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.     

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.     

Methodological Considerations in Soil Ingestion Estimation

Abstract

Methodological considerations play an important role in forming population estimates of soil ingestion in children. Two important areas of controversy are the hypothesized log-normal distribution of ingested soil and the identificatbn and appropriateness of methods for handling outliers. Each of these issues is discussed in the context of data collected on soil ingestion in Amherst, Massachusetts. Non-parametric methods are recommended as most suitable and appropriate for analysis of soil ingestion studies.     

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 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.     

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.     

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.     

Density Centrifugation Method for Recovering Rhizobium spp. from Soil for Fluorescent-Antibody Studies

A density centrifugation procedure has been developed as a replacement for soil flocculation and clarification steps employed in quantitative fluorescent-antibody studies on Rhizobium in soils. Near-quantitative recovery of added cells of two strains of Rhizobium japonicum and two strains of R. phaseoli was achieved from six soils with various properties. It is proposed that this technique may prove useful in separating other soil microorganisms from soil particles in ecological studies employing fluorescent-antibody techniques.     

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.     

Heavy Metals Bioconcentration from Soil to Vegetables and Assessment of Health Risk Caused by Their Ingestion

The present study was undertaken to assess the non-carcinogenic human health risk of heavy metals through the ingestion of locally grown and commonly used vegetables viz. Raphanus sativus (root vegetable), Daucus carota (root vegetable), Benincasa hispida (fruit vegetable) and Brassica campestris leaves (leafy vegetable) in a semi-urbanized area of Haryana state, India. Heavy metal quantification of soil and vegetable samples was done using flame atomic absorption spectrophotometer. Lead, cadmium and nickel concentration in vegetable samples varied in range of 0.12–6.54 mg kg^?1, 0.02–0.67 mg kg^?1 and <0.05–0.41 mg kg^?1, respectively. Cadmium and lead concentration in some vegetable samples exceeded maximum permissible limit given by World Health Organization/Food and Agriculture Organization and Indian standards. Much higher concentrations of Pb (40–190.5 mg kg^?1), Cd (0.56–9.85 mg kg^-1) and Ni (3.21–45.87 mg kg^?1) were reported in corresponding vegetable fields’ soils. Correlation analysis revealed the formation of three primary clusters, i.e. Cu–Cd, Cd–Pb and Ni–Zn in vegetable fields’ soils further supported by cluster analysis and principal component analysis. Bioconcentration factor revealed that heavy metals’ uptake was more by leafy vegetable than root and fruit vegetables. Hazard index of all the vegetables was less than unity; thus, the ingestion of these vegetables is unlikely to pose health risks to the target population.     

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.     

Improved PCR-Based Detection of Soil Transmitted Helminth Infections Using a Next-Generation Sequencing Approach to Assay Design

BackgroundThe soil transmitted helminths are a group of parasitic worms responsible for extensive morbidity in many of the world’s most economically depressed locations. With growing emphasis on disease mapping and eradication, the availability of accurate and cost-effective diagnostic measures is of paramount importance to global control and elimination efforts. While real-time PCR-based molecular detection assays have shown great promise, to date, these assays have utilized sub-optimal targets. By performing next-generation sequencing-based repeat analyses, we have identified high copy-number, non-coding DNA sequences from a series of soil transmitted pathogens. We have used these repetitive DNA elements as targets in the development of novel, multi-parallel, PCR-based diagnostic assays.Conclusions/SignificanceThe utilization of next-generation sequencing-based repeat DNA analysis methodologies for the identification of molecular diagnostic targets has the ability to improve assay species-specificity and limits of detection. By exploiting such high copy-number repeat sequences, the assays described here will facilitate soil transmitted helminth diagnostic efforts. We recommend similar analyses when designing PCR-based diagnostic tests for the detection of other eukaryotic pathogens.     

