British Geological Survey Overseas Geology Series

Technical Report No. WC/93/21

ODA/BGS RESEARCH AND DEVELOPMENT PROGRAMME

GAS GEOCHEMISTRY STUDIES AT POAS VOLCANO, COSTA RICA

MARCH 1992 AND JANUARY 1993 (PROJECT NO. 91/17)

R A NICHOLSON, M F HOWELLS P J BAXTER, S L CLEGG and J BARQUERO

This report was prepared for the Overseas Development Administration

Bibliographic Reference

R A Nicholson, M F Howells, P J Baxter, S L Clegg and J Barquero 1993. Gas Geochemistry at Poas Volcano, Costa Rica. March 1992 and January 1993.

Front Cover

The remnants of Poas crater lake, January 1993

ONERC 1993 Keyworth, Nottingham, British Geological Survey 1993

CONTENTS

1. SYNOPSIS

2. INTRODUCTION

3. FIELD STUDIES, GAS MONITORING AND SAMPLE COLLECTION

3.1 General Considerations

3.2 March 1992

3.3 January 1993

4. RESULrC AND DISCUSSION

4.1 Water Chemistry

4.1.1 Crater lake

4.1.2 Rainwaters

4.1.3 River and Drinking Waters

4.2 Gas Geochemistry

4.2.1 Fumarole Gases

4.2.2 Condensate from the volcanic plume

4.2.3 Diffusion Samplers

4.2.4 Impregnated Papers

4.2.5 Vegetation

5. CONCLUSIONS AND RECOMMENDATIONS

6. ACKNOWLEDGEMENTS

7. REFERENCES

GAS GEOCHEMISTRY STUDIES AT POAS VOLCANO, COSTA RICA

MARCH 1992 AND JANUARY 1993

R A Nicholson, M F Howells, P J Baxter", S L Clegg** and J Barquero***

1. SYNOPSIS

Volcanoes and earthquakes occur in many parts of the world. Primary phenomena are potentially catastrophic, whilst those of a secondary nature may be merely considered as background activity, or of sufficient intensity to cause long-term suffering to indigenous populations. The latter is the case at Poas volcano in Costa Rica, where increasing activity during the last decade has precipitated a change in the quiescent state of the volcano. Acid gases are apparently now being emitted more intensely than before, to the detriment of the health of people and domestic livestock living in the surrounding countryside; damage to crops and farm buildings is also comparatively serious.

A pilot study has been undertaken to determine the principal constituents of the gas emissions, the form in which they are most likely to be transported and the extent of the area within which they can be easily measured. In addition, the impact of the acid components, both gas and liquid (from the crater lake), on the composition of natural waters has also been considered.

The results obtained indicate that combined emissions of hydrochloric, hydrofluoric and sulphuric acids all contribute to the problems encountered, but seasonal changes and distance from the source also exert control. The acids appear to be variously transported as free acids, complexed species and aerosols, but the rate of transfer is heavily dependent on the prevailing atmospheric conditions. Rainfall effectively scrubs the acid gases during the wet season, thereby considerably ameliorating the immediate effects of the emissions. Continued longer-term monitoring is necessary to establish baseline information and recommendations for this are made.

* Dr P J Baxter, Coirsultant Occupatiorral Physician

Dept of Cornrn unity Medicine, Feiiirer 's, Gresham Rd, Cam bridge, CBI 2ES

Dr S L Clegg, Dept ofAtmospheric Chemistry

School of Environrn ental Sciences, University of East Aiiglia, Norwich, NR4 7TJ

Observatorio Vulcarroldgico y Sisrnoldgico de Costa Rica (OVSICORI- UNA)

Universidad Nacwrral, Apdo. 86 -3000, Heredia, Costa Rica

**

***

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2. INTRODUCTION

This work was carried out under Project No. 91/17 (Environmental aspects of radon and other gases in geothermal areas) as part of the ODA/BGS Research and Development Programme forming part of the UK Government's Aid Programme to developing countries.

The importance of geothermal resources as a relatively inexpensive source of energy has long been recognised. Project No. 87/15 was established to investigate the application of soil gas geochemistry techniques to the exploration for geothermal power sources in developing countries, and work undertaken in Costa Rica was reported by Ball (1988). Exploitation of such resources is however costly in the first instance, and therefore needs to be carefully assessed in terms of source quality. Geothermal steam contains many impurities such as carbon dioxide, radon and acid gases, some of which are potentially injurious to health, whilst others can cause corrosion in power plant components. Radon is known to cause lung cancer, and its association with high temperature steam provenances has been investigated in Kenya, Costa Rica and the Philippines, under the prcseiit projcct. The project was then extended to consider other harmful volcanic gas emissions and their epidemiological effects, in collaboration with a coiisultant occupational health physician based at the University of Cambridge. After discussion with geophysicists at the Open University, who had experience of working on Poas volcano for many years, it was decided to concentrate research at this particular location in Costa Rim, where a subtle change in activity of a dormant volcano is presently causing health problems and financial difficulties (through the direct effects of the volcanic gases on crops and domestic livestock) for local communities.

