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1.
Three relatively low-head hyperfiltration experiments were conducted to investigate clay membrane behavior at lower hydraulic heads. Dilute Cl− solutions (187 and 336 mg/L) were forced through thin layers of kaolinite (0.67–1.62 mm) under heads ranging between 1.45 and 1.89 m. At the end of each experiment, there was a Cl− concentration increase (15–29%) within the cell. The concentration increase is attributable to solute-sieving by the kaolinite. Calculated final concentration increases at the membrane face ranged between 1.51 and 1.65 times the initial concentration and the calculated values of the reflection coefficient ranged between 0.35 and 0.40. These experiments show that clays are capable of significant hyperfiltration effects at lower pressure heads more typical of natural aquifers. Consequently, hyperfiltration effects may need to be considered in a broad spectrum of shallow subsurface processes in which these effects have been regarded as inconsequential; particularly in perched aquifers.  相似文献   

2.
Transient models are needed to analyze time-dependent problems like hyperfiltration associated with head differences across clay barriers. Hyperfiltration (solute-sieving) effects create an increased concentration of natural groundwater solutes outside the clay barrier due to the inward head gradient. The purpose of our model is to predict solute buildup and distribution during hyperfiltration providing a basis for time analysis of solute migration. Required input parameters for the model include membrane properties like reflection coefficient, hydraulic conductivity, and solute concentration on the high-pressure side of the membrane before the onset of steady state, solution flux, and the effluent concentration. Model verification is based on published experimental results. The transient hyperfiltration model presented herein may prove useful in elucidating clay membrane (hyperfiltration) effects in the subsurface, however the sole purpose of this paper is to develop a transient model of hyperfiltration effects and test it by using published experimental data.  相似文献   

3.
It is widely recognized that clays and shales can demonstrate membrane properties. When a hydraulic head differential exists across a membrane-functioning clay-rich barrier, some of the solute is rejected by the membrane. This process is known as hyperfiltration. Some shallow geologic environments, including aquitards bounding shallow perched aquifers and unconfined aquifers, some river and stream beds, and some lake bottoms contain clay–soil mixes. Many engineering structures such as landfill liners, mixed soil augered barriers, and retention pond liners also consist of soil–clay mixes. No previous testing has been performed to investigate the likelihood that hyperfiltration may occur in such mixed soils. Therefore, we performed five experiments using different mixes of Na-bentonite and glass beads (100, 50, 25, 12 and 0% clay) to determine if any of these mixes exhibited membrane properties and to investigate what effect clay content had upon the membrane properties of the soil. Each mixture was compacted to 345 kPa and the sample mixtures were 0.58–0.97 mm thick. All the experiments used an approximately 35 ppm Cl− solution under an average 103 kPa hydraulic head. Experimental results show that all the simulated clay–sand mixtures do exhibit measurable membrane properties under these conditions. Values of the calculated reflection coefficient ranged from a low of 0.03 for 12% bentonite to 0.19 for 100% bentonite. Solute rejection ranged from 5.2% for 12% clay to a high of over 30% for the 100% clay. The 100% glass bead sample exhibited no membrane properties.  相似文献   

4.
Hyperfiltration is the ability of a membrane to retard the passage of a solute under a hydraulic head in excess of osmotic pressure. Disaggregated and recompacted shales, mudstones, clays and tuff have been shown to exhibit hyperfiltration-induced membrane effects in past experiments. However, limestone and dolomite have not previously been tested. Therefore, eight hyperfiltration experiments were performed on intact Burlington Limestone and Jefferson City Dolomite to assess the membrane properties of these lithologies. Four experiments were conducted on each lithology using 0.0050 and 0.0100 M Cl− solutions at heads of 0.50 and 1.00 m. Reflection coefficients, a measure of osmotic efficiency, ranged from 0.34 to 0.39 for the Burlington Limestone and 0.32 to 0.40 for the Jefferson City Dolomite. At the end of the hyperfiltration experiments, Cl− was concentrated within the cell above input concentrations by 85–95% for the Burlington Limestone and 79–105% for the Jefferson City Dolomite. An additional experiment passed 0.0020 M dissolved silica solution through the Burlington Limestone at a head of 0.965 bar (14 psi). The final concentration of silica within the cell was 0.0043 M dissolved silica at steady-state; a concentration 114% higher than the original input solution concentration. The reflection coefficient for this experiment was calculated to be 0.33. The results of these experiments suggest that membrane properties in these lithologies may be worthy of consideration in some geologic scenarios, including: (1) shallow or perched aquifers bounded by thin limestone or dolomite strata, (2) overpressured aquifers bounded by limestone or dolomite, (3) limestone or dolomite bounded aquifers with significant vertical components of flow, and (4) facies changes with significant lateral component of flow bounded by either lithology. Furthermore, the results suggest that silica cementation may be possible even under relatively low head conditions. Cementation due to hyperfiltration, even at shallow depths and low pressures should be further investigated. Similarly, other low permeability lithologies lacking a charged surface could potentially function as geologic membranes.  相似文献   

