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.     

A Guide to Interpreting Soil Ingestion Studies. 2. Qualitative and Quantitative Evidence of Soil Ingestion

Abstract

Four major studies have attempted to qualitatively and quantitatively assess the extent of soil ingestion in children using the soil tracer methodology. The validity of the estimates of soil ingestion of each study was reevaluated in light of the inherent strengths and limitations of study design and/or execution as well as via a novel methodology to estimate the soil recovery variance of each tracer which then lead to the estimation of soil ingestion detection limits of each tracer for studies performing mass-balance analyses. Based on these analyses it is concluded that the Binder et al. (1986) and Van Wijnen et al. (1990) studies provide no convincing evidence to support qualitative and quantitative estimates of soil ingestion due to inherent limitations of their respective study designs. The Davis et al. (1990) and Calabrese et al. (1989) studies displayed convincing qualitative evidence of soil ingestion. However, the results indicate that the median soil ingestion estimates of Davis et al. were less reliable than those of Calabrese et al. The range of detection limits vary according to the tracer and the assumption of acceptable precision in recovery estimation. The minimum detection level of soil ingestion in children in the Calabrese et al. study with a variance in recovery of 100% ± 20% was 16 mg day^?1 based on Zr.

These findings are of particular regulatory significance since they provide: (1) a method of assessing the level of detection inherent in soil ingestion studies, (2) a reevaluation of the major soil ingestion studies in light of new methodology, and (3) guidance for future studies so that detection capacity can now be included in the presentation of study findings.     

A Guide to Interpreting Soil Ingestion Studies. 1. Development of a Model to Estimate the Soil Ingestion Detection Level of Soil Ingestion Studies

Abstract

This paper provides a model to predict soil ingestion recovery values in soil ingestion studies either retrospectively or prospectively. The predictive equations generated from the model can be used to estimate minimum soil ingestion detection levels from soil ingestion studies which use mass-balance methods. The model is derived from data assessing soil recovery efficiencies in adults using eight different predictive tracer elements. The results constitute a methodology to determine minimum detection levels of soil ingestion, and hence have important regulatory significance.     

Soil ingestion rates in children identified by parental observation as likely high soil ingesters

Soil ingestion was estimated over seven consecutive days in a soil tracer‐based mass‐balance methodology in 12 children, aged 1 to 3 years. The children had been identified by parents as likely high soil ingesters based on retrospective observation of daily soil ingestion behavior over the past 30 d. While one of the 12 children displayed high soil ingestion (0.5 to 3.05 g/d) on 4 of 7 d, median soil ingestion rates for the 12 children were comparable to those published in other soil ingestion studies of young children selected for studies based on age and not assumed or observed soil ingestion behavior. The lack of ability of parental judgments to predict high soil ingestion rates represents both an important observation and a significant limitation in current methods to efficiently and accurately identify children who ingest high amounts of soil.     

Soil ingestion estimates for use in site evaluations based on the best tracer method

This paper develops a novel methodology, the Best Tracer method (BTM), that substantially overcomes the principal limitations (intertracer inconsistencies, and poor precision of recovery) of estimating soil ingestion by specific soil‐based tracers in massbalance studies. The BTM incorporates a biological and statistical framework that improves precision of recovery of tracer estimates, markedly reducing input‐output misalignment error resulting from a lack of correspondence between food tracer input and fecal tracer output.

This method is then used to re‐estimate the soil ingestion distribution of previously published soil ingestion estimates from two children studies (Calabrese et al. 1989; Davis et al., 1990) and one adult study (Calabrese et al., 1990). Revised estimates of soil ingestion are provided for each study. In addition, the results from the two children's studies are combined to form a single estimate of the soil ingestion distribution. These collective findings result in more reliable quantitative estimates of soil ingestion than trace element specific estimates, as well as providing improved understanding of current published soil ingestion studies, and improved methods that will enhance the design and interpretation of future soil ingestion studies.

