The catastrophic release of gas from Lake Nyos, Cameroon, in 1986 caused substantial but incomplete mixing of the stratified water column. The post-release evolution of water-column structure has been monitored through April 1992. Changes began immediately after the event as rainfall and inflow brought dilute fluid into the surface layer. Inflow and surface mixing have gradually deepened the chemocline. The Total Dissolved Solids (TDS) values in the upper 40 m of the water column have dropped from a few hundred mg/kg just after the release to <100 mg/kg. The chemocline is presently strongest at 50 m depth; 5 m below this, the TDS = 570 mg/kg. From 55 to 150 m depth is a gentle gradient in which TDS reaches 920 mg/kg. Little change in water-column chemistry has occurred in this depth interval since the release. Between 150 m depth and lake bottom at 210 m depth, a strong secondary chemocline has formed. Temperature, CO₂ concentration ([CO₂]), and TDS have all increased in the deepest layer in response to recharge by warm, mineralized water, reaching values of 25.0°C, 320 mmol/kg, and 1800 mg/kg, respectively, 1 m above lake bottom. Considering all these changes in part as a “recovery” process, it is possible to construct a model of the pre-release water column. The data indicate that the pre-release chemocline was at least 50 m deep. Above the chemocline was a dilute layer containing a seasonal thermocline; below the chemocline was probably a gradient zone(s) with correlated increases in TDS and [CO₂] and a secondary chemocline near lake bottom. Maximum values of TDS and [CO₂] calculated for pre-release bottom water are 2400 mg/kg and 430 mmol/kg, respectively, based on tritium data. From this pre-release structure, a model of the gas release is proposed that is consistent with available chemical and observational data. An important feature of the model is that disruption of the pre-existing stratification was much more extensive than previously proposed, and even the deepest water layers were involved in the event. This model is not intended to limit possible gas release mechanisms, and thus complete re-establishment of pre-1986 water-column conditions is not a prerequisite for a future release. Spontaneous instability could occur at lake bottom in <20 yr if dissolved gas pressures continue to increase in this zone by 0.5–1 bar/yr as they have for the last 6 yr.

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Lake Nyos released, a toxic aerosol of water and cabon dioxide, which killed an estimated 1700 people, in 1986. Since the amount of gas released represented only a small proportion of the gas which was dissolved in the lake and since the lake is currently being recharged with gas at an alarming rate, the only reliable way to avoid another disaster is to remove some of the gas. However, before this is attempted it is important to try to understand what triggered the ‘overturn’ which released gas in 1986, if we fail to do this we might find that the very process of removing gas could trigger the disaster which it is designed to prevent. There is general agreement that the best way to remove gas from Lake Nyos would be to install a pipe (or a number of pipes) with one end close to the lake bed. If gas-rich bottom water is then drawn up the pipe, by pumping water from the top, natural ‘gas-lift’ will be induced which will maintain flow within the pipe. And the gas which is released will cool the water as it rises. The practical problems are thus not ones of maintaining the flow of water within the pipe but of controlling the flow and disposing of the cold, partially degassed, water. If the water is returned onto the surface of the lake, as has been suggested, it will sink and could destabilise the lake. If it is discharged outside the lake basin, as has also been suggested, the amount of cold stream water retained by the lake may be increased and this too could destabilise the lake. Therefore it is proposed that the water should be discharged into a storage reservoir then reinjected at its level of neutral buoyancy. Once the amount of gas in the lake had been reduced to a safe level, then a pipe could be installed with one end near the lake bed and the other through the narrow natural dam in the north-west corner of the lake. Water flowing into the lake would continuously displace bottom water and any future buildup of gas in the deep water would thus be avoided.

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Investigations have been carried out on crater lakes in areas of recent volcanism in the Azores and in Italy, with the aim of detecting possible evidence of residual anomalies associated with past volcanic activities; data from crater lakes of Cameroon have been considered for comparison. Among the physical-chemical characters taken into account, the increases of temperature, ammonium and dissolved carbon dioxide with depth are interpreted as providing information about the contribution of endogene fluids to the lake water budgets. The greater extent of such evidence at Lakes Monoun and Nyos (Cameroon) appears associated with the disasters that occurred there during the last decade; some similarities observed at Lake Albano (Italy) suggest a potential instability also for this crater lake.

