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At least 613 records · Page 34Linked to original sources
USGS2017

Molecular characterization of a novel orthomyxovirus from rainbow and steelhead trout ( Oncorhynchus mykiss )

A novel virus, rainbow trout orthomyxovirus (RbtOV), was isolated in 1997 and again in 2000 from commercially-reared rainbow trout ( Oncorhynchus mykiss ) in Idaho, USA. The virus grew optimally in the CHSE-214 cell line at 15°C producing a diffuse cytopathic effect; however, juvenile rainbow trout exposed to cell culture-grown virus showed no mortality or gross pathology. Electron microscopy of preparations from infected cell cultures revealed the presence of typical orthomyxovirus particles. The complete genome of RbtOV is comprised of eight linear segments of single-stranded, negative-sense RNA having highly conserved 5′ and 3′-terminal nucleotide sequences. Another virus isolated in 2014 from steelhead trout (also O. mykiss ) in Wisconsin, USA, and designated SttOV was found to have eight genome segments with high amino acid sequence identities (89–99%) to the corresponding genes of RbtOV, suggesting these new viruses are isolates of the same virus species and may be more widespread than currently realized. The new isolates had the same genome segment order and the closest pairwise amino acid sequence identities of 16–42% with Infectious salmon anemia virus (ISAV), the type species and currently only member of the genus Isavirus in the family Orthomyxoviridae . However, pairwise comparisons of the predicted amino acid sequences of the 10 RbtOV and SttOV proteins with orthologs from representatives of the established orthomyxoviral genera and a phylogenetic analysis using the PB1 protein showed that while RbtOV and SttOV clustered most closely with ISAV, they diverged sufficiently to merit consideration as representatives of a novel genus. A set of PCR primers was designed using conserved regions of the PB1 gene to produce amplicons that may be sequenced for identification of similar fish orthomyxoviruses in the future.

Virus Research↗
USGS1978

Determination of dissolved boron in fresh, estuarine, and geothermal waters by d.c. argon-plasma emission spectrometry

A d.c. argon-plasma emission spectrometer is used to determine dissolved boron in natural (fresh and estuarine) water samples. Concentrations ranged from 0.02 to 250 mg l -1 . The emission—concentration function is linear from 0.02 to 1000 mg l -1 . Achievement of a relative standard deviation of ⩽ 3% requires frequent restandardization to offset sensitivity changes. Dilution may be necessary to overcome high and variable electron density caused by differences in alkali-metal content and to avoid quenching of the plasma by high solute concentrations of sodium and other easily ionized elements. The proposed method was tested against a reference method and found to be more sensitive, equally or more precise and accurate, less subject to interferences, with a wider linear analytical range than the carmine method. Analyses of standard reference samples yielded results in all cases within one standard deviation of the means.

Analytica Chimica Acta↗
USGS2001

Soil amplification maps for estimating earthquake ground motions in the Central US

The State Geologists of the Central United States Earthquake Consortium (CUSEC) are developing maps to assist State and local emergency managers and community officials in evaluating the earthquake hazards for the CUSEC region. The state geological surveys have worked together to produce a series of maps that show seismic shaking potential for eleven 1 X 2 degree (scale 1:250 000 or 1 in. ??? 3.9 miles) quadrangles that cover the high-risk area of the New Madrid Seismic Zone in eight states. Shear wave velocity values for the surficial materials were gathered and used to classify the soils according to their potential to amplify earthquake ground motions. Geologic base maps of surficial materials or 3-D material maps, either existing or produced for this project, were used in conjunction with shear wave velocities to classify the soils for the upper 15-30 m. These maps are available in an electronic form suitable for inclusion in the federal emergency management agency's earthquake loss estimation program (HAZUS). ?? 2001 Elsevier Science B.V. All rights reserved.

Engineering Geology↗
USGS1999

Sulfur-bearing coatings on fly ash from a coal-fired power plant: Composition, origin, and influence on ash alteration

Fly ash samples collected from two locations in the exhaust stream of a coal-fired power plant differ markedly with respect to the abundance of thin (≈0.1 μm) sulfur-rich surface coatings that are observable by scanning electron microscopy. The coatings, tentatively identified as an aluminum-potassium-sulfate phase, probably form upon reaction between condensed sulfuric acid aerosols and glass surfaces, and are preferentially concentrated on ash exposed to exhaust stream gases for longer. The coatings are highly soluble and if sufficiently abundant, can impart an acidic pH to solutions initially in contact with ash. These observations suggest that proposals for ash use and predictions of ash behavior during disposal should consider the transient, acid-generating potential of some ash fractions and the possible effects on initial ash leachability and alteration.

