Matrix Pore Water Research Articles

Paleohydrogeology of the Horonobe area, Northern Hokkaido, Japan: Groundwater flow conditions during glacial and postglacial periods estimated from chemical and isotopic data for fracture and pore water

Understanding the difference in groundwater flow between glacial and interglacial periods is crucial for predicting the impact of future climate changes on groundwater movement. This study assesses the difference in groundwater flow between the last glacial period (LGP) and the postglacial period (PGP) in fractured mudstones of the Horonobe area, Japan, by combining the data for stable isotopes (δD and δ18O) and Cl− concentration of fracture and pore waters with radiocarbon (14C) age. The isotopic compositions of fractures and pore waters indicate that groundwater at 28–250 m deep in a borehole closest to the recharge area comprises meteoric water, recharged under the same climates as the present. The fracture water has isotopic compositions more similar to meteoric water than the matrix pore water near the fracture. The 14C age of fracture water suggests meteoric water recharge during the PGP. At greater depths in the borehole and sampling points in other boreholes, the isotopic compositions indicate the mixing of glacial meteoric and altered connate water, with the fracture water having comparable isotopic compositions with the matrix pore water. The recharge timing of meteoric water is inferred to be the LGP or before based on 14C dating. These results suggest that the meteoric water recharged during the PGP flows at a shallow depth, whereas the meteoric water recharged during the LGP intruded to greater depths. This result is consistent with previous inferences from surface geophysical and geological surveys that the depths of local valleys during the LGP were greater by < 50 m than the present ones and enhanced the downward hydraulic gradient. Combining the chemical and isotopic compositions of groundwater with 14C age helps assess the groundwater flow during the LGP and PGP in fractured rocks.

Simulating electrochemical migration and anion exclusion in porous and fractured media using PFLOTRAN[formula omitted

Multicomponent transport of dissolved charged species often involves electrostatic interactions among the different ions. These interactions are the result of the different diffusion rates of the chemical species, which create a diffusion potential and thus an electromigration term in the transport equation that is additive to the Fickian diffusion term due to the concentration gradient. The explicit consideration of the electromigration term involves the use of the Nernst–Planck equation, which tightly couples the transport of the charged species through the electro-diffusive term. Here, we have implemented the Nernst–Planck equation in the open source computer code PFLOTRAN. The advantages of the customised code, denoted as PFLOTRANNP, in comparison with other recent similar developments, are that (i) the implementation is extended also to the multiple continuum module of PFLOTRAN, which makes the code suitable for the modelling of electromigration processes in fractured rock, (ii) PFLOTRANNP leverages the parallelisation capabilities of one of the latest stable PFLOTRAN versions and thus it is suited for large-size simulations in supercomputers and (iii) the developed code is made available through a public repository. PFLOTRANNP was successfully validated against a standard benchmark involving 1D transport of a multicomponent electrolyte solution and a verification case, involving a fracture–matrix system, was also presented. Finally, PFLOTRANNP was used to interpret field and experimental data at Olkiluoto (Finland), which show that an imbalance exists between chloride concentration in the matrix pore water and in the fracture filling groundwater. The results of the simulations show that this imbalance is indeed caused by anion exclusion processes, which are more significant in gneiss rock types than in pegmatitic granitic rock.

Monitoring shale water uptake using 2D magnetic resonance relaxation correlation and SPRITE MRI

