Terrestrial algae: pioneer organisms of carbonate rock solutional weathering in South China karst.

The formation of soil in karst ecosystem has always been a scientific problem of great concern to human beings. Algae can grow on the exposed and non-nutrition carbonate surface, inducing and accelerating weathering of rock substrates, thus promoting soil formation. Yet the actual contribution of algae to solutional weathering intensity remains unclear. In this study, we performed weathering simulation experiment on two algae species (Klebsormidium dissectum (F.Gay) H.Ettl & G.Gärtner and Chlorella vulgaris Beijerinck), which were screened from carbonated rock surfaces from a typical karst region in South China. The results showed: (1) both algae have solutional weathering effect on carbonate rock, (2) there is no difference of solutional intensity observed, yet the solutional modes are different, suggesting different ecological adaptative strategies, (3) algae on carbonate rocks have higher carbonic anhydrase activity (CAA) and secrete more extracellular polysaccharide (EPS), accelerating rock weathering. (4) The absolute dissolution amount of carbonate rock with algae participation is 3 times of that of without algae. These results indicate the significant impact of terrestrial algae on carbonate rock solutional weathering and provides quantitative evidence that terrestrial algae are pioneer species. It also contributes to our further understanding of soil formation in karst ecosystems in South China.

Mechanistic analysis of matrix-acid treatment of carbonate formations: An experimental core flooding study.

Despite the extensive works carried out on optimal design of the acidizing operations, the detailed mechanisms of the wormholes formation and propagation within the rock structure and their effects on optimum acid injection rate have not been well studied in the available literature. In this work, high pressure-high temperature (HP-HT) acid injection experiments and computed tomography (CT) scan imaging were performed by HCl 15 wt% to discover the mechanisms underlie the creation of wormholes and their extension in the carbonate rocks. The pressure drop profiles and permeability variations before and after acidizing process were employed to identify the optimum acid injection rate. As a final point, core effluent samples were collected and analyzed for justification of the HP-HT experimental results. For performed HP-HT experiments, acid injection rate of 7 cm3/min was obtained as the optimal acid injection rate. The maximum permeability improvement, Kf/Ki = 11.2, was achieved at the optimum acid injection rate corresponded to the minimum acid breakthrough volume, as well as the acid consumption. The results obtained from the CT scan analysis show that the wormhole created in core C.4 is close to the optimal conditions with a single distinctive wormhole to bypass the damage. At acid injection rates lower or higher than the optimum injection rate, the shape of the wormholes changes to conical at very low injection rates and ramified at high injection rates. The highest concentration of calcium at effluent samples was observed for the minimum and maximum injection rates (1 and 15 cm3/min), respectively. The maximum contact time and highest contact area between the acid and rock were attained at minimum and maximum acid injection rates, respectively. As this work was performed at realistic oilfield conditions, it can be used for effective plan, execution and optimization of the acidizing operation in carbonate reservoirs.

Iron coatings on carbonate rocks shape the attached bacterial aquifer community.

Most studies of groundwater ecosystems target planktonic microbes, which are easily obtained via water samples. In contrast, little is known about the diversity and function of microbes adhering to rock surfaces, particularly to consolidated rocks. To investigate microbial attachment to rock surfaces, we incubated rock chips from fractured aquifers in limestone-mudstone alternations in bioreactors fed with groundwater from two wells representing oxic and anoxic conditions. Half of the chips were coated with iron oxides, representing common secondary mineralization in fractured rock. Our time-series analysis showed bacteria colonizing the chips within two days, reaching cell numbers up to 4.16 × 105 cells/mm2 after 44 days. Scanning electron microscopy analyses revealed extensive colonization but no multi-layered biofilms, with chips from oxic bioreactors more densely colonized than from anoxic ones. Estimated attached-to-planktonic cell ratios yielded values of up to 106: 1 and 103: 1, for oxic and anoxic aquifers, respectively. We identified distinct attached and planktonic communities with an overlap between 17 % and 42 %. Oxic bioreactors were dominated by proteobacterial genera Aquabacterium and Rhodoferax, while Rheinheimera and Simplicispira were the key players of anoxic bioreactors. Motility, attachment, and biofilm formation traits were predicted in major genera based on groundwater metagenome-assembled genomes and reference genomes. Early rock colonizers appeared to be facultative autotrophs, capable of fixing CO2 to synthesize biomass and a biofilm matrix. Late colonizers were predicted to possess biofilm degrading enzymes such as beta-glucosidase, beta-galactosidase, amylases. Fe-coated chips of both bioreactors featured more potential iron reducers and oxidizers than bare rock chips. As secondary minerals can also serve as energy source, they might favor primary production and thus contribute to subsurface ecosystem services like carbon fixation. Since most subsurface microbes seem to be attached, their contribution to ecosystem services should be considered in future studies.

