CONICET | Buscador de Institutos y Recursos Humanos

The Guaraní Aquifer System (GAS) is the largest transboundary aquifer system in South America. It extendsfor some 1200000 km2 over four countries: Brazil, Argentina, Paraguay, and Uruguay. The aquifer is formed bysandstones and confined by basalts in about 90 % of its extent. Sandstones outcrop along aquifer edges, deepeningtoward the center of the basin, where they can reach a maximum thickness of some 600 m and depths of 2200 m.The GAS contains an enormous volume of water. However, it is not well known, so that it is hard to assess theimpact of exploitation. This is a sensitive issue because the aquifer is shared by four countries.To address this and other issues, the four countries developed the Environmental Protection and SustainableDevelopment Project for the GAS, a multidisciplinary effort that culminated with a series of technical/managementproducts. One of the outcomes of the project was the regional groundwater flow model for the entire aquifer,presented in this paper.The GAS was simulated as a single continuous sandstone unit with no flow interactions either with overlayingbasalts nor with underlying permic deposits. These conceptual simplifications about the layer structure of theGAS imply an essentially two dimensional flow regime. Moreover, no information was available regarding headvariations along the vertical. The code TRANSIN II, which allows automatic parameter calibration, was used inthe study.Recharge was applied in outcropping areas along the aquifer boundaries, while local and regional discharge wassimulated through the aquifer boundaries and along numerous stream reaches in contact with the aquifer. Thehypothesis that the GAS may discharge through selected reaches along the Uruguay and Paraguay Rivers wasalso tested. Pumping amounted to 1040 hm3/year. The calibration strategy consisted on automatically calibratinghydraulic conductivity and recharge rates using 114 measured piezometric levels, aiming at obtaining a goodmatch between simulated and measured levels and conductivity values coherent with the current knowledge of theaquifer.In this work, five zonations were defined to parametrize conductivity, each producing an alternative conceptualmodel. The number of zones ranged from one, i.e. homogeneous case, to thirty one.Simulated piezometric levels were within the calibration target defined according to the quantity and qualityof available data. The best fit between measured and simulated levels was obtained for the largest number ofconductivity zones. The model water balance indicated that flows through the system are small compared to thewater volume in storage and to the annual discharge of the Paraná and Uruguay Rivers.Calibrated conductivities for all five scenarios were higher in the central region of the modeled area, with valuesabove the range typically expected for sandstones, even considering scale effects. This result would indicate theneed for the model to conduct flows in that area through, for instance, a preferential flow zone or a connection withoverlying geologic units. Therefore, two additional simulations representing geologic structures were evaluated:first by introducing a hydraulic connection between the GAS and overlying basalts, second by incorporatingtwo regional geologic structures into the thirty one hydraulic conductivity zoning configuration with the aim atreproducing the compartamentalization concept introduced recently by several authors. As a result, calibrationerrors were reduced, flow patterns modified locally and conductivity values better adjusted to reality.Despite the significant progress done on the conceptual model, and particularly in the numerical model, it isnecessary to continue and deepen the analysis of the geology and structures, as well as recharge and discharge ofwater flows, to increase the reliability of the conceptual model and therefore the numerical model.

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