Poas is a composite stratovolcano rising 1400m above its surroundings in the populated Cordillera Central of Costa Rica. The last major eruptions occurred in 1952/53, when phreatic activity resulted in the loss of the crater lake, and culminated in the growth of a 45m high pyroclastic cone, which subsequently became the focus of continuous fumarolic activity. A new lake formed in 1967 and maintained a depth of about SOm, circulating mildly acidic brines at about 40°C. Apart from minor fluctuations in level and temperature, and a brief period of geyser activity during 1978/79, the system appeared stable until early 1986 when a long-term decrease in the lake level commenced. Intense geysering again occurred from June 1986 to July 1987, and these events heralded the start of a new magmatic cycle, which has been studied by a variety of geophysical and geochemical techniques, such as those of Brown et al. (1989), Barquero and Feriifindez (1990) and Rowe et al. (1992).

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However, the environmental hazards posed by the escape of high temperature volcanic gases and aerosols are only now beginning to be appreciated.

A preliminary visit to Poas volcano by BGS staff was undertaken in November 1991, after discussions with geophysicists from the Open University and a consultant epidemiologist from the University of Cambridge. During this visit, useful contacts were also established with local scientists (OVSICORI-UNA) responsible for the constant monitoring of Costa Rican volcanoes. Rain adversely affected the sampling programme, but nevertheless sufficient data were gathered, and the results obtained from both field and laboratory studies were reported by Nicholson et al. (1992). This earlier report contains some information not repeated here, and reference to the earlier text should be made if improved clarification of certain aspects is required.

Since completion of the 1992 report, two further periods of fieldwork have taken place. These occurred in March 1992 and January 1993, and were designed to monitor the changing characteristics of the volcano and its environs, particularly the gas plume and water catchments, under dry-season conditions. The application of statistical techniques by the School of Environmental Sciences at the University of East Anglia, to some of the crater lake data acquired during the preliminary field excursion, encouraged us to examine the possibility of coiidciising the aerosols formed in the gas cloud above the lake. Therefore during January 1993, condensate in the volcanic plume was sampled using a device suspended from a wire stretched across the crater above the lake.

The gradual decrease in crater lake water level over the last few years at last ensured that it was possible to collect condensate samples relatively safely, during the January 1993 visit, from one of the previously-submerged fumaroles. However, constant blockage of the sampling tube occurred, due to spattering by mud boiling inside the hardened mud-dome containing the open vent.

3. FIELD STUDIES, GAS MONITORING AND SAMPLE COLLECTION

3.1 General Considerations

The preliminary visit in November 1991 took place at the end of the wet season in Central America, and some of the proposed sampling activities were curtailed because of heavy rain, Nicholson et al. (1992). Poas volcano comprises few andesitic lavas separated by layers of unconsolidated materials, and during periods of exceptional rainfall some areas become liable to spontaneous collapse. Parts of

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the crater area were therefore considered inaccessible for safety reasons, and it was decided to repeat the exercise when it could reasonably be expected that somewhat drier conditions would be encountered. This would also allow direct comparison of data for the chemical composition of both gas and water samples between wet and dry seasons.

3.2 March 1992

There was a marked difference between the conditions encountered on this occasion and those experienced during the earlier fieldwork. Little or no rain fell throughout the entire visit, only light precipitation from persistent afternoon cloud-cover caused any problems. However, acid fallout from the volcanic plume appeared to be more intense in the drier air, and brief interviews with local people confirmed this as typical of the dry seasonal effects of the volcano. In populated areas downwind, several kilometers from the summit, there was a definite sulphurous smell in the air, although the highest concentrations of sulphur dioxide measured, using an ME1 personal toxic gas monitor, did not exceed 0.3 - OSppm over periods of a few minutes (STEL - Short Term Exposure Limit 2.Oppni). Data subsequently obtained from long-term (600 hours exposure) diffusive gas samplers showed comparable values (see below).

Colleagues from OVSICORI-UNA were not available to accompany us to the crater immediately following our arrival due to commitments elsewhere, and therefore a preliminary visit to the volcano entailed a brief study of conditions on the upper rim only. On this occasion the wind was gusting to 8.0m/s from the NNE, but locally within the caldera the direction was variable. Sulphur dioxide was measured using the toxic gas monitor, the highest temporal concentration recorded being 34.5ppm, which necessitated the wearing of breathing masks for short periods. Stinging sensations, caused by acid precipitation on exposed skin, were pronounced. This was subsequently confirmed by collecting the acid- fallout on impregnated filter papers, and measuring the sulphate, chloride and fluoride contents. One of the OVSICORI-UNA rainfall sampling points is located on this rim (Borde Este).

The exposed western flank, approximately lkm downwind, is completely denuded of vegetation on the inner-facing slopes, and only supports low scrub elsewhere. This flank is also one of the monitoring points for rainfall (Pelon). Wind direction here was again variable, apparently blowing from all points of the compass within the space of a few minutes, but wind speeds were very low at zero to 4.0m/s. Low cloud

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within the crater obscured the view, so that it was impossible to know when the volcanic plume was drifting towards the sampling area, but values of sulphur dioxide measured with the toxic gas monitor did not exceed 0.2ppm at any one time. However, Draeger short-term diffusion tubes emplaced above the open ground showed a total flux of 10ppm sulphur dioxide over a four-hour period, i.e. 2.5ppm per hour. There was no indication of the presence of either hydrogen chloride or hydrogen sulphide at this location, and it is assumed that both were below the detection limit of the diffusion tube method.

Similar studies carried out at the Mirador public viewing platform, which is much closer to the source but at right angles to the prevailing wind direction, gave zero concentrations for all three acid gases, even after five hours of exposure. The obvious signs of corrosion of metal objects in this area, such as the remains of the corrugated shelter previously used to house gas sampling equipment, indicates that this situation is not normal, and it is known that localised meteorological conditions drastically affect the circulation of air masses in the region of the crater. The Mirador viewing station is another collection point for rainwater.

