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Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada
Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.—Discussion
Abstract Recent development of fluvial facies models has been due to improved description of natural river and floodplain processes and deposits using: (1) ground-penetrating radar (GPR) combined with cores and trenches to describe modern deposits in 3D; (2) study of frozen rivers to allow easy access to the entire channel belt and procurement of undisturbed cores; (3) optically stimulated luminescence (OSL) for improved dating of deposits; (4) high-resolution remote sensing over large areas and at short time intervals in order to determine temporal changes in channel and floodplain geometry due to erosion and deposition; (5) new measuring equipment such as acoustic Doppler current profilers (ADCP), high-resolution multibeam sonar, and GPS, for measuring surface topography, flow, and sedimentary processes. However, there is still a lack of studies of river geometry, flow, and sedimentary processes at the all-important high flow stages, especially on big rivers and floodplains. Laboratory studies of bed geometry, flow, and sediment transport, erosion, and deposition have been undertaken for a range of scales, from small bedforms such as ripples, dunes, and antidunes, to bars and channels, to whole channel belt–floodplain systems. Controls on river and floodplain mechanics such as sediment supply, base level, and tectonism have also been evaluated. However, there are scaling problems with laboratory experiments that become more acute as the scale of the system increases. The new field and laboratory data have allowed development of new qualitative and quantitative fluvial depositional models. Such models account for the fact that: (1) there are different superimposed scales of fluvial forms and associated stratasets in rivers and floodplains; (2) the geometry and mode of migration of any scale of fluvial form (e.g., dune, bar, channel, channel-belt) is closely related to the geometry and internal character of the associated strataset, which allows development of generalized depositional models for the different scales; (3) changes in flow stage over various time scales affect the nature of deposits. These new models use consistent descriptive terminology and dispel many of the extant misconceptions about fluvial deposits. Quantitative, process-based models of fluvial deposits exist, but are not well developed, especially for the longer-term and larger-scale processes and deposits. Process-based models of the effects of tectonism, climate, and base-level change on fluvial deposits are in their infancy. Furthermore, most models are difficult to test. These problems with quantitative models are due to lack of appropriate quantitative data, and difficulties in mathematical modeling of complex natural systems. As a result of this, stochastic models are commonly used to represent fluvial stratigraphy, given initial data from wells, cores, and geophysical surveys. Development of quantitative models is essential if we are to understand and predict the nature and spatial distribution of ancient fluvial deposits, and to characterize aquifers and hydrocarbon reservoirs for subsurface fluid flow simulations. Such development will require more studies of rivers and floodplains during floods, and more mathematical sophistication.
Front Matter
Abstract The spatial variation of sedimentary aquifer properties (e.g., porosity and permeability) must be characterized in order to develop accurate models of groundwater flow and solute transport. Aquifer characterization should ideally involve the following steps: (1) analysis of borehole logs, cores, and hydraulic testing data to determine the sedimentological nature and origin of the strata, and their hydraulic properties; (2) stratigraphic correlation of borehole logs and cores in order to assess the lateral continuity of distinctive sediment types (facies) between boreholes; (3) use of geophysical profiles to assess the orientation and structural continuity of sequences of strata, and to recognize distinctive geophysical patterns that can be related to distinctive sedimentary facies; (4) modeling of the geometry and distribution of sedimentary facies in the volume between boreholes, and (5) distribution of properties such as porosity and permeability as a function of sedimentary facies. Unfortunately, hydrogeologists rarely incorporate information on the sedimentology of aquifers, and shallow geophysical methods are not routinely used for aquifer characterization. Furthermore, techniques for accurately modeling the sedimentary facies or hydrofacies in three dimensions are still under development. However, recent use of combined sedimentological and geophysical techniques (e.g., GPR, high-frequency seismic, resistivity, electrical conductivity) have helped to describe the spatial variation in the porosity and permeability of sedimentary aquifers. These advances are leading to quantitative, 3-D stratigraphic models that provide a link among spatial variation in porosity and permeability, different scales of stratification (defined mainly by variations in grain size, shape, and fabric), and variation in geophysical parameters such
Integration of Sedimentologic and Hydrogeologic Properties for Improved Transport Simulations
