Equilibrium Partitioning Model Research Articles

Determination of aqueous concentrations of polycyclic aromatic hydrocarbons (PAHs) in an urban stream

The concentrations of freely dissolved polycyclic aromatic hydrocarbons (PAHs) in an urban stream at high-flow and base-flow were estimated by an equilibrium partitioning model (EPM), and by use of lipid-filled semipermeable membrane devices (SPMDs), then compared to direct measurements made on bulk (unfiltered) water samples. The SPMD method was slightly more effective in detecting smaller three- and four-ring PAHs and the EPM method was slightly more effective in detecting five and six-ring PAHs. Although the SPMDs sequestered slightly fewer of the large PAHs, they sampled more compounds of a size range likely to be bioavailable to aquatic organisms. Estimates of the concentrations of freely dissolved PAHs for the EPM and the SPMD methods were similar when compared under the same flow regimes. The SPMD method also provided a time-integrated average measure of freely dissolved PAHs that cannot be easily duplicated with conventional sampling procedures. Concentrations of freely dissolved PAHs (EPM method) as well as concentrations of total PAHs at high-flow averaged about 20 times greater than at base-flow. This suggests the potential for immediate toxicological impacts on stream biota is greater at high-flow because of increased concentrations of bioavailable PAHs. For individual PAHs, differences between bulk and freely dissolved concentrations were most evident at high-flow for high K ow compounds strongly partitioned to suspended solids. Thus, in hazard assessments for aquatic biota it is important to employ measures of freely dissolved PAHs during storm water runoff events.

Laboratory effects of microcosm size and the pesticide chlorpyrifos on benthic macroinvertebrate colonization of soft estuarine sediments

A 42-d flow-through experiment was conducted to evaluate the effects of the organo-phosphate pesticide, chlorpyrifos, and microcosm size (small: 144 cm 2; large: 400 cm 2) on benthic estuarine macroinvertebrate colonization. Nested central and perimeter (outside margin) cores were used to assess animal distribution within microcosms. Fine-grained, organically-rich (approximately 4.0% organic carbon and 20% dry wt) sediments were nominally fortified with chlorpyrifos controls, low (1.0) and high treatments (10.0 μg −1 wet sediment). Large microcosms contained a significantly (p < 0.05) higher average taxa richness (10.9) than small microcosms (8.6) but small microcosms contained a significantly greater average animal density (295.8; numerical abundance adjusted to unit area) than large microcosms (120.5). Density of the polychaete, Neanthes succinea, the amphipod, Corophium acherusicum, and the barnacle, Balanus sp., was significantly greater in small microcosms but density of Ensis minor was significantly greater in large microcosms. In small and large microcosms, respectively, densities averaged significantly greater numbers in perimeter cores (e.g. 203.1 and 75.1) vs central cores (71.9 and 45.4). Average density decreased significantly with increasing chlorpyrifos concentration from controls (326.8), to low (123.8) and high (78.8) treatments. The density decrease was significantly related only to C. acherusicum whose densities decreased from controls (285.8) to low (88.5) and high (43.9) dosed microcosms. Application of an equilibrium partitioning model indicated that density of C. acherusicum was sensitive to an estimated interstitial water concentration of approximately 0.48 μg liter −1. Non-metric multidimensional scaling ordination analyses provided important insights into response patterns not available through ANOVA procedures. A permutation procedure (ANOSIM) detected a significant size effect ( p < 0.0001) and a significant effect between controls and low ( p < 0.042) and high doses ( p < 0.013) but not between low and high chlorpyrifos treatments ( p < 0.465). A single species, C. ascherusicum, as in the ANOVA analyses, dominated contributions to community average percent dissimilarity in most combinations of microcosm size and chlorpyrifos treatment effects (range: 8.4–21.9%). Community structure differed significantly in several combinations of microcosm size, core position and chlorpyrifos treatment. Results confirm earlier work that intrinsic design factors influence benthic macroinvertebrate community structure and determine taxa available to pesticide exposure in microcosms.