On the volcanic dome in the bottom of the main crater, values of 2.5ppm for hydrogen chloride and 5.Oppni for sulphur dioxide were obtained with short-term diffusion tubes. There was no indication of hydrogen sulphide. During sampling of the low-temperature (SSOC) dome fumaroles, using the evacuated Giggenbach vessels, the atmospheric sulphur dioxide concentration frequently exceeded 20ppm, and occasionally reached 50ppm. The maximum value determined was 7lppm, which is three times higher than that previously recorded in November 1991. Within the crater the wind direction was highly variable, and although close to the source the gases, it was often possible to work for prolonged periods without a breathing mask. It was not possible to approach any of the high-temperature fumaroles venting through the remnants of the crater lake, due principally to the presence of acid waters and thixotropic sediments. However, it was possible for staff from

of

OVSICORI-UNA, who are familiar with the inherent dangers of the area, to collect lake water samples. These samples have been routinely collected on approximately a monthly basis as part of the continuing geochemical monitoring of the volcano.

Samples of acid-fallout within the crater were collected on 125mm filter papers impregnated with sodium formate and glycerol, and placed flat on a polyethylene sheet for one hour, and also by pumping 1 litre of air through 25mm filter papers contained in a filter holder. This latter experiment was repeated at approximately

6

quarter-hourly intervals over a period of four hours, at a point on the crater rim diametrically opposite to the Borde Este, during which time the atmospheric sulphur dioxide concentration varied between zero and 3.0ppm. Wind speed was relatively gentle and direction highly variable, there was also persistent light rainfall.

To confirm the earlier data obtained on acid precipitation at locations around the volcano, long-term diffusion tubes were again emplaced at sites near the summit, and also at distances of up to seven kilometers on the downwind side. These remained in place for almost one month, and were subsequently retrieved by OVSICORI-UNA staff and returned to the UK for analysis in the Department of Trade and Industry's Warren Spring Laboratory.

A few samples of decaying tree vegetation were collected from open land on a dairy farm near the summit. This farm frequently receives acid rain as wind direction varies, and was the location at which the highest concentration of sulphur dioxide had previously been measured using the diffusion tube technique.

Natural water samples wcre collectcd from a varicty of provenances around the volcano, to determine the distribution of normal lcvels of cations and anions; sources included piped drinking waters and rivcrs. All the water samples remained unfiltered. Waters had previously only been collected from rivers known, or believed, to drain the summit crater region. By sampling more widely it was hoped to obtain a better overall understanding of the chemistry of the main catchments involved. The Rio Gata was blue when the sample was collected, but the reason is unknown as subsequent analysis was equivocal.

3.3 January 1993

Thermodynamic calculations in the School of Environmental Sciences at the University of East Anglia (UEA), on the chemical composition of the crater lake waters, showed that it was important to measure the composition of the aerosol droplets in the vapour cloud above the lake. A device to sample these droplets was designed and built in the BGS workshops (Platel). The framework was fabricated from 316 stainless-steel and PVC, as it was thought that these materials would ensure rigidity combined with the inertness of the plastic components necessary to prevent sample contamination.

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During the actual sampling, a 0.5km long by 1.6mm diameter stainless-steel cable was suspended between two sides of an upper ledge above the crater lake, across that part of the crater above the main area of fumarolic activity. The cone of the sampler was filled with crushed ice (transported to the crater in a vacuum flask), and the device deployed along the wire into the vapour cloud. Acid droplets condensed on the outside of the cold PVC cone and were collected in a polyethylene bottle; three samples of condensate were obtained for analysis. The temperature of the outside of the cone was probably just above freezing point, and therefore ideal for condensing the volcanic vapours; the ice/water mix remained cold sufficiently long enough to allow sampling for at least one hour on each occasion. Attempts to improve the rate of collection, through lowering the temperature still further by the addition of sodium chloride to the crushed ice, failed due to the build-up of a solid deposit of frost on the outside surface of the cone. The sampler was recovered along the suspended wire by the use of a reel containing a fifty pound breaking strain nylon fishing line. The strong winds experienced whilst sampling occasionally caused problems during retrieval due to the nylon line wrapping itself around the stainless-steel wire and ultimately breaking. It was then necessary to climb down into the bottom of the crater near the lake and lower the sampler until it could removed from the stainless-steel wire by hand.

Water samples were collected from many of the sites sampled in March 1992, in order to confirm the earlier analytical data, and note any major variations. The names of some of the previously unknown rivers and their tributaries were also substantiated. The Rio Gata again appeared blue (Plate 2), but no explanation for this phenomenon was offered by local counterparts, as there was apparently no known upstream source of contamination, other than the volcano. A series of rainwater samples, collected on a regular basis during 1992, from the Mirador (Public Viewpoint), Edificio (Visitor Centre), Botos (Extinct Crater), Borde Este (East Side of Upper Rim), Pelon (Western Flank) and Potrero (Near Entrance Gate), were sub-sampled for confirmatory analyses in the BGS laboratories. Similarly, samples of water from the crater lake collected between February 1992 and January 1993 were also returned to the BGS laboratories. Sample locations are indicated in Figure 1. One of the samples of lake water collected during this fieldwork was preserved with sodium tetrachloromercurate, to stabilise the dissolved sulphur dioxide as the disulphitomercurate complex, for subsequent spectrophotometric determination by the method of West and Gaeke (1956). All the water samples remained unfiltered.

