25

Ijen Volcanic Complex The Ijen complex, situated in East Java near the city of Banyuwangi, is the easternmost volcanic centre in the island of Java. The large caldera complex hosts a

large number of volcanic edifices of which Ijen and Raung are the most active. The Ijen crater (Kawah Ijen) contains the world�s largest lakes of highly acid (pH<0.5) and mineralised volcanic water. A permanent solfatara on the lakeshore continuously produces native sulfur, which is mined by local workers. Occasional outbursts of phreatic activity, centred within the lake, have

formed the main threat in recent times. Apart from potential dangers of lahars, it has been known since long that the acid nature of the water also generates environmental problems. In 1921 a dam was built to regulate the water level, but water percolates through the porous wall, and forms the headwaters of a 40 km long acid river. After a first stretch in within the caldera, it breaks through the caldera rim and reaches an inhabited and cultivated alluvial plain before reaching the Java Sea. In this area, virtually all of the acid river water is used for irrigation. Extensive coffee plantations cover much of the highland within the caldera. The crater lake and surroundings are a natural park of great scenic beauty. Together with hot springs and waterfalls in the caldera catchment, it attracts an increasing number of tourists.

Geological setting The Ijen caldera has a diameter of about 14-16 km. Its northern margin is clearly visible as a typical caldera escarpment with a steep inner slope and elevations ranging from 850 to 1559 m. The southern and eastern walls are covered by the marginal volcanoes of Suket (2950 m), Jampit/Pendil (2338 m), Rante (2644 m), Merapi (2799 m), Ijen (2386 m), Pawenen (2123 m) and Ringgih (1965 m). The lowest part of the caldera (near Blawan village) has an altitude of 850 m, suggesting that the maximum depth of the caldera is about 700 m. Inside the caldera the topography is dominated by a large number of extinct volcanic cones: Cilik (1872 m), Pendil (2375 m), Anyar (1276 m), Genteng (1712 m), Gelaman (1726 m), Kukusan (1994 m), Papak (2099 m), Widodaren (2100 m), Blau (1774 m), Gendingwaluh (1519 m), Lingker (1630 m) and Kunci (1788 m).

Figure 3.1

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Figure 3.3. Schematic overview of the history of the Ijen caldera after Van Bemmelen (1941) and Sitorius (1990). Pre-caldera activity is supposed to have started prior to 300,000 years ago, probably forming a large single stratovolcano (Old Ijen) with an estimated altitude of 3500 m. Lavas and pyroclastics of these deposits disconformably overlie Miocene limestone. The caldera formation is associated with the eruption of a large volume (~80 km3) of pyroclastic flow deposits, which reach a thickness of 100-150 m and are most widespread on the northern slope of the complex. The event occurred some time

before 50,000 years ago, based on a K-Ar date (50±20ka) of a lava flow of Mt. Blau, which is considered to be the oldest post-caldera unit. This age also constrains the formation of lakes on the caldera floor. Lake sediments comprising shales, sand and river channel deposits are exposed in the northern area near Blawan. Post-caldera

Figure 3.2

27

activity (phreatomagmatic, phreatic, strombolian and plinian) produced the rim cones, which are generally composite edifices, and the inner cones, which are predominantly constructed by cinders. These younger volcanoes produced the ash and scoria cones, lava flows, pyroclastic flow and surge deposits and debris avalanche material that now cover the caldera flow. Radiocarbon dating of the pyroclastic flow deposits (Sitorus, 1990) yielded ages of >45,000 BP (Jampit), 37,900±1850 (Suket), 29,800±700 (Ringgih), 24,400±460 (Old Pawenen), 21,100±310 (Malang) and 2,590±60 (Ijen).

Figure 3.4. Plot of SiO2 versus K2O for lavas from the Ijen complex (Sitorus, 1990) Lavas from the Ijen Complex show a large variation in SiO2 contents (46-63 wt.%) ranging from basalt to dacite (Fig. 3.4). Basalts are medium to high-K, whereas most andesites plot in the high-K field. Pre-caldera and caldera products show a large scatter. Compositions of lavas from individual post-caldera centres are generally coherent, but collectively do not yield well-defined trends in variation diagrams. Plagioclase, orthopyroxene, clinopyroxene and Fe-Ti oxides are common phenocrysts, while olivine is restricted to relatively mafic post-caldera rocks. Biotite is only found in the Glaman lava dome.

