Direct Soil Ingestion Research Articles

Insights into the prediction uncertainty of machine-learning-based digital soil mapping through a local attribution approach

Abstract. Machine learning (ML) models have become key ingredients for digital soil mapping. To improve the interpretability of their predictions, diagnostic tools such as the widely used local attribution approach known as SHapley Additive exPlanations (SHAP) have been developed. However, the analysis of ML model predictions is only one part of the problem, and there is an interest in obtaining deeper insights into the drivers of the prediction uncertainty as well, i.e. explaining why an ML model is confident given the set of chosen covariate values in addition to why the ML model delivered some particular results. In this study, we show how to apply SHAP to local prediction uncertainty estimates for a case of urban soil pollution – namely, the presence of petroleum hydrocarbons in soil in Toulouse (France), which pose a health risk via vapour intrusion into buildings, direct soil ingestion, and groundwater contamination. Our results show that the drivers of the prediction best estimates are not necessarily the drivers of confidence in these predictions, and we identify those leading to a reduction in uncertainty. Our study suggests that decisions regarding data collection and covariate characterisation as well as communication of the results should be made accordingly.

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) soil contamination in Lausanne, Switzerland: Combining pollution mapping and human exposure assessment for targeted risk management

In December 2020, high soil concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) were discovered across large parts of Lausanne, Switzerland. Concentrations reached up to 640 ng TEQWHO-2005/kg dry weight. The most likely source was a former municipal waste incinerator. A three-step, multidisciplinary approach to human health risk assessment was conducted to determine the potential population exposure to PCDD/Fs and identify appropriate preventive measures. First, exposure scenarios were developed based on contaminated land uses. Second, the toxicological risks of different scenarios were evaluated using a toxicokinetic model estimating increases in blood serum PCDD/F concentrations over background concentrations from the general population's food consumption. Third, a detailed geostatistical mapping of PCDD/F soil contamination was performed. Stochastic simulations with an external drift and an anisotropic model of the variogram were generated to incorporate the effects of distance from emission source, topography, and main wind directions on the spatial distribution of PCDD/Fs in topsoil. Three main scenarios were assessed: i) direct ingestion of soil by children in playgrounds; ii) consumption of vegetables from private gardens by children and adults; and iii) consumption of food from livestock and poultry raised on contaminated soil. The worst exposure scenario involved the consumption of eggs from private hen houses, resulting in PCDD/F concentrations in serum an order of magnitude higher than might normally be expected. No relevant increases in serum concentrations were calculated for direct soil ingestion and vegetable consumption, except for cucurbitaceous vegetables. Combining mapping and exposure scenario assessment resulted in targeted protective measures for land users, especially concerning food consumption. The results also raised concerns about the potential unsafe consumption of products derived from animals raised on land with PCDD/F concentrations only moderately over environmental background levels.

Exploring the Use and Appeal of Playpens to Protect Infants from Exposure to Animals, Animal Feces, and Dirt in Rural Ethiopia.

.The persistence and pervasiveness of growth stunting in low- and middle-income countries spur reexamination of disease transmission pathways related to water, sanitation, and hygiene. Animal feces constitute a more important reservoir of enteric pathogens in homes in low-income countries than previously recognized, and exploratory object mouthing and direct ingestion of soil and animal feces represent underexplored exposure pathways. The effectiveness, adoption, constraints, and scale-up potential of measures for reducing infant and young children’s exposure to fecal pathogens are recently beginning to be systematically explored. This mixed methods study tested the feasibility and appeal of using playpens to establish a hygienic “safe zone” for infants in rural Ethiopia. We conducted home trials of three playpen designs, including two models made from locally available materials through user-centered design. After using playpens for several weeks, caregivers reported extensive benefits, ranging from perceived safety to developmental and hygiene benefits for infants and relief from physical stress and worry for caregivers. We observed many playpens contaminated with Escherichia coli after weeks of use, though at concentrations below those of the common room floor on which infants might otherwise have played. Caregivers reported daily playpen use, but for intervals likely insufficient to protect infants from pathogen exposure affecting growth. We determine that playpens alone cannot plausibly protect infants from environmental contamination, but our results support further exploration of the potential benefits and commercial viability of scaling up use of playpens in rural, agricultural households as part of a comprehensive approach to child development and women’s empowerment.

