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ABSTRACT: Phthalates are widely used as plasticizers, and improved ability to predict emissions of phthalates is of interest because of concern about their health effects. An experimental chamber was used to measure emissions of di-2-ethylhexyl-phthalate (DEHP) from vinyl flooring, with ammonium sulfate particles introduced to examine their influence on the emission rate and to measure the partitioning of DEHP onto airborne particles. When particles were introduced to the chamber at concentrations of 100 to 245 μg/m3, the total (gas + particle) DEHP concentrations increased by a factor of 3 to 8; under these conditions, emissions were significantly enhanced compared to the condition without particles. The measured DEHP partition coefficient to ammonium sulfate particles with a median diameter of 45 ± 5 nm was 0.032 ± 0.003 m3/μg (95% confidence interval). The DEHP-particle sorption equilibration time was demonstrated to be less than 1 min. Both the partition coefficient and equilibration time agree well with predictions from the literature. This study represents the first known measurements of the particle-gas partition coefficient for DEHP. Furthermore, the results demonstrate that the emission rate of DEHP is substantially enhanced in the presence of particles. The particles rapidly sorb DEHP from the gas phase, allowing more to be emitted from the source, and also appear to enhance the convective mass-transfer coefficient itself. Airborne particles can influence SVOC fate and transport in the indoor environment, and these mechanisms must be considered in evaluating exposure and human health.
Environmental Science & Technology 03/2013; · 4.80 Impact Factor
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ABSTRACT: Background: Limited data are available to assess human exposure to thousands of chemicals currently in commerce. Information that relates human intake of a chemical to its production and use can help inform understanding of mechanisms and pathways that control exposure and support efforts to protect public health.Objectives: We introduce the intake-to-production ratio (IPR) as an economy-wide quantitative indicator of the extent to which chemical production results in human exposure.Methods: The IPR was evaluated as the ratio of two terms: aggregate rate of chemical uptake in a human population (inferred from urinary excretion data) divided by the rate that chemical is produced in or imported into that population's economy. We used biomonitoring data from the U.S. Centers for Disease Control and Prevention along with chemical manufacturing data reported by the U.S. Environmental Protection Agency, as well as other published data, to estimate the IPR for nine chemicals in the United States. Results are reported in units of parts per million, where 1 ppm indicates 1 g of chemical uptake for every million grams of economy-wide use.Results: Estimated IPR values for the studied compounds span many orders of magnitude from a low of 0.6 ppm for bisphenol A to a high of > 180,000 ppm for methyl paraben. Intermediate results were obtained for five phthalates and two chlorinated aromatic compounds: 120 ppm for butyl benzyl phthalate, 670 ppm for di(2-ethylhexyl) phthalate, 760 ppm for di(n-butyl) phthalate, 1,040 ppm for para-dichlorobenzene, 6,800 ppm for di(isobutyl) phthalate, 7,700 ppm for diethyl phthalate, and 8,000-24,000 ppm (range) for triclosan.Conclusion: The IPR is well suited as an aggregate metric of exposure intensity for characterizing population-level exposure to synthesized chemicals, particularly those that move fairly rapidly from manufacture to human intake and have relatively stable production and intake rates.
Environmental Health Perspectives 12/2012; 120(12):1678-1683. · 7.04 Impact Factor
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ABSTRACT: The emission of di-2-ethylhexyl phthalate (DEHP) from vinyl flooring (VF) was measured in specially-designed stainless steel chambers. In duplicate chamber studies, the gas-phase concentration in the chamber increased slowly and reached a steady state level of 0.8~0.9 µg/m3 after about 40 days. By increasing the area of vinyl flooring and decreasing that of the stainless steel surface within the chamber, the time to reach steady state was significantly reduced, compared to a previous study (1 month versus 5 months). The adsorption isotherm of DEHP on the stainless steel chamber surfaces was explicitly measured using solvent extraction and thermal desorption. The strong partitioning of DEHP onto the stainless steel surface was found to follow a simple linear relationship. Thermal desorption resulted in higher recovery than solvent extraction. Investigation of sorption kinetics showed that it takes several weeks for the sorption of DEHP onto the stainless steel surface to reach equilibrium. The content of DEHP in VF was measured at about 15% (w/w) using pressurized liquid extraction. The independently measured or calculated parameters were used to validate an SVOC emission model, with excellent agreement between model predictions and the observed gas-phase DEHP chamber concentrations.
