Hydrogeochemical surveillance at El Chichón volcano crater lake, Chiapas, Mexico

Hydrogeochemical surveillance at El Chichón volcano crater lake, Chiapas, Mexico

Journal of Volcanology and Geothermal Research 285 (2014) 118–128 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Re...

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Journal of Volcanology and Geothermal Research 285 (2014) 118–128

Contents lists available at ScienceDirect

Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores

Hydrogeochemical surveillance at El Chichón volcano crater lake, Chiapas, Mexico María Aurora Armienta a,⁎, Servando De la Cruz-Reyna a, Silvia Ramos b, Nora Ceniceros a, Olivia Cruz a, Alejandra Aguayo a, Flor Arcega-Cabrera c a b c

Instituto de Geofísica, Universidad Nacional Autónoma de México, Circuito exterior, C.U. México D.F. 04510, México Universidad de Ciencias y Artes de Chiapas, Centro de Monitoreo Volcanológico-Sismológico y Subsecretaría de Protección Civil del Gobierno del Estado De Chiapas México Facultad de Química, Unidad Sisal, Universidad Nacional Autónoma de México, Puerto de Abrigo Sisal, Yucatán 97355, México

a r t i c l e

i n f o

Article history: Received 16 February 2014 Accepted 11 August 2014 Available online 19 August 2014 Keywords: Volcanic lakes Volcano monitoring Hazard assessment Hydrogeochemistry El Chichón

a b s t r a c t El Chichón volcano has an eruptive record of at least 12 major eruptions in the Holocene, the latest one in March– April 1982 causing the worst volcanic disaster in the history of Mexico. After about 6 centuries of quiescence, this eruption destroyed a large dome and opened a 1 km wide crater. A lake, formed within the crater shortly after the eruption, has been an important source of information about the evolution of the post-eruptive processes. The fluctuations of the crater lake water physicochemical parameters, observed since 1983, have allowed in identifying hydrothermal waters and H2S-rich gases, influenced by tectonic and meteorological effects, as the main contributors to its composition. Here we propose some methods to help in assessing the state of the volcano derived from the relative contribution of these factors as an easy to implement volcanic surveillance method in potentially active volcanoes with crater lakes, or other volcano-influenced water sources. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The management of volcanic hazard should include systematic monitoring and interpretation of diverse parameters providing information about the internal condition and processes within a volcanic system. Pre- and syn-eruptive hydrogeochemical changes have been observed at several volcanoes (Hirabayashi et al., 1982; Takahashi et al., 1988; De la Cruz-Reyna et al., 1989; Martini et al., 1991; Gíslason et al., 1992; Shevenell and Goff, 1995; Tilling and Jones, 1996; Fischer et al., 1997; Quattrocchi et al., 2000; Varekamp et al., 2001; Federico et al., 2004; Armienta et al., 2008; Capasso et al., 2014). Nowadays, hydrogeochemical monitoring represents an important tool (Marrero et al., 2005; Rouwet et al., 2009; Capasso et al., 2014; Ingebritsen et al., 2014), that jointly with other seismic, geodetic and gravimetric monitoring data supply valuable information to reduce the degree of non-uniqueness of the interpretations. Such information furnishes a more accurate evaluation of the more likely volcanic activity scenarios, making it possible to issue adequate and on-time warnings of changes in a volcano condition. Of particular interest are volcanoes maintaining crater lakes, as in some cases waters of such lakes have shown precursory chemical changes reflecting volcanic unrest. The Yugama crater lake located in the summit of Mt Kusatsu-Shirane volcano showed chemical changes 1 year in advance to its 1976 eruption (Ossaka et al., 1980), and after the explosive ⁎ Corresponding author. E-mail addresses: [email protected]fisica.unam.mx (M.A. Armienta), [email protected] (S. Ramos), [email protected] (F. Arcega-Cabrera).

http://dx.doi.org/10.1016/j.jvolgeores.2014.08.011 0377-0273/© 2014 Elsevier B.V. All rights reserved.

eruptions occurred in 1982 and 1983 (Ohba et al., 2008). Additionally, sulfate concentrations increased and polythionate decreased before its 1982 eruption (Takano and Watanuki, 1990). Similarly, polythionate concentrations decreased before a phreatic eruption at Poás volcano, and increased before the development of magmatic activity (Rowe et al., 1992). At Ruapehu volcano Mg/Cl ratio in the lake water increased during and after eruptive periods (Giggenbach, 1983). At Popocatépetl volcano, sulfate and Mg/Cl increased prior to the onset of its current eruptive period (Armienta et al., 2000). Recently, at Dominica, Lesser Antilles, chemistry of geothermal systems was included as a tool for volcano monitoring (Joseph et al., 2011) The eruption of El Chichón volcano in 1982 was one of the worst disasters occurring in México in the past century, causing nearly 2000 fatalities (De la Cruz-Reyna and Martin del Pozzo, 2009). Less than 1 month after the eruption, three small lakes formed in the crater, that later coalesced into a single, larger one that remains to the present, with significant fluctuations in shape and dimensions. The chemical characterization of the lake water was first reported by Casadevall et al. (1984) and diverse studies of the lake chemistry have been carried out afterwards (Armienta and De la Cruz-Reyna, 1995; Taran et al., 1998; Armienta et al., 2000; Tassi et al., 2003; Rouwet et al., 2004, 2008, 2009; Taran et al., 2008; Morton-Bermea et al., 2010; Cuoco et al., 2013). We have monitored the chemical composition of El Chichón volcano crater lake for about 25 years. First sampling campaigns following the lake formation were irregular, and often after long time intervals at one lake shore location only; however the sampling frequency increased

M.A. Armienta et al. / Journal of Volcanology and Geothermal Research 285 (2014) 118–128

afterwards and other 5 sampling points were included since 2003. The information that may be extracted from any evolving processes indeed depends on the sampling frequency. The Nyquist–Shannon sampling theorem states that it is only possible to observe processes evolving over times twice the sampling period (Nyquist, 2002), condition that may become awkward in an irregular sampling. From the balance between estimated lake volumen, precipitation and spring waters input, Taran and Rouwet (2008), and Rouwet et al. (2014) concluded that El Chichón lake has an estimated residence time of 2 months, thus requiring a monthly sampling to detect hydrogeochemical changes related to that process. However, longer sampling periods should provide information on seasonal and non-seasonal longer duration cycles. In the present work the lake water chemistry evolution is discussed aiming to develop an improved methodology for volcanic activity level assessment at El Chichón, understood as a measure of the changes in the level of interaction between magmatic gases and the lake water. Specific observations of chemical species concentrations and data processing methods are proposed as part of a monitoring protocol to assess the level of volcanic activity, and associated hazards. The proposed methodology is focused on straightforward procedures, useful even in a framework of limited technological conditions, providing basic factors for hazard assessment and an increased capability to issuing early warnings to civil protection authorities.

