The central region of Mexico is characterized by a volcanic range where 14 active volcanoes can be found, together with lacustrine depressions where two of the main lakes in the country are located: Chapala and Cuitzeo. These lakes are part of the Lerma river Basin, crossing Central Mexico from east to west, starting in the center of the country and reaching the Pacific Ocean in the west. Around the Cuitzeo depression one of the main mountain heights is the Los Azufres range, where the second in electric production geothermal field in Mexico is found. The drainage basin of Lerma River has been recognized as one of the most polluted in Mexico due to the proliferation of industrial development and the use of fertilizers and pesticides in the agricultural local practices. Recent studies of chemical, radioisotopic and bacteriological concentration levels in different wells and springs belonging to the Upper Lerma river basin have shown that the recharge zone is complex[2,3].
Due to the neighboring of Los Azufres geothermal field, geochemical surveys were performed around Cuitzeo Lake since the early eighties[4,5]. These surveys showed four zones interesting for geothermal uses.
In Mexico low temperature geothermal resources are slightly used for direct applications in spite of its availability in rural zones, in this study an analysis of the possibilities of low-medium geothermal resources around Cuitzeo Lake is presented together with a geochemical and radon data interpretation.
MATERIALS AND METHODS
Cuitzeo Lake is located in the northern part of Michoacan state. Recent volcanism
has occurred at this state, where the youngest volcano, Paricutin, was born
in 1943 (Fig. 1). The main regional geological formations
are from Tertiary and Quaternary periods. Michoacan hydrology is composed by
upper Lerma River, the central lake zone and the Balsas River. The Cuitzeo basin
having 3977 km2 is one of the largest lakes of the zone. The main
landform of the sampling zone is formed by the Cuitzeo depression. The southern
region of the lake has been reported to have neutral-alkaline groundwater type;
the recharge zones are located at the border of the hills at the east of Zinapecuaro
and west of Morelia. The qualitative direction of underground flows, deduced
from chemical water composition are from south to north in the southern part
of the Cuitzeo Lake while at Querendaro, in the southeastern part of the lake,
the flow is southeast to northwest following the local faulting[6,7].
||Physico-chemical composition of the water samples from the
sites around Cuitzeo Lake. Sampling temperatures (T) are given in °C.
Concentration values for Na, K, Ca, Mg, HCO3, SO4,
Cl, B and SiO2 are given in mg kg-1
||Location of Cuitzeo Lake including several studied sites
The weather is moderate with summer rains (May-October) giving an average annual
precipitation of 906 mm and the environmental temperature ranges from 10 to
The western part of the Cuitzeo Lake is located about 30 km from the Los Azufres geothermal field. The geochemical and radon data were obtained from sites around the Cuitzeo Lake and Los Azufres (100°39-101°20 W and 19°46-19°59 N) at an average altitude of 1850 m. Data from Jeruco (J), Cuitzeo (C), Copandaro (CO), Panteon (PA), El Salitre (ES), Los Baños (LB), San Agustin del Pulque (SAP), San Juan Tararameo (SJT), San Agustin del Maiz (SAM), Araro (AR), Mariano Escobedo (ME) and Santa Rita (SR) (Fig. 1 and Table 1).
Data from 1983, 1990, 2001 and 2003[2,4,5,8] were analyzed to investigate the main geochemical features, the radon behavior and the estimation of reservoir temperatures.
The chemical data were classified and plotted on a Schoeller diagram to see through the shapes of the curves if the samples are or are not related to each other.
The Giggenbach, Cl¯-HCO3¯-SO42¯
and the Na-K-Mg triangular plots were also obtained for the studied sites in
order to classify the waters according to the dominant ions and to estimate
reservoir temperatures. The Cationic Composition Geo-thermometer (CCG) was included
in the Na-K-Mg plot[11,12]. The silica mixing model was used to investigate
the fraction of hot water in the samples.
Isotopic data from some samples were compared to the global meteoric water line and to the composition of the Los Azufres geothermal wells considering data for the natural state reservoir fluids.
Groundwater radon data were obtained during field campaigns performed each three months in 2001 and 2003 at ten monitoring stations located at the sites indicated in Fig. 1.
RESULTS AND DISCUSSION
Chemical composition of samples is given in Table 1. Neutral to alkaline pH values were measured. Relative Cl-SO4-HCO3 compositions for the samples indicate that only Araro samples (AR-1, AR-2) are located in the area related to mature waters. Steam heated waters were found at San Agustin del Pulque (SAP), San Juan Tararameo 2 (SJT-2) and San Agustin del Maiz (SAM). All the other samples are located on the bicarbonate region and they are known as peripheral waters due to the absorption of deep CO2 and to the mixing with shallower waters (Fig. 2).
