生物降解对土壤气中苯衰减系数及筛选值的影响

以砂质土壤为例,采用Bio-vapor软件计算了生物降解对苯的a_i-s(衰减系数)及筛选值的影响,并对关键影响参数〔c_s(污染源苯质量浓度)、LT(建筑底板与污染源距离)、L_a(好氧土层厚度)和k_w(生物降解系数)〕进行分析. 结果表明:当c_s≥5×105mg/m3时,生物降解对a_i-s基本无贡献；当c_s≤1×104 mg/m3时,生物降解可导致a_i-s降低1~2个数量级,但降幅随c_s和LT的变化不明显；当c_s介于二者之间时,生物降解对a_i-s的作用受LT变化的影响较明显,LT升高1个数量级时,生物降解可导致a_i-s降低2个数量级. 生物降解对a_i-s的作用受L_a影响比较明显,L_a由0.50m增至1.50m时,生物降解可导致a_i-s降低2个数量级. Bio-vapor软件预测的砂质土壤条件下L_a的最大值为0.63m,低于现场普遍测试结果(1.50m),表明该模型预测结果可能过于保守,实际项目中可通过测试土壤气中各组分的纵向分布确定L_a. 当c_s≤5×104 mg/m3时,k_w由0.033h-1增至2.000h-1,生物降解将导致a_i-s降低2个数量级. 因此,同一概念模型下考虑生物降解时土壤气中苯筛选值高于不考虑生物降解时1~2个数量级. 

Impact of Biodegradation on Attenuation Factor and Screening Level of Benzene in Soil Vapor

Effects of biodegradation on the attenuation factor (a_i-s ) and screening level of benzene in the soil vapor were studied by using the one dimensional model, Bio-vapor for sandy soil, in particular on critical influential factors of biodegradation were simulated and discussed, including source intensity (c_s), vertical separation distance between building foundation and source (LT), depth of aerobic zone in soil (L_a), biodegradation rate (k_w ). The results indicated that contribution from biodegradation of benzene to a_i-s was negligible, when c_s was higher than 5×105 mg/m3. When c_s was less than 1×104 mg/m3, a_i-s was 1-2orders of magnitude reduction when the aerobic biodegradation process was included compared to the non-biodegradation case, but the reduction is insensitive to the change of c_s and LT. When c_s falls in between, a_i-s decreases by 2orders of magnitude as LT increases by 1order of magnitude, and LT is a sensitive parameter. L_a is a critical factor in determining the biodegradation effect, and bio-attenuation decreases by 2orders of magnitude as L_a just increases from 0.50to 1.50m. Compared with the field measured results for L_a, which were always deeper than 1.50m, the modeled results for the sandy soil was 0.63m, indicating that Bio-vapor is conservative in predicting the depth of aerobic zone. Therefore determination of L_a through measuring the soil vapor profile in practical project is recommended. Bioattenuation was more sensitive to k_w for light contaminated sites, and a_i-s decreases by 2orders of magnitude as k_w increases from 0.033to 2.000h-1 when c_s is no more than 5×104 mg/m3. The vapor screening value for benzene is 1-2orders of magnitude lower when biodegradation is considered for the same conceptual model scenario with no biodegradation.