Partitioning of 2,2‘,4,4‘-Tetrachlorobiphenyl by the Dissolved Organic Matter in Oxic and Anoxic Porewaters

Until recently, the dissolved organic matter (DOM) of pore waters was an overlooked pool of organic matter important to the fate and transport of hydrophobic organic chemicals (HOCs). The aqueous solubility enhancement and the porewater DOM partition coefficient (KPWDOM) of 2,2‘,4,4‘-tetrachlorobiphenyl (TeCB) were compared in samples of anoxic porewater and that same porewater after aeration. Aeration of anoxic porewater increased TeCB partitioning by porewater DOM. Despite a decrease in DOM concentration (coagulated by precipitated iron(III) oxidized during porewater aeration), oxic (aerated) porewaters had greater solubility enhancement and up to an order of magnitude larger KPWDOM than anoxic (unaltered) porewaters. Anoxic porewaters with no detectable dissolved oxygen and 45 mg/L dissolved iron(II) had the smallest TeCB solubility enhancement and KPWDOM. Using experimentally determined KPWDOM values, a three-phase equilibrium-partitioning model predicts the following: (1) the distribution of HOC fre...

Development and Verification of a Bioaccumulation Model for Organic Contaminants in Benthic Invertebrates

A novel nonequilibrium, steady-state model is presented to predict the bioaccumulation of organic chemicals by filter feeding and detritivorous benthic invertebrates. This model accounts for chemical disequilibria between overlying water, diet and sediment, biomagnification, and benthic invertebrate feeding preferences and strategies. The results of a field study of PCB congener bioaccumulation in various benthic invertebrate species in western Lake Erie are reported to verify the model. A comparison of model-predicted and field data demonstrate that the predictability of this model is better than that of the widely used equilibrium partitioning model for as sessing bioaccumulation in benthic organisms and for developing sediment quality criteria.

Effects of In Vivo induction and inhibition of phase 1 and 2 enzymes on the toxicity of malathion to tilapia

A 42-d flow-through experiment was conducted to evaluate the effects of the organo-phosphate pesticide, chlorpyrifos, and microcosm size (small: 144 cm2; large: 400 cm2) on benthic estuarine macroinvertebrate colonization. Nested central and perimeter (outside margin) cores were used to assess animal distribution within microcosms. Fine-grained, organically-rich (approximately 4.0% organic carbon and 20% dry wt) sediments were nominally fortified with chlorpyrifos controls, low (1.0) and high treatments (10.0 μg−1 wet sediment). Large microcosms contained a significantly (p < 0.05) higher average taxa richness (10.9) than small microcosms (8.6) but small microcosms contained a significantly greater average animal density (295.8; numerical abundance adjusted to unit area) than large microcosms (120.5). Density of the polychaete, Neanthes succinea, the amphipod, Corophium acherusicum, and the barnacle, Balanus sp., was significantly greater in small microcosms but density of Ensis minor was significantly greater in large microcosms. In small and large microcosms, respectively, densities averaged significantly greater numbers in perimeter cores (e.g. 203.1 and 75.1) vs central cores (71.9 and 45.4). Average density decreased significantly with increasing chlorpyrifos concentration from controls (326.8), to low (123.8) and high (78.8) treatments. The density decrease was significantly related only to C. acherusicum whose densities decreased from controls (285.8) to low (88.5) and high (43.9) dosed microcosms. Application of an equilibrium partitioning model indicated that density of C. acherusicum was sensitive to an estimated interstitial water concentration of approximately 0.48 μg liter−1. Non-metric multidimensional scaling ordination analyses provided important insights into response patterns not available through ANOVA procedures. A permutation procedure (ANOSIM) detected a significant size effect (p < 0.0001) and a significant effect between controls and low (p < 0.042) and high doses (p < 0.013) but not between low and high chlorpyrifos treatments (p < 0.465). A single species, C. ascherusicum, as in the ANOVA analyses, dominated contributions to community average percent dissimilarity in most combinations of microcosm size and chlorpyrifos treatment effects (range: 8.4–21.9%). Community structure differed significantly in several combinations of microcosm size, core position and chlorpyrifos treatment. Results confirm earlier work that intrinsic design factors influence benthic macroinvertebrate community structure and determine taxa available to pesticide exposure in microcosms.