2. El Chichón Volcano. Geological setting and eruptive history El Chichón is an active trachyandesitic volcano located in the Chiapanecan Volcanic Arc, so isolated within a sedimentary mountain range that was not even recognized as a volcano until 1928 (Müllerried, 1933). It is located at 17.36° N, 93.23° W, in a tropical forest zone having an average precipitation around 4000 mm/yr (as a mean of the National Meteorological Service stations located in towns around the volcano). The volcano stands over a basement of middle Cretaceous to middle Miocene marine limestones, claystones and sandstones (Damon and Montesinos, 1978; Canul and Rocha, 1981; Canul et al., 1983; Duffield et al., 1984; García-Palomo et al., 2004; Macías et al., 2008; Garduño-Monroy et al., 2014). There are three main models explaining the El Chichón origins: The subduction process of the Cocos plate under the North American plate, complicated by the awkward geometry of the plate boundary fault system and a close interaction with the Caribbean plate (Damon and Montesinos, 1978; Luhr et al., 1984; Mora et al., 2007; Manea and Manea, 2008; Mazot et al., 2011); the subduction of a different southwest dipping slab from the Gulf of Mexico resulting from the collision of Yucatán block with Mexico (Kim et al., 2011); a rift magmatism related to the melting and dehydration of that slab (Arce et al., 2014). At least 12 major eruptions have occurred at El Chichón in the Holocene, and more precisely in the past 8000 years, with repose intervals lasting between 100 to 600 years (Tilling et al., 1984; Espíndola et al., 2000; Macías et al., 2003, 2008; De la Cruz-Reyna and Martin Del Pozzo, 2009; Layer et al., 2009; Tilling, 2009; Mendoza-Rosas and De la Cruz-Reyna, 2010). In March–April 1982, El Chichón volcano suddenly reawakened with a weeklong series of eruptive outbursts reaching VEI 5, producing the worst volcanic disaster in the recorded history of Mexico, killing nearly 2000 people, and causing severe economic loss (Yokoyama et al., 1992; De la Cruz-Reyna and Martin Del Pozzo, 2009; Tilling, 2009; De la Cruz-Reyna and Tilling, 2014). This eruption, occurred after about 550 years of quiescence (Macías et al., 2003), destroying a dome about 1.5 km diameter in its base and 300 m height, and forming a 1 km wide, 200 m deep crater. The final, most explosive phases devastated an area about 10 km around the volcano, and caused heavy ashfalls on locations several hundred kilometers from the summit (Varekamp et al., 1984). A few weeks later, on April 25, three lakes were observed in the crater that later merged into a bigger lake, first observed in November, 1982 (Casadevall et al., 1984).

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The analysis of the El Chichón eruptive history has allowed to estimate the probabilities of future eruptions (Mendoza-Rosas and De la Cruz-Reyna, 2010), and several scenarios are possible: a long quiescence period; an effusive dome-emplacement phase that may turn explosive due to magma water interactions, or an open vent major explosive eruption such as those occurred 900, 2000 and 2400 yr BP (Macías, 2007). What kind of precursors may we expect for each possible scenario? Whatever those precursors are, they would make sense only when compared to a background activity and its evolution. Here we intend to outline such evolving background, and propose a viable methodology to continue the acquisition and processing of data. Such methods must account for the difficult logistic and sampling conditions at El Chichón crater lake, and the limited funds that may realistically become available in a permanent hydrogeochemical surveillance program. 3. The crater lake After its coalescence, the early shallow lake (1–3.3 m deep) covered an area of about 1.4 × 105 m2 on the central crater floor (Casadevall et al., 1984). However, its shape and dimensions have changed along the years (Rouwet, 2011). The chemistry of El Chichón crater lake water has also fluctuated since the first sampling in 1983 reported in Casadevall et al. (1984). Lower acidity was measured in 1986 (pH of 2.5) with respect to 1983 (pH 0.56) (Armienta et al., 2000). Afterwards, pH values have been varying around 2.5. Proportion and concentrations of main ions have shown strong changes along the years. The crater-lake was acid, calcium chloride type in 1983. Afterwards, predominance of main anions has fluctuated between chloride and sulfate, and among magnesium, sodium, and calcium for main cations (Armienta and De la Cruz-Reyna, 1995; Armienta et al., 2000, 2008). Armienta et al. (2000) concluded that the magmatic contribution to the crater lake water had a decreasing trend between 1983 and 2000. Hydrogeochemical changes have been attributed to diverse processes such as variations of the hydrothermal water supply and the volcanic gas input, as well as precipitation–dissolution and oxidation–reduction reactions within the lake water (Armienta et al., 2008). Tassi et al. (2003) related the behavior of the magmatic–hydrothermal system of El Chichón to the interaction between a deep magmatic source and a shallow cold aquifer, combined with changes in the permeability of the system. Rouwet et al. (2009) and Peiffer and Taran (2013) attribute the nonhomogeneous permeability to changes in the crater lake size covering more pumiceous sands. Taran et al. (1998, 2008) and Rouwet et al. (2008, 2009) related changes in the crater-lake chemistry to the activity and influence of near-neutral geyser-like springs (Soap Pool) into the lake water. 4. Sampling and analytical methods Sites sampled from January 2003 to April, 2014 were located along the eastern shore of the lake, as illustrated in Fig. 1. Temperature and pH have been measured in the field with a Hanna pH-meter calibrated in situ with buffer solutions at the same temperature of the water before each reading. Location M6 has been sampled since 1983. Some partial analytical results have been reported elsewhere (Armienta and De la Cruz-Reyna, 1995; Taran et al., 1998; Armienta et al., 2000, 2008). The changing dimensions of the lake makes impossible to sample exactly the same points in every field campaign. Moreover, in some occasions, the increased size of the lake prevented reaching some of the sampling points. Chemical analyses of main ions have been performed at the Laboratorio de Química Analítica, Instituto de Geofísica, UNAM, following Standard methods (APHA, 1995, 2005). Sodium and potassium were measured by atomic emission spectroscopy adding LiNO3 to control ionization. Concentrations of calcium and magnesium were determined by complexometric titration with EDTA. Sulfates were measured by turbidimetry. Chloride and fluoride concentrations were determined by

