Search

In order to evaluate the natural attenuationof chlorinatedethenes, especially trichloroethylene (TCE) anddichloroethylenes (DCEs) in an aquifer, the effects ofdispersion-and-dilution and biodegradation on the concentrations of TCE and DCEs were investigated using numerical simulation. The resultsobtained in this study can be summarized as follows: (1) In an aquifer with high biodegradation rate of DCEs (first order rateconstantof DCEs is over 2.0×10-3/d), the concentrationratioof DCEs to TCE is less than 1.0. (2) In an aquifer with highdispersion-and-dilution rate (groundwater velocity is over 0.6m/d-1.0m/d), the concentration ratio of DCEs to TCE is less than 1.0. (3) When the first order rate constantof DCEs is less than 1.0×10-3/d and the groundwater velocity is less than 0.4m/d-0.7m/d, DCEs tend to accumulate in an aquifer.

本稿では10年以上の長期にわたって継続している地下水揚水法による土壌・地下水浄化対策の評価を行った. また, 汚染物質であるトリクロロエチレンの分解による自然減衰効果についても考察した. 現場では長期にわたる揚水処理を続けた結果, 汚染源及びその周辺の浅層地下水の浄化をほぼ完了した. その過程で汚染物質濃度が環境基準よりも高い時期には揚水量が増えると浄化率が上がり, 環境基準を下回り始めると浄化率の向上が顕著でなくなる傾向を示した. 一方, 自然減衰効果としては, 分解生成物のジクロロエチレン濃度がトリクロロエチレン濃度の約64%に保たれていること, および地下水濃度が環境基準値以下になるまでの修復期間が自然減衰効果により約2年は早まっていることが明らかになった.

Recently, the assessing of the natural attenuation capacity in the aquifer, which is called the monitored natural attenuation (MNA), is attempted as a remediation technique for groundwater contamination by the volatile organic compounds (VOCs). The natural attenuation includes the processes of biodegradation, dispersion, dilution, sorption, and volatilization of contaminants. Especially the biodegradation process is often recognized as a dominant destructive attenuation mechanism. In this paper, the characteristics of biodegradation in the aquifer and the effect of natural attenuation capacity were evaluated by the field survey and the numerical simulation at the groundwater remediation site. As a result, the biodegradation rates of trichloroethylene (TCE) and dichloroethylenes (DCEs) were calculated and it was cleared that DCEs were degraded faster than TCE.

Groundwater contamination due to chlorinated hydrocarbons like trichloroethylene and tetrachloroethylene has been a great environmental issue in Japan. Because these substances are volatile and little soluble in water, and can exist in unsaturated zone as vaporized, dissolved and undiluted non-aqueous phases, they migrate as gas-water-liquid three phase flow in subsurface. Recently, the air sparging coupled with the soil vapor extraction as the physical remediation technologies has been applied to a site contaminated with chlorinated hydrocarbons. The air sparging technology facilitates the contaminant volatilization with air injected into saturated zone and the soil vapor extraction technology extracts the contaminant in soil and injected gas. Therefore, in order to establish these technologies, it is indispensable to understand the mechanism of gas-water two phase flow in subsurface.

Chlorinated hydrocarbons (CHCs) are highly volatile and little soluble in water, and can exist in unsaturated zone as vaporized, dissolved and undiluted non-aqueous phases. Therefore, it is of great importance to understand transport mechanism of CHCs with gas advection and diffusion in unsaturated zone, and then numerical analysis for vertical transport of CHCs in unsaturated zoneincluding effect of rainfall was conducted. Vertical one-dimensional fluid flow equations and mass transport equations for water and gas phases, and two layer model for mass transfer between the phases were employed in the model presented. The results showed that, though gas phase transport is usually dominant during dry period in unsaturated zone, transport in water phase overcomes that in gas phase during rainwater percolating period.