http://netedu.xauat.edu.cn/jpkc/netedu/jpkc2009/szylyybh/content/wlzy/1/Review%20of%20Arsenic%20in%20natural%20water%20-%20Smedly.pdf

A review of the source, behaviour and distribution of arsenic in natural waters

Solid phase microextraction with thermal desorption using fused silica optical fibers

The science of the total environment

The use of equilibrium expressions for sorption to natural particles in fate and transport models is often invalid due to slow kinetics. This paper reviews recent research into the causes of slow sorption and desorption rates at the intraparticle level and how this phenomenon relates to contaminant transport, bioavailability, and remediation. Sorption kinetics are complex and poorly predictable at present. Diffusion limitations appear to play a major role. Contending mechanisms include diffusion through natural organic matter matrices and diffusion through intraparticle nanopores. These mechanisms probably operate simultaneously, but the relative importance of each in a given system is indeterminate. Sorption shows anomalous behaviors that are presently not well explained by the simple diffusion models, including concentration dependence of the slow fraction, distributed rate constants, and kinetic hysteresis. Research is needed to determine whether adsorption/desorption bond energies may play a role alon...

Mechanisms of Slow Sorption of Organic Chemicals to Natural Particles

found that field-scale dispersivities are several orders of.magnitude greater than lab-scale values for the same material; it is generally agreed that this difference is a reflection of the influence of natural heterogeneities which produce irregular flow patterns at the field scale. Consequently, laboratory measurements of dispersivity cannot be used to predict field values of dispersivity. Instead field-scale tracer tests are sometimes conducted to estimate dispersivity at a particular site. Early efforts to document the scale dependence of dispersivity [Lallemand-Barres and Peaudecerf, 1978; Anderson, 1979; Pickens and Grisak, 1981; Beims, 1983; Neretnieks, 1985] were based on field values of dispersivity reported in the literature and the test scales associated with those values. These studies were useful in that they indeed documented field evidence of the scale effect, but they were lacking in that they did not assess the reliability of the data presented. Because we felt that the data would be more

A Critical Review of Data on Field-Scale Dispersion in Aquifers

A tracer solution containing Cl and Ca was passed through a large intact cylindrical sample of fractured clayey glacial till obtained from a depth of 5 m in a fresh excavation. The sample was intersected by two orthogonal sets of fractures oriented parallel to the long axis of the column. Breakthrough curves for Cl and Ca show that these species were significantly retarded in relation to the average fracture flow water velocity as they passed through the column. The retardation is attributed to the effect of diffusion of the solutes from the fractures into the porous matrix of the till. The relative positions of the Ca and Cl breakthrough curves are attributed to the combined influence of a smaller effective diffusion coefficient for Ca than for Cl and adsorption of Ca in the matrix during the diffusion process. Relatively good agreement with the experimental data was achieved in mathematical simulation using the finite element solute transport model described by Grisak and Pickens (1980). Effective diffusion coefficients for Cl and Ca in the till matrix were found to be 5.0 × 10−7 and 1.9 × 10−7 cm2/s, respectively. The lack of total continuity between fracture sets was accounted for in the model by using an effective fracture spacing and effective aperture width based on the concepts of flow in planar fractures in sets of simple geometry.

Solute transport through fractured media: 2. Column study of fractured till

Attenuation of nitrate in aquitard sediments of southern Ontario

We describe and evaluate two previously undocumented methods of bulk permeability (K) determination and compare results obtained with results of conventional smaller-scale tests on the same thick clayey till deposit. Analysis of the downward propagation of seasonal water table fluctuations yielded bulk K of approximately 10−10 m s−1, in close agreement with results of laboratory consolidation and permeameter tests, slug tests, and distribution of vertical hydraulic gradient with depth in the deposit. This agreement suggests that for such materials, high-gradient tests conducted at small scales of distance and time can provide reasonable estimates of bulk K. The results also imply groundwater residence times in the till of thousands of years. The flow pattern observed near a large excavation in the till was consistent with the initial recoveries of piezometers installed in much smaller boreholes, assuming perturbation of hydraulic head in the formation due to borehole excavation. Time scales of these perturbations, which prevent interpretation of measured hydraulic head using conventional well hydraulic methods, varied from months to tens of years.

A multiscale study of the permeability of a thick clayey till

At three industrial sites in Ontario, New Hampshire, and Florida, tetrachloroethylene (PCE) and trichloroethylene (TCE), released decades ago as dense nonaqueous phase liquids (DNAPLs), now form persistent source zones for dissolved contaminant plumes. These zones are suspended below the water table and above the bottoms of their respective, moderately homogeneous, unconfined sandy aquifers. Exceptionally detailed, depth-discrete, ground water sampling was performed using a direct-push sampler along cross sections of the dissolved-phase plumes, immediately downgradient of these DNAPL source zones. The total plume PCE or TCE mass-discharge through each cross section ranged between 15 and 31 kg/year. Vertical ground water sample spacing as small as 15 cm and lateral spacing typically between 1 and 3 m revealed small zones where maximum concentrations were between 1% and 61% of solubility. These local maxima are surrounded by much lower concentration zones. A spacing no larger than 15 to 30 cm was needed at some locations to identify high concentration zones, and aqueous VOC concentrations varied as much as four orders of magnitude across 30 cm vertical intervals. High-resolution sampling at these sites showed that three-quarters of the mass-discharge occurs within 5% to 10% of the plume cross sectional areas. The extreme spatial variability of the mass-discharge occurs even though the sand aquifers are nearly hydraulically homogeneous. Depth-discrete field techniques such as those used in this study are essential for finding the small zones producing most of the mass-discharge, which is important for assessing natural attenuation and designing remedial options.

Mass and Flux Distributions from DNAPL Zones in Sandy Aquifers

High-nitrogen loadings of rivers and aquifers systems are a major concern because of potential effects on human health and water quality impacts such as eutrophication of lakes and coastal zones. T...

/pdf/abnormally-high-ammonium-of-natural-origin-in-a-coastal-ykxpbcx469.pdf

John A. Cherry

Papers

Solute transport through fractured media: 2. Column study of fractured till

Attenuation of nitrate in aquitard sediments of southern Ontario

A multiscale study of the permeability of a thick clayey till

Mass and Flux Distributions from DNAPL Zones in Sandy Aquifers

Abnormally High Ammonium of Natural Origin in a Coastal Aquifer-Aquitard System in the Pearl River Delta, China