Ingestion of Fecal Bacteria along Multiple Pathways by Young Children in Rural Bangladesh Participating in a Cluster-Randomized Trial of Water, Sanitation, and Hygiene Interventions (WASH Benefits)

Quantifying the contribution of individual exposure pathways to a child’s total ingestion of fecal matter could help prioritize interventions to reduce environmental enteropathy and diarrhea. This study used data on fecal contamination of drinking water, food, soil, hands, and objects and second-by-second data on children’s contacts with these environmental reservoirs in rural Bangladesh to assess the relative contribution of different pathways to children’s ingestion of fecal indicator bacteria and if ingestion decreased with the water, sanitation, and hygiene interventions implemented in the WASH Benefits Trial. Our model estimated that rural Bangladeshi children <36 months old consume 3.6–4.9 log10 most probable number E. coli/day. Among children <6 months, placing objects in the mouth accounted for 60% of E. coli ingested. For children 6–35 months old, mouthing their own hands, direct soil ingestion, and ingestion of contaminated food were the primary pathways of E. coli ingestion. The amount of E. coli ingested by children and the predominant pathways of E. coli ingestion were unchanged by the water, sanitation, and hygiene interventions. These results highlight contaminated soil, children’s hands, food, and objects as primary pathways of E. coli ingestion and emphasize the value of intervening along these pathways.

Geogenic nickel exposure from food consumption and soil ingestion: A bioavailability based assessment

Accumulation and oral bioavailability of nickel (Ni) were rarely assessed for staple crops grown in high geogenic Ni soils. To assess exposure risk of geogenic Ni, soil, wheat, and rice samples were collected from a naturally high background Ni area and measured for Ni oral relative bioavailability (RBA, relative to NiSO4) using a newly developed mouse urinary Ni excretion bioassay. Results showed that soils were enriched with Ni (80.5 ± 23.0 mg kg−1, n = 58), while high Ni contents were observed in rice (2.66 ± 1.46 mg kg−1) and wheat (1.32 ± 0.78 mg kg−1) grains, with rice containing ∼2-fold higher Ni content than wheat. Ni-RBA was low in soil (14.8 ± 7.79%, n = 18), but high in wheat and rice with rice Ni-RBA (85.9 ± 19.1%, n = 9) being ∼2-fold higher than wheat (46.1 ± 21.2%, n = 16). A negative correlation (r = 0.61) was observed between Ni-RBA and iron content in rice and wheat, suggesting the low iron status of rice drives its high Ni bioavailability. The higher Ni accumulation and bioavailability for rice highlights that rice consumption was a more important contributor to daily Ni intake compared to wheat, while Ni intake from direct soil ingestion was negligible. This study suggests a potential health risk of staple crops especially rice when grown in high geogenic Ni areas.

Bioavailability and bioaccessibility in soil: a short review and a case study

In industrialized countries, there is a growing concern about the possible negative effects on human health induced by high levels of heavy metals in soil. It is recognized that high levels of heavy metals are not necessarily indicative of the adverse effects. These effects are related to bioavailable fractions, which are involved in plant uptake and transfer to the food chain. Bioavailability is a complex issue that is essential to evaluate to determine if heavy metals present in soil may pose hazards to humans and the environment. In the case of direct ingestion of soil, it is essential to consider also bioaccessibility. Bioavailability and bioaccessibility are related to several soil processes and may be largely determined by soil characteristics. This review deals with the influence of soil properties on metal bioavailability and bioaccessibility. A case study on bioavailability and bioaccessibility of heavy metals is reported, considering a large uncontaminated area influenced by deposition from a cement plant.

Soil ingestion among young children in rural Bangladesh.

