Derivation of Risk Management Criteria for Chemicals of Unknown Toxic Potency at Contaminated Sites

Environmental investigations of former industrial sites often detect the presence of chemicals for which no soil criteria exist and for which regulatory agencies have not derived estimates of toxic potency. This poses a considerable problem for making informed risk management decisions involving sites where such chemicals are present. As a result, a methodology has been developed for making risk-based decisions for chemicals of unknown toxic potency in soil at contaminated sites. The method is based on principles and procedures used by the US Food and Drug Administration (USFDA), the US Environmental Protection Agency (USEPA) and the Canadian Council of Ministers of the Environment (CCME). After analyzing the data on hundreds of carcinogenic and non-carcinogenic substances, the USFDA and other leading researchers have concluded that, if no toxicological data is available on a chemical, exposures less than 1.5?µg/person/day (i.e., 0.02?µg/kg body weight/day) are unlikely to result in appreciable health risks even if the substance was later found to be a carcinogen. To develop maximum soil concentrations that will be protective of human health (i.e., Risk Management Criteria or RMC), the above exposure limit of 0.02?µg/kg body weight/day has been assumed to be protective of risks from exposure to chemicals lacking toxicological data. Using a stochastic risk assessment model for estimating exposures to chemicals from contaminated sites, our analyses indicate that a soil concentration of 2?µg/g would be protective of human health for land uses that include residential, commercial, and industrial development provided no major indirect pathways exist at the site. If indirect pathways exists (e.g., vapor infiltration of soil gases, uptake of chemicals into garden produce, etc.), alternate RMC could be developed, that include such indirect pathways, using the methodology provided in this paper. Used by experienced risk assessors, the approach is a scientifically defensible screening method that will preclude many chemicals from unnecessary evaluation, while allowing risk assessors to focus efforts on chemicals of greater concern and make informed risk management decisions.     

A Deterministic Method for Deriving Site-Specific Human Health Assessment Criteria for Contaminants in Soil

A method for deriving Site-Specific Assessment Criteria (SSAC) for use when considering risk to human health from chronic exposure to heavy metals (except lead), metalloids, and organic substances in soil, with application to the United Kingdom (UK), is described. The SSAC represents the soil concentration above which an unacceptable risk to human health may be indicated.

The method considers the UK standard land uses (residential with and without plant uptake, allotment gardens, and commercial/industrial) by applying the default exposure factors and algorithms provided. Non-standard land uses can be also considered. Site-specific determinations of contaminant bioaccessibility or of plant-to-soil concentration factors may be used if available.

The method adopts the risk-based source-pathway-receptor pollutant linkage framework and a deterministic methodology. Exposure pathways considered are direct ingestion of soil and dust, consumption of home grown or allotment vegetables, ingestion of soil attached to such vegetables, inhalation of soil vapors outdoors and inhalation of soil vapors indoors. A test for the significance of the dermal pathway is also included.

It is not intended that the method be used to generate or replace UK Soil Guideline Values, because this can only be done by the appropriate authoritative bodies within the UK (Department of the Environment, Food and Rural Affairs and the Environment Agency).     

Benzene Degradation Coupled with Chlorate Reduction in a Soil Column Study

Perchlorate and chlorate are electron acceptors that during reduction result in the formation of molecular oxygen. The produced oxygen can be used for activation of anaerobic persistent pollutants, like benzene. In this study chlorate was tested as potential electron acceptor to stimulate benzene degradation in anoxic polluted soil column. A chlorate amended benzene polluted soil column was operated over a period of 500 days. Benzene was immediately degraded in the column after start up, and benzene removal recovered completely after omission of chlorate or a too high influent chlorate concentration (22 mM). Mass balance calculations showed that per mole of benzene five mole of chlorate were reduced. At the end of the experiment higher loading rates were applied to measure the maximal benzene degradation rate in this system; a breakthrough of benzene was not observed. The average benzene degradation rate over this period was 31 μmol l⁻¹ h⁻¹ with a maximal of 78 μmol l⁻¹ h⁻¹. The high degradation rate and the necessity of chlorate indicate that oxygen produced during chlorate reduction indeed is used for the activation of benzene. This is the first column study where benzene biodegradation at a high rate coupled with anaerobic chlorate reduction is observed.     

