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At least 1,009 records · Page 56Linked to original sources
USGS2010

Gravity-driven groundwater flow and slope failure potential: 2. Effects of slope morphology, material properties, and hydraulic heterogeneity

Hillslope morphology, material properties, and hydraulic heterogeneities influence the role of groundwater flow in provoking slope instability. We evaluate these influences quantitatively by employing the elastic effective stress model and Coulomb failure potential concept described in our companion paper (Iverson and Reid, this issue). Sensitivity analyses show that of four dimensionless quantities that control model results (i.e., Poisson's ratio, porosity, topographic profile, and hydraulic conductivity contrast), slope profiles and hydraulic conductivity contrasts have the most pronounced and diverse effects on groundwater seepage forces, effective stresses, and slope failure potentials. Gravity-driven groundwater flow strongly influences the shape of equilibrium hillslopes, which we define as those with uniform near-surface failure potentials. For homogeneous slopes with no groundwater flow, equilibrium hillslope profiles are straight; but with gravity-driven flow, equilibrium profiles are concave or convex-concave, and the largest failure potentials exist near the bases of convex slopes. In heterogeneous slopes, relatively slight hydraulic conductivity contrasts of less than 1 order of magnitude markedly affect the seepage force field and slope failure potential. Maximum effects occur if conductivity contrasts are of four orders of magnitude or more, and large hydraulic gradients commonly result in particularly large failure potentials just upslope from where low-conductivity layers intersect the ground surface.

Water Resources Research↗
USGS2010

Flow resistance under conditions of intense gravel transport

A study of flow resistance was undertaken in a channelized reach of the North Fork Toutle River, downstream of Mount St. Helens, Washington. Hydraulic and sediment transport data were collected in flows with velocities up to 3 m/s and shear stresses up to 7 times the critical value needed for bed load transport. Details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance. The plane gravel bed persists up to stresses ∼3 times critical, at which point, irregular bed forms appear. Bed forms greatly increase flow resistance and cause velocity profiles to become distorted. The latter arises as an effect of flows becoming depth-limited as bed form amplitude increases. At very high rates of bed load transport, an upper stage plane bed appeared. Velocity profiles measured in these flows match the law of the wall closely, with the equivalent roughness being well represented by k s = 3 D 84 of the bed load. The effects noted here will be important in very large floods or in rivers that are not free to widen, such as those cut into bedrock.

Washington↗
USGS2010

A solute flux approach to transport in heterogeneous formations: 2. Uncertainty analysis

Uncertainty in the mass flux for advection dominated solute movement in heterogeneous porous media is investigated using the Lagrangian framework developed in paper 1 by Dagan et al. (this issue). Expressions for the covariance of the mass flux and cumulative mass flux are derived as functions of the injection volume and sampling area size relative to the scale of heterogeneity. The result is illustrated for solute advection in three types of heterogeneous porous media: stratified formations, two‐ and three‐dimensional porous media; small perturbation approximation is used for the two‐ and three‐dimensional cases. Variances of the mass flux and cumulative mass flux are evaluated as functions of the injection volume (area) scale versus log‐hydraulic conductivity integral scale. The greatest decrease in coefficient of variation (CV) of the mass flux is for the source scale 1–5 times the hydraulic conductivity integral scale; further increase in the source size decreases CV comparatively less. The variance of the cumulative mass flux (or total discharge) indicates that for the source size of 20 hydraulic conductivity integral scales, the transport conditions are almost ergodic. The present results also indicate that the cumulative mass flux is a relatively robust quantity for describing field‐scale solute transport.