Environmental Science & Technology 10/2012; · 4.80 Impact Factor
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ABSTRACT: A systematic and efficient strategy is needed to assess and manage potential risks to human health that arise from the manufacture and use of thousands of chemicals. Among available tools for rapid assessment of large numbers of chemicals, significant gaps are associated with the capability to evaluate exposures that occur indoors. For semivolatile organic compounds (SVOCs), exposure is strongly influenced by the types of products in which these SVOCs occur. We propose methods for obtaining screening-level estimates for two primary SVOC source classes: additives in products used indoors and ingredients in products sprayed or applied to interior surfaces. Accounting for product use, emission characteristics, and the properties of the SVOCs, we estimate exposure via inhalation of SVOCs in the gas-phase, inhalation of SVOCs sorbed to airborne particles, ingestion of SVOCs sorbed to dust, and dermal sorption of SVOCs from the air into the blood. We also evaluate how exposure to the general public will change if chemical substitutions are made. Further development of a comprehensive set of models including the other SVOC-containing products and the other SVOC exposure pathways, together with appropriate methods for estimating or measuring the key parameters (in particular, the gas-phase concentration in equilibrium with the material-phase concentration of the SVOC in the product, or y(0)), is needed. When combined with rapid toxicity estimates, screening-level exposure estimates can contribute to health-risk-based prioritization of a wide range of chemicals of concern.
Environmental Science & Technology 08/2012; 46(20):11171-8. · 4.80 Impact Factor
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ABSTRACT: Hypolimnetic oxygenation systems (HOx) are being increasingly used in freshwater reservoirs to elevate dissolved oxygen levels in the hypolimnion and suppress sediment-water fluxes of soluble metals (e.g. Fe and Mn) which are often microbially mediated. We assessed changes in sediment microbial community structure and corresponding biogeochemical cycling on a reservoir-wide scale as a function of HOx operations. Sediment microbial biomass as quantified by DNA concentration was increased in regions most influenced by the HOx. Following an initial decrease in biomass in the upper sediment while oxygen concentrations were low, biomass typically increased at all depths as the 4-month-long oxygenation season progressed. A distinct shift in microbial community structure was only observed at the end of the season in the upper sediment near the HOx. While this shift was correlated to HOx-enhanced oxygen availability, increased TOC levels and precipitation of Fe- and Mn-oxides, abiotic controls on Fe and Mn cycling, and/or the adaptability of many bacteria to variations in prevailing electron acceptors may explain the delayed response and the comparatively limited changes at other locations. While the sediment microbial community proved remarkably resistant to relatively short-term changes in HOx operations, HOx-induced variation in microbial structure, biomass, and activity was observed after a full season of oxygenation.
FEMS Microbiology Ecology 04/2012; 80(1):248-63. · 3.41 Impact Factor
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Environmental Science & Technology 02/2012; 46(7):4252. · 4.80 Impact Factor
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ABSTRACT: One of the primary goals of hypolimnetic oxygenation systems (HOx) from a drinking water perspective is to suppress sediment-water fluxes of reduced chemical species (e.g., manganese and iron) by replenishing dissolved oxygen (O(2)) in the hypolimnion. Manganese (Mn) in particular is becoming a serious problem for water treatment on a global scale. While it has been established that HOx can increase sediment O(2) uptake rates and subsequently enhance the sediment oxic zone via elevated near-sediment O(2) and mixing, the influence of HOx on sediment-water fluxes of chemical species with more complicated redox kinetics like Mn has not been comprehensively evaluated. This study was based on Mn and O(2) data collected primarily in-situ to characterize both the sediment and water column in a drinking-water-supply reservoir equipped with an HOx. While diffusive Mn flux out of the sediment was enhanced by HOx operation due to an increased concentration driving force across the sediment-water interface, oxygenation maintained elevated near-sediment and porewater O(2) levels that facilitated biogeochemical cycling and subsequent retention of released Mn within the benthic region. Results show that soluble Mn levels in the lower hypolimnion increased substantially when the HOx was turned off for as little as ∼48 h and the upper sediment became anoxic. Turning off the HOx for longer periods (i.e., several weeks) significantly impaired water quality due to sediment Mn release. Continual oxygenation maintained an oxic benthic region sufficient to prevent Mn release to the overlying source water.