ABSTRACT Traditional geostatistical approaches for estimating distributions of hydraulic conductivity fail to reflect sharp contrasts that occur at boundaries between different stratigraphic units, thus limiting the accuracy of contaminant-transport models. We present an approach to incorporate a stratigraphic framework into geostatistical simulation at the scale of a plume, to better represent aquifer heterogeneity. The approach was developed and tested at the Schoolcraft Bioremediation Site in southwestern Michigan, where detailed estimates of aquifer properties were needed to accurately simulate multi-component reactive transport and to design an effective bioremediation strategy. The sediments at the site were deposited as glaciofluvial outwash downstream of the Kalamazoo Moraine, and consist mainly of fine to medium sands with interbedded gravels and silts. A series of 18-meter-long continuous cores was collected in the vicinity of the bioremediation-system delivery wells. These cores were assessed for sedimentary facies, grain-size distribution, porosity, and hydraulic conductivity. Sedimentologic measurements from outcrop analogs supplemented the core data from the site. On the basis of the core data, the aquifer was separated into four stratigraphic units, and the measured conductivity values were geostatistically interpolated within each stratigraphic unit. The stratigraphically based estimates of hydraulic conductivity were used as input to a high-resolution, three-dimensional model of groundwater flow and solute transport in the region. The model with stratigraphic interpolation provided better transport predictions for an injected tracer pulse than models that do not incorporate the stratigraphy.
Influence of Incised-Valley-Fill Deposits on Hydrogeology of a Stream-Dominated Alluvial Fan
ABSTRACT Coarse-grained, incised-valley-fill (IVF) deposits of the Kings River alluvial fan, located southeast of Fresno, California, strongly influence local recharge and groundwater flow, and thus contaminant transport, within the alluvial-fan aquifer. Alluvial-fan sequence stratigraphic concepts provide a framework for predicting the geometry and internal facies associations of the IVF and surrounding depositional sequences. Three-dimensional Markov chain models of spatial variability of facies distributions were developed for each sequence and the uppermost IVF. Facies distributions in individual sequences were simulated separately to avoid geostatistical correlation across unconformity boundaries. These individual realizations were combined to produce a final, three-dimensional, multi-scale model of aquifer hydrofacies distributions. Modeling of groundwater flow and solute transport within this stratigraphic framework indicates that the coarse-grained IVF significantly influences groundwater flow and contaminant transport in several ways. First, the high degree of gravel/sand body connectivity within the IVF results in rapid groundwater flow relative to the surrounding, generally finer-grained fan deposits. Second, the coarse-grained nature and relatively high hydraulic conductivity of these sediments enhances vertical flow and recharge. Finally, modeling indicates that groundwater and contaminants generally flow from the IVF deposits into the adjacent alluvial-fan deposits. Thus, the IVF not only results in rapid downward and horizontal movement of contaminants but also routes non-point-source contaminants into adjacent deposits. This significantly increases the vulnerability of aquifer sediments adjacent to the IVF to contamination.
Sensitivity of Groundwater Flow Patterns to Parameterization of Object-Based Fluvial Aquifer Models
ABSTRACT Object-based models offer a convenient means of numerically simulating the structure of sedimentary aquifers. Multiple parameters control the shapes, proportions, and spatial relationships of the objects defining the different facies in the resulting simulations. A sensitivity analysis is performed here to determine the sensitivity of several different performance measures of groundwater flow in a fluvial-basin aquifer to the parameterization of the object-based model. Unconditional realizations of the permeability field are created through an object-based model that simulates multiple facies (channel, levee, splay, and floodplain deposits) in a fluvial depositional system. A single permeability value is assigned to each facies. For each realization, Latin hypercube sampling is used to draw the input parameters from predefined distributions characterizing the uncertainty inherent in these parameters. Groundwater flow is simulated parallel to the principal channel azimuth, and the groundwater flow patterns are determined by tracking streamlines through multiple realizations of the simulated aquifer. Results on groundwater flow include points on the distributions of travel times, tortuosity of flow paths, and dispersion of the flow patterns. The values of the input parameters are compared to the results on groundwater flow using a generalized sensitivity analysis to provide a quantitative technique for the assessment of parameter sensitivity. Results of this sensitivity analysis show that a first-order characterization goal must be determination of the proportions of the high-permeability (channel) and low-permeability (floodplain) facies within the aquifer. To a lesser degree, the performance measures of groundwater flow are sensitive to the channel sinuosity, thickness, and width-to-thickness ratio. These results can be used to provide guidance towards more efficient characterization of fluvial depositional settings for groundwater studies.