Prevalence of Soil Mouthing/Ingestion among Healthy Children Aged 1 to 6

Ingestion of non-food items/mouthing behavior results in exposure of children to contaminants in soil/dust. We characterize the prevalence of such behaviors in healthy children. The relative frequency of such behaviors was assessed by parent interviews for 533 children age 1 to 6. Thirty-eight percent of children put soil in their mouths at least monthly, 24% at least weekly, and 11% daily. High-risk behavior decreased quickly for children aged 2 or more, but was still reported at least monthly by 3 to 9% of parents of children up through age 6. Highest outdoor object mouthing rates occur among 1-year-old children, who are reported to play daily in sand/dirt and have generally high levels of mouthing. Such children may have higher soil/dust ingestion and higher exposure to contaminants when soil/dust contains lead or other agents. These high-risk groups may help focus educational interventions and/or risk assessments.     

Monte Carlo Vadose Zone Model for Soil Remedial Criteria

Many vadose zone models are available for environmental remediation, but few offer the procedures for verifying model predictions with field data and for dealing with uncertainties associated with model input parameters. This article presents a modified model combining a one-dimensional vadose-zone transport model and a simple groundwater mixing model with a function of Monte Carlo simulation (MCS). The modified model is applied to determine soil remedial concentrations for methyl tertiary butyl ether (MTBE). The modified model generates a distribution of MTBE ground-water concentrations at the point of compliance. This distribution can be used to estimate the risk of exceeding groundwater quality standard given soil remedial concentrations. In a case study, soil remedial concentration for MTBE is established to be 5?µg/kg, with a 95% and 10?µg/kg with a 50% probability that groundwater concentration will not exceed the water quality objective of 13?µg/L. Furthermore, this study uses MCS to investigate uncertainties of model input parameter hydraulic conductivity (K). One set of data (K1) is based on the results of hydraulic conductivity laboratory tests, and the other (K2) is based on the results of slug tests conducted in the field. As expected, the laboratory data show smaller K values than the field data. The comparison of the MCS results obtained from the two sets of K data indicates that the MTBE groundwater concentrations calculated based on K1 are generally 160 to 625% greater than those calculated based on K2 at the same percentiles of the MCS distribution. A higher soil remedial concentration of9jig/kg is then calculated based on the MCS results from K2 at 95%ile and 19?µg/kg at 50%ile.     

Investigation of Dermal Contact with Soil in Controlled Trials

A series of laboratory, greenhouse, and field experiments have been conducted at the University of Washington to improve the empirical grounding of dermal/soil pathway risk calculations. This article presents results from controlled trials, conducted in a greenhouse, in which volunteers engaged in activities in soil amended with a fluorescent marker. Activities included transplanting of bedding plants and laying of pipe by adults and children's play. Soil contact on hands, forearms, lower legs, and faces was examined using both fluoro-metric and gravimetric measurements. Results provide information on pre- and postactivity loadings, the extent of contact associated with the selected activities, and the effects of clothing, activity duration, and soil moisture. Preactivity loadings were consistent with previously reported observations. Following activity, skin coverage was found to be substantially incomplete except on hands. Local soil loadings may therefore deviate markedly from mean values obtained by washing skin surfaces. A protective effect of clothing was observed but may reflect the relatively short duration of the experiments and the use of freshly laundered clothing. No significant trend in soil loading with exposure duration was found, although some evidence of increasing surface area involvement with time was observed. Finally, wet soil activities produced consistently higher loadings on volunteers' hands than dry soil activities.     

Toxicological Considerations of Contaminants in the Terrestrial Environment for Ecological Risk Assessment

The terrestrial environment acts as a “sink” for contaminants that have been purposely or accidentally released into the environment. Science and policy that support protective measures for terrestrial ecosystems have run behind those of aquatic toxicology and water quality concerns. As a result ecological risk assessment (ERA) involving terrestrial environments tends to be conducted at a simplistic level, relying on numeric targets (soil quality criteria) as a basis for decision-making. However, soil criteria for ecological receptors are somewhat deficient in terms of the numbers available and the data that supports these numbers. Direct toxicity assessments (DTA) for terrestrial environments, such as those used for water quality evaluations, can provide additional useful information about the toxicity and bioavailability of mixtures of contaminants present in soils. This article outlines the approaches used for assessing the toxicity of soil contaminants in terrestrial environments and critiques their advantages and pitfalls.     