Fluid storage and transport in shale and clay formations is important in many areas of application, including hydrocarbon recovery from, and waste fluid storing in, these reservoirs. The multiscale heterogeneous nature of shales is largely responsible for these rocks being considered unconventional and complicated. Researchers often rely on imaging methods to provide spatial information to characterize their heterogeneity.SEM and nano-CT are powerful methods that have been used to give images of shale nanopores with fields of view (FOV) of 100 μm. However, this limited FOV has raised questions about the validity of simulations based on these methods to scale up and represent macroscopic shale properties. Here, we have used, for the first time, SPRITE Magnetic Resonance Imaging (MRI) methods to give core plug size shale images. The imaging method resolves water and oil signals to give separate images of each in shales.“As received” shale samples were used to acquire 1D and 3D oil and water images to characterize natural fluid distribution. Water uptake experiments were conducted to probe fluid displacement in shales. Measurements included gravimetric, Magnetic Resonance (MR) T1-T2∗ relaxation correlation, and SPRITE imaging methods. Water and oil images showed that sample scale heterogeneities are imposed by the shale beddings. Water uptake experiments (1) showed the capability of the T1-T2∗ method to identify MR signals from shale fracture water and matrix pore water, and (2) illustrated wettability control of spatial water distribution in shales during spontaneous imbibition.

Quantifying groundwater flow variability in a poorly cemented fractured sandstone aquifer to inform in situ remediation

This study applies innovative methods to characterize and quantify the magnitude of groundwater flow in a fractured and variably cemented sandstone aquifer to inform an in-situ remediation strategy for trichloroethene (TCE) contamination. A modified active-distributed temperature sensing (A-DTS) approach in which fiber optic cables were permanently grouted in the borehole was used to quantify groundwater flow rates. Two additional tracer tests were conducted: 1) fluorescein tracer injection followed by rock coring and sampling for visual mapping and porewater analysis, and 2) deployment of passive flux meters in conventional monitoring wells to evaluate groundwater velocity and mass flux distributions. Forced gradient injection of fluorescein tracer suggests a dual porosity flow system wherein higher rates of groundwater flow occur within discrete features including highly permeable bedding planes and fractures, with slower flow occurring within the rock matrix. Tracer was observed and detected in the unfractured matrix porewater >1.5 m away from the injection well. Beyond this distance, >6 m radially away from the injection hole, tracer was primarily detected within and adjacent to high transmissivity fractures serving as preferential flow paths. The Darcy flux calculated using active distributed temperature sensing (A-DTS) shows depth-discrete values ranging from 7 to 60 cm/day, with average and median values of 23 and 17 cm/day, respectively. Passive Flux Meters (PFMs) deployed in three conventional monitoring wells with slotted screens and sand filter packs showed groundwater flux values ranging from 2 to 11 cm/day, with an overall average of 4 cm/day and are likely biased low due to spreading in the sand pack. The study results were used to inform an in-situ remediation system design including the proposed injection well spacing and the amendment delivery approach. In addition, the results were used to build confidence in the viability of delivering an oxidant to the rock matrix via advective processes. This is important because 1) the matrix is where the majority of the TCE mass occurs, and 2) it provides insights on processes that directly affect remedial performance expectations given advective delivery to preferential pathways and the matrix overcomes diffusion only conditions.

Effects of vugs on resistivity of vuggy carbonate reservoirs

Abstract The pore structure characteristic parameters of vuggy carbonate reservoirs were extracted, and matrix resistivity of vuggy reservoir was calculated by the percolation network simulation. A segmented cross-scale resistivity calculation method was established, in which the finite element method was used to simulate the resistivity of vuggy reservoirs. The mathematical models of vug porosity and water saturation with rock resistivity in vuggy carbonate reservoir were established, and the relationships between them were obtained. Experimental results verified the reliability of the simulation results. The method presented provides new technical means and research method for the resistivity log interpretation of vuggy carbonate reservoirs. The matrix porosity, vug porosity and matrix pore water saturation are the key factors determining the resistivity of reservoir rocks.

