A natural gradient experiment on solute transport in a sand aquifer: 1. Approach and overview of plume movement
TL;DR: A large-scale field experiment on natural gradient transport of solutes in groundwater has been conducted at a site in Borden, Ontario as discussed by the authors, where over 19,900 samples have been collected over a 3-year period.
Abstract: A large-scale field experiment on natural gradient transport of solutes in groundwater has been conducted at a site in Borden, Ontario. Well-defined initial conditions were achieved by the pulse injection of 12 m3 of a uniform solution containing known masses of two inorganic tracers (chloride and bromide) and five halogenated organic chemicals (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2-dichlorobenzene, and hexachloroethane). A dense, three-dimensional array of over 5000 sampling points was installed throughout the zone traversed by the solutes. Over 19,900 samples have been collected over a 3-year period. The tracers followed a linear horizontal trajectory at an approximately constant velocity, both of which compare well with expectations based on water table contours and estimates of hydraulic head gradient, porosity, and hydraulic conductivity. The vertical displacement over the duration of the experiment was small. Spreading was much more pronounced in the horizontal longitudinal than in the horizontal transverse direction; vertical spreading was very small. The organic solutes were retarded in mobility, as expected.
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TL;DR: The Borden aquifer was examined in great detail by conducting permeability measurements on a series of cores taken along two cross sections, one along and the other transverse to the mean flow direction as discussed by the authors.
Abstract: The spatial variability of hydraulic conductivity at the site of a long-term tracer test performed in the Borden aquifer was examined in great detail by conducting permeability measurements on a series of cores taken along two cross sections, one along and the other transverse to the mean flow direction. Along the two cross sections, a regular-spaced grid of hydraulic conductivity data with 0.05 m vertical and 1.0 m horizontal spatial discretization revealed that the aquifer is comprised of numerous thin, discontinuous lenses of contrasting hydraulic conductivity. Estimation of the three-dimensional covariance structure of the aquifer from the log-transformed data indicates that an exponential covariance model with a variance equal to 0.29, an isotropic horizontal correlation length equal to about 2.8 m, and a vertical correlation length equal to 0.12 m is representative. A value for the longitudinal macrodispersivity calculated from these statistical parameters using three-dimensional stochastic transport theory developed by L. W. Gelhar and C. L. Axness (1983) is about 0.6 m. For the vertically averaged case, the two-dimensional theory developed by G. Dagan (1982, 1984) yields a longitudinal djspersivity equal to 0.45 m. Use of the estimated statistical parameters describing the ln (K) variability in Dagan's transient equations closely predicted the observed longitudinal and horizontal transverse spread of the tracer with time. Weak vertical and horizontal dispersion that is controlled essentially by local-scale dispersion was obtained from the analysis. Because the dispersion predicted independently from the statistical description of the Borden aquifer is consistent with the spread of the injected tracer, it is felt that the theory holds promise for providing meaningful estimates of effective transport parameters in other complex-structured aquifers.
1,307 citations
TL;DR: A review of the current state of knowledge about the behavior of colloids in porous media and their role in contaminant transport can be found in this paper, where the authors identify some important future research needs.
Abstract: Publisher Summary It is noted that colloidal particles can be effectively transported through subsurface porous media under certain hydrogeochemical conditions. If present in large concentrations, mobile colloids can act as carriers for strongly sorbing contaminants and thereby provide an unretarded transport pathway for contaminants that are otherwise strongly retarded. This potential transport pathway can be considered in risk assessments of sites heavily contaminated with toxic chemicals, such as certain radionuclides, heavy metals, and hydrophobic organic compounds. The chapter reviews the current state of knowledge about the behavior of colloids in porous media and their role in contaminant transport. The chapter also identifies some important future research needs. Colloids are commonly defined as small particles or other entities with dimensions roughly between 1 nm and 1 μm. These size limits to dissolved molecules on one side and to larger suspended particles on the other side. Colloidal particles remain stable in suspension over long time periods, unless they coagulate to form larger aggregates or deposit onto surfaces of larger grains.
661 citations
TL;DR: New knowledge developed under the NCP on the sources, occurrence and pathways of contaminants (organochlorines, Hg, Pb and Cd, PAHs, artificial radionuclides) are highlighted and compelling evidence for close connectivity between the global emission of contaminants from industrial and agricultural activities and the Arctic is provided.
656 citations
TL;DR: In this paper, the three-dimensional movement of a tracer plume containing bromide and chloride is investigated using the data base from a large-scale natural gradient field experiment on groundwater solute transport.
