Surface water - groundwater relationships assessment as a tool for environmental management in the Aji stream basin (Argentina)

FÁTIMA NOELIA BÉCHER QUINODÓZ; MONICA BLARASIN; HÉCTOR PANARELLO

Rendiconti Online della Società Geologica Italiana

Società Geologica Italiana

Lugar: Roma; Año: 2016 vol. 39 p. 480 - 480

2035-8008

In the South of the Córdoba province (Argentina), the most used hydrological resource is groundwater from the unconfined aquifer, which quite often presents scarce suitability for several uses due to high values of salts, sulfates, Arsenic and fluorides. This situation and the high salinity of confined aquifers, makes it necessary to explore other possibilities, for example, conjunctive use of groundwater and surface water. Thus, the objective of this work is to show the dynamic and geochemical relationships between surface water and groundwater in the medium-low basin of the Ají stream. This information was interpreted to obtain a hydrogeochemical conceptual model to be used as a tool for environmental management. The conceptual model was supported by the use of stable 18O/16O and 2H/1H isotopes and was validated through numerical modeling and the preliminary results of 222Rn activity (in situ measurements). The stream, whose upper basin is situated in the Comechingones Mountains, flows 70 km into the Pampean plains, always with sodium bicarbonate fresh water type (460 - 1,400 µS/cm). Although in some parts of the piedmont the stream recharges the unconfined aquifer, in the plain it is of the gaining type. The aquifer presents a diverse geochemical pattern, with fresh water to salt water (720 to 14,000 µS/cm), from sodium bicarbonate to sodium chloride type. The isotopic results show that the surface water and the groundwater samples are lined up in a mixture line. The geochemical modeling with Phreeqc software revealed that in the mixing models between stream and aquifer, for hydraulically linked samples, the aquifer only provides 2-4 per cent of the total stream flow. This was validated through 222Rn results, which led us to the estimation of about 2 percent of groundwater supply for the season in which the study was performed (end of summer). A preponderance of surface water mass with high velocity, that can dissolve the solutes from the aquifer by means of hydrodynamic dispersion, is interpreted. The fresh surface water, which remains almost unchanged when flowing through the plain, becomes a vital water supply offer. In this sense, 31 wells located beside the stream have already been built to generate induced recharge (surface fresh water towards the aquifer). This water is used for urban activities in the nearest town of about 10,000 inhabitants.