Phreeqc interactive calculation
The saturation index with respect to calcite (SIc) can indicate that
whether the precipitation of travertine may occur or not (Dilsiz, 2006).
Minvielle et al. (2015) proposed that the saturation index with respect
to calcite (SIc) is a significant parameter that can be used to study
hydrochemistry of karst systems through calcium-carbonate equilibriums.
The values of saturation index with respect to the anhydrite, aragonite,
calcite, dolomite, gypsum and halite in the Heinitang hot springs are
determined by Phreeqc Interactive 3.1.4 (Parkhurst and Appelo, 1999),
and they are presented in Table 4.
Table 3 SI values with respect to minerals in the hot water
samples.
The values of saturation index with respect to aragonite range from
-0.41 to 0.99. The values of saturation index with respect to calcite
range from -0.29 to 1.11. The values of saturation index with respect to
dolomite range from -1.14 to 2.22. The SI values with respect to
anhydrite and gypsum are similar and in the range from -4.1 to -3.6. The
SI values with respect to halite are the lowest and in the range of
-7.48 to -6.59 (Table 3). The SI values with respect to aragonite,
calcite and dolomite have the similar tendency, which are positive in
water samples S2*, S2, S3, S4* and S4. The solution is in a state of
saturation. However, the SI values with respect to aragonite, calcite
and dolomite are negative for the S1, S5 and S6 water samples that
almost not deposit travertine (Fig. 8). The results show that the SI
values with respect to aragonite, calcite and dolomite can be affected
by similar factors, and they are significant for the precipitation of
travertine.
Fig. 8. SI values with respect to the anhydrite, aragonite,
calcite, dolomite, gypsum and halite in the Heinitang hot spring
samples.
As shown in Figure 8, compared with water samples S2* and S4*, the SI
values with respect to aragonite, calcite and dolomite decrease in water
samples S2 and S4. The SI values with respect to aragonite and dolomite
decrease from positive to negative. The SI values with respect to
calcite decrease from 0.8 to 0.06 and from 1.11 to 0.1, respectively.
This indicates that the saturation index with respect to aragonite,
calcite and dolomite can decrease with the precipitation of travertine
from the hot spring. This may be the reason why travertine stops growing
near these vents.
The main compositions of travertine are aragonite and calcite, and their
molecular formulas are both CaCO3 (Pentecost, 2005).
Based on the analyses above, it is clear that the concentration of
calcium has a significant effect on the deposition of travertine.
However, the concentration of calcium only represents the total calcium
content, the concentration of free calcium and ion pairs of calcium are
still unknown. Therefore, we want to know whether the concentrations of
ion pairs of calcium have a significant effect on the precipitation of
travertine. The main compositions of calcium include Ca,
CaHCO3+,
CaCO30,
CaSO40, CaOH+ and
CaHSO4+ in the Heinitang hot spring.
Shen et al. (1993) suggested that the total concentration of calcium can
be calculated by:
mCa (O) = mCa +\(\mathrm{m}_{\mathrm{\text{CaHCO}}_{\mathrm{3}}^{\mathrm{+}}}\)+\(\mathrm{m}_{\mathrm{\text{CaCO}}_{\mathrm{3}}^{\mathrm{0}}}\)+\(\mathrm{m}_{\mathrm{\text{CaSO}}_{\mathrm{4}}^{\mathrm{0}}}\)+\(\mathrm{m}_{\mathrm{\text{CaOH}}^{\mathrm{+}}}\)+\(\mathrm{m}_{\mathrm{\text{CaHSO}}_{\mathrm{4}}^{\mathrm{+}}}\) (5)
where m represents molality, O refers to the total concentration.
The concentration of free calcium and ion pairs of calcium are
calculated by Phreeqc Interactive 3.1.4 (Parkhurst and Appelo, 1999),
and are listed in Table 4.
Table 4 Concentrations of the free calcium and ion pairs of
calcium in the Heinitang hot spring.
As shown in Table 5, the concentration of free calcium (Ca) is the
highest in the water samples, with the concentration from 1.89 to 2.52
mmol /L, and the concentrations of ion pair
CaHCO3+ and
CaCO30 range from 0.22 to 0.32 mmol /L
and from 0.004 to 0.097 mmol /L, respectively. Ion pair concentrations
of CaSO40, CaOH+ and
CaHSO4+ are extremely low. The
concentration of CaSO40 ranges from
4.69x10-4 to 1.13x10-3 mmol/L, the
concentration of CaOH+, from
2.2x10-7 to 6.56x10-6 mmol/L, and
the concentration of CaHSO4+, from
3x10-10 to 1.63x10-8 mmol/L.
