From this article it appears that is was Marcellus related.
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Drilling halted after spill into NW Pa. reservoir
JOHNSONBURG, Pa. (AP) — A company has halted Marcellus drilling in one northwestern Pennsylvania town after a kind of clay used to expedite the process spilled into a local reservoir.
Hunt Marcellus Operating Co. tells the Bradford Era (http://bit.ly/vf5OUf ) that it stopped drilling in Johnsonburg after some bentonite gel was released into Silver Creek and made its way into a drinking water reservoir controlled by the Johnsonburg Municipal Authority.
Bentonite is a form of impure clay that easily absorbs water and is used to facilitate gas and oil well drilling.
It wasn't immediately clear how much of the substance got into the reservoir.
But water authority chairman Eric Poague says the substance is not expected to affect the safety of the water — though it could make it cloudy — because bentonite is a "non-toxic substance that comes from the earth."
Information from: The Bradford Era, http://www.bradfordera.com
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