Review of existing terrestrial bioaccumulation models and terrestrial bioaccumulation modeling needs for organic chemicals

Protocols for terrestrial bioaccumulation assessments are far less-developed than for aquatic systems. This article reviews modeling approaches that can be used to assess the terrestrial bioaccumulation potential of commercial organic chemicals. Models exist for plant, invertebrate, mammal, and avian species and for entire terrestrial food webs, including some that consider spatial factors. Limitations and gaps in terrestrial bioaccumulation modeling include the lack of QSARs for biotransformation and dietary assimilation efficiencies for terrestrial species; the lack of models and QSARs for important terrestrial species such as insects, amphibians and reptiles; the lack of standardized testing protocols for plants with limited development of plant models; and the limited chemical domain of existing bioaccumulation models and QSARs (e.g., primarily applicable to nonionic organic chemicals). There is an urgent need for high-quality field data sets for validating models and assessing their performance. There is a need to improve coordination among laboratory, field, and modeling efforts on bioaccumulative substances in order to improve the state of the science for challenging substances.

Pre-anthropocene mercury residues in North American freshwater fish

Mercury (Hg) has been entering the environment from both natural and anthropogenic sources for millennia, and humans have been influencing its environmental transport and fate from well before the Industrial Revolution. Exposure to Hg (as neurotoxic monomethylmercury [MeHg]) occurs primarily through consumption of finfish, shellfish, and marine mammals, and regulatory limits for MeHg concentrations in fish tissue have steadily decreased as information on its health impacts has become available. These facts prompted us to consider 2 questions: 1) What might the MeHg levels in fish tissue have been in the pre-Anthropocene, before significant human impacts on the environment? and 2) How would these pre-Anthropocene levels have compared with current regulatory criteria for MeHg residues in fish tissue? We addressed the first question by estimating pre-Anthropocene concentrations of MeHg in the tissues of prey and predatory fish with an integrated Hg speciation, transport, fate, and food web model (SERAFM), using estimated Hg concentrations in soil, sediment, and atmospheric deposition before the onset of significant human activity (i.e., ≤2000 BCE). Model results show MeHg residues in fish varying depending on the characteristics of the modeled water body, which suggests that Hg in fish tissue is best considered at the scale of individual watersheds or water bodies. We addressed the second question by comparing these model estimates with current regulatory criteria and found that MeHg residues in predatory (but not prey) fish could have approached or exceeded these criteria in some water bodies during the pre-Anthropocene. This suggests that the possibility of naturally occurring levels of Hg in fish below which it is not possible to descend, regardless of where those levels stand with respect to current regulatory limits. Risk management decisions made under these circumstances have the potential to be ineffectual, frustrating, and costly for decision makers and stakeholders alike, suggesting the need for regulatory flexibility when addressing the issue of Hg in fish.

Acquisition of polychlorinated biphenyls (PCBs) by Pacific chinook salmon: an exploration of various exposure scenarios

In 2011, as part of an update to its state water quality standards (WQS) for protection of human health, the State of Oregon adopted a fish consumption rate of 175 g/day for freshwater and estuarine finfish and shellfish, including anadromous species. WQS for the protection of human health whose derivation is based in part on anadromous fish, create the expectation that implementation of these WQS will lead to lower contaminant levels in returning adult fish. Whether this expectation can be met is likely a function of where and when such fish are exposed. Various exposure scenarios have been advanced to explain acquisition of bioaccumulative contaminants by Pacific salmonids. This study examined 16 different scenarios with bioenergetics and toxicokinetic models to identify those where WQS might be effective in reducing polychlorinated biphenyls (PCBs)--a representative bioaccumulative contaminant--in returning adult Fall chinook salmon, a representative salmonid. Model estimates of tissue concentrations and body burdens in juveniles and adults were corroborated with observations reported in the literature. Model results suggest that WQS may effect limited (< approximately 2 ×) reductions in PCB levels in adults who were resident in a confined marine water body or who transited a highly contaminated estuary as out-migrating juveniles. In all other scenarios examined, WQS would have little effect on PCB levels in returning adults. Although the results of any modeling study must be interpreted with caution and are not necessarily applicable to all salmonid species, they do suggest that the ability of WQS to meet the expectation of reducing contaminant loadings in anadromous species is limited.