Hydrothermal fluid alteration of the Ordovician epigenetic karstification reservoir in the Tahe Oilfield, Tarim Basin, NW China.

In recent years, the Ordovician carbonate reservoirs in the Tarim Basin have received attention due to continuous advancements in of deep strata exploration for oil and gas. The Ordovician carbonates in the Tahe Oilfield have experienced multiple phases of tectonic movement and ancient karst action to form reservoirs consisting of multi-scale spaces such as large caves, fractures, and dissolution pores. This study identifies indicators of atmospheric freshwater karst and hydrothermal karst in the Ordovician carbonate rocks of the Tahe Oilfield by comparing and analyzing lithological observation, geochemical data, fluid inclusions, logging interpretation, and seismic data. The spatial and temporal distribution of karst reservoirs are summarized, and the results show that the stage of tectonic movement and pulsating uplift of strata occurred in the early Caledonian and Hercynian orogenies, and the carbonate rocks were uplifted to the surface and large-scale atmospheric freshwater karstification. In the deeper strata, the thermal convection of fluids caused by volcanic activity accelerated the thermochemical sulfate reduction (TSR), and the generated H2S gas accompanied the upward transport of hydrothermal fluids, which further dissolved and modified the original karst system, increasing the storage space. However, away from the heat source, calcite (positive Eu anomaly, higher 87Sr/86Sr ratio, lower δ18O value,) reprecipitated due to the temperature reduction, so the role of hydrothermal activity in reservoir reconstruction is limited. The two karst action patterns indicated that epigenetic karstification is an important process for forming carbonate reservoirs in the Tahe Oilfield and the basis of hydrothermal karst reservoir formation. This study demonstrates the research and exploration value of karstification for the geo-energy field, which could benefit sustainable development in the Tarim Basin.

Cyclic confining pressure and rock permeability: Mechanical compaction or fines migration.

The apparent rocks permeability, measured under cyclic loading, tends to constantly decrease with each subsequent loading/unloading cycle, the main reason for this, the researchers consider irreversible deformations. However, this understanding is not complete, as attention is not paid to the method of determining the permeability. Under cyclic loading, permeability is measured by fluid flow, which is able to carry colloids and they can clog pore throats and reduce permeability, but this is not taken into account in the study of porous rocks. Colloidal migration is also not taken into account in routine core tests, although colloids can significantly reduce the apparent permeability of rocks during migration. Also it is impossible to determine exactly which of the factors (mechanical compaction or colloid migration) contributed to the change in apparent permeability. The purpose of this research is to provide experimental evidence that under cyclic loading, permeability decreases not only due to mechanical compaction, but also due to colloid migration within the porous medium. For this task, a methodology has been developed, the feature of which is that between loading/unloading cycles, the core is blowning with a large pressure gradient. It has been established that under cyclic loading, the apparent permeability of rock samples changes as a result of colloid migration up to 20 %. The dynamics of permeability becomes unpredictable and weakly depends on the confining pressure when changing the direction and regime of the gas flow. At the end of the multi-cycle confining pressure test, the apparent permeability of 3 out of four core samples, despite the decrease during injection, recovered to the initial value which also confirms that the apparent permeability has affected by colloid migration. A mechanism for changing the apparent permeability during gas injection and cyclic confining pressure is proposed. The outcomes indicate that colloid migration should be taken into account during core tests not only in cyclic loading but also in routine coreflooding tests.

The Rehbinder Effect in Testing Saturated Carbonate Geomaterials.

Carbonate geomaterial samples were tested for uniaxial compressive strength and tensile strength under air-dried and distilled-water-wet conditions. When tested for uniaxial compression, samples saturated with distilled water showed 20% lower average strength than that of air-dried samples. In the indirect tensile (Brazilian) test, samples saturated with distilled water showed 25% lower average strength than that of dry samples. In comparison with air-dried conditions, when the geomaterial is saturated with water, the ratio of the tensile strength to the compressive strength is decreased, mainly due to the decrease in the tensile strength caused by the Rehbinder effect.

