The commission is expected to adopt a resolution formally appointing him on Wednesday.
Tambini issued a brief statement saying that for 30 years, his professional life has been "dedicated to water. I believe my career experiences, water resources knowledge, leadership skills, and core values will serve me well."
He said he could not comment further or conduct DRBC-related activities "until I officially conclude my relationship" with Pennsylvania American.
The commission oversees water quantity and quality for the watershed, the drinking water source for 15 million people.
Its voting members are the governors of Pennsylvania, New Jersey, New York and Delaware, plus a federal representative.
It has remained divided on the issue of natural gas. Nearly four years ago, the commission instituted a moratorium on drilling until regulations could be adopted. Environmental groups want a permanent moratorium. Gov. Corbett has pushed vigorously for his fellow commissioners to pass regulations.
The president of the Marcellus Shale Coalition, David Spigelmyer, said the industry group looks forward "to working constructively and collaboratively with Mr. Tambini on critical energy-related issues, specifically advancing common sense, workable regulations that permit safe shale development in portions of northeastern Pennsylvania."
The Delaware Riverkeeper, Maya van Rossum, said she did not know Tambini. She said the executive director has an obligation to put protection of the people and the watershed first. She called it "a high hurdle of responsibility . . . that includes shedding any and all allegiances to industry, private interests, and particular political entities."
The commission's chief administrative officer, Richard C. Gore, will head the agency until Aug. 1.