Groundwater dating with sulfur hexafluoride: Methodology and field comparison with tritium and hydrodynamic methods

In the present study, performed partly under the BASELINE Project carried out within the EU Fifth Framework Programme, a simple methodology of SF6 analyses in groundwater samples has been developed and interpretation procedures tested. The current detection limit of our analytical system is approximately 0.02 fmol/l. Samples are collected in glass bottles (ca. 2.4 l) without contact with the atmosphere and are transported to the laboratory where the dissolved gases are extracted using the headspace method. The extracted SF6 is measured using gas chromatograph equipped with an electron capture detector. Two confined Polish sandy aquifers of Tertiary age were investigated within the project. The hydrochemistry (major, minor and trace components) and environmental tracers (3H, 14C, 13C, 2H, 18O, F-11, F-12, F-113 and SF6) were measured. Tritium and SF6 ages were estimated from time records of the tracers using the lumped-parameter approach. The hydrodynamic modeling was performed with the Visual-MODFLOW, flow times estimated with the MODPATH, and the space distributions of tritium and SF6 modeled with the MT3D. The presented work summarizes the results obtained so far in the Bogucice Sands aquifer, which belongs to the medium-size aquifers in Poland with the area of 176 km+2. The initial hydrodynamic and transport modeling yielded in some wells much lower and in other wells much higher SF6 and tritium concentrations than those measured. This means that hydrodynamic and transport models do not yield unique solutions unless calibrated with the tracer data. For the two most distant wells, where the stable isotope and 14C data indicate a glacial age, the initial modeling with the MODPATH yielded 2 ka. However, glacial ages are easily obtainable in the model by adequate changes in the hydraulic conductivity of the permeable and semi-permeable layers. Tracer data explain the existence of three different hydrochemical zones. Young waters with distinct tritium and SF6 contents are aerobic, and of HCO3-Ca-and HCO3-SO4-Ca types. In that zone slightly increased Na and Cl contents as well as the highest concentrations of SO4 and nitrates are observed due to anthropogenic influences. Older waters are deeper, under the confining cover, in the zone of anaerobic conditions. Dissolved oxygen, nitrate and uranium contents are reduced, and Fe, Mn and NH4 contents increase in that zone. In the third zone, waters are of glacial age and of HCO3-Ca-Na or HCO3-Na types with TDS values higher than 1 g/l and Na content higher than 200 mg/l, due to either admixtures of older waters, or still going on freshening of marine sediments