Limits in soil ingestion data: The potential for imputing data when soil ingestion estimates are below the detection limit

The formation of a soil ingestion distribution based on pooling data from current soil ingestion studies is appealing. An important issue in forming such a distribution is what to do with negative soil ingestion estimates for particular subjects, because they comprise approximately 10 to 40% of the total soil ingestion estimates. A method of correcting for the negative estimates of soil ingestion is to make use of the “soil ingestion detection limit”;. An appropriate methodology for forming estimates of such detection limits is available in the literature. This paper discusses appropriate use of the existing soil ingestion detection limit methodology in forming a pooled database using current soil ingestion study data. The discussion focuses attention on the current limitations of children's soil ingestion data and potential pitfalls in applying the detection limit model when generating a soil ingestion distribution. In summary, currently available soil ingestion data are not sufficiently reliable to impute individual soil ingestion estimates below the detection limit. Research directed toward identifying and quantifying individual error in soil ingestion estimates is needed to overcome this limitation.     

Issues in Exposure and Dose Assessment for Epidemiology and Risk Assessment

Epidemiologic studies have been effective in identifying human environmental and occupational hazards. However, most epidemiologic data has been difficult to use in quantitative risk assessments because of the vague specification of exposure and dose. Toxicologic animal studies have used applied doses (quantities administered, or exposures with fixed duration) and well characterized end points to determine effects. However, direct use of animal data in human risk assessment has been limited by uncertainties in the extrapolation. The applied dose paradigm of toxicology is not suited for cross species extrapolation, nor for use in epidemiology as a dose metric because of the complexity of human exposures. Physiologically based pharmacokinetic (PBPK) modeling can estimate the time course of tissue concentrations in humans, given an exposure-time profile, and it has been used for extrapolating findings from animals to humans. It is proposed that human PBPK modeling can be used in appropriately designed epidemiologic studies to estimate tissue concentrations. Secondly, tissue time courses can be used to form dose metrics based on the type and time course of adverse effects. These dose metrics will strengthen the determination of epidemiologic dose-response relationships by reducing misclassification. Findings from this approach can be readily integrated into quantitative risk assessment.     

A modeling procedure to predict cancer risks associated with inhalable oil dust from Kuwait oil lakes

A model was constructed to estimate cancer risks associated with PM₁₀‐bound polycydic aromatic hydrocarbons (PAHs) from Kuwait oil lakes. The design of the risk model was based on a conceptual “chain of events”; leading from levels of PAH compounds in oil lakes, erosion of oil dust and input into the atmosphere, to contaminant concentration in air, to actual human exposure in residential areas. Uncertainties in the “chain of events”; model were addressed using Monte Carlo techniques. To identify the exposure duration of concern [duration beyond which risk becomes unacceptable (i.e. Risk > 10^‐6)], four exposure durations were tested 10, 20, 40, 70 years. 40 years was identified to be the exposure duration of concern based on the 95th percen‐tile of the risk distribution. As a result, the acceptability of risk was specified in terms of a single constraint on the 95^th percentile of the risk distribution evaluated after 40 years of exposure: 0 < Risk (40 y)₀.₉₅ ≤ 10^‐6. Based on this constraint, it was estimated that a removal rate of 217, 793.27 m³/year to be an adequate action for risk management. The northern oil lakes were identified as the lakes of most concern when inhalation exposures are considered.     