Water Resources Research↗
USGS2010

A solute flux approach to transport in heterogeneous formations: 1. The general framework

It is common to represent solute tranport in heterogeneous formations in terms of the resident concentration C (x, t), regarded as a random space function. The present study investigates the alternative representation by q , the solute mass flux at a point of a control plane normal to the mean flow. This representation is appropriate for many field applications in which the variable of interest is the mass of solute discharged through a control surface. A general framework to compute the statistical moments of q and of the associated total solute discharge Q and mass M is established. With x the direction of the mean flow, a solute particle is crossing the control plane at y = η, z = ζ and at the travel (arrival) time τ. The associated expected solute flux value is proportional to the joint probability density function (pdf) g 1 of η, ζ and τ, whereas the variance of q is shown to depend on the joint pdf g 2 of the same variables for two particles. In turn, the statistical moments of η, ζ and τ depend on those of the velocity components through a system of stochastic ordinary differential equations. For a steady velocity field and neglecting the effect of pore‐scale dispersion, a major simplification of the problem results in the independence of the random variables η, ζ and τ. As a consequence, the pdf of η and ζ can be derived independently of τ. A few approximate approaches to derive the statistical moments of η, ζ and τ are outlined. These methods will be explored in paper 2 in order to effectively derive the variances of the total solute discharge and mass, while paper 3 will deal with the nonlinear effect of the velocity variance upon the moments of η, ζ and τ

Water Resources Research↗
USGS2010

Legal mechanisms for protecting riparian resource values

Riparian resources include the borders of rivers, lakes, ponds, and potholes. These border areas are very important for a number of reasons, including stream channel maintenance, flood control, aesthetics, erosion control, fish and wildlife habitat, recreation, and water quality maintenance. These diverse functions are not well protected by law or policy. We reviewed law and policies regarding endangered species habitat designation, land use planning, grazing management, water allocation, takings, and federal permits and licenses, along with the roles of federal, state, and local governments. We discuss the politics of implementing these policies, focusing on the difficulties in changing entrenched water and land use practices. Our review indicates a lack of direct attention to riparian ecosystem issues in almost all environmental and land use programs at every level of government. Protection of riparian resource values requires a means to integrate existing programs to focus on riparian zones.

Water Resources Research↗
USGS2010

Solute transport with multisegment, equilibrium-controlled, classical reactions: Problem solvability and feed forward method's applicability for complex segments of at most binary participants

The feed forward (FF) method derives efficient operational equations for simulating transport of reacting solutes. It has been shown to be applicable in the presence of networks with any number of homogeneous and/or heterogeneous, classical reaction segments that consist of three, at most binary participants. Using a sequential (network type after network type) exploration approach and, independently, theoretical explanations, it is demonstrated for networks with classical reaction segments containing more than three, at most binary participants that if any one of such networks leads to a solvable transport problem then the FF method is applicable. Ways of helping to avoid networks that produce problem insolvability are developed and demonstrated. A previously suggested algebraic, matrix rank procedure has been adapted and augmented to serve as the main, easy-to-apply solvability test for already postulated networks. Four network conditions that often generate insolvability have been identified and studied. Their early detection during network formulation may help to avoid postulation of insolvable networks.

Water Resources Research↗
USGS2010

Large-scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 3. Hydraulic conductivity variability and calculated macrodispersivities

Hydraulic conductivity ( K ) variability in a sand and gravel aquifer on Cape Cod, Massachusetts, was measured and subsequently used in stochastic transport theories to estimate macrodispersivities. Nearly 1500 K measurements were obtained by borehole flowmeter tests and permeameter analyses of cores. The geometric mean for the flowmeter tests (0.11 cm/s) is similar to that estimated from other field tests. The mean for the permeameter tests (0.035 cm/s) is significantly lower, possibly because of compaction of the cores. The variance for the flowmeter (0.24) is also greater than that for the permeameter (0.14). Geostatistical analyses applying negative exponential models with and without nuggets reveal similar spatial correlation structures for the two data sets. Estimated correlation scales range from 2.9 to 8 m in the horizontal and from 0.18 to 0.38 m in the vertical. Estimates of asymptotic longitudinal dispersivity (b.35–0.78 m) are similar in magnitude to that observed in the natural gradient tracer test (0.96 m) previously conducted at this site.