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Eruptive activity The present activity is restricted to Ijen volcano, which has hosted an acid crater lake for at least 200 years. Documented historic eruptions did not produce juvenile magmatic products but were predominantly phreatic in nature. The following summary is based on Kusumadinata (1979) and Volcanic Activity Reports of the Smithsonian Institution Global Volcanism Program: 1796 Phreatic eruption 1817 15-16 January: phreatic eruption (flooding of mud towards Banyuwangi, fairly large

volume of lake water discharged into Banyupahit River) 1917 25 February-14 March: lake seemed to boil; repeated phreatic eruptions, mud

thrown up to 8-10 m above the lake surface 1921-1923 Increasing lake water temperature; steaming gases above water surface 1936 5-25 November: phreatic eruption producing lahar similar to that of 1796 and 1817 1952 22 April: steam eruption up to 1 km high; mud thrown up to 7 m above the lake

surface 1962 13 April: 7 m high eruption; gas bubbles on lake surface, about 10 m in diameter

18 April: bubbling water up to 10 m high, changing of watercolour 1976 30 October: bubbling water at Silenong for 30 minutes 1991 15,21,22 March: bubbling water and changing of water colour, 25-50 m high gas

outpouring at high velocity; this activity was recorded as seismic tremor between 16 and 28 March.

1993 3,4,7 July and 1 August: phreatic eruptions, changing of lake water colour, water outpouring, booming noise, clotted steam; all centred in the middle of the lake

1994 3 February: minor phreatic eruption from the south part of the lake. Coincident with the eruption, the lake level rose ~1 m.

1997 Late June: period of increased seismic activity; changing of lake water colour; gas bubbles and areas of up welling; strong sulphuric odour; birds were seen falling into the water; one or more sulfur workers near the summit reported dizziness and headaches.

1999 28 June: two phreatic eruptions at the Seating location. An accompanying detonation was heard at the sulfur-mining site 2 km from the summit and volcanic tremor was recorded with an amplitude of 0.5-1 mm. The following week, 6-12 July, yellow-grey sulfur emissions were observed from the crater and a loud "whizz" noise was heard. The crater lake's water was brownish-white and had sulfur agglutinate floating on the surface. Seismicity had increased starting in early April. The number of B-type events remained high (more than 34/week) for most of the period through mid-June. Seismicity then gradually declined through mid-July, after which the weekly number of B-type events remained stable at an average of 9/week. During the period of 18 May through the week ending on 21 June a "white ash plume" rose 50-100 m.

Volcanic hazards VSI has distinguished three types of hazard areas. A danger area that can be struck by lava flows, pyroclastic flows, eruptive lahars, volcanic bombs and exhalation of poisonous gases covers 65000 km2. It includes the low-lying terrains within the caldera and the Banyuputih river valley down to the coast, and has a population of about 12000 (1985). An alert area threatened by ejecta (bombs and air fall) covers an area with a diameter of 8 km around the crater. A second alert area where rain lahar might be expected is defined by river valleys inside the caldera as well as on the

29

northeastern outer flanks. The areas cover 68.5 km2 and host a population of 73000 in about 68 villages (1985). Mild phreatic eruptions in the lake that occasionally occur pose threats within the crater area.

Figure 3.5. Hazard map showing the areas threatened by lahars, pyroclastic flows, lavas and air fall deposits.

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The crater lake The lake (2200 m a.s.l) has a regular oval shape (600 x 1000 m), a surface area of 41 x 106 m2 a volume estimated between 32 and 36 x 106 m3. In 1921 a dam was built by the Dutch to regulate the water level and prevent catastrophic overflows during the rainy season. Originally sluices were used but these constructions are not operational anymore. Similarity between the topographic maps of 1920 (Kemmerling, 1921) and 1994 (VSI) suggests that the morphology of the crater has not changed much in recent history despite the repeated phreatic eruption events. In contrast, the morphology of the crater-lake bottom has undergone significant changes. Depth soundings in 1925 recorded a maximum depth of 198 m at the deepest point, which was then located east of the centre. In 1938 the deepest point had moved westward with the result that the lake was deeper in the centre (~200 m) and in several points in the western half. Recent depth measurements carried out in 1996 (Takano, unpublished data) suggest that maximum depths are slightly less (Fig. 3.6).

Figure 3.6. Depth contours from a survey in 1996 (Takano, unpublished data).

Detailed monitoring by Dutch volcanologists (e.g., Stehn, 1930) revealed a clear relation between rainfall, lake water level and lake temperature. Yearly precipitation in the Ijen area is variable with maximum amounts op to 2.5 meters. There are significant fluctuations between a dry (May-October) and a wet season (November-April) when the lake level may rise by up to 4 meters. Surface lake temperatures are always higher than air temperature and generally decrease in the rainy season. Lake level increases and accompanying temperature drops are also observed after short-term periods of heavy rainfall.