5.
Osmosis has been cited as a mechanism for explaining anomalously high fluid pressures in the subsurface. Clays and shales act as membranes, and osmotic flux across these units may result in pressures sufficiently high to explain these anomalies. The theoretical osmotic pressures as calculated solely from solution properties can be quite large; however, it is not yet resolved whether these geologic membranes are sufficiently ideal to generate such pressures.Osmotic efficiencies of a Na-bentonite membrane were measured by a series of hyperfiltration experiments using various molarities of NaCl at two different porosities. The highest osmotic efficiency (0.8912) occurred at the lower porosity and the lowest NaCl input solution. The lowest measured osmotic efficiency (0.0423) occurred at the high porosity and the highest NaCl input concentration.The osmotic efficiencies obtained from the hyperfiltration experiments correlate very favorably with the Fritz-Marine Membrane Model. This model predicts that the maximum osmotically-induced hydraulic pressures in the subsurface should occur across shales having low porosities and high cation exchange capacities in which the unit separates solutions of brackish waters.  相似文献   

6.
Several recent laboratory studies and field investigations have indicated that shales and compacted clay minerals behave as semipermeable membranes. One of the properties of semipermeable membranes is to retard or prevent the passage of charged ionic species through the membrane pores while allowing relatively free movement of uncharged species. This phenomenon is termed salt filtering, reverse osmosis, or ultrafiltration.This paper shows how one can proceed from the ion exchange capacity of clay minerals and, by means of Donnan membrane equilibrium concept and the Teorell-Meyer-Siever theory, develop a theory to explain why and to what extent ultrafiltration occurs when solutions of known concentration are forced to flow through a clay membrane. Reasonable agreement between theory and laboratory results were found. The concentration of the ultrafiltrate was always greater than predicted because of uncertainty in values of some parameters in the equations.Ultrafiltration phenomena may be responsible for the formation of some subsurface brines and mineral deposits. The effect should also be taken into consideration in any proposal for subsurface waste emplacement in an environment containing large quantities of clay minerals.  相似文献   

7.
Membrane behavior in naturally occurring and engineering systems refers to the restriction of solute migration through a membrane relative to the migration of the solvent. Hyperfiltration is the net flux that results when two solutions of different concentration are separated by a membrane and an external force is applied in excess of the osmotic pressure. Clay membranes containing layered fabric have higher efficiencies than membranes with random fabrics. Therefore, low-permeability rocks with a foliated fabric might exhibit membrane properties. Four hyperfiltration experiments were conducted on samples of Darrington Phyllite from the Easton Metamorphic Suite of the Northwest Cascades, Washington (USA). Chloride solutions were passed through thin, intact discs at relatively low heads. At the end of the experiments, dissolved chloride concentrations had increased 110–140 % and calculated reflection coefficients ranged from 0.87 to 0.88; this was attributed to partial solute rejection by the phyllite. Natural scenarios in which phyllite might exhibit membrane properties include (1) shallow perched aquifers bounded by phyllite, (2) overpressured aquifers bounded by phyllite, (3) phyllite-bounded aquifers with significant vertical groundwater flows, and (4) ultrafiltration during metamorphic devolatilization at depth. Membrane processes exhibited by phyllite may also contribute to the formation of some low-temperature ore bodies.  相似文献   

8.
Information on the distribution of subsurface temperature and hydraulic heads at 24 observation wells in and around the Tokyo Lowland, the eastern part of the Tokyo Metropolitan area, were examined to make clear the relationship between groundwater and the subsurface thermal environment in the urban area. Minimums in temperature–depth logs due to subsurface temperature increasing at shallow parts were recognized in 21 wells. This fact shows subsurface temperature is affected by ground surface warming in almost all of this area. Deeper than minimums, where the effects of surface warming became relatively small, regional variation is observed as follows: high temperatures are shown in the central part to the southern part, and low temperatures shown in the inland to eastern part. The high temperature area corresponds to an area where the lower boundary of groundwater flow is relatively shallow. This area corresponds also to an area with severe land subsidence resulting from excessive groundwater pumping. It is considered that this high temperature area is formed by the effects of upward groundwater flow affected by hydrogeological conditions and pumping. On the other hand, a comparison between past data (1956–1967) and present data (2001–2003) revealed widespread decreasing temperature in the inland area. This is explained by downward groundwater flow based on an analysis of temperature–depth logs. This fact suggests that subsurface temperature is not only increasing from the effects of surface warming but also decreasing from the effects of groundwater environment change due to pumping.  相似文献   