Massachusetts↗
USGS2010

Solute transport with multiple equilibrium-controlled or kinetically controlled chemical reactions

A new approach is applied to the problem of modeling solute transport accompanied by many chemical reactions. The approach, based on concepts of the concentration space and its stoichiometric subspaces, uses elements of the subspaces as primary dependent variables. It is shown that the resulting model equations are compact in form, isolate the chemical reaction expressions from flow expressions, and can be used for either equilibrium or kinetically controlled reactions. The implications of the results on numerical algorithms for solving the equations are discussed. The application of the theory is illustrated throughout with examples involving a simple but broadly representative set of reactions previously considered in the literature. Numerical results are presented for four interconnected reactions: a homogeneous complexation reaction, two sorption reactions, and a dissolution/precipitation reaction. Three cases are considered: (1) four kinetically controlled reactions, (2) four equilibrium-controlled reactions, and (3) a system with two kinetically controlled reactions and two equilibrium-controlled reactions.

Water Resources Research↗
USGS2010

Relation of nickel concentrations in tree rings to groundwater contamination

Increment cores were collected from trees growing at two sites where groundwater is contaminated by nickel. Proton-induced X ray emission spectroscopy was used to determine the nickel concentrations in selected individual rings and in parts of individual rings. Ring nickel concentrations were interpreted on the basis of recent concentrations of nickel in aquifers, historical information about site use activities, and model simulations of groundwater flow. Nickel concentrations in rings increased during years of site use but not in trees outside the contaminated aquifers. Consequently, it was concluded that trees may preserve in their rings an annual record of nickel contamination in groundwater. Tulip trees and oaks contained higher concentrations of nickel than did sassafras, sweet gum, or black cherry. No evidence was found that nickel accumulates consistently within parts of individual rings or that nickel is translocated across ring boundaries.

Water Resources Research↗
USGS2010

Fat fractal scaling of drainage networks from a random spatial network model

An alternative quantification of the scaling properties of river channel networks is explored using a spatial network model. Whereas scaling descriptions of drainage networks previously have been presented using a fractal analysis primarily of the channel lengths, we illustrate the scaling of the surface area of the channels defining the network pattern with an exponent which is independent of the fractal dimension but not of the fractal nature of the network. The methodology presented is a fat fractal analysis in which the drainage basin minus the channel area is considered the fat fractal. Random channel networks within a fixed basin area are generated on grids of different scales. The sample channel networks generated by the model have a common outlet of fixed width and a rule of upstream channel narrowing specified by a diameter branching exponent using hydraulic and geomorphologic principles. Scaling exponents are computed for each sample network on a given grid size and are regressed against network magnitude. Results indicate that the size of the exponents are related to magnitude of the networks and generally decrease as network magnitude increases. Cases showing differences in scaling exponents with like magnitudes suggest a direction of future work regarding other topologic basin characteristics as potential explanatory variables.

Water Resources Research↗
USGS2010

Use of chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age‐dating tools: The alluvium and terrace system of central Oklahoma

The use of Chlorofluorocarbons (CFCs) as an age‐dating tool and tracer in shallow groundwaters has been investigated. New methodology for field sampling and preserving groundwaters containing parts per trillion concentrations of the CFCs, F‐1l and F‐12, is presented. Samples are analyzed by purge‐and‐trap gas chromatography with an electron capture detector. Physical and chemical processes that can alter natural concentrations (air‐water equilibrium) of CFCs were investigated to assess dating uncertainties. CFC model recharge ages appear to be defined within 2 years under optimum conditions. The method was applied to central Oklahoma to demonstrate the usefulness of CFCs as (1) an age‐dating tool of shallow groundwaters, (2) a tracer of sewage effluent in surface and shallow groundwaters, and (3) a tracer of shallow groundwater. Results of dating indicate two primary recharge periods in central Oklahoma over the past 45 years that correspond to the wet periods 1945–1960 and 1967–1975.