Profiles of chloride in matrix porewater as natural tracer for matrix diffusion in crystalline rocks

Matrix porewater from low permeable Grimsel granodiorite was successfully characterised using indirect methods applied to originally saturated core samples. Core samples were taken from a 17 m long borehole originating from a tunnel of the Grimsel Test Site into the crystalline bedrock intersecting a tectonic shear zone with a water-conducting fracture. Matrix porewater chloride profiles on the meter scale were determined on both sides of the water-conducting fracture. To evaluate transport processes within the bedrock formation, a series of diffusive model calculations were performed, which to fit the porewater data. Boundary and initial conditions were varied according to the geological conditions, whereas other required parameters such as the connected porosity and pore diffusion coefficients were determined by laboratory experiments on the cores and extrapolated to in situ conditions. The main conclusions can be summarized as follows: (1) Chloride porewater profiles at the meter scale can be simulated using diffusive transport models. This provides evidence that diffusive exchange with active fractures occurs over a range of a few meters in the low-permeable crystalline bedrock; (2) the best fit of the diffusion profile was achieved by a model approach, which takes asymmetric initial Cl-concentrations into account. This indicates that prior to the activation of the present water-conducting fracture, the porewater system in the bedrock was already active showing a concentration gradient in chloride; (3) the water-conducting fracture was activated at least between 850 and 1700 years before present, with a best-fit 1200 years before present; and (4) the hydraulic were affected by the construction of the rock laboratory 20 years ago, resulting in a rapid dilution of the fracture groundwater by advection.

Chloride diffusion in pore water in Olkiluoto veined gneiss and pegmatitic granite from a structural perspective

ABSTRACTSpent nuclear fuel from Finnish power plants is planned to be deposited deep in the crystalline bedrock in Olkiluoto, Finland. The bedrock and more specifically the elemental composition of ground water, which is composed of the fracture water and the matrix pore water, needs to be well characterized to assess the risks inherent to the long term safety of the site. To this end, it is valuable to investigate elemental composition of the matrix pore water since it tends to conserve hydrogeological signals for longer time spans compared to open fracture waters.In this study, the chloride concentration of matrix pore water in veined gneiss (VGN) and pegmatitic granite (PGR) samples were investigated. Chloride was out-diffused from the naturally saturated rock cores into deionized water. Chloride pore diffusion coefficients were derived by modelling the chloride breakthrough curves obtained from the out-diffusion experiments. Two component modelling gave best fit to the experimental results. There two diffusion coefficients were (9±2)×10-11 m2/s and (0.5±0.1)×10-11 m2/s for PGR and (2.5±0.5)×10-11 m2/s and (0.4±0.1)×10-11 m2/s for VGN. Porosity distribution and total porosities of the rock samples were studied with the C-14-PMMA autoradiography. Porosity for PGR was found to be 0.6 % with large mineral transecting fissures, and porosity for VGN was found to be 0.7 % with highly porous mineral clusters connected to each other via grain boundaries and intragranular pores. The findings here show that heterogeneity has to be taken into account in modelling to find better agreement with the experimental results. C-14-PMMA autoradiography results indicate dual-component behavior for diffusion in PGR and VGN which were used in the modelling.

Determination of hexavalent chromium concentrations in matrix porewater from a contaminated aquifer in fractured sedimentary bedrock

A new method for quantification of hexavalent chromium (Cr(VI)) in the porewater of rock core samples from contaminated sedimentary bedrock has been developed here. The method combines alkaline extraction with cation exchange column separation followed by determination of Cr concentrations by inductively coupled plasma mass spectrometry (ICP-MS). A porewater detection limit of 45μg/L was determined by performing extractions on uncontaminated samples, and accounts for dilution of porewater volumes by the extraction solution. Recoveries of Cr(VI) in quality control (QC) samples were greater than 90% and there was no significant interference from Cr(III). Relative standard deviations (RSD) were less than 10% for QC samples spiked with Cr(VI), and 2 to 47% (average of 21%) for replicate analyses of core samples. Cr(VI) analyses were conducted on depth-discrete core samples collected at intervals of <0.3m from sandstone and siltstone bedrock within a contaminated groundwater plume. Groundwater samples were collected using multilevel well ports and were also analyzed for Cr(VI) concentrations. Significant Cr(VI) anomalies were observed in the rock matrix of the core samples. Overall, we observe general agreement in the Cr(VI) concentrations between the samples of immobile rock-matrix porewater and the samples of groundwater which is mobile in rock fractures. This method provides a viable procedure for determination of Cr(VI) concentration in bedrock porewater, and these datasets are valuable for developing conceptual models, assessing plume transport and fate, and for considering remedial options.

