CONICET | Buscador de Institutos y Recursos Humanos

The shallow geology and local hydrogeological systems of theArgentine Gran Chaco are poorly known. This area is part of the Chaco-Pampean Plain in the Andean foreland and is predominantly known bythe subsurface data. Otumpa hills constitute a gentle topography locatedin the study area (26°- 28° S; 63°- 61° W) with N-S orientation. Thismorphology represents a transition zone between the western and easternlimits of the Guaraní and Yrendá-Toba-Tarijeño aquifer systems, respectively.Here, we report the results of two west-east profiles across Otumpahills (northern profile at 26º40? S; southern profile at 27º S) consistingin vertical electrical soundings and audiomagnetotelluric soundings. Weutilized 1-D and 2-D inversion techniques to obtain resistivity modelsfrom the measured data. The models defined several geoelectric layers,which were very consistent in both methods and in both profiles. Weidentified the upper geoelectric layer UL (> 20 Ωm), the semi-conductivegeoelectric layers SC, SC2 and SC3 (6-20 Ωm), the conductive geoelectriclayers C and C2 (4-8 Ωm) and the highly conductive geoelectric layersHC and HC2 (< 5 Ωm). SC and SC2 showed the best hydrogeologicalpossibilities of presenting freshwater. These layers lie on very porousaeolian sandstone and very silty loess. This is because they are hosted inthe Pampa formation northward, in the Chaco formation southwestward,and in the Chaco or Tacuarembó formations eastward, which are laterallyuplifted. The shallowest SC is 10 meters thick while SC2 is 100 metersthick. Northward, SC2 provides continuity to an analogous geoelectriclayer identified eastward. Several boreholes corroborate the presence offreshwater at both levels. HC and C in the audiomagnetotelluric profilesand C2 in the vertical electrical soundings profile suggest the presence ofhigh-saline water (< 8 Ωm). This 100-meter-thick hydrogeological levelis the most exploited in the area and is hosted by the Pampa and Chacoformations. However, the saturated zone for all layers is unknown.HC2, the deepest hydrogeological level (between 250 and 350 meters), ishosted by the Chaco and Tacuarembó formations and could be relatedto the thermal high-saline water found southeastward at a depth of 350meters depth. We correlated the shallowest levels (SC, SC2, HC, C andC2) with the Toba aquifer system that may pass eastward the Guaraníaquifer system because the Tacuarembó Formation is the host there andthe deepest level HC2 could also be related to the Guaraní aquifer system.The results provide a new shallow geological-geophysical model of theOtumpa hills area that reveals aquifers more associated with lithologyand topography than with stratigraphy. A slight ductile deformation isevidenced by the geometric shapes of HC2 and SC2 and by the gentle tiltsthat accompany the slopes (SC2, HC and SC). Finally, the relevance of thiswork resides in the local aquifers identification and providing informationto improve its usually poor management in this region. Moreover, furtherstudies are likely to provide new insights into the hydrological interactionbetween the Guaraní and Yrendá-Toba-Tarijeño aquifer systems of theSouth American region.Palabras clave: Guaraní aquifer system, Yrendá-Toba-Tarijeño aquifersystem, Otumpa hills, audiomagnetotelluric, vertical electrical sounding.