British Columbia Mine Reclamation Symposia

Tools for assessing soil metal bio-availability 2010

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TOOLS FOR ASSESSING SOIL METAL BIO-AVAILABILITY  Dale Doram   Golder Associates Ltd. #300, 10525-170 St. Edmonton, AB. T5P 4W2  ABSTRACT  Metals or trace elements are natural constituents and occur throughout the biosphere. While metals or trace elements are naturally present in our environment, in some instances they are elevated above provincial or federal generic soil quality guidelines due to industrial processes like air emissions. Theoretically, elevated soil metal concentrations above generic guidelines mean there could be potential local impacts to terrestrial or aquatic receptors.  Since generic regulatory guidelines are derived by analyzing “total” soil metals using strong acid digests, they do not consider the chemical form and bio- availability of the specific metals on a site-specific basis. Using case studies and the literature, the limitations of using generic soil metal guidelines to assess site-specific receptor impacts will be discussed. Two tools for evaluating metal bio-availability, sequential extraction and bio-assays, are discussed. These tools can be used in setting site-specific remediation objectives for a site that exceeds generic regulatory criteria.  Key words:  metals, remediation, sequential extraction, bio-availability, bio-assays.  INTRODUCTION  Metals or trace elements are natural constituents and occur throughout the biosphere (Kabata-Pendias 2000).  In chemistry, a metal (Greek: Metallo, Μέταλλο) is an element, compound, or alloy characterized by high electrical conductivity.  Many types of metals (alkali, transition, post-transition) occur in nature. The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic at low concentrations (Sparks 2005).  Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), copper (Cu), zinc (Zn) and lead (Pb).  Heavy metals occur naturally in the earth's crust and cannot be degraded or destroyed (Sparks 2005). Sources of exposure for metals include direct ingestion or via food, drinking water and air. As trace elements, some heavy metals (e.g., copper, selenium, zinc) are essential to both plant and human health (Kabata-Pendias 2000). However, at higher concentrations they can lead to toxicity (Kabata-Pendias 2000, Sparks 2005). Heavy metal toxicity in humans could result from drinking-water contamination (e.g., lead pipes), high ambient air concentrations near emission sources, or intake via the food chain (Sparks 2005).  Heavy metal toxicity can increase over time through bio-accumulation (Kabata-Pendias 2000, Sparks 2005). Bio-accumulation means increasing the concentration of a chemical in a biological organism over time, compared to the chemical's concentration originally in the environment (Sparks 2005).  Heavy metals can enter the terrestrial environment by industrial and consumer waste (bio-solid composts), or from acidic conditions breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater (Sparks 2005, Figure 1).   Figure 1 Metal-Human Relationships in the Environment  The impacts of heavy metals on soil quality could include direct and indirect effects. Direct effects would include effects on soil organisms such as bacteria and earthworms. Research to date suggests there is contradictory evidence whether metals in soil containing municipal solid waste composts may harm soil microorganisms (Woodbury 1997). Indirect effects would include any impacts on plant germination and growth.  The impacts of heavy metals on water quality would derive from the leaching of the metals in surface runoff or soil erosion which could transport metal containing soil, to surface water bodies. Research to date suggests that heavy metals applied to soils through bio-solids (BS), are relatively immobile in soil at pH levels above 6.0, and are therefore unlikely to leach into surface or groundwater systems (Williams et al., 1980). In addition, the availability of the metals decreases with time, due to reactions with clays, organic matter and oxides of iron (Williams et al. 1980).  EVALUATING METALS IN SOIL AND ANALYTICAL LIMITATIONS  If a site is suspected of containing metals, the following general approach is used:   Take soil sample of site and of appropriate background areas use proper QA/QC, chain-of- custody procedures;  Submitting the soil samples to the lab and doing a full metal screen; and  Comparing the results to either provincial guidelines or CCME soil metal criteria (i.e., for lead CCME Criteria is 70 mg/kg).  If the soil metal values exceed regulatory criteria, the following approach is typically used:   Remediate the site by remove the soil to a landfill (expensive); or  Undertaking a risk assessment to develop site specific regulatory criteria (also expensive).  