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Fig. 1. Sampling points on the Eastern shore of the El Chichón crater lake. The locations are approximate, as the lake persistently changes size and shape.

Table 1a Concentration (mg/L), pH and conductivity (Λ S/cm) ranges in the sampled points 2003–2014 (samples M2–M6). Date

pH

T °C

Λ μS/cm

SO4

Cl

Na

K

Ca

Mg

B

SiO2

January 2003 April 2003 May 2003 June 2003 July 2003 August 2003 November 2003 May 2004 July 2004 October 2004 February 2005 October 2005 January 2006 October 2006 May 2007 September 2007 January 2008 March 2008 April 2008 January 2009 February 2009 March 2009 October 2009 December 2009 January 2010 March 2010 May 2010 October 2010 March 2011 December 2011 March 2012 July 2012 September 2012 March 2013 February 2014 March 2014 April 2014

2.5–2.57 2.18–2.24 1.87–1.99 2.11–2.12 2.24–2.73 2.21–2.25 2.43–2.47 2.28–2.64 1.97–2.71 2.4–2.44 2.31–2.4 2.32–2.35 2.13–2.23 2.35–2.58 2.27–2.45 2.34–2.59 2.46–2.64 2.22–2.3 2.38–2.51 2.29–2.32 2.34–2.43 2.26–2.31 2.09–2.18 2.4–2.48 2.4–2.49 2.2–2.26 2.1–2.18 2.24–2.56 2.23–2.28 2.65–2.76 2.38–2.55 2.27–2.51 2.30–2.40 2.26–2.29 2.38–2.47 2.43–2.46 2.38–2.56

31–35 29–35 35–47 n.dat. 34–70 31–41 32–41 40–42.4 38–50 34–53 34–50 28–30 30.4–36.0 32.7–41.1 36–41.4 34.4–54.2 31.8–38.2 31.7–40.7 32–38 29.6–42.9 26.2–39.8 28.8–39.1 34.8–45.8 32.1–36.9 23.9–28.9 28.4–46.1 32.9–45.5 20.1–28.8 38.6–40.6 30.2–39.9 31.4–40.1 32.6–40.7 32.9–40.6 30.2–34.2 38–42.5 41.5–44 39–44.2

1952–2080 3710–3950 3980–4360 4170–4440 1188–2940 2540–2640 2010–2280 6350–7980 5470–8100 4080–4700 3420–4550 2420–2510 3620–3800 1458–2150 3040–6860 4720–6140 2000–2780 2990–3340 2320–2800 4100–4200 3980–4390 4450–4670 2850–3160 3070–3390 3030–3670 4030–4760 5150–5880 1509–2460 3780–4810 1930–2480 4330–4550 4250–6140 3210–4120 3170–3520 4200–4880 4830–5470 5380–6680

327–357 702–759 857–919 920–991 468–570 429–538 410–436 268–494 395–541 375–396 560–639 400–422 688–763 239–430 560–619 558–710 344–436 400–473 341–500 498–596 485–536 521–549 505–558 455–474 452–508 593–685 880–1017 275–393 469–529 356–427 522–547 450–681 454–610 608–638 482–540 581–623 519–608

24–32 45–58 52–72 52–75 8–33 16–18 5–10 1645–1913 1713–2068 785–808 325–400 16–18 17–18 3–19 1275–1660 933–1380 165–283 238–308 188–381 350–400 495–600 448–490 76–88 160–204 256–292 329–395 321–410 30–53 647–914 313–478 940–1095 725–1065 335–495 86–100 832–927 1185–1380 935–1361

25–28 42–52 52–68 48–68 28–34 20–24 10–17 711–904 710–824 363–401 240–322 15–19 30–31 9–24 543–783 479–581 77–110 123–167 115–202 202–217 215–298 231–252 63–65 95–117 120–134 187–212 229–278 29–47 349–435 153–264 493–538 464–547 214–312 77–80 395–459 541–626 540–644

5.6–7.1 10–12.3 12.6–16.9 14.2–16.7 7.6–9.5 5.4–5.6 3.1–5.8 121–145 119–142 66.9–68.3 41–47.3 4.2–5.6 7.9–9 2.7–27.7 95–130.6 76.7–94.2 15.7–21.6 22.5–29.6 21.7–36.3 34.7–37.3 37.3–53.3 42–44.3 12.3–13.1 13.7–20.8 22.5–24.3 34.1–37.6 44–49.7 6.7–9.7 60.4–73.1 27–43 91.2–96 80–96 39.7–55.9 15.7–16.9 65–70 91.5–99.2 90–107.4

22–32 42–49 47–60 57–64 28–53 26–28 19–23 309–355 327–342 152–160 92–120 20–26 35–77 16–38 244–315 214–230 43–69 60–88 70–108 90–102 89–114 97–105 46–65 54–60 64–73 86–95 111–122 28–54 147–167 83–93 174–190 99–153 91–105 46–57 187–214 242–270 241–274