Impact of fluid–rock chemical interactions on tracer transport in fractured rocks

In this paper, we investigate the impact of chemical interactions, in the form of mineral precipitation and dissolution reactions, on tracer transport in fractured rocks. When a tracer is introduced in fractured rocks, it moves through the fracture primarily by advection and it also enters the stagnant water of the surrounding rock matrix through diffusion. Inside the porous rock matrix, the tracer chemically interacts with the solid materials of the rock, where it can precipitate depending on the local equilibrium conditions. Alternatively, it can be dissolved from the solid phase of the rock matrix into the matrix pore water, diffuse into the flowing fluids of the fracture and is advected out of it. We show that such chemical interactions between the fluid and solid phases have significant impact on tracer transport in fractured rocks. We invoke the dual-porosity conceptualization to represent the fractured rocks and develop a semi-analytical solution to describe the transient transport of tracers in interacting fluid-rock systems. To test the accuracy and stability of the semi-analytical solution, we compare it with simulation results obtained with the TOUGHREACT simulator. We observe that, in a chemically interacting system, the tracer breakthrough curve exhibits a pseudo-steady state, where the tracer concentration remains more or less constant over a finite period of time. Such a pseudo-steady condition is not observed in a non-reactive fluid-rock system. We show that the duration of the pseudo-state depends on the physical and chemical parameters of the system, and can be exploited to extract information about the fractured rock system, such as the fracture spacing and fracture-matrix interface area.

Some aspects of release and transport of gases in deep granitic rocks: possible implications for nuclear waste repositories

Radon, radium and helium data from three sites in granitic rock in Sweden (Forsmark and Laxemar) and Finland (Olkiluoto) from depths greater than 1,000 m were interpreted by a model that describes how daughter nuclides, including helium from uranium and thorium decay, escape from minerals, enter the pore water in the rock matrix and diffuse to the seeping water in the fractures in the rock. The radium concentrations are on the order of <30 Bq/l of water that has recently infiltrated and then emerged from the rock. Radon concentrations are mostly several orders of magnitude larger. The model predicts values in the same range. The fair agreement between model results, field data and laboratory data over a scale spanning micrometres over meters to kilometres, and time scales of days to millions of years, shows that the micropores of the rock matrix are connected even at depths down to and beyond a kilometre. Molecular diffusion in the matrix pore water is a key migration mechanism. Laboratory-derived sorption coefficients for radium are of the same magnitude as those needed in the modelling of the in situ data to give good agreement.

Re-evaluating dual-porosity effects at the site of a seminal groundwater modelling study: Tilmanstone, southern England

Abstract The first numerical model of solute transport to incorporate Fickian diffusive exchange between mobile fracture water and immobile porewater for an actual case of groundwater contamination at catchment scale was applied to the Chalk aquifer at Tilmanstone in SE England by Bibby ( Water Resources Research , 1981, 17 , 1075–1081). The unconfined aquifer at Tilmanstone had been contaminated by coalfield brine leaking from disposal lagoons operating throughout much of the twentieth century. Recent observations show that the Bibby model underestimates dual-porosity diffusive retardation, and hence underestimates the persistence of contamination, probably by several decades. 2D representation of the aquifer in plan ignored the hydrostratigraphy, and model calibration was limited by the lack of time-variant paired profiles of fracture water and porewater. Vertical profiles through the Chalk determined by packer testing, borehole dilution testing and geophysical logging, together with a new depth profile of chloride concentration in Chalk matrix porewater, are described. The hydrostratigraphy is interpreted in relation to the Chalk lithostratigraphy of SE England, and incorporated into a vertical-section model of chloride transport along the axis of the valley, consistent with the new and historical profiles of fracture water and porewater chloride concentrations. New predictions of the longevity of the chloride contamination at Tilmanstone are presented.