Between 1980 and 1993 temperatures roughly averaged at about 40oC with a maximum of 46 and a minimum of 32oC (see Delmelle, 1995). Earlier records show lower temperatures, e.g., seasonal fluctuations between 20 and 40oC in 1930 (Stehn, 1930). Temperatures of 34-37oC were measured in the dry seasons of 1996-97. In general, temperatures are fairly homogeneous under normal activity and do not

31

change significantly with depth. A transect across the lake made in August 1997 showed temperatures being ~1o lower in the centre than near the shores (Sumarti, 1998). Increases in water level and temperature have been observed during the short-lived periods of higher hydrothermal activity (bubbling and phreatic eruption events, cf., BGVN reports).

Water chemistry The Ijen lake is an extreme example of acid sulfate-chloride type crater-lake waters carrying a high load of dissolved elements. A very low pH value of ~0.2 is accompanied by high loads of TDS (~100 g/kg). The high acidity and high concentrations of SO4, Cl and F can be attributed to magmatic volatiles at the lake bottom.

Repeated sampling in the 1990s (Delmelle and Bernard, 1994; Delmelle et al., 2000; Sumarti, 1998; Sumarti and Van Bergen, unpublished data), in combination with earlier data, showed that bulk compositions have remained fairly constant over the last 60 years (Table 3.1). A survey in 1997 also revealed that concentrations do not vary markedly with depth (Sumarti, 1998; Takano et al., unpublished data). These observations indicate that the lake is well mixed and that most chemistry-controlling processes are generally stable. However, monthly monitoring in 1997-1998 (Sumarti, 1998) showed compositional changes in response to seasonal influences (Fig 3.8). From September 1997 till January 1998 element concentrations remained fairly constant despite a decrease in water level of almost 4 meters. A subsequent increase in water level, apparently induced by rainfall, was accompanied by decreasing concentrations of most elements. An anomalous increase in Cl and F observed in March 1998 was accompanied by a deviating high temperature, which might reflect an increase in magmatic fluid input through the vents that feed the lake.

Figure 3.7. Concentrations of cations in the lake water are close to calculated concentrations (solid dots) assuming isochemical dissolution of 60gr of basaltic andesite in 1 litre of water. (A. Bernard, unpublished).

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Table 3.1 Chemical compositions (mg/kg) of Ijen crater lake, Banyupahit-Banyuputih river and tributaries (Delmelle et al., 2000).

Figure 3.8 Seasonal fluctuations of element concentrations in Kawah Ijen lake (Sumarti, 1998).

Crater lake Banyupahit Banyuputih tributaries near Blawan

Locality upper creek bridge bridge Blawan Asembagus Kalisengon

Elevation (m) 2350 2350 2350 1990 1750 1500 1200 950 160

Date ?-Sep-62 10-Dec-93 10-Aug-96 10-Dec-93 13-Aug-96 14-Dec-93 13-Aug-96 13-Aug-96 13-Aug-96 24-Dec-93 24-Dec-93 24-Dec-93

Label ZEL62-2* IJ-93 IJ-96 BP93-3 BP96-6 BP93-5 BP96-7 BP96-8 BP96-11 BLW-1

T (°C) n.r. 41.6 35.6 25.8 20.1 17.8 21.6 24.4 24.2 24.4 22.1 24.4pH 0.02 0-0.2 0-0.2 0.74 0.5 0.76 1.87 4.29 6.41 7.73 7.86

TDS (g/kg) 91.4 104.1 107.0 111.9 49.9 54.0 53.2 6.4 0.9 0.9 0.3 0.5Na 1100 734 1160 825 589 517 583 150 45 200 16 32K 1051 1306 1473 1419 828 947 859 69 18 46 8.1 15

Mg 699 729 630 821 327 527 359 151 37 74 20 52Ca 888 1197 968 815 696 725 718 351 90 87 42 101Sr n.r. 10.9 6.3 10 6.3 8 6.3 2.3 0.5 n.a. n.a. n.a.Ba n.r. 0.11 0.16 0.13 <0.1 0.05 <0.1 <0.1 <0.1 n.a. n.a. n.a.B n.r. 35.9 53 46.5 24 23.4 24 2.6 0.6 1.6 n.a. n.a.Al 4913 4781 5413 5010 3058 2972 3163 424 29 0.09 0.13 0.29

SiO2 72 180 161 106 124 116 126 137 98 131 70 73Fe 1858 2076 2062 2515 1466 2099 1511 77 7.2 <1 0.08 0.12Mn n.r. 33 40 38 23 24 24 8 1.2 <1 <1 <1Ti 23.4 4.5 6.3 23.2 22 24.9 22 1.3 0.1 n.a. n.a. n.a.V n.r. n.a. 11.1 n.a. 7.7 n.a. 0.7 0.2 <0.1 n.a. n.a. n.a.