LYSOSOMAL PHYSIOLOGY IN TETRAHYMENA : III. Pharmacological Studies on Acid Hydrolase Release and the Ingestion and Egestion of Dimethylbenzanthracene Particles

The ingestion of ¹⁴C-labeled 9,10-dimethyl-1,2-benzanthracene particles, the extracellular release of acid phosphatase, ribonuclease, and α-glucosidase, and the egestion of preingested dimethylbenzanthracene particles by Tetrahymena taken from logarithmically growing cultures and resuspended in a dilute salt solution were followed in the presence of several pharmacologic agents. Serotonin, caffeine, and, to a lesser extent, dibutyryl cyclic AMP increased the rate of particle ingestion, but did not alter the rate of release of the three acid hydrolases studied. Added catecholamines did not affect either particle ingestion or acid hydrolase release, but particle ingestion was inhibited by the catecholamine antagonists, dichloroisoproterenol, desmethylimipramine, reserpine, and phenoxybenzamine. These drugs also increased the release of acid phosphatase and ribonuclease in 5-h incubations. Desmethylimipramine acted within 1 h to increase acid hydrolase release, but the effect of dichloroisoproterenol developed more slowly and was secondary to a change in cellular content of the hydrolases. Desmethylimipramine increased the energy of activation for the release of acid phosphatase, while dichloroisoproterenol did not. Both of these drugs enhanced the egestion of preingested dimethylbenzanthracene particles, supporting the view that acid hydrolase release occurs through a cytoproct egestion mechanism. Particle ingestion was also inhibited by colchicine, vinblastine, and cytochalasin B, but these agents had no effect on acid hydrolase release, thus further differentiating the properties of the ingestion mechanism from those of the egestion mechanism. It appears that both microtubules and microfilaments play a role in the ingestion process and that this process may be controlled in part by a cyclic AMP-mediated serotoninergic and adrenergic system.     

Large Sample Area and Size Are Needed for Forest Soil Seed Bank Studies to Ensure Low Discrepancy with Standing Vegetation

A large number of small-sized samples invariably shows that woody species are absent from forest soil seed banks, leading to a large discrepancy with the seedling bank on the forest floor. We ask: 1) Does this conventional sampling strategy limit the detection of seeds of woody species? 2) Are large sample areas and sample sizes needed for higher recovery of seeds of woody species? We collected 100 samples that were 10 cm (length) ×10 cm (width) ×10 cm (depth), referred to as larger number of small-sized samples (LNSS) in a 1 ha forest plot, and placed them to germinate in a greenhouse, and collected 30 samples that were 1 m×1 m×10 cm, referred to as small number of large-sized samples (SNLS) and placed them (10 each) in a nearby secondary forest, shrub land and grass land. Only 15.7% of woody plant species of the forest stand were detected by the 100 LNSS, contrasting with 22.9%, 37.3% and 20.5% woody plant species being detected by SNLS in the secondary forest, shrub land and grassland, respectively. The increased number of species vs. sampled areas confirmed power-law relationships for forest stand, the LNSS and SNLS at all three recipient sites. Our results, although based on one forest, indicate that conventional LNSS did not yield a high percentage of detection for woody species, but SNLS strategy yielded a higher percentage of detection for woody species in the seed bank if samples were exposed to a better field germination environment. A 4 m² minimum sample area derived from power equations is larger than the sampled area in most studies in the literature. Increased sample size also is needed to obtain an increased sample area if the number of samples is to remain relatively low.     

Soil carbon stocks in plantations and natural forests of the sub-tropics

PurposeSoil carbon (C) storage plays an important role in the mitigation of atmospheric CO₂ emission. Soil C pools under different vegetation are distinct and need to be investigated. However, there are still large quantities of data shortages, which should be remedied by field and systematic studies.Materials and methodsSoil was collected at 0–10 cm depth from subtropical natural vegetation and plantations both in southeast China and southeast Queensland, Australia, respectively. Soil samples were assayed for soil organic C; organic N and inorganic N; and mineralization of SOC; total C, N, and P; and pH.Results and discussionOur results suggested soil C concentrations in natural vegetation ranged from 6.25% to 9.20%, whereas soil C concentrations in plantations ranged from 1.08% to 2.69%. No significant differences were found among vegetation along altitudinal gradients, whereas plantations with different tree species had different soil C concentrations, being higher in broadleaf-species plantations than in coniferous-species plantations. But there were no differences in soil C between single-species plantations and mixed-species plantations. Soil C concentrations in plantations were correlated with soil moisture, soil pH and dissolved organic C concentrations; Whereas soil C concentrations in natural vegetation were significantly correlated with soil moisture, soil pH and NO₃⁻ concentrations.ConclusionsThese results can contribute to the remedy of data shortages and provide the data necessary for model projections and informed decisions in the future.     