Predicting the Effect of Moisture on Vapor-Phase Sorption of Volatile Organic Compounds to Soils

Sorption from the vapor phase is an important factor controlling the transport of volatile organic compounds (VOCs) in the unsaturated zone. Accurate description of sorption from the vapor phase to soil in the unsaturated zone therefore is intrinsic to predicting the ultimate fate of contaminants. Vapor-phase sorption of several VOCs onto seven soils at oven-dried, air-dried, typical field moisture and water-saturated conditions was measured using a batch-equilibrium headspace method. An equilibrium partitioning model was developed to predict the effect of soil moisture content on vapor-phase sorption. The model was based on the soil pore-size distribution and BET isotherm sorption parameters under oven-dried and water-saturated conditions. The model was shown to accurately predict observed experimental results.

Eutrophication increases the association of PCB to dissolved organic matter in marine microcosms

Partitioning of 14C-2,2′,4,4′-TCB (tetrachlorobiphenyl) between sediment, and water and suspended particles was studied in eutrophic and oligotrophic benthic microcosms, respectively. Microcosms consisted of glass aquaria with homogenized clay sediment, circulated with natural seawater (60 m, 34‰). Organic enrichment in the eutrophic treatment was obtained by adding a phytoplankton culture of the marine diatom Phaeodactylum tricornutum into the water column; the oligotrophic aquaria received only filtered seawater. 14C-TCB was added to the water using a generator column technique. Phytoplankton and TCB were continuously added during an exposure phase of 8 days, followed by a semi-static phase in which TCB and phytoplankton were left to equilibrate and sediment during one month. Presence of phytoplankton induced a higher concentration of TCB in the water column. Higher concentration of TCB in the water was not caused by higher amounts of TCB bound to algal cells but due to an association of TCB with an increased production of dissolved organic matter and bacteria in the eutrophic treatment. TCB concentration in the sediments was not significantly different between treatments. The observed partitioning of TCB between water and sediment was in accordance with a predicted distribution, based on the equilibrium partitioning model.

Modelling the accumulation of hydrophobic organic chemicals in earthworms

In this paper a method is developed which can be used to estimate the body burden of organic hydrophobic chemicals in earthworms. In contrast to the equilibrium partitioning theory, two routes of uptake are incorporated: uptake from interstitial water and dietary uptake. Although many uncertainties still remain, calculations show that for earthworms steady state body burdens are mainly determined by uptake from interstitial water. Under most circumstances, the contribution of dietary uptake is small, except for hydrophobic chemicals (log Kow > 5) in soils with a high organic matter (OM) content of ≈ 20 %. Under those conditions, estimates of the steady state body burden calculated with the equilibrium partitioning model, in which only uptake from interstitial water is taken into account, might result in a small underestimation of the real body burden of chemicals in earthworms.

Sediment Porewater Partitioning of Polycyclic Aromatic Hydrocarbons in Three Cores from Boston Harbor, Massachusetts

Polycyclic aromatic hydrocarbon (PAH) concentrations were measured in sediments and porewaters isolated from three cores from Boston Harbor, MA. Measured porewater PAH concentrations were significantly lower than the concentrations predicted by two- and three-phase equilibrium partitioning models. We hypothesize that only a fraction of the measured sediment PAH concentrations was available to partition rapidly into sediment porewaters.

Organic carbon partitioning as a basis for predicting the toxicity of chlorpyrifos in sediments

AbstractThe objective of this study was to evaluate an organic carbon partitioning model for predicting bioavailability of the organophosphate chlorpyrifos in sediments, in support of the development of a sediment‐quality criterion for the pesticide. Initial 10‐d water‐only toxicity tests were conducted with the midge Chironomus tentans to define the sensitivity of this species to chlorpyrifos. Two uncontaminated sediments with differing organic carbon contents (approximately 3 and 8.5%) were spiked with varying amounts of chlorpyrifos designed to result in equilibrium interstitial (pore) water concentrations that would bracket the effects concentrations observed in the water‐only exposures. Ten‐day toxicity tests with Chironomus tentans were conducted with the spiked sediments under conditions similar to those in the water‐only exposure. Based on predicted pore‐water concentrations of chlorpyrifos, results of the water‐only and sediment tests were in close agreement. The water‐only LC50 of chlorpyrifos to Chironomus tentans was 70 ng/L, whereas LC50 values based on predicted pore‐water concentrations in the two test sediments were 40 and 70 ng/L. Total concentrations of chlorpyrifos measured in pore water from the test sediments were always at least an order of magnitude greater than predicted pore‐water concentrations of the pesticide. However, upon correction for that fraction of the measured chlorpyrifos potentially bound to DOC in the pore water, measured and predicted chlorpyrifos concentrations were in much closer agreement, with the former typically about threefold greater than the latter. Overall, these results suggest that within the range of organic carbon tested in the present study, an equilibrium partitioning model based on organic carbon is appropriate for predicting the bioavailability of sediment‐associated chlorpyrifos to benthic invertebrates.

