Modeling of non-isothermal multi-component reactive transport in field scale porous media flow systems

A general 2-D finite element multi-component reactive transport code, TRANQ UI, was developed, using a sequential iteration approach (SIA). It is well suited to deal with complex real-world thermo-hydro-geochemical problems fo r single-phase variably water saturated porous media flow systems. The mode l considers a wide range of hydrological and thermodynamic as well as chemi cal processes such as aqueous complexation, acid-base, redox, mineral disso lution/precipitation, gas dissolution/ex-solution, ion exchange and adsorpt ion via surface complexation. Under unsaturated conditions only water flow is considered, although gas pressures are allowed to vary in space in a dep th-dependent manner specified by the user. In addition to the fully iterati ve sequential approach (SIA), a sequential non-iterative approach (SNIA), i n which transport and chemistry are de-coupled, was implemented and tested. The accuracy and numerical performance of SIA and SNIA have been compared using several test cases. The accuracy of SNIA depends on space and time di scretization as well as on the nature of the chemical reactions. The capabi lity of the code to model a real case study in the field is illustrated by its application to the modeling of the hydrochemical evolution of the Llobr egat Delta aquitard in northeastern Spain over the last 3500 years during w hen fresh-water flow from a lower aquifer displaced the native saline aquit ard waters. Manzano and Custodio carried out a reactive transport model of this case study by using the PHREEQM code and considering: water flow, aque ous complexation, cation exchange and calcite dissolution. Their results co mpare favorably well with measured porewater chemical data, except for some of the cations. Our code is not only able to reproduce the results of prev ious numerical models, but leads to computed concentrations which are close r to measured data mainly because our model takes into consideration redox processes in addition to the processes mentioned above. A number of sensiti vity runs were performed with TRANQUI in order to analyze the effect of err ors and uncertainties on cation selectivities. (C) 1999 Elsevier Science B. V. All rights reserved.