Cu n.r. 0.48 1 0.53 0.39 0.73 0.7 0.72 0.01 n.a. n.a. n.a.Zn n.r. n.a. 4.1 n.a. 3.6 n.a. 3.6 0.7 <0.05 n.a. n.a. n.a.Pb n.r. 3.6 2.3 3.6 2.4 3 2.4 <0.5 <0.5 n.a. n.a. n.a.

SO4 60223 70154 71309 74279 32673 34430 35010 3695 442 228 73 166F n.r. 1604 1045 2077 480 764 473 64 8 n.a. <1 <1Cl 20561 21218 22630 23924 9551 10778 10323 1229 145 81 33 71

Fe3+/Fe2+ 0.24 0.22 n.a. 0.27 n.a. 1.36 n.a. n.a. n.a. n.a. n.a. n.a.

.hot spring Kalisat

Jul 96 Sept 97 Oct 97 Nov 97 Dec 97 Jan 97 Feb 98 Mar 98

0.0

1.0

2.0

3.0

4.0

5.0

Con

cent

atio

ns (l

og m

g/l)

Al

S

Cl

F

Fe

MgCa-K-Na

Ti

BSi

0 -20 -220 -285 -380 -405 -320 -145Water level (cm)

Dry season Rainy season

33

Native sulfur, suspended solids and sediments Aggregates of sulfur spherules can occasionally be seen floating on the water surface. Individual globules are up to 5 mm in diameter and are generally hollow. They contain tiny (1-20 micrometer) inclusions of sulfide crystals consisting of:

pyrite - FeS2 bismuthinite - Bi2S3 Sb-rich enargite or luzonite (Cu3AsS4) stannite - Cu2FeSnS4

The native sulfur globules may originate from liquid sulfur pools on the lake bottom and brought to the surface by discharging gases, or they formed in the water by oxidation of H2S-rich gas bubbles.

Native sulfur is a rare component in the lake sediments, which mainly consists of "-cristobalite and less abundant amorphous silica, quartz, gypsum, anatase (or rutile), barite and some pyrite. The same minerals as well as celestite have also been identified as suspended solids in the lake water.

Layers of fine-grained sulfur-rich lake sediments are exposed on the inner crater wall. They contain pyroclastic sulfur, which thought to originate from the ejection of bottom material during explosive phreatic events.

Figure 3.9 Saturation state of the lake with respect to several mineral phases, and saturation of sulfide minerals as a function of water temperature, to illustrate possible conditions in the hydrothermal sub-lake system. Minerals are just saturated when SI (=log (IAP/K) = 0 and undersaturated when SI<0.

Polythionates and the 1993 crisis Total polythionate concentrations (Delmelle and Bernard, 1994; Delmelle et al., 2000; Sumarti, 1998) range between ~400 (1990) and ~1000 (1996). Reported values always show S5O6>S4O6>S6O6, suggesting that SO2/H2S ratios of feeding gases are <1.05 (cf., Takano et al., 1994). In contrast to other chemical parameters, the polythionate contents have fluctuated significantly in the 1990s. Compared to data on a sample of September 1990, concentrations were somewhat higher on 15 May 1993. A further increase was observed on 15 June. This was followed by an increase in seismic activity near the end of that month, which preceded phreatic eruptions on 3, 4 and 7 July. Polythionate concentrations were

34

again higher on 7 July. Seismicity was variable from 8 to 31 July, while lake-water temperature rose from 39 to 40oC. On 1 August two phreatic eruptions occurred. Because of the seismic activity, a warning was issued to the local population, sulfur-mine workers and tourists and the area around the crater was closed. No injuries were reported during any of the eruptions. The seismicity gradually decreased during 2-21 August, as did polythionate concentrations in a sample taken on 31 August. The water temperature through most of August was 39-41°C. These June-July-August events suggest a possible relation between polythionate contents and phreatic explosions for the Ijen system. However, the relations are not entirely clear. Seismic activity had returned to a normal level by 10 September. Still, polythionates showed an increase on 10 and 17 September, accompanied by an increase in temperature (up to 44oC), after which the concentrations dropped to lower values on 11 October. Delmelle et al. (2000) attribute this to an increased discharge rate of magmatic fluids at the lake bottom as a result of the earlier vent-clearing explosions. Maximum polythionate concentrations were observed in a sample of July 1996 (Sumarti, 1998). The levels were found to be lower again in September 1997 after a seismic crisis that occurred in June-July of that year.

