On the sources of salinity in groundwater under plain areas. Insights from δ 18 O, δ 2 H and hydrochemistry in the Azul River basin, Argentina

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1 On the sources of salinity in groundwater under plain areas. Insights from δ 18 O, δ 2 H and hydrochemistry in the Azul River basin, Argentina M.E. Zabala a, M. Manzano b, M. Varni a, P. Weinzettel a a Instituto de Hidrología de Llanuras, Azul, Argentina b Technical University of Cartagena, Spain Abstract. The Azul River basin, with some 6200 km 2, is located in the plains of Buenos Aires Province, Argentina. The Azul River flows along 160 km from the Tandilia Range, in the SW, to the Channel 11, in the NE. Average annual precipitation is 1005 mm ( ); mean reference evapotranspiration is 1090 mm. The geology consists of Miocene to recent sediments, mostly sands and silts with some clay and calcrete layers, overlying crystalline rocks and marine sediments. The water table is shallow and groundwater in the aquifer upper 30 m displays an increasing salinity from SW to NE. Previous hypothesis to explain salinity was infiltration of evapo-concentrated surface water, as the small soil slope in the northern basin (<0.2%) induces rainfall accumulation in lowlands, were water evaporates prior infiltration. But recent chemical and isotopic data reveal two salinity sources: evaporation of recent recharge water, and mixing with old saline groundwater of yet unknown origin. 1. Introduction In large and medium size sedimentary basins groundwater salinity changes in both horizontal and vertical direction due to different reasons. In arid and template zones an inverse zonality can be seen, with less saline waters overlain by more saline ones due to evapotranspiration and to dissolution by rain water of salts formed in the soil surface during the dry season. Increased salinity downwards flow can commonly be attributed to growing residence time and/or to upward transfer of deeper, more saline groundwater flows; lateral transfer of groundwater from adjacent basins can also produce horizontal and vertical salinity changes. In plain areas vertical hydraulic gradients are usually very subtle as to identify flow senses and salinity provenances; hydrochemical and isotope tracers are very useful to assess salinity sources in these cases. 2. Geology and conceptual hydrogeological model The Azul River basin, with some 6200 km 2, is located in the plains of Buenos Aires Province, Argentina (Fig. 1). The Azul River flows along 160 km from the Tandilia Range, in the SW, to the man-made Channel 11, in the NE, which in turns drains to the Salado River, an afluent of the Paraná River. Average annual precipitation is 1005 mm ( ); mean reference evapotranspiration is 1090 mm [1]. The aquifer is formed by Quaternary-age sediments of the Pampeano and Post-Pampeano formations. These are silts, sandy silts, and clayed silt, overlain by fine sands and silts of various origins, mainly aeolian and fluvial [2]. Calcretes are common at shallow depths within thesde sediments. According to the U. S. Soil Taxonomy [3] most of the soils in the basin are Paleudol petrocalcic. Bedrock outcrops occur in the upper basin (SW) and consists of metamorphic rocks, granite, tonalite, migmatite and quartzite [4]. These rocks constitute the lower boundary of the water table aquifer and occur at a depth of 120 m in Azul city. From this city to the northern limit of the basin and beyond, the Quaternary sediments are underlain by the Miocene, wedge shape marine sediments of the Paraná 1