Fuel↗
USGS2003

Changes in homologous and heterologous gap junction contacts during maturation-inducing hormone-dependent meiotic resumption in ovarian follicles of Atlantic croaker

Homologous (granulosa cell-granulosa cell) gap junction (GJ) contacts increase in ovarian follicles of Atlantic croaker (Micropogonias undulatus) during the early (first) stage of maturation, but their profile during the second stage [i.e., during maturation-inducing hormone (MIH)-mediated meiotic resumption] is unknown. The profile of homologous GJ contacts during the second stage of maturation in croaker follicles was examined in this study and compared to that of heterologous (granulosa cell-oocyte) GJ, for which changes have been previously documented. Follicles were incubated with human chorionic gonadotropin to induce maturational competence (first stage), and then with MIH to induce meiotic resumption. The follicles were collected for examination immediately before and after different durations of MIH exposure until the oocyte had reached the stage of germinal vesicle breakdown (GVBD; index of meiotic resumption). Ultrathin sections were observed by transmission electron microscopy, and homologous and heterologous GJ contacts were quantified along a 100-??m segment of granulosa cell-zona radiata complex per follicle (three follicles/time/fish, n=3 fish). Relatively high numbers of both types of GJ were observed before and after the first few hours of MIH exposure (up to the stage of oil droplet coalescence). GJ numbers declined during partial yolk globule coalescence (at or near GVBD) and were just under 50% of starting values after the completion of GVBD (P<0.05). These results confirm earlier observations that GVBD temporally correlates with declining heterologous GJ contacts, and for the first time in teleosts show that there is a parallel decline in homologous GJ. The significance of the changes in homologous and heterologous GJ is uncertain and deserves further study. ?? 2003 Elsevier Science (USA). All rights reserved.

General and Comparative Endocrinology↗
USGS1981

Speciation of aryloxyethoxyethyl benzyl dimethyl ammonium salts by glass capillary gas chromatography and high-performance liquid chromatography

Using glass capillary gas chromatography (GC) and ion-pair reversed-phase high-performance liquid chromatography (HPLC), a pair of structurally similar microbicides, benzyl diisobutylphenoxyethoxyethyl dimethyl ammonium chloride and its cresoxy-analogue, were effectively resolved and accurately quantified with a high degree of selectivity and specificity. Optimization of conditions in the chromatographic systems to attain desirable baseline separations of the components of interest is described. The minimum detectable quantities as determined are in the neighborhood of 10 ng for the quaternary ammonium salts by HPLC-UV detection, 0.1 ng for the cyanamide derivatives of the salts by GC-alkaline flame-ionization detection and 1–2 pg for the trichloroethyl carbamate derivatives of the salts by GC-electron-capture detection. In conjunction with cation-exchange chromatography for sample enrichment and purification, the glass capillary GC and ion-pair reversed-phase HPLC methods presented here are particularly suitable to separate and simultaneously analyze for the aryloxyethoxyethyl benzyl dimethyl ammonium compounds concerned in environmental samples. It appears that these chromatographic techniques possess potential applicability to the separation and quantification of other classes of quaternary ammonium compounds with closely related structures that are not separable by other methods.

Journal of Chromatography↗
USGS1985

Retention behavior of long chain quaternary ammonium homologues and related nitroso-alkymethylamines

Several chromatographic methods have been utilized to study the retentionbehavior of a homologous series of n -alkylbenzyldimethylammonium chlorides (ABDAC) and the corresponding nitroso- n -alkylmethylamines (NAMA). Linear correlation of the logarithmic capacity factor ( k ') with the number of carbons in the alkyl chain provides useful information on both gas chromatographic (GC) and high-performance liquid chromatographich (HPLC) retention parameters of unknown components. Under all conditions empolyed, GC methodology has proved effective in achieving complete resolution of the homologous mixture of NMA despite its obvious inadequacy in the separation of E-Z configurational isomers. Conversely, normal-phase HPLC on silica demonstrates that the selectivity (a) value for an E-Z pair is much higher than that for an adjacent homologous pair. In the reversed-phase HPLC study, three different silica-based column systems were examined under various mobile phase conditions. The extent of variation in k ' was found to be a function of the organic modifier, counter-ion concentration, eluent pH, nature of counter-ion, and the polarity and type of stationary phase. The k '&mdash;[NaClO 4 ] profiles showed similar trends between the ABDAC and the NAMA series, supporting the dipolar electronic structures of the latter compounds. Mobile phase and stationary phase effects on component separation are described. The methodology presented establishes the utility of HPLC separation techniques as versatile analytical tools for practical application.

