基于保护地下水的土壤修复目标层次化制订方法

按照复杂程度将污染物从土壤经地下水迁移到下游饮水井的过程划分为3个层次,采用分层评估框架建立了基于保护地下水的土壤修复目标的制订方法,并利用该方法确定了某污染场地的土壤修复目标值. 结果表明,在污染场地下游200 m处的饮水井内水质标准不降低的前提下,随着评价层次的不断提高,需要修复的污染物由4种(苯、甲苯、乙苯、二甲苯)减至1种(苯),待修复土方量由23.1×104 m3降至4.7×104 m3,可极大地节约修复成本. 该场地污染土壤的第一层次和第二层次修复目标值与部分国家/地区的有关标准限值较为一致；第三层次修复目标值考虑的污染物迁移过程更加完整,更能反映场地的实际情况. 参数的敏感性分析表明,对土壤修复目标值计算结果影响最大的参数为土壤有机质质量分数、土壤有机碳-水分配系数和入渗速度. 在确定修复方案时,应该通过试验或补充调查获取这些参数,以降低结果的不确定性. 研究显示,将第三层次评估结果作为该污染场地的修复目标能充分保证下游饮水井内水质满足要求,并且可以避免过度修复. 

A Tiered Approach to Calculating Soil Remediation Targets Based on Groundwater Protection

Abstract: A tiered assessment approach was utilized to develop a framework for deriving soil remediation targets for groundwater protection, after analyzing the processes by which contaminants migrate from soil to downstream wells. The framework involves three tiers according to the complexity of contaminant migration. The soil remediation targets of a petrochemical contaminated site were then calculated based on the approach. According to the results, based on the premise that the downstream protection of the well (200 m from the site) showed no decrease, with the continuously upgrading assessment tier, the number of contaminants needing removal was reduced from four (benzene, toluene, ethylbenzene and xylene) to one (benzene). Furthermore, the earthwork volume pending remediation was reduced from 23.1×10⁴ m³ to 4.7×10⁴ m³, saving most of the remediation cost. The remediation targets of Tier 1 and Tier 2 were consistent with relevant standards of some countries and regions. Moreover, the remediation targets of Tier 3 were more relaxed and closer to the actual site characteristics compared with the standards. After analyzing the model parameter sensitivity, the remediation targets were impacted greatly by parameters including soil organic matter content, soil organic carbon-water partition coefficient and infiltration speed. The parameters should be obtained by testing or supplementary investigation under permitting conditions, in order to reduce the uncertainty of the results. The results showed that formulating the remediation targets with the Tier 3 assessment can meet the requirements for water quality in the downstream wells and avoid excessive remediation.