Ingestion of soil and dust is a pathway of children’s exposure to several environmental contaminants, including lead, pesticides, and fecal contamination. Empirically based estimates of central tendency for soil consumption by children in high-income countries range from 9 to 135 dry mg/day. Using a Monte Carlo simulation, we modeled the mass of soil directly and indirectly ingested per day by rural Bangladeshi children and identified the parameters that influence the mass ingested. We combined data from observations of direct and indirect ingestion among children with measurements of soil mass on the children’s hands, mother’s hands, and objects to quantify soil ingestion/day. Estimated geometric mean soil ingestion was 162 dry mg/day for children 3–5 months, 224 dry mg/day for children 6–11 months, 234 dry mg/day for children 12–23 months, 168 dry mg/day for children 24–35 months, and 178 dry mg/day for children 36–47 months old. Across all age groups, children placing their hands in their mouths accounted for 46–78% of total ingestion and mouthing objects contributed 8–12%. Direct ingestion of soil accounted for nearly 40% of soil ingested among children 6–23 months old. Sensitivity analyses identified that the parameters most affecting the estimates were the load of soil on the child’s hand, the frequency of hand-to-mouth contacts while not eating, and, for children 6–23 months old, the frequency of direct soil ingestion. In a rural, low-income setting, children’s soil consumption was substantially more than the estimates for children in high-income countries. Further characterizing soil ingestion of children in low-income contexts would improve assessments of the risks they face from soil-associated contaminants.

Exposure to Livestock Feces and Water Quality, Sanitation, and Hygiene (WASH) Conditions among Caregivers and Young Children: Formative Research in Rural Burkina Faso.

.Livestock farming is common in low-income settings as a source of income and animal-sourced food. However, there is growing evidence of the harmful health effects of proximity of animals to infants and young children, especially through exposure to zoonotic pathogens. Poultry ownership is almost universal in rural Burkina Faso. Poultry feces are a significant risk factor for enteric diseases that are associated with child undernutrition. To investigate the extent of exposure to livestock feces among young children and caregivers, we conducted direct observations of 20 caregiver–child dyads for a total of 80 hours (4 hours per dyad) and recorded water quality, sanitation, and hygiene (WASH)-related behaviors. We also undertook in-depth interviews with these caregivers and focus group discussions with separate groups of men and women who were poultry farmers. Poultry and other livestock feces were visible in all 20 and 19 households, respectively, in both kitchen areas and in the household courtyards where children frequently sit or crawl. Direct soil ingestion by young children was observed in almost half of the households (45%). Poor handwashing practices were also common among caregivers and children. Although latrines were available in almost all households, child feces disposal practices were inadequate. This body of research suggests an urgent need to adapt conventional WASH and livestock interventions to reduce the exposure of infants and young children to livestock feces.

Tissue distribution and oral exposure risk assessment of heavy metals in an urban bird: magpie from Central Iran.

Direct ingestion of soil and/or soil attached to the food items is a potential rout for wildlife exposure to contaminants. In this study, bioaccumulation of five heavy metals (HMs) in internal tissues of an urban bird (Pica pica) collected from Aran-O-Bidgol City, Central Iran and their related soil were investigated. A total of 15 magpie specimens were collected in autumn 2013 and then their internal tissues were digested using a mixture of HNO3 and H2O2, and finally, concentrations of HMs were detected by ICP-OES. In addition, in order to show level of HM exposure risk to magpie, an exposure risk assessment was modeled. Results indicated that HMs were accumulated as follows: liver > kidney > muscle. Zn and Cu were significantly higher in magpie's tissues collected from agricultural site; on the other hand, Pb and Cd were significantly higher in industrial site (p < 0.05). Level of Cd in male's livers (2.11μg/g dw) was significantly higher than in females (1.85μg/g dw) (p < 0.05). Levels of Cd, Pb, and Ni in liver, muscle, and kidney, respectively, were significantly higher in adults than in subadults (p < 0.05). Soil exposure doses of all HMs were lower than tolerable daily intake (Zn 4.35, Cu 1.34, Ni 5.65, Pb 0.35, and Cd 0.53). The calculated hazard quotations (HQs) for HMs were as follows: Pb > Zn > Cu > Ni > Cd and for all HMs were at no risk level (HQ < 1). The amounts of hazard index for three sites were as follows: urban (1.032) > agriculture (0.943) ≥ industry (0.941) and only for urban area was at low risk (1 < HQ < 2). It seemed that birds living in a safe environment and/or HM contaminations in soil separately had no negative effects on magpies. We can also suggest that low levels of HMs in magpie's tissues can be due to low levels of HMs in soil.

