Comparing Deterministic and Probabilistic Risk Assessments. A case study at a closed steel mill in southern Sweden (7 pp)

Background, Aims and Scope   Contaminated land is a high priority environmental problem in most of Europe and North-America. Sweden is no exception and generic guideline values have been developed for the initial assessment, but site-specific assessments are also needed. The generic guideline values are not applicable when the exposure conditions are different from the typical Swedish conditions or when the site contains a particularly sensitive ecosystem. The Swedish guideline values have, like in many other countries, been set by using deterministic point estimates for all variables and constants in the used multimedia model. The same approach is common also for site-specific assessments, and a limitation is that it fails to quantify variability and uncertainty. Probabilistic risk assessment provided a method to deal with this problem. Variability and uncertainty in the input parameters (variables or constants) are described by probability distributions, and likewise the output (risk or exposure) is presented as a probability distribution. A substantial number of probabilistic risk assessments for contaminated land at sites in North America, Europe and Asia have been published. However, an extensive review of the literature did not identify any study where probabilistic risk assessment was applied to a site contaminated by an iron or steel industry. Here we will describe such a case, where we have compared a deterministic point estimate with a probabilistic risk assessment for six elements and benzo[a]pyrene. Methods   The site had different metallurgical plants in operation for more than 100 years. Most parts of the steel mill were closed by the mid 1980s, and today the site is used by small-sized enterprises. The soil is contaminated with metals from the previous industrial operations. The present owner plans to develop the site and has therefore initiated extensive investigations of soil contamination. Sixty-two soil samples collected between 1997 and 2000 provided a good coverage of the whole site, and were analyzed for the content of different elements and polycyclic aromatic hydrocarbons (PAH). The exposure assessments were focused on six elements with high concentrations compared to the generic guideline values; arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), copper (Cu) and zinc (Zn). In addition, benzo[a]pyrene was included due to the high toxicity and comparatively high concentrations. Variability and uncertainty were characterized in a Monte Carlo simulation of exposures (10 000 iterations), and the exposures were evaluated with two land use scenarios; less sensitive use and sensitive use. Results and Discussion   The deterministic point estimates and the probabilistic estimates of the 95th percentile are in approximately the same ranges in the scenario of less sensitive land use. It is only the exposure for arsenic that is slightly above the toxicological reference value (TRV) in the deterministic assessment. In the probabilistic assessment, the exposure for all elements is below the TRV. The results for sensitive land use are applicable to a scenario where the site is developed for general housing. The deterministic point estimates and the probabilistic estimates of the 95th percentile are also here in approximately the same ranges, but the exposure exceeds the TRV for arsenic, cadmium and lead. Drinking water, vegetables grown on site and soil ingestion are the major exposure pathways for this scenario. In this assessment, the estimated intake distributions are applicable to a randomly selected individual. The probability distributions used here to characterize the different soil parameters are typically representing both variability and uncertainty, and the same is true the majority of the exposure variables. We therefore decided not to attempt to separate variability and uncertainty at this stage, but with additional data from a more in-depth site investigation it might be possible to achieve this. Conclusions and Outlook   To the best of our knowledge, this study is the first report on a probabilistic risk assessment on a former iron and steel works site. The materials handled by this industry were less toxic than for many other metallurgical operations, but contaminants may still severely limit the options for future land use. This case study shows that probabilistic exposure estimates for a set of soil contaminants can be quite similar to deterministic point estimates. The main difference is instead to be found in the additional information obtained with the probabilistic assessment. The sensitivity analyses show pathways and input variables that contribute most to variations in the total intake of each contaminant, e.g. dermal contact and ingestion of soil, vegetables and drinking water. This information can be used both in the planning of future land use and for active measures to reduce current exposure. The probabilistic assessment also provides information on the magnitude of exposure and the margin of safety. This information may facilitate risk communication between decision-makers and stakeholders. The presentation of results from probabilistic risk assessments is only briefly discussed in the literature and here we see a need for research and opportunities for enhancement. The choice of data analytical tools may then be of importance, since more complex multimedia models are rather difficult to decipher when implemented within traditional spreadsheet software. Some of the research needs are identified here and in a previous review article in this journal.     

基于保护水环境的场地地下水风险评估模拟应用研究

以江苏省某溶剂化工场地为例，利用英国污染地块水文地质风险评估模型研究了该场地地下水污染物对场地内及周边水环境的影响。比较了稳态和非稳态地下水溶质迁移模型(Ogata Banks、Domenico、Time variant Domenico)对污染物在自然衰减条件下从污染源向场地边界迁移过程模拟。结果表明：该场地的地下水中苯、乙苯、氯苯、1,2-二氯苯、1,4-二氯苯、氯仿超过了地下水质量Ⅲ类标准，但场地内污染物对场地边界地下水的潜在环境风险基本可以忽略。Ogata Banks与Domenico模型对于污染物在含水层的自然衰减下的迁移模拟表明污染物在30 m内的迁移过程中浓度下降较快，在30 m外的迁移过程中污染物的浓度较低且衰减不明显。Domenico模型计算较为简洁，在风险评估中具有较强的适用性；敏感性分析表明合规点至污染源距离x、含水层容重ρ、含水层孔隙度n、有机碳含量f_oc、有机碳分配系数K_oc、水力传导系数K、水力梯度i均影响污染物在含水层的自然衰减迁移过程，通过精细化水文地质调查获取上述参数将有助于提高风险评估结果的精确度。     

Regulation of Contaminated Soils in Spain - A New Legal Instrument (4 pp)