Assessment of Potential Cancer Risk from Consumption of PCBs Bioaccumulated in Fish and Shellfish.

We evaluated the potential cancer risk to adults from ingesting polychlorinated biphenyls (PCBs) in fish and shellfish using an equilibrium partitioning model of PCB bioaccumulation in the aquatic animal. Estimated potential cancer risk to humans increased exponentially with increasing hydrophobicity of the PCB. However, the addition of food-chain sources of PCBs was necessary to cause potential cancer risk to exceed 10(-6). Environmental degradation of the PCB reduced cancer risk by reducing the exposure concentration; 3.3 degradation half-lives were required to reduce cancer risk estimates by one order of magnitude. PCB biotransformation to nongenotoxic metabolites (no increase in the cancer slope factor) by the aquatic animal reduced cancer risk by reducing the steady-state concentration of PCBs in the edible tissue. Even relatively slow biotransformation (e.g., metabolic half-life of 100 days) reduced cancer risk estimates under the default model conditions. Nonequilibrium conditions, such as limited exposure time, reduced potential cancer risk by reducing contaminant concentrations in the aquatic animal. Risk assessment using toxic equivalency factors predicted substantially greater potential risk for specific congeners than for PCB mixtures. Our evaluation demonstrates that deviation from conventional assumptions used in risk assessment (e.g., negligible biotransformation and degradation; steady-state equilibrium) can significantly affect cancer risk estimates.

Sources of Uncertainty in the Application of the Equilibrium Partitioning Approach to Sediment Quality Assessment

One widely discussed method to assess biological impact of contaminated sediments is through the application of the equilibrium partitioning model. The partitioning model can be used to develop normalized chemical concentrations that can be correlated to biological effects resulting from sediment contamination. The application of the equilibrium partitioning model requires the measurement or estimation of several model variables. However, the combined uncertainties in the determination of these variables may limit the usefulness of this approach. Uncertainty in estimated pore water effects concentrations and analytical variability of three chemical measurements was investigated using Monte Carlo simulation. Results of Monte Carlo simulations indicated that the levels of analytical uncertainties in the study were acceptable. However, the results also revealed that analytical variability can significantly impair interpretation of the data and should be considered in the choice among different analytical methods. Investigation of duplicate field samples collected in the study indicated an additional source of uncertainty from the occurrence of spatially heterogeneous contaminant distributions.

The influence of organic matter quality on the toxicity and partitioning of sediment-associated fluoranthene

Organic matter in sediment is derived from many sources, including dead plants and animals, fecal matter, and flocculated colloidal organic matter. Chemical partitioning and toxicity of nonpolar organic contaminants is strongly affected by the quantity of sediment organic matter. The purpose of this study was to determine whether the quality of sediment organic matter affects partitioning and bioavailability of such contaminants. A base substrate, amended to a consistent organic carbon level (nominally 0.4%, measured 0.35% ± 0.11 sd POC [particulate organic carbon]) with five types of organic matter (a macrophyte, fecal matter of two invertebrate species, suspended particulate organic matter, and an organic-rich mud), was spiked with serial concentrations of the polynuclear aromatic hydrocarbon, fluoranthene. After a five-week equilibration period, the toxicity of the spiked substrates to an infaunal amphipod, Rhepoxynius abronius, was measured, and the distribution of fluoranthene between particulate and interstitial water phases (including total and freely dissolved interstitial water phases) was determined. The range of acute toxicity among organic matter–source treatments was small when based on total sediment concentrations (11.1–19.1 mg fluoranthene per kilogram dry sediment, nominal). Partitioning of fluoranthene between particulate-sorbed and interstitial water phases was not affected by organic matter quality, except when the source was fresh plant material. This variability may have been due to qualitative differences in the particulate/interstitial water partitioning in plant material as a source of sedimentary organic matter or an artifact in the method used to determine freely dissolved, interstitial water fluoranthene concentrations. Equilibrium partitioning models accurately predicted interstitial water concentrations of fluoranthene for the nonplant substrates once corrections were made for the solubility of fluoranthene in seawater. Our data suggest that equilibrium partitioning–based sediment quality criteria for fluoranthene may not need to correct for the quality of organic carbon in sediments and that these criteria can be applied to sediments with a particulate organic carbon content as low as 0.3%.

