Immobilization of nickel and other metals in contaminated sediments by hydroxyapatite addition

Batch experiments were conducted to evaluate the ability of hydroxyapatte (HA) to reduce the solubility of metals, including the primary contaminants of concern, Ni and U, from contaminated sediments located on the Department of Energy's Savannah River Site, near Aiken, SC. Hydroxyapatitie was added to the sediments at application rates of 0, 5, 15.8, and 50 g kg-1. After equilibrating in either 0.02 M KCl or 0.01 M CaCl2, the samples were centrifuged and the supernatants filtered prior to metal, dissolved organic C, and PO4 analyses. The treated soils were then air-dried and changes in solid-phase metal distribution were evaluated using sequential extractions and electron-based microanalysis techniques. Hydroxyapatite was effective at reducing the solubility of U and, to a lesser degree, Ni. Hydroxyapatite was also effective in reducing the solubility of Al, Ba, Cd, Co, Mn, and Pb. Sequential extractions indicate that HA transfers such metals from more chemically labile forms, such as the water-soluble and exchangeable fractions, by altering solid-phase speciation in favor of secondary phosphate precipitates. Hydroxyapatite effectiveness was somewhat reduced in the presence of soluble organics that likely increased contaminant metal solubility through complexation. Arsenic and Cr solubility increased with HA addition, suggesting that the increase in pH and competition from PO4 reduced sorption of oxyanion contaminants. Energy dispersive x-ray (EDXA) analysis conducted in the transmission electron microscope (TEM) confirmed that HA amendment sequesters U, Ni, Pb, and possibly other contaminant metals in association with secondary Al-phosphates.     

In situ treatment of metals in contaminated soils with phytate

Batch experiments were conducted to evaluate the ability of various forms of phytate, the hexaphosphoric form of myo-inositol (IP6), to immobilize U, Ni, and other inorganic contaminants in soils and sediments. A Ca-phytate precipitate (Ca(n)-IP6), dodeca sodium-phytate (Na12-IP6), and hydroxyapatite (HA) were added to contaminated soil at rates of 0, 10, 25, and 50 g kg(-1) and equilibrated in 0.001 M CaCl2. The samples were then centrifuged, the solution pH was measured, and the supernatants were filtered prior to analysis for dissolved organic carbon (DOC), U, Ni, P, and other inorganic contaminants, such as As, Cr, Se, and Pb. The residual sediments were air-dried prior to characterization by analytical electron microscopy and extraction with the Toxicity Characteristic Leaching Procedure (TCLP). The solubility of several metals (e.g., U, Pb, Cu) increased with increasing Na12-IP6 when compared with the nonamended control. In some cases immobilization was observed at the lowest Na12-IP6 application rate (10 g kg(-1)) with an increase in solubility observed at the higher rates, demonstrating the importance of metal to ligand ratio. In contrast, Ca(n)-IP6 and HA decreased the solubility of U, Ni, Al, Pb, Ba, Co, Mn, and Zn. For example, soluble U decreased from 2242 to 76 microg kg(-1) and Ni from 58 to 9.6 mg kg with the Ca(n)-IP6 addition, similar to the results observed for HA. Arsenic and Se solubility increased for HA and both forms of IP6, but to a much greater degree for Na12-IP6, suggesting that the increase in pH observed for HA and Na12-IP6, combined with added competition from PO4 and IP6 for sorption sites, resulted in the release of sorbed oxyanion contaminants. The analytical electron microscopy results indicated that metals such as U and Ni were closely associated with secondary Al-rich precipitates in the HA-treated soils, rather than unreacted HA. The analytical electron microscopy results were less definitive for the Ca(n)-IP6-treated soil, although the residual P-containing material was enriched in Al, with lesser amounts of U and Ni.     

Assessment of spatial variation of cesium-137 in small catchments

Surface contamination by bomb-derived and Chernobyl-derived 137Cs has been subject to changes due to physical decay and lateral transport of contaminated soil particles, which have resulted in an on-going transfer of radionuclides from terrestrial ecosystems to surface water, river bed sediments, and flood plains. Knowledge of the different sources of spatial variation of 137Cs is particularly essential for estimating 137Cs transfer to fluvial systems and for successfully applying 137Cs as an environmental tracer in soil erosion studies. This study combined a straightforward sediment redistribution model and geostatistical interpolation of point samples of 137Cs activities in soil to distinguish the effects of sediment erosion and deposition from other sources of variation in 137Cs in the small Mochovce catchment in Slovakia. These other sources of variation could then be interpreted. Besides erosion and deposition processes, the initial pattern of 137Cs deposition, floodplain sedimentation, and short-range spatial variation were identified as the major sources of spatial variation of the 137Cs inventory.     

