Double Funnel & Gate “Sparge Gate” Groundwater Plume Migration Control & Treatment
HMA was engaged in an environmental assessment of a former industrial site with significant petroleum volatile organic compounds (PVOCs) impacts. The site is positioned along 5,000 feet of riverbank. HMA conducted soil and groundwater profiling/sampling to understand the geology, contaminant transport, exposure pathways, and define the distribution of PVOCs in the subsurface. The Michigan Part 201 groundwater-surface water interface (GSI) pathway was of immediate concern to the MEGLE. Benzene concentrations in excess of 10,000 ug/L were found in the shallow aquifer upgradient of the GSI, which greatly exceeded the generic GSI criterion of 200 ug/L.
HMA reviewed various remedial technologies to control and mitigate the PVOCs in the shallow aquifer, which discharges to the adjacent river. The remedial goal sought short-term compliance with GSI criteria and long-term protection of the river from upland source areas. Significant aqueous-phase PVOC concentrations affected 1,350 feet of the river. A funnel-gate system was designed to channel the groundwater for treatment. MODFLOW was used to simulate groundwater flow and elevations of the proposed remedy to verify adequate funnel to gate ratios and the effects of groundwater mounding.
Low-permeability slurry walls were trenched parallel with the river to “funnel” the groundwater toward four treatment “gates”. Upland slurry walls were installed to stagnate high-concentration groundwater and increase flow pathways which promoted natural attenuation. The treatment gates were placed along the core of the groundwater plumes where soil excavation was needed adjacent to the river. They were constructed in the subsurface with a series of vaults and corridors, one system 126 feet long and the other 270 feet long. Air sparging occurred in the first corridor. Vaults had piping to control water elevations. The second long corridor allowed the potential for secondary treatment, but none was needed, as shown through system testing. This second corridor was then used to promote attenuation of any remaining PVOCs in the oxygenated waters. The treated water was released through the last vault to a “reconstructed aquifer” where the riverbank had been excavated. The reconstructed aquifer created a new GSI before reaching the river.
The system works on gravity flow of groundwater; thus no pumping of water to an aboveground system is needed. Valves in the vaults allow the system to be closed off for maintenance and cleaning. Introducing oxygen to the native groundwater with elevated total dissolved solids produces mineral precipitate which requires bi-annual removal. The open-topped design allows access for cleaning and promotes the longevity of the system’s planned operational life (>20 years). The system’s performance is monitored regularly and has successfully been treating the groundwater to less than GSI criterion.
- Groundwater with high concentrations of benzene and associated PVOCs venting to river.
- Private ownership with active operations.
- Installed slurry walls parallel to river to redirect (funnel) groundwater flow.
- Funnel and gate system constructed to treat groundwater below grade level to eliminate pumping.
- Primary treatment by air sparging to volatilize PVOCs.
- Flow through system treats groundwater and discharges to reconstructed aquifer adjacent to river.
- Groundwater concentrations treated from >10,000 ug/L benzene to less than GSI criterion.
- HMA provided system design, field construction management, system testing, and reporting to MEGLE.