[Geochemical Characteristics of Cd in Different Parent Soils in Karst Area and Prediction of Cd Content in Maize].

The naturally high background value region of Cd derived from the weathering of carbonate has received wide attention. Due to the significant difference in soil physicochemical properties, soil Cd content, and bioavailability of different parent materials in the karst area, there are certain limitations in using the total soil Cd content to classify the environmental quality of cultivated land. In this study, surface soil and maize samples of eluvium and alluvial parent material in typical karst areas were collected systematically; the contents of maize Cd, soil Cd, pH, and oxides were analyzed, the Cd geochemical characteristics of different parent soils and the influencing factors of their bioavailability were revealed, and scientific and effective arable land use zoning suggestions based on the prediction model were suggested. The results showed that the physicochemical properties of different parent material soils in the karst area were obviously different. The alluvial parent material soil had low Cd content but high bioavailability, and the maize Cd exceeding rate was high. The maize Cd bioaccumulation factor was significantly negatively correlated with soil CaO, pH, Mn, and TC, and the correlation coefficients were -0.385, -0.620, -0.484, and -0.384, respectively. Compared with the multiple linear regression prediction model, using the random forest model to predict the maize Cd enrichment coefficient had higher accuracy and precision. Furthermore, a new scheme for the safe utilization of cultivated land at the plot scale based on soil Cd and predicted crop Cd content was proposed in this study, making full use of arable land resources to ensure crop safety.

Experimental Study on the Dissolution Characteristics and Microstructure of Carbonate Rocks under the Action of Thermal-Hydraulic-Chemical Coupling.

Microdamage in a rock induces a change in the rock's internal structure, affecting the stability and strength of the rock mass. To determine the influence of dissolution on the pore structure of rocks, the latest continuous flow microreaction technology was used, and a rock hydrodynamic pressure dissolution test device simulating multifactor coupling conditions was independently developed. The micromorphology characteristics of carbonate rock samples before and after dissolution were explored using computed tomography (CT) scanning. To conduct the dissolution test on 64 rock samples under 16 groups of working conditions, 4 rock samples under 4 groups were scanned by CT under working conditions, twice before and after corrosion. Subsequently, the changes in the dissolution effect and pore structure before and after dissolution were quantitatively compared and analyzed. The results show that the dissolution results were directly proportional to the flow rate, temperature, dissolution time, and hydrodynamic pressure. However, the dissolution results were inversely proportional to the pH value. The characterization of the pore structure changes before and after sample erosion is challenging. After erosion, the porosity, pore volume, and aperture of rock samples increased; however, the number of pores decreased. Under acidic conditions near the surface, carbonate rock microstructure changes can directly reflect structural failure characteristics. Consequently, heterogeneity, the presence of unstable minerals, and a large initial pore size result in the formation of large pores and a new pore system. This research provides the foundation and assistance for predicting the dissolution effect and evolution law of dissolved pores in carbonate rocks under multifactor coupling, offering a crucial guide for engineering design and construction in karst areas.

Development and Application of Carbonate Dissolution Test Equipment under Thermal-Hydraulic-Chemical Coupling Condition.

The latest continuous flow micro reaction technology was adopted to independently develop carbonate rock dissolution test equipment. Carbonate rock dissolution tests were conducted under different temperatures, flow rates, and dynamic water pressure conditions to study the dissolution process of carbonate rocks under the coupling of heat-water-chemistry. The dissolution effect and development law of carbonate rocks were explored by quantitatively studying carbonate rock dissolution rate and chemical composition of karst water. The results showed that the self-designed dissolution test equipment has obvious advantages. After dissolution, carbonate rock specimens were damaged to varying degrees. The dissolution rate was proportional to water velocity and hydrodynamic pressure, with the velocity effect being greater than the hydrodynamic pressure effect. The pH value, conductivity, and Ca2+ ion content of the reaction solution gradually increased after dissolution. The development and application of the equipment have proved that, at low dynamic water pressures (2 MPa), the water flow velocity effect on the dissolution velocity was 1.5 times that when the dynamic water pressure was high (6 MPa); at a low water flow velocity of 15 mL/min, the dynamic water pressure effect on the dissolution velocity was three times that when the water flow velocity was high (75 mL/min). The development process is gradually becoming strong and stable. Its research has important theoretical significance and engineering application value to provide technical means and guarantee for the early identification, karst development, and safety evaluation of karst geological disasters.