Water Resources Research↗
USGS2010

A geochemical transport model for redox-controlled movement of mineral fronts in groundwater flow systems: A case of nitrate removal by oxidation of pyrite

A one-dimensional prototype geochemical transport model was developed in order to handle simultaneous precipitation-dissolution and oxidation-reduction reactions governed by chemical equilibria. Total aqueous component concentrations are the primary dependent variables, and a sequential iterative approach is used for the calculation. The model was verified by analytical and numerical comparisons and is able to simulate sharp mineral fronts. At a site in Denmark, denitrification has been observed by oxidation of pyrite. Simulation of nitrate movement at this site showed a redox front movement rate of 0.58 m yr −1 , which agreed with calculations of others. It appears that the sequential iterative approach is the most practical for extension to multidimensional simulation and for handling large numbers of components and reactions. However, slow convergence may limit the size of redox systems that can be handled.

Water Resources Research↗
USGS2010

The role of groundwater transport in aquatic mercury cycling

Mercury, which is transported globally by atmospheric pathways to remote aquatic environments, is a ubiquitous contaminant at very low (nanograms Hg per liter) aqueous concentrations. Until recently, however, analytical and sampling techniques were not available for freshwater systems to quantify the actual levels of mercury concentrations without introducing significant contamination artifacts. Four different sampling strategies were used to evaluate ground water flow as a mercury source and transport mechanism within aquatic systems. The sampling strategies employ ultraclean techniques to determine mercury concentrations in groundwater and pore water near Pallette Lake, Wisconsin. Ambient groundwater concentrations are about 2–4 ng Hg L −1 , whereas pore waters near the sediment/water interface average about 12 ng Hg L −1 , emphasizing the importance of biogeochemical processes near the interface. Overall, the groundwater system removes about twice as much mercury (1.5 g yr −1 ) as it contributes (0.7 g yr −1 ) to Pallette Lake. About three fourths of the groundwater mercury load is recycled, thought to be derived from the water column.

Wisconsin↗
USGS2010

Exact Scheffé-type confidence intervals for output from groundwater flow models: 2. Combined use of hydrogeologic information and calibration data

Calibration data (observed values corresponding to model-computed values of dependent variables) are incorporated into a general method of computing exact Scheffé-type confidence intervals analogous to the confidence intervals developed in part 1 (Cooley, this issue) for a function of parameters derived from a groundwater flow model. Parameter uncertainty is specified by a distribution of parameters conditioned on the calibration data. This distribution was obtained as a posterior distribution by applying Bayes' theorem to the hydrogeologically derived prior distribution of parameters from part 1 and a distribution of differences between the calibration data and corresponding model-computed dependent variables. Tests show that the new confidence intervals can be much smaller than the intervals of part 1 because the prior parameter variance-covariance structure is altered so that combinations of parameters that give poor model fit to the data are unlikely. The confidence intervals of part 1 and the new confidence intervals can be effectively employed in a sequential method of model construction whereby new information is used to reduce confidence interval widths at each stage.

Water Resources Research↗
USGS2010

The effect of streambed topography on surface-subsurface water exchange in mountain catchments

A numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography. At St. Kevin Gulch, an effluent stream in the Rocky Mountains of Colorado, subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well-defined flow paths through the alluvium. Stream water-filled flow paths in the alluvium (referred to as substream flow paths) returned to the stream a short distance downstream (1 to 10 m). Recharge to the substream flow paths occurred where stream water slope increased, at the transition from pools (<1%) to steeper channel units (5–20%). Return of substream flow paths to the stream occurred where stream water slope decreased, at the transition from steeper channel units to pools. A net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments. Along our study reach at St. Kevin Gulch the net inflow of water from subsurface to stream (1.6 mL s −1 m −1 ) underestimated the gross inflow (2.7 mL s −1 m −1 ) by 40%. The influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water-sediment contact times; these effects could have important consequences for solute transport, retention, and transformation in catchments.