Child soil ingestion in rural Ghana - frequency, caregiver perceptions, relationship with household floor material and associations with child diarrhoea.

The objectives of this work were to evaluate (i) the prevalence and frequency of caregiver-reported soil ingestion by children, (ii) whether household flooring material in the bedroom (earth vs. concrete) affected caregiver-reported soil ingestion, (iii) whether caregiver-reported soil ingestion was associated with caregiver-reported diarrhoea and (iv) caregivers' perceptions of their children ingesting soil. We conducted 309 household surveys in northern Ghana, including 529 children under five (249 children aged 6-36 months), and measured faecal contamination in soil from 31 households. Among all children, 15% were reported to have directly ingested soil in the past week, including 28% of children aged 6-36 months. Among children reported to have ingested soil, the median frequency was 14 times in the past week, and the median amount of soil ingested each time was half a handful. There was no association between household floor material and whether the caregiver observed a child directly ingesting soil. After adjusting for household floor material and other potential confounding variables, caregiver-reported soil ingestion was associated with caregiver-reported diarrhoea for children under five [adjusted odds ratio (adj. OR) = 3.13, 95% confidence interval (CI) 2.76-3.55] and children aged 6-36 months (adj. OR = 2.61, 95% CI 2.01-3.39). Approximately 83% of caregivers whose children ingested soil reported they thought it was unsafe and were more likely to report stopping their child from ingesting soil, but these responses did not affect the quantity of soil ingested. Our results suggest direct soil ingestion is associated with diarrhoea independent of household floor material, and separate interventions may be necessary to prevent exploratory soil ingestion.

A soil ingestion pilot study for teenage children in China

Soil ingestion by people is an important route of exposure to environmental contaminants. Studies on soil ingestion using tracer mass-balance method are mainly for young children and adults, scarcely for teenagers. In such case, the soil ingestion rate for teenagers recommended by regulators is generally identical to that of adults based on one activity pattern modeling study. However, teenagers are expected to have different soil ingestion exposure via ingestion pathway due to different activity patterns and exposure scenarios. We conducted a pilot study on soil ingestion in 30 teenage children aged 12.0–16.5 years from Hubei Province of China, using the best tracer method, with the results compared with previous soil ingestion studies. The estimated mean and median soil ingestion rates for teenagers in this study based on the best five tracers (Al, Ce, Sc, V, and Y) were 45.2 mg d−1 and 44.8 mg d−1 respectively, with the 95% confidence interval of the mean value ranging from 28.0 mg d−1 to 50.9 mg d−1. These soil ingestion rate estimates were slightly higher than the recommended values for adults by U.S. EPA, but lower than those for Chinese younger children observed in the other similar study. The result in this study is important to access the health risk resulting from exposure to toxic substances in soil via direct soil ingestion pathway by teenagers in China as well as other countries.

Health risk assessment of an abandoned herbicide factory site for transportation use in Dalian, China.

An abandoned herbicide factory site was used as an example of how planning should be considered for development of the site for transportation use in Dalian, China. Exposure pathways and parameters for three types of transportation use (land for a traffic hub, land for an urban road, and land for a subway) were developed. Twenty-five sampling sites were selected and 38 soil samples were collected in March 2015. Hexachlorobenzene and benzo(a)pyrene which were extracted by Soxhlet extraction and detected by gas chromatography mass spectrometry were the most significant pollutants detected. The maximum concentration of the two pollutants in the surface layer (0-0.5m) were 0.57 and 3.10mg/kg, and in the bottom layer (1.0m) were 2.57 and 3.72mg/kg, respectively. In this study, risk assessment results based on the established exposure scenario and parameters showed that there was a significant difference in traffic hub land use under specific exposure pathway and common insensitive land use exposure pathways (direct ingestion of soil, dermal contact with soil, and inhalation of soil-derived dust). Commonly considered hexachlorobenzene and benzo(a)pyrene carcinogenic risk values exceeded the maximum acceptable level (10-6) and were found to be 23.9-fold and 189-fold higher than the carcinogenic risk values, respectively. Parameter sensitivity analysis data showed that for transportation use, the two parameters "EFOa" and "OSIRa" were the most significant factors associated with variation of the carcinogenic risk value. For traffic hub land use, urban road land use, and subway land use, the main exposure pathways were through "inhalation of soil vapors outdoors (from surface soil)," "direct ingestion of soil," and "inhalation of soil vapors indoors (from bottom soil)," which contributed 84.75, 73.00, and 100.00% to the total risk value, respectively.

