A Methodological Approach for the Evaluation of Soil Pollution by Potentially Toxic Trace Elements

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in: JOURNAL OF GEOCHEMICAL EXPLORATION on 2019, available at: https://doi.org/10.1016/j.gexplo.2019.04.005” 1 1A METHODOLOGICAL APPROACH FOR THE EVALUATION OF 2SOIL POLLUTION BY POTENTIALLY TOXIC TRACE ELEMENTS 3Emilio Galán, Antonio Romero-Baena, Patricia Aparicio, Isabel González 4Departamento de Cristalografía, Mineralogía y Química Agrícola. Universidad de 5Sevilla, Prof. García González nº 1, 41012 Sevilla (Spain) 6 7ABSTRACT 8Risk assessment of soils polluted by potentially toxic trace elements 9generally is a complex procedure that includes many variables and 10 parameters, many of them difficult to assess. The proposed methodological 11 approach is an easier procedure to obtain reasonable security for the 12 potential risk of a soil contaminated by trace elements. This approach can 13 be successfully applied in most of the study cases, avoiding a sometimes 14 flawed risk analysis procedure, which many times leads to an unnecessary 15 declaration of soil pollution, resulting in mandatory and expensive 16 reclamation actions. 17 The protocol supposes that: a) high concentrations of potentially toxic trace 18 elements exceeding generic reference levels are not always a risk to human 19 health, and b) the mobile fractions of potentially toxic trace elements and 20 their bioavailability are the key factors to consider in any sort of risk 21 assessment, when such assessments are needed. 22 The proposal consists of successive steps that ranges from a simple 23 documentation and investigation of available information of the history, 24 present state of the potential polluted site (PPS), mapping and field works 25 to a detailed study that involves determination of local reference values, 26 geoaccumulation factors, soil parameters, mobility, bioavailability, 27 operational speciation, etc. For As, Hg, Cd, and Pb, particular procedures 28 are also proposed. 29 As can be deduced from the proposal, anomalously high concentrations of 30 potentially toxic trace elements in a soil, although they may exceed the 31 generic reference levels, do not always pose a health risk. Thus, risk 32 assessment is only necessary in a very few cases. 2 33 34 KEY-WORDS: Soil pollution assessment; potentially toxic trace elements; 35 generic reference level; bioavailability; risk analysis. 36 37 38 INTRODUCTION 39 Soil is a dynamic system that is capable of trapping potentially toxic trace 40 elements (PTTEs) (including heavy metals and metalloids) avoiding their 41 passing to more sensitive media such as the hydrosphere and the biosphere. 42 Thus, soil acts a protective barrier by immobilizing and accumulating 43 PTTEs. The capacity for trapping them is dependent on the soil properties 44 and the physical and chemical environmental conditions. When a soil 45 reaches critical loading levels, it can be considered as polluted (for 46 example, Rieuwerts et al. 1998; Cheng et al. 2001; Salvagio et al 2002, 47 Nickolson et al 2003, de Vries et al. 2013). The assessment of soil pollution 48 is of paramount importance for the protection of soil ecological functions 49 and for agriculture (Wuana and Okieimen, 2011). 50 Trace elements can accumulate in soils by either geogenic or anthropogenic 51 processes. In fact, PTTEs loadings in soils usually reflect both geogenic 52 and anthropogenic contributions. The presence of geogenic trace elements 53 in soils is largely determined by their geoavailability, which is that portion 54 of the total content of a chemical element or a compound in an earth 55 material that can be liberated to the surficial or near-surface environment 56 through mechanical, chemical, or biological processes (Plumlee, 1994). 57 Geoavailable trace elements typically reach the soil when they are released 58 from their parent rock because of weathering. Together with metals derived 59 from volcanic emissions or from the leaching of ore minerals, they 60 constitute the so-called “geogenic metals”. Generally, the amount of 61 geoavailable trace elements released from the parent rocks into soil is 62 negligible compared to the amount having an anthropogenic origin. The 63 highest levels of trace elements in soils that are inherited from the parent 64 rock are typically those of Cr, Mn and Ni, followed by Co, Cu, Zn and Pb; 65 moreover, As, Cd and Hg are usually present at very low concentrations. 66 Nevertheless, in some cases the geogenic contributions in soils can be as 67 high as 20% of the total (Ludden et al. 2015). 