13–17 16–17 25–31 24–30 17–24 13–16 8–9 50–71 75–87 24–29 22–24 9–12 23–24 6–16 34–50 38–46 14–26 14–20 13–22 24–33 19–27 18–24 25–30 13–24 26–29 21–25 29–36 10–21 21–24 21–25 32–43 37–53 17–22 20–28 41–54 34–46 46–54

b0.3 1.5–1.9 1.7–2.5 b0.3–1.8 0.9–3.6 1.2–1.9 b0.3 20.3–24.3 18.5–19.5 8.8–12.6 6.3–8.7 3.1–5.3 1.7–2 b0.3–1.7 16.9–22.6 12.4–19.9 3.5–5 3.8–4.9 1.6–3.2 5.9–6.8 7.7–9.3 7.9–8.3 b0.3–0.8 3–4.3 4–4.6 5–6.4 7.9–9 0.9–1.9 11.1–15.2 4.6–6.2 15.5–16.5 13.8–17.9 6.6–10.5 2.4–2.5 12.5–15.1 16.2–19.3 16–20

60–79 92–123 143–160 169–172 96–222 77–95 52–78 238–263 218–230 134–155 154–191 58–68 102–103 67–103 274–288 235–266 90–127 115–149 124–192 157–180 156–187 181–196 117–138 130–145 148–160 185–204 222–235 80–132 216–233 146–188 220–236 237–262 142–185 112–140 159–192 209–239 204–214

n.dat. = no data.

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Table 1b Concentration of major ions, silica, boron (mg/L), pH, and ionic balance at each sampled point of one field campaign per year. Date January 2003 M3 M4 M5 M6 Mean St dev

pH

Na

K

Ca

Mg

CO3

HCO3

SO4

Cl

SiO2

B

Bal

2.57 2.51 2.52 2.5

25.1 25.3 28.0 25.7 26.0 1.3

5.6 5.8 7.1 5.6 6.0 0.7

21.9 21.9 31.9 21.9 24.4 5.0

15.7 14.5 16.9 13.3 15.1 1.6

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

336.4 334.8 356.8 327.4 338.9 12.6

24.1 29.3 28.4 31.8 28.4 3.2

64.5 64.4 79.2 59.5 66.9 8.5

b0.3 b0.3 b0.3 b0.3

−9.79 −8.14 −5.29 −7.72

2.3 2.34 2.4 2.48 2.64

798.4 903.8 794.4 730.0 711.0 787.5 75.6

138.4 145.2 135.3 128.2 121.3 133.7 9.2

335.0 354.9 345.0 311.1 309.1 331.0 20.4

58.0 71.3 58.0 49.6 49.6 57.3 8.9

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

536.8 588.4 587.5 501.9 420.4 527.0 69.9

1912.5 2015.0 1820.0 1645.0 1650.0 1808.5 162.4

252.0 263.3 256.0 243.2 238.2 250.5 10.0

27.9 24.3 22.8 20.3 22.1 23.5 2.9

1.87 2.94 2.85 2.79 3.43

239.8 291.4 287.0 322.2 292.6 286.6 29.7

41.1 45.6 44.7 47.3 44.7 44.7 2.3

92.2 118.2 116.2 120.2 116.2 112.6 11.5

23.1 23.1 24.3 24.3 21.9 23.3 1.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

638.5 571.0 578.0 559.5 587.0 586.8 30.6

325.0 400.0 390.0 391.0 390.0 379.2 30.6

191.2 155.7 155.9 164.9 154.3 164.4 15.5

6.3 8.1 7.9 8.7 8.0 7.8 0.9

−1.14 6.77 6.24 10.13 5.91

13.1 24.0 15.1 23.1 8.9 16.9 6.5

3.4 5.2 3.7 4.8 2.7 4.0 1.0

20.8 37.7 20.8 22.4 16.0 23.6 8.2

8.8 16.0 10.2 9.2 5.8 10.0 3.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

378.8 429.8 328.0 283.4 239.0 331.8 75.5

5.6 7.9 7.4 18.8 3.5 8.6 5.9

67.3 103.4 76.5 81.4 70.1 79.7 14.3

0.3 b0.3 1.3 1.6 1.7 1.0 0.7

−10.70 −8.80 −8.48 −4.40 −7.51

September 2007 M2 2.34 M3 2.59 M4 2.4 M5 2.38 M6 2.4 Mean St dev

569.1 478.6 550.3 581.2 570.7 550.0 41.4

94.2 76.7 87.1 94.1 87.8 88.0 7.2

230.2 218.3 214.3 226.2 216.3 221.0 6.8

40.9 45.7 43.3 42.1 38.5 42.1 2.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

578.5 709.5 599.0 576.5 557.5 604.2 60.7

1307.5 932.5 1220.0 1380.0 1262.5 1220.5 171.5

245.0 265.9 239.7 234.9 238.1 244.7 12.4

19.9 12.4 16.7 19.2 19.6 17.6 3.2

−2.44 −1.32 −2.71 −4.42 −2.39

March 2008 M2 M3 M4 M5 M6 Mean St dev

126.0 123.1 166.9 129.0 131.9 135.4 17.9

22.5 22.5 29.6 23.4 25.0 24.6 3.0

60.3 60.8 87.7 63.8 64.3 67.4 11.5

14.2 14.2 20.2 14.5 14.5 15.5 2.6

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

471.7 473.2 463.6 437.3 400.1 449.2 31.0

243.0 238.0 308.3 243.0 242.0 254.9 29.9

115.4 119.5 148.7 126.5 131.8 128.4 13.0

3.9 3.8 4.9 3.9 3.9 4.1 0.5

−1.26 −1.67 3.18 0.38 2.05

February 2009 M2 2.34 M3 2.43 M4 2.34 M6 2.34 Mean St dev

215.0 236.2 298.0 228.3 244.4 36.8

37.2 42.5 53.2 40.2 43.3 7.0

91.3 106.2 114.1 89.3 100.2 11.9

19.2 22.3 27.1 20.4 22.3 3.5

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

536.0 485.0 504.3 529.9 513.8 23.6

495.0 500.0 600.0 500.0 523.8 50.9

155.8 186.9 180.7 161.4 171.2 14.9

7.8 8.0 9.3 7.7 8.2 0.7

−8.91 −4.30 −1.12 −7.42

May 2010 M2 M3 M4 M5 M6 Mean St dev

2.17 2.18 2.17 2.14 2.1

229.0 265.0 252.5 277.0 277.5 260.2 20.2

44.0 47.0 44.5 49.5 49.7 46.9 2.7

111.7 121.6 111.7 121.6 121.6 117.6 5.4

29.0 36.3 31.4 31.4 32.6 32.1 2.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

