The Vashon landfill is a 10-acre municipal solid waste facility that was formally closed during the summer of 2001, in accordance with WAC 173-351. The closure design report and construction were completed in accordance with WAC 173 – 351 and included agency interactions and submittals. Prior to closure, facilities at the site were brought into conformance with the King County Surface Water Design Manual requirements. In addition to the stormwater facilities and the installation of the final cover system, BHC Consulting has completed several programs which provide performance monitoring; observation and documentation of geophysical, geotechnical, and hydrogeological investigations; analysis of leachate storage; and treatment facilities.
Throughout this project, BHC has provided comprehensive consultant services and has worked closely with King County to meet agency deadlines. Project team reorganization (combining the roles of project manager and project engineer and utilizing staff engineer efforts) following Phase III has significantly reduced project costs. As a phased project, the closure and related activities included the following components:
PHASE I - Data Collection and Preliminary Design, including a preliminary design report with analysis of alternative cover systems, environmental control facilities, value engineering, and a site-wide hydrogeologic report.
PHASE II - 1999 Stormwater Improvements, including design, permitting, and construction management. Two stormwater detention ponds were constructed and two existing ponds were retrofitted to conform to the 1998 King County Surface Water Design Manual.
PHASE III - Final Closure, including design, permitting, and construction management. The work was completed in accordance with WAC 173 – 351 and included agency interaction and coordination. Efforts involved the design of a lining system that included a gas collection layer, a low permeability soil layer made from processed, on-site borrow materials. The design also included horizontal LFG wells and a vacuum break structure. Existing leachate collection and transmission facilities were modified to facilitate improved removal of leachate from the cell. Leachate pumping facilities were updated with water surface level monitoring and telemetry to transmit data to the transfer station. The final closure was supplemented with a closure construction report to document and certify the construction activities, in accordance with WAC 173-351.
PHASE IV - Leachate Management Evaluation, including an evaluation of alternative methods for leachate pretreatment, storage, and disposal. The work consists of the evaluation of leachate flows and pollutant loadings since final closure of the Vashon Landfill relative to pre-closure conditions and for the potential treatment of leachate at the Vashon WWTP.
PHASE V - Additional and Interim Services, including geotechnical explorations which utilized ground-penetrating radar and boreholes to delineate refuse that had been placed beyond the cell; evaluations of the existing leachate system which utilized intrinsically safe video technology; leachate system maintenance by jet-rodding existing leachate lines, both within the lined cell and beneath the liner; the initial phase of a landfill gas sampling and analysis program; and a post-closure plan and Operations and Maintenance manual in accordance with WAC 173-351.
PHASE VI - Environmental Control Facilities Evaluation and Improvements, including a site-wide evaluation of the existing environmental control facilities, compilation of information gathered in previous investigations, and an associated report. Five (5) regional aquifer-monitoring wells were constructed in November 2003. Information from well construction was used to refine the site geologic and hydrogeologic models. These models, along with the facilities evaluation, will be used to determine the desired location of potential future landfill gas and groundwater monitoring points and potential upgrades to the existing environmental control facilities.
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