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Geology topics

Robert P. Eganhouse

Publications and source records attributed to Robert P. Eganhouse.

17 recordsLinked to original sources
USGS2018

Reductive dechlorination rates of 4,4′-DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene) in sediments of the Palos Verdes Shelf, CA

Wastes from the world's largest manufacturer of DDT (1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene) were released into the Los Angeles County municipal sewer system from 1947 to 1971. Following primary treatment, the effluent was discharged through a submarine outfall system whereupon a portion of the DDT and associated degradation products were deposited in sediments of the Palos Verdes Shelf (PVS). Parent DDT is present only in trace amounts in the sediments today, the vast majority having been transformed to DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene) shortly following deposition. Previously believed to be inert, DDE is slowly being converted to DDMU (1-chloro-4-[2-chloro-1-(4-chlorophenyl)ethenyl]benzene) and DDMU to DDNU (1-chloro-4-[1-(4-chlorophenyl)ethenyl]benzene) via microbially-mediated reductive dechlorination (RDC). Kinetic and compositional data suggest that this process began sometime in the mid- to late 1970s. Rates of DDE RDC in shelf sediments are spatially variable and have proven difficult to determine accurately. This limits our ability to understand the factors controlling RDC rates and to predict the course of natural recovery. In the present study, concentrations of ten DDT compounds and twelve PCB (polychlorinated biphenyl) congeners were determined in cores collected at two locations on the PVS (stations 3C, 6C, ~7km and ~2km downcurrent from the outfalls, respectively). DDE inventories, normalized to those of non-degrading PCB congeners having similar physico-chemical properties, were modeled to yield first-order RDC rates for the period 1981–2010. Average rates at stations 3C and 6C were 0.044±0.004 and 0.008±0.002yr −1 , respectively, with depth-dependent RDC rates at station 3C (1992–2003) ranging from 0.0025 to 0.102yr −1 . Comparison of RDC and total loss (i.e., RDC+physical loss) rates suggests that the average per cent loss of DDE due to RDC is ~90% at station 3C (1981–2010) and ~57% at station 6C (1992–2010). Trajectories of adjusted molar inventories of DDE, DDMU, and DDNU were forecast using a first-order multi-step reaction series (M-SRS) model. The results for DDE are consistent with the normalization procedure; RDC rates at stations 3C and 6C were 0.036±0.002yr −1 and 0.010±0.001yr −1 , respectively. At station 6C, the DDE to DDMU transformation appears to be the rate limiting step in the reaction sequence, DDE k 1 → DDMU k 2 → DDNU k3 → unidentified compound(s), whereas at station 3C RDC rates for DDE and DDMU are roughly equivalent. At both locations the transformation rate of DDNU is 7–20 times that of the other steps. Estimated half-lives of DDE at stations 3C and 6C based on the M-SRS model results are ~19 and 72 years, respectively.

California↗
USGS2018

DDT and related compounds in pore water of shallow sediments on the Palos Verdes Shelf, California, USA