Journal of Chromatography↗
USGS2000

Environmental history and tephrostratigraphy at Carp Lake, southwestern Columbia Basin, Washington, USA

Sediment cores from Carp Lake provide a pollen record of the last ca. 125,000 years that helps disclose vegetational and climatic conditions from the present day to the previous interglaciation (120-133 ka). The core also contained 15 tephra layers, which were characterised by electron-microprobe analysis of volcanic glass shards. Identified tephra include Mount St. Helens Ye, 3.69 ka; Mazama ash bed, 7.54 ka; Mount St. Helens layer C, 35-50 ka; an unnamed Mount St. Helens tephra, 75-150 ka; the tephra equivalent of layer E at Pringle Falls, Oregon, <218 ka; and an andesitic tephra layer similar to that at Tulelake, California, 174 ka. Ten calibrated radiocarbon ages and the ages of Mount St. Helens Ye, Mazama ash, and the unnamed Mount St. Helens tephra were used to develop an age-depth model. This model was refined by also incorporating the age of marine oxygen isotope stage (IS) boundary 4/5 (73.9 ka) and the age of IS-5e (125 ka). The justification for this age-model is based on an analysis of the pollen record and lithologic data. The pollen record is divided into 11 assemblage zones that describe alternations between periods of montane conifer forest, pine forest, and steppe. The previous interglacial period (IS-5e) supported temperate xerothermic forests of pine and oak and a northward and westward expansion of steppe and juniper woodland, compared to their present occurrence. The period from 83 to 117 ka contains intervals of pine forest and parkland alternating with pine-spruce forest, suggesting shifts from cold humid to cool temperate conditions. Between 73 and 83 ka, a forest of oak, hemlock, Douglas-fir, and fir was present that has no modem analogue. It suggests warm wet summers and cool wet winters. Cool humid conditions during the mid-Wisconsin interval supported mixed conifer forest with Douglas-fir and spruce. The glacial interval featured cold dry steppe, with an expansion of spruce in the late-glacial. Xerothermic communities prevailed in the early Holocene, when temperate steppe was widespread and the lake dried intermittently. The middle Holocene was characterised by ponderosa pine forest, and the modem vegetation was established in the last 3900 yr, when ponderosa pine, Douglas-fir, fir, and oak were part of the local vegetation.

Conference Paper↗
USGS1969

Equipment and techniques for low-altitude aerial sensing of water-vapor concentration and movement

Progress in the development of equipment and techniques for making rapid measurements of moisture movement through the atmosphere over a large area is described. Airborne sensing elements measure relative humidity, temperature, and air currents. These data are telemetered to a ground-based station and recorded. A radar unit tracks the aircraft and electronically plots its position on a base map of the area being studied. Thus the distribution of atmospheric conditions can be directly related to the underlying terrain and vegetation features.

Remote Sensing of Environment↗
USGS1999

Relationships between the structure of natural organic matter and its reactivity towards molecular ozone and hydroxyl radicals

Oxidation reaction rate parameters for molecular ozone (O3) and hydroxyl (HO) radicals with a variety of hydrophobic organic acids (HOAs) isolated from different geographic locations were determined from batch ozonation studies. Rate parameter values, obtained under equivalent dissolved organic carbon concentrations in both the presence and absence of non-NOM HO radical scavengers, varied as a function of NOM structure. First-order rate constants for O3 consumption (k(O3)) averaged 8.8 x 10-3 s-1, ranging from 3.9 x 10-3 s-1 for a groundwater HOA to > 16 x 10-3 s-1 for river HOAs with large terrestrial carbon inputs. The average second-order rate constant (k(HO,DOC) between HO radicals and NOM was 3.6 x 108 l (mol C)-1 s-1; a mass of 12 g C per mole C was used in all calculations. Specific ultraviolet absorbance (SUVA) at 254 or 280 nm of the HOAs correlated well (r > 0.9) with O3 consumption rate parameters, implying that organic ??-electrons strongly and selectively influence oxidative reactivity. HO radical reactions with NOM were less selective, although correlation between k(HO,DOC) and SUVA existed. Other physical-chemical properties of NOM, such as aromatic and aliphatic carbon content from 13C-NMR spectroscopy, proved less sensitive for predicting oxidation reactivity than SUVA. The implication of this study is that the structural nature of NOM varies temporally and spatially in a water source, and both the nature and amount of NOM will influence oxidation rates.