943.0 879.5 915.0 1017.0 1006.5 952.2 59

321.0 330.0 344.0 370.0 410.0 355 35.9

235.1 226.9 221.8 232.5 225.4 228.3 5.4

7.9 8.2 8.4 8.9 9.0 8.5 0.5

−5.29 1.37 −3.08 −3.95 −3.97

1.99 2.01 2.04 2.03 2.2

397.0 435.0 428.0 348.0 397.0 401.0 34.4

69.8 73.1 73.0 60.4 67.7 68.8 5.2

147.0 167.2 157.0 157.0 145.0 154.6 8.9

21.0 23.2 24.4 24.4 14.7 21.5 4.1

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

491.0 528.0 468.5 503.4 443.0 486.8 32.6

803.0 914.0 843.0 647.0 770.0 795.4 98.8

217.0 215.8 232.7 221.0 231.4 223.6 8.0

14.0 13.4 15.2 14.4 11.0 13.6 1.6

7.06 7.09 4.40 8.37 6.67

May 2004 M2 M3 M4 M5 M6 Mean St dev

February 2005 M2 2.4 M3 2.31 M4 2.33 M5 2.34 M6 2.33 Mean St dev October 2006 M2 M3 M4 M5 M6 Mean St dev

April 2011 M2 M3 M4 M5 M6 Mean St dev

2.39 2.48 2.48 2.54 2.58

2.22 2.23 2.26 2.26 2.3

(continued on next page)

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Table 1b (continued) Date March 2012 M2 M3 M4 M5 M6 Mean St dev March 2013 M2 M3 M4 M5 M6 Mean St dev April 2014 M3 M4 M5 M6 Mean St dev

Ca

Mg

CO3

HCO3

93.3 91.2 91.6 92.0 96.3 92.9 2.1

173.7 173.7 187.6 189.5 181.6 181.2 7.5

35.9 35.9 32.3 43.1 33.5 36.1 4.2

0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

77.2 79.7 77.9 79.2 78.3 78.5 1.0

16.1 16.9 15.7 16.2 15.9 16.2 0.5

51.3 57.3 46.4 47.4 46.4 49.8 4.7

19.2 25.1 21.6 28.1 20.4 22.9 3.7

0.0 0.0 0.0 0.0 0.0 0.0 0.0

643.5 625.0 540.0 570.5 594.8 47.9

107.4 103.7 89.7 95.8 99.2 7.9

273.7 271.7 260.1 240.7 261.5 15.2

50.6 48.3 45.9 54.1 49.7 3.5

0.0 0.0 0.0 0.0 0.0 0.0

pH

Na

2.38 2.43 2.47 2.55 2.53

506.3 492.8 538.0 535.0 529.0 520.2 19.7

2.26 2.27 2.28 2.28 2.29

2.38 2.41 2.56 2.54

K

potentiometry with ion-selective electrodes, a 5 M solution of NaNO3 was added as ionic strength adjuster for chloride and a TISAB solution for decomplexing and adjusting the ionic strength for fluoride. Silica was measured by colorimetry following the molybdosilicic acid method. Boron was determined by colorimetry by its reaction with carminic acid with a lowest detection level of 0.3 mg/L. Analytical control showed less than 5% differences in duplicates. Box and whisker, Giggenbach plots, and water type were obtained using the geochemical program AquaChem (2005). 5. Results and discussion Concentration, pH and conductivity ranges of chemical species measured at different sites of the lake shore since 2003 are listed in Table 1a. The highest temperature was measured at site M1, located near a fumarole field (up to 93 °C in April, 2003). This site has always shown strong differences with the other sampled points, and its behavior is probably controlled by a different process. We thus do not include it in

SO4

Cl

SiO2

B

Bal

521.5 545 543.5 525.5 547.0 536.5 12.0

1022.5 940.0 1057.5 1095.0 1095.0 1042.0 64.5

220.5 222.4 225.5 235.9 224.7 225.8 6.0

15.6 15.6 15.5 16.5 16.2 15.9 0.4

0.63 1.62 0.01 −0.46 −2.46

0.0 0.0 0.0 0.0 0.0 0.0 0.0

615.9 626.7 616.0 638.0 608.0 620.9 11.6

97.0 86.5 99.0 100.0 92.2 94.9 5.6

117.3 140.0 116.4 114.7 112.3 120.1 11.3

2.4 2.4 2.4 2.4 2.5 2.4 0.0

−7.47 −4.37 −8.68 −7.77 −8.28

0.0 0.0 0.0 0.0 0.0 0.0

576.8 588.2 608.5 519.4 573.2 38.2

1361.3 1136.3 935.0 1021.3 1113.4 184.6

214.5 207.2 204.8 203.7 207.6 4.9

20.0 19.5 16.0 17.0 18.1 1.9

−1.86 3.36 4.29 4.94

Table 1a, as it reflects more of the Soap Pool (Rouwet et al., 2004, 2008) than the overall lake behavior. Lower temperatures, from 24 °C to 70 °C, were measured at all of the other sites. Most samples varied within a pH range from 1.9 to 2.8. Sulfate and chloride show the strongest fluctuations, from 239 to 1017 mg/L the former and from 3 to 2068 mg/L the latter. Sulfate was the predominant anion at some dates and chloride at others. Due to the low pH, bicarbonates were not detected at any date. Main cations have also shown strong variations; sodium ranged from 9 to 904 mg/L, calcium from 16 to 355 and Mg from 6 to 87 mg/L, without a single predominance along the time. Other species also varied within this period. Silica ranged from 52 to 288 mg/L and boron from non-detectable (b0.3 mg/L) to 24.3 mg/L. Table 1b lists the pH, main ions, SiO2 and B results of the sampled points of one of the field campaigns per year. The last column shows the ionic balance (considering pH) for each sampling site. The average and the standard deviations of the chemical species indicates that all of the sampled points M2, M3, M4, M5 and M6 show a similar behavior, thus providing a satisfactory representation of the lake water behavior.