For nearly two and a half decades following World War II, production wastes from the world's largest manufacturer of technical DDT (1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene) were discharged into sewers of Los Angeles County. Following treatment, the wastes were released via a submarine outfall system to nearshore coastal waters where a portion accumulated in shallow sediments of the Palos Verdes Shelf (PVS). An investigation of the pore-water geochemistry of DDT-related compounds (DDX) was undertaken in an effort to understand factors controlling the rate of reductive dechlorination (RDC) of the major DDT degradate, 4,4′-DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene). Equilibrium matrix-solid phase microextraction (matrix-SPMEeq) combined with automated thermal desorption-gas chromatography/mass spectrometry (TDGC/MS) was used to determine freely dissolved concentrations of ten DDX analytes in sediment cores collected from three locations on the PVS (stations 3C, 6C, 8C, which are 7 km, 2 km, and 0 km, respectively, downcurrent from the outfall system). Pore-water concentrations (pM) of the principal DDX compounds involved in RDC were: 3C-DDE: 6.0–24, DDMU (1-chloro-4-[2-chloro-1-(4-chlorophenyl)ethenyl]benzene): 11–160, DDNU (1-chloro-4-[1-(4-chlorophenyl)ethenyl]benzene): 1.8–68; 6C-DDE: 5.6–170, DDMU: 5.6–177, DDNU: 1.7–87; 8CDDE: 27–212, DDMU: 31–403, DDNU: 5.5–89. Variations in the spatial distribution of DDX analytes in pore water reflect several factors including proximity to the outfalls, RDC reaction rates, and natural variability in sedimentation and post-depositional transport processes. A comparison of pore-water data produced using matrix-SPME eq /TD-GC/MS and whole-core squeezing/solvent extraction/liquid injection-GC/MS indicates that the majority of the DDE in the upper sediment column (≤about 10 cm) is associated with dissolved/colloidal organic matter. Below that depth, freely-dissolved DDE predominates. The principal organic geochemical phase controlling sorption of DDE in PVS sediments are residual hydrocarbons, the vast majority of which originated from petroleum refinery wastes. Organic carbon-normalized sediment-water distribution coefficients (KOC) were calculated from solid-phase and pore-water concentrations of DDX and organic carbon. Log K OC values (L/kg) were relatively invariant across the shelf and with depth in the sediment column. Shelf-wide compound-specific coefficients (log K OC ) were: DDE: 7.5 ± 0.11, DDMU: 6.92 ± 0.13, DDNU: 6.37 ± 0.19. The spatial uniformity of K OC means that biological exposure and availability of the DDX compounds can, in principle, be estimated from solid-phase chemical measurements.

California↗
USGS2017

Weathering of oil in a surficial aquifer

The composition of crude oil in a surficial aquifer was determined in two locations at the Bemidji, MN, spill site. The abundances of 71 individual hydrocarbons varied within 16 locations sampled. Little depletion of these hydrocarbons (relative to the pipeline oil) occurred in the first 10 years after the spill, whereas losses of 25% to 85% of the total measured hydrocarbons occurred after 30 years. The C 6‐30 n ‐alkanes, toluene, and o ‐xylene were the most depleted hydrocarbons. Some hydrocarbons, such as the n‐ C 10–24 cyclohexanes, tri‐ and tetra‐ methylbenzenes, acyclic isoprenoids, and naphthalenes were the least depleted. Benzene was detected at every sampling location 30 years after the spill. Degradation of the oil led to increases in the percent organic carbon and in the δ 13 C of the oil. Another method of determining hydrocarbon loss was by normalizing the total measured hydrocarbon concentrations to that of the most conservative analytes. This method indicated that the total measured hydrocarbons were depleted by 47% to 77% and loss of the oil mass over 30 years was 18% to 31%. Differences in hydrocarbon depletion were related to the depth of the oil in the aquifer, local topography, amount of recharge reaching the oil, availability of electron acceptors, and the presence of less permeable soils above the oil. The results from this study indicate that once crude oil has been in the subsurface for a number of years there is no longer a “starting oil concentration” that can be used to understand processes that affect its fate and the transport of hydrocarbons in groundwater.

Minnesota↗
USGS2017

Final data report for factors controlling DDE dechlorination rates on the Palos Verdes Shelf: A field and laboratory investigation