Water Research↗
USGS2000

Characterization and speciation of mercury-bearing mine wastes using X-ray absorption spectroscopy

Mining of mercury deposits located in the California Coast Range has resulted in the release of mercury to the local environment and water supplies. The solubility, transport, and potential bioavailability of mercury are controlled by its chemical speciation, which can be directly determined for samples with total mercury concentrations greater than 100 mg kg-1 (ppm) using X-ray absorption spectroscopy (XAS). This technique has the additional benefits of being non-destructive to the sample, element-specific, relatively sensitive at low concentrations, and requiring minimal sample preparation. In this study, Hg L(III)-edge extended X-ray absorption fine structure (EXAFS) spectra were collected for several mercury mine tailings (calcines) in the California Coast Range. Total mercury concentrations of samples analyzed ranged from 230 to 1060 ppm. Speciation data (mercury phases present and relative abundances) were obtained by comparing the spectra from heterogeneous, roasted (calcined) mine tailings samples with a spectral database of mercury minerals and sorbed mercury complexes. Speciation analyses were also conducted on known mixtures of pure mercury minerals in order to assess the quantitative accuracy of the technique. While some calcine samples were found to consist exclusively of mercuric sulfide, others contain additional, more soluble mercury phases, indicating a greater potential for the release of mercury into solution. Also, a correlation was observed between samples from hot-spring mercury deposits, in which chloride levels are elevated, and the presence of mercury-chloride species as detected by the speciation analysis. The speciation results demonstrate the ability of XAS to identify multiple mercury phases in a heterogeneous sample, with a quantitative accuracy of ??25% for the mercury-containing phases considered. Use of this technique, in conjunction with standard microanalytical techniques such as X-ray diffraction and electron probe microanalysis, is beneficial in the prioritization and remediation of mercury-contaminated mine sites. (C) 2000 Elsevier Science B.V.

Science of the Total Environment↗
USGS2002

Challenges in observational seismology

Earthquake seismology became a quantitative scientific discipline after instruments were developed to record seismic waves in the late 19th century ( Dewey and Byerly, 1969 ; Chapter 1 by Agnew). Earthquake seismology is essentially based on field observations. The great progress made in the past several decades was primarily due to increasingly plentiful and high-quality data that are readily distributed. Our ability to collect, process, and analyze earthquake data has been accelerated by advances in electronics, communications, computers, and software (see Chapter 85 edited by Snoke and Garcia-Fernandez). Instrumental observation of earthquakes has been carried out for a little over 100 years by seismic stations and networks of various sizes, from local to global scales (see Chapter 87 edited by Lahr and van Eck). The observed data have been used, for example, (1) to compute the source parameters of earthquakes, (2) to determine the physical properties of the Earth's interior, (3) to test the theory of plate tectonics , (4) to map active faults, (5) to infer the nature of damaging ground shaking, and (6) to carry out seismic hazard analysis. Construction of a satisfactory theory of the earthquake process has not yet been achieved within the context of physical laws. Good progress, however, has been made in building a physical foundation of the earthquake source process, partly as a result of research directed toward earthquake prediction. This chapter is intended for a general audience. Technical details are not given, but relevant references and chapters in this Handbook are referred to. The first part of this chapter presents a brief overview of the observational aspects of earthquake seismology, concentrating on instrumental observations of seismic waves generated by earthquakes (i.e., seismic monitoring), and readers are referred to Chapter 49 by Musson and Cecic for noninstrumental observations. A few key developments and practices are summarized by taking a general view, since many national and regional developments have been chronicled in national and institutional reports (see Chapter 79 edited by Kisslinger). In the latter part of this chapter, the nature of seismic monitoring and some challenges in observational seismology are discussed from a personal perspective. Comments of a technical or philosophical nature are given in the Notes at the end of the chapter, and they are referenced by superscript numbers in the text.