3 68 69 Anthropogenic trace elements enter the soil via several pathways and 70 activities, including aerial deposition, acid mine drainage, smelting 71 operations, fertilizer and pesticide applications, sewage sludge amendment, 72 dredged sediment disposal, irrigation water and some urban activities 73 (González et al. 2011; Wuana and Okieimen, 2011). One of the principal 74 source of soil contamination is mining activity, which is considered as a 75 problem of global dimensions around the world (for example, Davies and 76 Ballinger, 1990; Lin et al., 2005; Aleksander-Kwaterczak and Hellios- 77 Rybicka, 2009; González et al., 2011; Madejón et al., 2011; Carvalho et al., 78 2012; Kossoff et al. 2012; Abreu et al., 2014b; Perez Sirvent et al 2014, 79 Romero et al., 2018). 80 The distinction among geogenic and anthropogenic trace elements in a soil 81 is of paramount importance for determining whether a soil is polluted 82 (Galán et al., 2014a; Petrik et al., 2018). Firstly, only the anthropogenic 83 contributions are of primary concern in soil pollution, because they are the 84 only ones that are legally considered as true contaminants. On the other 85 hand, geogenic elements usually are below threshold values (González et 86 al. 2011) and, even if these levels are exceeded, they usually show a low 87 availability, which reduce the risk for toxicity (Romero et al., 2012; Galán 88 et al., 2014a; Romero et al., 2018). 89 Distinguishing the contribution of geogenic and anthropogenic sources is 90 not always easy to achieve, particularly in mining areas. A good and 91 practical methodological approach for estimating the geogenic contribution 92 in a soil potentially polluted by PTTEs was recently proposed by Galán et 93 al. (2014a). That paper also discusses the limitations of other “classical” 94 approaches proposed for the estimation of the anthropogenic input versus 95 the background, namely the contamination factor (CF), the enrichment 96 factor (EF) and the geoaccumulation index (Igeo). 97 PTTEs can be mobilized by physical, chemical and biological vectors, and 98 they can leave soils by volatilization, dissolution, leaching or erosion. 99 When PTTEs are in a bioavailable (i.e., relatively soluble) form, they can 100 enter organisms (Adriano, 2001; Kabata-Pendias, 2004, 2011). Thus, the 101 contamination by trace elements represents one of the most prominent 102 environmental potential hazards to health, and the assessment of soil 4 103 pollution is one of the most important environmental issues. 104 Presently, most procedures for assessing soil pollution due to trace 105 elements are based on chemical analysis. When the total content of a PTTE 106 in a soil exceeds the Generic Reference Level (GRL) (usually referred as 107 screening level), i.e. the element concentration in soil below which no 108 hazard may be expected, the soil is considered as potentially polluted and a 109 risk analysis is mandatory (US-EPA, 2001; Carlon 2007). 110 However, the total content is not a significant value from a toxicological 111 point of view (Singh, 1997; de Groot et al., 1998), because trace elements 112 in soils interact with other components and thereby change their activity, 113 mobility and their derivative effects (Baker et al., 2003; Adriano et al., 114 2004). Even the “total” or pseudototal contents of trace elements in soils, 115 (those measured on aqua regia solutions) (Tarvainen and Kallio, 2002; 116 Albanese et al. 2007), which are used by the EU as indicators for 117 environmental policies, do not represent accurate information about the 118 available fractions of those elements. Instead, they often represent the 119 worst possible situation and overestimate the real risk. 120 In addition, each state or region calculates its own GRL using different 121 approaches, which result in very different values. Thus, this reference 122 should be just one aspect that should be considered in a more general 123 assessment. 124 On the other hand, classic “risk analyses” do not perform well in assessing 125 pollution by trace elements, due to the geogenic contribution (Galán et al., 126 2014a), and the most frequently applied programs may enter unwarranted 127 results (Galán et al., 2014b; Rivera et al., 2016). 128 Actually, the official regulations based on the criteria listed above are not 129 very accurate for soil polluted by trace elements, because the criteria are 130 not very scientifically sound. Therefore, a new approach for the 131 management of soil polluted by PTTEs is proposed. A good methodology 132 should take into account the mobility of the trace elements in the soil and 133 the bioavailable fraction, which is the most potentially toxic fraction 134 (Lanno et al., 1999), as well as information on the elements’ operational 135 and mineralogical speciation. Those data are also of utmost importance to 136 predict the remaining health risk after remediation actions. 