GROUNDWATER TRANSPORT OF CRATER-LAKE BRINE AT POAS VOLCANO, COSTA-RICA

Poas Volcano is an active stratovolcano in Costa Rica that has a lake in its active crater. The crater lake has high temperatures (50-90-deg rees-C), high acidity (pH almost-equal-to 0.0), and a high dissolved-s olids content (100 g/kg). The volcano has numerous freshwater springs on its flanks, but a few on the northwestern flank are highly acidic ( pH = 1.6-2.5) and have high dissolved-solids concentrations (2-22 g/kg ). This study analyzes the regional groundwater system at Poas and dem onstrates the likelihood that the water discharging from the acidic sp rings in the Rio Agrio watershed originates at the acidic crater lake. Both heat and solute transport are analyzed on a regional scale throu gh numerical simulations using the HST3D finite-difference model, whic h solves the coupled equations for fluid flow, heat transport, and sol ute transport. The code allows fluid viscosity and density to be funct ions of both temperature and solute concentration. The simulations use estimates for recharge to the mountain and a range of values and vari ous distributions of permeability and porosity. Several sensitivity an alyses are performed to test how the uncertainty in many of the model parameters affects the simulation results. These uncertainties yield a n estimated range of travel times from the crater lake to the Rio Agri o springs of 1-30 years, which is in close agreement with the results of tritium analyses of the springs. Calculated groundwater fluxes into and out of the crater lake are both about several hundred kg/s. These fluxes must be accounted for in water budgets of the crater lake.