Redistribution of contaminants by a fluctuating water table in a micro-porous, double-porosity aquifer: Field observations and model simulations

Large seasonal fluctuations of the water table are characteristic of aquifers with a low specific yield, including those fractured, double-porosity aquifers that have significant matrix porosity containing virtually immobile porewater, such as the Chalk of northern Europe. Where these aquifers are contaminated, a strong relationship between water table elevation and contaminant concentration in groundwater is commonly observed, of significance to the assessment, monitoring, and remediation of contaminated groundwater. To examine the processes governing contaminant redistribution by a fluctuating water table within the ‘seasonally unsaturated zone’, or SUZ, profiles of porewater solute concentrations have been established at a contaminated site in southern England. These profiles document the contaminant distribution in porewater of the Chalk matrix over the SUZ at a greater level of detail than recorded previously. A novel double-porosity solute transport code has been developed to simulate the evolution of the SUZ matrix porewater contaminant profiles, given a fluctuating water table, when the groundwater is initially contaminated and the SUZ is initially free of contamination. The model is simply characterised by: the matrix-fracture porosity ratio, the matrix block geometry, and a characteristic diffusion time. De-saturation and re-saturation of fractures is handled by a new approximation method. Contaminant accumulates in the upper levels of the SUZ, where it is less accessible to mobile groundwater, and acts as a persistent secondary source of contamination once the original source of contamination has been removed or has become depleted. The ‘SUZ process’ first attenuates the progress of contaminants in groundwater, and subsequently controls the slow release of contamination back to the mobile groundwater, thus prolonging the duration of groundwater contamination by many years. The SUZ process should operate in any fractured, micro-porous lithology e.g. fractured clays and mudstones, making this approach widely applicable.

Implications of halide leaching on 36Cl studies at Yucca Mountain, Nevada

Chlorine 36 was generated from nuclear tests in the 1950s and 1960s and has been used to identify fast flow paths at Yucca Mountain, the proposed repository for high‐level nuclear waste [Fabryka‐Martin et al., 1997, 1998]. Bomb pulse 36Cl, brought into the subsurface by infiltrating rainwater, presumably resides along fracture surfaces because of the extremely low rock matrix permeability. However, leaching a rock sample to extract this salt inevitably extracts pore water chloride (Cl) and rock matrix chloride, thereby making it difficult to obtain reproducible measurements or detect the specific bomb pulse signatures. Complexities introduced by these sources of older chloride include dilution of bomb pulse 36Cl/Cl ratios for samples from strata with a high Cl concentration, variations in measured ratios as a function of leaching time, rock chip size, and the differing effects of active leaching from those of passive leaching. This work provides both a conceptual model and a mathematical solution for the leaching processes and examines the role of sample leaching in the 36Cl studies of Yucca Mountain rocks. An analytical solution is derived for the diffusion of Cl and 36Cl in composite media (rock matrix and water) to accommodate variable diffusivity. This solution is subsequently used to develop a leaching model that takes into account bomb pulse signal, matrix pore water, and relatively hard to leach components (isolated fluid inclusion and mineral boundary salts). The model is then applied to samples from stratigraphic units at Yucca Mountain to obtain leachate concentrations from different setup methods (protocols), including duration, chip size, and gravitational settling of the water‐rock mixture. The model results show that the probability of detecting a 36Cl/Cl bomb pulse signal is severely diminished under longer leaching times and smaller rock fragment sizes and that leaching times of 1 to 10 hours are most likely to be successful in detecting a bomb pulse signal. Bomb pulse 36Cl/Cl ratios are more likely to be observed if pore water Cl concentrations were initially low prior to the introduction of bomb pulse carrying water.

