Recently, Monitored Natural Attenuation(MNA) has been introduced as a new concept of natural attenuationadding continuous monitoring to focus management in advanced countriesThis study evaluated the possibility and the degradation rate constant of contaminants in groundwater using analysis of hydrog...
Recently, Monitored Natural Attenuation(MNA) has been introduced as a new concept of natural attenuationadding continuous monitoring to focus management in advanced countriesThis study evaluated the possibility and the degradation rate constant of contaminants in groundwater using analysis of hydrogeological characteristic factors, hydrodistributional characteristic factors, electron acceptor known as a factor of natural attenuation assessments and TCE/DCE/VC concentration at a actual TCE contamined site. Also this study assessed the applicability and limitation of natural attenuation through estimation of temporal and spatial variation and attenuation effect of TCE contamination in groundwater around the major contaminated area using 3D modeling method. Hydrogeological characteristics of the area were northeastward groundwater flow from M mountain and D mountain, where the A division is located, to J stream and groundwater level that sensitively responds to rainfall. Variation in groundwater level in highlands is larger than that in downstream of the J River. In the subject area, the highest TCE concentration was 0.608 mg/l, and average value was 0.088mg/l. TCE showed high concentrationaround the A division and northeast area where groundwater flows and DCE as biodegradation product of TCE and VC as degradation product of DCE were found. cis-1,2-DCE was detected at 30 points and the highest concentration was 0.22mg/l. Petroleum organic compounds and total organic carbon to affect the rate of TCE biodegradation were lower than detection limit or detected in small quantities. The result of diffusion sampling showed that the TCE concentration at the lower layer of groundwater were higher/ than that at the upper layer of groundwater in upstream area (around the J division), otherwise in downstream area (J river) upper layer of groundwater showed higher level of TCE than lower layer of groundwater. It seems to be related with the groundwater characteristics of recharge and discharge area and transport characteristics of TCE. Tendency change of TCE concentration was investigated to assess the possibility of natural attenuation of TCE contaminated groundwater at the subject area. Total of 5 wells including 3 wells like GW01 around the A division and 2 wells like GW19 around the J river showed decrease in TCE concentration. Total 3 wells like MW06 located at northeast side of A division showed increase in TCE concentration. The concentration of dissolved oxygen and nitrates were high at upstream, but were low at downstream. Also, high level of dissolved Fe and Mn(> 1mg/l) were detected at the downstream It shows that this area is suitable for the biodegradation of chlorinated organic compounds like TCE. Also, high level of TCE and DCE were detected at the downstream, which means that TCE is naturally biodegradaded at anoxic condition. Natural attenuation assessments suggested by US EPA showed that 41.3% (19 points) of wells were limited biodegradation reactions and 2.2% (1 point) of wells were complete biodegradation reactions. Some wells showed continuous increase in the level of TCE and it seems to be the result of constant inflow of TCE from the lower layer of soil. At the wells showing decreasing tendency of TCE, the average rate of natural attenuation of TCE was 8.93x10-4 day-1, and a half-life was 2.13 year. At the wells showing decreasing tendency of DCE, the average rate of natural attenuation of DCE was 1.22x10-3 day-1, and a half-life was 1.56 year. In the case of GW01 well, it takes approximately 19 years to decrease TCE contamination to the level of domestic water standard of 0.03mg/l. To estimate transport and natural attenuation of TCE in groundwater, MODFLOW was used for groundwater transport modeling and RT3D was used for contaminants transport modeling. Modeling area was selected by 1,850 m x 1,750 m with boundaries of surrounding rivers and dividing ridges. As the result of groundwater transport modeling with discharge areas as J River, S river and tributaries, groundwater flowed from west side mountain ranges to J river at east and northeast area. Contaminants transport modeling was conducted according to the different scenarios like one considering only current concentration and the other considering the possibility of constant inflow of TCE from soil layers. Also respective scenarios were applied in consideration of absorption of contaminants, biodegradation reflecting the rate of natural attenuation, and application of treatment after pumping. In the case of no consideration of absorption and biodegradation, contaminant plume of TCE at the subject area would exist around J River