OLD FAULTS AREN'T OUR FAULT: PROPOSED RIO VALENCIANO DAM, JUNCOS, PUERTO RICO, SAFETY ISSUE OR PERCEPTION PROBLEM

The proposed Rio Valenciano Dam will be a 30-meter high dam that will close the valley of the generally-north-flowing Rio Valenciano at a point about 2.2 kilometers south of Juncos, Puerto Rico. The reservoir has a tributary area of 38.8 square kilometers and will supply an average of 14.4 million gallons of water per day (MGD) for treatment and distribution to the much-needed municipalities in the East Central Region. This area is rapidly developing and requires additional sources of water to meet current and future demand. The actual water supply in the area is scarce in spite of abundant rainfall. Current sources of water are predominantly direct river intakes with no storage capacity. This results in insufficient supplies during periods of drought.The Rio Valenciano reservoir site lies on the northern edge of the San Lorenzo batholith. This granitic rock was intruded into volcanic rocks about 65 million years ago (Ma). Just north of the batholith is one of two major faults crossing Puerto Rico, the Northern Puerto Rico Fault Zone (NPRFZ). This fault is 1 to 2 km north of the dam location. The NPRFZ trends east-southeast, has left lateral strike-slip motion, and can be traced for about 50 km (Jolly et al., 1998). Displacement along this fault has been estimated between 30 km (Briggs and Pease, 1968) to greater than 50 km (Jolly et al., 1998). The major movement on the NPRFZ has been determined by Jolly et al. (1998) to have occurred about 85 million years ago during upper Cretaceous time. However, movement on associated strike-slip faults can be seen cutting rocks as young as upper Paleocene (55 Ma) (Jolly, 1998) indicating movement took place over a prolonged period. Major movement according to Jolly (1998) would have preceded emplacement of the San Lorenzo batholith, but minor movements would have postdated it.Black & Veatch International (B&V) was contracted by the Infrastructure Financing Authority of Puerto Rico (AFI) to develop a Engineer-Procure-Construct bid documents for the dam, water transmission, water treatment plant, distribution, and associated appurtenant structures. A part of this contract included a geological and geotechnical investigation to develop a preliminary design for the dam upon which contractors could generate bids. This investigation included a review of the regional geology from the literature, geologic mapping in and around the site of the dam and reservoir, a core boring program along the proposed axis of the dam, and seismic refraction surveys.Based on the results of this study, an unrecorded fault has been postulated to cross the dam axis and generally paralleling the Rio Valenciano. This fault will be referred to as the Rio Valenciano Fault (RVF), and there are several pieces of indirect evidence for its existence. The RVF appears to trend along the course of the river to the south where it seems to connect with a fault parallel to the NPRFZ mapped by others (Cox and Briggs, 1973). To the north, there is topographic evidence from maps and air photos that the RVF continues up to the broad valley north of Juncos, where the NPRFZ is located. The evidence for the RVF being inactive is even more convincing than the evidence for its existence.