This data report provides a compilation of information developed over the last 6+ years by a multi-disciplinary, multi-institutional research team. The overall goal of this work has been to identify the biological, chemical, and physical factors that control rates of reductive dechlorination of DDE and DDMU in sediments of the Palos Verdes Shelf (PVS). More specific questions and objectives are delineated in the Scope of Work (section 12.1., Appendix 1). The study was composed of two parts: 1) field characterization studies, and 2) laboratory microcosm experiments. The goal of the field characterization studies was to define the conditions under which reductive dechlorination of DDE (and DDMU) is occurring in PVS sediments. This involved two separate cruises (2009, 2010) during which sediment cores, bottom water and other real-time field measurements (e.g., conductivity, temperature, depth of the water column) were acquired. The sediment cores were distributed among research team members for detailed chemical (R. Eganhouse, B. Orem, M. Reinhard), microbiological (A. Spormann), and physical (B. Edwards) analysis as well as for laboratory microcosm experiments (M. Reinhard). A team of collaborating USGS scientists generously contributed valuable information pertaining to geochronology (P. Swarzenski), the character of sedimentary geosorbent phases (P. Hackley), mineralogy (D. Webster), and grain-size characteristics (C. Sherwood) of PVS sediment samples. Together, this information will serve as framework for a conceptual model of natural degradation processes in the DDT-contaminated sediments on the PVS. These findings will enable the USEPA to gain a better understanding of the controls on reductive dechlorination and how dechlorination rates vary spatially and temporally. This, in turn, should facilitate decision making concerning the progress of natural attenuation and when monitoring at the site can be terminated. Toward that end, a brief Synthesis Report, summarizing and interpreting the acquired data, is being prepared and will be released in the coming year.

Palos Verdes Shelf↗
USGS2017

Final synthesis report for factors controlling DDE dechlorination rates on the palos verdes shelf: A field and laboratory investigation

This project was organized into separate field and laboratory studies aimed at answering “18 questions” in the original Scope of Work (cf., section 2 of this report, Background, for explanation). Because of some early results, certain questions became irrelevant and were, therefore, not pursued. In other cases, there simply was not enough time to complete the originally planned studies. On the other hand, additional work, not identified in any of the original “18 questions”, was carried out for purposes of addressing specific issues of concern to the USEPA (United States Environmental Protection Agency). Examples of the latter include: 1) the analysis of an expanded list of sediment cores for DDX (DDX refers to the ten DDT-related compounds of interest in this study; cf., Eganhouse et al., [1]) and selected PCB congeners to facilitate estimation of site-specific reductive dechlorination (RDC) and total loss rates, 2) analysis of gravity and box cores for trace elements to allow stratigraphic alignment, and 3) determination of the extent of mineralization of p,p’-DDE (1-chloro-2-[2,2-dichloro-1-(4- chlorophenyl)ethyl]benzene) in microcosm experiments. In this Executive Summary, we offer a brief recapitulation of what was learned about the factors controlling reductive dechlorination of p,p’-DDE in Palos Verdes Shelf (PVS) sediments using the “18 questions” as a structural guide. The summary is written in narrative form, but references to specific sections (corresponding to the “18 questions”) are identified parenthetically in the text so that the reader can explore the expanded answers that appear later in the report.

Palos Verdes Shelf↗
USGS2016

Determination of polydimethylsiloxane–water partition coefficients for ten 1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene-related compounds and twelve polychlorinated biphenyls using gas chromatography/mass spectrometry

Polymer-water partition coefficients (Kpw) of ten DDT-related compounds were determined in pure water at 25 °C using commercial polydimethylsiloxane-coated optical fiber. Analyte concentrations were measured by thermal desorption-gas chromatography/full scan mass spectrometry (TD–GC/MSFS; fibers) and liquid injection-gas chromatography/selected ion monitoring mass spectrometry (LI–GC/MSSIM; water). Equilibrium was approached from two directions (fiber uptake and depletion) as a means of assessing data concordance. Measured compound-specific log Kpw values ranged from 4.8 to 6.1 with an average difference in log Kpw between the two approaches of 0.05 log units (∼12% of Kpw). Comparison of the experimentally-determined log Kpw values with previously published data confirmed the consistency of the results and the reliability of the method. A second experiment was conducted with the same ten DDT-related compounds and twelve selected PCB (polychlorinated biphenyl) congeners under conditions characteristic of a coastal marine field site (viz., seawater, 11 °C) that is currently under investigation for DDT and PCB contamination. Equilibration at lower temperature and higher ionic strength resulted in an increase in log Kpw for the DDT-related compounds of 0.28–0.49 log units (61–101% of Kpw), depending on the analyte. The increase in Kpw would have the effect of reducing by approximately half the calculated freely dissolved pore-water concentrations (Cfree). This demonstrates the importance of determining partition coefficients under conditions as they exist in the field.