Figure 3 shows the relative Na-K-Mg content for the samples. Ground waters and springs are usually found close to the Mg corner while geothermal weir box samples are found on the full equilibrium line. As shown in the Fig. 3, only samples SAM, SAP, SJT-3 are in full equilibrium considering the CCG full equilibrium line, while AR-1, AR-2 and SJT-2 are in partial equilibrium. All the other samples correspond to immature waters. A mixing trend is observed among the samples indicating reservoir temperatures between 150 and 220°C, considering Giggenbach equilibrium line which corresponds to 120 to 190°C with respect to CCG equilibrium line (Fig. 3).
According to Schoeller diagram (Fig. 4) the studied waters
show different salinity and two main patterns regarding the Mg content. Low
Mg waters correspond to SAM, SJT-2, SJT-3, SAP, AR-1, AR-2 and SR indicating
their geothermal character. The high Mg content is related to mixing with cooler
and shallower waters. A wide range of SO4, Cl and B concentration
values were observed among the samples. In almost all the samples the low Mg
content corresponds to high chloride and boron contents.
Results for different geo-thermometers: K/Na, K/Mg; Na/K, Na-K-Ca with Mg correction, Cationic Composition Geo thermometer (CCG) and SiO2[11,13] are given in Table 2.
||Relation CI-SO4 HCO3 composition of
||Relative Na-K-Mg content for the samples. The Giggenbach
and CCG, equilibrium lines are also indicated
Temperature (T°C) calculations in
Table 2 were performed as follows (c means concentration in
mg kg-1): T (K/Na) = [1390/(1.75-log(cK/cNa))]-
273; T (K/Mg) = [4410/(14.0-log(cK2/cMg))]-
273; T (Na/K) ) =[1217/(1.483-log(cK/cNa))]-
273; T(Na/K/Ca) = [1647/(log (cNa/cK) +
β (log (cCa0.5/cNa) +2.06) +2.47)]-273;
β = 4/3 for T < 100°C, β = 1/3 for T > 100°C, c in this
case is given in moles kg-1; T(Na/K/Ca) )= [1178/(1.47
+ log(cNa/cK))]-273, this expression was used for samples
SJT-2, SJT-3, SAM, AR-1 and AR-2 containing a %Mg < 3.5; T CCG =
[10080/(5 log(cNa/cK) +2 log(cCa/cNa2)-
log(cMg/cNa2)+16.65)]-273, this expression
was suitable for samples LB and SR. T CCG = [16000/(3 log(cNa/cK)
+3 log(cCa/cNa2)- log(cMg/cNa2)+44.67)]-273,
this expression was used with samples: J, C-2, C-3, CO, PA, ES, SJT-1 and ME.
T CCG = [11140/(6 log(cNa/cK) + log(cMg/cNa2)+18.3)]-273,
this expression was used with sample SAP. T SiO2 =
-42.198 + 28.831 S- 3.6686E-4 S2 + 3.1665E-7 S3 +
77.034 log S where S is the SiO2 concentration of the sample; T SiO2
|| Schoeller diagram for the samples
||Reservoir temperatures (T°C) estimated by different geo-thermometers:
T (K/Na), T (K/Mg), T (Na/K),
T (Na/K/Ca)[13,, T CCG, T SiO2,
Cationic geo-thermometers provide a wide range of temperature values, which
is due to the different rates of re-equilibration of chemical constituents in
the water-rock reactions occurring at depth. Cationic geo thermometers that
include Mg usually give low reservoir temperatures because Mg has a fast re-equilibration
rate, this is the case of the following geo thermometers: K/Mg, Na-K-Ca with
Mg correction and CCG for samples where the Mg content is higher than 3.5% (samples
J, C-2, C-3, CO, PA, ES and ME). In addition, cationic geo thermometers are
more suitable when samples are classified as mature waters, as in samples AR-1
and AR-2, in which the estimated temperatures (Table 2) seem
to be more consistent, in spite of using geo thermometers based on Mg. Cooling
processes when ascending the waters to the surface should be considered in order
to assess which geo thermometers are more reliable to estimate reservoir temperatures
from spring data. Such cooling processes sometimes imply the mixing of deep
hot waters with shallower and cooler (Mg rich) waters. Considering Na/K geo-thermometers
qualitative results were as follows: Na/K provided the higher
temperatures compared to Na/K and these gave higher temperatures
than those obtained using CCG.