Abstract The spatial covariance of ln(k) can be modeled with a hierarchy of covariance structures corresponding to the organization of bedding within and among the lithofacies of a sedimentary sequence. Such a model accounts for the spatial correlation of ln(k) within and across bedding units defined at any one level ( Ritzi et al., 2004 ). This is related to correlation of ln(k) at a higher level (larger scale) through the spatial correlation of indicator variables representing the proportions, geometry, and juxtaposition patterns of the units at the lower level. In this paper the fitting of the components of the hierarchical model, written as nested functions, is considered in developing a hierarchical covariance model for use in estimation, simulation, or analytical derivation of macrodispersivity models. The least-squares criterion, along with parameter prior information and other weighted constraints, is used as the objective function of the inverse problem, which is solved by the Gauss-Newton-Levenberg-Marquardt method. The method is tested on synthetic data and illustrated with real data from a site with glaciofluvial sand and gravel deposits.
ABSTRACT Aquifer heterogeneity at small scales (meters to tens of meters) can be characterized with hydrofacies. We investigate the feasibility of translating lithofacies into hydrofacies by testing the hypothesis that the permeability frequency distributions of different lithofacies are distinct. We mapped 11 lithofacies and performed more than 1800 in situ permeability measurements at an outcrop exposing poorly cemented, nonmarine, clastic sediment. The lithofacies represent both channel and interchannel deposits, are both ribbon-form and tabular, and vary in grain size from clay to sandy gravel. For each lithofacies permeability sample, we calculated variograms to define correlation lengths that were used to select spatially uncorrelated subsamples from each sample. The frequency distributions of permeability subsamples from the various lithofacies were compared using nonparametric statistical tests. The statistical tests generally support the claim that the lithofacies permeability distributions are distinct from one another.
Abstract Electrical logs of various types have been used for decades in a wide variety of geoscience applications. Except for studies within a few meters of the land surface, these logs have been obtained using existing wells or boreholes. Recently, electrical conductivity (EC) sensors have been incorporated into direct-push equipment to obtain sedimentologic information in unconsolidated deposits without the need for existing wells and at a resolution (0.02 m) that has not been possible using conventional logging tools. The high resolution of this information, coupled with the speed at which it can be obtained, makes direct-push EC logging a valuable new tool for a wide variety of hydrostratigraphic studies. We document the utility of this approach in a detailed stratigraphic evaluation of a floodplain margin in a major river valley in the United States. Throughout the central United States, unconsolidated sequences underlying floodplains are typically composed of fining-upward glaciofluvial or Holocene sediments in which silt and clay overbank deposits cap coarser materials that serve as regionally significant aquifers. EC transects at a field site on the Kansas River floodplain show that this fine-grained cap may be truncated by, or interfingered with, coarser sediments at the floodplain margin. This increased stratigraphic complexity suggests that the depositional settings assumed for the more central portions of the floodplain may not be appropriate in the margin areas. The replacement of fine-grained sediments with coarser-grained materials at the margin of the floodplain has significant implications for groundwater recharge and solute movement. Interpretations made using the EC transects are consistent with results from an electromagnetic survey, as well as head and chemistry data. This work shows that direct-push EC logging can provide information about site stratigraphy at a level of detail that would be difficult to obtain with conventional approaches. This unprecedented level of detail enables important insights to be obtained regarding stratigraphic controls on groundwater flow and solute transport.