Journal of Chromatography A↗
USGS2015

Determination of 1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene and related compounds in marine pore water by automated thermal desorption-gas chromatography/mass spectrometry using disposable optical fiber

A method is described for determination of ten DDT-related compounds in marine pore water based on equilibrium solid-phase microextraction (SPME) using commercial polydimethylsiloxane-coated optical fiber with analysis by automated thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS). Thermally cleaned fiber was directly exposed to sediments and allowed to reach equilibrium under static conditions at the in situ field temperature. Following removal, fibers were rinsed, dried and cut into appropriate lengths for storage in leak-tight containers at -20°C. Analysis by TD-GC/MS under full scan (FS) and selected ion monitoring (SIM) modes was then performed. Pore-water method detection limits in FS and SIM modes were estimated at 0.05-2.4ng/L and 0.7-16pg/L, respectively. Precision of the method, including contributions from fiber handling, was less than 10%. Analysis of independently prepared solutions containing eight DDT compounds yielded concentrations that were within 6.9±5.5% and 0.1±14% of the actual concentrations in FS and SIM modes, respectively. The use of optical fiber with automated analysis allows for studies at high temporal and/or spatial resolution as well as for monitoring programs over large spatial and/or long temporal scales with adequate sample replication. This greatly enhances the flexibility of the technique and improves the ability to meet quality control objectives at significantly lower cost.

Journal of Chromatography A↗
USGS2012

Determination of nonylphenol isomers in landfill leachate and municipal wastewater using steam distillation extraction coupled with comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry

4-Nonylphenols (4-NPs) are known endocrine disruptors and by-products of the microbial degradation of nonylphenol polyethoxylate surfactants. One of the challenges to understanding the toxic effects of nonylphenols is the large number of isomers that may exist in environmental samples. In order to attribute toxic effects to specific compounds, a method is needed for the separation and quantitation of individual nonylphenol isomers. The pre-concentration methods of solvent sublimation, solid-phase extraction or liquid–liquid extraction prior to chromatographic analysis can be problematic because of co-extraction of thousands of compounds typically found in complex matrices such as municipal wastewater or landfill leachate. In the present study, steam distillation extraction (SDE) was found to be an effective pre-concentration method for extraction of 4-NPs from leachate and wastewater, and comprehensive two-dimensional gas chromatography (GC × GC) coupled with fast mass spectral data acquisition by time-of-flight mass spectrometry (ToFMS) enhanced the resolution and identification of 4-NP isomers. Concentrations of eight 4-NP isomers were determined in leachate from landfill cells of different age and wastewater influent and effluent samples. 4-NP isomers were about 3 times more abundant in leachate from the younger cell than the older one, whereas concentrations in wastewater effluent were either below detection limits or <1% of influent concentrations. 4-NP isomer distribution patterns were found to have been altered following release to the environment. This is believed to reflect isomer-specific degradation and accumulation of 4-NPs in the aquatic environment.