Background, Aims and Scope   This paper presents the new Spanish regulation on contaminated soils and its scientific basis. The regulation offers a regulatory framework for establishing industrial activities which may result in soil contamination, and presents the methodology for setting the generic reference levels of organic pollutants.Legal Framework   The Spanish regulation on contaminated soils is derived from the waste legislation and covers 101 industrial activities, as well as facilities handling significant amounts of hazardous chemicals (over 10,000 kg per year) or fuel (300,000 l fuel per year or storing 50,000 l fuel at any time). The regulatory framework includes initial declarations and a tiered system for selecting those soils requiring a proper site-specific risk assessment.Scientific Basis   The regulation is risk based, and covers human and environmental risks. The human health risk assessment focuses on chemical analysis; the selection of relevant exposure routes is associated to the soil uses. The environmental risk assessment includes chemical analysis and direct toxicity testing, and covers three main ecological receptors: Soil organisms, associated aquatic systems and terrestrial vertebrates. Low-risk threshold concentrations are established as generic reference levels; if exceeded, a site-specific risk assessment is required. The detection of a very high level of acute toxicity of soil or leachates led to the declaration of the soil as contaminated due to the capacity for contaminating the adjacent areas.Conclusion   Overall, the Spanish regulation offers a balance for combining regulatory needs, proper scientific basis and practicability. The use of European risk assessment protocols and the European legal framework would facilitate the pan-European extrapolation of this approach. The inclusion of direct toxicity testing as a legal method for classifying a soil as contaminated is considered a key element.     

Land Application of Wastewater in Brazil - A scientific challenge: Chemical Characterization of Soil at Populina, S?o Paulo State (9 pp)

Background, Aims and Scope   The rapid increase of wastewater worldwide and the consequently urgently needed treatment measures towards disposal purposes has led to an expansion of alternative treatment methods. Besides conventional and cost intensive systems, domestic wastewater treatment by overland flow process represents an effective and low-cost alternative. However, in addition to beneficial elements in wastewaters, there is a release of other elements of the Periodic Table into the environment, which can be considered as potentially toxic. This requires a reliable chemical characterization of the wastewater element composition as well as the soils onto which it will be applied. This paper aims at presenting the chemical fingerprint methodology to characterize concentrations of a large set of elements in a tropical ferralsol in order to provide information concerning chemical modifications after long-term application of wastewater by overland flow process. The results are also used to estimate environmental implications according to guiding values for soils in São Paulo State.Methods   The wastewater treatment in Populina has been operating for 18 years. Composite soil samples were taken in a treated plot and a control plot. Wastewater was chemically characterized by using AAS. Total C and total N of soil samples were determined by Delta Plus Mass Spectrometer. The remaining elements (32) were analyzed using ICP-OES. Grain size analysis was carried out after oxidation by H2O2, and soil dispersion with NaOH and Na4P2O7 treatments. pH was measured in water. Quality control of all measurements was verified by corresponding reference materials. Fingerprint graphs were generated by standardizing the values of the control plot and by graphically displaying positive/negative percentage deviations of the element values at the treated plot from values of the control site. Results and Discussion   Chemical analyses of the wastewater used showed high BOD, COD, TOC and N values, while the majority of metals were below the detection limit that is attributed to the limitations in the instrumental techniques (AAS) utilized. The comparison of the two selected sites revealed substantial changes of the element contents associated with long-term application of domestic wastewater. Fingerprint graphs of the treated field indicated higher values for the vast majority of macro and micro-nutrients (C, N, P, K, Ca, Mg, Mo, Zn, Cu), as well as for other elements, e.g. Ba, Sr, Na, Li, or La. Enrichments of ecotoxic elements (e.g. As or Pb) occurred in the surface horizon despite the low contents of those elements in the wastewater. Other elements analyzed, i.e. Ag, Be, Bi, Cd, Sb, Sn, W or Sc, that are partially considered to be toxic, did not show considerable differences. Due to limitations in the analytical techniques applied, these results do not imply that there is no existence or no release of elements from wastewater at trace or ultra-trace levels. It is expected that more sophisticated techniques will be available in the future. Typical soil elements (Ti, Al, Fe, V, Zr) show higher values at the control site suggesting higher contents of mineralogical components. The highly positive deviation of C-contents linked with higher values of many other elements in the treated soil suggest element enrichments associated with organic matter. Concerning environmental implications, the comparison with guiding values for an enhanced monitoring showed partly an excess of acceptable metal concentration levels. Proposed intervention values were not exceeded, except for with Ba. Comparisons with phytotoxic element concentrations in soils indicate critical values for As, Cu and Cr. Conclusion   Using multi-element analysis, the study represents a first approach to access wastewater application by overland flow on the basis of a rather large range of chemical elements in Brazil. The comparison of the two selected areas revealed distinctly higher values for the majority of elements in the treated plot. The study also highlighted the importance of organic matter for element retention. The environmental implications of domestic wastewater application to soil surfaces can be grave. Since also toxic elements are enriched in these treated soils, the areas can partly be classified as polluted, and require long-term monitoring and detailed investigations.Recommendation and Outlook   The analytical data obtained may only allow for some conclusions concerning long-term feasibility and geochemical preconditions of wastewater irrigation. Therefore, not only the total concentrations of metals are of interest, but also element speciation must be analyzed in order to predict, for instance, how long the ion uptake capacity of the soil will last. Aside from the analytical procedures used in speciation, theoretical chemical (thermodynamic) concepts such as the principles of Pourbaix diagrams should also be used to predict solubility, immobilization and bioavailability of different element species.