Water Resources Research↗
USGS2010

Spring climate and salinity in the San Francisco Bay Estuary

Salinity in the San Francisco Bay Estuary almost always experiences its yearly maximum during late summer, but climate variability produces marked interannual variations. The atmospheric circulation pattern impacts the estuary primarily through variations of runoff from rainfall and snowmelt from the Sierra Nevada and, secondarily, through variations in the near-surface salinity in the coastal ocean. While winter precipitation is the primary influence upon salinity in the estuary, spring climate variations also contribute importantly to salinity fluctuations. Spring atmospheric circulation influences both the magnitude and the timing of freshwater flows, through anomalies of precipitation and temperature. To help discriminate between the effects of these two influences, the record is divided into subsets according to whether spring conditions in the region are cool and wet, warm and wet, cool and dry, or warm and dry. Warm springs promote early snowmelt-driven flows, and cool springs result in delayed flows. In addition to effects of winter and spring climate variability operating on the watershed, there are more subtle effects that are transmitted into the estuary from the coastal ocean. These influences are most pronounced in cool and dry springs with high surface salinity (SS) in the coastal ocean versus cool and wet springs with low SS in the coastal ocean. A transect of SS records at stations from the mouth to the head of the bay suggests that the coastal ocean anomaly signal is attenuated from the mouth to the interior of the estuary. In contrast, a delayed, postsummer signal caused by winter and spring runoff variations from the upstream watershed are most pronounced at the head of the estuary and attenuate toward the mouth.

California↗
USGS2010

A methodology for quantifying and valuing the impacts of flow changes on a fishery

A quasi-population model for adult brown trout was developed for the Taylor River below the Taylor Park Reservoir in Colorado. This model allows the population to be predicted under alternative flow management regimes. The predicted population effects of two different flow release patterns were compared with the predicted population for the current reservoir operation regime. Changes in angler catch were imputed for these scenarios. The changes in catch were valued using estimates of willingness to pay obtained from anglers fishing at the site. Total angling effort was held constant. For both of the flow scenarios examined the difference in economic use value was limited. The relatively small changes in value predicted were shaped by the small changes in catch predicted and the high number of fish caught under current conditions.

Water Resources Research↗
USGS2010

Transport of volatile organic compounds across the capillary fringe

Physical experiments were conducted to investigate the transport of a dissolved volatile organic compound (trichloroethylene, TCE) from shallow groundwater to the unsaturated zone under a variety of conditions including changes in the soil moisture profile and water table position. Experimental data indicated that at moderate groundwater velocities (0.1 m/d), vertical mechanical dispersion was negligible and molecular diffusion was the dominant vertical transport mechanism. Under these conditions, TCE concentrations decreased nearly 3 orders of magnitude across the capillary fringe and soil gas concentrations remained low relative to those of underlying groundwater. Data collected during a water table drop showed a short-term increase in concentrations throughout most of the unsaturated zone, but these concentrations quickly declined and approached initial values after the water table was returned to its original level. In the deep part of the unsaturated zone, the water table drop resulted in a long-term decrease in concentrations, illustrating the effects of hysteresis in the soil moisture profile. A two-dimensional random walk advection-diffusion model was developed to simulate the experimental conditions, and numerical simulations agreed well with experimental data. A simpler, one-dimensional finite-difference diffusion-dispersion model was also developed. One-dimensional simulations based on molecular diffusion also agreed well with experimental data. Simulations which incorporated mechanical dispersion tended to overestimate flux across the capillary fringe. Good agreement between the one- and two-dimensional models suggested that a simple, one-dimensional approximation of vertical transport across the capillary fringe can be useful when conditions are appropriate.

Water Resources Research↗