Vesicle to Micelle Transitions of Egg Phosphatidylcholine Liposomes Induced by Nonionic Surfactants, Poly(oxyethylene) Cetyl Ethers1

Vesicle to micelle transitions of sonicated liposomes of egg yolk phosphatidylcholine (EPC) induced by a homologous series of nonionic surfactants, poly(oxyethylene) cetyl ethers [POE(n) cetyl ether], were investigated by using the method of turbidity titrations. The turbidities of the mixed dispersions of sonicated vesicles and surfactant were systematically measured as a function of the surfactant added for a wide range of lipid concentrations (from 0.51 to 6.35 mM EPC). From the titration curves, two threshold points representing onset and complete solubilization of liposomal membranes were determined as a probe for the effect of the length of ethylene oxide (EO) moiety on the phase behavior of ternary system of POE(n) cetyl ethers-EPC-excess water. Patterns of turbidity curves and the surfactant concentrations at two threshold points as well as widths of region between two transitions, where lamellar sheets and mixed micelles may coexist, mainly depended on the length of EO head group. With changing the lengths, solubilization of liposomes and phase diagram showed optimal behavior. That is, in the middle range of EO numbers, it resulted in narrowest coexistence region between onset and complete solubilization. Assuming the equilibrium partitioning model, critical effective molar ratios of surfactant to lipid, Rsat, free surfactant concentrations, Dw, and the partition coefficient of surfactant between bilayer and aqueous phase, K, in surfactant-saturated liposomes were quantitatively determined as a function of EO number. Effective ratios, Rsol, and free surfactant concentration in mixed micelles were also determined. In addition, the effects of CMC and HLB of surfactants on the solubilization of liposome were discussed.

Toxicity of fluoranthene in sediment to marine amphipods: A test of the equilibrium partitioning approach to sediment quality criteria

AbstractThe toxicity of fluoranthene in sediment to the marine benthic amphipods, Rhepoxynius abronius (Barnard) and Corophium spinicorne (Stimpson) was determined in relation to the equilibrium partitioning approach to the development of sediment quality criteria. Toxicity tests were conducted with well‐sorted fine sands at three levels of organic carbon (OC), 0.18, 0.31 and 0.48%. Measured interstitial water concentrations of fluoranthene less than 50 μg/L were highly correlated with predictions based on the equilibrium partitioning model. LC50s based on bulk (total) fluoranthene concentrations increased significantly with increasing sediment OC. LC50s based on fluoranthene concentrations in interstitial water were not significantly different between 0.18 and 0.48% OC or between 0.31 and 0.48% OC, but the LC50 at 0.31% OC was significantly higher than that at 0.18% OC. The regression of sediment OC on bulk fluoranthene LC50 was linear, indicating that the concentration of fluoranthene in interstitial water was constant at equitoxic conditions, as predicted by the equilibrium partitioning model. The 10‐d LC50 of fluoranthene in interstitial water (23.8 μg/L) was intermediate between the U.S. Environmental Protection Agency's (U.S. EPA) acute (40 μg/L) and chronic (16 μg/L) water quality values for fluoranthene. Within the limitations of these experiments (i.e., one chemical, two species, sandy sediment with low carbon content), the results indicate that sediment quality criteria derived from the equilibrium partitioning model and water quality criteria would protect sensitive benthic invertebrates. The epibenthic, tube‐dwelling Corophium was less sensitive to test sediments than the infaunal, free‐burrowing Rhepoxynius, possibly because of different routes of exposure to fluoranthene. There was a close correspondence between estimates of sediment quality for fluoranthene based on distinctly different methodologies including equilibrium partitioning, apparent effects threshold, toxicity tests applied to experimentally spiked sediment and toxicity tests applied to field‐collected sediment.