5 137 The proposal described in this paper summarizes a protocol which has as 138 its objective the creation of a decision tool for use by environmental 139 administration, enterprises and private owners to determine whether a soil 140 represents an unacceptable health risk because of its content of PTTEs 141 (particularly As, Cd, Cr, Co, Cu, Hg, Ni, Pb and Zn). The results of the 142 application of this protocol to a potentially polluted site may be used to 143 declare whether the site is polluted and whether it needs reclamation 144 actions, if so. 145 The proposal is based on the following criteria: the mobility, availability, 146 bioavailability and speciation of the trace elements; the soil parameters 147 controlling the trace element mobility; the assessed origin of the elements 148 in the soil (geogenic vs. anthropogenic origin); the regional and local 149 geochemical baselines of the site where the soil occurs; and the generic 150 reference levels. 151 The proposed protocol supposes that a) high concentrations of toxic trace 152 elements exceeding GRL are not always a risk to human health, and b) the 153 mobile fractions of toxic trace elements contained in a soil and their 154 bioavailability are the key factors to consider in any sort of risk assessment, 155 when such assessments are needed. 156 To permit better understanding of the protocol and its application, the 157 concepts discussed above will be developed briefly. 158 159 BASIC ISSUES AND TERMINOLOGY USED IN THE PROTOCOL 160 Trace elements are present at low concentrations (mg.kg−1) in the earth’s 161 crust, soils and plants. Many of them are essential for plant, animal and 162 human growth but can be toxic above certain concentration levels, which 163 differ from one element to another (Siegel, 2002). Occasionally, soil is 164 polluted with high concentrations of major elements such as Na, Fe or Al. 165 From all of the trace elements found in soil, the latest US Environmental 166 Protection Agency (EPA) list of priority pollutants includes the following 167 13 trace elements: Ag, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Se, Tl and Zn. 168 Among these elements, the most common in polluted soils are As, Cd, Cr, 169 Cu, Hg, Pb, and Zn. However, Co and Ni are also very abundant in some 6 170 types of soils; in spite of their lower toxicity, the protocol proposed will be 171 focused on those nine elements. The Agency for Toxic Substances and 172 Disease Registry (ATSDR) has determined the minimal risk level of these 173 elements; their more significant health toxic effects are summarized in 174 Table 1. 175 By definition, a pollutant is a contaminant, but not all contaminants are 176 pollutants (Chapman, 2007). A pollutant in a soil must always be found in 177 anomalous (higher than usual) concentrations and must have a harmful 178 impact on some organisms. Thus, an element (whether it is a contaminant 179 or not) must pass from the soil into the solution. That is, the element should 180 be available and the danger it poses will depend on its “availability”. The 181 more specific term bioavailability is the degree to which a contaminant in a 182 potential source is available for uptake (that is, movement into or onto an 183 organism) (Newman and Jagoe, 1994). 184 An easy method to define the soil concentration of trace elements available 185 for uptake by biota was applied to soils in Finland by Tarvainen and Kallio 186 (2002). This method consisted of the determination of “total” (hot aqua 187 regia extraction) and “bioavailable” (acetic acid, ammonium acetate and 188 EDTA extraction) trace element concentrations. In such cases, the 189 “bioavailable” concentrations were only 1-5% of the “total” concentrations. 190 There are a lot of methods for assessing the mobility and availability of 191 PTTEs, such as calcium chloride, calcium nitrate, ammonium nitrate or 192 ammonium acetate (Kabata-Pendias, 2004; Anjos et al., 2012; Abreu et al., 193 2012, 2014a, Pinto et al., 2015). Other simple methods commonly used are 194 extraction with neutral or slightly acidic water and extractions with DTPA 195 or 0.05 M EDTA at pH 7, which are particularly effective for assessing the 196 transferability of divalent cations to plants (Quevauvilier et al., 1998; 197 Kabata-Pendias, 2004; Abreu 2014b; Anjos et al., 2012; Monterroso et al., 198 2014; Pinto et al., 2015). These methods can only be used to discriminate 199 between residual metals and metals extracted by water or a mild organic 200 solvent. 