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我国现有的《建设用地土壤污染风险评估技术导则》 (HJ25.3-2019) (以下简称HJ25.3) [1]借鉴了美国国家科学院基于土壤中VOCs质量分数计算其呼吸暴露途径的健康风险评估方法,未考虑污染物对人体嗅觉感官的影响与刺激[2]。在实际应用中,当地块土壤中存在异味污染物时,即使健康风险可接受,仍会发生“异味扰民”现象[3]。马杰[4]在某农药污染地块调查中发现,地块土壤中污染物质量分数均低于《土壤环境质量 建设用地土壤污染风险管控标准 (试行) 》 (GB36600-2018) [5]筛选值,但仍散发出明显的刺激性气味。为控制污染地块中异味污染物的环境影响,提高居民生活质量,亟需开展针对异味污染物的风险评估与土壤修复目标值制定方法研究。
美国在污染地块风险评估过程中已关注异味污染物对人体嗅觉感官的影响[6]。2003年康涅狄格州发布的《修复标准法规 挥发标准》 (Remediation Standard Regulations Volatilization Criteria) [6]中将部分异味污染物的嗅阈值作为土壤污染物蒸气吸入暴露途径的室内空气质量浓度上限值。目前,国内已有学者开展了基于异味污染物嗅觉效应的土壤修复目标值 (或者土壤阈值) 制定方法研究。周友亚等[7]借助河北某化肥遗留场地,综合考虑氨氮对现场工作人员的嗅觉影响及健康风险评估结果,确定了土壤氨氮的风险控制值,并进一步上升为河北省地方标准《建设用地土壤污染风险筛选值》 (DB13T 5216-2020) [8]。张施阳等[9]以某退役污染地块为案例场地,构建了基于“异味土壤-挥发途径-嗅觉效应”的土壤苯系物异味评估方法,并利用污染地块长期和短期异味暴露效应模型,推导了地块苯系物土壤风险阈值。
我国污染场地相关导则推荐采用J&E模型进行呼吸暴露途径的风险评估,但J&E模型假定的VOCs在土壤气-固-液中的三相平衡分配理论忽略了土壤有机质等对污染物的吸附锁定效果,也未考虑向上迁移过程中的生物降解作用,故计算结果偏保守 [10-11]。针对这一问题,国内外许多研究机构开展了基于实测土壤气或土壤气挥发通量进行风险评估的研究工作。MCNEEL等[12]研究表明,基于土壤气挥发通量较之基于土壤中污染物质量分数采用J&E模型计算三氯乙烯与苯的健康风险低1~2个数量级。张蒙蒙等[13]基于土壤气挥发通量计算苯的健康风险相比基于土壤中污染物质量分数采用J&E模型计算的健康风险低1个数量级。李卫东等[14]采用挥发通量计算三氯甲烷的修复目标值高出基于J&E模型计算的修复目标值1个数量级。上述研究表明,基于实测土壤气或挥发通量进行健康风险评估并制定修复目标值,可在一定程度上规避J&E模型高估VOCs污染物实际风险造成过度修复的问题。但是,针对有恶臭气味的VOCs物质,如硫醚、氨氮等,不管是基于土壤中污染物质量分数还是基于土壤气和土壤气挥发通量,仅仅通过健康风险评估方法确定恶臭类VOCs土壤修复目标值难以保证不会发生“异味扰民”的舆情问题。
本研究以某农药污染地块二甲基二硫醚为研究对象,分别采集地块土壤和土壤气样品测定土壤二甲基二硫醚质量分数和土壤气挥发通量。基于J&E模型与土壤气挥发通量模型,以保护人体健康和防止异味扰民2个角度评价二甲基二硫醚的环境影响,并探索基于二甲基二硫醚嗅觉效应的土壤修复目标值的制定方法,以期为我国异味污染地块的环境管理提供参考。
基于嗅觉效应的土壤二甲基二硫醚修复目标值制定
Method of establishing soil remediation target value of dimethyl disulfide based on olfactory effect
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摘要: 我国现行风险评估导则多采用基于土壤中污染物质量分数的Johnson-Ettinger(J&E)模型评估VOCs呼吸暴露健康风险,但对于土壤中异味污染物的特殊性考虑不足。以某农药污染地块土壤中异味污染物二甲基二硫醚为研究对象,分析了二甲基二硫醚在土壤与土壤气中的赋存状态,采用土壤气挥发通量与J&E模型分别计算室内呼吸暴露途径的健康与环境风险,探索基于异味污染物对人体嗅觉效应的土壤修复目标值制定方法。结果表明,土壤中二甲基二硫醚最高检出质量分数与土壤气挥发通量最大值检出点位一致,其余点位土壤气中二甲基二硫醚均有不同程度检出而土壤只有2个点位有检出;二甲基二硫醚作为挥发性比较强的有机物更容易赋存于土壤气相中,采用土壤气挥发通量测试结果来表征二甲基二硫醚的环境风险会更加客观可信。研究区域内二甲基二硫醚人体健康风险可接受,但室内暴露质量浓度最高值为5.28 mg·m−3,超过了污染物嗅阈值,存在异味引起的环境风险。基于嗅觉效应采用土壤气挥发通量制定的修复目标值为0.193 mg·kg−1,相比J&E模型提高了一个数量级,修复方量减少约29.75%。基于土壤气挥发通量综合考虑健康风险和嗅觉阈值进行风险评估与土壤修复目标值的制定,既能确保异味污染地块安全利用,又在一定程度上避免了过度修复的问题。本研究结果可为异味污染地块的环境管理提供参考。Abstract: The Johnson Ettinger (J&E) model based on soil pollutant mass fraction is mostly used to assess the health risk of inhalationexposure to VOCs of the current risk assessment guidelines in China. However, the specificity of odor pollutants in soil is narrowly considered. Herein, the health and environmental risks were calculated by the indoor inhalation exposure pathway via soil gas volatilization flux and J&E model. As an odor pollutant in soil of one pesticide contaminated site, dimethyl disulfide(DMDS) was chosen to explore and analyze occurrence conditions in soil and soil gas and the method of establishing the target of soil remediation due to the odour olfactory effect on humans. These results showed that the highest mass fraction of DMDS in soil were consistent