International Geophysics↗
USGS1997

An interactive program for computer-aided map design, display, and query: EMAPKGS2

EMAPKGS2 is a user-friendly, PC-based electronic mapping tool for use in hydrogeologic exploration and appraisal. EMAPKGS2 allows the analyst to construct maps interactively from data stored in a relational database, perform point-oriented spatial queries such as locating all wells within a specified radius, perform geographic overlays, and export the data to other programs for further analysis. EMAPKGS2 runs under Microsoft?? Windows??? 3.1 and compatible operating systems. EMAPKGS2 is a public domain program available from the Kansas Geological Survey. EMAPKGS2 is the centerpiece of WHEAT, the Windows-based Hydrogeologic Exploration and Appraisal Toolkit, a suite of user-friendly Microsoft?? Windows??? programs for natural resource exploration and management. The principal goals in development of WHEAT have been ease of use, hardware independence, low cost, and end-user extensibility. WHEAT'S native data format is a Microsoft?? Access?? database. WHEAT stores a feature's geographic coordinates as attributes so they can be accessed easily by the user. The WHEAT programs are designed to be used in conjunction with other Microsoft?? Windows??? software to allow the natural resource scientist to perform work easily and effectively. WHEAT and EMAPKGS have been used at several of Kansas' Groundwater Management Districts and the Kansas Geological Survey on groundwater management operations, groundwater modeling projects, and geologic exploration projects. ?? 1997 Elsevier Science Ltd.

Computers & Geosciences↗
USGS2000

Identification of a small, naked virus in tumor-like aggregates in cell lines derived from a green turtle, Chelonia mydas, with fibropapillomas

Serial cultivation of cell lines derived from lung, testis, periorbital and tumor tissues of a green turtle (Chelonia mydas) with fibropapillomas resulted in the in vitro formation of tumor-like cell aggregates, ranging in size from 0.5 to 2.0 mm in diameter. Successful induction of tumor-like aggregates was achieved in a cell line derived from lung tissue of healthy green turtles, following inoculation with cell-free media from these tumor-bearing cell lines, suggesting the presence of a transmissible agent. Thin-section electron microscopy of the cell aggregates revealed massive collagen deposits and intranuclear naked viral particles, measuring 5095 nm in diameter. These findings, together with the morphological similarity between these tumor-like cell aggregates and the naturally occurring tumor, suggest a possible association between this novel virus and the disease. Further characterization of this small naked virus will clarify its role in etiology of green turtle fibropapilloma, a life-threatening disease of this endangered marine species.

Journal of Virological Methods↗
USGS2002

Trace element abundances in major minerals of Late Permian coals from southwestern Guizhou province, China

Fourteen samples of minerals were separated by handpicking from Late Permian coals in southwestern Guizhou province, China. These 14 minerals were nodular pyrite, massive recrystallized pyrite, pyrite deposited from low-temperature hydrothermal fluid and from ground water; clay minerals; and calcite deposited from low-temperature hydrothermal fluid and from ground water. The mineralogy, elemental composition, and distribution of 33 elements in these samples were studied by optical microscopy, scanning electron microscope equipped with energy-dispersive X-ray spectrometer (SEM-EDX), X-ray diffraction (XRD), cold-vapor atomic absorption spectrometry (CV-AAS), atomic fluorescence spectrometry (AFS), inductively coupled-plasma mass spectrometry (ICP-MS), and ion-selective electrode (ISE). The results show that various minerals in coal contain variable amounts of trace elements. Clay minerals have high concentrations of Ba, Be, Cs, F, Ga, Nb, Rb, Th, U, and Zr. Quartz has little contribution to the concentration of trace elements in bulk coal. Arsenic, Mn, and Sr are in high concentrations in calcite. Pyrite has high concentrations of As, Cd, Hg, Mo, Sb, Se, Tl, and Zn. Different genetic types of calcite in coal can accumulate different trace elements; for example Ba, Co, Cr, Hg, Ni, Rb, Sn, Sr, and Zn are in higher concentrations in calcite deposited from low-temperature hydrothermal fluid than in that deposited from ground water. Furthermore, the concentrations of some trace elements are quite variable in pyrite; different genetic types of pyrites (Py-A, B, C, D) have different concentrations of trace elements, and the concentrations of trace elements are also different in pyrite of low-temperature hydrothermal origin collected from different locations. The study shows that elemental concentration is rather uniform in a pyrite vein. There are many micron and submicron mosaic pyrites in a pyrite vein, which is enriched in some trace elements, such as As and Mo. The content of trace element in pyrite vein depends upon the content of mosaic pyrite and of trace elements in it. Many environmentally sensitive trace elements are mainly contained in the minerals in coal, and hence the physical coal cleaning techniques can remove minerals from coal and decrease the emissions of potentially hazardous trace elements. ?? 2002 Elsevier Science B.V. All rights reserved.