Finding A Niche for Soil Microbial Toxicity Tests in Ecological Risk Assessment

Soil microbial toxicity tests are seldom used in ecological risk assessments or in the development of regulatory criteria in the U.S. The primary reason is the lack of an explicit connection between these tests and assessment end-points. Soil microorganisms have three potential roles with respect to ecological assessment endpoints: properties of microbial communities may be end-points; microbial responses may be used to estimate effects on plant production; and microbial responses may be used as surrogates for responses of higher organisms. Rates of microbial processes are important to ecosystem function, and thus should be valued by regulatory agencies. However, the definition of the microbial assessment endpoint is often an impediment to its use in risk assessment. Decreases in rates are not always undesirable. Processes in a nutrient cycle are particularly difficult to define as endpoints, because what constitutes an adverse effect on a process is dependent on the rates of others. Microbial tests may be used as evidence in an assessment of plant production, but the dependence of plants on microbial processes is rarely considered. As assessment endpoints are better defined in the future, microbial ecologists and toxicologists should be provided with more direction for developing appropriate microbial tests.     

A Quick,Onsite Test for Delineating Arsenic Contaminant Plumes in Soil

Dipping vats were used routinely in the southeastern U.S. in the early 1900s to eradicate the cattle fever tick. The legacy is many dip vat sites with arsenic (As)-contaminated soil and ground water. Assessing the extent of these As plumes can be time consuming and expensive. We describe a quicker and less expensive, onsite test for soil As. It is a modification of a commercially available test designed for As in water, taking about 10?min to complete, allowing large plume areas to be delineated in a single day. An As contaminant plume in Alachua County, FL, was delineated using the quick test. Soil samples taken from a large grid encompassing the plume were analyzed in the laboratory for As to confirm the results obtained with the quick test. The comparison showed that the quick test very accurately delineated the outer boundary of the plume as well as zones of higher As concentration within the plume.     

Bioaccessibility and other key parameters in assessing oral exposure to PAH-contaminated soils and dust: A critical review

Soil ingestion is an important pathway for human exposure to polycyclic aromatic hydrocarbon (PAH)-contaminated soils and dust for children (via ingesting hand residue) as well as for adults (via occupational exposure). An appropriate selection of exposure parameter values is essential for having an accurate risk assessment. This review addresses key parameters for estimating oral exposure to PAH-contaminated soils/dust, discusses their variability and uncertainty, and provides recommendations for value selection. Bioaccessibility (contaminant fraction solubilized in gastro-intestinal tract, available for entering bloodstream and reaching target organs) and soil ingestion rate are two key parameters for exposure assessment (usually characterized by large variability and/or uncertainty), followed by exposure frequency/duration and body weight.     

Probability and Persistence of High Pollutant Concentrations in Soils: A Modeling Study and Implications for Exposure and Risk Assessment

Risk assessments for environmental pollutants have relied upon steady-state models that do not represent the variability of pollutant transport and fate processes, thus predictions are unlikely to reflect the true variability in pollutant concentrations. Such models cannot be used to estimate the probability, magnitude and duration of short- to intermediate-term and high-concentration events that might lead to adverse acute impacts. In this study, a numerical model is used to simulate pollutant accumulation in surface soils at six U.S. locations that result from atmospheric deposition and leaching. Historical (50 year) precipitation data drive the model. Model predictions are filtered and analyzed to identify high pollution events (exceeding specific concentration thresholds) and their occurrence probability and duration. Predicted concentrations at each site varied by a factor of 100 over time and by a factor of five among the six locations. The frequency and duration of high pollution events also differed by locations and concentration threshold. In general, larger thresholds lead to less frequent events and shorter durations. The proposed method allows estimates of the probability of occurrence and duration of high pollution events, providing information that complements the steady-state methods.     

Physiological Daily Inhalation Rates for Free-Living Individuals Aged 1 Month to 96 Years,Using Data from Doubly Labeled Water Measurements: A Proposal for Air Quality Criteria,Standard Calculations and Health Risk Assessment