Journal of Chromatography A↗
USGS2011

Loss of volatile hydrocarbons from an LNAPL oil source

The light nonaqueous phase liquid (LNAPL) oil pool in an aquifer that resulted from a pipeline spill near Bemidji, Minnesota, was analyzed for volatile hydrocarbons (VHCs) to determine if the composition of the oil remains constant over time. Oil samples were obtained from wells at five locations in the oil pool in an anaerobic part of the glacial outwash aquifer. Samples covering a 21-year period were analyzed for 25 VHCs. Compared to the composition of oil from the pipeline source, VHCs identified in oil from wells sampled in 2008 were 13 to 64% depleted. The magnitude of loss for the VHCs analyzed was toluene ≫ o-xylene, benzene, C 6 and C 10–12 n-alkanes > C 7 –C 9 n-alkanes > m-xylene, cyclohexane, and 1- and 2-methylnaphthalene > 1,2,4-trimethylbenzene and ethylbenzene. Other VHCs including p-xylene, 1,3,5- and 1,2,3-trimethylbenzenes, the tetramethylbenzenes, methyl- and ethyl-cyclohexane, and naphthalene were not depleted during the time of the study. Water–oil and air–water batch equilibration simulations indicate that volatilization and biodegradation is most important for the C 6 –C 9 n-alkanes and cyclohexanes; dissolution and biodegradation is important for most of the other hydrocarbons. Depletion of the hydrocarbons in the oil pool is controlled by: the lack of oxygen and nutrients, differing rates of recharge, and the spatial distribution of oil in the aquifer. The mass loss of these VHCs in the 5 wells is between 1.6 and 7.4% in 29 years or an average annual loss of 0.06–0.26%/year. The present study shows that the composition of LNAPL changes over time and that these changes are spatially variable. This highlights the importance of characterizing the temporal and spatial variabilities of the source term in solute-transport models.

Minnesota↗
USGS2011

Long-term natural attenuation of crude oil in the subsurface

The time frame for natural attenuation of crude oil contamination in the subsurface has been studied for the last 27 years at a spill site located near Bemidji, Minnesota, USA. Data from the groundwater contaminant plume show that dissolved benzene concentrations adjacent to the oil decreased by 50% between 1993 and 2007. To assess how this decrease is related to benzene concentrations in the crude oil, samples of oil were bailed from floating oil in five wells and analysed for volatile components. Compared to reference oil collected from the pipeline in 1984, benzene concentrations in the well located farthest downgradient in the oil have decreased an average of 50%. Benzene and ethylbenzene depletion are linearly correlated with oil saturation in the pore space suggesting that dissolution is the primary removal mechanism and biodegradation within the oil body is insignificant.

Conference Paper↗
USGS2010

Assessment of PDMS-water partition coefficients: implications for passive environmental sampling of hydrophobic organic compounds

Solid-phase microextraction (SPME) has shown potential as an in situ passive-sampling technique in aquatic environments. The reliability of this method depends upon accurate determination of the partition coefficient between the fiber coating and water (K f ). For some hydrophobic organic compounds (HOCs), K f values spanning 4 orders of magnitude have been reported for polydimethylsiloxane (PDMS) and water. However, 24% of the published data examined in this review did not pass the criterion for negligible depletion, resulting in questionable K f values. The range in reported K f is reduced to just over 2 orders of magnitude for some polychlorinated biphenyls (PCBs) when these questionable values are removed. Other factors that could account for the range in reported K f , such as fiber-coating thickness and fiber manufacturer, were evaluated and found to be insignificant. In addition to accurate measurement of K f , an understanding of the impact of environmental variables, such as temperature and ionic strength, on partitioning is essential for application of laboratory-measured K f values to field samples. To date, few studies have measured K f for HOCs at conditions other than at 20 degrees or 25 degrees C in distilled water. The available data indicate measurable variations in K f at different temperatures and different ionic strengths. Therefore, if the appropriate environmental variables are not taken into account, significant error will be introduced into calculated aqueous concentrations using this passive sampling technique. A multiparameter linear solvation energy relationship (LSER) was developed to estimate log K f in distilled water at 25 degrees C based on published physicochemical parameters. This method provided a good correlation (R2 = 0.94) between measured and predicted log K f values for several compound classes. Thus, an LSER approach may offer a reliable means of predicting log K f for HOCs whose experimental log K f values are presently unavailable. Future research should focus on understanding the impact of environmental variables on K f . Obtaining the data needed for an LSER approach to estimate K f for all environmentally relevant HOCs would be beneficial to the application of SPME as a passive-sampling technique.