2 formation, whose maximun thickness is around 750 m under the Salado River basin, to the N of the Azul River basin. In some places of the Azul basin, under the Paraná formation appear the Eocene to Miocene continental sediments of the Olivos formation. While the Olivos formation has usually fresh, good quality groundwater, the Paraná formation has very saline water, even saltier than sea water [5]. FIG. 1. Location of the study area, the Azul River basin. The conceptual model to date indicates that in the upper basin (SW) predominate groundwater recharge processes (downward fluxes) and in the NE area groundwater discharge processes, with could be fed by upward fluxes from deeper formations underlain the Quaternary. Also, in this area, due to the shallowness of the water table evaporation-evapotranspiration processes could be produced in the upper layers of the aquifer. General ground-water-flow directions in the upper 30 m of aquifer are from SW to NE, as determined from a two-depths monitoring network tapping the phreatic surface (<6 m depth) and the piezometric level at 30 m. But small vertical gradients are observed, which would induce vertical flows dominantly downwards in the S and upwards in the N (Fig. 2). 3. Materials and methods Chemical analysis of major and some minor components, and isotopic analyses of 18 O and 2 H in 62 groundwater samples taken in February 2007 were studied to deduce the origin of groundwater composition and to contribute to trace the groundwater flow pattern. The samples were taken in a two-depths monitoring network of the Instituto de Hidrología de Llanuras (IHLLA). 43 samples are from the phreatic zone network (identified with plain numbers in all figures), which is between 3 and 6 m depth, and 19 samples are from the 30 m depth network (identified with numbers precedeed by I). The monitoring network consist in coupled emplacements covering the whole Azul River basin; the location of the borehole couples is shown in Fig Results and discussion The samples shown in this work come from a particular survey (February 2007), but they are reasonnably representative of groundwater composition in the aquifer of the Azul River basin at the two studied depths. This representativity is known from the study of temporal and lateral changes of chemical data from more than 15 surveys performed in the two-depths monitoring networks of the IHLLA. Some temporal changes (not shown here) whose origin is under study are observed in the 2

3 phreatic waters. They seem to obey mostly to the recharge history (rainfall and infiltration) prior each sampling survey. The chemistry of the 30 m depth groundwaters is very stable along time for each particular borehole, and only some minor oscilations have been observed. Any significative chemical change has been observed at reginal scale at both sampling depths. FIG. 2. Water table elevation contour lines; location of boreholes from the two-depths monitoring network (3 to 6 m and 30 m depth); piezometric hydrographs at two depths in three representative emplacements. Boreholes 12B and 19B have intermediate depth (around 15 m). Groundwater in the aquifer upper 30 m displays an increasing salinity from SW to NE (Fig. 3). Up to know the hypothesis to explain salinity was infiltration of evapo-concentrated surface water, as the small soil slope in the northern basin (<0.2%) induces rainfall accumulation in lowlands, were water evaporates prior infiltration. The present work was undertaken to check the salinity sources, among other objectives. The contents of major and some minor components, as well as some ionic ratios in groundwater at the two observational depths (phreatic water and 30 m), point to two salinity sources: 1) evaporation of recent recharge water (mostly found in the southern sector of the basin) and mixing with saline groundwater of unknown origin (mostly found in the centre and to the N of the basin). The last process contributes to the salinity not only of the 30 m depth waters but also to the phreatic waters (Fig. 4). The evolution of Na/Cl ionic ratio versus Cl content (Fig. 5) allows to distinguish two groups of waters, arbitrarily named A and B. Group A waters is mostly found in the southern sector of the basin and at both sampling depths. These waters show a variable Na/Cl ratio but always >>1, wile their Cl content does not change. Their salinity is assumed to be the result of local rain infiltration dissolving mostly carbonates and silica, and exchanging Ca-Mg and Na [6]. Group B waters, mostly found to the centre and N of the basin and also at both sampling depths, display a decreasing Na/Cl ratio as Cl increases, approaching Na/Cl ~1. This is assumed to be the result of mixing the type A waters with a saline groundwater different to the locally recharged water and having salinity ratios similar to conventional sea water. They are from boreholes deeping between 15 m (12B and 19B in Fig. 5, left) and 30 m (I12 and I14 in Fig. 5, right). The hydrogeological origin of this saline groundwater has yet to be studied, though the potential candidate is the marine Paraná formation. 3