The solfatara A strongly active solfatara field is present in the southeastern part of the crater near the lakeshore, where sulfur-mining activities have existed during many decades. The fumarolic gases are led through pipes where native sulfur is deposited upon cooling. The sulfur is collected by local workers and carried out of the crater, after passing a weighing station on the outer flanks.

Gas temperatures fluctuate around 200oC ranging between 169oC in 1993 and 244oC in 1979 (Delmelle et al., 2000). Temperature fluctuations seem to be independent of seasonal changes in rainfall. Also, no significant changes occurred during the period of unrest in July-September 1993. Chemical compositions of samples collected between 1979 and 1996 (Table 3.2) are typical for high-temperature gas from arc volcanoes. In terms of SO2-H2S-H2O equilibria the gas phase is supersaturated with respect to elemental sulfur (Se) despite the fact that surfaces of the pipes are coated with Se. Possibly, hydrolysis of previously deposited Se influences the gas composition.

Compared to earlier data, samples taken in 1996 have lower H2/H2O ratios, higher H2O/St, lower HCl concentrations and higher SO2/H2S ratios. They plot close to the H2/H2O gas buffer suggesting equilibrium in the H2-H2O-H2S-SO2 system near the surface. The differences with previously collected samples may be attributed to an enhanced addition of H2O-rich vapour, adsorption of HCl into groundwater and air oxidation at near-surface levels.

The N2/He ratios are variable. They range between volcanic arc-type magmatic values to lower ratios that could reflect changes in the influx of mantle helium or in the contribution of subducted or assimilated sediments. The 3He/4He ratio of 7.4 Ra falls within the range commonly found in arc volcanoes.

sample date T(oC) H2O CO2 SO2 H2S HCl HF He Ne H2 Ar O2 N2 CH4 CO

M40 ?/07/79 244 764000 225616 2289 9133 n.r. n.r. 1.2 n.r. 22.2 n.r. n.r. n.r. 1.4 33.5M57 ?/07/79 244 843000 146795 2873 7348 n.r. n.r. 1.38 n.r. 21.4 n.r. n.r. n.r. 0.8 20.3IJG1 17/09/93 187 864000 114000 8600 11500 1900 n.a 0.18 n.a. 2.2 1.52 10.4 386 0.1 n.a.IJG2 26/07/95 169 904800 77300 760 16000 4850 9 0.65 0.011 4.5 12.02 23.08 1199 0.1 <0.02IJG3 26/07/95 169 890270 90860 270 15070 5870 9 0.65 0.002 19.8 1.36 3.02 460 0.2 <0.02IJG4 09/08/96 217 968680 27830 1270 3080 560 3.3 0.03 n.a. 0.25 0.03 <1.66 13.7 10.2 0.05IJG5 09/08/96 217 969160 27520 1270 3030 460 3.3 0.03 n.a. 0.26 0.07 <1.71 19.5 7.8 <0.05IJG6 09/08/96 217 966520 29810 1450 3300 450 3.6 0.03 n.a. 0.29 0.03 <1.73 19.8 9.0 <0.05

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Table 3.2 Gas compositions (micromol/mol) of Ijen fumaroles (Delmelle et al., 2000 and references therein)

Spring waters Hot springs occur at several locations near Blawan village. They produce relatively dilute (TDS=1-2 g/kg) and near neutral (pH=~6.4) water at temperatures up to ~50oC. According to SO4-Cl-HCO3 contents the discharges are classified as neutral HCO3-SO4 waters. The springs are sometimes associated with deposits of travertine. Based on SO4-Cl relations, Delmelle et al. (2000) suggested that the Blawan hot springs might represent a mixture of acidic brines, similar to that of the summit hydrothermal system, with HCO3 steam-heated waters produced in the shallow part of the system.

Isotopic compositions

The crater-lake water is strongly enriched in 18O and D relative to local meteoric water (Fig. 3.10). The isotopic compositions plot close to the field of subduction-related volcanic gas (VAG). The isotopic relations suggest that the isotopic signatures reflect the combined effect of mixing of magmatic fluid with meteoric water and kinetic fractionation at the lake surface. The position of the fumarolic gas condensates can be explained by mixing of groundwater with remobilised lake brine or high-temperature volcanic steam. Compositions of the Blawan hot springs are consistent with meteoric water as the principle source.

Figure 3.10 Stable isotope signatures of crater lake water, fumarolic gas, springs and local meteoric waters (Delmelle et al, 2000).

