TRANSIENT MIXING CELL APPROACH FOR MODELLING GROUNDWATER FLUXES IN COMPLEX HYDROGEOLOGICAL BASINS

A transient Mixing Cell Model (MCM) was developed for assessing groundwater fluxes in complex hydro-geological basins prevailing transient groundwater flow system. It is aimed for complex systems with vague sub-aquifer structure, lack of hydro-geological information, where the boundaries and hydrological conditions along the boundaries are not sufficiently distinct, and where it is difficult to construct, solve and calibrate a hydrological model based on the continuity approach. The transient MCM method is based on a spatial and temporal distribution of dissolved ions and isotopes in a transient multi-compartmental flow model pattern. The MCM enables the assessment of groundwater fluxes between sub-aquifers associated with groundwater recharge and the groundwater storage capacity, prevailing in each compartment. The underlying goal of the sub-division of the aquifers into discrete sub-hydrological systems is to create hydrological units, in which each of the compartments has relatively homogeneous and fixed values for the hydrological parameters, concentrations and hydraulic heads variables. In this way we attain a situation (stemming from the definition of “compartment”) in which the gradients of hydraulic heads or concentrations of solutes are projected onto the boundaries between compartments. The model assumes mixing of solutes in the compartments of the system, due to the presence of water from various sources with differing solute compositions. As a result, the different sources of water are mixed, and a new solute composition is created which is considered unique to the water in the cell at that specific point in time. A set of water and solute balance equations was formulated for every dissolved conservative constituent in the compartments. This constitutes a set of large quantities of constraints, under which, in order to identify and quantify the unknown groundwater fluxes in the aquifer, it was necessary to solve the so-called “Inverse Problem” by using a process of optimization. The Least Squares (LS) mathematical algorithm E04NCF (by NAG®) was found to be the most suitable and effective for solving the compartmental model, capable of accommodating several orders of magnitude differences among the unknown parameters while limiting the solution to only positive values of water fluxes.