Trace metals in river bed sediments: An assessment of their partitioning and bioavailability by using multivariate exploratory analysis

In this study, river bed sediments were submitted to a BCR sequential extraction, together with three bioavailability tests: a weak acid elutriate (HCl), a physiologically based extraction test (PBET) and a toxicity characteristic leaching procedure (TCLP). The most remarkable features of the BCR procedure were: i) Mn and Zn showed the highest proportion of the F1 exchangeable fraction; ii) in addition to Fe, Pb and Zn were the metals with the higher percentages in the F2 reducible fraction; iii) Fe and Cu were the elements with the highest proportion of the F3 oxidizable fraction; iv) the application of Principal Component Analysis to the metals in each of the three fractions did not show clear associations between metals and sediment components considered as metal scavengers, v) considering the sum of the three BCR fractions, the elements showed a decreasing availability of: Fe > Pb > Zn > Cu > Mn > Ni > Cr. The single extractions followed a decreasing extractability order of: HCl > PBET > TCLP and they were far from the extractability deduced from the sum of fractions in the BCR extraction.     

Uptake and release of cesium-137 by five plant species as influenced by soil amendments in field experiments

Phytoextraction field experiments were conducted on soil contaminated with 0.39 to 8.7 Bq/g of 137Cs to determine the capacity of five plant species to accumulate 137Cs and the effects of three soil treatments on uptake. The plants tested were redroot pigweed (Amaranthus retroflexus L. var. aureus); a mixture of redroot pigweed and spreading pigweed (A. graecizans L.); purple amaranth (A. cruteus L.) x Powell's amaranth (A. powellii S. Watson), referred to here as the amaranth hybrid; Indian mustard [Brassica juncea (L.) Czern.]; and cabbage (Brassica oleracea L. var. capitata). For control plants, the concentration ratios (CR) of 137Cs were greatest for redroot pigweed and the amaranth hybrid, with average CR values of 1.0 +/- 0.24 and 0.95 +/- 0.14, respectively. The lowest value was for Indian mustard at 0.36 +/- 0.10. The soil treatments included (i) application of NH4NO3 solution to the soil after plants had matured, (ii) addition of composted manure to increase organic matter content of the soil, (iii) combination of the manure and ammonium solution treatments, and (iv) controls. The ammonium solution gave little overall increase in accumulation of 137Cs. The use of composted manure also had little influence, but the combination of the composted manure with application of ammonium solutions had a distinctly negative effect on plant uptake of 137Cs. On average the fraction of 137Cs taken up from the soil was reduced by 57.4 +/- 1.2% compared with controls. This was the result of release of competing ions, primarily Ca, from the manure and was observed across all five plant species tested. The application of ammonium solution took place in the last two weeks before harvest. The reduction of plant 137Cs content, by addition of the ammonium solution, as it interacted with the manure, indicates that substantial quantities 137Cs can be released from the shoots of plants as a result of sudden changes in soil solution chemistry.     

Soil erosion and sediment sources in an Ohio watershed using beryllium-7, cesium-137, and lead-210

Soil cores and suspended sediments were collected within the Old Woman Creek, Ohio (OWC) watershed following a thunderstorm and analyzed for 7Be, 137Cs, and 210Pb activities to compare the effects of till vs. no-till management on soil erosion and sediment yield. The upper reaches of the watershed draining tilled agricultural fields were disproportionately responsible for the majority of the suspended sediment load compared with lower in the watershed (2.0-7.0 metric tons/km2 [Mg/km2] vs. 1.2-2.6 Mg/km2). About 6 to 10 times more sediment was derived from the subbasins that are predominantly tilled (6.8-12.4 Mg/km2) compared with the subbasins undergoing no-till practices (0.5-1.1 Mg/km2). In undisturbed soils the 210Pb activities decreased with movement toward the bottom of the cores to the constant supported 210Pb value at a depth of about 10 cm. There was a subsurface maximum in 137Cs activity within the top 10 cm. In contrast, the 210Pb and 137Cs distributions in soils that are currently or were previously tilled were nearly homogeneous with depth, reflecting continuing or previous mixing by plowing. The activities of 210Pb and 7Be were linearly correlated and were higher in suspended sediments derived from no-till subbasins than those derived from tilled subbasins, indicating that the soil surface is the source of suspended sediment. This study demonstrates that no-till farming results in decreases in soil erosion and decreases in suspended sediment discharges and that those eroded sediments have a radionuclide signature corresponding to the tillage practice and the depth of erosion.     