Abstract: The three-dimensional movement of a tracer plume containing bromide and chloride is investigated using the data base from a large-scale natural gradient field experiment on groundwater solute transport. The analysis focuses on the zeroth-, first-, and second-order spatial moments of the concentration distribution. These moments define integrated measures of the dissolved mass, mean solute velocity, and dispersion of the plume. Moments are estimated from the point observations using quadrature approximations tailored to the density of the sampling network. The estimators appear to be robust, with acceptable sampling variability. Estimates of the mass in solution for both bromide and chloride demonstrate that the tracers behaved conservatively, as expected. Analysis of the first-order moment estimates indicates that the experimental tracer plumes traveled along identical trajectories. The horizontal trajectory is linear and aligned with the hydraulic gradient. The vertical trajectory is curvilinear, concave upward. The total vertical displacement is small, however, so that the vertical component of the mean solute velocity vector is negligible. The estimated mean solute velocity is identical for both tracers (0.091 m/day) and is spatially and temporally uniform for the first 647 days of travel time. After 647 days of transport, the plume apparently encountered a relatively large-scale heterogeneity in the velocity field, leading to a distinct vertical layering, and slowing the rate of advance of the center of mass of the plume as a whole. The estimated horizontal components of the covariance tensor evolve over time in a manner consistent with the qualitative shape changes observed from plots of the concentration data. The major principal axis, initially aligned roughly perpendicular to the hydraulic gradient, rotates smoothly over time until it is nearly aligned with the mean solute velocity vector, as the plume itself elongates and orients its long axis with the direction of movement. Plots of the components of the covariance tensor as functions of time show evidence of what is commonly called “scale-dependent” dispersion: the rate of growth of the covariance over time is not linear. The theoretical results of G. Dagan (1984) calibrate well to the estimated covariance data for the first 647 days of transport. The calibrated values of the parameters of the hydrualic conductivity distribution closely match independently measured values from the site. The asymptotic longitudinal dispersivity obtained from the calibration is 0.49 m, although asymptotic conditions were apparently not reached. The estimated covariance terms for the last sampling session, 1038 days after injection, are inconsistent with the earlier data and with the Dagan model, particularly for the transverse and off-diagonal components. This behavior is probably attributable to the observed large-scale heterogeneity in the velocity field.
641 citations
TL;DR: In this article, the authors explored the use of multiple isotopic tracers to evaluate the processes involved in nitrate attenuation in ground water and found that reduced sulfur and carbon are electron donors for nitrate reduction.
Abstract: This study explores the use of multiple isotopic tracers to evaluate the processes involved in nitrate attenuation in ground water. δ15N and δ18O are used to provide information about the role of denitrification on nitrate attenuation, and δ34S, δ18O, and δ13C are used to evaluate the role of reduced sulfur and carbon as electron donors for nitrate reduction. The focus of this study is a zone of significant NO3−1 attenuation occurring in a sand aquifer impacted by septic system contamination. The NO3−1 pattern, the ground water flow system, and changes in other chemical parameters suggest that the NO3−1 depletion is caused by denitrification. This is supported by the nitrate δ15N and δ18O data which both show significant isotopic enrichment as NO3−1 depletion proceeds along the flow path. The increase of sulfate and dissolved inorganic carbon observed in the zone of nitrate attenuation suggests that reduced sulfur in addition to carbon is also involved in denitrification. This is supported by a trend toward depleted sulfate δ34S and δ18O values in the zone of sulfate increase, which reflects the input of sulfate formed by the oxidation of biogenic pyrite present in the aquifer sediments. The trend toward depleted δ13 values in the zone of increasing dissolved inorganic carbon reflects the input of organic carbon into this carbon pool. Chemical mass balance indicates that carbon is the dominant electron donor; however, this study demonstrates the effectiveness of using multiple isotopic tracers for providing insight into the processes affecting nitrate attenuation in ground water.
543 citations
References
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01 Jan 1972
TL;DR: In this paper, the Milieux poreux Reference Record was created on 2004-09-07, modified on 2016-08-08 and the reference record was updated in 2016.
Abstract: Keywords: Ecoulement souterrain ; Milieux poreux Reference Record created on 2004-09-07, modified on 2016-08-08
12,446 citations
TL;DR: In this article, the authors investigated the sorption of hydrophobic compounds (aromatic hydrocarbons and chlorinated polycyclic aromatic compounds) spanning a concentration range in water solubility from 500 parts per trillion (ppt) to 1800 parts per million (ppm) on local pond and river sediments.
2,593 citations
TL;DR: The Borden aquifer was examined in great detail by conducting permeability measurements on a series of cores taken along two cross sections, one along and the other transverse to the mean flow direction as discussed by the authors.
Abstract: The spatial variability of hydraulic conductivity at the site of a long-term tracer test performed in the Borden aquifer was examined in great detail by conducting permeability measurements on a series of cores taken along two cross sections, one along and the other transverse to the mean flow direction. Along the two cross sections, a regular-spaced grid of hydraulic conductivity data with 0.05 m vertical and 1.0 m horizontal spatial discretization revealed that the aquifer is comprised of numerous thin, discontinuous lenses of contrasting hydraulic conductivity. Estimation of the three-dimensional covariance structure of the aquifer from the log-transformed data indicates that an exponential covariance model with a variance equal to 0.29, an isotropic horizontal correlation length equal to about 2.8 m, and a vertical correlation length equal to 0.12 m is representative. A value for the longitudinal macrodispersivity calculated from these statistical parameters using three-dimensional stochastic transport theory developed by L. W. Gelhar and C. L. Axness (1983) is about 0.6 m. For the vertically averaged case, the two-dimensional theory developed by G. Dagan (1982, 1984) yields a longitudinal djspersivity equal to 0.45 m. Use of the estimated statistical parameters describing the ln (K) variability in Dagan's transient equations closely predicted the observed longitudinal and horizontal transverse spread of the tracer with time. Weak vertical and horizontal dispersion that is controlled essentially by local-scale dispersion was obtained from the analysis. Because the dispersion predicted independently from the statistical description of the Borden aquifer is consistent with the spread of the injected tracer, it is felt that the theory holds promise for providing meaningful estimates of effective transport parameters in other complex-structured aquifers.
1,307 citations
TL;DR: In this article, a method for estimating the equilibrium sorption behavior of hydrophobic pollutants was developed for estimating KOC from water solubility (including crystal energy) and octanol/water partition coefficients.
1,221 citations
876 citations