HYDRAULIC CONTAINMENT OF LANDFILL GAS DRIVEN VOCS IN GROUNDWATER USING A STATE OF THE ART COMPUTERIZED CONTROL SYSTEM AT THE LOS REALES LANDFILL

摘要

Chlorinated volatile organic compounds (VOCs) were detected in monitor wells at the 260-acre Los Reales Landfill in Tucson, Arizona. The landfill, located within a State Superfund area, is the only operating landfill for the City of Tucson. Delineation of VOC impacted groundwater, which extends 1,500 feet off-site, was completed in 1996. Groundwater modeling was conducted to predict hydraulic capture zones and placement of extraction wells. This resulted in the design and construction of ten extraction wells to hydraulically contain the VOCs in groundwater. The remediation system includes a VOC stripping unit and a state-of-the-art electrical control system. The system is remotely operated, providing significant cost savings to the City through reduced operation and maintenance (O&M) costs. Treated groundwater is used to supplement water usage for landfill operations. The remaining treated groundwater is reinjected into the regional aquifer to enhance containment of the plume. The remediation system was also designed to allow for zero discharge of emissions. This resulted in the City not needing an air quality permit from the local regulatory agency. The employment of state-of-the art electrical control systems allows for increased system performance and ease of data acquisition. A Programmable Logic Controller (PLC) is used to control all system equipment and ensure safe operation. One key to the system is the use of Variable Frequency Drives (VFD) to control the extraction pumps in each well, in conjunction with down well pressure transducers and the PLC to control the groundwater depression produced by the groundwater extraction. The extraction pumps are automatically operated at a frequency that achieve the proper groundwater depression. By controlling the groundwater depression, the extraction pumps operate below their full capacity significantly reducing the cost of electricity over the life of the project. A standard personal computer running Human Machine Interface (HMI) and standard database software allow the system to be remotely controlled and monitored via modem. By remote data acquisition, the need for site visits are minimized and the long term operational costs of the system are substantially reduced. During design of the remediation system, the City of Tucson contracted with a gas to energy company to extract methane for production of electrical power. Review of historical records, soil gas surveys, and site data suggested the source of VOCs in groundwater was due to vapor phase migration of VOCs within the landfill gas. Removal of landfill gas, in conjunction with hydraulic containment of impacted groundwater, should shorten the required time necessary for site remediation. Results of ongoing groundwater and landfill gas monitoring during remediation will be assessed in early 1999 to determine the effectiveness of both systems.