Age and flow pattern of groundwater in a Jurassic limestone aquifer and related Tertiary sands derived from combined isotope, noble gas and chemical data

Abstract

Multi-tracer study of the Malm (Upper Jurassic) limestone aquifer in north-western part of Cracow, Poland, revealed the existence of much older waters than those estimated from Darcy's law. The following environmental tracers were used: 3H, 14C, δ 13C, δ 18O, δ 2H, 4He, Ne, Ar, Kr, Xe, 3He/ 4He and 40Ar/ 36Ar in combination with water chemistry. The natural drainage of unconfined parts of the aquifer is by springs and streams, with a dominant presence of modern and pre-bomb era Holocene waters, whereas the confined part is drained only by upward leakage through thick Miocene clays in river valleys, mainly in the Vistula (Wisła) river valley. As a consequence, the confined part contains much older waters. Their glacial ages are indicated by δ 18O and δ 2H values significantly more negative than those found for modern recharge and by noble gas temperatures reduced by ca. 4.5 °C when compared to the present-day mean annual air temperatures. Quantitative age interpretation of 14C is regarded unreliable due to isotope exchange between dissolved and solid carbonates as suggested by δ 13C values of DIC in the range of −0.6 to −6.1‰ for the confined part of the aquifer. Similarly, quantitative 4He dating turned out to be unreliable, though 4He excess values (0.93–5.45×10 −6 cm 3 STP/g) and very low 14C contents (0.0–5.5 pmc) suggest glacial ages. Changes in hydrochemistry also indicate a long-lasting water–rock interaction probably dominated by diffusion-controlled exchange with overlying and underlying formations. Admixture of older water ascending from underlying formations is observed at two sites. That older water is also supposed to be of Quaternary age as the 40Ar/ 36Ar of the mixture remains equal to the atmospheric ratio. Great tracer ages are shown to result mainly from the delay of solute velocity with respect to the velocity of mobile water, caused by diffusive exchange between mobile water in the fissures (porosity of 0.0001–0.001) and stagnant water in the matrix (porosity of ∼0.06). This stagnant water in the porous matrix is the main water reservoir in the Malm aquifer. In the erosion structures of the Malm limestones, close to the Cracow centre on the southern side of the Vistula river, Tertiary sands are deposited under clay cover. Prior to this study, the origin and age of mineral water exploited from these sands was controversial. However, tracer data indicated meteoric water recharged at the end of the last glacial, and excluded an admixture of connate marine water from adjacent formations. In one well a 10% admixture of modern water was observed with the mean age of about 30 years as determined from the lumped-parameter modelling of the tritium data. The recharge is supposed to take place indirectly through nearby Malm horsts and/or by seepage through Miocene clays in unidentified areas, with dissolution of evaporites as the main source of chemical components. The glacial ages of waters in the confined parts of the Malm aquifer and in Tertiary sands indicate their low vulnerability to anthropogenic pollution.