201 The distribution coefficient between soil and water governs the 202 bioavailability of trace elements to organisms contacting the soil. Weakly 203 bound elements are highly bioavailable, and strongly bound elements are 204 less bioavailable. Therefore, the application of these methods may yield 7 205 information on the short-term risk posed by some elements. However, the 206 value of this coefficient depends upon both the properties of the element 207 species and the components of the soil. 208 Soil parameters influence the mobility of trace elements and the sensitivity 209 to contaminant influx. Factors that contribute to the mobility and 210 availability of trace elements and their ability to pass to other ecosystems 211 include pH (Evans et al., 1995), texture, cation exchange capacity, redox 212 potential (Chuang et al., 1996), and salinity (Du Laing et al., 2009), as well 213 as the abundances of carbonates (Martínez and Motto, 2000), clay minerals 214 (Bradl, 2004), organic matter (Guo et al., 2006), and Fe and Mn oxides and 215 hydroxides (Covelo et al., 2007). In general, the strongest adsorption of 216 PTTEs occurs, excluding oxy-anions, in those soils with higher pH, cation 217 exchange capacity, and abundances of organic matter and clay minerals 218 (Sposito, 1989; Galán, 2000; González et al. 2011). 219 To provide a rough estimation of the distribution of trace elements in the 220 different soil phases and their relative mobility, a more complex extraction 221 method can be used, specifically sequential chemical extractions. Such 222 methods use different specific extractants that are based on complex 223 reactions. The result is an operational speciation that leads to the 224 partitioning of the elements into the main soil phases and their relative 225 mobility (Tessier et al.,1979; Ure et al. 1993). 226 For mining soils, specific sequential extraction procedures combined with 227 differential X-ray diffraction (DXRD) have been proposed to detect which 228 minerals are dissolving in each leach (Dold, 2003). This procedure is more 229 robust because it combines information about chemical speciation with 230 information about mineralogical speciation. However, in some polluted 231 soils, trace elements are usually associated with minor phases that are 232 difficult to detect by X-ray diffraction (XRD). Other techniques, such as 233 scanning electron microscopy combined with energy dispersive X-ray 234 spectroscopy (SEM-EDS), can be coupled to sequential extractions and can 235 be used to determine mineralogical speciation (González et al., 2011). 236 However, the plant-available fraction of an element in soil also depends on 237 its chemical speciation, i.e., its oxidation state, and its functional 238 speciation, which distinguishes between different molecular forms. As 239 shown in Table 1, for some PTTEs, such as As, Cr, and Hg, it is also 8 240 convenient to determine the chemical and/or functional speciation. The 241 analytical techniques for determining the chemical speciation of these 242 elements are summarized in Table 2 (Galán et al., 2012). 243 Although claiming that a soil is polluted is purely an administrative action, 244 to demonstrate pollution is not an easy task. Contamination could also be 245 defined as the presence of an anomalously high level of a given element in 246 the soil. However, since no soil in the world is pristine and completely 247 uncontaminated, it is quite difficult to determine if soils have been directly 248 contaminated or not. 249 The term contamination threshold usually defines the concentration 250 (mg/kg) of trace elements in soil above which certain actions are 251 recommended or enforced (Carlon, 2007). They represent the highest levels 252 allowed in unpolluted soils. In any case, thresholds must always be checked 253 against toxicological tests before they are deemed accurate. Additionally, 254 any soil with element levels exceeding their respective thresholds is not 255 necessarily polluted, even if it contains trace elements at concentrations 256 over their regional means. Only a properly conducted toxicological study 257 can determine the exact allowable bioavailable amount of a trace element 258 and its toxicity. 