A summary of sulfur isotope compositions of lake water, hot springs, fumaroles and native sulfur is shown in Fig. 3.11. The lake SO4, which has a δ34S value of 22-23 per mil, can be explained by disproportionation of magmatic SO2 at temperatures of 250-280oC. Native sulfur from floating globules, pyroclastic sulfur and sediments have constant depleted δ34S values of -4.1 to -3.2 per mil. The process and timing of Se formation in the lake is difficult to assess. Field observations and chemical modelling (Delmelle and Bernard, 1994) led Delmelle et al. (2000) to suggest that native sulfur is currently not formed in the lake. If there is a Se pool on the lake bottom, it may represent old remobilised sulfur-rich sediment or the disproportionation product of magmatic SO2 at high temperature.

The measured sulfur isotopic compositions of SO2 and H2S in the subaerial fumaroles correspond with a δ34S value for total sulfur of ~0 per mil, which is lower

36

than that of magmatic sulfur in island arcs (~5 per mil). This may reflect remobilisation of Se present in the system below the surface.

Figure 3.11 Variation in sulfur isotope compositions of Ijen lake water, fumarolic gas components, native sulfur and lava. Figure 3.12 Sulfur isotope fractionation between sulfate and native sulfur for Ijen in comparison with other crater lakes.

37

Figure 3.13 Simplified model of the Ijen crater lake (A. Bernard, unpublished). The lake chemistry is determined by dissolution of magmatic volatiles, fluid-rock interaction, evaporation of the lake water, dilution by meteoric water and recycling of lake water through seepage into the subsurface hydrothermal system. The lake acts as chemical condenser for volatiles and as a calorimeter trapping heat supplied by a shallow magmatic reservoir. Magmatic volatiles can be supplied to the crater lake system by direct injection of magmatic vapours (SO2, H2S, HCl and HF) via subaqueous fumaroles or via hot brines entering at the lake bottom.

38

Figure 3.14 Drainage system of the Banyupahit river, tributaries and locations of hot springs in the Ijen caldera.

39

150 860 865 890 970 1290 1625 1790 1870 1970 2030 2050Altitute (m. asl)

Kalisengon Kalisat

-1.0

0.0

1.0

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Conc

entra

tions

(lo

g m

g/l)

AlFe

B

MgK

pH

Ca

S, Cl

discharges of tributaries

Asembagus seepagestreams

The acid Banyupahit-Banyuputih River Between 1850 and 2050 meters a.s.l. several streamlets of seepage water from the Ijen lake form the headwaters of a hyperacid river that runs northward until it reaches the coastal plain near Asembagus after covering a distance of about 40 km. The first stretch within the caldera is called Banyupahit (�bitter water�). Shortly before breaking through the caldera wall near Blawan village, two major tributaries (Kalisat and Kalisengon) and discharges from hot springs change the river�s chemistry and appearance and give it a whitish colour caused by a milky-white precipitate. From there on, the river is called Banyuputih (�white water�).

Near the summit area where the initial discharge rate is about 50 litres/second, the water composition and pH (<0.5) are similar to that of the lake. Water compositions and acidity show a downstream trend with strong jumps of about two pH units at the tributary inlets, largely due to dilution with near neutral water. At an irrigation dam in the coastal plain near Liwung village (Asembagus area), discharge rates have increased about thousand-fold and pH values in the dry season may range between 3 and 4.5. Downstream trends in dissolved elements are shown in Fig. 3.15. Figure 3.15 Downstream trends in pH and concentrations of dissolved elements along the Banyupahit-Banyuputih river in September 1997 (from Sumarti, 1998) Apart from the effects of dilution, element concentrations are regulated by precipitation of saturated mineral phases, adsorption on solids and water-rock interactions. Gypsum can be found in extensive deposits near the seepage points, and also occurs in the first stretch (down to point BYP-7). Yellowish and whitish easily

40

soluble minerals can be observed on the riverbanks and apparently formed as a result of evaporation. They have been identified as several different hydrated aluminium sulfate compounds (Zelenov, 1969; Delmelle and Bernard, 2000):

alunogen - Al2(SO4)3.16H2O pickeringite - MgAl2(SO4)4.22H2O tamarugite - NaAl(SO4)2.6H2O kalinite - KAl(SO4)2.11H2O (possibly a dehydration product of K-alum - KAl(SO4)2.12H2O)

Alteration of streambed rock results in the formation of jarosite (KFe(SO4)2(OH)6) and barite (BaSO4) rich in Sr and Pb. The milky-white suspension that forms at the confluence with the tributaries (BYP3-6) is X-ray amorphous and contains Al, S, O, Fe, K and minor Si (Delmelle and Bernard, 2000). These authors proposed that it consists of a basic aluminium hydroxysulfate associated with minor amounts of silica and a Fe-bearing phase, while Zelenov (1963) suggested the presence of an aluminium hydroxide in the form of bayerite and hydrargillite. Silica and Fe-bearing phases are abundant further downstream near Liwung.