Exploring the Risk Thresholds of Soil Heavy Metals in Carbonate and Non-carbonate Rock Areas: The Case of Qianjiang District in Chongqing, China.

To determine whether the national soil heavy metal standards (GB 15618-2018) are applicable to some carbonate and non-carbonate zones in Southwest China, rice and rhizosphere soil samples were collected in Chongqing and analyzed for heavy metal contents, pH, and other chemical parameters. In addition, regression analysis was also used to predict the risk threshold of soil heavy metals. The Cd risk screening value in GB 15618-2018 was strict for alkaline soils (pH > 7.5) as compared to those revealed in carbonate and non-carbonate areas, while the calculated pollution threshold for Cd in acidic soils (pH ≤ 5.5) in the non-carbonate area was lower than that in GB 15618-2018. Therefore, to improve the applicability of the evaluation results, a soil-crop system evaluation is recommended.

Insight into the kinetic analysis of acid mine drainage treated by carbonate rock.

Acid mine drainage (AMD) has caused a great impact on soil, surface water, groundwater, plants or other organisms in the mining environment because of its high acidity, high sulphate content, and contains a variety of heavy metals. AMD treated by carbonate rocks have been regarded as a feasible technology for pollution control and applied widely in mine area. However, to date, the kinetics of the reaction between carbonate rock and AMD have not been investigated, resulting in the lack of systematic theoretical guidance for the implementation of this technology. In this study, effects of carbonate particle sizes and reaction temperature on AMD treatment were investigated. The dissolution efficiency of Ca2+ was used to quantitatively reflect extent of reaction, and the leaching kinetics was analysed based on the shrinking core model. The results showed that carbonate rocks with a particle size of 0.5-1.0 mm had the best pH-enhancing performance for AMD and highest removal efficiency for Fe3+ (>98.00%), while the Mn2+ and SO42- were temperature sensitive. The diffusion of solid product layer was the controlling step of the leaching reaction, and the apparent activation energy of the reaction was 12.63 kJ·mol-1.

Study on the precipitation of iron and the synchronous removal mechanisms of antimony and arsenic in the AMD under the induction of carbonate rocks.

The ecological environment can be severely polluted and destroyed by the acid mine drainage (AMD) generated during the exploration and utilization of minerals. However, neutralized by carbonate rocks (a natural material in Karst regions), the AMD secondary iron flocs containing a large number of iron oxides or hydroxide can be precipitated in AMD. The metal ions, such as antimony (Sb) and arsenic (As), can be effectively removed by these neutralizing products. In this paper, the neutralization reaction of different acid solutions in an iron-antimony-arsenic system was induced by carbonate rocks to explore the removal effect of metals during this neutralization process. Meanwhile, taking the release amounts of iron (Fe), Sb, and As as well as the phase transformation of minerals at different pH levels as stability indexes, we quantitatively analyzed the chemical stability of AMD neutralizing products (secondary iron flocs) containing Sb and As under the typical acid-base environment (pH = 3.0 ~ 9.0) of AMD and other waters. Results showed that the neutralization reaction with carbonate rocks induced the co-precipitation of Fe with Sb and As. When the concentration ratio of Fe, Sb, and As was 30:1:1, the pH of AMD raised from 3.0 to 7.28 within 72 h, and the three elements were removed by 99%, 85%, and 90%, respectively. After soaking the AMD secondary iron flocs in an acid environment (pH = 3.0) for 30 days, the release amount of Fe reached its peak of 0.070 mg/g. Then, when the pH value increased to 4.0, the As and Sb showed their maximum release amounts of 14.90 µg/g and 19.19 µg/g, respectively. In addition, under acidic conditions, these AMD secondary iron flocs were easily transformed into the goethite with better crystallinity and higher structural stability. This study could help reveal the development of the secondary mineral during the treatment of AMD by carbonate rocks and understand the release characteristics of metals from AMD secondary products containing Sb and As, which sheds light on and provides theoretical foundations for the passive treatment of AMD containing these two elements in the future.

Differential Analysis of Volumetric Strain Method Characterization in the Context of Phase Change of Water in Carbonate Rocks.