Bioavailability and health risk assessment of potentially toxic elements in Thriasio Plain, near Athens, Greece.

Elevated concentrations of potentially toxic elements (PTEs) are usually found in areas of intense industrial activity. Thriasio Plain is a plain near Athens, Greece, where most of the heavy industry of the country has been situated for decades, but it also is a residential and horticultural area. We aimed at measuring the levels of PTEs in soils and indigenous plant species and assessing the health risk associated with direct soil ingestion. Samples of soils at roadsides and growing plants were collected from 31 sites of that area. Concentrations of Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, V and Zn were measured in both soils (as pseudo-total) and aerial plant tissues. We found that As, Cd, Cr, Cu, Ni, Pb and Zn were higher than maximum regulatory limits. Element concentrations in plants were rather lower than expected, probably because indigenous plants have developed excluder behaviour over time. Copper and Zn soil-to-plant coefficients were highest among the other elements; for Cu this was unexpected, and probably associated with recent Cu-releasing industrial activity. Risk assessment analysis indicated that As was the element contributing more than 50% of the health risk related to direct soil ingestion, followed by Cr, Pb, and, surprisingly, Mn. We concluded that in a multi-element contamination situation, elevated risk of PTEs (such as As, Cr and Pb) may reduce the tolerance limits of exposure to less-toxic elements (here, Mn).

Bioavailability and risk assessment of potentially toxic elements in garden edible vegetables and soils around a highly contaminated former mining area in Germany

Although soil contamination by potentially toxic elements (PTEs) in Europe has a history of many centuries, related problems are often considered as having been dealt with due to the enforcement of tight legislations. However, there are many unsolved issues. We aimed to assess PTE levels in highly contaminated soils and in garden edible vegetables using human health risk indices in order to evaluate the availability and mobilization of arsenic (As), copper (Cu), manganese (Mn), mercury (Hg), lead (Pb), and zinc (Zn). In four gardens in Germany, situated on, or in the vicinity of, a mine dump area, we planted beans (Phaseolus vulgaris ssp. nanus), carrots (Daucus sativus) and lettuce (Lactuca sativa ssp. capitata). We examined soil-to-plant mobilization of elements using transfer coefficient (TC), as well as soil contamination using contamination factor (CF), enrichment factor (EF), and bioaccumulation index (Igeo). In addition, we tested two human health risk assessment indices: Soil-induced hazard quotient (HQS) (representing the “direct soil ingestion” pathway), and vegetable-induced hazard quotient (HQV) (representing the “vegetable intake” pathway). The studied elements were highly elevated in the soils. The values in garden 2 were especially high (e.g., Pb: 13789.0 and Hg: 36.8 mg kg−1) and largely exceeded the reported regulation limits of 50 (for As), 40 (Cu), 400 (Pb), 150 (Zn), and 5 (Hg) mg kg−1. Similarly, element concentrations were very high in the grown vegetables. The indices of CF, EF and Igeo were enhanced even to levels that are rarely reported in the literature. Specifically, garden 2 indicated severe contamination due to multi-element deposition. The contribution of each PTE to the total of measured HQS revealed that Pb was the single most important element causing health risk (contributing up to 77% to total HQS). Lead also posed the highest risk concerning vegetable consumption, contributing up to 77% to total HQV. The presence of lead in both cases was followed by that of As, Mn and Hg. We conclude that in multi-element contamination cases, along with high-toxicity elements (here, Pb, As and Hg) other elements may also be responsible for increasing human health risks (i.e., Mn), due to the possibility of adverse synergism of the PTEs.

Effect of particle size on risk assessment of direct soil ingestion and metals adhered to children's hands at playgrounds.