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As the crater lake water level has diminished over the past few years, it has gradually become easier to approach certain parts of the area around the lake with care, indeed OVSICORI-UNA staff have been undertaking sampling of the crater lake for many years on a routine basis. It was therefore encouraging to discover that it was at last possible to approach one of the higher temperature fumaroles (117oC) relatively safely to sample the condensate for gas composition (Plate 3). However, only one sample was retrieved due to persistent blocking of the quartz sampling tube by mud boiling within the hardened mud-dome capping the volcanic conduit. Pressure within the mud-dome changed constantly during the sampling, and it was not possible to determine at this stage whether or not the sample had become contaminated by air.

Samples of acid-fallout, on the crater rim at the Borde Este rainwater sampling site, were collected on 125mm filter papers impregnated with (a) sodium formate and glycerol and (b) potassium hydroxide and glycerol. These papers were placed flat on a polyethylene sheet for one hour. Similar experiments were carried out by pumping 1 litre of air through much smaller 25mm filter papers contained in a filter holder. During the sampling period the atmospheric sulphur dioxide concentration varied between zero and 4.Oppni, indicating the variability of the wind direction about the point of sampling.

4. RESULTS AND DISCUSSION

Major and trace elements in the water samples were determined by inductively coupled plasma optical emission spectrometry, and the anions by a combination of ion chromatography, flow injection analysis, selective ion electrode and titrimetric techniques. All water data are shown in Tables 1 - 4. The major components of the gas confined above the condensate sample were determined by gas chromatography, and the anions by titrimetry and selective ion electrode. Anions absorbed on impregnated papers were dissolved using selective reagents and measured by spectrophotometry and selective ion electrode.

4.1 Water Chemistry

The pH of the lake water samples was not determined, as the strength of the acid had undoubtedly increased significantly with the fall in lake level. Previous investigations had shown that acid concentrations were generally high, with negative pH values (pH t - O S ) , knowledge of which are important for ion speciation studies.

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Only three of the natural water samples indicated any tendency towards acidity, the Rio Agrio and Rio Desague having been noted previously, but the pH of 5.86 recorded on the Rio Gata perhaps indicated an additional escape route for summit drainage.

4.1.1 Crater Lake

Preliminary thermodynamic calculations of the state of Poas crater lake only considered those species with concentrations >O. lmol dm-3. Hydrogen ion concentration was obtained from the charge balance, and in the absence of any very recent data a temperature of 85OC was assumed. All fluoride not complexed with A13+ was presumed to be present as free hydrofluoric acid (HF), which is a logical assumption in such highly acidic waters.

The Pitzer (1973) thermodynamic model was used to determine speciation and ionic activities in the crater lake samples collected during 1991. Total ionic strengths and calculated partial pressures of hydrogen chloride (HC1) are shown in Table 5. In general, the dissolved acids (HCl and H2SO4) make up the bulk of the dissolved

material; consequently the partial pressures of HC1 and HF are functions of the total ionic strength.

Table 5

Date 19Apr 6 June 26 July 23Aug 24Sept 1Nov

Total ionic strength

3.70 1.69 3.34 3.00 4.18 4.53

Fraction due to acids 60% 73% 81% 79% 81% 80% (H+, ci-, SO&

pHCl (10e5 atm) 7.0 2.0 13.3 8.3 22.0 28.0

0.039 0.056 0.040 0.040 0.029 0.023 0.077 0.122 0.130 0.130 0.140 0.140 0.011 0.013 0.023 0.020 0.025 0.024

0.120 0.073 0.037 0.047 0.037 0.038

3.96 6.50 4.40 4.40 3.80 2.93

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Calculations show that the HSO4- -SO,$- speciation in solution is an important

influence on acid gas partial pressure by controlling the amount of free H+ available. Typically 30% of total H+ in the lake is present as HSO4-. Sulphate speciation also

influences the concentrations of metal ions which have weakly-soluble sulphates. For example, the calculated activity product of calcium sulphate is relatively constant over all the samples, indicating that it is in thermodynamic equilibrium with the mineral phase.

It is recognised that temperature gradients exist within the lake, as localised heating undoubtedly occurs where fumarolic activity is maximal. Changes in temperature affect chemical equilibria, and have a marked effect on the partial pressures of HC1 and HF, and therefore the amounts outgassed. A increase or decrease in lake temperature of lOOC results in approximately a factor of two change in partial pressure for both gases, i.e. increasing temperature increases the partial pressures and vice versa. An HC1 partial pressure of 7 x 10-5 atm implies an emission of 1500 tonne year-l km-2 from simple volatilisation of HC1 from the lake water, assuming a transfer coefficient of 5 x 10-3 m s-1. The fluoride is heavily complexed by A13+ in solution and its partial pressure, although comparable to that of HC1, is likely ultimately to be much less. Gas release from the lake is therefore presumed to be highly variable over the entire surface, and greatly influenced by isolated hots po t s.

From the above observations it was apparent that there were a number of interesting seasonal changes in the lake chemistry that merited further investigation. However, it was not clear whether the greater impact of aerosols in the dry season could be explained without further measurements, because it was impossible to distinguish between simple volatilisation of material from the lake and bubble-bursting processes. This distinction could be made if data were available on the composition of aerosol droplets above the lake, and as indicated above, the gas cloud was successfully sampled during subsequent fieldwork (see under gas geochemistry section for the results of these experiments).

