The hydrogen and oxygen isotopes of water and the carbon isotope
composition of dissolved inorganic carbon (DIC) from different aquifers
at an industrial site, highly contaminated by organic pollutants
representing residues of the former gas production, have been used as
natural tracers to characterize the hydrologic system. On the basis of
their stable isotope compositions as well as the seasonal variations,
different groups of waters (precipitation, surface waters, groundwaters
and mineral waters) as well as seasonably variable processes of mixing
between these waters can clearly be distinguished. In addition,
reservoir effects and infiltration rates can be estimated. In the
northern part of the site an influence of uprising mineral waters within
the Quaternary aquifers, presumably along a fault zone, can be
recognized. Marginal infiltration from the Neckar River in the cast and
surface water infiltration adjacent to a steep hill on the western edge
of the site with an infiltration rate of about one month can also be
resolved through the seasonal variation. Quaternary aquifers closer to
the centre of the site show no seasonal variations, except for one
borehole close to a former mill channel and another borehole adjacent to
a rain water channel. Distinct carbon isotope compositions and
concentrations of DIC for these different groups of waters reflect
variable influence of different components of the natural carbon cycle:
dissolution of marine carbonates in the mineral waters, biogenic,
soil-derived CO2 in ground- and surface waters, as well as additional
influence of atmospheric CO2 for the surface waters. Many Quaternary
aquifer waters have, however, distinctly lower delta(13)C(DIC) values
and higher DIC concentrations compared to those expected for natural
waters. Given the location of contaminated groundwaters at this site but
also in the industrially well-developed valley outside of this site, the
most likely source for the low C-13(DIC) values is a biodegradation of
anthropogenic organic substances, in particular the tar oils at the
site.