstrated enhanced transport compared to ideal latex colloid spheres in both saturated and unsaturated quartz sand columns. Oil droplets (mean diameter 0.74 ± 0.03 µm, density 0.92 g cm(-3), ζ-potential -34 ± 1 mV) were injected simultaneously with latex microsphere colloids (FluoSpheres; density 1.055 g cm(-3), diameters 0.02, 0.2, and 1.0 µm, ζ-potentials -16 ± 1, -30 ± 2, and -49 ± 1, respectively) and bromide into natural quartz sand (ζ-potential -63 ± 2 mV) via short-pulse column breakthrough experiments. Tests were conducted under both saturated and unsaturated conditions. Breakthrough of oil droplets preceded bromide and FluoSpheres. Recovery of oil droplets was 20% greater than similarly sized FluoSpheres in the saturated column, and 16% greater in the 0.18 ± 0.01 volumetric water content (VWC) unsaturated column. Higher variability was observed in the 0.14 ± 0.01 VWC column experiments with oil droplet recovery only slightly greater than similarly sized FluoSpheres. The research presents for the first time the direct comparison of colloidal oil droplet transport in porous media with that of other colloids, and demonstrates transport under unsaturated conditions. Based on experimental results and theoretical analyses, we discuss possible mechanisms that lead to the observed enhanced mobility of oil droplets compared to FluoSpheres with similar size and electrostatic properties.