Hydrochemical parameters
The precipitation of travertine is controlled by many factors, which can
be grouped into 4 primary fields: (1) hydrochemistry, (2) hydrodynamics,
(3) microbiology and (4) depositional environment. Hydrochemical
conditions are the most important factors affecting the deposition of
travertine (Zhou et al., 2017). The hydrochemical factors of the hot
springs mainly includes the concentrations of the major ions and
CO2, the partial pressure of CO2 (Table
1), \(\mathrm{\gamma}\)Ca/\(\mathrm{\gamma}\)HCO3and\(\ \mathrm{\gamma}\)Mg/\(\mathrm{\gamma}\)Ca (Table 2), the
saturation index with respect to anhydrite, aragonite, calcite,
dolomite, gypsum and halite (Table 3) and the concentration of
CaCO30 (Table 4).
Hydrochemical
constituents
Pentecost (1999) proposed that the concentrations of Ca and
HCO3 are the most important factors that affect the
travertine deposition, since the travertine mainly consist of calcite
(CaCO3), and almost all of the travertine deposits
contain more than 90% of calcium carbonate. Na concentrations of the
Heinitang hot springs range from 165 to 185 mg/L, and for the
concentrations of Ca are in the range between 85 and 115 mg/L (Table 1).
The concentrations of K and Mg are relatively low, ranging from 23 to 24
mg/L and from 8 to 26 mg/L, respectively. Compared with water samples
S2* and S4*, the concentrations of Na and K in the water samples S2 and
S4 are almost constant, Ca concentrations increase and Mg contents in
the water samples S2 and S4 decrease (Fig. 5a). HCO3 is
dominant in the anions of the water samples, and its concentrations are
high and range from 780 to 800 mg/L (Fig. 5b). The concentrations of
SO4 and Cl are relatively low (Fig. 5b). The
concentrations of SO4 are lower than 1 mg/L, and the
concentrations of Cl range from 50 to 70 mg/L in the water samples. The
concentrations of HCO3, SO4 and Cl are
almost unchanged, indicating that there is no significant evaporation
from the hot water or other sources of water supply. In addition, the
concentrations of F in the water samples are relatively high (3.8-5
mg/L) (Fig. 5c), which are much higher than the World Health
Organization (WHO) maximum guideline value (1.5 mg/L). The F
concentration in natural waters depends on such factors as temperature,
pH, presence or absence of complexing or precipitating ions and
colloids, solubility of fluorine-bearing minerals, anion exchange
capacity of aquifer materials (OH for F), the size and type of
geological formations traversed by water, and the amount of time that
water is in contact with a particular formation (Apambire et al., 1997).
Minerals which have the greatest effect on the hydrogeochemistry of
fluoride are fluorite, apatite, micas, amphiboles, certain clays and
villiaumite. Fluorite is the main mineral controlling aqueous fluoride
geochemistry in most environments. However, there are some notable
exceptions in sedimentary basin environments (Chae et al., 2006).
Fig. 5. (a) Cations, (b) anions, (c) F concentrations, (d) TDS,
(e) CO2 concentration and PCO2 of the
Heinitang hot water samples, (f) the relationship between pH and
PCO2.
As a measure of water salinity, the TDS may be affected by different
lithologies and geochemical conditions (Pasvanoğlu, 2013). Relatively
high TDS values of the hot springs in western Yunnan are common. The TDS
values are above 750 mg/L in the Heinitang hot springs (Fig. 5d),
indicating that the hot spring water undergoes a deep circulation and
longer residence time.
In general, the precipitation of travertine on the earth’s surface is
described by the overall reaction (Dandurand et al., 1982):
Ca2++2HCO3- →
CaCO3(s) + H2O+CO2 (g)
(2)
In this reaction, CaCO3 does not deposit immediately
when CO2 outgases or is consumed. Therefore, the
solution loses CO2 according to the following reaction:
H++HCO3- →
H2CO3→
H2O+CO2 (g) (3)
Consequently, the CaCO3 becomes supersaturated gradually
and precipitates:
Ca2++CO32- →
CaCO3 (s) (4)
This shows that the rate of CO2-outgassing can affect
the deposition of travertine. Outgassing of CO2 occurs
during surface flow as the travertine-depositing hot springs reach
equilibrium with the atmosphere, which contributes to the formation of
travertine (Lorah and Herman, 1988; Liu et al., 1995). Jones et al.
(2005) suggested that the precipitation of travertine was driven by
rapid CO2 degassing of CO2-rich spring
waters along the flow path of hot springs. The concentrations
and
the partial pressure of CO2 are high and show high
variability in the Heinitang hot springs. The values of
CO2 concentrations and partial pressure range from 20 to
80 mg/L and from 0.04 to 0.5 atm, respectively (Fig. 5e). The calculated
PCO2 values are significantly higher than that of the
atmosphere (0.04% atm). The release of CO2 is not only
controlled by PCO2, but also related to the flowing
path, the flowing rate and the location of spring. As mentioned above,
the deposition of travertine from hot spring vents S2, S3 and S4 stopped
during a period of time, this can be caused by the decrease in discharge
of the spring’s vents. After the artificial construction of the pools,
the spring waters flow along the wall of the pool, increasing the area
in contact with the air and accelerating the releasing rate of
CO2, which results in the deposition of travertine. This
indicates that the high values of CO2,
PCO2 and the conditions of
CO2-outgassing are necessary factors for the
precipitation of travertine from the hot springs.
pH of water is a common chemical parameter affecting the deposition of
travertine, along with several other factors, such as the hydrochemical
compositions of the water, temperature and PCO2 (Zhou et
al., 2014). Shen et al. (1993) reported that pH values and
PCO2 are not linearly related, but have a logarithmic
relationship under the standard condition and equilibrium with respect
to calcite in the solution. The observed values of pH of the water
samples range from 6 to 8 and plot on the scatter diagram (correlation
index R2 = 0.9766) (Fig. 5f). pH values can reflect
the values of PCO2 to some degree.
Table 2 The ratios of Ca and HCO3, Mg and Ca,
and the hydrochemical type of the Heinitang hot spring “*” represents
the water samples which were collected in 2013.