With respect to children, the data indicate that the Calabrese et al. (1989) study provides the most reliable estimates of soil ingestion based on the estimated precision of recovery. However, estimates for the combined data of the Calabrese et al. (1989) and Davis et al. (1990) studies include all available children's data from mass balance studies, and thus provide more robust estimates. The collective data suggest that the median child in these studies ingested 30–40 mg/day of soil, while the upper 95% estimate is approximately 200 mg/day. Current data are insufficient to distinguish the children's soil ingestion distribution from that of adults. The revised and improved estimates of soil ingestion for children and adults have important implications for contaminant exposure estimates used in site evaluation risk assessment procedures.     

Biasing Factors for Simple Soil Ingestion Estimates in Mass Balance Studies of Soil Ingestion

Soil ingestion estimates from mass balance soil ingestion studies can be used in Monte Carlo Risk assessment. We develop and describe a simulation model based on four mass balance soil ingestion studies that enables food ingestion, soil ingestion, and transit time to be mimicked. We use the simulation to evaluate potential biases that exist in current estimates of the distribution of daily soil ingestion in children (constructed from subject specific average daily estimates). The simulation identifies the importance of the study duration on the bias in the upper percentile soil ingestion estimates, indicating that the 95% soil ingestion estimate may be positively biased by over 100%. Misspecification of play areas for soil sampling is shown to have no biasing effect, and absorption of trace elements in food of up to 30% is shown to bias the soil ingestion distribution by less than 20 mg/d. The results, based on Al and Si trace element estimates, define the limits of previously published soil ingestion estimates, and provide insight for future study design and estimation methods.     

Soil Ingestion Distributions for Monte Carlo Risk Assessment in Children

Monte Carlo environmental risk assessment requires estimates of the exposure distributions. An exposure of principal concern is often soil ingestion among children. We estimate the long-term (annual) average soil ingestion exposure distribution using daily soil ingestion estimates from children who participated in a mass-balance study at Anaconda, MT. The estimated distribution is accompanied by uncertainty estimates. The estimates take advantage of developing knowledge about bias in soil ingestion estimates and are robust. The estimates account for small particle size soil, use the median trace element estimate for subject days, account for the small sample variance of the median estimates, and use best linear unbiased predictors to estimate the cumulative long term soil ingestion distribution. Bootstrapping is used to estimate the uncertainty of the distribution estimates. The median soil ingestion is estimated as 24?mg/d (sd = 4?mg/d), with the 95 percentile soil ingestion estimated as 91?mg/d (sd = 16.6?mg/d). Strategies are discussed for use of these estimates in Monte Carlo risk assessment.     

Assessment of Potential Human Health Risks from Arsenic in CCA-Treated Wood

Chromated copper arsenate (CCA) is a chemical preservative used to treat wood for a variety of outdoor uses, including decks, fencing, and play structures. This article describes a methodology to quantify exposures to arsenic from CCA-treated wood. Exposure was evaluated for ingestion and dermal contact with arsenic-containing residue on treated wood surfaces (dislodgeable arsenic), and ingestion, dermal contact, and inhalation of soil containing arsenic originating from treated wood structures. Standard approaches were used to quantify exposures to arsenic in soil. In the absence of standard approaches for exposures to dislodgeable residue, an empirical approach was developed, extrapolating from studies of soil loadings on hands and soil ingestion rates to estimate the amount of dislodgeable residue on hands that is subsequently ingested. Results from animal studies were used to develop relative bioavailability estimates for dislodgeable and soil arsenic. A focused sensitivity analysis demonstrated that the assumptions used regarding hand loading and subsequent incidental ingestion of dislodgeable arsenic had the most significant impact on the results. This assessment indicates low uptake of arsenic into the body, resulting in incremental lifetime cancer risks within USEPA's target risk levels. We compare this approach to other methodologies used to assess exposures to treated wood.     