after 20 years. However, in the case of consideration of biodegradation, contaminant plume of TCE at the subject area would be disappeared within 4 years. If constant inflow of TCE from soil layers around A division exists, in the case of no absorption and no biodegradation, contaminant plume of TCE would be present at high concentration after 20 years. However, in the modeling of consideration of biodegradation, contaminant plume of TCE attenuated at high rate. Inthe modeling of consideration both of absorption and biodegradation, contaminant plume of TCE still existed after 20 years because absorption inhibited contaminants transport. In the modeling of no absorption and no biodegradation, wells located at downstream of groundwater flow showed increasing at first and decreasing later level of TCE contamination. However, in the modeling of consideration of biodegradation, every well showed rapid decrease in TCE concentration. It also showed that DCE and VC, biodegradation product of TCE, known for higher harmfulness than TCE to human health increased rapidly. DCE increased 5 times higher than current highest concentrationand VC increased about 54 times. In the modeling of no consideration of biodegradation, contaminant plume of DCE existed around the J River in small quantities after 10 years. However, in the modeling of consideration of only biodegradation, contaminant plume of DCE disappeared at all after about 5 years, and in the modeling of consideration both of biodegradation and treatment after pumping, contaminant plume of DCE disappeared at all after about 3 years. In the modeling of consideration of biodegradation, contaminant plume of VC produced during DCE degradation presented much higher level of concentration than current situation after 10 years. It seems because of rapid increase of VC plume resulted from the rapid biodegradation of DCE produced from biodegradation of TCE. It is estimated that VC would exist longer than DCE because of slow rate of biodegradation. Therefore, to apply natural attenuation of TCE contaminated groundwater, continuous monitoring and management of transport and production of biodegradation product like DCE and VC are needed. The result of this study is from the facts that contamination sources were limited around the A division at the subject area, and the rate of natural attenuation calculated from part of wells confirmed the possibility of natural attenuation was applied to whole subject area, so the result could be overestimated and further studies are required about factors of natural attenuation assessments at various sites.
Recently, Monitored Natural Attenuation(MNA) has been introduced as a new concept of natural attenuationadding continuous monitoring to focus management in advanced countriesThis study evaluated the possibility and the degradation rate constant of contaminants in groundwater using analysis of hydrogeological characteristic factors, hydrodistributional characteristic factors, electron acceptor known as a factor of natural attenuation assessments and TCE/DCE/VC concentration at a actual TCE contamined site. Also this study assessed the applicability and limitation of natural attenuation through estimation of temporal and spatial variation and attenuation effect of TCE contamination in groundwater around the major contaminated area using 3D modeling method. Hydrogeological characteristics of the area were northeastward groundwater flow from M mountain and D mountain, where the A division is located, to J stream and groundwater level that sensitively responds to rainfall. Variation in groundwater level in highlands is larger than that in downstream of the J River. In the subject area, the highest TCE concentration was 0.608 mg/l, and average value was 0.088mg/l. TCE showed high concentrationaround the A division and northeast area where groundwater flows and DCE as biodegradation product of TCE and VC as degradation product of DCE were found. cis-1,2-DCE was detected at 30 points and the highest concentration was 0.22mg/l. Petroleum organic compounds and total organic carbon to affect the rate of TCE biodegradation were lower than detection limit or detected in small quantities. The result of diffusion sampling showed that the TCE concentration at the lower layer of groundwater were higher/ than that at the upper layer of groundwater in upstream area (around the J division), otherwise in downstream area (J river) upper layer of groundwater showed higher level of TCE than lower layer of groundwater. It seems to be related with the groundwater characteristics of recharge and discharge area and transport characteristics of TCE. Tendency change of TCE concentration was investigated to assess the possibility of natural attenuation of TCE contaminated groundwater at the subject area. Total of 5 wells including 3 wells like GW01 around the A division and 2 wells like GW19 around the J river showed decrease in TCE concentration. Total 3 