Fig. 2. Precipitation (mm of rainwater at Pichucalco pluviometric station) and crater lake water conductivity at M6 (μS/cm) with time.

M.A. Armienta et al. / Journal of Volcanology and Geothermal Research 285 (2014) 118–128

Fig. 3. Box and Whisker plot of main ions from 1985 to 2014 in sample point M6.

Available precipitation data from the nearest pluviometric station at Pichucalco, 20 km to the NNE of the crater do not show correlation with conductivity on the sampling dates (Fig. 2), a somewhat unexpected result, as rainfall should be an important source of the lake water. For example Terada et al. (2012) observed a 2-month lag in the increase of the lake water influx from groundwater after the rainy season at Aso volcano. However, at El Chichón volcano crater lake no significant conductivity changes have been observed during or after rainy seasons (e.g. Armienta et al., 2000; Rouwet et al., 2008), suggesting that the decreasing trend and the variations in conductivity with time are related to causes other than meteorological. Concentration ranges of main ions at site M6, with the longest data series since 1985, are shown in the Box-and-Whisker diagram depicted for that point (Fig. 3). Sodium and chloride showed the largest concentration range. Variations of the main ions chemistry resulting in various water types since 1985 for M6 are summarized in Table 2. One data point from 1983 is also included in the table in spite of being collected from a different point, located about 150 m SW from M6, as it reflects the composition of the lake shortly after the eruption. Calcium and chloride were the main ions in that lake sample collected by Casadevall et al. in January, 1983 (Casadevall et al., 1984). This type of water reappeared 10 years later in January, 1993. Sodium, calcium and chloride have predominated in most M6 samples. Sulfate has also reached relatively high concentrations at M6. The Giggenbach diagram (Giggenbach, 1988)

123

shows that all samples correspond to immature waters (Fig. 4). The three highest-conductivity samples measured since 1983 share a similar chemical fingerprint. Samples from January 1983 and January 1993 with 83,800 μS/cm and 46,400 μS/cm respectively are classified as calcium chloride type, the next high-conductivity sample with 41,100 μS/cm (collected in May, 1992) is Na–Ca–Cl type and also includes sodium as a main ion. A measure of the water–rock interaction is provided by the degree of neutralization (DON) developed by Varekamp et al. (2000). Fig. 5 shows the DON values calculated for M6 samples along time. A zig-zag type pattern has been observed since 1986, and the amplitude of the fluctuations seems to increase with time, particularly after 2001, indicating a highly variable lake dynamics (Rouwet, 2011). In addition, samples with a high DON (above 90%) have sodium and chloride as main ions (Tables 2 and 3), except the sample collected in January, 1993, a Ca–Cl type. This suggests a major influence of the hydrothermal system under the volcano for high-DON samples, henceforth referred as the “H group”. Stimac et al. (2004) considered that higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO4 and F at Mount Pinatubo reflected an important dilute hydrothermal contribution to the lake. Lower DON values (from 47% to 80%) were measured on El Chichón samples at other dates; all of them had sulfate as one of the main anions and some included chloride. Cations had a predominance of sodium, calcium or both. Presence of sulfate as one of the main anions in this group, henceforth called the “G group”, suggests the influence of H2S or SO2 or both producing SO2− upon dissolution 4 and oxidation in water. Some samples had characteristics between these two groups and could not be preferentially ascribed to any of them. Correlation between size of the lake and occurrence of each type of water (G or H) at certain dates could not be established from sporadic visual reports or lake size measurements (considering data reported by Rouwet et al., 2004). However, samples belonging to the H group were persistently obtained from larger lakes (up to 170,500 m2 in May, 2004) with respect to those belonging to the G group (up to 62,600 m2 in November, 2003), suggesting a hydrothermal influence on the lake dimension. An attempt to correlate chloride and boron concentrations using G samples collected in site M6 showed a low correlation (Fig. 6a). However, if only samples from the H group are considered, a higher correlation is observed (Fig. 6b), indicating a common source for both species in

Table 2 Crater-lake water types corresponding to site M6 at sampled dates. Water Type

Dates

Ca–Cl

January, 1983 January, 1993 November, 1985, September, 1986 November, 1991 May, 1992, August, 1992, March, 1996 January, 1997, April, 1998, April, 2000 January, 2001, April, 2001, June, 2001, July, 2001, September, 2001, May, 2004, July, 2004, April, 2000 January, 2002, March, 2013 May, 1995, March, 2002, August, 2002, October, 2004, May, 2007, September, 2007, January, 2009, February, 2009, March, 2012, February, 2014, March, 2014, April, 2014 February, 2005, April, 2005, January, 2006, January, 2008, March, 2008, April, 2008, October, 2009, December, 2009, January, 2010, March, 2010, May, 2010 December, 2002 May, 2003, June, 2003 May, 2003, June, 2003, July, 2003, January, 2006, April, 2006, October, 2006 April, 2003 August, 2003, November, 2003, October, 2005, January, 2006, May, 2006, June, 2006 October, 2010 July, 2012

Ca–Mg–Cl Ca–Na–SO4–Cl Na–Ca–Cl

Na–SO4–Cl Na–Ca–Cl–SO4

Na–Ca–SO4–Cl

Na–SO4 Na–Ca–SO4 Ca–SO4 Ca–SO4–Cl SO4 Ca–Na–SO4 Na–Cl–SO4

Fig. 4. Giggenbach diagram of samples since 1983 at site M6. Results for 1983 were reported by Casadevall et al. (1984).