Under such highly acid conditions it would be expected that any uncomplexed anionic species will be present as the free acids. Figure 2 shows the distribution of fluoride, chloride and sulphate in the lake waters throughout 1992, although difficulties in sampling from the same location on each occasion may have slightly affected the overall results. The much higher concentrations prevailing throughout

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the drier months (Dec - April) indicate the gradual evaporation of the remnants of the lake, followed by dilution with rainwater, and eventual loss through drainage during the wet season. It is understood that rain was much heavier than usual during December 1992, and similar conditions were experienced during the January 1993 fieldwork. This obviously affected the concentrations of dissolved ions, which would normally be expected to start increasing as the lake waters disappeared.

The sample preserved by the addition of sodium tetrachloromercurate showed that there was approximately 70mg/l free SO2 in solution. The presence of free unreacted gases such as SO2 and H2S is to be expected, since oxidationheduction

reactions of sulphur species will be a continuing process within the system.

4.1.2 Rainwaters

Figures 3 - 8 show the distribution of the same parameters in some of the rainwater samples collected during the 1992 wet season. However, it should be noted that the Botos sample is not strictly a rainwater, as it is taken from an extinct crater lake which is kept filled by rainfall, but which overflows constantly via a semi- permanent stream. All of these samples are generally recovered on a fortnightly basis, but localised meteorological conditions directly affect the amount of summit rainfall recorded. Therefore the dataset is always likely to be inconsistent, because sufficient sample is not necessarily obtained from every station on a regular basis.

There was an apparent event in September which dramatically affected the output of HC1 and HF. Chloride and fluoride are high at the Mirador, Borde Este and Botos, and also increased slightly in Poas crater lake at the same time. This may have been caused by a minor increase in activity, but could also have resulted from a brief period of drier weather which allowed greater evaporation. Volatilisation of HC1 and HF again increased significantly towards the end of the wet season, although this was not accompanied by a proportionate increase in SO2 emission, probably due

to the greater stability of sulphate ions in the lake waters. Botos lake was exceptional in December in that it showed a decrease in chloride content but a large increase in fluoride content; the reason is not known.

4.1.3 River and Drinking Waters

The distribution of trace elements in the natural waters is principally influenced by the composition of the main catchments, much of which is either primary volcanic

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material or weathered products of volcanic origin. The majority of the rivers exhibit neutral to weakly alkaline characteristics, and the variation of trace element content between the two sets of samples collected in March 1992 and January 1993 is minimal, being affected by seasonal dilution effects only. All data are shown in Table 4. However, three of the samples are distinctly acid, those of the Rio Agrio (bitter river) and Rio Desague having already been noted from earlier work, but the acidity of the Rio Gata was not recognised previously, because samples were not collected during the November 1991 fieldwork. The very low pH value, and the high levels of major and trace elements in the Rio Agrio, are thought to originate from Poas crater. This is confirmed by the rate of loss of lake fluids during recent years, caused by evaporation through increased power output, and seepage through the crater walls, Brown et al. (1991).

According to Figure 1, the sources of the Desague and Gata are both on the same side of the volcano as the Agrio, and although the pH's are significantly higher (between 5 and 6) , the possibility that some of the acid waters from the summit area are reaching tributaries feeding these two rivers cannot be ignored. The levels of dissolved halides and sulphate particularly are indicative of acid conditions, even though the absolute concentrations are much lower. The concentrations of fluoride, chloride and sulphate in the main catchmcnts arc shown in Figure 9, and the distinctly higher levels in the Desague and Gata are prominent. There are probably still some streams that have not been sampled due to their seasonal nature, but indications are that losses from the crater lake are confined to the WNW slopes i.e. towards the Bajos del Toro. The source of the drinking water supplied to this area is not known, but it may not be local as it is seemingly unaffected by the presence of acid river conditions nearby.

Apart from the small isolated community in the Bajos del Toro comprising farms, private dwellings, a school, a medical centre and shops and bars, the countryside is relatively devoid of human habitation. The area to the north of the community (several kilometers distant) is destined to be a major reservoir, and development work is already well underway. Because of its intended location this reservoir may ultimately be a catchment for some of the waters draining the NW slopes of Poas volcano. However, it is not known whether due account is being taken of the acid nature of the waters, and hence the implications for subsequent water quality.

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4.2 Gas Geochemistry

4.2.1 Fumarole Gases

During the March 1992 fieldwork, low temperature fumarole (SSOC) samples were collected from the top of the volcanic dome, using an inverted funnel buried in the ash. However, the oxygen and nitrogen content of the gases determined subsequently by gas chromatography indicated that the samples were contaminated by air, thereby confirming data obtained previously from an adjacent area.

As outlined above, in January 1993 it was possible to sample a higher temperature system near the crater lake, although the presence of boiling mud did cause problems with blockage of the quartz sampling tube. Only after return to the UK was it possible to establish that although apparently one good sample was collected, the headspace gas was again contaminated by atmospheric components. The oxygen content was 0.25% and the nitrogen content almost 20%, indicating that at least a quarter of the sample was probably air. A similar sample collected from the same fumarole in November 1992 by the University of Florence contained no oxygen and less than 0.1% nitrogen. Better sealing of the joint betwecii the sampling tube and the sampling vessels, and an improved technique for flushing the connecting pipe are therefore necessary. Our sample also contained approximately 0.3% w/v chloride and 35% w/v sulphate, but poor reproducibility between determinations for total sulphur, prevented accurate calculation of the average oxidation state, and hence estimation of the individual proportions of SO2 and H2S. The reasons for the poor

agreement are not fully understood, but interference from other species in solution is suspected, perhaps related to the presence of clay minerals from the boiling mud.

