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and Steinerne Rinne Erasbach) or the calcium-
bicarbonate-(sulphate) type (Westerh ¨fer Bach and
Reinsgraben) characterized by Ca
2þ
concentrations
of 2 - 5 mmol L
21
and high total alkalinities ranging
from 4 - 6 meq L
21
(Table 2; Jacobsen & Usdowski
1975; Usdowski et al. 1979; Dreybrodt et al. 1992;
Arp et
sites where high calcium loss is achieved equili-
bration of the carbonate system in course of precipi-
tation accounts for a minor decrease in saturation
values and pH (Fig. 3).
Similar to the calcium profile, strontium and
barium concentrations show a downstream decrease
suggesting that minor amounts are co-precipitated
with the calcite. While the concentrations of ions
such as Na
þ
,K
þ
,Mg
2þ
and Cl
2
keep very constant
throughout the course of all streams, in case of the
Westerh ¨fer Bach, the detected decrease of SO
22
concentrations also indicates incorporation of
sulphate during carbonate precipitation. From
elemental analysis of the tufa carbonate, sulphur
incorporated in calcite here amounts to 0.2 - 0.3
percent by weight. Additionally, PO
32
decreases
during the course of the stream (WB), which is
either consumed by primary production or also
bound to the precipitated carbonate phases. Where
analysed, NO
3
2
and NO
2
2
concentrations appear to
be constant (WB, DB, RG), indicating that nitrogen
is not limiting with regard to primary production.
al.
2001b;
Baier 2002; Shiraishi
et
al.
2008a, b).
Both streams originating in the evaporite-
containing Muschelkalk-Group aquifer (WB and
RG) are comparatively enriched in Ca
2þ
(3.0 -
5.2 mmol L
21
), Mg
2þ
(1.1 - 1.8 mmol L
21
), and
SO
22
(1.8 - 3.9 mmol L
21
). Also, these streams
have high concentrations of strontium and barium.
The Deinschwanger Bach and Erasbacher rivulet
(SR) discharge from Mg-poor limestone aquifers of
the Weißjura-Group resulting in lower Ca
2þ
con-
centrations (1.8 - 3.5 mmol L
21
) and Mg
2þ
con-
centrations (0.1 - 1.2 mmol L
21
), and an order
of magnitude lower SO
22
concentrations. On the
other hand, enhanced nitrate, nitrite, and phosphate
concentrations in case of the Deinschwanger Bach
are due to agriculture in its catchment area (Table 2).
Annual and diurnal cycles
Hydrochemical trends during the course
of the streams
Waters emanating from the spring sites are rather
constant in pH and temperature throughout the
year. Depending on air temperature, stream water
temperatures increase downstream in spring and
summer time and decrease in winter time. Yet,
main hydrochemical parameters generally show
only minor seasonal variation (Jacobson &
Usdowski 1975; Shiraishi et al. 2008b). Non-cyclic
changes in water chemistry in case of the Westerh ¨-
fer Bach reflect a dependence on rainfall and runoff
intensity rather than seasonal fluctuations (Fig. 4).
Here, intense rainfall in the catchment area leads
to enhanced underground dissolution of evaporites
within the Muschelkalk-strata resulting in increased
sulphate and calcium concentrations which are, in
turn, balanced by slightly lower total alkalinity
(Fig. 4). Nevertheless, the different hydrochemical
conditions hardly affect the commonly observable
pattern of calcite supersaturation and calcium loss
along the flow path (Jacobson & Usdowski 1975;
Dreybrodt et al. 1992; Shiraishi et al. 2008b;
Fig. 3). Also, the highly turbulent streams do not
reveal a clear diurnal change in bulk water hydro-
chemistry with exception of a diurnal temperature
cycle (Shiraishi et al. 2008b).
The common hydrochemical evolution, especially
that of the carbonate system, along the flow path
of the investigated streams is displayed in
Figure 3. In general, the streams start at initially
near-neutral pH (7.0 - 7.3), high partial CO
2
pressure (PCO
2
¼ 10000 - 15000 matm), and con-
sequently low saturation with respect to calcite,
the solely precipitating carbonate mineral phase.
Calcite saturation of the spring waters, defined
as the saturation index SI
Calcite
¼ log V
Calcite
¼
log(ion activity product fCa
2þ
gfCO
22
g/
solubility product K
calcite
), amounts to
SI
Calcite
¼ 0.0 - 0.25, indicating that spring waters
are already saturated to slightly supersaturated.
Because of CO
2
-degassing, the initially high PCO
2
of the spring waters rapidly decreases to a level of
about 800 - 1100 matm along a flow path of some
tenths to some hundreds of meters. This PCO
2
decrease is coupled with an increase of pH from
c. 7.3 at the spring sites to maximum values of
8.3 - 8.4. According to the rise in pH, calcite super-
saturation increases along the flow path attaining
maximum values of SI
Calcite
¼ 0.9 - 1.2 (i.e. 8- to
15-fold supersaturation V
Calcite
), and CaCO
3
pre-
cipitation becomes evident by the remarkable loss
in Ca
2þ
and total alkalinity. First occurrences of
tufa carbonates coincide with the beginning of this
decrease in calcium and alkalinity. Both, calcium
and total alkalinity further decline downstream
in fast flowing reaches or at major tufa cascades
where PCO
2
drop is enhanced. At downstream
Conclusions
(1)
Spring waters of all investigated streams
have a much higher CO
2
pressure than the
atmosphere and rapidly start degassing
of CO
2
when leaving the spring site. Equili-
bration of the carbonate system results in
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