|| Specific enthalpy (kJ kg-1) vs silica (mg kg-1)
In contrast, silica geo thermometers are based on silica solubility that depends
only on temperature. Two expressions were used in Table 2
to estimate reservoir temperatures: T SiO2 that considers
equilibrium with quartz and T SiO2 that considers
equilibrium with chalcedony and is more suitable for spring data. Slightly higher
temperatures are estimated with the quartz geo thermometer compared to the results
obtained with chalcedony. As shown in Table 2 the silica temperatures
indicate moderate reservoir temperatures between 90 and 200°C for all the
Figure 5 is a silica versus specific enthalpy plot, where the samples have been represented as well as the amorphous silica and quartz solubility curves. A linear tendency for the samples is observed which shows the occurrence of a mixing process. The fitted straight line for the samples was calculated by the minimum square method. This line is explained by assuming that the reservoir liquid at 220°C is cooled to 131°C by boiling and subsequently is mixed with non-thermal waters to give the sample compositions. This boiling process is shown by moving the reservoir liquid point to the hot component point. The point for separated vapor is named Steam in the figure. From Fig. 6 the silica concentration for the reservoir liquid is estimated as 336.4 mg kg-1 and the specific enthalpy was 943.6 J g-1. The fraction of hot component in the samples was estimated to be about 50% in AR-2, SJT-3, SAP and SAM and around 30% for AR-1, while the rest of samples are constituted by larger fractions of shallower cooler waters.
|| Deuterium vs oxygen-18 () of some samples
|| Radon concentration values (Bq L-1)
Isotopic results (δ 18 O, δD) of some springs are shown in Fig. 6 where data for the Los Azufres reservoir fluids at natural state were included. All the samples show the oxygen-18 shift characteristic of geothermal fluids. Araro samples show a similar composition as compared to the Los Azufres fluids indicating a relationship between them. Tello and Quijano indicated that Araro could be a discharge of the Los Azufres fluids, the present results seem to confirm this hypothesis.
The average and standard deviation of groundwater radon concentration values
for each monitoring station are shown in Fig. 7. The average
radon concentration values ranged from 0.88 to 3.66 Bq L-1. These
values, relatively low, indicate a rapid transit from recharge to the output
of springs and wells[2,3] even if the stations are located in different
geological environments around the lake.
|| Alternative uses of geothermal energy as a function of reservoir
temperature as reported by Lindall
Effectively, stations Jeruco (J), Cuitzeo (C-2 and C-3) are located on pyroclastic
flow deposits from local monogenetic volcanism at the northwestern shore of
the lake. Station San Juan Tararameo (SJT), at the central part of the lake,
corresponds to lacustrine deposits while Santa Rita (SR) and Copandaro (CO)
are located on andesitic rocks in the southern part of the lake. However, all
of them are associated to normal faulting that form the semi-graben of Cuitzeo
It is worth mentioning that the lower radon values in groundwater were found at El Salitre (ES), Copandaro (CO) and Mariano Escobedo (ME), three stations located on one of the main local geological faults. This behavior is explained considering that in such sites gas emanations occur. As radon is partitioned to the gas phase the liquid phase becomes depleted.
The higher radon values correspond to the sites where higher reservoir temperatures and gas emanations were found due to the high efficiency fluid flow that eventually transports the radon to the surface.
In Table 3 the possible alternative uses of geothermal energy as a function of reservoir temperature are given. For the studied sites temperatures, there are many possible applications of geothermal fluids, including electric generation by conventional or binary cycles at Araro.
Analysis of chemical data from Cuitzeo wells and springs suggests that one
or more geothermal reservoirs could occur. Araro waters are probably related
to the Los Azufres geothermal fluids. Chemical geo-thermometers provided a wide
range of temperatures for the reservoir, from 165 to 220°C which are able
for electric generation and a wide range of direct applications. A model based
on silica and enthalpy was obtained indicating a mixing process between hot
deep fluids with shallower, cooler waters in different proportions. AR-2, SAM,
SAP and SJT samples present about 50% of hot component in the mixture. Radon
results indicated a high efficient fluid flow transport in the zones where higher
reservoir temperatures were estimated.
The authors acknowledge Mr. Adrian Patiño for technical assistance and partial financial support from CONACYT projects 40858 and 12445.