BRITISH GEOLOGICAL SURVEY

TECHNICAL REPORT WC/92/68 Overseas Geology Series

ODA/BGS R & D PROGRAMME

Volcanic gas investigations in the Philippines May-August 1991 (Project No. 91/17)

P D Roberts

Front cover illustration Soil gas sampling, Taal Volcano, Philippines

Author P D Roberts BGS Keyworth

This report was prepared for the Overseas Development Administration

Bib1 iographic reference Roberts, P D. 1992. Volcanic gas investigations in the Philippines May-August 1991 (Project No. 91/17). British Geological Survey Technical Report WCt92t68.

0 NERC copyright 1992 Keyworth, Nottingham British Geological Survey 1992

TECHNICAL REPORT WC/92/68

ODA/BGS R & D PROGRAMME

Volcanic gas investigations in the Philippines May-August 1991 (Project No. 91/17)

P D Roberts

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CONTENTS

Summary

Introduction

Methods and equipment

Field locations

Results

Discussion

Conclusions

Acknowledgements

References

Figures 1 - 17

Tables 1 - 6

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SUMMARY

Visits to the Philippines were made in 1991as part of an ODA funded project (12ET 'Environmental aspects of radon and other gases in geothermal areas'). Investigations of gases emanating from volcanic areas in Kenya, Philippines and Costa Rica have been carried out and have concentrated on the radioactive gases radon and thoron. Other hazardous gases have also been measured where appropriate.

The work in the Philippines was carried out with the co-operation and assistance of the Philippine Mines and Geosciences Bureau, the Philippine Institute of Volcanology and Seismology and the Philippine Nuclear Research Institute.

Measurements of radon and thoron had been taken in the Philippines as part of the project in 1990 when Taal volcano, a geothermal power plant and the Philippines fault were visited. During the work described here, following requests from the Philippines, more detailed investigations were carried out at Taal volcano. In addition, other volcanoes and another geothermal power station were also visited.

The concentrations recorded of radon and thoron confirmed the low level of risk from these radioactive gases and their daughters. Other hazardous gases determined were found to be at low concentrations.

The data collected at Taal showed a relationship between the gas emissions and changes of activity at the volcano, which was thought to be close to eruption during the visit. Training in BGS techniques of gas geochemistry was given to Philippine counterparts and some BGS equipment was retained in the Philippines to enable the continuation of measurements at Taal and to allow the opportunity for further soil gas investigations by the co-operating organisations in the Philippines.

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1. INTRODUCTION

This report presents the results of fieldwork carried out in the Philippines during visits in May and July/August 1991. The work formed part of a Research and Development Project (12ET, 'Environmental aspects of radon and other gases in geothermal areas') to investigate radon and other hazardous gases emitted from volcanic and geothermal sources which is funded by the Overseas Development Administration . The itinerary and general details of this work have been described elsewhere (Roberts, 1992).

The techniques of gas sampling and analysis were similar to those adopted in previous work in Kenya and the Philippines. Equipment capable of detecting and measuring a range of gases was used, with the emphasis on measurement of the radioactive gases radon and thoron. The methods employed have been developed within Geochemistry Group of BGS and used extensively in the UK and overseas.

Data was obtained from soils, volcanic vents, and a geothermal power station. A series of readings taken at one volcano demonstrated the link between gas emissions and the level of underlying volcanic activity. In general the new data confirmed the low level of hazard from the gases determined in the Philippines, and emphasised the potential for volcanic hazard prediction of the techniques employed.

Office, laboratory facilities, field assistance and transport were provided by the Philippine Mines and Geosciences Bureau (MGB), volcanological information and field assistance by the Philippine Institute of Volcanology and Seismology (Philvolcs). Towards the end of the visit field comparison trials were carried out with radon and thoron detection equipment used by the Philippines Nuclear Research Institute (PNRI) and training in BGS soil gas sampling techniques given to the co-operating organisations.