Studies on the Growth of Basidiomycetes in Soil

Fructifications of basidiomycetes have been collected at LakenheathWarren, and thirty-nine species have been obtained from fivegrassland soils. The different soils have been found to havedifferent species growing upon them and most species occur onthe alkaline-slightly acid grasslands. Highly acid grasslandat Lakenheath has a poor fungus flora. Studies have been made of the regions in the soil in which themycelia of certain of these basidiomycetes occur, and attemptshave been made to isolate these fungi from soil. Some specieshave been found to inhabit the litter zone rather than the minerallayers of the soil. A few form extensive rhizomorph systemswhich may penetrate deeply into the soil. A few species of ringfungi, such as Marasmius oreades, Psalliota arvensis, and Tricholomanudum, have been found to possess a well-defined mycelial zonein the soil, from which the fungus could be isolated. Profile studies have shown that the mycelial zones of thesespecies contain a restricted population of microfungi, bothfewer species and fewer colonies, as compared with the normalsoil around. Ascomycetes, such as Arachniotus, Chaetomium, Gymnoascus,and Penicillium, have been isolated more frequently from mycelialzones than from normal soil.     

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.     

Contamination and health risk assessment of trace elements in soil at play centers of urban low-income settings

Abstract

Young children are considered critical receptors of potentially toxic trace elements (PTEs) by non-dietary ingestion of contaminated soil. The study assessed the potential enrichment of soil and the health risk of PTEs to 471 children less than seven years via non-dietary soil ingestion at six Early Childhood Development Centers (ECDCs) in urban low-income settings. The total concentrations of PTEs were determined by ICP-AES after wet acid digestion. The extent of soil contamination with PTEs and their source apportionment were assessed by the enrichment factor (EF). The US-EPA risk assessment model was used to determine the risk of PTE exposure by children. Multivariate statistical analyses and the EF suggested anthropogenic origin of PTEs in playgrounds and indoors, especially Cd and Pb from atmospheric deposition. Indoor floor dust at ECDCs was enriched (significant to extreme) with PTEs of anthropogenic origin imported from the outside environment. Children at the six ECDCs were not at significant non-carcinogenic risk of PTEs in soil and dust through non-dietary ingestion. The study setting is typical of urban child play centers in low-income countries which needs regular risk assessment and the enforcement of legislation in order to reduce the exposure of children to PTEs.     

几株赤潮甲藻的摄食能力

采用荧光标记的方法,在营养盐限制条件下,对6株赤潮甲藻对荧光标记的海洋细菌(FLB)、金藻(FLA)和两种粒径分别为0.5μm和2.0μm的荧光微球(FM0.5和FM2.0)4种摄食对象的摄食进行了比较研究。研究结果表明,除了东海原甲藻对4种摄食对象均没有摄食外,其它5株甲藻,微小亚历山大藻、链状亚历山大藻、塔玛亚历山大藻、海洋原甲藻和微小原甲藻均具有摄食能力,但对摄食对象的选择和摄食率有差异,多数摄食率是在4 h达到最大,白天的摄食能力强于夜间。研究说明了在营养盐限制环境中,有些具有兼性营养能力的甲藻对细菌和/或更小浮游植物的摄食能力可能对维持和促进其生长具有不可忽视的作用。     

Enhanced Removal of Lead from Soil Using Biosurfactant Derived from Edible Oils

ABSTRACT

Lead contamination in soil due to anthropogenic activities has amplified and therefore, remediation is of prime significance due to its nonbiodegradability and toxicity effects. This study focuses on lead removal from the soil collected from a rifle range using biosurfactants produced from native microorganisms and edible oils. Native microorganisms in contaminated soil served as a source for biosurfactant production aided by edible vegetable oils such as palm oil and gingelly oil. Preliminary isolation and characterization studies indicated the presence of Pseudomonas aeruginosa that produced biosurfactant and removed lead simultaneously. Batch adsorption experiments showed 96%–99.6% of lead adsorption following Langmuir isotherm model. Lead desorption of 23.6% occurred without biosurfactant. Whereas in the presence of biosurfactants, enhanced desorption of 62.3% was observed. Of both palm oil and gingelly oil derived biosurfactants, the former reached a lead removal efficiency of 93.6% indicating the feasibility and effectiveness of the biosurfactants for contaminated site remediation.