4.2.2 Condensate from the volcanic plume

The three samples of condensate had relative compositions of solute species that were very similar (Table 6). The differences in overall concentration can be explained in terms of condensation of variable amounts of pure water, leading to overall dilution or concentration. If the levels of metal ions are ignored, then the dilution is calculated to be a factor of about 0.005 - 0.05.

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Table 6

Element

Si Fe Na Al ca Pb Ni K Cr F Cl SO4

Sample 1 (mg/l) Sample 2 (mg/I)

132 9.8 10.5 2.6 8.1 19.5 12.8 7.7 25 608 4360 45.3

244 15.8 15.1 22.4 102 10.6 19.5 12.2 36.7 115 8540 244

Sample3 (mgll)

56 12.3 3.5 1.9 17.3 6.9 2 1.9 2.4 62.1 1570 28

The composition of the condensate was compared with averaged lake water, normalised to the lake water Na+ concentration, in order to establish where the concentrations of fluoride and chloride were enhanced relative to what would be expected from the lake water only. Cation composition on this basis is generally quite similar to the lake water, indicating that some of the liquid collected is physically-generated aerosol. Silica, lead, nickel and chromium are all enhanced, but the latter two elements are undoubtedly contamination from the stainless steel components of the sampling device. The high levels of lead may originate from the crater lake, but the transfer mechanism is a matter for conjecture; the silica is probably from wind transported volcanic dust particles. The severity of the acid conditions in the gas cloud was recognised from the health aspects, but was not sufficiently well identified prior to selecting the materials for constructing the sampling device.

Of the anions, chloride and fluoride are enhanced in the condensate, while sulphate is found in about the same relative concentration as in the lake water. Chloride is enhanced by up to 8 times and fluoride by about 4 times. It therefore appears that most of the HC1 and HF in the aerosol over the lake come from volatilisation at the lake surface, followed by dissolution into the aqueous aerosol. Earlier calculations suggested that if complexation of fluoride by Al3+ in the lake were ignored, then HF would be about 2 times - 4 times as volatile as HC1. However, it is found that HC1

15

concentrations in the aerosol are enhanced by a larger factor than HF. This is consistent with a high degree of complexation in the lake as indicated earlier.

The fact that sulphate is not significantly enhanced in the condensate suggests that production of aerosol acidity by dissolution and oxidation of SO2 is quite small relative to the direct transfer of HCl and HF. However, because H2SO4 is not

volatile, as the aerosol drifts away from the volcano the HCl and HF will probably evaporate and disperse more readily, especially under dry conditions, leaving the non-volatile H2SO4. There is therefore still about 2 x 10-3 M H2SO4 in the

aerosol to precipitate further away from the volcano, and this concentration will increase as the aerosol moves away to drier conditions and the water evaporates.

4.2.3 Diffusion samplers

The blank control tubes were emplaced in the Bajos del Toro some six kilometers from Poas summit, and the others in locations where they would receive constant exposure, subject to the prevailing winds. The tubes were left in place for 600 hours, almost half as long again as in the earlier cxperiment. All data obtained are shown in Table 7, the November 1991 data being included for comDarison.

Table 7

Location March'92 @g SOdm3)

Control Tubes 4 G j o n 249 San Luis 730*

Trojas 124 Farm 499* Mirador 156

Nov'91 @g SOdrn3)

0.4 250 299 60 314 117

* Approximate data only due to normal tube capacity being exceeded

The enhanced levels of SO2 observed at certain locations are very pronounced, even

the control area of the Bajos del Toro exhibiting an order-of-magnitude increase. The release of acid gases and aerosols into the environment is much greater during the drier months, due principally to the loss of the beneficial scrubbing effects of rainfall near the summit source. Local farmers complained of aggravated respiratory

16

problems during the dry season, and also damage to cash crops such as coffee. WHO guideline values are 30 pg/m3 for plants and 50 pg/m3 for humans (annual average), and 100 p/m3 for plants and 125 pg/m3 for humans (24-hour average). Therefore it can be seen that the recommended levels are being routinely exceeded, in some locations by substantial margins.

4.2.4 Impregnated Papers

The use of simple chemical absorption techniques was considered appropriate to monitor the short-term variation of gas dispersion at certain locations around the crater (Figure 1). During the March 1992 visit, 1 litre volumes of air were pumped through individual filter papers impregnated with sodium formate, at pre-determined intervals over a period of four hours, during which time aerial SO2 concentration

was monitored. The papers were then sealed in polyethylene bags and returned to the laboratory for analysis. Each paper was cut into two pieces, one being leached with distilled water for chloride and sulphate determination, and the other with a special buffer solution to extract the fluoride. The results are shown in Table 8.

