Hydrogeochemistry in two adjacent areas in the Pannonian Basin (southeast-Hungary)

Groundwaters from the Quaternary, Pliocene and Pontian layers in two adjace nt basin areas of Hungary (Szeged and Koros) and from the intermediate base ment high, all belonging to the central part of the Pannonian Basin, have b een investigated chemically and isotopically. Most of the waters are NaHCO3 type with significant quantitative and qualitative regional differences fo r the Szeged and Koros areas, the intermediate high, and the deepest studie d lithostratigraphic zone (the base of the Pontian stage). Hydrogen and oxy gen isotopes indicate that most of the waters have a paleometeoric origin. The period of infiltration for the entire studied depth interval (75-2500 m ) was within the last cold period in the Szeged and intermediate areas, and during a preceding warmer period in the Koros area. Over the entire studie d region, waters in the deepest lithostratigraphic position are mixtures of paleometeoric waters and deep waters squeezed out from the thick sequence of fine-grained (Pannonian) sediments that underlie the Pontian thermal wat er aquifer. In the Szeged and intermediate areas, the effect of the upward seepage from the deeper (Pannonian) layers was overprinted by the recharge in the last cold period. In the Koros area, due to the lack of infiltration during the last cold period, the mixing with Pannonian waters is seen up t o a depth of 1000 m. Based on Sr-87/Sr-86 ratios, water flow systems were s eparated. In the Szeged and intermediate areas, a shallower and a deeper fl ow systems were distinguished, one in the Pleistocene and Pliocene formatio ns, and another in the Pontian layers. In the Koros area, most of the water s belong to the same flow system that occupies the depth interval from the base of the Pontian (about 2500 m) up to 1000 m. The two shallowest samples studied (above a depth of 1000 m) show lower Sr-87/Sr-86 ratios. (C) 1999 Elsevier Science B.V. All rights reserved.