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Fig. 5. Degree of Neutralization (DON) of M6 samples from 1983 to 2014.

that group. Besides, boron concentrations were higher in group H (from 9 to 188 mg/L) than in group G (from non-detectable to 4 mg/L). On the other hand chloride did not show a correlation with sulfate for any of Table 3 Concentrations of Cl−, B, Na+ and SiO2 (mg/L), and DON. (Degree of neutralization) in G and H samples. G-type samples Date Dec-02 Apr-03 May-03 June-03 July-03 Aug-03 Nov-03 Oct-05 Jan-06 Apr-06 May-06 June-06 Oct-06 Jan-08 Mar-08 Apr-08 Oct-09 Dec-09 Jan-10 Oct-10 Mar-13

Cl− 31.8 86.7 53.7 75.1 31 17.1 7.22 16 17.5 15.4 16.3 15.6 3.5 169 242 247.5 88 181.5 257.5 29.8 99.2

B

Na+

SiO2

DON

b0.3 1.77 2.18 1.75 1.02 1.45 b0.3 3.06 2.05 1.45 1.84 b0.3 1.7 3.8 3.96 1.55 b0.3 3.45 4.04 0.86 2.50

25.7 52.2 56.2 60 29.8 19.5 10.7 14.5 30.2 26.8 32.4 25.3 8.9 83.3 131.9 155 63.1 98.9 120.1 28.7 78.3

59.5 123.2 143 171 95.9 76.7 55.6 58.1 102.8 97.7 112.1 94.1 70.1 126.7 131.8 153.4 124.7 135.5 160.2 86.6 112.3

58.3 62.4 47.4 64.4 67.3 58.1 59.1 47.2 54.1 76.7 71.5 60.2 47.1 78.7 61.6 77.3 48.3 76.6 80.1 53.2 68.5

165 115 188 50 37 37 66 42 48 33.9 50.3 14.13 11.3 20.6 18.5 8.83 16.9 19.6 16.2 18 17

3400 3000 1325 1839 1189 1239 1859 1207 1463 1183 1445 637.3 379.3 711 709.5 362.6 542.6 581.2 529 580 570

273 550 250 251 212 207 357 243 303 296 260.8 172 136.5 238.2 229.6 155.3 282 234.9 225 210 204

98.0 96.9 98.6 99.0 93.1 92.1 96.7 96.5 97.3 91.5 96.7 89.9 89.4 95.6 96.7 90.6 89.7 90.5 92.5 93.0 93.5

the groups, indicating independent sources and/or different geochemical processes for each ion. Giggenbach diagrams might also discern waters classified as H or G. While G samples are close to the Mg corner (Fig. 7a), H samples show higher degree of water–rock interaction (Fig. 7b). These plots indicate a lower maturity of G samples with respect to H samples. Tassi et al. (2003) attributed the chemical composition of waters rich in Ca and SO4, near the Mg corner to the interaction of low-salinity groundwaters of meteoric origin with volcanic gases containing H2S and SO2. Fig. 8 graphically shows the differences among the above described groups H and G, and the earliest state of the lake (1983, point A) in a Sulfate-Conductivity-DON diagram. Most samples from group H have the highest DON and Cl, comprising 31.3% of the measured samples at site M6. On the other hand, samples from group G, comprising 31.3%

H-type samples May-92 Aug-92 Jan-93 Mar-96 Jan-97 Apr-98 Apr-00 Jan-01 Apr-01 June-01 July-01 Sep-01 Mar-02 May-04 July-04 Oct-04 May-07 Sep-07 Mar-12 Mar-14 April-14

12,250 6900 13,200 4220 3740 2885 3500 2700 3400 3180 3590 1575 908 1650 1712.5 785 1275 1380 1095 1325 1021

Fig. 6. a. Chloride (mg/L) vs boron (mg/L) concentrations in G samples collected at site M6. b. Chloride (mg/L) vs boron (mg/L) concentrations in H samples collected at site M6.

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125

Fig. 8. Log(Cl) vs log(SO4) vs log(DON) 3-D Diagram. Samples collected since 1983 to 2014 at sampling point M6.

Fig. 7. a. Giggenbach diagram for samples in G group. b. Giggenbach diagram for samples in H group.

et al., 2013) may be causing some of the observed chemical changes at El Chichón. Mazot and Taran (2009), identified high CO2 emission zones along main faults in the lake of El Chichón from a CO2 flux measurement survey developed in March, 2007. The observed chemical fluctuations with time might then be attributed to variations in the lake input from the hydrothermal aquifer (H group with a high DON) probably induced by tectonically-related changes of the transport capability of fractures and faults in the lake floor. Indeed, the lake water sample collected in March 2007 (reported by Peiffer et al., 2011) corresponds to H type water with a DON of 93. Waters more influenced by gas dissolution (G type) may correspond to periods when only reduced permeability or small cracks allow gas flow through the floor, and the hydrothermal water input is relatively low or even absent. Plotting of samples from M6 in the classification diagram, developed by Varekamp et al. (2000), and modified by Armienta et al. (2008), which differentiates volcanoes with various degrees or kinds of activity, shows clear differences among samples collected at various dates (Fig. 9). Samples falling within the active volcanoes area of the diagram were collected from January 1983 to July 2001 most of them correspond to the H group (May and August, 1992; January, 1993; March, 1996; January, 1997; April, 1998; April, 2000; January, April, June and July,

of the samples, have lower DON and Cl values. Samples with characteristics of both groups (M) are located between both groups accounting for 35.8% of the total. The outlier point A stands for 1.5% of the prevalent conditions for the total samples and may be considered an extreme value in the measured interval, corresponding to the first sample collected after the lake formation in January 1983. The DON and other chemical fluctuations in time thus reveal a dynamic lake receiving chemical inputs from various sources. The hydrothermal system under El Chichón volcano seems to have a strong influence on the lake water dynamics, as observed by Armienta and De la Cruz-Reyna (1995) based on its general chemical characteristics, and later confirmed by other isotopic and trace element determinations (Taran et al., 1998; Tassi et al., 2003; Rouwet et al., 2009). A possible controlling factor is the local tectonic activity, since some important active faults such as the “Catedral” fault crosses the crater (GarduñoMonroy et al., 2014). For example, strong chemical variations resulting from a decrease in the hydrothermal input within a short time, probably had a seismic origin, as observed in the boiling lake at Dominica (Erouscilla et al., 2011). A similar influence from active tectonic faults, such as Catedral (Drouin, 2012; Drouin et al., 2012; Gómez-Vázquez

Fig. 9. Classification diagram (Varekamp et al., 2000; Armienta et al., 2008) of samples collected at site M6 between January 1983 and July 2001 (triangles), and between September 2001 and April 2014 (diamonds). The point marked A (open triangle) stands for the outlier point A in Fig. 8. The region enclosing all of the other points represents fluctuations of the hydrothermal-dominated system.