Modernized technological processes or increasing demands on building materials force the scientific community to analyze in more detail the suitability of individual raw materials and deposits. New or modernized research methodologies make it possible to better understand not only the geometrical structure of the pore space of materials but also the processes taking place in them and the interaction of many factors at the same time. Despite the extensive literature in the field of research on capillary-porous materials, scientists still face many challenges because not everything is known. Carbonate rocks are the most common (one-tenth of Earth's crust) sedimentary rocks. Analysis of the test results obtained with the use of the modernized differentia analysis of volumetric strain (DAVS) methodology allows for a better adjustment of rock deposits to the products that can be produced from them. In this manner, it is possible that it will contribute to a more rational use of exhaustible rock deposits and not only carbonate ones. This research subject is of great importance for modern science, which was also noted in many of science publications.

Lipopeptide production by Serratia marcescens SmSA using a Taguchi design and its application in enhanced heavy oil recovery.

Biosurfactant production at reactor level by Serratia marcescens SmSA was optimized and evaluated to enhance the heavy oil recovery on carbonate rocks. Temperature, agitation, and carbon/nitrogen (C/N) ratio were evaluated to optimize biosurfactant production by using a Taguchi (L9) design. The best conditions (C/N ratio: 6, 25 °C, and agitation: 100 rpm) were used to scale up the biosurfactant production with a 3-L bioreactor. The best aeration for biosurfactant production was 0.66 volume of air per volume of liquid per minute (vvm), producing the lowest surface tension (26 mN/m) in 14 h, with a biosurfactant yield of 14.26 g/L as a crude product and 2.85 g/L as a purified product, and a critical micelle concentration of 280 mg/L. The biosurfactant was characterized as a lipopeptide, and it was stable under extreme conditions: pH (2-12), salinity up to 200 g/L, and temperature up to 150 °C confirmed by thermogravimetric analysis. Enhanced oil recovery test was carried out with a carbonate core and heavy oil under reservoir conditions, obtaining an additional recovery of 8%, due to reduced interfacial tension and modified wettability of the rock. These findings highlight the potential application of S. marcescens SmSA biosurfactant in enhanced oil recovery.

Comparison of the Properties of Natural Sorbents for the Calcium Looping Process.

Capturing CO2 from industrial processes may be one of the main ways to control global temperature increases. One of the proposed methods is the calcium looping technology (CaL). The aim of this research was to assess the sequestration capacity of selected carbonate rocks, serpentinite, and basalt using a TGA-DSC analysis, thus simulating the CaL process. The highest degrees of conversion were obtained for limestones, lower degrees were obtained for magnesite and serpentinite, and the lowest were obtained for basalt. The decrease in the conversion rate, along with the subsequent CaL cycles, was most intense for the sorbents with the highest values. Thermally pretreated limestone samples demonstrated different degrees of conversion, which were the highest for the calcium-carbonate-rich limestones. The cumulative carbonation of the pretreated samples was more than twice as low as that of the raw ones. The thermal pretreatment was effective for the examined rocks.

Cadmium risk in the soil-plant system caused by weathering of carbonate bedrock.

Enrichment of cadmium (Cd) during weathering and pedogenesis of carbonate rocks has resulted in large areas of lands with soil Cd concentrations exceeding the official guidelines in China and other countries. However, it is reported in many studies that the risk of soil contamination by Cd from this natural process can be neglected as most of the Cd is not bioavailable. Noticing that the previous studies focused only on eluvial areas but not on lowland alluvium, where Cd from the eluvial areas can be transported and accumulated, we selected the Qingyang county in Anhui province, where there are two small drainage basins developed wholly on carbonate bedrock, to compare the Cd speciation and activity between eluvium and alluvium, and to evaluate the risk of Cd pollution to the latter. By the application of systematic sampling and analysis of the bedrock, soil, and rice grain samples, and in comparison with the previously acknowledged "high background with low mobility" area in Guizhou, it was found that soil developed from alluvium has both higher total Cd and higher mobile Cd proportion than soil from the upland eluvium. A very high percentage of rice grain samples (51%) grown on the alluvial soil exceeded the food standard for Cd (0.2 mg kg-1). Therefore, the spatial division of alluvium and eluvium should be the first step in the assessment of the Cd risk in carbonate regions, and special attention should be given to soil developed from alluvium.

Temporal Variations of Water Chemistry in the Wet Season in a Typical Urban Karst Groundwater System in Southwest China.

