A computer program for the simulation of evaporation of natural waters to high concentration

EQL/EVP is a FORTRAN 90 program that simulates the evaporation of dilute waters as well as concentrated brines. The code calculates stepwise the composition of the evaporated solution and the amounts of precipitated minerals. Activity coefficients are based on the Pitzer's interaction model which allows calculation to high ionic strength. The Newton–Raphson method is used to solve a set of linear mass-balance and non-linear mass-action equations. The simulation may be carried out in equilibrium mode where minerals are allowed to redissolve into the solution or in fractional crystallization mode where minerals are removed as they precipitate. Temperature dependence of various parameters and mineral solubility products are tentatively included between 0 and 50°C. The code processes all invariant points: those where the activity of water is constrained by several minerals and the invariant end-points. The code may be a useful tool for understanding hydrological and geochemical processes in arid regions.