9.
Graf (1982) has discussed possible mechanisms for ground water pressure differences large enough to drive subsurface reverse osmosis. Osmotic pressure differences were calculated assuming ideal membrane properties. However, consideration of non-ideal membrane properties indicates that actual subsurface osmotic pressure differences will be considerably smaller than those calculated by Graf (1980), and thus reverse osmosis may occur under more commonly observed hydrogeologic conditions.  相似文献   

10.
Hyperfiltration (reverse osmosis) and subsequent precipitation of minerals from the hyperfiltrated solution are processes that potentially decrease the hydraulic conductivity of porous media. These processes were demonstrated by hyperfiltrating NiSO4 solutions through fine-grained sandstone. The mineral precipitates occur in very small (mm sized) layers at the high-pressure side of the samples where they create zones of lowered hydraulic conductivity (2–3 orders of magnitude lower than initial). The total amount of precipitates is very small compared to the dissolved mass which was passed through the membrane. Hyperfiltration-induced precipitates and the resulting lowering of hydraulic conductivities were observed at solute saturations as low as 10%. Nevertheless, at saturations higher than 50% the conductivity reduction strongly increased. Full reversibility of the hydraulic conductivity reduction by extensive re-flushing with water was only obtained at low initial solute saturations (10%). This indicates that precipitated minerals in many pores are susceptible only to very slow, diffusion-controlled re-dissolution.  相似文献   

11.
A conservative solute tracer experiment was conducted in Indian Creek, a small urban stream in Philadelphia, Pennsylvania to investigate the role of subsurface properties on the exchange between streamwater and the hyporheic zone (subsurface surrounding the stream). Sodium Bromide (NaBr) was used as a conservative tracer, and it was monitored in the surface water at two stations and in the upper bed sediments (shallow hyporheic zone extending from 7.5 to 10 cm below the streambed). The hydraulic conductivity (K) of the upper bed sediments and the lower bed sediments (10–12.5 cm below the streambed) was measured in situ. High tracer concentrations were observed in the upper layer at locations where the hydraulic conductivity of the upper layer was larger than that of the lower layer. Low concentrations in the upper layer were observed in the converse case. A statistically significant relationship between the mass retained in the upper layer and the difference of K values between layers was observed.  相似文献   

12.
In the Bicholime, Goa, there is an iron-ore mine surrounded by villages and small towns. Exploitation of such an enormous amount of iron ore may disturb the local groundwater table, thereby causing groundwater depletion. However, these effects are dependent mainly on the geological formations in the region and the mining method. The effects of Bicholime mining on the surrounding groundwater resources and subsurface strata conditions are the main objective of this study. For this purpose, ground penetrating radar surveys were carried out to study the possible effects of mining on the hydrological regime of this open-pit mine, and the subsurface conditions in the Bicholime opencast iron-ore mine, Goa. The results of these surveys show that laterite layers exist from the surface to depths varying from 5 to 22 m at all study locations in the mining area, and in most places clay layers exist, except for the GPR section along the profiles DD1 and EE1 in the village at depths of 8-22 m. Due to the presence of impervious clay layers, mining and dewatering in the leasehold areas will most likely not affect the surrounding hydrological regime of this mining area. It is concluded that no hydrological connection exists between the open pits and the surrounding villages, so that sufficient groundwater is available in the village area, in spite of the enormous extraction of iron ore from the underground.  相似文献   

13.
铁路深埋隧道区岩溶发育特征研究   总被引：9，自引：2，他引：7  
对圆梁山隧道毛坝向斜段揭露溶洞进行分析的基础上 ,结合向斜区岩溶分布的勘察 ,对毛坝段深层岩溶发育特征及影响因素进行了研究。认为岩溶水在水力坡度或压力水头 ,尤其是水重力的作用下有不断向深层运移的趋势 ,所以在水具有侵蚀性的前提下 ,岩溶也具有向深层发育的趋势 ,并在厚层隔水底板及排泄基准面附近形成岩溶较发育带。而岩溶分布的层位及构造部位 ,在同一地质单元中的浅层和深层区具有类似性  相似文献   

