Joseph N. Ryan

Colloid mobilization and transport in groundwater

Abstract Recent field and laboratory experiments have identified colloid-facilitated transport of contaminants as an important mechanism of contaminant migration through groundwater. For colloid-facilitated transport to be important, three criteria must be met: (1) colloids must be generated; (2) contaminants must associate with the colloids; and (3) colloids must be transported through the groundwater. Significant progress in the understanding of colloid generation (by mobilization of existing colloids) and transport (limited by deposition) in model colloid and collector systems has been made in the past few decades. This knowledge of the model systems, however, is inadequate for prediction of colloid behavior in natural groundwater systems. An understanding of colloid behavior in natural systems is essential for predicting the potential for colloid-facilitated transport in a given groundwater. This review presents theories describing colloid mobilization, deposition, and transport, laboratory experiments in model systems designed to test these theories, and applications of these theories to colloid mobilization and transport experiments in natural groundwater systems. Emphasis is placed on mobilization of existing colloids by chemical and physical perturbations, the kinetics and dynamics of colloid deposition (filtration) and the “blocking” effect, and the effect of surface chemical heterogeneities on colloid deposition and transport.

Influence of Dissolved Organic Matter on the Environmental Fate of Metals, Nanoparticles, and Colloids

Influence of Dissolved Organic Matter on the Environmental Fate of Metals, Nanoparticles, and Colloids George R. Aiken,* Heileen Hsu-Kim, and Joseph N. Ryan U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303, United States Department of Civil and Environmental Engineering, Duke University, Box 90287, Durham, North Carolina 27708, United States Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, Colorado 80309, United States

Colloid Movement in Unsaturated Porous Media: Recent Advances and Future Directions

Investigations of colloid movement through geologic materials are driven by a variety of issues, including contaminant transport, soil-profile development, and subsurface migration of pathogenic microorganisms. In this review, we address recent advances in understanding of colloid transport through partially saturated porous media. Special emphasis is placed on features of the vadose zone (i.e., the presence of air–water interfaces, rapid fluctuations in porewater flow rates and chemistry) that distinguish colloid transport in unsaturated media from colloid transport in saturated media. We examine experimental studies on colloid deposition and mobilization and survey recent developments in modeling colloid transport and mass transfer. We conclude with an overview of directions for future research in this field.

The reversibility of virus attachment to mineral surfaces

Virus transport through groundwater is limited by attachment to mineral surfaces and inactivation. Current virus transport models do not consider the implications of the reversibility of virus attachment to minerals. To explore the reversibility of virus attachment to mineral surfaces, we attached PRD1, a bacteriophage considered to be a good model of enteric viruses, to quartz and ferric oxyhydroxide-coated quartz surfaces over a range of pH values in equilibrium “static columns.” Following attachment, we detached the viruses by replacing the pore solution with solutions of equal and higher pH. The extent of virus attachment followed an attachment “edge” that occurred at a pH value about 2.5–3.5 pH units above the pHiep of the mineral surfaces. Viruses attached below this edge were irreversibly attached until the pH of the detachment solution exceeded the pH value of the attachment edge. Viruses attached above this edge were reversibly attached. Derjaguin-Landau-Verwey-Overbeek (DLVO) potential energy calculations showed that the attachment edge occurred at the pH at which the potential energy of the primary minimum was near zero, implying that the position of the primary minimum (attractive or repulsive) controlled the equilibrium distribution of the viruses. The results suggest that the reversibility of virus attachment must be considered in virus transport models for accurate predictions of virus travel time.

Effect of solution chemistry on clay colloid release from an iron oxide-coated aquifer sand

This research compared the influence of (1) dissolution of iron oxides and (2) alteration of electrostatic interactions on the mobilization of colloids in a clay- and iron oxide-coated sand obtained from an Atlantic Coastal Plain aquifer in which colloids have been found suspended only in anoxic groundwater. The sediment was flushed with solutions of varying ionic strength, pH, and reductant and surfactant concentrations, and the steady-state rates of clay colloid release and iron oxyhydroxide dissolution were measured. The clay release rates were directly related to the calculated detachment energies and unrelated to rates of iron(III) oxide dissolution, indicating that electrostatic interactions dominated the binding of colloids to grain coatings. Mobilization of colloids by iron(III) oxide dissolution through reductive dissolution was limited by high ionic strength. Flushing of the sediment by a natural groundwater with high dissolved organic carbon concentration released clay without rapidly dissolving iron oxides. 59 refs., 8 figs., 3 tabs.

