Extensive and increasing rates of leakage have been observed at the Mount Santo Tomas reservoir. The exact locations of leakage are not known and several attempts to repair possible sites have not been effective. Neither cement grouting of the earthen walls at the sides of the masonry dam nor the application of a layer of bentonite clay over a portion of the floor of the dam were successful. Geological studies revealed the presence of several faults
and porous limestone formations with possible large caverns under the floor of the reservoir. One of the faults runs directly under the masonry dam. These areas are the principal suspects.