Environmental Science & Technology↗
USGS2009

Pore-water chemistry from the ICDP-USGS coer hole in the Chesapeake Bay impact structure--Implications for paleohydrology, microbial habitat, and water resources

We investigated the groundwater system of the Chesapeake Bay impact structure by analyzing the pore-water chemistry in cores taken from a 1766-m-deep drill hole 10 km north of Cape Charles, Virginia. Pore water was extracted using high-speed centrifuges from over 100 cores sampled from a 1300 m section of the drill hole. The pore-water samples were analyzed for major cations and anions, stable isotopes of water and sulfate, dissolved and total carbon, and bioavailable iron. The results reveal a broad transition between fresh and saline water from 100 to 500 m depth in the post-impact sediment section, and an underlying syn-impact section that is almost entirely filled with brine. The presence of brine in the lowermost post-impact section and the trend in the dissolved chloride with depth suggest a transport process dominated by molecular diffusion and slow, compaction-driven, upward flow. Major ion results indicate residual effects of diagenesis from heating, and a pre-impact origin for the brine. High levels of dissolved organic carbon (6-95 mg/L) and the distribution of electron acceptors indicate an environment that may be favorable for microbial activity throughout the drilled section. The concentration and extent of the brine is much greater than had previously been observed, suggesting its occurrence may be common in the inner crater. However, groundwater flow conditions in the structure may reduce the salt-water-intrusion hazard associated with the brine.

Special Paper of the Geological Society of America↗
USGS2008

Susceptibility of synthetic long-chain alkylbenzenes to degradation in reducing marine sediments

Long-chain alkylbenzenes (LCABs) synthesized for production of alkylbenzene sulfonate surfactants have been used as molecular markers of anthropogenic waste for 25 years. Synthetic LCABs comprise two classes, the tetrapropylene-based alkylbenzenes (TABs) and the linear alkylbenzenes (LABs). LABs supplanted TABs in the mid-1960s because of improved biodegradability of their sulfonated analogs. Use of LCABs for molecular stratigraphy depends on their preservation in sediments over decadal time scales. Most laboratory and field studies suggest that LABs degrade rapidly under aerobic conditions but are resistant to degradation when oxygen is absent. However, recent work indicates that LABs may not be as persistent under reducing conditions as previously thought. To assess the potential for degradation of LCABs in reducing sediments, box cores collected in 1992 and 2003 near a submarine wastewater outfall system were analyzed using gas chromatography/mass spectrometry. The TABs were effectively preserved; differences between whole-core inventories were within analytical error. By contrast, whole-core inventories of the LABs decreased by about 50−60% during the same time interval. Based on direct comparison of chemical inventories in coeval core sections, LAB transformation rates are estimated at 0.07 ± 0.01 yr −1 . These results indicate that caution should be exercised when using synthetic LCABs for reconstruction of depositional records.

Environmental Science & Technology↗
USGS2008

Assessment of 1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene (DDE) transformation rates on the Palos Verdes Shelf, CA

In 1953, the world's largest producer of DDT, Montrose Chemical Corporation, began to discharge process wastes into sewers of the Los Angeles County Sanitation Districts (LACSD), California. By 1971, when the sewer connection was terminated, approximately 1,500-2,000 metric tons of DDT had been introduced to the LACSD treatment plant in Carson, CA. After treatment, effluent from this plant was released to the ocean through a submarine outfall system on the Palos Verdes Shelf (PVS) near Los Angeles, resulting in the accumulation of highly contaminated marine sediments. Numerous investigations of the PVS have been undertaken since the late 1960s, but few have focused on the biogeochemical fate of DDT and its transformation products. In the early 1990s, it was shown that DDE, the major DDT compound in the sediments, was being reductively dechlorinated by microorganisms resident in sediments on the PVS. The U.S. Geological Survey undertook a study in cooperation with the U.S. Environmental Protection Agency to provide a better understanding of the range of reductive dechlorination rates on the PVS and the environmental factors that control them. Existing data show that rates of reductive dechlorination are variable spatially. A comparison of data from two cores collected approximately 7 kilometers downcurrent from the outfall systems in 1992 and 2003 yielded an average first-order transformation rate of approximately 0.05 yr-1. A multistep reaction model suggests that inventories of DDE in PVS sediments at the study site will continue to decline, whereas the inventory of the metabolite DDNU will reach a maximum around 2014.