259 A first step in evaluating the pollution content of a soil is to compare its 260 concentrations of PTTEs with background values. The geochemical 261 background of a chemical element represents its concentration in 262 uncontaminated soil. However, because it is virtually impossible to 263 determine, it is usually replaced by the geochemical baseline, which is the 264 average of superficial geochemical variations of the concentration at the 265 time of sampling (Salminen and Tarvainen, 2001; Baize and Sterckeman, 266 2001; Tarvainen and Kallio, 2002). It is influenced by subsoil lithology and 267 the presence of diffuse anthropic contamination. Thus, it is the sum of the 268 background value and the effect of diffuse anthropogenic contamination. 269 A more accurate baseline is the regional geochemical baseline, or the 270 geochemical background level for each region and element, as determined 271 for the specific geological context of the region (de Vivo et al., 1998; Galán 272 et al., 2008). These values allow us to discriminate between natural and 273 anthropogenic contamination. Additionally, they allow the potential 274 allowable load of an element in a specific soil to be determined from 15 480 A similar reasoning as that used for Pb may be done if there is a significant 481 mobile fraction of Cr and data on chemical speciation is advisable. The site 482 should be considered to be potentially polluted if the total Cr is less than 483 (or equal) to the GRL for Cr(VI) or less than the mobile Cr in the local 484 area. Otherwise, there is a real hazard. 485 Arsenic is a very toxic element, and it is very common (and sometimes 486 abundant) in soils. Its mobility is mainly controlled by “amorphous” and 487 crystalline Fe and Mn oxy-hydroxides (absorbents), and pH and Eh 488 variations are also very important. As(III) is ten times more toxic than 489 As(V), and in a soil with As anomalies, both must be assessed (using 490 chemical speciation) because their activities (mobility and toxicity) are 491 very different ( Bissen and Frimmel, 2003; Kabata-Pendias, 2011)). As 492 shown in Figure 4, the measurement of pH, Eh, and Fe and Mn oxy- 493 hydroxides in the soil; mobility in water at pH 5, 7 and 9; and 494 bioavailability using EDTA, as well as sequential extraction and 495 mineralogical speciation data, are necessary. Then, a similar assessment of 496 the results, as in the case of Cr, may be done to conclude if a real hazard 497 exists in the soil, and a risk analysis can be performed based on the use of 498 the soil (Figure 4). 499 The oxidation states of mercury are Hg0, Hg2+and Hg2 2+. The first is volatile 500 at room temperature. This metal in soils can be free, adsorbed, chelated, or 501 in mineral forms (i.e., as carbonate, hydroxide, or sulfide phases). 502 However, in the presence of dissolved organic matter (DOM) or salts, Hg 503 can form complexes. Its mobility and bioavailability depend on oxidation 504 state, which is controlled by pH, Eh, DOM and salinity (Schuster, 1991; 505 Finzgar et al., 2007; Kushwaha et al., 2018), and these are significant 506 parameters to be examined in the soil. Hg bioavailability is low in general 507 and decreases in soils containing DOM and sulphur (Figure 5) 508 Consequently, in detailed investigations of soils with high Hg anomalies, 509 the cited soil parameters must be determined. It is advisable to assess 510 bioavailability with EDTA and CaCl2. For the sequential extraction, it is 511 advisable to use the method indicated in Table 2, as well as to determine 512 the total Hg0 concentration. 513 For this metal, a real hazard exists when the mobile Hg fraction is greater 16 514 than the GRL, greater than local area’s mobile Hg, or when Hg0 is greater 515 than the Hg GRL. Surprisingly little attention is paid to this dangerous 516 metal (Fig. 5). 517 CONCLUSIONS 518 The proposed method is a) easy to apply, b) gives reasonable assessments 519 of the potential risk posed by soil contaminated by potentially toxic trace 520 elements, c) can be successfully applied to almost any kind of case study, 521 c) avoids a risk analysis procedure that may lead to a declaration that a soil 522 is polluted and to expensive remediation actions that are not really needed, 523 and d) saves time and money for administrators, enterprises and private 524 owners relative to “classical” assessments based on risk analysis. 525 As can be deduced from the proposal, anomalously high concentrations of 526 potentially toxic elements in a soil, although they may exceed the generic 527 reference levels, do not always pose a health risk. 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Supplemental Guidance for Developing Soil Screening 809 Levels for Superfund Sites OSWER 9355.4-24 810 WHO, 2005. Air Quality Guidelines Global Update. World Health 811 Organization. 