14.
Hyperfiltration is sometimes cited as a mechanism to explain high degrees of calcite cementation at shale/sandstone contacts. To test this cementation mechanism, a series of experiments were performed in which solutions undersaturated with respect to calcite were hydraulically forced through a Ca-bentonite at different flow rates. Calcite precipitate was observed on the bentonite membrane from hyperfiltrated stock solutions having initial calcite saturation indices of 0.91 and 0.59. Supersaturation conditions at the clay's high-pressure interface are likely provided by establishment of a concentration polarization layer.In the subsurface, the driving force for hyperfiltration is a differential hydraulic pressure gradient acting across a shale membrane. This hydraulically-induced flux of solution causes a build-up of solute at the shale's high-pressure interface to levels that may exceed saturation indices of common cementing minerals like calcite. Although the source of the hydraulic pressure is likely due to compaction within the sedimentary pile, directional flow constraints suggest that hyperfiltration-induced precipitation of calcite occurs at sand/shale boundaries away from areas of active compaction.  相似文献   

15.
Abstract: Stratigraphic controls on the formation and distribution of gas hydrates were examined for sediments from a BH-1 well drilled in the landward slope of the Nankai Trough, approximately 60 km off Omaezaki, Japan. Three lithologic units were recognized in the 250 m-thick sequence of sediments: Unit 1 (0–70 mbsf) consists of calcareous silt and clay with thin volcanic ash layers, Unit 2 (70–150 mbsf) consists of calcareous silt and clay with volcanic ash and thin sand layers, and Unit 3 (150–250 mbsf) consists of weakly consolidated calcareous silt and clay with thick and frequent sand layers. Soupy structures and gas bubbles in the sediments indicate the presence of two hydrate zones between 40 and 130 mbsf and below 195 mbsf. Nannofossil biostratigraphy and magnetostratigraphy indicate that the sequence recovered at the BH-1 well is mostly continuous and represents sediments deposited from 0 to 1.5 Ma. Calculation of the sedimentation rate reveals a condensed section between 65 and 90 mbsf. The inferred distribution of gas hydrates in the BH-1 well appears to be strongly controlled by the stratigraphy and lithology of the sediments. Thick, gently inclined sand layers in Unit 3 provide a conduit for the migration of gases from deeper regions, and are considered responsible for the formation of the hydrate zone below 195 mbsf. At shallower levels, thin, gently inclined sand layers are also considered to allow for the migration of gases, leading to the formation of the upper hydrate zone between 40 and 130 mbsf. The overlying sub-horizontal silt and clay of the condensed section, truncating the underlying gently inclined sand and silt/clay layers, may provide an effective trap for gases supplied through the sand layers, further contributing to hydrate formation in the upper hydrate zone.  相似文献   

16.
INTRODUCTIONAsandduneisanyaccumulationofsand grainsshapedintoamoundorridgebythewindunderthein fluenceofgravity .Sanddunesarecomparabletootherformsthatappearwhenafluidmovesoveraloosebed .Dunesarefoundwhereverloosesandiswindblown(Bagnold ,196 0 ) .MostofIraqianlandsareaffectedbywinderosion ,theformationandmovementofsanddunes ,particularlyinthemiddleandsouthofIraq .Thetotalareacoveredbysanddunesisapproximately 2mil lionshectares ,andthreatensmanyothermillionshectares (DougramejiandKaul,197…  相似文献   

17.
Efforts to map the lithology and geometry of sand and gravel channel‐belts and valley‐fills are limited by an inability to easily obtain information about the shallow subsurface. Until recently, boreholes were the only method available to obtain this information; however, borehole programmes are costly, time consuming and always leave in doubt the stratigraphic connection between and beyond the boreholes. Although standard shallow geophysical techniques such as ground‐penetrating radar (GPR) and shallow seismic can rapidly obtain subsurface data with high horizontal resolution, they only function well under select conditions. Electrical resistivity ground imaging (ERGI) is a recently developed shallow geophysical technique that rapidly produces high‐resolution profiles of the shallow subsurface under most field conditions. ERGI uses measurements of the ground's resistance to an electrical current to develop a two‐dimensional model of the shallow subsurface (<200 m) called an ERGI profile. ERGI measurements work equally well in resistive sediments (‘clean’ sand and gravel) and in conductive sediments (silt and clay). This paper tests the effectiveness of ERGI in mapping the lithology and geometry of buried fluvial deposits. ERGI surveys are presented from two channel‐fills and two valley‐fills. ERGI profiles are compared with lithostratigraphic profiles from borehole logs, sediment cores, wireline logs or GPR. Depth, width and lithology of sand and gravel channel‐fills and adjacent sediments can be accurately detected and delineated from the ERGI profiles, even when buried beneath 1–20 m of silt/clay.  相似文献   