Thermodynamic modelling (Delmelle and Bernard, 2000) points to saturation of amorphous silica, "-crystobalite and barite along the entire length of the river, whereas gypsum and celestite are (near) saturation only up to the junctions with the tributaries. The water is (nearly) saturated with respect to jarosite and jurbanite when approaching the first confluence. Ferric hydroxide, basaluminite, alunite, gibbsite and kaolinite are predicted to become (super)saturated below these points.

It should be noted, however, that concentrations of dissolved elements and saturation patterns are strongly influenced by seasonal variations in rainfall, which induces considerable changes in discharge rates of both the main river and tributaries. Monthly monitoring results (Sumarti, 1998; Sumarti and Van Bergen, unpublished data) show clear concentration decreases after the onset of the rainy season (Fig. 3.15). At a near-source location (e.g., BYP-9), this can be largely attributed to

Jul 96 Sept 97 Oct 97 Nov 97 Dec 97 Jan 98 Feb 98 Mar 98

-1.0

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Con

cent

ratio

n (l

og m

g/l)

Fe

Al

ClSCa

SiK

F

Mg

B

Dry season Rainy season

Figure 3.15 Seasonal fluctuations in element concentrations in the Banyupahit river at BYP-4, after the Kalisat tributary (Sumarti, 1998)

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dilution, whereas strongly enhanced removal of Fe and Al at BYP-4 (after the entrance of Kalisat) apparently results from precipitation of Fe,Al-bearing minerals at increased pH values.

Environmental implications and possible health risks When entering the coastal plain near Asembagus, water of the Banyuputih river is used for irrigation of 3500 hectare of agricultural land where rice and sugar cane are the main crops. The area is inhabited by about 100,000 people. In the dry season all of the water is diverted from the main riverbed. In general, the acidity of the river water near the irrigation dam at Liwung, where the discharge rate is about 4 m3/sec, is similar to that at the waterfall near Blawan, suggesting little dilution along the passage through the caldera wall and outer flanks. pH-values varying between 3 and 4.5 have been measured during several campaigns in the late 1990s, and are probably controlled to a significant extent by discharge rates of the neutral tributaries near Blawan. During increased water discharges and floods in the rainy season the river follows its original bed and reaches the sea. As local reports indicate, fish and other near-shore organisms perish at these occasions. According to 1996 data (Delmelle and Bernard, 2000) about 150 tons of SO4, 2.8 tons of F, 50 tons of Cl, 10 tons of Al, 34 tons of SiO2, 420 kg of Mn, 35 kg of Ti and 4 kg of Cu are discharged daily into the irrigation network. Long-term activity of the lake and river system must have brought millions of tons of heavy metals to the coastal area.

A preliminary survey (Sumarti, 1998) has shown that wells used for drinking water have high contents of fluoride. Tooth problems in local residents are widespread and may signal health effects from exposure to fluoride. Because the river water contains ~10ppm F, it is conceivable that it affects the quality of ground water in the surrounding areas. Other potentially toxic elements that require attention include Al, Mn, As, Cd, Hg, Pb, Se, Cu, and Zn.

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Field stops Stop 1 � Ijen crater lake and solfatara; climbing to the crater rim is relatively easy along a frequently used footpath, passing the weighing station of the sulfur workers. On the rim there is a superb view of the lake. The descent along the inner crater wall follows a steep trail with steps in many places. Depending on the wind direction, the strong action of the odorous sulfur-rich gases can be annoying for breathing. It is recommended to bring some mouth/nose protection and sufficient drinking water. There will be an opportunity to take lake water samples. Stop 2 � The acid Banyupahit river; several locations will be visited along the river within the caldera, including waterfalls with a pH of ~0.5-3. Upstream points are locations with deposition of gypsum and Al-sulfates. Downstream points near Blawan village include the stretch where strong jumps in pH values are caused by the influx of neutral tributaries. If time allows, the coffee plantation and one of the coffee factories will be visited as well. Stop 3 � The acid Banyuputih river and irrigation area in the coastal plain near Asembagus. References and sources used Bemmelen, R.W. (1949) The Geology of Indonesia, General Geology of Indonesia

and adjacent archipelagos, Government Printing office, the Hague. vol. 1A. Bulletin of the Netherlands Indies Vulcanological Survey, Vol. IV, 1936-1938,

Mining and Geological Department Bandung). Bulletin of the Volcanological Survey of Indonesia, edition 100, Republik Indonesia,

Departemen Perindustrian Dasar/Pertambangan, Djawatan Geologi-Bandung. Delmelle, P. (1995) Geochemical, isotopic and heat budget study of two volcano-

hosted hydrothermal system: the acid crater lake of Kawah Ijen, Indonesia, and Taal, Philippines, volcanoes, unpublished, Ph.D. thesis, Universite Libre de Bruxelles.

