Equivalent Representative Fractured Network for Modeling Groundwater Flow
Fractured media feature, e.g., dead end pathways, lack of flow interconnections and preferential infiltration paths. These are, somewhat, not in line with the assumptions allowing the implementation of a continuum based balance partial differential equations. We thus developed a lumped parameter modeling (LPM) approach to assess the groundwater flow patterns through the fractured granites of Porto Alegre, Southern Brazil. Observations consist of stationary groundwater levels mostly localized at part of the domain, and geometrical properties of surface lineaments over the 30 by 35 Km study domain. Evaluation of their hydraulic head values revealed the existence of isolated lineament clusters, which could not be detected a priori and generated not acceptable flow or no-flow zones. We thus subdivided the domain into a map of Representative Elementary Area (REA) cells, chosen so that the ratio of lineament lengths summation over that of the cell area remains practically unchanged between two consecutive subdivisions. The actual surface lineaments were replaced by an Equivalent Representative Fracture Network (ERFN). The evaluated steady state hydraulic head was then compared between two LPM solutions referring to: 1) the intersection points of the ERFN; 2) nodes each being the locus of all ERFN intersection points within the same former REA cell. Computation addressed by the latter solution was significantly less intense. Both approaches lead to hydraulic head isolines consistently similar to those interpolated on the basis of the observed groundwater levels. One available evident of a Nitrate polluted well and its plausible contamination source within the study domain, show that it is oriented with the predicted flow direction along the ERFN.