Using legacy data to relate biological condition to cumulative aquatic toxicity in the Willamette River Basin (Oregon, USA)

In the Willamette River Basin (Oregon, USA), various residential, municipal, industrial, and agricultural activities produce physical, biological, and chemical stressors that may impinge on the basin's aquatic ecosystems. For > 30 years, numerous water-quality and biological-condition data have been accumulated by often disparate monitoring programs. This diagnostic analysis explored whether these legacy data could be used to correlate the presence of chemical stressors with biological condition impacts with the understanding that association is not necessarily causation. Other natural or anthropogenic stressors that may also impact biological conditions were not considered in this study. Acute-toxicity indices were calculated separately for trace metals and organic chemicals detected in surface waters between 1994 and 2010 and then compared with land-use metrics and vertebrate- and invertebrate-assemblage indices from surveys conducted basin-wide during this same period. Half of the possible relations between land use, biological condition, and toxicity were statistically significant at p ≤ 0.10. These results suggest that conditions for aquatic receptors improve either as agricultural or urban land decreases or as forested land increases and that chemical mixtures (primarily involving pesticides) may have impacted components of the basin's aquatic ecosystems. There may be a need for strengthened chemical-management practices on agricultural and urban lands and for maintaining undisturbed forested land to limit chemical migration into adjacent waters. Although these results indicate some utility for legacy data, they also suggest that a more defensible assessment of chemical stressors requires a program specifically designed for that purpose.

A state-wide survey in Oregon (USA) of trace metals and organic chemicals in municipal effluent

Oregon's Senate Bill 737, enacted in 2007, required the state's 52 largest municipal wastewater treatment plants (WWTP) and water pollution control facilities (WPCF) to collect effluent samples in 2010 and analyze them for persistent organic pollutants. These facilities are located state-wide and represent a variety of treatment types, service population sizes, geographic areas, and flow conditions. Of the 406 chemicals ultimately analyzed, 114 were detected above the level of quantification (LOQ) in at least one sample. Few persistent pollutants were found possibly because of their diversion from effluent via sorption to sludge (solids phase) or high LOQs for certain chemicals. Several pesticides, as well as benzene and phenol degradation products, all previously unreported in effluent, were detected. Ten polychlorinated biphenyls (PCB) congeners were present at low concentrations in ≤ 10 samples, while polychlorinated naphthalenes and dioxins/furans were not detected at all. Twenty-one polybrominated diphenyl ether (PBDE) congeners were found, nine of which have been reported in Osprey eggs in Oregon and Washington. Methylmercury was present in 65% of samples, with average and maximum concentrations of 0.18 and 1.36 ng/L, respectively. Although they are generally assumed to be innocuous by-products of sewage treatment, additional research is needed on potential impacts to aquatic ecosystems of high loadings of coprostanol and cholesterol. These results suggest that effluent, rather than just receiving waters, should itself be analyzed for a wide range of contaminants in order to understand how upstream sources, conveyed through WWTPs and WPCFs, could be impacting aquatic ecosystems.

Spatially explicit ecological exposure models: a rationale for and path toward their increased acceptance and use

Spatially explicit wildlife exposure models have been developed to integrate chemical concentrations dispersed in space and time, heterogeneous habitats of varying qualities, and foraging behaviors of wildlife to give more realistic wildlife exposure estimates for ecological risk assessments. These models not only improve the realism of wildlife exposure estimates, but also increase the efficiency of remedial planning. However, despite being widely available, these models are rarely used in baseline (definitive) ecological risk assessments. A lack of precedent for their use, misperceptions about models in general and spatial models in particular, non-specific or no enabling regulations, poor communication, and uncertainties regarding inputs are all impediments to greater use of such models. An expert workshop was convened as part of an Environmental Security Technology Certification Program Project to evaluate current applications for spatially explicit models and consider ways such models could bring increased realism to ecological exposure assessments. Specific actions (e.g., greater accessibility and innovation in model design, increased communication with and training opportunities for decision makers and regulators, explicit consideration during assessment planning and problem formulation) were discussed as mechanisms to increase the use of these valuable and innovative modeling tools. The intent of this workshop synopsis is to highlight for the ecological risk assessment community both the value and availability of a wide range of spatial models and to recommend specific actions that may help to increase their acceptance and use by ecological risk assessment practitioners.