California↗
USGS2004

Molecular markers and their use in environmental organic geochemistry

Molecular markers are organic substances that carry information about sources of organic matter or contamination. The source/marker relation can be used to indicate the presence of a given source material (qualitative), or, under appropriate conditions, to estimate the amount of a source material (quantitative source apportionment) in the environment. Assemblages of markers can also be used as process probes. In this instance, systematic differences and/or similarities in the physical-chemical properties of markers are coupled with compositional changes in marker composition to infer the operation of natural processes. This paper provides an overview of what molecular markers are, what types of markers are present in the environment, the requirements for the use of markers, and some common applications. To illustrate how molecular markers can answer specific environmental questions, three case studies are presented. The first case study examines the impact of municipal waste on a large urban harbor (Boston Harbor). Linear alkylbenzenes (unreacted residues of linear alkylbenzenesulfonate surfactants) and coprostanol (a fecal indicator) provide information on the sources and likely transport pathways of municipal wastes in a complex hydrologic system. The marker data are also used to estimate the proportion of sewage-derived poly chlorinated biphenyls (PCBs) in polluted harbor sediments. The second case study concerns a portion of the continental shelf off southern California (Palos Verdes) where discharge of municipal wastewaters has led to extensive contamination of sediments and biota. Long-chain alkylbenzenes (surfactant residues), PCBs and the pesticide, DDT (dichlorodiphenyltrichloroethane), are used to develop sedimentation rate estimates for several time periods by molecular stratigraphy. These results, when combined with other information, allow conclusions to be drawn about the most likely transport pathway of sediments at the study site and to predict the fate of historically deposited contaminants. Finally, an investigation of a crude-oil spill in Bemidji, MN illustrates how monoaromatic hydrocarbons can be exploited as process probes, providing insights into the relative importance of different attenuation processes in a contaminated aquifer. The results show that natural attenuation of the monoaromatic hydrocarbons is occurring at this site and is dominated, not by physical and/or chemical processes, but by biodegradation.

Book chapter↗
USGS2002

Sources and magnitude of bias associated with determination of polychlorinated biphenyls in environmental samples

Recently compiled data on the composition of commercial Aroclor mixtures and ECD (electron capture detector) response factors for all 209 PCB congeners are used to develop estimates of the bias associated with determination of polychlorinated blphenyis. During quantitation of multlcomponent peaks by congener-specific procedures error is introduced because of variable ECD response to isomeric PCBs. Under worst case conditions, the magnitude of this bias can range from less than 2% to as much as 600%. Multicomponent peaks containing the more highly and the lower chlorinated congeners experience the most bias. For this reason, quantitation of Σ PCB in Aroclor mixtures dominated by these species (e.g. 1016) are potentially subject to the greatest error. Comparison of response factor data for ECDs from two laboratories shows that the sign and magnitude of calibration bias for a given multicomponent peak is variable and depends, in part, on the response characteristics of individual detectors. By using the most abundant congener (of each multicomponent peak) for purposes of calibration, one can reduce the maximum bias to less than 55%. Moreover, due to cancellation of errors, the bias resulting from summation of all peak concentrations (i.e. Σ PCB) becomes vanishingly small (<1.2%). In contrast, bias associated with determination of Σ PCB as Aroclor equivalents by the traditional Aroclor method is potentially large (>200%) and highly variable in sign and magnitude. In this case, bias originates not only from the incomplete chromatographic resolution of PCB congeners but also the overlapping patterns of the Aroclor mixtures. Together these results illustrate the advantages of the congener-specific method of PCB quantitation over the traditional Aroclor Method and the extreme difficulty of estimating bias incurred by the latter procedure on a post hoc basis.

Analytical Chemistry↗