The quantity of heavy metals in soil is measured after 2-mm sieving in Japan for risk assessment of direct soil ingestion. A study was conducted on the relationship between the size of soil particles and quantity of heavy metals in soil, and the particle-size distribution of soil adhered to children's hands, and the risks of direct intake of soil considering the particle sizes ingested were evaluated. The results showed that smaller particles had a tendency to contain more heavy metals than bigger ones, that the particle size of approximately 90% of the soil particles from playgrounds adhered to children's hands was less than 100 μm, and that 2-mm sieving in preparation for measuring heavy metal content caused underestimation of the risk of direct soil intake. The amount of heavy metals on children's hands after playing outside was investigated. Various metals and soil were adhered to their hands, and the amount of soil adhered could be estimated from the concentration of metals. To develop accurate risk assessment, the particle-size distribution of ingested soil and more detailed scenarios of soil intake are necessary.

Field evaluations on soil plant transfer of lead from an urban garden soil.

Lead (Pb) is one of the most common contaminants in urban soils. Gardening in contaminated soils can result in Pb transfer from soil to humans through vegetable consumption and unintentional direct soil ingestion. A field experiment was conducted in 2009 and 2010 in a community urban garden with a soil total Pb concentration of 60 to 300 mg kg. The objectives of this study were to evaluate soil-plant transfer of Pb, the effects of incorporation of a leaf compost as a means of reducing Pb concentrations in vegetables and the bioaccessibility of soil Pb, and the effects of vegetable cleaning techniques on the Pb concentrations in the edible portions of vegetables. The amount of compost added was 28 kg m. The tested plants were Swiss chard, tomato, sweet potato, and carrots. The vegetable cleaning techniques were kitchen cleaning, laboratory cleaning, and peeling. Compost addition diluted soil total Pb concentration by 29 to 52%. Lead concentrations of the edible portions of vegetables, except carrot, were below the maximum allowable limits of Pb established by the Food and Agriculture Organization and the World Health Organization. Swiss chard and tomatoes subjected to kitchen cleaning had higher Pb concentrations than laboratory-cleaned plants. Cleaning methods did not affect Pb concentrations in carrots. Bioaccessible Pb in the compost-added soils was 20 to 30% less than that of the no-compost soils; compost addition reduced the potential of transferring soil Pb to humans via vegetable consumption and direct soil ingestion. Thorough cleaning of vegetables further reduced the potential of transferring soil Pb to humans.

Formative research on hygiene behaviors and geophagy as part of interventions to improve infant growth

The predominant pathways for fecal‐oral transmission of bacteria for infants may differ from adults and these have not been studied. We conducted direct observation of 23 caregiver‐infant pairs for a total of 130 hours and recorded WASH related behaviors. We identified “potential key vectors” for fecal‐oral transmission of bacteria and enumerated Escherichia coli, Total coliforms, Enterobacteriaceae and total aerobic counts. In addition to infant's fingers, food, and drinking water, three infants actively ingested soil 11.3 ±9.2 times (mean ±SD) and two ingested chicken feces 2± 1.4 times in 6 hours. Infants’ hands were rarely washed and never with soap unless as part of a bath. Drinking water was contaminated with E. coli in 12/22 (54.5 %) households, while no infant food samples tested positive for E coli. Quantitatively, a year old infant ingesting 1 gram chicken feces in a day, 20 grams of soil from a laundry area of the kitchen yard and 400 ml of contaminated water consumes 4,700,000 to 23,000,000, 440 to 4,240 and 400 to 1,200 E. coli counts, respectively, from these sources. These findings suggest that predominant pathways for fecal‐oral transmission of bacteria among infants include direct ingestion of soil and chicken. Behavior change education should include breaking socio‐cultural beliefs that soil ingestion is beneficial for infant health, which are common throughout the world.Grant Funding Source : Department for International Development (DFID), United Kingdom

Human health risks of Pb and As exposure via consumption of home garden vegetables and incidental soil and dust ingestion: A probabilistic screening tool