Water Research 12/2011; 45(19):6381-92. · 4.86 Impact Factor
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ABSTRACT: Organotins (OTs) are additives widely used as thermal and light stabilizers in polyvinyl chloride (PVC) plastics. OTs can leach into water flowing through PVC pipes. This work examines the leaching rates of two potentially neurotoxic OTs, dimethyl tin (DMT) and dibutyl tin (DBT), from PVC pipe. Water was circulated in a closed loop laboratory PVC pipe system. Using a gas chromatograph-pulsed flame photometric detector (GC-PFPD), the change in concentrations of DMT and DBT in the water in the system was monitored over time and allowed to reach equilibrium. OT concentration as a function of time was analyzed using a mechanistic leaching rate model. The diffusion coefficient for OT in the PVC pipe material, the only unknown model parameter, was found to be 9 × 10(-18) m(2)/s. This value falls within with the range of values estimated from the literature (2 × 10(-18) to 2 × 10(-17) m(2)/s) thus increasing confidence in the leaching rate model.
Environmental Science & Technology 08/2011; 45(16):6902-7. · 4.80 Impact Factor
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ABSTRACT: Hypolimnetic oxygenation systems (HOx) are increasingly used in lakes and reservoirs to elevate dissolved oxygen (O(2)) while preserving stratification, thereby decreasing concentrations of reduced chemical species in the hypolimnion. By maintaining an oxic zone in the upper sediment, HOx suppress fluxes of reduced soluble species from the sediment into the overlying water. However, diminished HOx performance has been observed due to HOx-induced increases in sediment O(2) uptake. Based on a series of in situ O(2) microprofile and current velocity measurements, this study evaluates the vertical O(2) distribution at the sediment-water interface as a function of HOx operation. These data were used to determine how sediment O(2) uptake rate (JO2) and sediment oxic-zone depth (z(max)) were affected by applied oxygen-gas flow rate, changes in near-sediment mixing and O(2) concentration, and proximity to the HOx. The vertical sediment-water O(2) distribution was found to be strongly influenced by oxygenation on a reservoir-wide basis. Elevated JO2 and an oxic sediment zone were maintained during continuous HOx operation, with z(max) increasing linearly with HOx flow rate. In contrast, JO2 decreased to zero and the sediment became anoxic as the vertical O(2) distribution at the sediment-water interface collapsed during periods when the HOx was turned off and near-sediment mixing and O(2) concentrations decreased. JO2 and z(max) throughout the reservoir were found to be largely governed by HOx-induced mixing rather than O(2) levels in the water column. By quantifying how JO2 and z(max) vary in response to HOx operations, this work (1) characterizes how hypolimnetic oxygenation affects sediment O(2) dynamics, (2) contributes to the optimization of water quality and management of HOx-equipped lakes and reservoirs, and (3) enhances understanding of the effect of mixing and O(2) concentrations in other systems.
