统计方法在污染场地修复验收评估中的应用

为合理评估污染场地修复验收结果,运用2种统计学方法——平行样t检验和UCL(置信上限)评估方法,对修复案例场地土壤样品的检测结果进行评估. 结果表明:采用传统的逐一对比方法,有8个采样点的w(As)和1个采样点的w(Hg)超过修复目标；平行样t检验法显示,这9个超标采样点中,S48采样点w(As)和S1采样点w(Hg)略微超过修复目标,是由采样与实验室误差引起,可视为已经达到修复效果；95%UCL评估方法结果则显示,w(As)和w(Hg)的95%UCL分别为23.81和5.83mg/kg,均小于对应的修复目标值,只有S26采样点w(As)大于修复目标值的2倍,可判定该场地土壤中除S26采样点w(As)外,其他均已达标. 研究说明:①运用适当的统计分析方法对修复效果进行评估,可节省不必要的修复成本；②传统的逐一对比方法适用于修复场地面积较小、采样数量有限的情况,平行样t检验则可剔除采样过程或实验室分析误差导致的超标值的影响；③95%UCL评估方法可从整体上判断场地修复效果,适合于修复面积较大、采样数量相对较多的情况,应用时注意95%UCL应小于修复目标值,污染物含量的最高检测值应不超过修复目标的2倍且不出现局部连续超标区域. 

Case Application of Statistical Analysis in Remediation Validation of Contaminated Sites

Two statistical methods, t-test analysis using replicate samples and upper confidence limit (UCL), were applied in evaluation of attainment of cleanup standards in a site remediation contaminated by the heavy metal. The results showed that the site did not reach the remediation goals by eight arsenic samples and one mercury sample exceeding the cleanup standards, if the strict no exceedance rule is applied. The t-test analysis using replicate samples further indicated that exceedance of As content in S48soil sample and Hg content in S1soil sample were contributed by the field sampling or laboratory errors, hence Hg content meet the cleanup. The 95% upper confidence limits of As and Hg were respectively calculated as 23.81and 5.83mg/kg, which were below the remediation goals. However, the S26surrounding could not reach the remediation goals since As content in S26soil sample was two times greater than the cleanup standard. The results are listed as follows:1) The potential saving in remediation cost can reach by using statistical analysis in evaluation of attainment of cleanup standards for contaminated site remediation. 2) The traditional one by one observation comparison method is appropriate for small sites with limited samples. T-test analysis using replicate samples can be used for evaluating errors possible contributed from laboratory and filed sampling. 3) Upper confidence limit (UCL) can be applied to verify the attainment of the cleanup standard for large sites with more samples in conjunction with specific guidelines:95%UCL is less than the remediation goals; the maximum concentrations not more than 2times of the cleanup standard; exceedance area is discontinuous.