What are the implications of small regulatory exceedances of metals? What happens if background sites metal concentrations are also above regulatory criteria? Since the standard laboratory procedure for analyzing metals in soil uses “strong acid digests” to measure “total metals”, what is the “bio-availability” of the metals of concern at a site to specific receptors? Various investigators have identified limitations to conventional metal analytical procedures which include:   Standard metal tests use “strong acid digests” to provide “Total metals”; and  Metal geochemistry is complex and “Total” values may not show impacts to receptors.  While regulatory guidelines assume that the “total metal content” is bio-available for setting regulatory criteria, this assumption may be over conservative in protecting human and environmental health (EPA 2010). Using the bio-availability approach in conjunction with “total values” can reduce uncertainty in risk assessments and avoid unnecessary remediation expenses (EPA 2010).  Two tools for evaluating site-specific metal bio-availability, sequential extraction and bio-assays, will be discussed in case studies.  SEQUENTIAL EXTRACTION BACKGROUND  Elements in soils are associated with a number of physico-chemical forms that in turn influence availability (Ahnstroma and Parkera 2010). Sequential chemical extraction techniques have been widely used to examine these physicochemical forms, and thus to better understand the processes that influence element availability (Shuman 1985).  In a sequential extraction procedure (SEP), a soil sample is treated with a series of progressively harsher reagents to dissolve increasingly refractory forms (Ahnstroma and Parkera 2010). Ideally, the reagents are chosen to selectively attack a specific soil compartment with minimal dissolution of non-targeted phases. In practice, however, the discrete extraction of any given phase may be unachievable (Tessier et al. 1979). Sequential extractions have been used to characterize forms of metals that have different mobilities (Tessier et al. 1979; Shuman 1985; Mench et al. 1994). In the Tessier method (Tessier et al. 1979), different reagents are used to extract the various forms of metals variously defined as exchangeable, carbonate, iron and manganese oxide, organic and residual. The assumptions used to interpret these data are that the readily plant-available metals are the water soluble and exchangeable form, while the potentially available metals are in carbonate, iron and manganese oxide, and organic forms. The residual form of metals is considered to be unavailable to plants and has been used in some cases to be the measure of background concentration (Tessier et al. 1979).  SEQUENTIAL EXTRACTION CASE STUDY: INDUSTRIAL SITE IN AGRICULTURAL AREA  An industrial site in BC was investigated and a full metal suite was analyzed including arsenic, cadmium, chromium, copper, lead, nickel, silver, thallium, and zinc. There were several metals that exceeded regulatory criteria so sequential extraction was selected to study metal bio-availability at the site. The sequential extractions were undertaken by Vizon SciTec Inc. on surface horizons and the following metals were studied:  arsenic, cadmium, chromium, copper, lead, nickel, silver, thallium, and zinc. The pH values for the surface horizons samples used for sequential extraction ranged from 3.8 to 5.7.  The objective of the analysis was to determine which soil variables influence the amount of various soil metal fractions, namely, water soluble, exchangeable, carbonate, organic, oxide, and residual. All fractions were examined for the metals of interest:  lead, zinc, arsenic and cadmium.  One of the metals, zinc will be used to illustrate the use of sequential extraction to determine plant bio- availability. The total Zn concentration at the site was 411 mg/kg and the CCME Agriculture Criteria is 200 mg/kg. The question was how much Zn is “bio-available” at this site and would this level of zinc be toxic to agricultural crops?  At the site a modified Tessier method was used to separate metals into six fractions (Table 1, Figure 2).  Table 1 Sequential Extraction Procedure for Industrial Site Fraction # Fraction Reagents Availability to Plants 1 Water soluble De-ionized water Highly available 2 Exchangeable 1 M sodium acetate (pH 8) Available 3 Carbonate bound 1 M sodium acetate (pH 5) Potentially available 4 Fe-Mn Oxides bound 0.04M hydroxylamine hydrochloride (pH 2) Potentially available 5 Organic/sulphide bound 0.02M HNO3/H2O2 + H2O2 + ammonium acetate in 20% HNO3 Potentially available 6 Residual 7.5 mL HCL + 2.5 mL HNO3 Unavailable    Figure 2 Metal Fractions Evaluated in Sequential Extraction Procedures  In this extraction procedure, the water soluble and exchangeable are considered the quickly bio-available fraction, while potential bio-available fractions are the (organic, Fe/Mn oxide and carbonate fractions (Figure 2).  