Water Research 06/2011; 45(12):3692-703. · 4.86 Impact Factor
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ABSTRACT: Problems related to low levels of dissolved oxygen (O 2) in aquatic ecosystems are growing on a global scale (Jankowski et al. 2006; Zimmerman et al. 2008). O 2 depletion in stratified waters is largely controlled by sediment O 2 uptake, particularly in organic-rich environments (Higashino et al. 2004). Flux of O 2 across the sediment–water interface (SWI) may be governed by near-sediment hydrodynamic processes or by O 2 consump-tion within the sediment (Gundersen and Jørgensen 1990; Jør-gensen and Boudreau 2001; Glud et al. 2007). Turbulence in the bottom boundary layer (BBL) controls the thickness (δ DBL) of the diffusive boundary layer (DBL), the millimeter-scale region immediately above the sediment that typically regulates mass transport of O 2 to the SWI in non-advective, water-side–controlled systems (Jørgensen and Revsbech 1985; Lorke et al. 2003; Bryant et al. 2010). Within the sediment, the dis-tribution of O 2 is then determined by a balance between the amount of O 2 supplied via diffusion and/or other transport processes (e.g., bioturbation) and the amount of O 2 used by biogeochemical oxidation processes (Berg et al. 2003). Quanti-fying the O 2 flux into the sediment, or the sediment O 2 uptake Abstract Sediment–water fluxes are influenced by both hydrodynamics and sediment biogeochemical processes. However, fluxes at the sediment–water interface (SWI) are almost always analyzed from either a water-or sediment-side per-spective. This study expands on previous work by comparing water-side (hydrodynamics and resulting diffusive boundary layer thickness, δ DBL) and sediment-side (oxygen consumption and resulting sediment oxic zone) approaches for evaluating diffusive sediment oxygen uptake rate (J O2) and δ DBL from microprofiles. Dissolved oxy-gen microprofile and current velocity data were analyzed using five common methods to estimate J O2 and δ DBL and to assess the robustness of the approaches. Comparable values for J O2 and δ DBL were obtained (agreement within 20%), and turbulence-induced variations in these parameters were uniformly characterized with the five methods. J O2 estimates based on water-side data were consistently higher (+1.8 mmol m –2 d –1 or 25% on average) and δ DBL esti-mates correspondingly lower (–0.4 mm or 35% on average) than those obtained using sediment-side data. This devi-ation may be attributed to definition of the sediment–water interface location, artifacts of the methods themselves, assumptions made on sediment properties, and/or variability in sediment oxygen-uptake processes. Our work emphasizes that sediment-side microprofile data may more accurately describe oxygen uptake at a particular loca-tion, whereas water-side data are representative of oxygen uptake over a broader sediment area. Regardless, our over-all results show clearly that estimates of J O2 and δ DBL are not strongly dependent on the method chosen for analysis.
Limnology and oceanography, methods 04/2010; · 1.53 Impact Factor
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ABSTRACT: Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (J O2), diffusive boundary layer thickness (d DBL), and sediment oxic zone depth (z max) in situ. High temporal-resolution microprofiles across the sediment–water interface and current velocity data within the bottom boundary layer in a medium-sized mesotrophic lake were obtained during a 12-h field study. We resolved the dynamic forcing of a full 8-h seiche cycle and evaluated J O2 from both sides of the sediment–water interface. Turbulence (characterized by the energy dissipation rate, e), the vertical distribution of dissolved oxygen across the sediment–water interface (characterized by d DBL and z max), J O2 , and the sediment oxygen consumption rate (R O2) are all strongly correlated in our freshwater system. Seiche-induced turbulence shifted from relatively active (e 5 1.2 3 10 28 W kg 21) to inactive (e 5 7.8 3 10 212 W kg 21). In response to this dynamic forcing, d DBL increased from 1.0 mm to the point of becoming undefined, z max decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and J O2 decreased from 7.0 to 1.1 mmol m 22 d 21 over a time span of hours. J O2 and oxygen consumption were found to be almost equivalent (within , 5% and thus close to steady state), with R O2 adjusting rapidly to changes in J O2 . Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating J O2 . Dissolved oxygen (O 2) is one of the most critical ecological parameters affecting natural aquatic systems with benthic diversity, ecosystem health, and overall water quality all negatively influenced by depleted O 2 levels (Wetzel 2001; Stachowitsch et al. 2007). The amount of O 2 taken up by the sediment largely governs O 2 depletion in stratified waters with organic-rich sediment (Bouldin 1968; Veenstra and Nolen 1991). Sediment O 2 uptake is a function of both physical limitations on O 2 transfer to the sediment and sediment O 2 consumption processes (Jørgensen and Bou-dreau 2001). Resolving the vertical distribution of O 2 at the sediment–water interface (SWI) allows for the quantification of the sediment O 2 uptake flux (J O 2), which is a fundamental parameter for the characterization of O 2 dynamics in aquatic systems (Wetzel 2001). Consequently, considerable effort has been devoted to elucidating the water-side and sediment-side factors controlling sediment O 2 uptake in freshwater and marine systems (Bouldin 1968; Jørgensen and Revsbech 1985; Boudreau 2001). Molecular diffusion typically becomes the controlling transport process for dissolved species (e.g., O 2) at approximately 1 mm above the SWI in nonadvective systems (e.g., cohesive freshwater and marine sediment; Jørgensen and Revsbech 1985; Røy et al. 2004). This
Limnology and oceanography 03/2010; 55(2):950-964. · 3.42 Impact Factor
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ABSTRACT: Because of the ubiquitous nature of phthalates in the environment and the potential for adverse human health effects, an urgent need exists to identify the most important sources and pathways of exposure.