Electrochemical monitoring of aflatoxin M1 in milk samples using silver nanoparticles dispersed on α‐cyclodextrin‐GQDs nanocomposite

Aflatoxins are potential food pollutants produced by fungi. One of important toxins is aflatoxin M1 (AFM1). A great deal of concern is associated with AFM1 toxicity. In the present study, an innovative electrochemical interface for quantitation of AFM1 based on ternary signal amplification strategy was fabricated. In this work, silver nanoparticles was electrodeposited onto green and biocompatible nanocomposite containing α‐cyclodextrin as conductive matrix and graphene quantum dots as amplification element. Therefore, a multilayer film based on α‐cyclodextrin, graphene quantum dots, and silver nanoparticles was exploited to develop a highly sensitive electrochemical sensor for detection of AFM1. Fully electrochemical methodology was used to prepare a transducer on a glassy carbon electrode, which provided a high surface area toward sensitive detection of AFM1. The surface morphology of electrode surface was characterized by high‐resolution field emission scanning electron microscope. The proposed sensing platform provides a simple tool for AFM1 detection. Under optimized condition, the calibration curve for AFM1 concentration was linear in 0.015mM to 25mM with low limit of quantification of 2μM. The practical analytical utility of the modified electrode was illustrated by determination of AFM1 in unprocessed milk samples.     

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).     

Identifying Metals Contamination in Soil: A Geochemical Approach

The geochemical evaluation methodology described in this paper is used to distinguish contaminated samples from those that contain only naturally occurring levels of inorganic constituents. Site-to-background comparisons of trace elements in soil based solely on statistical techniques are prone to high false positive indications. Trace element distributions in soil tend to span a wide range of concentrations and are highly right-skewed, approximating lognormal distributions, and background data sets are typically too small to capture this range. Geochemical correlations of trace versus major elements are predicated on the natural elemental associations in soil. Linear trends with positive slopes are expected for scatter plots of specific trace versus major elements in uncontaminated samples. Individual samples that may contain a component of contamination are identified by their positions off the trend formed by uncontaminated samples. In addition to pinpointing which samples may be contaminated, this technique provides mechanistic explanations for naturally elevated element concentrations, information that a purely statistical approach cannot provide. These geochemical evaluations have been successfully performed at numerous facilities across the United States. Removing naturally occurring constituents from consideration early in a site investigation reduces or eliminates unnecessary investigation and risk assessment, and focuses remediation efforts.     

Health Risk Assessment of Fluoride Exposure in Soil,Plants, and Water at Isfahan,Iran

Fluoride is a potentially toxic element, with a narrow range of tolerable amounts taken up via food or drinking water. To evaluate F content in surface soils, 255 topsoil samples (0–20 cm) in an area of 6800 km² in Isfahan province of central Iran were collected. Crop plants and randomly sampled water samples from wells were evaluated during the spring and summer seasons. Total F concentration in 96% of soil samples was lower than the global suggested average of 200 mg kg^?1. The mean F concentration of water samples in the study area was 0.05 and 0.3 mg L^?1 in summer and spring, respectively. Fluoride concentrations in different plant species were in the range of normal values. The total hazard quotient (HQ) for both population groups via consumption of cereals, vegetables, and water; incidental ingestion of soil; inhalation of soil particulates; and dermal contact with water and soil was less than 1.0, resembling no obvious risk. It is suggested that neither age group in Isfahan province will experience a significant potential health risk through their dietary intake of cereals, vegetables, and water; ingestion of soil; inhalation of particulates; and dermal contact.     

Revisiting Dust and Soil Ingestion Rates Based on Hand-to-Mouth Transfer

Soil ingestion rates calculated using a tracer-based mass balance approach may carry considerable study errors, insensitivities, and “artefacts” of analysis that result in significant uncertainty. These same soil ingestion rates are often used as surrogates for dust ingestion rates. Therefore, a more direct and mechanistic method was developed to estimate soil and dust ingestion rates. The soil and dust ingestion rates were calculated using measures of: particle loading to indoor surfaces; fraction transferred to the hands; hand surface area; fraction of hand surface area that may be mouthed or contact food; frequency of hand-to-mouth events, amount dissolved by saliva; and exposure time. Adapted specifically for the Canadian context, estimated mean indoor dust ingestion rates range from 2.2 mg/d for teenagers to 41 mg/d for toddlers; mean soil ingestion rates range from 1.2 mg/d for seniors to 23 mg/d for children. Combined soil and dust ingestion rates ranged from 3.8 mg/d for seniors to 61 mg/d for toddlers. These ingestion rates are lower than values adopted by most agencies. These ingestion rates are mechanistic, can be adjusted on a site-specific basis, can be modified into an hourly rate and are presented as a more realistic alternative to traditional mass balance approaches.

