Ms. Greathouse’s experience was not an isolated case. More than a quarter century of efforts by some lawmakers and regulators to force the federal government to police the industry better have been thwarted, as E.P.A. studies have been repeatedly narrowed in scope, and important findings have been removed.

For example, the agency had planned to call last year for a moratorium on the gas-drilling technique known as hydrofracking in the New York City watershed, according to internal documents, but the advice was removed from the publicly released letter sent to New York.