Uptake of cesium-137 and strontium-90 from contaminated soil by three plant species; application to phytoremediation

A field test was conducted to determine the ability of three plant species to extract 137Cs and 90Sr from contaminated soil. Redroot pigweed (Amaranthus retroflexus L.), Indian mustard [Brassica juncea (L.) Czern.], and tepary bean (Phaseolus acutifolius A. Gray) were planted in a series of spatially randomized cells in soil that was contaminated in the 1950s and 1960s. We examined the potential for phytoextraction of 90Sr and 137Cs by these three species. Concentration ratios (CR) for 137Cs for redroot pigweed, Indian mustard, and tepary bean were 2.58, 0.46, and 0.17, respectively. For 90Sr they were substantially higher: 6.5, 8.2, and 15.2, respectively. The greatest accumulation of both radionuclides was obtained with redroot pigweed, even though its CR for 90Sr was the lowest, because of its relatively large biomass. There was a linear relationship between the 137Cs concentration in plants and its concentration in soil only for redroot pigweed. Uptake of 90Sr exhibits no relationship to 90Sr concentrations in the soil. Estimates of time required for removal of 50% of the two contaminants, assuming two crops of redroot pigweed per year, are 7 yr for 90Sr and 18 yr for 137Cs.     

Pulsed redistribution of a contaminant following forest fire: cesium-137 in runoff

Of the natural processes that concentrate dispersed environmental contaminants, landscape fire stands out as having potential to rapidly concentrate contaminants and accelerate their redistribution. This study used rainfall simulation methods to quantify changes in concentration of a widely dispersed environmental contaminant (global fallout 137Cs) in soils and surface water runoff following a major forest fire at Los Alamos, New Mexico, USA. The 137Cs concentrations at the ground surface increased up to 40 times higher in ash deposits and three times higher for the topmost 50 mm of soil compared with pre-fire soils. Average redistribution rates were about one order of magnitude greater for burned plots, 5.96 KBq ha(-1) mm(-1) rainfall, compared with unburned plots, 0.55 KBq ha(-1) mm(-1) rainfall. The greatest surface water transport of 137Cs, 11.6 KBq ha(-1) mm(-1), occurred at the plot with the greatest amount of ground cover removal (80% bare soil) following fire. Concentration increases of 137Cs occurred during surface water erosion, resulting in enrichment of 137Cs levels in sediments by factors of 1.4 to 2.9 compared with parent soils. The elevated concentrations in runoff declined rapidly with time and cumulative precipitation occurrence and approached pre-fire levels after approximately 240 mm of rainfall. Our results provide evidence of order-of-magnitude concentration increases of a fallout radionuclide as a result of forest fire and rapid transport of radionuclides following fire that may have important implications for a wide range of geophysical, ecosystem, fire management, and risk-based issues.     

Radiocaesium accumulation in stemwood: integrated approach at the scale of forest stands for contaminated Scots pine in Belarus

Twenty years after the Chernobyl accident, root uptake from the surface layers of contaminated forest soils plays a major role in radiocaesium ((137)Cs) transfer to the trees and accumulation in perennial compartments, including stemwood. Trustworthy long-term predictions (modelling) of stemwood contamination with (137)Cs should accordingly be based on a reliable picture of this source-sink relationship. Considering the complexity of the processes involved in (137)Cs cycling in forest stands, elementary ratios like transfer factors (TF) were shown to be not very relevant for that purpose. At the tree level, alternatives like the wood immobilisation potential (WIP) have therefore been proposed in order to quantify the current net (137)Cs accumulation in stemwood. Our objective was here to compare WIP values determined for a series of contaminated forest stands in Belarus with the corresponding pools of (137)Cs available in the soil for root uptake. The comparison reveals that both indices are quite proportional, whatever the forest ecosystem features. This corroborates the relevancy of WIP as an indicator of the current (137)Cs root uptake by the trees, which could accordingly help to improve the existing models of (137)Cs cycling and the long-term management of contaminated forest ecosystems.     