Therefore, the CaSO40,
CaOH+ and CaHSO4+can be ignored, and only the free calcium (Ca),
CaHCO3+ and
CaCO30 are considered.
The concentrations of Ca and CaHCO3+in the water samples collected in 2013 were relatively low. The
concentrations of free calcium and
CaHCO3+ in the water samples collected
in 2018 were relatively high, which were similar in all the water
samples (S1, S2, S3, S4, S5 and S6) (Fig. 9a). However, the tendency of
the CaCO30 content in the water
samples is completely different from those of Ca and
CaHCO3+. The concentrations of
CaCO30 in the water samples collected
in 2013 were significantly higher than those in the water samples
collected in 2018. In addition, the concentrations of
CaCO30 in water samples S2, S3 and S4
were relatively high, the concentrations of
CaCO30 were relatively low for water
samples S1, S5 and S6 (Fig. 9b). As mentioned above, the size of
travertine in 2018 was significantly larger than that in 2013 near the
hot spring vents S2 and S4. Therefore, it is known that the
concentration of CaCO30 decreases and
the concentrations of Ca and CaHCO3+almost remain constant with the precipitation of travertine. This
indicates that the deposition of travertine can be greatly affected by
the concentration of CaCO30, while the
concentrations of Ca and CaHCO3+ have
little influence on the precipitation of travertine. The high
concentration of CaCO30 in the hot
spring may be more conducive to the deposition of travertine.
Fig. 9. Concentrations of free Ca,
CaHCO3+ (a) and
CaCO30 (b) of the Heinitang hot
spring.
Figures 8 and 9 show that the higher the concentration of
CaCO30, the higher the saturation
index with respect to calcite (SIc), aragonite (SIa) and dolomite (SId)
wound be. However, they are not linearly related, but have an
exponential relationship (Fig.10). The curve of
CaCO30-calcite, with a steep slope, is
almost the same as CaCO30-aragonite
except the intercept. The curve of
CaCO30-dolomite is relatively flat
with a higher intercept. The intercept represents the corresponding
concentration of CaCO30 when the
saturation index with respect to calcite (SIC),
aragonite (SIA) and dolomite (SID) are 0
(the solution is in equilibrium with calcite, aragonite and dolomite).
When the calcite, aragonite and dolomite in the solutions are in
equilibrium, the corresponding concentrations of
CaCO30 are 0.008 mmol/L,0.01 mmol/L
and 0.012 mmol/L (Fig. 10), respectively. This indicates that with the
increasing CaCO30 concentration in the
Heinitang hot spring, calcite first reaches the dissolution equilibrium
compared with dolomite and aragonite.
In addition, the concentrations of
CaCO30 in water samples S2* and S4*
collected in 2013 are higher than those of water samples S2 and S4
collected in 2018. The saturation index with respect to dolomite,
calcite and aragonite are positive and the value of saturation index
with respect to dolomite is significantly higher than those of calcite
and aragonite, which may deposit firstly. However, with the
precipitation of dolomite, the concentrations of
CaCO30 and the saturation index with
respect to dolomite, calcite and aragonite wound decrease. Therefore,
the values of saturation index with respect to dolomite and aragonite
are negative in water samples S2 and S4. The value of saturation index
with respect to calcite also decreases but is positive. The
precipitation of active travertine consisting of calcite occurs in 2018.
This indicates that the precipitation of travertine is affected by the
deposition of CaCO30. The relationship
between the concentration of CaCO30and the saturation index with respect to dolomite, calcite and aragonite
can be used to simply identify the composition of travertine. When the
concentration of CaCO30 is higher than
0.018 mmol/L, the travertine may consist of dolomite, calcite and
aragonite, and the concentration of dolomite is higher than calcite and
aragonite. When the CaCO30concentration is between 0.008 and 0.018 mmol/L, calcite is the main
component of travertine. When the concentration of
CaCO30 is lower than 0.008 mmol/L, the
precipitation of travertine will hardly occur near the hot spring’s
vents.
Fig. 10. Relationship between SIc, SIa, SId values and
CaCO30 in the hot water samples.