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Fig. 10. Relative contribution of the H and G systems illustrated by the (SO4 + Cl)/Cl ratio vs time since 1983 at M6.

2001). Samples falling near the limit between active and quiescent samples also belong to the H group (September, 2001; May and July, 2004; May and September, 2007). On the other hand, samples markedly to the left of that limit belong to group G, with the furthermost to the left collected in October, 2006. The last sample included in this study, from April, 2014, is also located in the quiescent zone. The apparent paradox of having G samples falling in the “quiescent” region and H samples in the “active” region of the Varekamp diagram may be explained considering that El Chichón volcano condition after the eruption has been strongly controlled by a hydrothermal system, and both, G and H samples, represent fluctuations around a boundary region in the diagram reflecting relatively small and sporadic contributions of magmatic gas (e.g. Tassi et al., 2003). An important precursor signaling increased magmatic influence could thus be new measurements falling away from the region defined in Fig. 9 toward the point marked A (open triangle). Such point corresponds to the outlier point A in Fig. 8, the earliest sample obtained 10 months after the end of the El Chichón eruption (Casadevall et al., 1984). Although the hydrogeology of the aquifer system feeding the lake is not the aim of this study, the characteristics and variations in the chemistry of the lake waters along the time are consistent with a model proposed by Peiffer et al. (2011) that includes a deep and a shallow aquifer with different chemical compositions; besides, the influence of rain cannot be overseen in this tropical zone. Rouwet et al. (2008) attributed chloride occurrence in the lake waters to the discharge of the so called “Soap Pool” springs only, and predicted a zero chloride content by 2009. The presence of Cl-enriched H waters in April, 2014 (1021 mg/L) suggests that other sources may be feeding chloride to the lake, or that

the “Soap Pool” is still contributing to the chloride budget of the lake. Furthermore, the size of the lake in March–April, 2014, has been one of the largest ever observed. Fig. 10 shows the large-amplitude fluctuations of the (SO4 + Cl)/Cl ratio in the period 1983–2001. The large peaks in November 1985, May 1992, and Jan 1993 may correspond to outbursts of the hydrothermal system water. Further fluctuations in 2003 and 2007 may be related to Cl discharges from the Soap Pool, as reported by Rouwet et al. (2004, 2008). A cluster analysis was performed to find groups of parameters with similar geochemical features or attributes during the last 10 years. Results are shown in Fig. 11. Three groups can be recognized: the first one involves major ions, except sulfates; the second group sulfates and conductivity; and the third fluoride and pH. Therefore, groups of variables that could be considered as more sensitive to the internal ongoing processes in the geochemistry of the lake could be fluoride – pH, and sulfate – conductivity since they show lesser similarity with the other parameters, as revealed by their higher linkage distances in the dendogram of Fig. 11. Remaining variables show a tight grouping meaning less variability, i.e., more space–time stability. The measured variables may thus reflect the underlying processes with different degrees of relevance. Some may be irrelevant or redundant, while others may be highly significant or “master variables” (Salomons, 1995; Mao, 2005), as minor fluctuations of these variables relate to significant changes of the system (Manahan, 2007). A factor analysis shows the formation of two factors which explain the 73% variability of the chemical characteristics of the lake from 2003 to 2014 (Table 4). The first factor is formed by the main ions, and the second

Table 4 Factor analysis for El Chichón sampling sites. Log transforming was performed for all variables, pH excepted. Significant values (N0.7) in boldface.

Fig. 11. Ward 1-Pearson r cluster analysis (dendogram) of the analyzed parameters in samples collected at the six crater lake shore sites from 2003 to 2014.

Parameter

Factor 1 Major ions

Factor 2 Sulfates

pH Conductivity SO4 Cl F Na K Ca Mg B SiO2 % Total variance

0.03 0.41 0.04 0.81 0.37 0.92 0.95 0.88 0.72 0.89 0.87 51%

−0.89 0.60 0.78 0.42 −0.14 0.33 0.27 0.35 0.45 −0.09 0.00 22%

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one by the negatively correlated pH and sulfates. This analysis could help to set monitoring priorities when sampling and analytic resources are limited.

6. Conclusions The fluctuations of physicochemical parameters observed since 1983 by the hydrogeochemical monitoring of the crater lake water of El Chichón volcano appear to have two main contributions: hydrothermal waters and H2S-rich gases, and are also influenced by meteorological effects. Crater lake water composition fluctuates from hydrothermally influenced, higher DON, sodium–chloride type to lower DON, sulfate type. Larger lakes seem to be correlated with the more hydrothermal-influenced waters. These conditions may be considered as representatives of several dominant processes. The transitions from one condition to the other are probably controlled by external tectonic factors such as pre-existing faults with seismic or aseismic displacements. Unfortunately, the available seismic information is not sufficient to establish the influence of the seismic component. However, the persistent displacement observed by geodetic methods such as direct EDM measurements (Gómez-Vázquez et al., 2013) and InSar surveys (Drouin, 2012; Drouin et al., 2012) strongly support this possibility. The position of H-samples within the active volcanic zone in the Varekamp classification diagram for volcanic lake waters suggests that frequent and systematic monitoring should continue to detect a possible evolution toward the A-point implying a chloride increase, and a pH decrease. The statistical analysis reveals that sulfate concentrations that have increased before eruption episodes at other volcanoes and that reflect even small changes of the volcanic state should be an important parameter to monitor at El Chichón lake water. Persistent monitoring of the crater lake water faces serious logistical problems, including hazardous long stays within the crater required to sample all of the points. A minimum, yet useful monitoring may be achieved sampling the representative point M6 as frequently as possible, measuring out at least water temperature, sulfate and chloride concentrations, and pH. Finally, monitoring is a multitask activity. No single parameter is sufficient to support decision-making. Anomalies in the crater lake water chemistry must be interpreted in the context of other parameters such as seismicity, deformation, and others.

Acknowledgments The authors are grateful to the reviewers: Jose Luis Macias and Dmitri Rouwet, and to the editor Alessandro Aiuppa for their valuable comments that have greatly improved this paper. This project has been partially funded by DGAPA-PAPIIT-UNAM IN-106312-3 project.

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