日本福岛核事故后四川省部分生态系统和食物链的γ核素放射性水平调查

本文调查和掌握了四川省部分生态系统中放射性水平,分析了日本福岛核泄漏事故中放射性核素对四川省主要环境介质的影响和放射性核素的迁移和富集情况,了解此次事故对部分食物链和生态系统的影响程度。对气溶胶、沉降物、土壤、植物、动物、人类食物这一链条样品中的238U、232Th、226Ra、40K、137Cs、131I 6种放射性核素进行检测,结果表明,天然放射性核素238U、232Th、226Ra、40K的检测结果均属于环境正常水平,人工放射性核素137Cs、131I低于仪器检出限。所有样品的检测结果均符合相关国家标准,与环境背景值相当,日本福岛核事故在短时间内,未对四川省生态环境和食物链造成放射性影响。     

Predicting inter-taxa differences in plant uptake of cesium-134/137

For (134/137)Cs, and many other soil contaminants, research into transfer to plants has focused on particular crops and phytoremediation candidates, producing uptake data for a small proportion of all plant taxa. Despite the significance of differences in uptake between plant taxa, the capacity of soil-to-plant transfer models to predict them is currently confined to those taxa for which data exist, there being no method to predict uptake by other taxa. We used residual maximum likelihood (REML) analysis on data from experiments (including 89 plant taxa from China plus 32 phytoremediation candidates) together with data from the literature, to construct a database of relative (134/137)Cs concentrations in 273 plant taxa. The REML (134/137)Cs concentrations in plants are not normally distributed but significantly clustered. Analysis of variance (ANOVA), coded with a recent ordinal phylogeny for flowering plants, showed that plant taxa do not behave independently for (134/137)Cs concentration because 42 and 15% of inter-taxa differences are associated with phylogeny above the species and ordinal level, respectively. In general, Eudicots, and especially the Caryophyllales, Asterales, and Brassicales, have high (134/137)Cs concentrations, while the Fabales and Magnoliids, in particular Poales, have low (134/137)Cs concentrations. Plants of the stress-tolerant ruderal (S-R) growth strategy sensu Grime have, in general, high concentrations of Cs, while those of the competitive (C) and generalist (C-S-R) strategies have low concentrations, although these effects are less pronounced than those of phylogeny. Plant phylogeny and growth strategy might thus be used to predict a significant portion of inter-taxa differences in plant uptake of (134/137)Cs.     

Phosphorus extractability of soils amended with stockpiled and composted cattle manure

Managing fertilizer applications to maintain soil P below environmentally unacceptable levels should consider the contribution of manure and synthetic fertilizer sources to soluble and extractable forms of P. Our objective was to evaluate soil and manure characteristics and application rates on P extractability in recently amended soils. Five soils of the U.S. southern High Plains were amended with beef cattle manures, composted beef manure, and inorganic fertilizers [Ca(H(2)PO(4))(2) or KH(2)PO(4)] at five rates and incubated under controlled conditions. Mehlich 3-, Olsen (NaHCO(3))-, Texas A&M extractant (TAM)-, and water-extractable P were determined for the soils after selected incubation periods. Except for TAM and some water extractions, P extractability as a function of total P applied was linear (P < 0.001) for a wide range of application rates. Mehlich-3, NaHCO(3), and water P extraction efficiencies of KH(2)PO(4)-amended soils averaged 22, 34, and 115% greater (P < or = 0.036), respectively, than efficiencies of soils amended with manures except for the Texline (calcareous) loam and Pullman clay loam soils. Phosphorus extraction efficiencies decreased with time for KH(2)PO(4)-amended soils (P < 0.05) but remained stable or increased for manure-amended soils during the 8-wk incubation period. Across all soils and manure sources, changes in water-extractable P per unit increase in Mehlich 3-, NaHCO(3)-, and TAM-extractable P averaged 100, 85, and 125% greater, respectively, for inorganic as compared with manure-amended soils. These source-dependent relationships limit the use of agronomic soil extractants to make correct inferences about water-extractable P and dissolved P in runoff.     

Reservoir sedimentation and environmental degradation: assessing trends in sediment-associated trace elements in Grenada Lake, Mississippi

Sediments impounded within flood control reservoirs are potentially important archives of environmental and geomorphic processes occurring within drainage basins. The concentrations of select sediment-associated trace elements were assessed within the impoundment of Grenada Lake, a relatively large flood control reservoir in Mississippi with a history of contaminant bioaccumulation in fish. The post-construction sediments (after 1954) are discriminated from the pre-construction sediments (before 1954) based on depth variations in sediment texture and 137Cs emissions. The concentrations of select trace elements of the post-1954 sediments all are statistically greater than the pre-1954 sediments, and these same sediments also are enriched in clay. Once these concentrations are normalized by clay content, all trace elements in the post-1954 sediments are lower in concentration than the pre-1954 normalized sediments. Moreover, the trace elements when normalized by clay or Al content show virtually no change vertically (over time) within the reservoir impoundment. This suggests that the sources of these sediment-associated trace elements within Grenada Lake, whether natural or anthropogenic, have not changed appreciably over the lifespan of the reservoir and that the degradation of sedimentologic and ecologic indices within the lake are due to the sequestration of clay or clay-sized materials.     

