Evaluation of pond/aquifer flow exchanges using local discretization and contrasting different boundary condition in MODFLOW. Case of Santa Olalla pond (SW Spain)

Almonte-Marismas is a coastal aquifer situated in Doñana Natural Park (Southwestern of Spain, Huelva). It supports one of the most important wetland areas in Europe due to its biodiversity, size and strategic location. Nowadays, the aquifer suffers serious threats due to the large amount of water extraction that takes place in the area due to the high demand for water that exists for the supply of tourism and irrigation.

There is a flow model of the regional aquifer which is used to support the water management Administration. However, this model does not take into account groundwater interactions with local ponds. Santa Olalla pond is a hypogenic wetland that, on a regional scale, it receives the discharge of the Almonte-Marismas aquifer. This fact allows it to maintain a permanent water regime without suffering a reduction in its volume of water. Despite of that, the intense pumping in the zone could affect it and be a risk in the future.

The objective of this study is the identification of an appropriate model structure to characterize and implement the Santa Olalla Pond in the current steady-state model of the regional aquifer of Doñana employing ModelMuse interface. For this purpose, different boundary conditions (LAKE and DRAIN packages) were contrasted to represent the pond, combined with different local grid refinement (LGR2 package). The contrast criteria to assess the goodness of the numerical representation have been the piezometric heads in the wells situated in the surroundings of the pond and the stage levels and water balance of the pond.