[Supplementary materials are available for this article. Go to the publisher's online edition of Human and Ecological Risk Assessment to view the free supplementary files.]     

Soil Ingestion Estimates for Children in Anaconda Using Trace Element Concentrations in Different Particle Size Fractions

This investigation assessed the effect of soil particle size on soil ingestion estimates of children residing at a superfund site. Earlier research indicated that wide intertracer variability in soil ingestion estimates are based on soil concentrations with a soil particle size of 0 to 2?µm was markedly reduced when the estimates were based on soil tracer concentrations for a soil particle size of 0-250?µm. The reduced intertracer variation was principally attributed to changes in soil concentrations of only three of the soil tracers (i.e., Ce, La, Nd) which became concentrated in the finer particle size by approximately 2.5 to 4.0-fold. It was hypothesized that the intertracer agreement in soil ingestion estimates may continue to improve if the estimates are based on concentrations of tracers at finer particle sizes assuming that children ingest finer particles and that the above three tracers would continue to be further concentrated in the finer sized soil particles. The principal findings indicate: 1. The soil concentrations of Al, Si, and Ti do not increase at the two finer particle size ranges measured. 2. The soil concentrations of Ce, La, and Nd increased by a factor 2.5 to 4.0 in the 100 to 250?µm particle size range when compared with the 0 to 2?µm particle size range. No further substantial increase in concentration was observed in the 53 to 100 |jm particle size range. 3. The soil ingestion estimates are consistently and markedly changed only between the estimates based in 0 to 2?µm and 100 to 250?µm for Ce, La, and Nd. These changes reduced the intertracer variability in estimating soil ingestion, suggesting that the children eat finer soil particle sizes. 4. Because the particle sizes for all tracers (except Zr) were only modestly affected at the 53 to 100?µm range, it was not possible to confidently resolve the particle size of soil ingested by the children. 5. Residual intertracer variability in soil ingestion estimates based on Ce, La, Nd are likely to be significantly affected by non-food, non-soil sources of these tracers (i.e., source error). 6. Soil ingestion estimates of this study will be more reliable when derived from the finer-sized particles.     

What proportion of household dust is derived from outdoor soil?

This report estimated the amount of outdoor soil in indoor dust in the Calabrese et al. (1989) children soil ingestion study via the use of statistical modelling. The estimate used data on outdoor soil and indoor dust in the homes of 60 children with eight tracer elements (Al, Ba, Mn, Si, Ti, V, Y, and Zr). The model estimated that 31.3% of indoor dust had an origin of outdoor soil. Based on a previous report (Stanek and Calabrese, 1992) on differential soil from dust ingestion in the Calabrese et al. (1989) study and the data of the present analysis, the median outdoor soil ingestion of the Calabrese et al. (1989) study should be revised downward by 35%. For the three most reliable tracers, the median soil ingestion estimates would be reduced from 29 to 19 mg/d for Al, 55 to 36 mg/d for Ti, and 16 to 10 mg/d for, Zr.     

Evaluation of beta-D-glucan density in blood after drinking an extraction element of Agaricus blazei murill]

beta-D-Glucan (beta-G) measurement in blood is useful for prompt diagnosis, selection of treatment and therapeutic evaluation for deep fungal infection. A patient with high beta-G blood was not improved by strong anti-fungal treatment in our hospital, and the beta-G levels in the blood fell immediately after discontinuing ingestion of the agaricus mushroom extracted element (sennseiro). To verify the elevation of beta-G concentration in the blood after intake of an agaricus mushroom extraction element, beta-G concentration in the blood was measured 4 days after 9 volunteers drank 2 g of agaricus mushroom extraction element, "SSG plus 35" twice a day for 3 days. A low value of beta-G in blood was detected in one person (11.1%), which demonstrated that beta-G concentration in the blood increased by oral ingestion of the agaricus element although the reason was unclear why it was detected in only one person. Taken together, when high beta-D-glucan is identified by uncertain cause or in spite of enough anti-fungal medication, it is necessary to confirm whether a patient has ingested health food containing an agaricus mushroom element or other fungus elements to avoid needless treatment with anti-fungal medicine.     

