Many technologies can be used to clean up compounds in the environment, but not all technologies are suitable for every site. Before the cleanup phase of a project begins, state environmental regulations require that multiple technologies be screened to determine whether they will be effective at a given site. Technologies that pass this screening process must then be further evaluated using specific criteria established by the regulations. These criteria include effectiveness, reliability, technical difficulty, relative cost, short-term and long-term risks, timeliness, and comparative benefits.
For the Site, National Grid conducted a Phase III screening and evaluation of the following categories of technologies:Technologies that would build a barrier to limit contact with the compounds, or to limit their migration. Technologies that would treat the compounds in place, i.e., without removing them from the Site. Technologies that would remove the compounds from the Site so they could be recycled, treated, or disposed of at an approved facility.
Because of the complexity of the conditions at the Site, National Grid selected a combination of technologies to address soil and groundwater in the upland areas and sediments and porewater in the harbor. National Grid presented the selected approach in its February 2012 Remedial Action Plan. It includes the following elements:
- Harbor sediments containing visible coal tar or elevated PAH compounds from the former MGP operations will be dredged and transported off the Site for proper treatment and recycling.
- After the sediments have been dredged, a cap consisting of specially designed layers of material will be placed in the harbor near some of the seawalls to treat porewater. (Porewater refers to water found between particles of sediment.)
- Soil with MGP-related compounds or coal tar residue will be excavated and transported off the Site for proper treatment and disposal.
- After soil with MGP-related compounds has been excavated from the park, a cap of clean soil will be placed over the excavation.
- Where soil cannot be excavated (such as beneath the existing National Grid building), a new seawall will be built to serve as a barrier between the coal tar in the soil and the harbor.
- The deep DNAPL below the city park, National Grid, and Coast Guard properties will be pumped to the surface, from where it will be moved off-Site for disposal.
Legal restrictions called Activity and Use Limitations will be placed on some of the deeds for upland properties. These restrictions will require maintenance of the caps installed during the cleanup, and will also provide guidelines for future construction in areas affected by the MGP.