18.
Many hazardous waste sites in the south Louisiana Gulf Coast have been emplaced in sediments of Plio-Pleistocene to Recent age. Because of the fining upward nature of these regressive-transgressive fluvial-deltaic sequences and the purported confining capabilities of the shallow clay layers within them, this area would seem to be ideal for the location of surface waste landfills. However, detailed geologic mapping at a site in southeastern Louisiana documents how the three-dimensional distribution of sediment types and early diagenetic features, both of which were ultimately controlled by depositional history, can increase effective vertical permeability of finegrained sequences. Many bodies of sand that appear to be isolated in standard geotechnical cross sections can be shown to be part of spatially complex three-dimensional distributary networks, with fine-grained sediments representing overbank and backswamp deposits. Some clay layers are actually a composite of thinner clay beds, each subjected to subaerial exposure and the development of secondary porosity related to soil formation. There has been documented leakage of wastes down through the clays, and a recent study indicates that the effective vertical hydraulic conductivity of the clay layers exceeds 10–5 cm s–1, or from one to four orders of magnitude higher than values measured on samples from cores of the same sediment. An understanding of the depositional framework, facies architecture, and diagenetic history of geologic materials underlying waste disposal sites in Louisiana is required for rational development of monitoring and remediation plans.  相似文献   

19.
Osmotic phenomena refer to water and solute transport processes that occur when transport of solute molecules or ions is restricted by the porous medium relative to that of water molecules. Chemical osmosis and reverse osmosis/ultrafiltration are osmotic phenomena. The studies of “coupled flow” caused by chemical osmosis have been widely applied in many fields, such as earth science, environmental science and civil engineering. This paper provided a review of the considerable advances in the field of chemical osmosis and clay semipermeable membrane since the 1950s. We summarized the research progress of chemical osmosis in clayey sediments into three aspects: theoretical basis, experimental research and numerical model. In particular, the laboratory equipment and measurement methods of the chemico-osmotic efficiency coefficient σ were described,. The existing discontinuous models based on the ‘diffusive double layer’ theory were summerized, as well as the various control factors of σ. It increases with Cation Exchange Capacity (CEC), compaction pressure and decreases by the increasing of porosity and solution concentration. This paper also summerized the contimuum models based on non-equilibrium thermodynamics, which are used to explain and predict anomalies of hydraulic head pressure and salinity in clayey environments. For the future development of this discipline, it is critical to find a reliable method to confirm the σ value. It is also critical to emphasize the research on chemical osmosis in complex conditions and the influence of chemical osmosis on groundwater flow and solute transpotation. China has just stepped into this research area and more efforts should be made if significant progress is desired. This review will be helpful to further research on groundwater numerical simulation integrated with chemical osmosis in China.  相似文献   

20.
Macropores developed in barrier layers in soil covers overlying acid-generating waste rock may produce preferential flow through the barrier layers and compromise cover performance. However, little has been published on the effects of preferential flow on water balance in soil covers. In the current study, an inclined, layered soil cover with a 10-cm-wide sand-filled channel pathway in a silty clay barrier layer was built over reactive waste rock in the laboratory. The channel or preferential flow pathway represented the aggregate of cracks or fissures that may occur in the barrier during compaction and/or climate-induced deterioration. Precipitation, runoff, interflow, percolation, and water content were recorded during the test. A commercial software VADOSE/W was used to simulate the measured water balance and to conduct further sensitivity analysis on the effects of the location of the channel and the saturated hydraulic conductivity of the channel material on water balance. The maximum percolation, 80.1% of the total precipitation, was obtained when the distance between the mid-point of the channel pathway and the highest point on the slope accounted for 71% of the total horizontal length of the soil cover. The modeled percolation increased steadily with an increase in the hydraulic conductivity of the channel material. Percolation was found to be sensitive to the location of the channel and the saturated hydraulic conductivity of the channel material, confirming that proper cover design and construction should aim at minimizing the development of vertical preferential flow in barrier layers. The sum of percolation and interflow was relatively constant when the location of the channel changed along the slope, which may be helpful in locating preferential flow pathways and repairing the barrier.  相似文献   

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