Sample No

1 2 3 4 5 6 7 8 9 10 11 12 13 Lava Dome Lava Dome

F- (mg/l)

0.15 0.11 0.10 0.09 0.10 0.10 0.15 0.22 0.20 0.25 0.31 0.29 0.32 0.40 0.50

Table 8

C1- (mg/l)

nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

so4 (n1g/l)

0.64 0.53 0.42 0.87 0.54 0.66 0.54 0.56 0.55 1.79 1.63 1.76 1.99 3.65 7.87

Aerial SO2

3.0 0.4 0.4 0.6 0.2 0.2 0.4 0.3 0.6 1.5 1 .o 0.8 1.8 10.0 15 .O-20.0

nd = not detected

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It is to be expected that there would be an gradual increase in both fluoride and sulphate content, which is consistent with the general overall increase in the measured aerial SO2 concentration. The papers were kept sealed until used, and

therefore contamination is easily ruled out. The reason for there being no detectable HC1 at this point is unknown, but may either be due either to the specificity of the sodium formate used to impregnate the papers, or that little or no HCl was reaching the sampling point. The latter seems unlikely as fluoride species were present, and HF is more soluble in water than is HC1, and would therefore be presumed to be scrubbed more efficiently by atmospheric water vapour. Two further 1 litre volumes of air were sampled on the top of the lava dome within the crater, at aerial SO2

concentrations of 10-20ppm. Significant fluoride and sulphate were measured, but again there was no indication of HC1 (Table 8). Short-term diffusion tube measurements at this point indicated 10ppm HCl, and therefore the inference is that the sodium formate with which the papers were impregnated was only collecting the fluoride and sulphate. The variability of the data is directly related to the localised movement of air masses within the crater.

The four larger diameter filter papers laid on polyethylene sheets for one hour on the top of the lava dome were each used to dctermine pH, fluoride, chloride and sulphate. Subsequent analysis of a distilled water leachate from one these papers indicated an acidic pH of 5.4, against a blank value of 6.4. Other leaching solutions were used to extract the anions, but of these only fluoride and chloride were present in any significant quantities at approximately 35ppm and 500ppm respectively. This is at variance with the data from the pumped papers, and appears to indicate that immediate localised fallout of HC1 as hydrochloric acid is occurring close to the point of generation.

During January 1993 these experiments were repeated using, additionally, papers impregnated with potassium hydroxide. The collection and preservation of both fluoride and sulphate were successful, but again the chloride did not seem to be retained using either sodium formate or potassium hydroxide. This seems inconceivable, and it must therefore be assumed that chloride is present either in very low atmospheric concentrations, which is unlikely, or in such a form that it is being rapidly removed from the air before reaching the sampling stations. This requires further investigation.

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4.2.5 Vegetation

The samples of wood from the decaying trees were weighed, broken into smaller fragments where possible, and allowed to soak in deionised water for up to four days. The solutions were then filtered, diluted to volume in a volumetric flask, and 5ml portions removed for the determination of the retained sulphate concentration. Results varied between 33mg/kg and 330mg/kg, and contradicts earlier work which showed that sulphate only seemed to concentrate in living tissue. However, these new data may be a feature of dry season conditions which allow acid precipitation products to accumulate in vegetation without being leached by continuous rainfall. A much wider study, incorporating other types of vegetation, is necessary to substantiate these findings e.g. dead and living vegetation and inter-species comparisons.

5. CONCLUSIONS AND RECOMMENDATIONS

It has been shown that there is a wide variation in the amounts of acid contaminants dispersing into the environment from Poas crater lake. The damage to vegetation is readily visible immediately downwind of thc volcano, but there are undoubtedly potentially more serious effects of the emissions at greater distances from the summit, and from the health aspect, invisible to the naked eye. The principal agricultural and epidemiological problems are likely to arise from acid precipitation of both HCl and HF, both of which readily disperse from the source during the dry season because of their high volatility. However, sulphur species measured as sulphur dioxide have been monitored at least seven kilometers from the summit, and may therefore cause more severe health problems in the long term. This is due ultimately to higher strength sulphur acids being formed through evaporation of water during vapour transport. The formation of acid aerosols has long been suspected, and these preliminary measurements have indicated that although the amounts may be small, they are a pathway through which acid gases escape to the environment.

The enhanced levels of sulphur dioxide measured at some localities are very conspicuous, and emphasise the problems created when a change in wind direction discretely alters the fallout zone of the volcanic plume. During the wet season, many areas seem to be protected from the effects of the acidity, presumably because rainfall effectively scrubs the main components close to their source. This is adequately demonstrated by the differences in data obtained for the diffusion tube

19

measurements in the control area. In the Bajos del Toro, sulphur dioxide showed an order-of-magnitude increase in background levels in the dry season (April 1992) over those obtained during the wet season (November 1991). The release of acid gases and aerosols into the environment is therefore apparently much greater during the drier months. Local farmers complained of aggravated respiratory problems during the dry season, and also damage to important (for Costa Rica) crops such as coffee. Visible foliar damage to coffee plantations and other vegetation several kilometers from the summit of Poas was indeed apparent on each of the field trips, and therefore invisible damage to soil profiles may also be expected, with obvious consequences for plant health. World Health Organisation guideline values for SO2

are 30 pg/m3 for plants and 50 ,&m3 for humans (annual average), and 100 lUglm3 for plants and 125 &m3 for humans (24-hour average). Therefore by reference to Table 7 it is obvious that the recommended levels are being exceeded by significant margins in some locations monitored.