2. METHODS AND EQUIPMENT

2.1 Gas Equipment The techniques of collection and analysis of gases employed for this work are based on those developed within Geochemistry group, BGS which have been employed in Kenya and the UK (Ball and others, 1983a, 1983b; Ball and others, 1991; Ball and Clark, 1992) and previously in the Philippines (Roberts, 1990, 1991). The principal instruments used were a Pylon radon gas detector, capable of on-site radon (Rn-222) and thoron (Tn, Rn- 220) determination, and an Orsat gas analyser, to measure carbon dioxide and oxygen, modified for field use (Ball and others, 1983b, 1989). Other gas determinations were made utilising Drager tubes, a proprietary gas sampling system relying on sensor colour changes to give semi-quantitative data.

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2.2 Soil gas Soil traverse and single site measurements were carried out in all areas visited. Soil gas was extracted from 20-50 cm below surface using a soil gas probe hammered in to the soil, the gas then being passed into the analytical equipment. Radon, thoron, carbon dioxide and oxygen can be measured within 10 minutes and this ability to determine gas concentrations quickly enables a number of sites to be measured within a short period of time.

2.3 Vents. Gas from vents was collected by placing a large inverted plastic funnel over the gas source and piping the gas, through a cooling tube, filter and hand pump, to the analytical equipment. The funnel was sealed against atmospheric gas entry where possible with earth packed around the funnel edge. The cooling tube comprised a coiled l m length of brake pipe tubing. Where the gases were exceptionally hot a wet cloth was wrapped around the coil to improve cooling. A conical flask was employed where necessary to act as a steam condenser.

2.4 Geothermal Power Station Sampling was carried out within the generator buildings, and in one case outside. Each generator set is fed by steam from wells in the geothermal field and this supply has a gas separator attached. Some of this steam is fed to a condenser with a tap to allow routine gas sampling. The gas was sampled from this point and fed through a conical flask condenser into the Pylon or ORSAT apparatus.

3. FIELD LOCATIONS

Determinations were carried out at Taal volcano, S of Manila (Figs. 1, 2). Others were obtained at Mayon volcano, Tiwi geothermal power plant and environs in S Luzon (Fig. 2) and at Mt. Pinatubo, NW of Manila (Figs. 1, 2).

3.1 Taal volcano Taal volcano is located in central Luzon Island 60km south of Manila (Fig. 3), 90-110km east of the Manila trench, and to the east of the Bataan lineament (Wolfe and Self, 1982, Fig. 2). This zone of weakness is marked by andesitic to calc-alkaline strato-volcanoes such as Mt. Natib, Mt. Pinatubo (the most northerly), Mt. Mariveles and others. Further east of Taal, 140-175km east of the Manila trench, lie several volcanoes, alkalic in style, such as Mt. Arayat, Laguna de Bay, and Mt. Banahaw, site of a geothermal energy plant investigated as part of the project in 1990 (Roberts 1990). Their origin is thought to be related to subduction beneath Luzon Island.

The volcano, andesitic in composition, lies between the two groups described above, and an estimated 500km3 of tephra have been ejected from this 3000 years old volcano- tectonic depression. Historic eruptions have been phreato-magmatic for the most part, although an aa type lava flow was erupted during 1988-89. Many eruptions have been very violent leading to widespread destruction and loss of life; those within the last 200 years have been restricted to the island containing the main crater (Fig. 4), and during the last 30 years to the SW of the island.

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During 1990 gas data was collected from a traverse to the north of the main crater across a N-S trending fracture, marked by an outcrop of altered pyroclastics, on the Daang Kastila ridge (Fig. 4). In 1990 there was little or no visible activity at the volcano either within or outside the crater, but in the early part of 1991 the lake temperature rose, seismic activity increased and vents emitting steam and gas appeared within and outside the northern crater edge. Extensive bubbling could be observed in the crater lake. These changes in activity were monitored by the Philvolcs from the Pira-Piraso observatory (Fig. 4) located on the north coast of the island.

During work described here, more detailed readings were taken along the traverse on four occasions, and several readings taken at vents within and outside the crater in addition to 'background' measurements collected at the Pira-Piraso observatory. As a result of heavy rain (July and August are within the local rainy season) the crater was visited on only two out of four possible visits.

