Tracking reactive pollutants in large groundwater systems by particle-based simulations

Worldwide, great amounts of human and financial resources are being invested to protect and secure clean water resources. Especially in arid and semi-arid regions civilization depends on the availability of freshwater from the underlying aquifer systems where water quality and quantity are often dramatically deteriorating. Main reasons for the mitigation of water quality are extensive fertilizer use in agriculture and waste water from cities and various industries. It may be assumed that climate and demographic changes will add further stress to this situation in the future. One way to assess water quality is to model the coupled groundwater and chemical system, e.g.to assess the impact of possible contaminant precipitation, absorption and migration in subsurface media. Currently, simulating such scenarios at large scales is a challenging task due to the extreme computational load, numerical stability issues, scale-dependencies and spatially and temporally infrequently distributed or missing data, which can lead e.g. to in appropriate model simplifications and additionally uncertainties in the results. The simulation of advective-dispersive mass transport is usually solved by standard finite differences, finite element or finite volume methods. Particle tracking is an alternative method and commonly used e.g. to delineate contaminant travel times, with the advantage of being numerically more stable and computational less expensive. Since particle tracking is used to evaluate groundwater residence times, it seems natural and straightforward to include reactive processes to track geochemical changes as well. The main focus of the study is the evaluation of reactive transport processes at large scales. Therefore, a number of new methods have been developed and implemented into the OpenGeoSys project, which is a scientific, FEM-based, open source code for numerical simulation of thermo-hydro-mechanical-chemical processes in porous and fractured media (www.opengeosys.org). The presented methods describe the coupling concept that enables an interaction of mass transport simulations and particle tracking methods with geochemical reactions (i.e. OpenGeoSys coupled to PHREEQC and/or BRNS modules). The presentation addresses and discusses the issues and challenges of particle based reactive transport modelling which has been applied and tested in a high-performance computing environment during a case study of modeling groundwater flow and transport of the eastern half of the Arabian Peninsula (2.2 mio km2).