with the highest detected level of soil gas volatilization flux at the same sampling point, and the remaining sampling points showed different degrees of detection of DMDS in soil gas without two points detected DMDS in soil. DMDS was more likely to be deposited in the soil gas phase and it would be more objective and reliable to the utilization of soil gas volatilization flux test results to characterize the environmental risk of DMDS. The human health risk of DMDS was acceptable in the investigation region, but the maximum indoor exposure concentration was 5.28 mg·m−3, which exceeded the odour threshold of pollutants, and there were environmental risks caused by odour. The remediation target value based on olfactory effect was 0.193 mg·kg−1, and the volatilization flux of soil gas exceeds the order of magnitude of J&E model, and the remediation cost could be reduced for around 29.75%. The risk assessment and soil remediation target value could improve the safe use of odour contaminated sites and block excessive remediation due to the soil gas volatilization flux integrated with the health risk and olfactory threshold. This research can provide advanced theoretical reference for the environmental management of odor contaminated sites.
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表 1 模型参数定义及取值
Table 1. Definition and value of model parameters
参数 取值 单位 数据来源 参数 取值 单位 数据来源 LB 3 m HJ25.3[1] θt 0.42 无量纲 实测值 ER 1/4 320 次·s−1 HJ25.3[1] foc 0.002 9 无量纲 实测值 ED 25 a HJ25.3[1] A 0.054 2 m2 实测值 AT 9 125 d HJ25.3[1] T 604 800 s 实测值 θacrack 0.26 无量纲 HJ25.3[1] UF 1 无量纲 [21] θwcrack 0.12 无量纲 HJ25.3[1] MF 1 无量纲 [21] DAIR 14.5 m3·d−1 HJ25.3[1] H 0.044 5 无量纲 [24] η 0.000 5 无量纲 HJ25.3[1] Koc 220 L·kg−1 [25] EF 250 d·a−1 HJ25.3[1] Dair 0.096 cm2·s−1 [26] BW 61.8 kg HJ25.3[1] Dwater 1.26×10−5 cm2·s−1 [26] ρ 1.58 kg·dm−3 实测值 NOAEL 5.5 mg·kg−1·d−1 [27] θwater 0.3 无量纲 实测值 Codor 0.046 mg·m−3 [28] θair 0.12 无量纲 实测值 表 2 研究区域内各点位二甲基二硫醚质量分数
Table 2. Mass fraction of dimethyl disulfide at each point in the research region
采样点位 土壤中污染物
质量分数/ (mg·kg−1)土壤气挥发通量/
(mg·m−2·s−1)A1 ND 1.95×10-7 A2 ND 3.25×10-7 A3 ND 1.43×10-6 A4 ND 1.14×10-4 A5 0.46 1.78×10-4 A6 7.64 1.24×10-3 A7 ND 9.11×10-7 A8 ND 2.28×10-7 A9 ND 4.88×10-7 注:“ND”表示土壤中二甲基二硫醚未检出。 表 3 校正因子计算结果
Table 3. Correction factor calculation results
监测点 Cair-Flux/
(mg·m−3)Cair-J&E/
(mg·m−3)模型校正因子 1 2.81×10−4 3.45×10−2 $ {\rm{CF}}= \dfrac{{C}_{\mathrm{a}\mathrm{i}\mathrm{r}-\mathrm{F}\mathrm{l}\mathrm{u}\mathrm{x}}\text{ (}\text{95\%UCL}\text{) }}{{C}_{\mathrm{a}\mathrm{i}\mathrm{r}-\mathrm{J}\&\mathrm{E}}\text{(95\%UCL)}}= 0.348 $ 2 4.68×10−4 3.45×10−2 3 2.06×10−3 3.45×10−2 4 1.64×10−1 3.45×10−2 5 2.56×10−1 3.18×10−1 6 1.79×100 5.28×100 7 1.31×10−3 3.45×10−2 8 3.28×10−4 3.45×10−2 9 7.03×10−4 3.45×10−2 注:95% UCL表示 95%置信区间上限值。 表 4 基于嗅觉效应的修复目标值
Table 4. Remediation target value based on olfactory effect
计算模型 修复目标值/
(mg·kg−1)修复土方量/
m3基于Cs采用J&E模型 0.067 23 292 基于实测土壤气挥发通量
(校正因子)0.193 16 362 -
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