3.2 Mount Pinatubo The1745m high Mount Pinatubo volcano, situated1OOkm NW of Manila (Fig. 2), was classed as 'live' but dormant until early 1991 and the most recent eruption was thought to have taken place in the 15th century. Mt. Pinatubo is an andesitic to dacitic strato- volcano, was without a crater and had an infilled caldera. Activity prior to 1991 had consisted of a small steaming vent, but during April 1991 activity increased and produced several extremely active steaming vents. This was accompanied by enhanced seismic activity indicating upward movement of magma within the volcano. At this stage, in early May 1991, an unsuccessful attempt was made by the author to reach the vents. On June 9th the volcano erupted violently and the explosive plinian phreato- magmatic activity over the next few months deposited ash and tephra over a wide area, causing immense devastation and loss of life. These eruptions continued for over two months at an intense level; this has now declined substantially although there have been periodic temporary increases in activity.

Gas data was collected during two visits from within recent ash fall about 50 - 1OOcm thick, about 5km to the N W of the eruptive centre. Sequences of ash samples were collected at several locations for later geochemical analysis.

3.3 Mayon volcano Situated in the southern part of Luzon Island, Mayon volcano (Fig. 2) is one of the volcanoes in a belt to the east of the Philippines fault (Fig. 3), which form a linear cluster related to movement in the Philippine trench. This classic cone shaped volcano, over 2400m high is alkaline in composition and currently classed as active but quiescent with some steaming from the central crater. There have been several destructive eruptions this century, the last occurring in 1968.

Unfortunately time did not allow the summit to be gained to allow sampling of the vents, but samples of soil gas were measured on the northern flanks to within 15Om of the summit. A series of readings were also taken at the Philvolcs station, at a height of over 700m on the north-eastern flank of the volcano.

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3.4 Tiwi geothermal field and power station Tiwi geothermal field lies in S Luzon, 20km to the north-west of Mayon volcano, lies below the NE slope of the extinct andesitic Mt. Malinao volcano (Fig. 2). The power station generates 330MW of power and was the first major geothermal power station in the Philippines, commencing operation in 1979. Steam is fed to three generating plants, each containing two 55MW generators and five of the six steam supplies were measured. A few km to the SE of the power plant lies an area, overlying the geothermal reservoir, where hot springs were present until a steam explosion several years ago disrupted the water source. Soil gas measurements were taken in this area to determine the composition of gases percolating from the reservoir beneath.

4. RESULTS

The radon, thoron and carbon dioxide values and their ratios will be considered here. Changes in oxygen values are normally the inverse of carbon dioxide, and the sum with carbon dioxide can indicate the amount of gas flow. Radon and thoron values are based on counts taken over three minutes, and as a result of counting statistics the determination of low concentrations can produce large uncertainties.

4.1 Taal The traverse north of the crater was surveyed four times between May and August 1991 when radon, thoron, carbon dioxide and oxygen were determined. The results are summarised in Figs. 5-9. and tabulated in Table 1. Steaming vents within the crater were sampled on two occasions and other vents outside the crater to the north were tested during the same days as the traverse, the data gathered is displayed in Figs.10-13 and Table 2. In addition sites at the Pira-Piraso Philvolcs station on the N shore of the island were sampled during each visit, this data is shown in Table 3 and within the Figures 10- 13.

Within the crater, Drager gas sampling equipment was used to determine a range of gases and volatiles and the data from these is shown in Table 4.

4.1.1 Traverse Radon

The data from the traverse clearly shows the change in activity at Taal between May and August (Fig. 5). If the values from the ridge top site are considered, a rapid rise is seen in radon content from 52.7 Becquerels/litre (Bq/l) in May, to the maximum of 155.5Bq/l on 5th July, followed by a small decline to 139.4Bq/l on July 25th. This is followed by a larger decline to 72.7Bq/l by 7th August. To either side of the ridge crest, values are lower and this reflects the low soil gas transport expected away from the fracture zone along the ridge crest, nevertheless similar relative changes in radon levels over time are seen.

