Title : Groundwater flow and transport modeling for assessing the coastal managed aquifer recharge (MAR). A case study
Abstract:
In coastal aquifers, there is a natural freshwater-saltwater equilibrium relationship that can be altered by human action by reducing the discharge of freshwater into the sea as a result of pumping extractions. This subterranean advance of seawater can occur in different ways, the most frequent being: i) the horizontal advance of the freshwater-saltwater interface inward and ii) the vertical ascent in the form of a saline cone ("upconing") of the interface in favor of wells in which intensive pumping takes place.
Considering this process, this study aimed to identify the effect of managed aquifer recharge (MAR) on upconing using mathematical flow and transport modeling, to determine an optimal MAR scenario for marine intrusion control, taken as a case study of the coastal aquifer of La Plana de Castellón (Spain).
As materials and methods, the necessary information was collected for updating the conceptual hydrogeological model, and, based on the said model; the data processing was carried out for its use in modeling with Visual MODFLOW Flex for the simulation of groundwater flow. Based on the results, the flow conditions were calibrated by the traditional trial and error procedure, in order to obtain a Normalized Mean Squared Error (RMS-N) equal to or less than 10%. Subsequently, SEAWAT, a code based on MODFLOW/MT3DMS, was used to simulate the three-dimensional flow of groundwater with variable density along with solute transport based on Total Dissolved Solids (TDS) concentrations and to identify both upconing like the recharge water plume of MAR. For these simulations, piezometric and hydrochemical information was available from a MAR test using wells and surplus surface water.
The groundwater flow model was obtained and considering the conceptual hydrogeological model, it was observed that the results were coherent, obtaining RMS-N values ??between 6.6% and 8.4%. Concerning the simulation of transport with TDS, this was carried out considering both a scenario without MAR and with MAR, to appreciate the recharge water plume and its effect on the system.
It was possible to verify that the water coming from the lateral groundwater inlets produces a natural attenuation on the system and that together with the MAR there is a positive and notorious effect on the quality of the aquifer water by reducing its salinity, in addition to producing a rise of piezometric levels. With this information, hypothetical scenarios were proposed taking into account different recharge flows and recharge duration, identifying the best configuration to achieve attenuation of saltwater intrusion on a shorter time scale.
Audience Take Away:
- The example presented in this study is the use of mathematical modeling to support decision-making. Modeling can be a very valuable tool because it allows you to compare different management models and check which one can bring more benefits.
- The modeling makes it possible to study both processes that have occurred and are occurring, and also to predict their behavior under innumerable future scenarios.
- The modeling can be applied to any field of knowledge. You have to look for software that meets the objectives of the modeling. Today there is a large number of open-access software.
