Spring distribution in Quaternary deposits, South Carpathians, Romania: isotope composition, chemistry and radiocarbon dating

The investigated area is part of a plateau situated at around 350m elevation in the south-western part of the South Carpathians. The area is crossed from north-west to south-east by dry valleys, which cut Pleistocene and Quaternary clastic deposits. Water is flowing across these valleys temporarily, after strong storms or during rainy periods. A groundwater geochemistry study was carried out on water samples collected from different springs and wells. The study was motivated by the fact that springs have represented the only source of potable water in the region, only recently completed by a few wells.

Spring distributions and geological data reveal the presence of a multi-layered system situated in the Lower Quaternary deposits. Precipitations and meteorological parameters were monitored in the region for a period over 10 years. The mean δ¹⁸O and δD values of groundwater reflect the yearly weighted mean of the isotopic composition of precipitation, demonstrating locally derived recharge. The aquifers are situated at different depths in clastic deposits; the shallower aquifers are affected by evaporations during the drought periods of the summer.

The water samples have concentrations of 157 to 852 mg/l for anion and 55 to 308 mg/l for cations, with TDS between 212 and 1157 mg/l. The total dissolved salts limit proposed in the guideline of WHO is 1200 mg/l, above the limit water having a bad taste, all the measured samples are below this limit.

The Piper ternary diagrams for spring water indicate that the dominant hydrochemical types is HCO₃^--Ca⁺²-Mg⁺² with transition toward higher SO₄^-2 and Mg⁺²  contents to the deeper aquifer. The anions vary from HCO₃^- with transition to no dominant- and with Cl^- contents higher for the shallower aquifer. The sequence of abundance of cations is generally Ca²⁺>Mg⁺²>K⁺>Na⁺ and for anions: HCO₃^->SO₄^-2>Cl^->NO₃^->F^-. The Gibbs diagrams indicate rock weathering as a major driving force for driving the groundwater ionic chemistry in the study area. Radiocarbon dating of DIC (dissolved inorganic carbon) indicates a sub recent recharge of the aquifers.

The vertical and lateral variations in groundwater chemistry may vary, and are influenced by lateral lithologic variation of the Quaternary clastic deposits. This is an unpredictable quality factor when taking the decision for the drinking water drill locations.