The carbon dioxide values (Fig. 6) reach a maximum of 32.5% on 25th July on the ridge crest, more than six times higher than that recorded on 10th May, this peak is flanked by values of 25.5% on 5th July and 24% in August. The profile along the traverse is different from that of radon, with generally higher levels to the west side of the ridges

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crest compared to those of the east side. On 10th May the CO2 values peak at 25m W, rather than at the ridge crest.

The values vary between 4.3Bq/l and 15.6Bq/l (Fig. 7), and all sites, with the exception of the ridge top, show a maximum value on 25th July. The ridge top site values vary greatly but show an inverse relationship with radon so that high values are present in May and August; in contrast, the low value on the 25th July (4 Bq/l) and the absence of detectable thoron on 5th July may show the dilution effect of very high gas rates at the ridge top site.

Ratios The radonlthoron ratios (Fig. 8) show wide variation and highlight the massive relative increase in radon concentrations on the ridge crest during July. However the radon- carbon dioxide ratios (Fig. 9), which effectively correct the radon values for the flow of the carrier carbon dioxide gas, show the highest value for the ridge top site on 10th May, followed by a steady decline. This peak on 10th May is seen at all sites E of the crest and the data shows that the 'corrected' radon flow in general is higher on the E side of the ridge. These ratios illustrate the complimentary nature of the carbon dioxide and radon measurements.

Thoron

4.1.2 Vents The data recorded from the steaming vents shows similar changes over time compared with the traverse. However values are significantly different since gas transmission is much more rapid through the vents than within soils.

This was recorded from a steaming vent within the crater on two occasions, 11th May and 6th July and shows (Table 2, Fig. 10) a rise in radon from 2.3Bq/l to 9.1Bq/l (the value of 15.2Bqll in Table 2 may be a result of other factors, see below). These values are very much lower than those determined at the vent about 60-70m to the north, outside the crater, where values of between 52.9 and 69.5 Bq/l were recorded. The values from this vent show a radon maximum on 25th July. In contrast, values from the vent further to the north (400m N) are lower and show a decline throughout the period of measurement from 38.2Bq/l in May to 31.2Bq/l in August. This may reflect the fact that the gas flow at this relatively distant site was low and had almost ceased by the last determination in August and there may have been little direct connection with deep gas flows at this stage.

The carbon dioxide values, shown in Figure 11, provide a possible explanation for the low radon values recorded from the crater vent. The carbon dioxide concentrations from the crater vents are extremely high, over 95% and vent gas flow rates were observed to be very high, thus diluting the radon content. A lower value of 77% determined on 6th July (Table 2) almost certainly results from air leakage into the collection system, this also casts doubt on the radon and thoron values from the same determination at that vent. The vent 50m N of the crater shows carbon dioxide values which reflect those of radon, showing a gradual rise until 25th July and a slight fall thereafter. However values at the more distant vent are low and show little variation, only rising slightly from 5.5% in May to 7% in August.

These values show the highest recorded for thoron at Taal, 39.5Bq/l, the May value for the crater vent (Fig. 12). However, this was followed by a much lower value of 7.4 Bq/l

Radon

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Thoron

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in July. A coherent picture emerges from the vent 50m N of the crater where thoron values rise from 9Bq/l in May to 15.1Bq/l on 25th July, followed by a decline to 8.6Bq/l in August. However at the more northerly vent values decline from 7.4Bq/l in May to 3.5Bq/l on 25th July and then rise to 5Bq/l in August.

The Rn/Tn ratios are very low for the crater vent ( ~ 2 ) but much larger outside the crater, reflecting the much lower gas flow rates outside the crater (Fig. 13). The vent 50m N shows a steady decline with ratios falling from 5.9 in May to 4.6 on 25th July. In contrast the Rn/Tn ratio for the more northerly vent increases from 6.9 on the 10th May to 10.7 on 25th July, followed by a decrease to 6.2 in August. Radon/C02 ratios (Table 2) differ from this pattern in that both vents outside the crater show a steady decline from a maximum in May, the more northerly vent showing a lower rate of decline.

