Geologic and hydrologic data collected at test holes NC-1 and NC-3, Guaynabo and San Juan, eastern Puerto Rico

Abstract

Test holes NC-1 and NC-3 were drilled in the municipalities of Guaynabo and San Juan, respectively, during 1986, as part of a study of the aquifers in the Northern Coastal Province of Puerto Rico. These test holes were drilled to the depths of 635 and 375 feet below land surface, respectively. Hydrologic and geologic data collected include continuous core, water samples, head measurements, geophysical logs, and estimates of the hydraulic properties of the water-bearing zones. Detailed petrological and microfaunal analyses of the core were used to determine the mineralogical content, ages, and paleoenvironments of deposition. The core recovered from test hole NC-1 was from two formations of middle Tertiary age: these were, in descending order, the Cibao Formation and Mucarabones Sand. The core recovered from test hole NC-3 was from three formations: these were, in descending order, the Cibao Formation, the Mucarabones Sand, and undifferentiatcd igneous rocks of early Tertiary or Late Cretaceous age. Five and seven artesian water-bearing zones were encountered in test holes NC-1 and NC-3, respectively. The water-bearing zones in the two test holes contained brackish water. INTRODUCTION Aquifers of the Northern Coastal Province of Puerto Rico are part of a coastward-thickening wedge of highly karstified platform carbonates and minor clastic rocks of Oligocene to Holocene age. This sequence flanks the central mountains of Cretaceous and early Tertiary age, and extends near Aguada in the western part of the island to Loiza, about 30 mi east of San Juan and comprises an area of approximately 700 mi2. Limited hydrologic and geologic data are available for the subsurface coastal areas of northern Puerto Rico. As part of a cooperative study between the U.S. Geological Survey and the Commonwealth of Puerto Rico (TorresGonzalez and Wolansky, 1984), 15 test holes were drilled (fig. 1) to better understand the geologic and hydrologic characteristics of the aquifers and confining units of the Northern Coastal Province. The results of these test holes are being documented in a series of reports. 1 U.S. Geological Survey, Water Resources Division, San Juan, Puerto Rico. 2 Department of Geology, University of New Orleans, Louisiana. 1