Meeting the challenge of identifying persistent pollutants at the state level

In 2007, the State of Oregon enacted legislation aimed at identifying persistent pollutants that could pose a threat to waters of the State and then reducing their discharge by means of a comprehensive pollution prevention program. This legislation defined a persistent pollutant as one that is toxic and persistent or bioaccumulative; a broad definition that required evaluation of an extensive number and variety of chemicals. The Oregon Department of Environmental Quality, in consultation with a science workgroup, implemented a 12-step process for identifying and prioritizing persistent pollutants consistent with this definition. This process is characterized by (a) maximum overall transparency in its conduct, including extensive public involvement, (b) 3 levels of objective and predefined criteria for categorization of a chemical as a persistent pollutant, (c) full disclosure of values and sources for all physicochemical data used for comparison with these criteria, and (d) clear acknowledgement when a chemical was identified as a persistent pollutant for reasons other than these criteria alone. This process was used to identify those chemicals relevant as persistent pollutants and to then prioritize them in terms of their relative ability to adversely impact waters of the state, with special emphasis on impacts to aquatic receptors. An initial list of 2130 chemicals was compiled from existing lists. Criteria for toxicity, persistence, and bioaccumulative potential were defined and then used with 2 different chemical property evaluation models (PBT Profiler and EPISuite) to produce a final list of 118 chemicals. The final list includes several legacy pollutants but also contains numerous current-use pharmaceuticals, personal care products, and pesticides, approximately half of which appear only once or not at all on lists compiled by others. Although it drew from the experience of others, assembling this list proved to be an exemplar of science in the service of policy.

Carcinogenic risk as the basis for fish advisories: a critique

Fish advisories are important tools in public health practice and are primarily used to translate fish contaminant levels into consumption recommendations for consumers. Even when a targeted advisory is issued, it may alter broad food consumption patterns among the public, including diminishing intake of fish-based protein and polyunsaturated fatty acids. Such alterations may have both positive (e.g., reduced exposure to contaminants) and negative (e.g., loss of health benefits or cultural traditions associated with consuming fish) consequences. Currently, a fish advisory may be based on the potential for either noncarcinogenic or carcinogenic endpoints. Consumption recommendations based on a cancer outcome are likely to be highly restrictive, potentially diminishing opportunities for the recognized health benefits associated with a fish-rich diet. This possibility causes us to raise 3 arguments against using cancer risk as the basis for fish consumption advisories. First, the benefits of fish consumption are widely recognized. Second, the standard methodology to predict cancer risk is likely to overestimate actual risk, often by orders of magnitude. Third, the public's real and perceived concerns about cancer may result in unintended consequences, such as avoidance of fish altogether. As an alternative to cancer-based advisories, we suggest that future advisories incorporate a multidisciplinary public health framework focused on avoiding noncarcinogenic health outcomes and encouraging the public to consume a balanced diet rich in fish. We also suggest that decision makers need to 1) understand which elements of the advisory process are science and which are implicit or default policy, 2) consciously consider whether these policy elements are appropriate for their particular situation, and 3) if not, be willing to make and defend alternative policy choices.

Recommendations for the development and application of wildlife toxicity reference values

Toxicity reference values (TRVs) are essential in models used in the prediction of the potential for adverse impacts of environmental contaminants to avian and mammalian wildlife; however, issues in their derivation and application continue to result in inconsistent hazard and risk assessments that present a challenge to site managers and regulatory agencies. Currently, the available science does not support several common practices in TRV derivation and application. Key issues include inappropriate use of hazard quotients and the inability to define the probability of adverse outcomes. Other common problems include the continued use of no-observed- and lowest-observed-adverse-effect levels (NOAELs and LOAELs), the use of allometric scaling for interspecific extrapolation of chronic TRVs, inappropriate extrapolation across classes when data are limited, and extrapolation of chronic TRVs from acute data without scientific basis. Recommendations for future TRV derivation focus on using all available qualified toxicity data to include measures of variation associated with those data. This can be achieved by deriving effective dose (EDx)-based TRVs where x refers to an acceptable (as defined in a problem formulation) reduction in endpoint performance relative to the negative control instead of relying on NOAELs and LOAELs. Recommendations for moving past the use of hazard quotients and dealing with the uncertainty in the TRVs are also provided.