This sampling equipment was used to determine a range of volatile chemicals such as HC1, H2SO4, HCN, H2S, Hg, mercaptans and hydrocarbons, in gases emitted from the vent within the crater. Results show that the content of these gases is very low in general (Table 3) and most compounds tested were below detection limits. There appears to have been an increase in H2S content from May to July, and this could be confirmed readily by smell. HCN is a significant component and increases from about 25ppm to over 30ppm between the two determinations. Hydrocarbons are present in trace amounts only and the value for mercaptans in May most probably represents an interference effect within the sensor.

Ratios

Drager - tube data

4.1.3 Philvolcs Pira-Piraso station The soil values recorded here served as a 'background' check for the traverse measurements but show some significant variations as well (Table 3). The values plotted on Figure 10 are derived from the 'tree' site.

The values recorded at the 'tree' site (Table 4) show a gradual increase from 1.2 Bq/l in May to 5.0 Bq/l in August. The data from the 'grass' site are low on 25th July (1.8Bq/l) and 6th August (0.9Bq/l). The well known effects of rain on radon content is shown clearly in the data recorded on 25th July when values for the 'grass' site rise from 1.8Bq/l to over 6Bq/l following heavy and persistent rain which had sealed the soil surface, this increase was still evident the next day when 5.5Bq/l was recorded.

Soil gas values at the Philvolcs station rise from 0.5% in May to over 2% in all determinations thereafter (Fig. 11). This change may indicate a general increase in diffuse gas flow throughout the island.

The values range from 3.2Bq/l to 6.9Bq/l and do not show any consistent pattern (Fig.

Radon

Carbon Dioxide

Thoron

12). Ratios

The radon/thoron ratios recorded are low and show a rise at the tree site from 0.3 to 1.3 during the period of measurements reflecting the increase in radon concentrations. The radon/C02 ratios do not show any clear pattern.

4.1.4 Equipment comparison - traverse During 6/7th August a comparison was carried out between equipment and methods of the Philippine Nuclear Research Institute (PNRI), and those of BGS, used to determine

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radon and thoron in soil. PNRI used a soil gas sampling method based on simply inserting a metal spike into the ground using a sledgehammer and determining the radon and thoron with an EDA radon monitor of the same type employed by the author during 1990 (Roberts, 1990). Reading were taken using the PNRI method, the BGS soil probe and Pylon, and by the PNRI radon monitor through the BGS soil probe. The latter was in order to obtain inter-calibration data since the BGS soil probe was left in the Philippines so that monitoring work could continue at Taal using the PNRI radon monitor and BGS methods. The comparative data for radon and thoron is illustrated in Figure 14 with the three different determinations illustrated together.

4.2 Mayon and Tiwi

4.2.1 Mayon Radon and Thoron

Few soil gas determinations were possible at Mayon, and these were taken on the N and N W flanks of the volcano. All the radon and thoron values are low, with a maximum of 3.4Bq/l for radon and 1.9Bq/l for thoron (Table 5, Fig. 15). As the actual alpha counts recorded were low, the uncertainties in the concentrations are high and range from about 17% to over 50% for radon. No clear pattern emerges, there is a wide variation in values both at the Philvolcs station and high on the volcano, and those recorded at the latter sites are all lower than those at the Philvolcs station. Thoron is very variable but in general is higher at the Philvolcs station. The maximum thoron value was recorded on the intermediate ridge site (1.5Bq/l ).

The values were variable and generally low (Fig. 15) especially at the high altitude site and no clear pattern emerges. The ridge site has the highest concentration of 5%.

The radon/thoron ratios are variable and reflect the uncertainties in the determinations rather than any real differences. Radon/carbon dioxide ratios are all low, with the highest value of 2.3 recorded on 16th July.

Carbon Dioxide

Ratios

4.2.2 Tiwi and Environs Radon, thoron and carbon dioxide were the only gases determined at Tiwi power station. Results from the Philippines in 1990 had established that radon daughters were below detection limits in geothermal power stations, and no attempt was made to measure these separately. Other hazardous gases are monitored by the power station authorities.