The structurally bound fraction is considered not bio-available (Figure 2).  Table 2 Summary of Sequential Extraction Metal Fractions (%) Metal Highly Bioavailability and Mobility Moderately Bioavailability and Mobility Low or Nil Bioavailability and Mobility Water Soluble Fraction (%) Exchangeable Fraction (%) Carbonate Fraction (%) Oxide Fraction (%) Organic Fraction (%) Residual Fraction (%) Zinc 2 3 14 39 8 34  In interpreting the zinc sequential the following was considered. The quickly bio-available fractions (water soluble +exchangeable fractions) were found to equal about 5% of the “total zinc” content of 411 mg/kg (21 mg/kg, Table 2). The question investigated was whether 21 mg/kg of zinc would be toxic to agricultural crops?  Kabata-Pendias (2000) reported that >300 mg/kg of “bio-available” zinc is toxic to barley. In addition, according to the Ontario Ministry of Agriculture and Food, 15 to 25mg/kg of “available” zinc is adequate for agricultural crop growth. Soil chemistry (pH, etc) can affect plant availability of zinc and other metals.  In relation to the zinc levels at this site, it was concluded that they wouldn’t be toxic to agricultural crops even though they exceed regulatory criteria.  If aquatic receptors were present at a site like this, then evaluating the “water soluble” levels against known regulatory criteria for the protection of aquatic life could be undertaken.  BIO-ASSAYS BACKGROUND  Conventional chemical analysis of soil may not isolate all of the potential toxic constituents in metal containing media such as bio-solid compost. An alternative to conventional chemical analysis is the toxicity-based approach, which utilizes the responses of living organisms (bioassays), to detect the presence of toxic constituents. If there is a negative response of the bioassays to the material in question, then chemical analysis can be undertaken to isolate the specific toxic substances causing the responses.  A bioassay is a technique by which organisms (i.e., whole plants or animals), biological systems (e.g., tissues), or biological processes (e.g., enzymatic activity) are used to measure the biological effects of a substance (EPA 1987).  Typically, a hazardous site bioassay involves laboratory testing of soil, soil leachates, water, or sediment samples using a standard array of test organisms under controlled laboratory conditions (EPA 1987).  Bioassays have two important advantages over chemical analyses (EPA 1987).  They can directly measure the effects of a known or suspected contaminant on biota, and they are, in general, inexpensive compared to complete chemical analyses.   Living organism bioassays can also provide the following advantages when determining the toxicity of materials, compared to chemical analysis alone (Keddy et al. 1994):   Living organisms reflect the effects of the positive and negative chemical impacts, because they respond to the biologically active components of complex materials;  Bioassays provide a more direct measurement of toxicity compared to chemical analyses, because the results are an integration of all environmental variables and contaminants; and  Bioassay end-points are quantitative measures of toxicity.  Why consider bio-assays in soil metal bio-availability investigation?:   Environment Canada and EPA recognize bioassays for evaluating metal impacts; and  They are useful for small exceedances of regulatory criteria (e.g. biosolid composts where As, Cd, Zn can be elevated).  BIO-ASSAY CASE STUDY BIO-SOLID COMPOST  In the late 1990’s TransAlta and the City of Edmonton proposed the worlds largest co-compost facility in Edmonton, Alberta (Doram and Bateman 1998).  At that time, TransAlta planned to utilize and market compost, from the proposed City of Edmonton composting project, as a soil amendment (Doram and Bateman 1998). The compost product was from a mixture of Municipal Solid Waste (MSW) and bio- solids (BS) and was named Nutri-Plus. Nutri-Plus was to be marketed as a soil amendment for the agriculture and horticulture industries. As part of the product registration and project planning process, TransAlta wanted to evaluate whether there are any potentially toxic constituents in the compost that could affect surface water and the terrestrial (soil/plant) agricultural land ecosystem (Doram and Batemen 1998). The potential impact of heavy metals from the compost was identified as an important issue by stakeholders and TransAlta. Total heavy metal analysis of nine batches of compost samples from a pilot program undertaken in 1996, verified that heavy metals such as arsenic, cadmium, lead and mercury are present in the compost.  