Reactive transport model for the ambient unsaturated hydrogeochemical system at Yucca Mountain, Nevada

To assist a technical review of a potential application for a geologic repository, a reactive transport model is presented for the ambient hydrogeochemical system at Yucca Mountain (YM). The model simulates two-phase, non-isothermal, advective and diffusive flow and transport through one-dimensional matrix and fracture continua (dual permeability) containing ten kinetically reactive hydrostratigraphic layers. We developed a thermodynamic interpretation of ambient groundwater chemistry at YM from limited analytical data and then combined it with insights from experimental and site-specific data to craft a conceptual model describing thermodynamic and kinetic relationships in the mineral/glass–water–gas system. The model was calibrated by adjusting uncertain thermodynamic and kinetic parameters to reflect observed trends at YM. The following calibration criteria were used to ensure that model predictions are consistent with hydrochemical and petrologic data from YM: (1) calculated multicomponent matrix pore water compositions do not vary significantly from reinterpreted analytical data; (2) simulated variations in matrix silica concentrations with depth are bounded by the observed analytical range; and (3) feldspars and glass dissolve while clays and calcite precipitate. The largest changes in calculated species concentrations occur in response to glass dissolution. Simulated matrix groundwater compositions at the depth of the potential repository are largely inherited from percolating waters rather than controlled by in situ chemical reactions. Predictions about groundwater flow pathways in the ambient YM system are sensitive to assumptions regarding percolation flux and fracture–matrix interactions. Confidence in complex reactive transport models of the thermally perturbed YM repository system depends on successful representation of ambient system conditions.

Environmental assessment of waste matrices contaminated with arsenic

The use of equilibrium-based and mass transfer-based leaching tests has been proposed to provide an integrated assessment of leaching processes from solid wastes. The objectives of the research presented here are to (i) validate this assessment approach for contaminated soils and cement-based matrices, (ii) evaluate the use of diffusion and coupled dissolution–diffusion models for estimating constituent release, and (iii) evaluate model parameterization using results from batch equilibrium leaching tests and physical characterization. The test matrices consisted of (i) a soil contaminated with arsenic from a pesticide production facility, (ii) the same soil subsequently treated by a Portland cement stabilization/solidification (S/S) process, and (iii) a synthetic cement-based matrix spiked with arsenic(III) oxide. Results indicated that a good assessment of contaminant release from contaminated soils and cement-based S/S treated wastes can be obtained by the integrated use of equilibrium-based and mass transfer-based leaching tests in conjunction with the appropriate release model. During the time scale of laboratory testing, the release of arsenic from the contaminated soil matrix was governed by diffusion and the solubility of arsenic in the pore solution while the release of arsenic from the cement-based matrices was mainly controlled by solubilization at the interface between the matrix and the bulk leaching solution. In addition, results indicated that (i) estimation of the activity coefficient within the matrix pore water is necessary for accurate prediction of constituent release rates and (ii) inaccurate representation of the factors controlling release during laboratory testing can result in significant errors in release estimates.

Radioactive waste containment in indurated shales: comparison between the chemical containment properties of matrix and fractures

Abstract The chemical containment properties of poorly consolidated, plastic clays have received considerable attention, particularly during investigations related to radioactive waste disposal. However, the chemical containment properties of indurated shales have received rather less consideration. This paper summarizes those properties of consolidated shales which make them suitable for the chemical containment of radionuclides, using the example of investigations at the Tournemire research site in the south of France. The problems associated with investigating such low-permeability media are highlighted, and solutions to these problems are presented. At this research site, tectonic processes have induced centimetre-scale fractures and decimetre-scale faults in the shales, whereas excavation of a tunnel has generated metre-scale fracturing. Some low water flows through the fracture networks have been detected. However, the Tournemire shales are characterized by very low matrix pore-water contents, and any limited fluid mobility through the matrix is caused mainly by diffusion. Slowly flowing waters in fractures are chemically analogous to these matrix pore-waters. The hydrochemistry is of Na-Cl-( HCO 3 − ) type, and the waters are slightly alkaline, strongly reducing and of low ionic strength. The resemblance between the chemistries of pore- and fracture-waters arises from the buffering of the pore-water chemistry by matrix mineral/water interactions, and possible subsequent diffusion of the pore-water into the fractures. The chemical buffering mechanisms controlling the water chemistries (especially pH, alkalinity and Eh ), are discussed and modelled. The chemical conditions will minimize the mobility of many radionuclides in both shale matrix and in fractures. Chemical evidence from the fracture-filling calcite is consistent with this hypothesis and demonstrates the past immobility of U and Cs. Batch experiments carried out in gloveboxes under a N 2 atmosphere and with synthetic fracture-water show weaker retention of Cs by the calcite fillings than by the shale matrix. Due to its very low porosity, the shale matrix should behave like a membrane filter for colloids, further reducing the mobility of radionuclides. In fractures, interface structures between matrix walls and calcite veins should also hinder colloid migration (except perhaps in occasional centimetre-scale apertures).