Reported disappearance rates of oral doses of doubly labeled water (²H₂O and H₂ ¹⁸O) in urine, monitored by gas-isotope-ratio mass spectrometry for an aggregate period of over 30,000 days and completed with indirect calorimetry and nutritional balance measurements, have been used to determine physiological daily inhalation rates for 2210 individuals aged 3 weeks to 96 years. Rates in m³/kg-day for healthy normal-weight individuals (n = 1252) were higher by 6 to 21% compared to their overweight/obese counterparts (n = 679). Rates for healthy normal-weight males and females drop by about 66 to 75% within the course of a lifetime. Infants and children between the age of 3 weeks to less than 7 years inhale 1.6 to 4.3 times more air (0.395 ± 0.048 to 0.739 ± 0.071 m³/kg-day, mean ± S.D., n = 581) than adults aged 23 to 96 years (0.172 ± 0.037 to 0.247 ± 0.039 m³/kg-day, n = 388). The 99th percentile rate of 0.725 m³/kg-day based on measurements for boys aged 2.6 to less than 6 months is recommended for air quality criteria and standard calculation for non-carcinogenic compounds pertaining to individuals of any age or gender (normality confirmed using the Shapiro-Wilk test, p ≥ 0.05). This rate is 2.5-fold more protective than the daily inhalation estimate of 0.286 m³/kg-day published by the Federal Register in 1980 (i.e., 20 m³/day for a 70-kg adult). It ensures that very few newborns aged 1 month and younger, less than 1% of infants aged 2.6 to less than 6 months and of course no older individuals up to 96 years of age inhale more toxic chemicals than associated safe doses which are not anticipated to result in any adverse effects in humans, when air concentration reaches the resulting air quality criteria and standard values. This rate is also protective for underweight, overweight, and obese individuals. Finally, as far as newborns are concerned, a rate of 0.956 m³/kg-day based on the 99th percentile estimates is recommended for short-term criteria and standard calculations for toxic chemicals that yield adverse effects over instantaneous to short-term duration.     

Ecological Risk Assessment of Radiological Exposure to Depleted Uranium in Soils at a Weapons Testing Facility

The potential for unacceptable risks to biota from radiological exposure to depleted uranium (DU) in soils was evaluated at two sites where DU weapons testing had been conducted in the past. A screening risk assessment was conducted to determine if measured concentrations of DU-associated radionuclides in site soils exceed radionuclide levels considered protective of biota. While concentrations of individual radionuclides did not exceed acceptable levels, total radionuclide concentrations could result in potentially unacceptable doses to exposed biota. Thus, a receptor-specific assessment was conducted to estimate external and internal radiological doses to vegetation and wildlife known or expected to occur at the sites. Wildlife evaluated included herbivores, omnivores, and top-level predators. Internal dose estimates to wildlife considered exposure via fugitive dust inhalation and soil and food ingestion; root uptake was the primary exposure route evaluated for vegetation. Total doses were compared with acceptable dose levels of 1.0 and 0.1 rad/day for vegetation and wildlife, respectively, with potentially unacceptable risks indicated for doses exceeding these levels. All estimated doses were below or approximated acceptable levels, typically by an order of magnitude or more. These results indicate that current levels of DU in soils do not pose unacceptable radiological risks to biota at the sites evaluated.     

Ecotoxicity Test Data for Total Petroleum Hydrocarbons in Soil: Plants and Soil-Dwelling Invertebrates

Ecotoxicity benchmarks for petroleum mixtures can be used in a screening-level ecological risk assessment. Data from studies evaluating the toxicity of total petroleum hydrocarbons (TPH) to plants and soil invertebrates were reviewed for possible application to soil benchmark development. Toxicity data included LOAECs; estimated EC25s, EC20s, and LC50s; effective concentrations that caused greater than a 20% level of effect; and NOAECs. The variabilities in petroleum material, chemical analytical methodology, age of hydrocarbon-soil contact, nutrient amendment, and measured effects levels did not permit much meaningful aggregation of the data. Tenth, twenty-fifth, and fiftieth percentiles of toxicity and no-effects data are presented for unaggregated results within studies. Effects on invertebrates often occurred at concentrations of TPH lower than those associated with effects on plants. Lighter mixtures generally were associated with lower ranges of effects concentrations than heavier crude oil. Few aged and non-aged samples were available from the same study, and these did not show obvious trends regarding toxicity. Similarly, the addition of nutrients to promote bioremediation was not observed across studies to alter effective or nontoxic concentrations in a systematic way. Existing toxicity data are not sufficient to establish broadly applicable TPH ecotoxicity screening benchmarks with much confidence, even for specific mixtures.     

