Simulation of ground-water flow in the water-table aquifer near Barceloneta, Puerto Rico

A two-dimensional, steadystate digital flow model of the water-table limestone aquifer near Barceloneta, in north-central Puerto Rico, was developed: (1) to estimate the effects of additional ground-water withdrawals on the aquifer water levels and, (2) to determine whether hydraulic gradients relative to mean sea level would be reversed as a result of the additional pumpage, thereby initiating a potential for seawater intrusion. The model was calibrated to the ground-water levels of February 1982. Observed heads were within 10 feet of the model-computed heads. Adjusted regional values of aquifer transmissivity ranged from 1,000 to 300,000 feet squared per day. Localized cavernous conditions in the Aymamon Limestone have resulted in aquifer transmissivities in excess of 2,000,000 feet squared per day. Solution conduits, however, were not considered in the steady-state regional ground-water flow analysis of the model. The proximity of the physical boundaries of the aquifer combined with the relative low value of specific yield and high value of transmissivity, causes the ground-water system to reach steady-state conditions within the first 150 days after stresses begin. Recharge is the most sensitive parameter in the model. Calibrated values of aquifer recharge were as much as 25 inches per year in the outcrop areas of the Aguada Limestone. During any intense drought, ground-water discharges to Cano Tiburones and Rio Grande de Manati will be significantly reduced. Model results show that if 1982 ground-water withdrawals at Cruce Davila near the center of the study area are doubled (from 3.5 to 7.0 million gallons per day) water levels would decline a maximum of about 5 feet. A maximum pumpage of 6 million gallons per day would minimize the potential for seawater intrusion in the Cruce Davila area. Simulated pumpage, in addition to the present (1982) pumpage of 11 million gallons per day from the entire study area was varied from 7 to 29 million gallons per day. Additional withdrawal of 7 million gallons per day caused a maximum water level decline of about 4 feet; 17 million gallons per day caused a decline of 16 feet. At an increase in withdrawal of 29 million gallons per day ground water flow to the Cano Tiburones would cease as would flow in the Rio £rande de Manati downstream of Highway 2.