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The first of a series of water-borne surveys of bathymetry, water column temperature and water column chemistry of Ruapehu Crater Lake was undertaken in February of 1991 to ascertain the behaviour of the lake during quiescent periods. Results show the present maximum depth of the lake to be 134 m, and the central vent to be overlain by a large pool of molten sulfur (>50 m in diameter and ≥6 m deep). Maximum recorded temperature in the sulfur was 177°C. The lake showed little stratification with respect to volatile constituents, indicating that the vent was in effect totally sealed with respect to the transfer of magmatic gases during the 3 month quiescent period prior to sampling. Minor thermal inversions characterised the upper 60 m of the water column in several locations, whereas water column chemical analyses showed this interval to be largely isochemical. A small but abrupt increase in TDS below 60 m depth and a thermal gradient of 0.2°C/m between 60 and 130 m suggest that the lake was thermally convecting over this interval. This convection is thought to be driven by thermal inputs from the underlying sulfur, including conductive heat transfer and discharge of recirculating lake water from the upper portion of the vent. The results suggest that a gas buildup in the water column is unlikely to form in the present state of the vent-lake system, and therefore that Lake Nyos-type gas release eruptions are unlikely to occur.

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Crater lakes with active subaqueous fumaroles often contain molten sulfur pools on the lake floor. Volcanic gases passing through the sulfur pools carry hollow spherules of solidified molten sulfur to the surface of crater lakes. This sulfur dissolves SO₂ and H₂S gases and releases these gases into the water. The sulfur also contains homocyclic sulfur (cycl. S_x, x = 6–16) and probably sulfane monosulfonates. The concentration of cyclic S₇ increases with increasing temperature between 120 and 175°C, which is useful to estimate the temperatures of subaqueous molten sulfur pools. The gases drastically lower viscosity of the molten sulfur. This may be due to blockage of growing long-chain sulfur molecules by the dissolved gases. Thus a jump in viscosity at 159°C observed for pure sulfur is not likely to be present in subaqueous molten sulfur at crater lakes. Based on the chemistry and morphology of sulfur slicks, activity of subaqueous fumaroles can be divided into four stages (I–IV), each of which may serve for qualitative in situ monitoring of crater lakes. At Stage I, no molten sulfur pools exist on the lake floor and fumaroles discharge low temperature gases (<119°C) containing only traces of SO₂; at Stage II, subaqueous molten sulfur pools (119°C < T < 150°C) are formed, releasing yellow hollow spherules of sulfur with no tails; at Stage III, the fumarolic temperature increases to >150°C, resulting in an increase in molten sulfur viscosity; and at Stage IV, frequent phreatic or geyser-like eruptions are observed. The molten sulfur pools are dispersed into pieces on the lake floor at this stage.

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Significant changes in temperature and water chemistry were observed in Lake Yugama, a crater lake at Kusatsu-Shirane volcano, Japan, between 1988 and 1993. Heat, water and chloride budgets were evaluated with a box model. Until 1989, heat and chloride inputs into the lake were 3∼10 MW and 1∼3.5 tons/day, respectively; but these increased to 19–25 MW and 4–12 tons/day by 1990. The strong correlation between the Cl^– concentration and lake water pH suggests input of a HCl solution or HCl vapor as the cause of the Cl^– change. These changes coupled with the high level of seismicity observed around the volcano in 1990 seem to be consistent with the formation of fractures in the surrounding area of a cooling magma.

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Kelut volcano has a notorious history of hazardous eruptions, five this century, the last in 1990. The hazards, generally lahar-related, are generated from the crater lake at its summit. Geophysical and geochemical monitoring of the volcano and lake on a continuous basis is ensured by the VSI. The results of monitoring of geochemical parameters including temperature, pH and the concentrations of sulphate, boron, magnesium, chloride and Mg/Cl ratio of the crater lake showed a significant increase about 4 months prior to the 1990 eruption, reflecting an increase in activity of the volcano.

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Three crater lakes occur on top of the dormant stratovolcano Keli Mutu in Flores (Indonesia). The lakes contain cool (20–30°C) acid-sulfate-chloride brines and have high TDS with relatively high Zn and Pb contents. Two lakes show plume-like upwellings, probably fed by subaqueous fumaroles. A river on the mountain slope contains fluids that escaped from the lakes through seepage. The fluid compositions have varied strongly over time for S and the halogens (the volcanic-gas derived elements), whereas many of the major cations (rock-derived elements) show a net increase in the lakes over this century. Speciation and mineral saturation modeling, bulk sediment analyses, and micro-probe sediment analyses indicate that the lakes are reducing at depth, and all three lakes are close to saturation with gypsum/anhydrite. One lake is also saturated with Fe-oxides, while the other two are thought to be saturated with a suite of Cu and As minerals. The most acid lake is also saturated with native sulfur and possibly pyrite. Mass balance estimates based on fresh volcanic rocks and ashes, the dissolved element load and the lake sediment chemistry strongly suggest that the lakes are partly fed by underlying geothermal circulation systems. The lakes are thus not closed reaction vessels with dissolution of local rock debris by fluids acidified by volcanic gases, but they draw a rock-derived element flux from deeper levels. Mass transfer is large in these dynamic systems, including the transport of tonnes of ore metals over a period of centuries.