812 Wuana RA & Okieimen FE (2011) Heavy Metals in Contaminated Soils: A 813 Review of Sources, Chemistry, Risks and Best Available Strategies 814 for Remediation. Ecology. doi:10.5402/2011/402647. 815 816 TABLES AND FIGURES CAPTIONS 817 Table 1 Toxicological parameters and carcinogenic classification of the 818 most common elements found in polluted soils. (In terms of duration, acute 819 = 1 to 14 days, intermediate = 15 to 364 days, and chronic = 1 year or 820 longer). IARC (1997); ATSDR (2016) 821 822 Table 2 Analytical techniques recommended for determining the chemical 823 speciation of As, Cr, Hg and Pb (Modified from Galán et al., 2012) 824 825 Table 3 Generic reference levels for different elements and countries 826 827 Fig. 1 Flow-sheet diagram for the management of potentially polluted soils, 828 including the study stages 829 830 Fig. 2 Flow-sheet diagram for the management of sites potentially polluted 831 by lead 832 833 Fig. 3 Flow-sheet diagram for the management of sites potentially polluted 834 by chromium 835 836 Fig. 4 Flow-sheet diagram for the management of sites potentially polluted 837 by arsenic 838 839 Fig. 5 Flow-sheet diagram for the management of sites potentially polluted 840 by mercury ASSESSMENT OF NEW DATA AND AVAILABLE INFORMATION Source of Pollution Magnitude of the Problem Affected Area DETAILED SAMPLING OF THE SITE 2 Chemical Analyses of Potentially Toxic Elements (PTEs) Soil Parameters ASSESSMENT OF RESULTS REMEDIATION ACTIONS MONITORING, SUPERVISION AND CONTROL (PPS) 3 POLLUTED SOIL DECLARATION Chemical Analysis of PTEs Enrichment and Geoaccumulation Factors Comparison with Target and Threshold Values, Local Baseline, Generic Reference Levels Soil Parameters Mobility/Bioavailability Operational Speciation Geostatistical Study and Mapping of Anomalies POTENTIAL POLLUTION (Immobile Pollutants) REAL HAZARD (Mobile Pollutants) RISK ANALYSIS + POTENTIALLY UNPOLLUTED SOIL SAMPLING FOR POLLUTION CHECKING SITE SAMPLING - DETAILED INVESTIGATION Depending on the use MANAGEMENT OF POTENTIALLY POLLUTED SOIL PRELIMINARY INVESTIGATION 1 DOCUMENTATION: previous information about the Potentially Polluted Site (PPS). Location, mapping, infrastructure, land uses, previous data, soil properties, etc. FIELD WORK Site survey, mapping checking/correction, pollutant distribution mapping, etc. HYPOTHESIS FORMULATION Possible Pollutants Spatial Distribution Heterogeneity SAMPLING FOR LOCAL BASELINE UNPOLLUTED SOIL POTENTIALLY POLLUTED SOIL Table 2 Analytical techniques recommended for determining the chemical speciation of As, Cr, Hg and Pb (Modified from Galán et al., 2012) Element Species Analytical method As As (III) As (V) Extraction solid-liquid with hydroxylamine hydrochloride and measurement by high pressure liquid chromatography (HPLC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) Cr Cr (III) Cr (VI) Pre-formation of the Cr (III)-AEDT complex and subsequent separation of Cr (VI) by ICP-MS coupled ion chromatography. Hg Hg0 Hg (II) Monomethylmercury MMHg+ Dimethylmercury-DMHg Diethylmercury- DEHg For the analysis of elemental mercury (Hg0) (volatile) thermosorption in a quartz furnace under a nitrogen stream and analysis of the mercury vapor by atomic fluorescence are recommended. Organic mercury species and inorganic mercury (Hg2+) can be determined by HPLC-ICP-MS or a coupled gas chromatography system with AFS detector. Pb R4Pb, R3Pb+, R2Pb2+ Gas chromatography - mass spectrometry (GC-MS) analysis with subsequent detection by atomic absorbance spectroscopy (GC-AAS). In a previous step, derivatization of the polar species of lead is carried out by alkylation using the Grignard reaction or ethylation in an aqueous medium with sodium tetraethyl borate. In addition to this technique, it is also recommended to use GC-MS and GCICP-MS systems, which provide good results with high sensitivity. Table 3 Generic reference levels for different elements and countries. Andalusian (Spain)1 The Netherlands2 USA3 Italy4 Element Industria l Non industrial Other uses Intervention Industria l Residentia l Industria l Residentia l As 40 36 36 55 2 0,4 50 20 Cd 750 75 25 12 900 70 15 2 Co 250 25 24 240 250 20 Cu 10000 3130 595 190 Total Cr 10000 10000 10000 380 3400 230 800 150 Cr(VI) 100 20 20 15 2 Hg 15 6 6 10 14 10 5 1 Ni 10000 1530 1530 210 23000 1600 500 120 Pb 2750 275 275 530 750 400 1000 100 Zinc 10000 10000 10000 720 340000 23000 1500 150 1Decreto 18/2005; 2Denneman, C. (1999); 3US EPA (2001); 4Decreto Ministeriale 25 ottobre 1999