Using emissions of di(2-ethylhexyl) phthalate (DEHP) from vinyl flooring (VF) as an illustrative example, we describe a fundamental approach that can be used to identify the important sources and pathways of exposure associated with phthalates in indoor material.
We used a three-compartment model to estimate the emission rate of DEHP from VF and the evolving exposures via inhalation, dermal absorption, and oral ingestion of dust in a realistic indoor setting.
A sensitivity analysis indicates that the VF source characteristics (surface area and material-phase concentration of DEHP), as well as the external mass-transfer coefficient and ventilation rate, are important variables that influence the steady-state DEHP concentration and the resulting exposure. In addition, DEHP is sorbed by interior surfaces, and the associated surface area and surface/air partition coefficients strongly influence the time to steady state. The roughly 40-fold range in predicted exposure reveals the inherent difficulty in using biomonitoring to identify specific sources of exposure to phthalates in the general population.
The relatively simple dependence on source and chemical-specific transport parameters suggests that the mechanistic modeling approach could be extended to predict exposures arising from other sources of phthalates as well as additional sources of other semivolatile organic compounds (SVOCs) such as biocides and flame retardants. This modeling approach could also provide a relatively inexpensive way to quantify exposure to many of the SVOCs used in indoor materials and consumer products.
Environmental Health Perspectives 02/2010; 118(2):253-8. · 7.04 Impact Factor
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ABSTRACT: The leaching of organotin (OT) heat stabilizers from polyvinyl chloride (PVC) pipes used in residential drinking water systems may affect the quality of drinking water. These OTs, principally mono- and di-substituted species of butyltins and methyltins, are a potential health concern because they belong to a broad class of compounds that may be immune, nervous, and reproductive system toxicants. In this article, we develop probability distributions of U.S. population exposures to mixtures of OTs encountered in drinking water transported by PVC pipes. We employed a family of mathematical models to estimate OT leaching rates from PVC pipe as a function of both surface area and time. We then integrated the distribution of estimated leaching rates into an exposure model that estimated the probability distribution of OT concentrations in tap waters and the resulting potential human OT exposures via tap water consumption. Our study results suggest that human OT exposures through tap water consumption are likely to be considerably lower than the World Health Organization (WHO) "safe" long-term concentration in drinking water (150 microg/L) for dibutyltin (DBT)--the most toxic of the OT considered in this article. The 90th percentile average daily dose (ADD) estimate of 0.034 +/- 2.92 x 10(-4)microg/kg day is approximately 120 times lower than the WHO-based ADD for DBT (4.2 microg/kg day).
Risk Analysis 11/2009; 29(11):1615-28. · 2.37 Impact Factor
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ABSTRACT: A two-room model is developed to estimate the emission rate of di-2-ethylhexyl phthalate (DEHP) from vinyl flooring and the evolving gas-phase and adsorbed surface concentrations in a realistic indoor environment. Because the DEHP emission rate measured in a test chamber may be quite different from the emission rate from the same material in the indoor environment the model provides a convenient means to predict emissions and transport in a more realistic setting. Adsorption isotherms for phthalates and plasticizers on interior surfaces, such as carpet, wood, dust, and human skin, are derived from previous field and laboratory studies. Log-linear relationships between equilibrium parameters and chemical vapor pressure are obtained. The predicted indoor air DEHP concentration at steady state is 0.15 microg/m3. Room 1 reaches steady state within about one year, while the adjacent room reaches steady state about three months later. Ventilation rate has a strong influence on DEHP emission rate while total suspended particle concentration has a substantial impact on gas-phase concentration. Exposure to DEHP via inhalation, dermal absorption, and oral ingestion of dust is evaluated. The model clarifies the mechanisms that govern the release of DEHP from vinyl flooring and the subsequent interactions with interior surfaces, airborne particles, dust, and human skin. Although further model development, parameter identification, and model validation are needed, our preliminary model provides a mechanistic framework that elucidates exposure pathways for phthalate plasticizers, and can most likely be adapted to predict emissions and transport of other semivolatile organic compounds, such as brominated flame retardants and biocides, in a residential environment.