A model for the presence of polychlorinated biphenyls (PCBs) in the Willamette River Basin (Oregon)

The Willamette River drains a 32,000 km2 basin (Basin) in northwestern Oregon. Owing to their persistence and toxicity, polychlorinated biphenyls (PCBs) in resident fish within the Basin at levels above consumption advisory thresholds are a human and environmental health concern. This concern may trigger a Willamette River Total Maximum Daily Load (TMDL) for PCBs, at which time both their probable sources and the mechanism by which they came to be distributed throughout the Basin will be of considerable regulatory interest. Deposition within the Basin of some portion of global primary (1930-1980) and secondary (post-1980) emissions arriving via long-range advective transport in the atmosphere was posited as an explanation. This proposition was explored with a seasonally responsive, dynamic mass balance watershed-scale model that estimated concentrations of toxicologically relevant congeners (PCB-077, -118, -169) in various environmental media over a 90-year period, assuming advective inflow to the Basin's atmosphere to be the only PCB congener source. Model results suggest that rising air concentrations, and associated advective inflows, from increasing primary emissions between 1930 and 1975 (PCB-118 peak inflow, 1970, approximately equal to 11 kg y(-1)) and declining primary and secondary emissionsthereafter, could have yielded congener concentrations observed in air, soil, and fish between 1993 and 2003. The possibility that observed concentrations may be obtainable entirely with inputs from global legacy sources raises questions as to the efficacy of a TMDL directed primarily at local point or area sources. Better characterization of potential sources, and collection of additional soil and air data combined with more sophisticated modeling, appear to be necessary precursors to any PCB TMDL for the Willamette River.

Exposure assessment and risk of gastrointestinal illness among surfers

Surfing is a unique recreational activity with the possibility of elevated risk for contracting gastrointestinal (GI) illness through ingestion of contaminated water. No prior studies have assessed exposure from ingestion among surfing populations. This study estimated the magnitude and frequency of incidental water ingestion using a Web-based survey and integrated exposure distributions with enterococci distributions to predict the probability of GI illness at six Oregon beaches. The mean exposure magnitude and frequency were 170 ml of water ingested per day and 77 days spent surfing per year, respectively. The mean number of enterococci ingested ranged from approximately 11 to 86 colony-forming units (CFU) per day. Exposure-response analyses were conducted using an ingested dose model and two epidemiological models. Risk was characterized using joint probability curves (JPC). At the most contaminated beach, the annualized ingested dose model estimated a mean 9% probability of a 50% probability of GI illness, similar to the results of the first epidemiological model (mean 6% probability of a 50% probability of GI illness). The second epidemiological model predicted a 23% probability of exceeding an exposure equivalent to the U.S. Environmental Protection Agency (EPA) maximum acceptable GI illness rate (19 cases/1000 swimmers). While the annual risk of GI illness for Oregon surfers is not high, data showed that surfers ingest more water compared to swimmers and divers and need to be considered in regulatory and public health efforts, especially in more contaminated waters. Our approach to characterize risk among surfers is novel and informative to officials responsible for advisory programs. It also highlights the need for further research on microbial dose-response relationships to meet the needs of quantitative microbial risk assessments (QMRA).

Environmental management with knowledge of uncertainty: a methylmercury case study

In Oregon's Willamette River Basin, health advisories currently limit consumption of fish that have accumulated methylmercury to levels posing a potential health risk for humans. Under the Clean Water Act, these advisories represent an impairment of the beneficial use of fish consumption and create the requirement for a mercury total maximum daily load. A percent load reduction for total mercury was determined by comparing mercury levels in surface water to a water column guidance value linked to the protection of specified beneficial uses. In this case study, we discuss how probabilistic (Monte Carlo) methods were used to quantify uncertainty in the water column guidance value, how they provided decision makers with knowledge as to the probability of any given water column guidance value affording human health protection for methylmercury, and how this knowledge affected decisions as to a mercury load reduction for the Willamette River Basin. Through consultations with stakeholders, a water column guidance value of 0.92 ng/L (a median for higher trophic level fish) was chosen from among a suite of values of differing probabilities. The selected water column guidance value, when compared with ambient total mercury levels, indicated that a 50% probability of achieving the tissue criterion would require a load reduction of about 26%. Having and working with an explicit knowledge of uncertainty was not easy for many decision makers or stakeholders. However, such knowledge gave them more informed choices, a better understanding of what a specific choice of water column guidance value could mean in terms of achieving protectiveness, and led to a lower load reduction than suggested by a purely deterministic analysis. Nonetheless, more attention must be given to developing management, communication, and regulatory frameworks that can effectively use the greater knowledge of uncertainty afforded by probabilistic methods.

