Documentation on the Numerical Aspects in Forming and Solving the Equations

Hello,

There are many papers on Toughreact but I am searching for a documentation/paper on forming the equations i.e. accumulation, convection, reaction terms and solving these equations. I guess initially the flow equations are solved and then the geochemical equations are solved as batch reaction. However, I was wondering which equations they use, how they form the equations, how they compute the derivatives, what the scheme of solving the problem is. How do they handle very small concentration and how do they take out the equations and the independent parameters if the components dissappear? I would appreciate if you could recommend some papers. As there are so many papers published, it would take some time to find what I am looking for. Thank you very much for your help beforehand. I am not interested in discretization or dual porosity formulations.