Delmelle, P. and Bernard, A. (1995) Geochemistry, mineralogy and chemical modelling of the acid crater lake of Kawah Ijen volcano, Indonesia, Geochim. Cosmochim. Acta, Res. 58: 2445-2460.

Delmelle, P. and Bernard, A. (2000) Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia. J. Volcanol. Geotherm. Res., 97: 55-75

Delmelle, P., Bernard, A., Kusakabe., Fischer, T. and Takano, B. (2000) Geochemistry of the magmatic-hydrothermal system of Kawah Ijen volcano, East Java, Indonesia. J. Volcanol. Geotherm. Res., 97: 31-53.

Kaswanda, Wikartadioura, Djuhara, Martono and Sumpena (1993) Volcanic Hazard Map of Ijen Volcano, East Java scale 1:100,000. Volcanological Survey of Indonesia.

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Kemmerling, G.L.L and Woudstra, H.W. (1921) De geologie en geomorphologie van den Idjen en analyse van merkwaardige watersoorten op het Idjen hoogland, Uitgegeven door de Koninklijke Natuurkundige Vereniging bij G. Kolff and Co., Batavia-Weltevreden, 162 pp. (in Dutch)

Kusumadinata, K., Hadian, R., Hamidi, S., and Reksowirogo, L.D. (1979) Data dasar gunungapi Indonesia, Direktorat Vulkanologi, Direktorat Jenderal Pertambangan Umum, Departemen

Newhall, Ch., and Dzurizin, D. (1988) Historical unrest at large calderas of the world, U.S. Geological Survey Bulletin 1855, vol. 1, 598 pp.

Simkin, T., Siebert, L., Bridege, D., Newhall, Ch., and Latter J.H. (1981) Volcanoes of the world, Hutchinson Ross Publishing Company, Stroudsburg, Pennsylvania, 232 pp.

Sitorus, K. (1990) Volcanic stratigraphy and geochemistry of the Idjen caldera complex, east Java, Indonesia, unpublished, Master thesis, Victoria Univ. of Wellington, New Zealand, 148 pp.

Stehn, Ch.E. (1928) Volcanic phenomena during the months of May 1928, Bull. Netherl. Ind. Vulc. Surv. No. 7: 15-19.

Stehn, Ch.E. (1930) Volcanic phenomena during the months of May and June 1930, Bull. Netherl. Ind. Vulc. Surv. P. 15-19.

Stehn, Ch.E. (1931) Volcanic phenomena during the months of March and April 1930, Bull. Netherl. Ind. Vulc. Surv. P. 50-62.

Stehn, Ch.E. (1936) Volcanic phenomena during the months of November and December 1936, Bull. Netherl. Ind. Vulc. Surv. Vol. 78: 37-52.

Sumarti, S. (1998) Volcanic pollutants in hyperacid river water discharged from Ijen crater lake, East Java, Indonesia. Unpublished MSc thesis Utrecht University, 77pp.

Sumaryo E., Suryo, I., Magdalena, M., Mulyati, B., Purwoto, Yance, W. B. (1995) Laporan pengamatan dan pendataan kependudukan Kawah Ijen 3-17 Mei 1994, Direktorat Vulkanologi, unpublished.

Suratman (1993) Laporan kegiatan Kawah Ijen 3 Juli 1993, Direktorat Vulkanologi, unpublished.

Van Padang, M.N. (1938) Volcanic phenomena during the months of May and June 1938, Bull. Netherl. Ind. Vulc. Surv. Vol. 84: 99-109.

Wikartadipura, S. (1971) Laporan Pemeriksaan Daerah bahaya/ Waspada Sementara Kawah Idjen pada Lereng Utara-Timur, Volcanological Survey of Indonesia, Energy and Mining Department, Indonesia (unpublished).

Zelenov, K. K. (1969) Aluminium and titanium Kava Ijen volcano crater lake, International geological review, Res. 11: 84-93.