The values for radon recorded from the geothermal station show good agreement within each plant (Fig. 16) but the values for plant B (35.8 - 37.4Bq/l) are significantly lower than those from A and C (over 50 Bq/l). Normally thoron would not be detected when measuring gas from within geothermal plants by virtue of the long transport times involved from the wells to the gas sampling point. However, thoron was detected at plant C from the N end sample, and it is possible that gas transit times are unusually short for this generator. Unfortunately no detailed information for the well distribution pattern and steam routing was available. The two soil gas determinations SW of the plant, show the highest radon values obtained in S Luzon of over 80 Bq/l. Thoron was not detected at these sites, implying a relatively long transport time for the soil gas.

Radon and Thoron

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Carbon Dioxide All carbon dioxide values within the power plant were greater than 95%, as expected, and indicate that the gas samples were uncontaminated with air. SW of the plant, soil values of 52% and 95% show that active gas transport is taking place.

4.3 Mt. Pinatubo The sole gas determination from Pinatubo was obtained from within fresh ash. This showed a radon and thoron content of 2.1Bq/l and 1.4Bq/l respectively, and low values for radon would be expected from this material since uranium daughters will not yet be in equilibrium. Carbon dioxide and oxygen concentrations of 5.5% and 12.5% respectively were obtained for this site.

5 DISCUSSION

The results from Taal are of special interest since they illustrate the changes in gas emission from this volcano while it was undergoing major internal changes, and indeed was being treated by the Philippine authorities as an imminent eruption risk. Other data from Taal, collected by Philvolcs, such as seismic activity (Fig. 17), shows the greatly enhanced activity between March and July 1991, in particular two peaks, April and mid- June, declining thereafter steadily until a low value was reached in late July. It is worth noting that the radon value determined on the ridge top on 10th May was already higher than that recorded in 1990, (52.7 Bq/l, May 1991; 41.3 Bq/l, Nov. 1990) reflecting the increase in activity that had taken place. The first two visits to Taal in 199lcorresponded to periods of moderately active seismicity, though the second, on the 5th July, was during the period of declining activity. The last two visits, 25th July and 6th August, took place when activity had declined to a relatively low level. As can be seen from the graph of radon data from the Taal traverse, these changes in activity are not reflected in a simple way in the gas data. The radon data (ridge top), for example, contains a peak at 5th July, with only a slightly lower value for 25th July. The lowest value, in contrast, is that of 10th May. However, if the Rn/C02 ratio data is considered, a more consistent picture emerges, where the peak is reached on 5th July and the May value is much higher than the 25th July and 7th August values. These changes reflect much more closely the seismic activity variations and demonstrates the importance of monitoring both radon and carbon dioxide in these circumstances. Since the ridge top site had no soil and the altered rock probably offered a direct link to the fractures for gas, this site cannot be regarded as typical of soil measurements. However, the data from the traverse sites away from the ridge top, taken in soil without known fractures beneath, show the same relative changes and demonstrate that soil gas emissions even some distance from the crater can be used as an indicator of volcanic activity. The two main crater vent Rn/C02 ratios also conform with the seismic patterns, in that the May ratio value is the highest and the 6th July value lower. The crater vents probably have a more direct connection with the magma and should reflect any changes in volcanic activity more directly. However, the values from the vents outside the crater show a different pattern reflecting a less direct contact with the magma beneath.

However, the changes in volcanic activity were more complex than the simple seismic data implies, since field observation of ground temperatures and the Rn/Tn ratio data

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from the vents indicated that the focus of activity was moving northwards in July, towards the position of the traverse. This has been confirmed during a recent increase in activity (Feb. 92) when steaming was observed in the area of the traverse for the first time (De Luna and others, 1992).

At the Pira-Piraso station tree site the May soil gas Rn/CO2 ratios were high, decreasing thereafter (apart from the rain affected 26th July value) and Rn/Tn ratios show the inverse of this. It is possible that this reflects the changes in volcanic activity and if so, there are important implications for using these gases means of monitoring activity remotely from any vents. Unfortunately it was impossible during the visit to establish with certainty whether monitoring the gases at the station would also detect an increase in activity, it is hoped that future work will attempt to establish this.

