Flow Routing in the Karst of Puerto Rico

90% of Puerto Rico's 2400 km(2) of karst is located near the Atlantic Coast. The north coast limestone aquifer consists of Tertiary carbonates with an unconfined upper aquifer and a confined lower aquifer. The transmissivity of the lower aquifer is an order of magnitude smaller than the upper aquifer. About half of the recharge is rainfall on karst uplands that travels north to near-coast springs and wells. Approximately 148,000 m(3)/day is withdrawn from the lowland areas, some of which contain a variety of chemical contaminants (e.g., phthalates) introduced in the populated lowlands. Widespread use of septic tanks and of caves and dolines for trash disposal is also a threat of unknown magnitude to water quality. Although the general flow paths of the upper recharges are known, and scores of streams in caves (of hundreds) have been mapped in higher elevations, the specific routes to the final discharges are not well understood. Dye traces, and isotope comparison of different sectors of the lower discharge area, have not proved particularly useful to identify them, so some efforts have focused on defining the characteristics of accessible cave system flows to see whether they can provide useful analogs. Geologic strike and dip are major controls of cave passage orientations, followed by faulting or major joint sets.