Abstract Preferential flow paths in shallow groundwater systems can be characterized by intensive tracer experiments, but these are expensive and time consuming to carry out. Geophysical surveys, such as ground-penetrating radar (GPR), have also been used to detect the presence of preferential flow paths in these systems, but the results have rarely been compared. Tracer experiments and GPR surveys were combined in two shallow alluvial gravel aquifer systems to better detect the presence of, and characterize, shallow groundwater. The two groundwater systems had differing hydraulic conductivities, dispersivities, and degrees of heterogeneity. Aquifer parameters (flow velocity, hydraulic conductivity, dispersivity) were derived from the tracer data using the method of temporal moments. Preferential flow paths were inferred using data on tracers and pesticide concentrations. The radar image showed preferential flow paths with trends similar to those identified from the tracer experiments and pesticide leaching trials. The combination of these techniques increased the confidence in the final interpretation.
Time-Lapse Geophysics For Mapping Fluid Flow in Near Real Time: Results From a Controlled Mesoscale Experiment
Abstract Accurate modeling of subsurface processes requires densely spaced values of hydrological properties, biological activity, and geochemical conditions, as well as the changes in these values. Obtaining these high-resolution property values invasively is cost prohibitive and likely infeasible. The only practical way to obtain these values is through geophysical imaging tools (e.g., Sauck et al., 1998 ). While the best we can do with single geophysical surveys is obtain a map of physical properties, time-lapse 3D geophysical surveys can provide access to processes in near real time information that can be used in making decisions on a range of subsurface environmental mitigation efforts. While the use of time-lapse geophysics has become well accepted, especially in near-surface geophysics, the processing of this data in real time for optimal use of the information is still in its infancy. A controlled mesoscale experimental setting was created at Columbia University and used for an implementation of an approach in which geophysical data are processed to automatically provide near-realtime images that provide insight into ongoing processes. A controlled injection of canola oil in a large sand tank yielded a suite of 162 3D datasets that were processed in near real time, demonstrating the practicality of this approach to yield information on fluid flow in real time.
The Use of Ground-Penetrating Radar for Characterizing Sediments Under Transient Flow Conditions
Abstract Understanding how heterogeneous sedimentary deposits affect fluid flow and contaminant transport has been greatly improved through outcrop studies. Such studies have aided in hydrogeological site characterization by identifying sedimentary structures that greatly impact flow and transport. The use of geophysical methods for delineating structures relevant to flow and transport such as fast paths, which control the initial breakthrough of contaminants, or sand-rich regions, which retard reactive contaminants, is beginning to receive much attention. In particular, ground-penetrating radar (GPR) was recently evaluated for this purpose within the framework of an aquifer analog study. Using models derived from digitized outcrop images, GPR simulations showed that field data were largely affected by non-uniform water saturation in addition to sediment heterogeneity. In the present work, we begin to explore the possibility of using GPR during transient flow to further aid in hydrogeological site characterization. A case study is presented in which a digitized outcrop image is chosen for the simultaneous simulation of variably saturated flow and GPR (for crosshole and surface reflection configurations). The sensitivity of the fluid distribution and of the GPR response to model parameters is investigated for the outcrop model during steady state and induced transient flow (ponding infiltration). In some cases, synthetic time-lapsed measurements are shown to offer information pertinent to hydrogeological site characterization.
Characterization of Heterogeneity in Unsaturated Sandstone Using Borehole Logs and Cross-Borehole Tomography
Abstract Characterization of the spatial variability of hydraulic properties of an aquifer is essential for reliable modeling of the fate of contaminants in the subsurface. Many geophysical methods offer the potential to derive such information, because of the high spatial density of sampling and the, albeit indirect, relationship between many geophysical and hydraulic parameters. In particular, borehole-to-borehole imaging may provide high-resolution sampling at scales which will permit detailed site characterization. Here, we examine the high-resolution spatial variability of electric and electromagnetic properties in the vadose zone at a specific field site in the Triassic Sherwood Sandstone aquifer in the UK, using cross-borehole radar. Assessment of spatial variability in the vadose zone is achieved through computation of experimental semivariograms of geophysical properties obtained from inter-well tomograms. The variability is compared with that deduced from conventional geophysical well-logging tools. The stratified nature of the site is clearly identified by gamma-log measurements. Single-hole and cross-hole geophysical measurements involving electrical properties are affected by both the stratification and the hydrological forcing conditions at the surface, and all show a similar spatial correlation structure. These data suggest that a representation of one-dimensional recharge processes is appropriate at the site. Data on moisture content from cross-hole radar are quantitatively compared against the results of stochastic unsaturated-flow simulations, accounting for the lithological spatial variability as described by gamma logs. The results demonstrate how improved conceptualization of the hydrological model of the site is achieved through incorporation of such geophysical data. Such a methodology permits improved assessment of mechanisms of recharge and transport at the site.
