PRELIMINARY CHEMICAL AND ISOTOPIC ANALYSIS OF GROUNDWATER SYSTEMS IN THE NORTHERN BOHEMIAN CRETACEOUS BASIN, CZECH REPUBLIC

Three groundwater systems have been identified in the 3,750 m2 northern Bohemian Cretaceous Basin, Czech Republic. Basin sediments are bounded by the Decin fault zone along the Czech-German Border to the north and the Stredohori Fault zone to the south. The lower portion of the basin (300 m) is filled with mostly sandy and claystone layers, whereas the upper portion of the basin (400 m) is filled with finer grained material. Three aquifer systems have been identified. The aquifers, which from top to bottom are Quaternary alluvial, C-D, and A-B, corresponds to stratigraphic units.

Quaternary aquifer waters tend to be of the Ca2+-SO42--HCO3- type and a mean TDS of 818 mg/l (range 59 to 5140 mg/l). C-D aquifer waters tend to be of the Ca2+-HCO3- type with lesser amounts of Na+ and SO42- and have a mean TDS of 371 mg/l (range 36 to 5070 mg/l). A-B aquifer waters tend to be of the Ca2+-SO42--HCO3- type with lesser amount of Na+ and Mg2+ and have mean TDS of 663 mg/l (range 46 to 3180 mg/l. The chemical compositions of waters in each aquifer evolve along their flow paths with accounts for the large range in TDS and solute compositions.

A-B aquifer waters have a mean d13C composition of -8.9 o/oo, whereas C-D and Quaternary aquifer waters have mean d13C compositions -14.7 and -16.8 o/oo, respectively. The more negative compositions are interpreted as the result of soil zone CO2 gas and the composition of A-B aquifer water is interpreted as the result of additional carbonate mineral dissolution in the aquifer. The computer code NETPATH has been used to model flow path chemical evolutions and to evaluate interactions between aquifer systems.

Preliminary stable isotope and tritium data suggest A-B aquifer waters are older and recharged under cooler climatic conditions than C-D and Quaternary aquifer waters. A-B aquifer waters (mean d18O and d2H = -10.02 and -72.23 o/oo, respectively) plot more negatively relative the global meteoric water line (GMWL) than C-D aquifer waters (d18O and d2H = -9.76 and -69.34 o/oo, respectively) and Quaternary aquifer waters (d18O and d2H = -9.44 and -67.19 o/oo, respectively). A-B aquifer waters have a mean tritium content of only 0.52 TU and both C-D and Quaternary aquifer waters have mean tritium contents of about 6 TU. Final analysis of groundwater flow patterns and aquifer interactions wait pending Carbon-14 data.