Environmental Science and Technology 05/2009; 43(7):2374-80. · 5.23 Impact Factor
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ABSTRACT: Oxygenation systems, such as bubble-plume diffusers, are used to improve water quality by replenishing dissolved oxygen (DO) in the hypolimnia of water-supply reservoirs. The diffusers induce circulation and mixing, which helps distribute DO throughout the hypolimnion. Mixing, however, has also been observed to increase hypolimnetic oxygen demand (HOD) during system operation, thus accelerating oxygen depletion. Two water-supply reservoirs (Spring Hollow Reservoir (SHR) and Carvins Cove Reservoir (CCR)) that employ linear bubble-plume diffusers were studied to quantify diffuser effects on HOD. A recently validated plume model was used to predict oxygen addition rates. The results were used together with observed oxygen accumulation rates to evaluate HOD over a wide range of applied gas flow rates. Plume-induced mixing correlated well with applied gas flow rate and was observed to increase HOD. Linear relationships between applied gas flow rate and HOD were found for both SHR and CCR. HOD was also observed to be independent of bulk hypolimnion oxygen concentration, indicating that HOD is controlled by induced mixing. Despite transient increases in HOD, oxygenation caused an overall decrease in background HOD, as well as a decrease in induced HOD during diffuser operation, over several years. This suggests that the residual or background oxygen demand decreases from one year to the next. Despite diffuser-induced increases in HOD, hypolimnetic oxygenation remains a viable method for replenishing DO in thermally-stratified water-supply reservoirs such as SHR and CCR.
Water Research 02/2009; 43(6):1700-10. · 4.86 Impact Factor
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ABSTRACT: Soluble metals such as iron (Fe) and manganese (Mn) often reach problematic levels in water-supply reservoirs during summer stratification following the onset of hypolimnetic hypoxia. The behavior of soluble and particulate Fe and Mn was studied following the installation of a hypolimnetic oxygenation system in Carvins Cove Reservoir, a water-supply impoundment managed by the Western Virginia Water Authority. During oxygenation, manganese concentrations were very low in the bulk hypolimnion (<0.05 mg l(-1)), but high concentrations (>2.0 mg l(-1)) were still observed in the benthic region close to the sediment, despite near-sediment dissolved oxygen concentrations in excess of 5.0 mg l(-1). Oxygenation appears to affect the location of the oxic/anoxic boundary sufficiently to restrict substantial transport of soluble Mn to the bulk water of the hypolimnion. However, the position of the oxic/anoxic boundary was not uniformly affected along the reservoir bottom, allowing horizontal transport of soluble Mn from higher elevations in contact with hypoxic sediments. During one summer, when the oxygen system was turned off for a month, the soluble Mn in the bulk hypolimnion increased substantially. Oxygen concentrations were quickly restored after the system was turned back on, but elevated levels of soluble Mn persisted until the sedimentation rate of detritus through the hypolimnion increased. When operated without interruption, the oxygenation system was able to reduce the bulk average hypolimnion soluble Mn concentration by up to 97%, indicating that source water control of soluble Mn and Fe can be accomplished with hypolimnetic oxygenation in water-supply reservoirs.
Water Research 12/2008; 43(5):1285-94. · 4.86 Impact Factor
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ABSTRACT: When properly designed, hypolimnetic aeration and oxygenation systems can replenish dissolved oxygen in water bodies while preserving stratification. The three primary devices are the airlift aerator, Speece Cone, and bubble-plume diffuser. In each device, gas bubbles in contact with water facilitate interfacial transfer of oxygen, nitrogen, and other soluble gases. However, early design procedures for airlift aerators were empirical, while most bubble-plume models did not account for stratification or gas transfer. Using fundamental principles, a discrete-bubble model was first developed to predict plume dynamics and gas transfer for a circular bubble-plume diffuser. The discrete-bubble approach has subsequently been validated using oxygen transfer tests in a large vertical tank and applied successfully at full-scale to an airlift aerator as well as to both circular and linear bubble-plume diffusers. The performance of each of the four completely different full-scale systems (on a scale of 10 m or more) was predicted based on the behavior of individual bubbles (on a scale of about 1 mm). The combined results suggest thatthe models can be used with some confidence to predict system performance based on applied air or oxygen flow rate, initial bubble size, and, in the case of bubble plume diffusers, near-field boundary conditions. The discrete-bubble approach has also been extended to the Speece Cone, but the model has not yet been validated due to a lack of suitable data. The unified suite of models, all based on simple discrete-bubble dynamics, represents the current state-of-the-art for designing systems to add oxygen to stratified lakes and reservoirs.