International Journal of Coal Geology↗
USGS2003

Applicability of tetrazolium salts for the measurement of respiratory activity and viability of groundwater bacteria

A study was undertaken to measure aerobic respiration by indigenous bacteria in a sand and gravel aquifer on western Cape Cod, MA using tetrazolium salts and by direct oxygen consumption using gas chromatography (GC). In groundwater and aquifer slurries, the rate of aerobic respiration calculated from the direct GC assay was more than 600 times greater than that using the tetrazolium salt 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl tetrazolium chloride (INT). To explain this discrepancy, the toxicity of INT and two additional tetrazolium salts, sodium 3′-[1-(phenylamino)-carbonyl]-3,4-tetrazolium]-bis(4-methoxy-6-nitro) benzenesulfonic acid hydrate (XTT) and 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), to bacterial isolates from the aquifer was investigated. Each of the three tetrazolium salts was observed to be toxic to some of the groundwater isolates at concentrations normally used in electron transport system (ETS) and viability assays. For example, incubation of cells with XTT (3 mM) caused the density of four of the five groundwater strains tested to decline by more than four orders of magnitude. A reasonable percentage (>57%) of cells killed by CTC and INT contained visible formazan crystals (the insoluble, reduced form of the salts) after 4 h of incubation. Thus, many of the cells reduced enough CTC or INT prior to dying to be considered viable by microscopic evaluation. However, one bacterium ( Pseudomonas fluorescens ) that remained viable and culturable in the presence of INT and CTC, did not incorporate formazan crystals into more than a few percent of cells, even after 24 h of incubation. This strain would be considered nonviable based on traditional tetrazolium salt reduction assays. The data show that tetrazolium salt assays are likely to dramatically underestimate total ETS activity in groundwater and, although they may provide a reasonable overall estimate of viable cell numbers in a community of groundwater bacteria, some specific strains may be falsely considered nonviable by this assay due to poor uptake or reduction of the salts.

Massachusetts↗
USGS1997

Quantitative analysis of major elements in silicate minerals and glasses by micro-PIXE

The Guelph micro-PIXE facility has been modified to accommodate a second Si(Li) X-ray detector which records the spectrum due to light major elements (11 ??? Z ??? 20) with no deleterious effects from scattered 3 MeV protons. Spectra have been recorded from 30 well-characterized materials, including a broad range of silicate minerals and both natural and synthetic glasses. Sodium is mobile in some of the glasses, but not in the studied mineral lattices. The mean value of the instrumental constant H for each of the elements Mg, Al, and Si in these materials is systematically 6-8% lower than the H-value measured for the pure metals. Normalization factors are derived which permit the matrix corrections requisite for trace-element measurements in silicates to be based upon pure metal standards for Mg, Al and Si, supplemented by well-established, silicate mineral standards for the elements Na, K and Ca. Rigorous comparisons of electron microprobe and micro-PIXE analyses for the entire, 30-sample suite demonstrate the ability of micro-PIXE to produce accurate analysis for the light major elements in silicates. ?? 1997 Elsevier Science B.V.

Nuclear Instruments and Methods in Physics Researc↗
USGS2001

Progression of natural attenuation processes at a crude-oil spill site . I. Geochemical evolution of the plume

A 16-year study of a hydrocarbon plume shows that the extent of contaminant migration and compound-specific behavior have changed as redox reactions, most notably iron reduction, have progressed over time. Concentration changes at a small scale, determined from analysis of pore-water samples drained from aquifer cores, are compared with concentration changes at the plume scale, determined from analysis of water samples from an observation well network. The small-scale data show clearly that the hydrocarbon plume is growing slowly as sediment iron oxides are depleted. Contaminants, such as ortho -xylene that appeared not to be moving downgradient from the oil on the basis of observation well data, are migrating in thin layers as the aquifer evolves to methanogenic conditions. However, the plume-scale observation well data show that the downgradient extent of the Fe 2+ and BTEX plume did not change between 1992 and 1995. Instead, depletion of the unstable Fe (III) oxides near the subsurface crude-oil source has caused the maximum dissolved iron concentration zone within the plume to spread at a rate of approximately 3 m/year. The zone of maximum concentrations of benzene, toluene, ethylbenzene and xylene (BTEX) has also spread within the anoxic plume. In monitoring the remediation of hydrocarbon-contaminated ground water by natural attenuation, subtle concentration changes in observation well data from the anoxic zone may be diagnostic of depletion of the intrinsic electron-accepting capacity of the aquifer. Recognition of these subtle patterns may allow early prediction of growth of the hydrocarbon plume.

Journal of Contaminant Hydrology↗