4 FIG. 3. Horizontal salinity evolution at two depths (phreatic zone and 30 m) in the upper part of the aquifer in February Transect location and borehole numbers are in Fig. 2. FIG. 4. Cl content evolution in groundwater at the phreatic zone and at 30 m depth in February Salinity increase in the lowly mineralised waters seems to be the effect of evapotranspiration during infiltration, and may be also from the water table; salinity increase in the most saline waters point to mixing with saline groundwaters similar to sea water. The mixing affects both the shallow (left) and deeper (right) waters. Samples 12B and 19B (left) are from boreholes of intermediate depth (15 m). The evolution of SO4/Cl ionic ratio versus electrical conductivity (EC) at the two studied depths points to the existence of reduction processes in most of the phreatic and 30 m depth groundwaters (Fig. 6). But many waters not reduced from both depths seems to show the signature of local recharge mixing with a more saline groundwater. The δ 18 O vs Cl graph (Fig. 7) shows clearly the two salinity sources induced from the chemistry study. Only for reference purposes, the theoretical mixing line between a sea-like salinity groundwater and a fresh groundwater representative of local recharge has been drawn. Water from borehole 31 has been chosen as representative of unmodified local recharge, but most probably the real fresh end member is phreatic groundwater somewhat heavier than average rainfall due to concentration by evapotranspiration. The expected δ 18 O signature of local rain in phreatic waters could be something around -5.7 to Also the saline end member could be saltier than sea water (as it is the porewater of the candidate Paraná formation). Thus, groundwater in many locations and depths (like the phreatic 13, 41, 27, 7, or the 15 m depth 12B, and the 30 m depth I9, I12 and I14) could have their salinity mostly from the saline end member, without or with very little effect of evapotranspiration. 4

5 FIG. 5. Na/Cl ionic ratio as a function of Cl content in phreatic and 30 m depth groundwater in February Numbers refers to the identification of boreholes in Fig 2. FIG. 6. SO4/Cl ratio evolution versus electrical conductivity in phreatic and 30 m depth groundwater in February Numbers refers to the identification of boreholes in Fig Conclusions and future work Groundwater salinity in the upper 30 m of the aquifer under the Azul River basin increases form SW to NE, following the flow sense of the river and what was supposed to be the main horizontal component of groundwater flow. Previous hypothesis to explain this salinity was infiltration of evapo-concentrated surface water, as the small soil slope in the northern basin induces rainfall accumulation in lowlands and evaporation prior infiltration. But the study of chemical and isotopic data at two different dephs (phreatic zone and 30 m) points to the existence of two sources of salinity: 1) evapotranspiration prior to and during recharge, and may be also from the water table, and 2) mixing of locally recharged water with a saline groundwater of different origin. Mineral dissolution and other processes like cation exchange also contribute to groundwater salinity, but to a minor extent compared to the other sources. Thus, preliminary results points to the existence of lateral groundwater transfer from deep formations in the northern part of the basin contributing to groundwater salinity, besides evaporation and transpiration in shallower layers. The hydrogeological origin of this saline groundwater is still under way, but seems to be the marine sediments of the Paraná formation. 5

6 The clear picture shown by the water stable isotops and by chemical ratios is promising about the successful use of other isotopic tools to assess solute sources and to trace groundwater flow pattern in the large Azul River basin aquifer. FIG. 7. δ 18 O and Cl in February They support the hypothesis of two salinity sources: evaporation of recent recharge water, and mixing with old saline groundwater with sea-water like salinity. Numbers refers to the identification of boreholes in Fig 2. ACKNOWLEDGEMENTS This research was funded by the Instituto de Hidrología de Llanuras (IHLLA), Argentina. The chemical analysis were performed by IHLLA staff, whose positive attitude is very much appreciated. The autors thank Dr Cristina Dapeña, at the INGEIS (Buenos Aires), for the isotopic analysis. REFERENCES [1] RIVAS, R. and CASELLES, V. A simpliffied equation to estimate spatial reference evaporation from remote sensing-based surface temperature and local meteorological data. Remote Sensing of the Environment 93 (2004) [2] FIDALGO, F., et al., Geología superficial de la llanura Bonaerense (Argentina). VI Argentinean Geological Congress, Proceedings, (1975) [3] Soil Survey Division Staff. Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture, Handbook 18 (1993). [4] GONZÁLEZ BONORINO, R. et al., Estudio geológico de las Sierras de Olavarría y Azul (Bs. As.). LEMIT, Serie II 63 (1956) [5] AUGE, M. Regiones hidrogeológicas. República Argentina. Provincias de Buenos Aires, Mendoza y Santa Fé (2004) [6] ZABALA, M.E. et al., Estudio preliminar del origen del fondo químico natural de las aguas subterráneas en la cuenca del arroyo del Azul. In: Hacia la gestión integral de los recursos hídricos en zonas de llanura, Volume I. M. Varni, I. Entraigas and L. Vives (eds.). (2010)

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