The consumption of home grown vegetables may represent a significant exposure pathway for arsenic (As) and lead (Pb) relative to direct incidental ingestion of soil, thus a probabilistic screening tool for estimating these exposures was developed using regression models relating co-located soil and home garden (HG) vegetable concentrations of Pb and As established from multiple independent studies and 2-dimensional Monte Carlo analyses. For high-quantity consumers of HG vegetables (i.e., the upper 95th percentile of consumers in the general population), the HG consumption pathway can be as significant as incidental soil and dust ingestion for inorganic As and, therefore, should be considered when developing generic health-based soil criteria in residential settings. Predicted Pb Hazard Quotient (HQ) estimates among young children resulting from HG consumption were 4- to 10-fold lower than exposures resulting from direct incidental soil and dust ingestion. The difference in soil/dust ingestion rates used to characterize young children (the 95th percentile of 202mg/d) versus a lifetime residential receptor (the 95th percentile of 30mg/d) was a primary factor contributing to the relative differences observed between HQ and incremental lifetime cancer risk (ILCR) resulting from these two exposure pathways for lead Pb and inorganic arsenic As, respectively.

Enhanced soil toxic metal fixation in iron (hydr)oxides by redox cycles

The possibility to foster toxic metals (Cd, Cu, Ni, Pb and Zn) fixation on Fe (hydr)oxides, either native or added as ameliorants, was investigated at the laboratory scale in two heavily polluted arable and grassland soils by inducing repeated cycles of the reduction and oxidation. Soil samples were treated with Fe-rich waste (FeW) alone or together with fresh organic waste (OW) to accelerate reduction step. The FeW (242 g Fe kg − 1 ) was an air dried sediment produced by treatment of olive mill waste waters and the OW (410 g C kg − 1 ) was obtained by source separate collection of solid urban waste. The efficiency of metals immobilization was evaluated in terms of biological availability. Phyto-availability was assessed either by 1 M NH 4 acetate extraction (exchangeable fraction) and by 0.005 M DTPA/0.1 M triethanolamine (pH 7.3) extraction, whereas hazard to humans from direct soil ingestion (oral bio-accessibility) was tested by the physiologically based extraction test (PBET). Aerobic incubation of soils treated with FeW + OW did not cause any significant change to the exchangeable fraction of toxic metals in both the arable and the grassland soils, whereas one to three redox cycles, applied after FeW + OW addition, reduced Cd and Cu in both soils and Pb in the arable soil below detection limits. Exchangeable Zn was reduced in the FeW + OW treated soil by 50 and 90% in the arable and grassland soils, respectively. DTPA-extractable Cd, Cu and Zn were unaffected by treatments when soils were maintained oxic, but on the contrary, decreased significantly after one to three redox cycles by 88%, 93%, 36% and 95%, 98%, 65% in the arable and grassland soil, respectively. Lead showed a different behaviour with a general tendency to increase its availability. This result could be explained with the progressive dissolution of Mn oxides during the redox cycles. DTPA-extractable Ni showed in the arable soil a progressive increase, but remained constant in the grassland soil. A further reduction in DTPA-extractable metals, particularly evident for Ni, Pb and Zn, was observed after 150 d ageing of treated soils. Soil treatments were also effective in reducing potential hazard from direct soil ingestion in grassland soil (95%, 100%, 27% and 30% for Cd, Cu, Pb and Zn respectively), whereas the effect was much lower in the arable soil. The mechanisms of metal fixation are compatible with chemisorption and co-precipitation on Fe (hydr)oxides. These results suggest that redox cycles can be applied to FeW treated soils to reduce biological availability of many toxic metals in neutral soils, but their efficiency varies according to the element and soil composition. The addition of OW generally fosters metal fixation by intensification of the reductive process.

都内公園土壌中重金属濃度の相関関係

The concentrations of heavy metals (V, Mn, Co, Cu, Zn, Cd, Ba, in addition to Pb which was reported elsewhere) in surface soil samples collected from 31 public playgrounds in a ward in Tokyo were determined by ICP mass spectrometry after HNO3/HF/HClO4 digestion. Mean concentrations of all metals but Ba exceeded the levels of uncontaminated soils in Japan reported in the previous literature indicating the presence of soil pollution. The exposure to V via direct soil ingestion was estimated to contribute more than that via dietary intake; however, it was not serious enough to warrant immediate attention. For other metals, dietary exposure accounted much more than soil ingestion. There were strong positive correlations between metal concentrations in the soil samples, particularly between V and Mn (r=0.97), V and Co (r=0.95), Pb and Cu (r=0.75), or Pb and Zn (r=0.84). These correlations suggested that the metals, to a certain extent, shared common pollution source.