Fluxes of water and solute in a coastal wetland sediment. 2. Effect of macropores on solute exchange with surface water

We investigated interactions between sediment physical structure and solute transport in an intertidal coastal wetland. Two distinct pore-size classes in the sediment were identified. Macropores had effective diameters greater than 100 μm and a normalized volume of 5%; matrix pores had effective diameters smaller than 100 μm and were the volumetrically dominant pore-size class (95%). We found that infiltration and evaporation-driven water fluxes were segregated between macropores and matrix pores, respectively, which had the effect of enhancing diffusive effluxes of chloride from the sediment to surface water. Chloride was highly concentrated relative to seawater in matrix porewater but was comparatively dilute in macropores. Concentration differences in pore-size classes declined with depth until indistinguishable below 10 cm. The segregated chloride distribution can be explained if recharge to the sediment occurred by downward infiltration in macropores and discharge occurred by an upward flux in matrix pores to satisfy evapotranspiration. Without disturbance by the downward infiltration flux in macropores, upward advection of chloride in matrix pores and evapoconcentration increased chloride concentrations in matrix pores to a level well above the concentration in seawater. The resulting high concentrations of chloride in matrix pores induced a large diffusive efflux of chloride into surface water that was sufficient to balance new input of chloride by infiltration of seawater in macropores (0.085 mmol Cl cm -2 day-'). Transport models that were constrained by water balance measurements at the field site explained both the exponential form of the vertical distribution of chloride in matrix pores and the rate of change in storage of chloride in sediment porewater over a 1 month period. We conclude that segregation of water and solute fluxes in two pore-size classes strongly influences sediment salinity of coastal wetlands, which has direct bearing on primary productivity of dominant vegetation and on exchange of dissolved nutrients and contaminants between intertidal wetlands and open water.

Field experiments in a fractured clay till: 2. Solute and colloid transport

A field tracer experiment was conducted in a lateral flow field in the weathered and highly fractured upper 6 m of a 40‐m‐thick clay‐rich till plain in southwestern Ontario. In the upper 3 m where fractures are closely spaced (&lt;0.13 m) the advancing front (C/C0 = 0.01) of the nonreactive solute tracers, bromide and 18O, migrated at rates of 0.01 to 0.07 m/d, over distances of 4 and 6 m and under a lateral hydraulic gradient of 0.24. In this same zone, two strains of colloid‐sized bacteriophage tracers migrated at rates of 2 to &gt;5 m/d. Simulations with a discrete fracture/porous matrix flow and transport model, which used the cubic law for flow in fractures, showed that diffusion of the solutes, but not the much larger colloids, into the matrix pore water between fractures is sufficient to cause the observed difference in solute and colloid transport rates. Transport‐derived and hydraulic conductivity‐derived fracture aperture values were similar, within a factor of 3 and falling mainly within a range of 5–40 μm. In the upper 3 m the solute tracers were evenly distributed between pore water in the fractures and the matrix, and as a result, solute transport can be closely approximated with an equivalent porous medium (EPM) approach. Below this depth, fractures are more widely spaced (0.13 to &gt;1 m) with concentration peaks tending to occur near visible fractures, and solute transport cannot be adequately described with an EPM approach.