Conceptual and numerical modeling approach of the Guarani Aquifer System

Abstract

In large aquifers, relevant for their considerable size, regional groundwater modeling remains challenging given geologic complexity and data scarcity in space and time. Yet, it may be conjectured that regional scale ground- water flow models can help in understanding the flow system functioning and the relative magnitude of water budget com- ponents, which are important for aquifer management. The Guaran ́ı Aquifer System is the largest transboundary aquifer in South America. It contains an enormous volume of water; however, it is not well known, being difficult to assess the impact of exploitation currently used to supply over 25 mil- lion inhabitants. This is a sensitive issue because the aquifer is shared by four countries. Moreover, an integrated ground- water model, and therefore a global water balance, were not available. In this work, a transient regional scale model for the entire aquifer based upon five simplified, equally plau- sible conceptual models represented by different hydraulic conductivity parametrizations is used to analyze the flow sys- tem and water balance components. Combining an increas- ing number of hydraulic conductivity zones and an appro- priate set of boundary conditions, the hypothesis of a con- tinuous sedimentary unit yielded errors within the calibra- tion target in a regional sense. The magnitude of the water budget terms resulted very similar for all parametrizations. Recharge and stream/aquifer fluxes were the dominant com- ponents representing, on average, 84.2 % of total inflows and 61.4 % of total outflows, respectively. However, leakage was small compared to stream discharges of main rivers. For in- stance, the simulated average leakage for the Uruguay River was 8 m3 s−1 while the observed absolute minimum dis- charge was 382 m3 s−1. Streams located in heavily pumped regions switched from a gaining condition in early years to a losing condition over time. Water is discharged through the aquifer boundaries, except at the eastern boundary. On av- erage, pumping represented 16.2 % of inflows while aquifer storage experienced a small overall increment. The model water balance indicates that the current rate of groundwa- ter withdrawals does not exceed the rate of recharge in a regional sense.