Selective solid-phase extraction using molecularly imprinted polymer for analysis of methamidophos in water and soil samples

An analytical methodology for the analysis of methamidophos in water and soil samples incorporating a molecularly imprinted solid-phase extraction process using methamidophos-imprinted polymer was developed. Binding study demonstrated that the polymer exhibited excellent affinity and high selectivity to the methamidophos. Evidence was also found by FT-IR analysis that hydrogen bonding between the CO(2)H in the polymer cavities and the NH(2) and P=O of the template was the origin of methamidophos recognition. The use of molecularly imprinted solid-phase extraction improved the accuracy and precision of the GC method and lowered the limit of detection. The recovery of methamidophos extracted from a 10.0 g soil sample at the 100 ng/g spike level was 95.4%. The limit of detection was 3.8 ng/g. The recovery of methamidophos extracted from 100 mL tap and river water at 1 ng/mL spike level was 96.1% and 95.8%, and the limits of detection were 10 and 13 ng/L respectively. These molecularly imprinted solid-phase extraction procedures enabled selective extraction of polar methamidophos successfully from water and soil samples, demonstrating the potential of molecularly imprinted solid-phase extraction for rapid, selective, and cost-effective sample pretreatment.     

Competing Factors of Compost Concentration and Proximity to Root Affect the Distribution of Streptomycetes

Streptomycetes are important members of soil microbial communities and are particularly active in the degradation of recalcitrant macromolecules and have been implicated in biological control of plant disease. Using a streptomycetes-specific polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (PCR-DGGE) methodology coupled with band excision and sequence analysis, we examined the effect of grape marc compost amendment to soil on cucumber plant–associated streptomycetes community composition. We observed that both compost amendment and proximity to the root surface influenced the streptomycetes community composition. A strong root selection for a soil-derived Streptomycete, most closely related to Streptomyces thermotolerans, S. iakyrus, and S. thermocarboxydus, was independent of compost amendment rate. However, while the impact of compost amendment was mitigated with increasing proximity to the root, high levels of compost amendment resulted in the detection of compost-derived species on the root surface. Conversely, in rhizosphere and non-rhizosphere soils, the community composition of streptomycetes was affected strongly even by modest compost amendment. The application of a streptomycetes-specific PCR primer set combined with DGGE analysis provided a rapid means of examining the distribution and ecology of streptomycetes in soils and plant-associated environments.     

Estimation of Sediment Ingestion Rates Based on Hand-to-Mouth Contact and Incidental Surface Water Ingestion

There is a lack of scientifically justified approaches for assessing sediment ingestion rates of people exposed to contaminated sediments. Consequently, a method was developed to estimate sediment ingestion rates from: (1) hand-to-mouth contact with sediments and (2) incidental ingestion of surface water containing suspended sediments. In the case of hand-to-mouth contact, a mechanistic approach was used based on established principles and assumptions previously used for estimation of soil and dust ingestion rates. A key modification of the approach was to account for greater adherence of sediments to hands as compared to soil and dusts. For estimation of sediment ingestion from surface water contact, a method was developed that considered the unique aspects of suspended sediments. The analysis indicated that hand-to-mouth contact is the dominant pathway for ingestion of sediment. When people use aquatic areas for recreational purposes, the analysis has indicated that mean sediment ingestion rates may range from 18 to 72 mg/h for various receptor age groups. For sites where people spend more than 1 h per day on a consistent basis in direct contact with sediments, the results indicate that sediment ingestion rates may be greater than those typically assumed in Canadian human health risk assessment guidance for soils.     

Scaling down the analysis of environmental processes: monitoring enzyme activity in natural substrates on a millimeter resolution scale

Natural environments often show high levels of spatial heterogeneity. With a methodology based on the immobilization of fluorescent substrates, the distribution of extracellular enzymes can be studied at a 2.3-mm resolution with a detection limit of 1.8 nmol · h(-1) · cm(-2). The method is applicable to environmental samples such as wood, litter, soil, or fungal colonies.