Equilibrium partitioning and bioaccumulation of sediment‐associated contaminants by infaunal organisms

AbstractThe utility and limits of applicability of a simple equilibrium partitioning model for predicting the maximum concentration of neutral organic compounds which can be accumulated by infaunal organisms exposed to a contaminated sediment were examined. Accumulation factors (AFs) for PCBs, the lipid normalized PCB concentration in organisms divided by the organic carbon normalized PCB concentration in sediments, were measured for PCBs in infaunal mollusks and polychaetes at field sites with a range of sediment Aroclor (A‐1254) and total organic carbon (TOC) concentrations. The average AFs for A‐1254 were found to be higher (x̄ = 4.94; range 3.76–7.27) at sites with lower contaminant concentrations (15.0–48.3 ng A‐1254/g dry sediment) than at more contaminated sites (328–9,200 ng/g), where AFs were lower (x̄ = 2.62; range 1.14–5.04). AF data grouped on the basis of sediment A‐1254 and TOC concentration differed statistically between, but not within each group. Significant differences in mean AFs were found between some species and between some PCB congeners. When all data were considered, the variability associated with AFs was lower than that found for bioaccumulation factors on a wet weight basis, indicating the utility of lipid and organic carbon normalization.

Modelling the environmental distribution of nonpolar organic compounds: The influence of dissolved organic carbon in overlying and interstitial water

The partitioning of nonpolar compounds among sediment solids, interstitial water, and dissolved organic carbon in overlying and interstitial waters of an aquatic system was studied using column equilibration techniques. The equilibrium distribution relationships for these environmental compartments were incorporated into the equilibrium partitioning model, CEPAC, developed by McCall et al. (1983).

Factors affecting pore water hydrocarbon concentrations in Puget Sound sediments

Abstract Polycyclic aromatic hydrocarbon (PAH) and aliphatic hydrocarbon concentrations have been determined for sediments and associated pore waters collected at 2 sites (11 stations) in Puget Sound, Washington (northwest U.S.A.). These sediments have been contaminated to varying degrees by hydrocarbons from a creosote plant and from various combustion sources. PAH were not detected in pore waters of sediments whose PAH were primarily derived from combustion and natural sources, even though pore water concentrations predicted from sediment concentrations and two-phase equilibrium partitioning models were above detection limits from most PAH. Equilibrium partition coefficients calculated from field aqueous and solid phase data from an area contaminated with creosote agreed with laboratory-derived coefficients to within a factor of ± 4. Pore water concentrations of creosote-derived aliphatic hydrocarbons increase with increasing concentration in bulk sediments. However, pore water concentrations of natural and contaminant aliphatic hydrocarbons are much higher than predicted by solubility data, possibly due to association with nonfilterable dissolved organic matter and colloids. Other major factors controlling hydrocarbon pore water concentrations include differential hydrocarbon sources, specific particle associations and solubility.

The role of small, low density particles on the partition of selected PCB congeners between water and suspended matter (North Sea area)

The dissolved and particulate suspended concentrations of five PCB congeners (with 3–7 chlorine atoms) in the Southern Bight (North Sea) are interpreted in terms of an equilibrium partitioning model. K d values for riverine/coastal samples generally increase with chlorine number of the congeners (K d = 10 4–10 6). Values are more than an order of magnitude larger at the low suspended matter concentrations offshore (< 1 mg·dm −3). This is interpreted in terms of the presence of different SPM fractions with different contents of PCB: a fraction consisting of low size/density particles carrying high contents of PCBs, dominates at low SPM concentrations. The contribution of particulate forms to the total PCB concentration per unit volume may, therefore, be significant even at low SPM concentrations.

Phosphate modulation of calcium transport by a calcium-phospholipid-phosphate complex of calcifying tissues

The ionophoretic activities of a stable Ca-PS-Pi complex were examined in a three-compartment lipid-aqueous phase transport system. It was found that the complex transported Ca2+ at a slower rate than free PS. Analysis of the transport process, using an equilibrium partitioning model, revealed that Ca2+ binding to the complex was the rate-limiting step. This inhibition was overcome by the addition of exogeneous Pi. It was concluded that the complex acted as an ionophore and that the presence of coordinated Ca and P atoms facilitated Pi modulation of Ca2+ transport.