An examination of ecological risk assessment and management practices

Ecological risk assessment has grown and evolved since the 1980s, as have new challenges (e.g. global climate change, loss of habitat and biodiversity and the effects of multiple anthropogenic chemicals on ecological systems) that need to be factored into the risk assessment processes. There is also an on-going shift from evaluating adverse health impacts on particular, often small scale, environments to undertaking more complex ecological assessments of whole populations and communities across ecologically meaningful landscapes. These trends are generating an increased demand for much more complex ecological assessments, making it increasingly clear that to achieve its potential as a management tool, methods must be developed to apply ecological risk assessment to larger and more complex scales. This paper reviews the development of the ecological risk assessment paradigm in the United States, identifies ways it is being applied and adapted in other countries, explores future research needs and practice improvements, and examines current issues that need to be considered in taking forward the scientific development of ecological risk assessment as a useful environmental management tool.

An assessment of anthropogenic source impacts on mercury cycling in the Willamette Basin, Oregon, USA

In Oregon's Willamette River Basin (Basin), methylmercury levels in fish triggered health advisories and required development of a mercury Total Maximum Daily Load (TMDL) for the Willamette River. A seasonally-responsive dynamic systems model is used to identify the principal sources of natural and anthropogenic mercury, the relative contributions of these sources to the river, the impact of hypothetical reductions in specific natural and anthropogenic sources on mercury levels in surface water, sediment, and fish tissue, and the degree to which any such changes would be clearly discernible to environmental managers and Basin stakeholders. Two scenarios are modeled: "PRES", which considered all currently known natural and anthropogenic mercury sources and "LEEM", which (hypothetically) eliminated all local, but not global, anthropogenic sources and greatly lowered native soil erosion rates. Elimination of local air emissions reduces runoff of air-deposited mercury by approximately 34% and advection from the Basin by approximately 12%, while lowering erosion rates reduces particulate runoff by approximately 57%, deposition from the water column to surficial sediment by approximately 33%, and fluvial load by approximately 24%; for a net reduction of 25.6% in the total mercury load to the river. Such hypothetical reductions bring methylmercury concentrations in predatory fish to levels that would allow restoration of fish consumption as a beneficial use. However, several factors, primarily technical feasibility and global sources, may impede attempts to attain this beneficial use. Actualizing the hypothetical 100% elimination of local anthropogenic sources and a >50% reduction in erosion could pose significant technical challenges. Because local anthropogenic emissions make relatively smaller contributions to the Basin than do persistent global sources (sources over which there is little, if any, possibility of local control), localized environmental management actions alone may not be adequate to address mercury impacts within the Basin.

Mercury levels and relationships in water, sediment, and fish tissue in the Willamette Basin, Oregon

In Oregon's Willamette River Basin (the Basin), health advisories currently limit consumption of fish that have accumulated methylmercury (MeHg) to levels posing a significant human health risk. These advisories created the requirement for a mercury total maximum daily load for the Basin, which required a greater understanding of the behavior, distribution, and levels of mercury and MeHg in the Basin. In 2002, the Oregon Department of Environmental Quality initiated a study to measure (using ultraclean techniques) mercury and MeHg levels in water, sediment, and fish samples collected throughout the Basin. Results from the Middle Fork (nominal background) suggested that naturally occurring surface-water concentrations of mercury and MeHg would on an annual average basis be expected in the range of 0.5 to 1.0 and 0.04 to 0.06 ng L(-1), respectively. Concentrations in the Coast Fork (Cottage Grove), which were markedly higher, are likely the result of historical mining discharges. The possibility exists that wetlands alone could contribute the dissolved MeHg levels (approximately 0.04 ng L(-1)) observed in the Main Stem. Mercury levels in sediment were similar, and near background, in the Main Stem, Coast Fork (Row River), and Middle Fork but significantly increased in the Coast Fork (Cottage Grove). Fish tissue mercury levels were typically highest in piscivorous and lowest in invertivorous species but highest in the Coast Fork (Cottage Grove). In the Coast Fork and Cottage Grove Reservoir, discharges from historical mercury mining activities appear to have significantly impacted water, sediment, and fish tissue levels; however these impacts do not appear to extend into the Main Stem. Basinwide mercury data are at present too spottily distributed to determine whether significant mercury point sources exist along the Main Stem.