Development of a 3-D Depositional Model of Braided-River Gravels and Sands to Improve Aquifer Characterization
Abstract Braided-river gravels and sands form important aquifers in the Quaternary fluvioglacial outwash deposits of many parts of the world. A detailed understanding of these deposits is vital for modeling groundwater flow and contaminant transport. A quantitative, 3D depositional model that can aid characterization of gravelly fluvial aquifers is developed based on existing published information and extensive new data from the Sagavanirktok River in northern Alaska. Sagavanirktok River deposits were studied using trenches, cores, wireline logs, porosity and permeability measurements, and ground-penetrating radar profiles. The mode of origin of the deposits was interpreted using knowledge of: (1) channel geometry and mode of erosion and deposition derived from annual aerial photos, and (2) bed texture and bed topography during erosion-deposition events (floods). Recognition of different scales of bedform and associated stratification is essential to the accurate modeling of fluvial deposits. Within a channel belt , the deposits of compound braid bars, point bars, and major channel fills are represented by compound sets of large-scale inclined strata. These compound sets fine upward, fine upward then coarsen upward, or show little vertical variation in grain size, and commonly have open-framework gravel near their bases. Unit bars and minor channel fills (associated with cross-bar channels) are represented by simple sets of large-scale inclined strata. These simple sets generally fine upward, and open-framework gravel commonly occurs at the bases and downstream ends of these sets. Superimposed simple sets form compound sets. Dunes and bed-load sheets that migrate over bars and in channels are represented by sets of medium-scale trough cross strata and gravelly planar strata, respectively. Cross strata in a medium-scale set can alternate between open-framework and closed-framework gravel. Ripples and upper-stage plane beds are represented by sets of small-scale trough cross-stratified sand and planar-laminated sand, respectively. At the top of the channel belt, these sands contain drifted plant remains, roots, and burrows. The 3-D depositional model represents the geometry and spatial distribution of the different scales of strata that occur in all river deposits. Furthermore, the length : thickness ratios of different scales of strata are similar to the length : height ratios of the formative bed forms (e.g., bars, dunes) and scale with the channel geometry, suggesting that the model can be applied to different scales of river deposits. Distributions of porosity and permeability are related to sediment textures and can be included in the model by predicting the spatial distribution of sediment textures within different scales of strata. Of particular importance is the distribution of high-permeability open-framework gravel strata that may be continuous for tens to hundreds of meters. Permeabilities of open-framework gravels can be two or three orders of magnitude greater than permeabilities of surrounding sediments, and significantly influence fluid flow and contaminant transport within the aquifer. Stochastic predictions of the spatial distribution of different scales of strata and their associated porosities and permeabilities in an aquifer will benefit from site-specific data (e.g., geophysical profiles, borehole logs, wireline logs, and pumping tests) combined with this three-dimensional model of gravelly fluvial deposits.
Back Matter
Abstract The spatial variation of sedimentary aquifer properties (e.g., porosity and permeability) must be characterized in order to develop accurate models of groundwater flow and solute transport. The purpose of this volume was to bring together examples of the most recent research by sedimentologists, geophysicists, and hydrogeologists working on characterization of aquifer heterogeneity. The volume can be considered to be an outgrowth of SEPM Concepts in Hydrogeology and Environmental Geology Volume 1, entitled Hydrogeologic Models of Sedimentary Aquifers, which aimed to show how sedimentological information can be used in aquifer characterization and can thus help solve hydrogeologic problems. The papers in this volume demonstrate that integration of sedimentological and geophysical techniques for purposes of aquifer characterization are still in their infancy but that developments are promising.