Simulation of groundwater flow and solvents transport in a coastal industrial site considering climate change

• Main Authors: Shu-YunLiang, 梁蜀昀
• Other Authors: Cheng-Haw Lee
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/79715528223521472710

博士 === 國立成功大學 === 資源工程學系 === 104 === SUMMARY The study examined groundwater hydrogeochemistry of the shallow aquifer by analyzing water quality of the monitoring wells set in the offshore industrial park. The Groundwater Modelling System (GMS) was used to construct a hydrological model. The tidal influence was considered in transient simulation to assess the influence of contamination monitoring system. Hydrogeologicae parameters, including top elevation, hydraulic conductivity and rainfall, were used together with data of the groundwater level for the numerical model. The assessment was made according to ten groundwater monitoring wells – both groundwater level and water quality were considered. In terms of the water level, there was a high frequency peak during each flood season. High-intensity or prolonged rainfall affected groundwater greatly. Seasonal changes of drought and flood were obvious. In terms of the water quality, the result illustrated a significant change of amplitude in well 4 and 8 as a consequence of semidiurnal tidal effects. Inconspicuous amplitude was found in well 1, 3, 5, 6, 7, 10, where were far away from the coast. Non-regularity signals were detected in well 2 and 9. It might be interpreted as an effect of engineering facilities. By modeling the flow and solute transport of the groundwater salt, the study indicated that conspicuous tidal affected well 8 and inconspicuous tidal affected well 3. The assessment showed water desalinization might take about fourteen years of continual rainfall leaching in the high chloride land reclamation area – twenty years if it goes through seawater intrusion. In relation to pollution caused by salt corrosion of old pipeline, the assessment indicated that BTEX and chloroform might take about five to six years of natural decay toward standard value and be continual contamination in decades. Key words: Climate Change、Time-Frequency Analysis、Solute Transport、Salinization、Collaborative Governance INTRODUCTION Coastal petrochemical industry areas, beucase of the heterogeneity and the uncertainty of the ground field, are mostly vulnerable to pollution. The situation becomes worse if taking the impact of climate change and extreme events into consideration. A high-risk contamination coastal industrial area is chosen in this study for assessment. By considering the temporal trends of groundwater, hydrological drivry factors under climate change and issues of groundwater safety, strategies of adaptation governance were discussed respectively. Time-frequency analysis, relationship between groundwater flow velocity and salt concentration, flow and solute transport model are also applied to access those issues. MATERIALS AND METHODS The study area was composed of clay, silt, sand, and gravel. The groundwater flow rate was approximately 2.7 × 10-4 cm/sec. Water level elevation varied between 0.11~2.5 m. Groundwater flow from the middle of study area to the embracing beach, which was the typical island-type-like distribution of groundwater lens. Other groundwater flows were affected by rainfall leaching and tidal effect. To understand the impact of flow regime, the study used time-frequency analysis to 10 monitoring wells’ groundwater level during May, 2010 to June, 2014. The study also applied Modflow＆MT3DMS simulation to access the possibility of chlorinated organic pollution. RESULTS AND DISCUSSION The temporal trends of groundwater showed the water level rose generally in all monitoring wells. There was a high frequency peak during each flood season – indicating the water levels of the monitoring wells were disturbed by heavy rainfall. High-intensity or prolonged rainfall affected groundwater greatly. Seasonal changes of drought and flood were obvious. In relation to the individual monitoring wells, there were significant changes of amplitude in well 4 and 8 as a consequence of semidiurnal tidal effects. Inconspicuous amplitude was found in well 1, 3, 5, 6, 7, 10, where were far away from the coast. Non-regularity signals were detected in well 2 and 9. It cound be interpreted as an effect of engineering facilities. By modeling the flow and solute transport of the groundwater salt, the study indicated that conspicuous tidal affected well 8 and inconspicuous tidal affected well 3. The terrace of this region was reclaimed from the sea. The initial measurement was 28,000~17,000 mg/L for chloride because groundwater here in 1994 was rather fresh but seawater. Normally, soil salinization increased in the drought season (rainfall precipitation is 114 mm/d) due to water evaporation. It decreased in the flood season (rainfall precipitation is 463 mm/d) as a result of rainfall leaching and desalination. In the monitoring well 3, it took about 14 years for the water quality to be desalinize to less than 3,000 mg/L chloride – in the situation of continual rainfall leaching without seawater intrusion. The desalinization time for monitoring well 8 was hard to estimate because of tidal effect or climate extreme. However, it was expected to have longer desalting time due to occasional recharge of chlorine concentration. The well 8 was also possible to maintain a high chlorine concentration. BTEX and chloroform, the most detected substances, were selected as the substance of interest in petrochemical contaminated sites for chlorinated organic assessment and single/ continuous modeling. The result showed some of the BTEX plume (xylene, benzene, toluene, and ethylbenzene) would be slightly rounded shape when the groundwater flow was very slow. Advection effect was not significant. It would take about 5 years for regulatory standard at a natural slope by single pollution. The result estimated more than two-times of contamination after 6 years in continuous pollution emission site. A single chloroform pollution incident (1.0 mg/L) could decline to 0.1 mg/L at a natural slope after 5 years, but 6.0 mg/L while continued the pollution emission and would take decades to regulatory standard without any treatment. CONCLUSION Since there was no significant pollution in the study area, we offered some recommendations from the precautionary perspectives. First, it was critical to set up the benefit assessment for monitoring wells in vulnerable polluted area, and to develop pollution potential index for coastal petrochemical industry. In addition, developing pollution potential map by geographic information system was critical. Having interrogate and repair pipeline in higher risk areas could effectively reduce the chance of contamination.