Analogous to the escape of gases into the environment there is also that of acid waters into the Rios Agrio and Desague. These waters potentially pose a much greater threat, in that they can directly contaminate groundwater supplies, with a consequential threat to the health of fauna and flora. Fluorine is well documented as being beneficial to the formation of skcletal tissue at low dose rates, but is equally well known to be injurious to health when present in excess. Waters containing high acid concentrations can mobilise many potentially toxic trace elements, and it is to be hoped that studies prior to the decision to site the reservoir on the NW side of Poas volcano took this aspect into account.

It should be emphasised that this has only been a pilot study, and there is an urgent need for much longer-term monitoring to establish baseline information. It is therefore recommended that:

(a) The sampling of fumarole gases should be repeated to substantiate the limited datasets available from past exercises. OVSICORI-UNA staff should be trained to undertake these analyses as part of their routine monitoring. Apart from gas sampling bombs, the remainder of the equipment necessary is believed to be available in-house. The situation relating to staff suitable for training is unknown as it is recognised that some new staff have been recruited recently.

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Although it was anticipated that the monitoring of gas emissions, using passive sampling techniques, would be carried out on a routine basis through a CEC-funded collaborative project with Costa Rica, this has not been possible to date. It is important to obtain reliable data for several cycles of wet and dry seasons, in order to monitor the mean fallout at selected locations relative to the source emissions. The uncertainties relating to the collection and determination of chloride species need to be resolved.

A co-ordinated sampling exercise should be mounted to determine the amounts of acid components being retained by various types of vegetation and soils around the volcano, to ascertain the area1 extent of the contamination. The continued monitoring of rain and river waters for pH, fluoride, chloride and sulphate by OVSICORI-UNA is therefore an important aspect of this.

The successful preliminary experiment to condense aqueous vapour from the gas cloud above the crater lake should be repeated, using a device constructed solely from inert materials, to minimise sample contamination. Only then will it be possible to obtain completcly reliable data for aerosol composition. This sampling and analysis could then be continued by OVSICORI-UNA as part of their routine monitoring of the volcano.

Only by understanding the magnitude of the problem, by establishing a comprehensive database of information through the constant monitoring of source gases and waters, and the principal pathways through which these contaminants disperse in the environment, will it eventually be possible to suggest improved measures to combat the principal effects of the secondary volcanic processes. It may even be feasible ultimately to predict long and short-term changes in the activity related to the changing chemistry of the gases and waters. These techniques might then be applicable to similar situations elsewhere in the world, where secondary volcanic activity is raising environmental issues.

ACKNOWLEDGEMENTS

Many people contributed to both the fieldwork and laboratory investigations for this project, and it is not practicable to acknowledge everybody individually. Due appreciation has been paid to those with significant inputs, by inclusion in the list of

21

authors. However, this list could easily have been extended to include Eduardo Malavassi and Erick Fernindez from OVSICORI-UNA, both of whom gave either assistance with field investigations aiid/or helpful advice. Similarly Barbara Vickers and other colleagues in the Analytical Geochemistry Group undertook multifarious chemical determinations, without which much of the above study would have been impossible. The late Professor Geoff Brown, and his colleague Dr Hazel Rymer, from the Open University also helped considerably by introducing us to the field area, and by providing geological information based on their personal experiences over many years working on Poas Volcano. Dr Peter Brimblecombe from the University of East Anglia contributed much to the research, by virtue of his extensive experience on atmospheric contaminants. Thanks are due to all those concerned. In addition, the comments of Dr John Bennett and Doug Miles on the text of this report are gratefully acknowledged.

7. REFERENCES

Ball T K, 1988. Soil gas geochemical methods applied to geothermal exploration. Report on visit to Costa Rica. ODA/BGS Subvention R gL D Programme 1987/88, Project 87/15. British Geological Survey Technical Report No WC/88/15R.

Barquero J and Fernhdez E, 1990. Erupciones de gases y sus consecuencias en e1 Volcan Poas, Costa Rica. Boletin de Vulcanologia Universidad Nacional, Costa Rica No 21, pp 13 - 18.

Brown G C, Rymer H, Dowden J, Kapadia P, Stevenson D, Barquero J and Morales L D, 1989. Energy budget analysis for Poas crater lake: implications for predicting volcanic activity. Nature, Vol 339, No 6223, pp 370 - 373.

Brown G C, Rymer H and Stevenson D, 1991. Volcano monitoring by microgravimetry and energy budget analysis. J of the Geol SOC of London, Vol 148, pp 598 - 593.

Martini M, 1993. Volcanic activity in Costa Rica. Smithsonian Institution-Global Volcanism Network Bulletin, Vol 18, No 1, pp 8 - 9.

Nicholson R A, Howells M F, Roberts P D and Baxter P J, 1992. Gas geochemistry studies at Poas Volcano, Costa Rica. British Geological Survey Technical Report W C/92/10.

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Pitzer K S, 1973. Thermodynamics of electrolytes 1. Theoretical basis and general equations. J of Phys Chem, Vol. 77, pp 268 - 277.

Rowe G L, Brantley S L, Fernandez M, Fernandez J F, Borgia A and Barquero J, 1992. Fluid-volcano interaction in an active stratovolcano: the crater lake system of Poas volcano, Costa Rica. J of Volcano1 and Geotherm Res, Vol49, pp 23 - 51.

West P W and Gaeke G C, 1956. Fixation of Sulphur Dioxide as Disulfitomercurate (11) and Subsequent Colorimetric Estimation. Anal Chem, Vol28, No 12, pp 1816 - 1819.

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