An overview of the Salmonella enteritidis risk assessment for shell eggs and egg products

This article summarizes a quantitative microbial risk assessment designed to characterize the public health impact of consumption of shell eggs and egg products contaminated with Salmonella Enteritidis (SE). This risk assessment's objectives were to: (1) establish the baseline risk of foodborne illness from SE, (2) identify and evaluate potential risk mitigation strategies, and (3) identify data gaps related to future research efforts. The risk assessment model has five modules. The Egg Production module estimates the number of eggs produced that are SE-contaminated. Shell Egg Processing, Egg Products Processing, and Preparation & Consumption modules estimate the increase or decrease in the numbers of SE organisms in eggs or egg products as they pass through storage, transportation, processing, and preparation. A Public Health Outcomes module then calculates the incidence of illnesses and four clinical outcomes, as well as the cases of reactive arthritis associated with SE infection following consumption. The baseline model estimates an average production of 2.3 million SE-contaminated shell eggs/year of the estimated 69 billion produced annually and predicts an average of 661,633, human illnesses per year from consumption of these eggs. The model estimates approximately 94% of these cases recover without medical care, 5% visit a physician, an additional 0.5% are hospitalized, and 0.05% result in death. The contribution of SE from commercially pasteurized egg products was estimated to be negligible. Five mitigation scenarios were selected for comparison of their individual and combined effects on the number of human illnesses. Results suggest that mitigation in only one segment of the farm-to-table continuum will be less effective than several applied in different segments. Key data gaps and areas for future research include the epidemiology of SE on farms, the bacteriology of SE in eggs, human behavior in food handling and preparation, and human responses to SE exposure.

A case study comparing static and spatially explicit ecological exposure analysis methods

Exposure to chemical contaminants must be estimated when performing ecological risk assessments. A previous article proposed a habitat area and quality conditioned population exposure estimator, E[HQ]P, and described an individual-based, random walk, Monte Carlo model (SE3M) to facilitate calculation of E[HQ]P. In this article, E[HQ]P was compared with exposure estimates from a baseline risk assessment that evaluated mink and great blue heron exposure to fluoride at a federal Superfund site. Calculation of E[HQ]P took into consideration a receptor's forage area, movement behavior, population size, and the areal extent and quality of suitable habitat. The baseline assessment used four methods that did (total and unit Tier 2) and did not (total and unit Tier 1) consider habitat area or quality; where "total" included all exposure units on site and "unit" only a given exposure unit. Total Tier 1 estimates were consistently higher than E[HQ]P (e.g., 169.1 mg/kg x d versus 21.6 mg/kg x d). Risk managers using total Tier 1 results for decision making would be unlikely to underestimate exposure; however, implementability of correspondingly lower remedial objectives could be challenging. Unit Tier 1 estimates were higher (e.g., 96.5 mg/kg x d versus 61.6 mg/kg x d) or lower (e.g., 3.5 mg/kg x d versus 51.1 mg/kg x d) than E[HQ]P depending on variations in landscape features. Total Tier 2 and E[HQ]P estimates were similar (e.g., 20.7 mg/kg x d versus 21.6 mg/kg x d) when an ecologically questionable average exposure was assumed. Unit Tier 2 estimates were consistently well below E[HQ]P (e.g., 17.8 mg/kg x d versus 61.6 mg/kg x d) when an average exposure was not assumed. Risk managers using unit Tier 1 or 2 results could be basing their decisions on potentially large underestimates of exposure. By forgoing average exposure assumptions, and explicitly addressing landscape heterogeneity, SE3M appears capable of yielding exposure estimates that are not as potentially misleading to risk managers as those produced with traditional averaging methods.