A Scaling Rule for Landscape Patches and How It Applies to Conserving Soil Resources in Savannas

Scaling issues are complex, yet understanding issues such as scale dependencies in ecological patterns and processes is usually critical if we are to make sense of ecological data and if we want to predict how land management options, for example, are constrained by scale. In this article, we develop the beginnings of a way to approach the complexity of scaling issues. Our approach is rooted in scaling functions, which integrate the scale dependency of patterns and processes in landscapes with the ways that organisms scale their responses to these patterns and processes. We propose that such functions may have sufficient generality that we can develop scaling rules—statements that link scale with consequences for certain phenomena in certain systems. As an example, we propose that in savanna ecosystems, there is a consistent relationship between the size of vegetation patches in the landscape and the degree to which critical resources, such as soil nutrients or water, become concentrated in these patches. In this case, the features of the scaling functions that underlie this rule have to do with physical processes, such as surface water flow and material redistribution, and the ways that patches of plants physically “capture” such runoff and convert it into plant biomass, thereby concentrating resources and increasing patch size. To be operationally useful, such scaling rules must be expressed in ways that can generate predictions. We developed a scaling equation that can be used to evaluate the potential impacts of different disturbances on vegetation patches and on how soils and their nutrients are conserved within Australian savanna landscapes. We illustrate that for a 10-km² paddock, given an equivalent area of impact, the thinning of large tree islands potentially can cause a far greater loss of soil nitrogen (21 metric tons) than grazing out small grass clumps (2 metric tons). Although our example is hypothetical, we believe that addressing scaling problems by first conceptualizing scaling functions, then proposing scaling rules, and then deriving scaling equations is a useful approach. Scaling equations can be used in simulation models, or (as we have done) in simple hypothetical scenarios, to collapse the complexity of scaling issues into a manageable framework. Received 8 December 1998; accepted 17 August 1999.     

A Model to Assess Restoration of Abandoned Pasture in Costa Rica Based on Soil Hydrologic Features and Forest Structure

An ecological functional assessment (EFA) was used on 10 southwest Costa Rica sites representing a chronosequence of formerly pastured lands to undisturbed tropical wet forest. Ecological functional assessment is a tool designed to assess wetland functions in the United States that was adapted to upland forests. Models to indicate characteristic soil hydrologic features and soil structure and aboveground spatial structure of habitat were used to examine the degree to which selected sites within the chronosequence approach the undisturbed condition of the natural forest. An index of the functional model for the maintenance of characteristic soil hydrologic features (such as infiltration, bulk density, etc.) showed that the 20‐year‐old secondary forest was at approximately 60% of the condition of the undisturbed sites, whereas active pasture was evaluated at approximately 20% of the reference undisturbed forest; 4‐ and 10‐year‐old sites were intermediate. The spatial structure of habitat model showed that 20‐year‐old secondary forest was approximately 50% of reference forest, whereas active pasture was approximately 10% of the condition of undisturbed forest; 4‐year‐old sites were evaluated at approximately 20% and 10‐year‐old sites at approximately 60% of the reference state. Overall the functional assessment process indicated that degraded tropical wet forest sites have recovered almost 60% of their functional qualities 10 years following pasture abandonment. These results indicate that EFA can be a useful technique for monitoring restoration programs in the tropics.     

Further Comparisons of Empirical and Epidemiological Data with Predictions of the Integrated Exposure Uptake Biokinetic Model for Lead in Children

Observed blood lead levels for young children from several communities are compared with blood lead levels predicted for those communities using the USEPA's Integrated Exposure Uptake Biokinetic (IEUBK) Model. In contrast to the compari sons described elsewhere, the blood lead levels observed in the communities con sidered here are not well represented by the model's predictions. The model's predictions for Midvale, UT; Sandy, UT; Cincinnati, OH; and a recent data set for Palmerton, PA, show considerable deviation from observation both for the geometric mean blood lead level and the percent of blood lead levels above 10?µg/dL. Various adjustments in the model to consider play area soils, site specific geometric standard deviations and the time children spend away from their homes do not substantially improve the comparisons to observation. It is difficult to predict a priori the data sets for which the model will yield adequate predictions. This reduces the value of the model for use in communities where blood lead measurements have not been made, and suggests that caution should be exercised when using the model to set soil lead cleanup levels or to predict the result of remediation.