wells like MW06 located at northeast side of A division showed increase in TCE concentration. The concentration of dissolved oxygen and nitrates were high at upstream, but were low at downstream. Also, high level of dissolved Fe and Mn(> 1mg/l) were detected at the downstream It shows that this area is suitable for the biodegradation of chlorinated organic compounds like TCE. Also, high level of TCE and DCE were detected at the downstream, which means that TCE is naturally biodegradaded at anoxic condition. Natural attenuation assessments suggested by US EPA showed that 41.3% (19 points) of wells were limited biodegradation reactions and 2.2% (1 point) of wells were complete biodegradation reactions. Some wells showed continuous increase in the level of TCE and it seems to be the result of constant inflow of TCE from the lower layer of soil. At the wells showing decreasing tendency of TCE, the average rate of natural attenuation of TCE was 8.93x10-4 day-1, and a half-life was 2.13 year. At the wells showing decreasing tendency of DCE, the average rate of natural attenuation of DCE was 1.22x10-3 day-1, and a half-life was 1.56 year. In the case of GW01 well, it takes approximately 19 years to decrease TCE contamination to the level of domestic water standard of 0.03mg/l. To estimate transport and natural attenuation of TCE in groundwater, MODFLOW was used for groundwater transport modeling and RT3D was used for contaminants transport modeling. Modeling area was selected by 1,850 m x 1,750 m with boundaries of surrounding rivers and dividing ridges. As the result of groundwater transport modeling with discharge areas as J River, S river and tributaries, groundwater flowed from west side mountain ranges to J river at east and northeast area. Contaminants transport modeling was conducted according to the different scenarios like one considering only current concentration and the other considering the possibility of constant inflow of TCE from soil layers. Also respective scenarios were applied in consideration of absorption of contaminants, biodegradation reflecting the rate of natural attenuation, and application of treatment after pumping. In the case of no consideration of absorption and biodegradation, contaminant plume of TCE at the subject area would exist around J River after 20 years. However, in the case of consideration of biodegradation, contaminant plume of TCE at the subject area would be disappeared within 4 years. If constant inflow of TCE from soil layers around A division exists, in the case of no absorption and no biodegradation, contaminant plume of TCE would be present at high concentration after 20 years. However, in the modeling of consideration of biodegradation, contaminant plume of TCE attenuated at high rate. Inthe modeling of consideration both of absorption and biodegradation, contaminant plume of TCE still existed after 20 years because absorption inhibited contaminants transport. In the modeling of no absorption and no biodegradation, wells located at downstream of groundwater flow showed increasing at first and decreasing later level of TCE contamination. However, in the modeling of consideration of biodegradation, every well showed rapid decrease in TCE concentration. It also showed that DCE and VC, biodegradation product of TCE, known for higher harmfulness than TCE to human health increased rapidly. DCE increased 5 times higher than current highest concentrationand VC increased about 54 times. In the modeling of no consideration of biodegradation, contaminant plume of DCE existed around the J River in small quantities after 10 years. However, in the modeling of consideration of only biodegradation, contaminant plume of DCE disappeared at all after about 5 years, and in the modeling of consideration both of biodegradation and treatment after pumping, contaminant plume of DCE disappeared at all after about 3 years. In the modeling of consideration of biodegradation, contaminant plume of VC produced during DCE degradation presented much higher level of concentration than current situation after 10 years. It seems because of rapid increase of VC plume resulted from the rapid biodegradation of DCE produced from biodegradation of TCE. It is estimated that VC would exist longer than DCE because of slow rate of biodegradation. Therefore, to apply natural attenuation of TCE contaminated groundwater, continuous monitoring and management of transport and production of biodegradation product like DCE and VC are needed. The result of this study is from the facts that contamination sources were limited around the A division at the subject area, and the rate of natural attenuation calculated from part of wells confirmed the possibility of natural attenuation was applied to whole subject area, so the result could be overestimated and further studies are required about factors of natural attenuation assessments at various sites.
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