A spatially and bioenergetically explicit terrestrial ecological exposure model

In typical exposure models, dose is a function of ingestion rate, which is a function of field metabolic rate and food energy availability. It is implicitly assumed that neither food energy nor ingestion rate is limited. This is unlikely to be true in the field. Poor habitat quality (expressed as limited or lacking food energy) or a physiologically limited maximum ingestion rate may collectively limit energy intake. A receptor may thus be as much at risk from lack of energy as from toxicant effects. To explore this possibility, an existing spatially explicit exposure model (SE3M) was enhanced to: 1) express 'habitat quality' in terms of gross energy available from a suite of habitat-specific food types, 2) track fulfillment of a receptor's daily energy needs as it traverses habitat patches with varying gross energy levels, 3) link intake of contaminants to food consumed to meet daily energy needs, and 4) track contaminant doses and resulting tissue residue levels as a receptor moves through habitat patches with differing levels of contamination. A feedback term through which chemical stressors affect a receptor's ability to intake and process energy was not considered at this time. The now spatially and energetically explicit exposure model, SE(4)M, provides a platform for exploring spatial and bioenergetic factors that may influence a receptor's acquisition of energy and contaminant tissue residues as it moves through space and time. An application of this model would be to provide predictions of tissue residue levels that are accessible to calibration or validation with empirical field data.

Generating probabilistic spatially-explicit individual and population exposure estimates for ecological risk assessments

Exposure to chemical contaminants in various media must be estimated when performing ecological risk assessments. Exposure estimates are often based on the 95th-percentile upper confidence limit on the mean concentration of all samples, calculated without regard to critical ecological and spatial information about the relative relationship of receptors, their habitats, and contaminants. This practice produces exposure estimates that are potentially unrepresentative of the ecology of the receptor. This article proposes a habitat area and quality-conditioned exposure estimator, E[HQ], that requires consideration of these relationships. It describes a spatially explicit ecological exposure model to facilitate calculation of E[HQ]. The model provides (1) a flexible platform for investigating the effect of changes in habitat area, habitat quality, foraging area, and population size on exposure estimates, and (2) a tool for calculating E[HQ] for use in actual risk assessments. The inner loop of a Visual Basic program randomly walks a receptor over a multicelled landscape--each cell of which contains values for cell area, habitat area, habitat quality, and concentration--accumulating an exposure estimate until the total area foraged is less than or equal to a given foraging area. An outer loop then steps through foraging areas of increasing size. This program is iterated by Monte Carlo software, with the number of iterations representing the population size. Results indicate that (1) any single estimator may over- or underestimate exposure, depending on foraging strategy and spatial relationships of habitat and contamination, and (2) changes in exposure estimates in response to changes in foraging and habitat area are not linear.

A Procedure for Performing Population-Level Ecological Risk Assessments

/ In 1997, Oregon enacted amendments to its state hazardous waste site cleanup law which emphasize risk-based remedial action decisions. In a departure from US EPA practice, the amended statute and associated rules require that protection of ecological receptors occur at the population level for all plants and animals not listed as threatened or endangered. By rule, the acceptable risklevel for populations of ecological receptors is a 10% or less chance that 20% or more of the total local population would receive an exposure greater than the toxicity reference value for a hazardous substance. This paper describes a practical procedure for performing population-level ecological risk assessments using a combination of relatively simple techniques. The procedure involves: (1) establishing a distribution of exposures and a contaminant-specific toxicity reference value, either as a point value or a distribution, for an individual receptor, (2) estimating the abundance of these receptors within their local populations, (3) estimating the probability of an individual receptor experiencing an exposure in excess of the toxicity reference value, (4) estimating the number of individual receptors in the local population likely to experience an exposure above the toxicity reference value greater than 10% of the time, and (5) determining whether this number is greater than 20% of the total local population.

A global biogeochemical budget for vanadium

Evidence that environmental levels of vanadium are increasing has raised concern over the injection of vanadium into the atmosphere from anthropogenic sources. A simple global mass balance model was developed to demonstrate the influence of anthropogenic vanadium on the global distribution of this trace metal. Vanadium in particulate emissions owing to man's industrial activities were estimated to comprise approximately 53% of total atmosphere vanadium loading and exceeded natural continental or volcanogenic dust by only a narrow margin. Oceanic deposition of vanadium adhering to anthropogenic particles was estimated to comprise approximately 5% of total ocean vanadium loading. There is no suggestion that these inputs of anthropogenic vanadium pose a significant global environmental threat. It is entirely possible, however, that anthropogenic vanadium inputs could pose an environmental hazard given a more restricted area and a specific set of unfavorable circumstances.