Sources and evolution of anthropogenic lead in dated sediments from Lake Clair, Québec, Canada

Two sediments cores were collected from the deepest part of Lake Clair (Québec, Canada) to assess the historical sources of Pb additions to the lake. The cores were collected by divers by carefully inserting a Plexiglas tube into the sediments. To determine the stratigraphic ages of the sediments, (210)Pb and (137)Cs activities were counted by gamma-ray spectroscopy. Lead concentrations and isotopic ratios were performed by inductively coupled plasma-mass spectrometry (ICP-MS), following digestion of the samples with a mixture of HF, HNO(3), and HClO(4) acids and Pb separation by anion-exchange chromatography. Starting at the middle of the 19th century, Pb content of the sediments increased until 1975. The maximum Pb enrichment factor of 35 times (relative to the natural background) was found in sediments deposited in 1975. At this time, excess Pb flux was estimated to be about 0.03 g m(-2) yr(-1). Before 1872, the Pb isotopic ratios were relatively stable (mean (206)Pb/(207)Pb = 1.20 +/- 0.01), reflecting the natural Pb background. Between 1872 and 1894, the source of anthropogenic Pb was highly radiogenic as shown by the Pb isotopic signatures of the sediments (mean (206)Pb/(207)Pb = 1.22 +/- 0.01), possibly reflecting deforestation and agricultural developments in the St.-Lawrence Valley. Between 1894 and 1937, widespread use of industrial and domestic charcoals may explain the isotopic composition of Pb accumulated in the sediments (mean (206)Pb/(207)Pb = 1.19 +/- 0.01). From 1937 to 1975, Pb isotopic compositions became less radiogenic ((206)Pb/(207)Pb from 1.18 to 1.17) even though elemental Pb abundance reached extremely high values (623 mg kg(-1)). This isotopic shift reflects increased use of alkyl-lead in gasoline. For sediments accumulated between 1967 and 1996, the U.S. contribution to anthropogenic Pb accumulated in Lake Clair sediments amounted to between 30 and 63%.     

Cesium-137 monitoring using mosses from W. Macedonia,N. Greece

¹³⁷Cs activities in mosses and substrate (soil, bark) collected from W. Macedonia, Greece were measured 20 years after the Chernobyl reactor accident. Archive material from previous studies was also used for comparison and diachronic estimation of the radio-contamination status. A gradual decrease was detected which depended on various factors such as the collected species, location, growth rate and substrate. Maximum accumulation capacity of ¹³⁷Cs was observed in the epilithic mosses in comparison to the epiphytic ones. The ¹³⁷Cs content in the bark of the two broad-leaved species (oak and fagus) was higher than that of the conifer (pinus). Bark specimens of about 50 cm height were in general more contaminated than those of 200 cm. Autoradiography revealed an amount of ¹³⁷Cs distributed more or less uniformly in moss thalli. The high ¹³⁷Cs activities found in mosses 20 years after Chernobyl suggest that these primitive plants are effective, suitable and inexpensive biological detectors of the distribution and burden of radionuclide fallout pattern.     

Tree-ring strontium-90 and cesium-137 as potential indicators of radioactive pollution

To examine whether tree rings can be used to detect or assess local historical 90Sr or 137Cs fallout, such as that resulting from the Hiroshima atomic bomb, radial distribution of 90Sr and 137Cs in trees was examined. We studied a gymnosperm [Japanese cedar, Cryptomeria japonica (L. f.) D. Don] and an angiosperm (Japanese persimmon, Diospyros kaki Thunb.) tree species from the vicinity of the atomic bomb hypocenter, and from other locations in Japan. A significant amount of 137Cs was detected in tree rings formed before 1945, indicating lateral migration of Cs. In contrast, the specific activity of 90Sr in the Hiroshima Japanese cedar showed the highest level in 1945, due to relatively immobile characteristics of Sr compared with Cs. Strontium-90 and Sr analyses in tree rings helped identify and distinguish between residual 90Sr activity from the Hiroshima atomic bomb and the atmospheric nuclear testing. This indicates the possibility of detecting or assessing previous local 90Sr pollution through with treering analysis.     