Membranes for the control of natural organic matter from surface waters

A range of nanofiltration (NF) modules was evaluated to determine rejection of disinfection by-product (DBP) precursors from low turbidity surface waters. Dissolved organic carbon (DOC), trihalomethane formation potential (THMFP), haloacetic acid formation potential (HAAFP), and chloral hydrate formation potential (CHFP) rejections averaged 90, 97, 94, and 86%, respectively. Rejections of bromide ion, an inorganic precursor, ranged from 40-80%. Pretreatment using microfiltration (MF) alone before NF provided some removal of turbidity but not enough to maintain the initial flux and recovery of the NF unit. NF runs were sustained over 30 days; however, some adverse changes in operational conditions were observed, and significant pressure increases were necessary to maintain flux. Precursor rejections by NF following MF varied little over time frames of up to 30 days. MF was only moderately eAective in particle removals, with virtually no DBP precursor removal provided by MF. Ultrafiltration (UF) alone did not exhibit significant changes in operational conditions over a 30-day time frame; however, only modest precursor (<30% DOC) removal was observed. 7 2000 Elsevier Science Ltd. All rights reserved

Determination of hydrologic pathways during snowmelt for alpine/subalpine basins, Rocky Mountain National Park, Colorado

Alpine/subalpine ecosystems in Rocky Mountain National Park may be sensitive to atmospherically derived acidic deposition. Two- and three-component hydrograph separation analyses and correlation analyses were performed for six basins to provide insight into streamflow generation during snowmelt and to assess basin sensitivity to acidic deposition. Three-component hydrograph separation results for five basins showed that streamflow contained from 42 to 57% direct snowmelt runoff, 37 to 54% subsurface water, and 4 to 13% direct rain runoff for the May through October 1994 study period. Subsurface contributions were 89% of total flow for the sixth basin. The reliability of hydrograph separation model assumptions was explored. Subsurface flow was positively correlated with the amount of surficial material in a basin and was negatively correlated with basin slope. Basins with extensive surficial material and shallow slopes are less susceptible to ecosystem changes due to acidic deposition than basins with less surficial material and steeper slopes. This study was initiated to expand the intensive hydrologic research that has been conducted in Loch Vale basin to a more regional scale.

Conservative and reactive solute transport in constructed wetlands

[1] The transport of bromide, a conservative tracer, and rhodamine WT (RWT), a photodegrading tracer, was evaluated in three wastewater-dependent wetlands near Phoenix, Arizona, using a solute transport model with transient storage. Coupled sodium bromide and RWT tracer tests were performed to establish conservative transport and reactive parameters in constructed wetlands with water losses ranging from (1) relatively impermeable (15%), (2) moderately leaky (45%), and (3) significantly leaky (76%). RWT first-order photolysis rates and sorption coefficients were determined from independent field and laboratory experiments. Individual wetland hydraulic profiles influenced the extent of transient storage interaction in stagnant water areas and consequently RWT removal. Solute mixing and transient storage interaction occurred in the impermeable wetland, resulting in 21% RWT mass loss from main channel and storage zone photolysis (10%) and sorption (11%) reactions. Advection and dispersion governed solute transport in the leaky wetland, limiting RWT photolysis removal (1.2%) and favoring main channel sorption (3.6%). The moderately leaky wetland contained islands parallel to flow, producing channel flow and minimizing RWT losses (1.6%). INDEX TERMS: 1890 Hydrology: Wetlands; 1871 Hydrology: Surface water quality; 3230 Mathematical Geophysics: Numerical solutions; KEYWORDS: constructed wetlands, OTIS, transient storage

Role of organic acidity in sorption of natural organic matter (NOM) to oxide surfaces

Abstract Very strong organic acids (acid groups ionized at pH 3.0) represent a significant portion of the total acidity (4–25%) in samples of organic matter isolated from two lakes, a landfill leachate, and a Krafft mill effluent. On the basis of model compound behavior and established complexation constants, these very strongly acidic groups are predicted to be the key to the formation of strong surface complexes between organic molecules and oxide surfaces. This prediction was confirmed for the sorption of aquatic natural organic matter (NOM) on to preformed iron oxide surfaces. A significant fraction of the carboxylic acidity in polymeric analogs to NOM is due to weak acids (which ionize above pH 8.0). If a portion of the weak acidity in aquatic NOM currently attributed to phenols were actually due to carboxylic acids, significant discrepancies in the understanding of NOM behavior could be reconciled.

Effects of Iron on Optical Properties of Dissolved Organic Matter

Iron is a source of interference in the spectroscopic analysis of dissolved organic matter (DOM); however, its effects on commonly employed ultraviolet and visible (UV-vis) light adsorption and fluorescence measurements are poorly defined. Here, we describe the effects of iron(II) and iron(III) on the UV-vis absorption and fluorescence of solutions containing two DOM fractions and two surface water samples. In each case, regardless of DOM composition, UV-vis absorption increased linearly with increasing iron(III). Correction factors were derived using iron(III) absorption coefficients determined at wavelengths commonly used to characterize DOM. Iron(III) addition increased specific UV absorbances (SUVA) and decreased the absorption ratios (E2:E3) and spectral slope ratios (SR) of DOM samples. Both iron(II) and iron(III) quenched DOM fluorescence at pH 6.7. The degree and region of fluorescence quenching varied with the iron:DOC concentration ratio, DOM composition, and pH. Regions of the fluorescence spectra associated with greater DOM conjugation were more susceptible to iron quenching, and DOM fluorescence indices were sensitive to the presence of both forms of iron. Analyses of the excitation-emission matrices using a 7- and 13-component parallel factor analysis (PARAFAC) model showed low PARAFAC sensitivity to iron addition.