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Oxygen, hydrogen, and sulfur isotope data for fluids and minerals associated with the crater lake of Poás Volcano, Costa Rica, are interpreted in the context of the chemical and hydrologic structure of the volcano. Oxygen and hydrogen isotope data were obtained for rain, spring, and river water, low-temperature fumarole condensates, and acid brines collected from the hot crater lake before its disappearance in April 1989. Flank river and spring waters whose solute compositions have been modified by volcanic and hydrothermal activity have, with one exception, isotopic compositions similar to local meteoric water. Acid chloride-sulfate brines of the summit crater lake are extremely enriched in ¹⁸O with respect to local meteoric water; in the most enriched brines ¹⁸O shifts are greater than 20‰. The ¹⁸O shift is related to a kinetic isotope effect associated with the intense evaporation at the surface of the lake. These same brines exhibit only minimal shifts in their D/H ratios. The apparent lack of deuterium fractionation in the brines is attributed to an increase in the flux of isotopically light steam into the crater lake and/or a decrease in the deuterium fractionation factor for evaporation that occurs at the surface of the lake. The decrease in deuterium fractionation is correlated with large increases in lake-brine acidity and dissolved solids concentration that preceded the disappearance of the lake. Sulfur isotope data are presented for H₂S and SO₂ gas collected from low temperature fumaroles; dissolved sulfate in spring, river, and crater lake waters; and native sulfur and gypsum found in the acid lake and active crater area. Δ_SO2-H2S for low temperature gases is approximately 24‰ indicating an equilibration temperature of 165°C. Δ_SO2-H2S for low temperature H₂S and lake brine sulfate is approximately 23‰, all indicating subsurface equilibration occurred at 265°C. The H₂S and native sulfur are both highly depleted in ³⁴S (δ³⁴S = –8 to –11‰). δ³⁴S values of ³⁴S-depleted H₂S and ³⁴S-enriched sulfate in lake brine are produced by disproportionation of SO₂ released by the shallow magma body. Native sulfur is formed by the oxidation of ³⁴S-depleted H₂S by non-sulfur-bearing oxidants such as atmospheric oxygen and ferric iron. Mass-balance calculations indicate that sulfitolysis of polythionic acids could also result in the deposition of significant quantities of native sulfur. Implications of the isotopic composition of present-day fluids observed at Poás Volcano with respect to the isotope systematics of acid-sulfate ore deposits are considered.

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Ten core samples and 78 dredge samples from Lake Mashu, a remote oligotrophic caldera lake in Hokkaido, Japan, were analyzed to reconstruct the history of hot spring activities on the lake bottom. The sedimentation rate was estimated using common tephra layers found both in Lake Mashu and in adjacent Lake Kussharo. From the depositional age of the upper common tephra (250 B.P.) obtained by ²¹⁰Pb dating of the Lake Kussharo core, the sedimentation rate of Lake Mashu was calculated to range 6.4 to 7.4 mg/cm² y. Iron and Mn that originated from hot springs on the lake bottom are oxidized in the oxic lake water to form Fe and Mn-rich layers. Iron is accumulated in the central parts of basins, whereas Mn is accumulated in relatively peripheral areas. The horizontal separation of these elements in the sediment is attributed to the difference in oxidation rate. Manganese content in pore water increases with depth at a site of Mn accumulation. This indicates that Mn is mobilized and deposited forming Mn concentrated layer just above Fe concentrated layer. X-Ray photoelectron spectroscopic examination revealed that Fe was in the form of ferric oxyhydroxide, and that Fe and Mn were concentrated on the surface of sediment particles. Annual depositional rates of Fe and Mn correspond approximately to the reported rates of supply from lake-bottom springs. Therefore Fe and Mn contents in the sediment indicate the variations in hot spring activity at the lake bottom. This activity has fluctuated in intensity over the last 700 years, but now appears to be diminishing.

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