Euradwaste '08 - EU Bookshop - Europa

topes of carbon in DIC, DOC and SOC were evaluated. This work is described in detail in [2]. Here
only few major aspects concerning the formation of DOC in the clay/lignite layer are touched.
In Figure 5 on the left the correlation between biogenic DIC and DOC is plotted, assuming that the
13 C content of the source water is of inorganic origin with -27 ‰ and additional DIC is of organic
origin. Besides data for NA8, which well is probably not strongly connected to the clay/lignite water,
the other data follow a line, with increase of biogenic DIC from infiltration waters NA10 and
RP1 to the waters from the clay/lignite layer. It is one important indication that in this layer DOC is
formed by additional release of DIC, as it is observed at other sites, e.g. 11].
Concerning the mechanism of DOC release important information can be gained from other isotope
signatures. A strong indicator for microbial degradation of organic matter by sulphate reduction is
the � 34 S signal in dissolved sulphates. Since sulphate reducing bacteria had already been detected at
the site, appropriate analyses were performed in groundwater from the infiltration area and from the
clay/lignite layers.
The results show that � 34 S-values in waters from the clay-lignite layer are in a range between
16.4‰ and 24.6 ‰, i.e. strongly increased compared to the values between -8.5 and 3.48 observed
in the wells from the infiltration area (see Figure 5, right). The substantial enrichment of 34 S in
these boreholes is a clear indication that microbial sulphate reduction occurs in the clay/lignite layers.
The microbial sulphate reduction is accompanied by isotope fractionation. The lighter isotope
32 S is preferentially metabolised by the microbes leaving residual sulphate in the solution enriched
in 34 S, which is observed here. With the initial sulphate concentration in the infiltration area and the
concentration after sulphate reduction in the clay lignite an enrichment factor can be calculated by
use of the Rayleigh equation. Based on the data from all wells shown in Figure 5 and the corresponding
sulphate concentrations an enrichment factor of 11‰ was calculated [2]. Compared to
other investigations this value is comparably low, but not unusual for bacterial sulphate reduction.
Enrichment factors in a similar range between of ~10 ‰ have been observed e.g. in [12].
DOC [mg/l]
5
4
3
2
1
NA8
NA10
RP1
NA6
NA4
RP2
NA13
NA7
NA12
0
5 15 25 35 45
DIC biogenic [mg/l]
Figure 5: Correlation of biogenic DIC with DOC (left) and � 34 S values in boreholes from the infiltration
area (red circle) and clay/lignite layers(yellow circle) [2]
In order to better understand the interrelation between SOC and DOC, in particular to understand
the relatively low DOC concentrations below 5 mg C/l in the water from clay/lignite layer com-
349
0.2
-8.5
3.48
20.11
23.5
20.4
16.43
24.63