It is important to investigate temporal variations of water chemistry for the purpose of improving water quality in karst groundwater systems. Groundwater samples were collected daily under various land uses of Guiyang. Major ions and stable carbon isotope composition of dissolved inorganic carbon (δ13CDIC) were analyzed to understand the biogeochemical processes. The water chemistry was dominated by Ca2+, Mg2+, HCO3-, and SO42-, which mainly derived from the dissolution of carbonate rocks (limestone and dolomite) and oxidation of sulfide. The groundwater was defined as of the HCO3-Ca Mg and HCO3·SO4-Ca·Mg type, according to its hydrochemical characteristics. Results suggested that hydrochemical concentrations changed quickly, in response to rainfall events. The fast response revealed that karst groundwater was easily impacted by rainfall and anthropogenic inputs according to temporal variation of water chemistry. The distribution of DIC (dissolved inorganic carbon) and δ13CDIC showed that DIC is mainly sourced from soil CO2(g) influx and carbonate dissolution. δ13CDIC and major ions ratios suggested that carbonate minerals were dissolved by H2SO4 at groundwater in wooded area, contributing an important source for DIC due to the slight enrichment of heavy δ13CDIC. More negative δ13CDIC values were observed after rainfall reflected the fact that soil CO2(g) and organic carbon oxidation influxes accounted for a large share during DIC formation. Various δ13CDIC and hydrochemical patterns were observed under various land use and human activity conditions. Meanwhile, relative high nitrate loads were found in groundwater after rainfall, suggesting high anthropogenic inputs following rainwater as having side effects on water quality. This study suggests that water chemistry and isotopic proof provide a better understanding of water quality and carbon dynamics responding to rainfall events in the karst groundwater systems.

Sources and transformations of nitrate constrained by nitrate isotopes and Bayesian model in karst surface water, Guilin, Southwest China.

Surface water suffering from nitrate (NO3-) contamination in karst area is not only harmful to human health as drinking water but can also affect the process of carbonate rock weathering, so it is crucial to trace the sources and transformations of NO3- in karst surface water. In this study, an investigation of water chemical data and NO3- isotopes (δ15N and δ18O) was used to elucidate the transformations of NO3- and quantify a proportional apportionment of NO3- sources of individual potential sources (incl. soil organic nitrogen (SON), atmospheric precipitation (AP), manure and sewage wastes (M&S), and chemical fertilizer (CF)) in the Lijiang River (typical karst surface water), Guilin, Southwest China. δ15N-NO3- and δ18O-NO3- values of water samples from the Lijiang River range from 2.14 to 13.50‰ (mean, 6.59‰) and from - 2.44 to 6.97‰ (mean, 3.76‰), respectively. A positive correlation between Cl- and NO3- but no correlations between NO3- and δ15N-NO3- or δ18O-NO3- are found and the δ18O-NO3- values fitted the theoretical δ18O-NO3- values produced from nitrification, suggesting that the genesis of NO3- in waters of the Lijiang River is affected by nitrification processes and the mixing process has a major effect on NO3- transportation. Results of the Bayesian stable isotope mixing model show that the M&S and SON are the main NO3- source through the whole year (accounting for ~ 61% and 65% of the total NO3- in the wet and dry season, respectively), followed by CF (~ 29%). Furthermore, we find that nitrification of nitrogen in fertilizers, soil, and manure and sewage can promote the carbonate rock weathering. The estimated contribution of such nitrification to the weathering of carbonate rocks accounts for about 11% of the total carbonate rock weathering flux (calculated by HCO3-) in the Lijiang River. This finding indicates that the weathering of carbonate rock is probably affected by nitrogen nitrification processes in karst catchment.

Transport of a PAH-degrading bacterium in saturated limestone media under various physicochemical conditions: Common and unexpected retention and remobilization behaviors.

Laboratory saturated columns packed with granular limestone grains were used to explore the retention and remobilization of functional bacteria FA1 under various physicochemical conditions. The unique surface properties of limestone and FA1 caused some unexpected phenomena. Solution IS, cation type, temperature and surface biological property all affected FA1 retention in the columns. The IS effect was temperature dependent and initial solution pH showed little influence due to the strong buffering ability of limestone. Perturbations of solution IS caused slight release of previously retained bacteria in some columns with NaCl as the background electrolyte, while increase in flow rate caused no release at all. When CaCl2 was the background, bacterial remobilization only occurred following both cation exchange and IS reduction. DLVO forces incorporating with surface roughness calculation were determined to assist with interpretation of interaction mechanisms. All the experimental evidences suggest the importance of cation bridging, cation exchange, surface roughness, and hydrophobic interaction in controlling bacterium transport in saturated limestone porous media.

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.