The final answer to the critical issue raised by the recent Peterborough Oil Company decision is that MTBE is an additive and is not, in MassDEP’s view, subject to the “oil exemption” under the Massachusetts Contingency Plan.
I have to say that I’ve done few posts in recent years that have prompted more immediate responses than those on this case. After yesterday’s little birdie suggesting that MassDEP might be taking the position that MTBE is subject to the oil exemption, another little birdie, this time from MassDEP, quickly contradicted the first one.
Not only is the second little birdie from the horse’s mouth, 
as it were, but she pointed me to the following MassDEP MCP Q&A from 2008:
QUESTION: 793. I have two release sites in Zone II’s whose groundwater is categorized as GW-1 solely because of the Zone II. They are a former and a current gasoline station located in a commercial district served by municipal water supply. There are no on-site or offsite impacts, however we have not achieved GW-1 standards at either location. We are trying to achieve closures via Method 3 risk characterizations. At one of the sites, all gasoline-related contaminants have been reduced to less than GW-1 standards except for MTBE. In regard to the new section dealing with Zone II’s under 40.0924(2)(b)3.a., which is applicable to Oil contamination, does Oil include MTBE associated with gasoline? (i.e. can this section be used for a site with residual MTBE contamination above GW-1 standards?)
ANSWER: It is not explicit in the definition of Oil that MtBE is excluded, but it is the Department’s intent that MtBE not be included. The attached link [which I’ve deleted because it appears to be broken] contains MassDEP’s response to comments on the proposed regulations. On page 7 of the comments you will see that MassDEP was asked to consider including MtBE under the proposal and our response indicates that we chose not to include MtBE. The reasoning for not including MtBE is because it does not biodegrade readily and within a short distance from the source area as do the petroleum constituents.
Seems right to me. End of story, finally?