The pilot study results suggested metals  such as As, Zn Pb were either close to or exceeded CCME Compost Criteria values (Doram and Batemen 1998).  The objectives of the study were:   To determine if composting meets applicable CCME Category A Compost Criteria to allow unlimited use;  To use the toxicity results as an indicator of the potential aquatic and terrestrial environmental impacts of applying the compost to agricultural land;  Identify composting process enhancements; and  To determine if compost can be used for TransAlta’s coal mine reclamation program.  Table 3 summarizes the tests used on the soil/compost mixture.  Table 3 Bio-Solid Compost Testing Procedure Tests  Acute Test?  Chronic Test?  Rationale Microtox  Yes   bacteria Daphnia Magna 48h  Yes   invertebrate Flathead Minnow 48h  Yes   fish Algae   Yes  eucaryotic Lettuce emergence, root elong.  Yes   sensitive vegetable Barley emergence, root elong.  Yes   sensitive cereal Earthworm survival 7 and 14d  Yes   soil lumbridae  The example lettuce seedling emergence results (Figure 3), shows that the soil/compost mix showed better seedling emergence than either the soil alone or the control.  This suggests that the bio-solid compost is aiding seedling emergence so in non-toxic to the soil (Doram and Batemen 1998).  The other bio-assay results supported these trends as none of the terrestrial (seedling emergence, earthworm survival, root elongation) or aquatic (Microtox, Daphnia Magna, algal growth, fathead minnow survival) biological indicators were found to be affected by the metal containing bio-solid compost (Doram and Batemen 1998).                        Figure 3: Lettuce Seedling Emergence Results CONCLUSIONS  The following were concluded from these Case Studies:   Conventional  soil metal results doesn’t  supply information on metal “bio-availability;  Sequential extraction is an alternative for evaluating metal “bio-availability” when analytical results are found to exceed regulatory criteria;  Bio-assays are useful for small regulatory criteria exceedances for substances like bio-solid composts; and  These options don’t replace standard Risk Assessment or Tier 2 Criteria development but are part of and overall tool box for evaluating soil metal “bio-availability’ that can be useful for assessing impacts to receptors.  REFERENCES  Ahnstrom, Z.S. and D.R. Parker. 2010.  Development and assessment of a sequential extraction procedure for the fractuionation of soil cadmium.  SSSA 63: 1650-1658.  Doram, D.R. and J. C. Bateman. 1998.  An evaluation of the potential terrestrial and aquatic toxicity of Nutri-plus compost when used as a soil amendment on agricultural land.  35 th  Annual Alberta Soil Science Workshop Proceedings.  Keddy, C., Greene, J. and Bonnell, M. 1997. A Review of Whole Organism Bioassays for Assessing the Quality of Soil, freshwater Sediment, and Freshwater in Canada. Scientific Series No. 198. Prepared for the CCME Subcommittee on Environmental Quality Criteria.  US Environmental Protection Agency (EPA). 1987.   Role of Acute Toxicity Bioassays in the Remedial Action Process at Hazardous Waste Sites.  Environmental Research Laboratory, Corvallis, OR.  US Environmental Protection Agency (EPA). 2010.  Bioavailability of Metals in Contaminated Soil and Dust.  US EPA website.  Kabata-Pendias, A.  2000.  Trace Elements in Soils and Plants.  Third Edition.  CRC Press.  Boca Raton, FL.  413p.  Mench, M.J., V.L Didier, M. Loffer, A. Gomez and P. Masson.  1994.  A Mimicked In-situ Remediation Study of Metal-contaminated Soils with Emphasis on Cadmium and Lead.  J. Envirom. Qual. 23:58.  In: Laperche, 2001.  Immobilization of Lead by In Situ Formation of Lead Phosphates in Soils.  Chapter 3 in Iksander, I. K.  2001.  Environmental restoration of Metals Contaminated Soils.  Lewis Publishers.  Shuman, L.M.  1985.  Fractionation Method for Soil Microelements.  Soil Science, 140:11-22.  Sparks, D. L. 2005.  Toxic metal in the environment: the role of surfaces.  Elements Vol 1 193-197.  Tessier, A, P.G.C. Campbell and M. Bisson.  1979.  Sequential Extraction Procedure for the Speciation of Particulate Metals.  Analytical Chemistry 51:844-851.  Williams, D., Vlamis, J., Pakite, A. and Corey, J. 1980. Trace element accumulation, movement, and distribution in the soil profile from massive application of sewage sludge. Soil Science 129: 119-132.  Woodbury, P. 1997. Municipal Solid Waste Composting: Key Aspects of Compost Quality Assurance. Cornell University Composting Fact Sheet 7. 


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