Environmental Science and Technology 01/2007; 40(24):7512-20. · 5.23 Impact Factor
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ABSTRACT: A model that predicts the emission rate of volatile organic compounds (VOCs) from building materials is extended and used to predict the emission rate of semivolatile organic compounds (SVOCs) from polymeric materials. Reasonable agreement between model predictions and gas-phase di-2-ethylhexyl phthalate (DEHP) concentrations is achieved using data collected in a previous experimental study that measured emissions of DEHP from vinyl flooring in two very different chambers. While emissions of highly volatile VOCs are subject to "internal" control (the material-phase diffusion coefficient), emissions of the very low volatility SVOCs are subject to "external" control (partitioning into the gas phase, the convective mass-transfer coefficient, and adsorption onto interior surfaces). The effect of SVOCs partitioning onto airborne particles is also examined. The DEHP emission rate is increased when the gas-phase concentration is high, and especially when partitioning to the airborne particles is strong. Airborne particles may play an important role in inhalation exposure as well as in transporting SVOCs well beyond the source. Although more rigorous validation is needed, the model should help elucidate the mechanisms governing emissions of phthalate plasticizers, brominated flame retardants, biocides, and other SVOCs from a wide range of building materials and consumer products.
Environmental Science and Technology 02/2006; 40(2):456-61. · 5.23 Impact Factor
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ABSTRACT: A model is developed that predicts exposure and absorbed dose for chemical contaminants in household drinking water via three pathways: inhalation, direct and indirect ingestion, and dermal penetration. Extensive probability distributions for building characteristics, activity and water use patterns, operating conditions of water devices, and physiological characteristics of the general population are developed. The impacts of different operating conditions on mass transfer coefficients for the shower, bath, washing machine, dishwasher, and faucet are established. Dichlorobromomethane, inorganic lead, and endosulfan, three compounds associated with adverse birth outcomes that have significantly different chemical properties, are selected for analysis. The primary exposure pathways for dichlorobromomethane are inhalation (62%) and ingestion (27%). Seventy percent of total exposure to endosulfan comes from ingestion, and 18% from dermal sorption with the remaining 12% due to inhalation. Virtually all (99.9%) of the exposure to lead occurs via ingestion. A nested Monte Carlo analysis shows that natural variability contributes significantly more (a factor of 10) toward total uncertainty than knowledge uncertainty (a factor of 1.5). Better identification of certain critical input variables (ventilation rate in the shower and bathroom, ingestion rate, the boiling water mass transfer coefficient, and skin permeability) is required.
Environmental Science and Technology 04/2004; 38(6):1799-806. · 5.23 Impact Factor
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ABSTRACT: Building materials may act as both sources of and sinks forvolatile organic compounds (VOCs) in indoor air. A strategy to characterize the rate of absorption and desorption of VOCs by diffusion-controlled building materials is validated. A previously developed model that predicts mass transfer between a flat slab of material and the well-mixed air within a chamber or room is extended. The generalized model allows a nonuniform initial material-phase concentration and a transient influent gas-phase concentration to be simultaneously considered. An analytical solution to the more general model is developed. Experimental data are obtained by placing samples of vinyl flooring inside a small stainless steel chamber and exposing them to absorption/desorption cycles of n-dodecane and phenol. Measured values for the material-air partition coefficient and the material-phase diffusion coefficient were obtained previously in a series of completely independent experiments. The a priori model predictions are in close agreement with the observed experimental data.
Environmental Science and Technology 10/2003; 37(17):3821-7. · 5.23 Impact Factor