90Sr、137Cs在某种包气带土壤中的迁移研究

本文叙述了放射性废物中具有代表性的裂变核素^90Sr、^137Cs在某种包气带土壤中的迁移情况研究。使用小型土柱的氚水淋洗实验研究土壤水力学性质，最后进行大型土柱实验研究核素在该包气带土壤中的迁移，并对实验情况进行了数学模拟。用该土壤原状土进行小型土柱的氚水淋洗试验，测得了土壤的水力弥散度为0．32cm，土壤有效孔隙度为0．35。经过290天的大型柱迁移试验表明，土壤对^90Sr的阻滞系数为220．4，在模拟实际降雨量的情况下，^90Sr的平均迁移速度为0．63cm/y，^137Cs在大型柱试验中没有明显迁移。数值模拟^90Sr、^137Cs迁移，得出经过上述大型柱试验相同的条件下，^90Sr、^137Cs迁移的峰位置基本和大型柱试验结果相同。     

Prediction of radionuclide aging in soils from the Chernobyl and Mediterranean areas

The aging of soil-pollutant interaction, which may lead to an increase in pollutant fixation, is the main driving force in the natural attenuation of contaminated soils. Here a test was evaluated to predict the aging of radiostrontium and radiocesium in soils from the Chernobyl and Mediterranean areas. After contamination, soils were maintained at various temperatures for up to 12 mo, with or without drying-wetting (DW) cycles. Changes in the quantity of radionuclide reversibly sorbed over time were monitored using an extraction test (1 mol L(-1) NH(4)Cl; 10 mL g(-1); 16 h). The fixed fraction could not be predicted from soil properties controlling the sorption step. Aging was not as relevant for Sr as for Cs. The time elapsed since contamination was the main factor responsible for the slight (up to 1.3-fold) decreases in Sr extraction yields. The additional effect of DW cycles was negligible. Instead, all factors accelerated Cs aging due to the enhancement of Cs trapping by clay interlayer collapse, with up to 20-fold increases in Cs fixation. The DW cycles also caused secondary effects on the Cs-specific sorption pool, which were beneficial or detrimental depending on the soil type. Extraction yields from laboratory aged samples agreed with those from field samples taken a few years after the Chernobyl accident. These results confirm the prediction capacity of the laboratory test and its usefulness in risk assessment exercises and in the design of intervention actions, particularly because neither fixation nor aging were related to the soil properties, such as clay content.     

Chemical extraction methods to assess bioavailable arsenic in soil and solid media

Soil ingestion by children is an important pathway in assessing public health risks associated with exposure to arsenic-contaminated soils. Soil chemical methods are available to extract various pools of soil arsenic, but their ability to measure bioavailable arsenic from soil ingestion is unknown. Arsenic extracted by five commonly used soil extractants was compared with bioavailable arsenic measured in vivo by immature swine (Sus scrofa) dosing trials. Fifteen contaminated soils that contained 233 to 17 500 mg kg(-1) arsenic were studied. Soil extractants were selected to dissolve surficially adsorbed and/or readily soluble arsenic (water, 1 M sodium acetate, 0.1 M Na2HPO4/0.1 M NaH2PO4) and arsenic in Fe and Mn oxide minerals (hydroxylamine hydrochloride, ammonium oxalate). The mean percent of total arsenic extracted was: ammonium oxalate (53.6%) > or = hydroxylamine hydrochloride (51.7%) > phosphate (10.5%), acetate (7.16%) > water (0.15%). The strongest relationship between arsenic determined by soil chemical extraction and in vivo bioavailable arsenic was found for hydroxylamine hydrochloride extractant (r = 0.88, significant at the 0.01 probability level). Comparison of the amount of arsenic extracted by soil methods with bioavailable arsenic showed the following trend: ammonium oxalate, hydroxylamine hydrochloride > in vivo > phosphate, acetate > water. The amount of arsenic dissolved in the stomach (potentially bioavailable) is between surficially adsorbed (extracted by phosphate or acetate) and surficially adsorbed + nonsurficial forms in Fe and Mn oxides (extracted by hydroxylamine hydrochloride or ammonium oxalate). Soil extraction methods that dissolve some of the amorphous Fe, such as hydroxylamine hydrochloride, can be designed to provide closer estimates of bioavailable arsenic.