At Tiwi power station the radon levels recorded (avge=53.8Bq/l) are lower than those recorded from Makban, central Luzon in 1991, where values ranged from 65 Bq/l to over 90 Bq/l. These radon levels confirm the low potential for the build up of harmful levels of radon and daughters, and the well ventilated nature of the generating stations assists in keeping these radioactive products low. The data recorded at Makban in 1990 indicated that the build up of radon daughters does not occur and that the health risk to workers from radon and daughters should be very low. As at Makban, the significantly different values of radon recorded from the generating plants within the power station may represent steam obtained from different parts of the geothermal field, and this variation of radon within geothermal fields has been noted in other work (Ball and Clark, 1992). The sites measured outside the power station show higher radon values, over 80 Bq/l, and an absence of thoron. This data, together with the relatively high carbon dioxide values, implies some gas transport from the reservoir below, but at a relatively slow rate since the thoron has decayed entirely before reaching surface. It is worth noting that the radon levels recorded in the soil here are higher than those recorded from vents at Taal, this indicates a higher radon generation rate within the bedrock, possibly resulting from geochemical differences, such as a higher uranium and radium content.

At Mayon volcano, in contrast to Taal, there is no evidence of widespread gas transport around the flanks of the volcano, all radon and carbon dioxide levels being very low. This may only reflect the quiescent state of Mayon at present but data taken over a time span would be needed to establish this. There is a contrast with the geothermal area to the north in that values at Mayon are considerably lower, but this almost certainly reflects the special nature of the soil sites at Tiwi where they overly the geothermal reservoir.

Comparison with PNRI radon equipment - Taal traverse The data obtained from the comparison of techniques and equipment illustrate the benefits of the BGS soil probe compared with the PNRI technique of merely hammering a spike into the ground. The radon levels recorded by PNRI method are very much lower as a result of the low efficiency of gas extraction with that technique. However, when the BGS soil gas sampling method and equipment was used good agreement was found between the two different types of radon monitoring equipment (Pylon, BGS; EDA, PNRI). This should mean monitoring of the same traverse carried out by Philvolcs and PNRI since August, using the BGS probe and EDA monitor, can be directly related to the

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measurements reported here. Philvolcs and PNRI have already successfully used the BGS soil gas probe left on loan in the Philippines to repeat the traverse surveys at Taal, and to extend measurements to other environments (De Luna and others, 1992).

6 CONCLUSIONS

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The health hazard of radon and daughters within geothermal power stations and around volcanoes was determined to be low in the Philippines. The data from the Philippines, together with that gathered from Kenya (Ball 1989, Ball and Clark 1992) and Costa Rica (Nicholson et al., 1992) has confirmed the low risk from radon, thoron and their radioactive daughters in geothermal power stations and close to volcanoes.

Non-radioactive gases, especially sulphur containing species (e.g. S02, H2S) can be a significant hazard, and high levels of these have been found at Poas volcano in Costa Rica during work as part of this project (Nicholson et al, 1992). However, high levels of these gases were not found within the Philippines and no vegetation or crop damage attributable to gas emissions was observed at any of the volcanoes studied in the Philippines.

Previous work elsewhere has shown links between radon emanations and volcanic activity (e.g. Cox, 1983; Thomas and others 1986) but have normally relied on techniques requiring a significant period of time between sample collection and analysis. This work has demonstrated the potential of BGS techniques, allowing rapid on-site determination of radon levels together with carbon dioxide. The methods described in this report could provide a useful additional way of monitoring the activity of volcanoes such as Taal and Pinatubo.

Philippine counterparts have now been trained in the BGS techniques and continue to monitor Taal volcano when possible, and have also applied the techniques to an active fault in the Manila area extending the work of the author over the Philippine fault system in 1990 (Roberts 1990; De Luna et al., 1991). MGB, Philvolcs and PNRI are now able to continue and extend use of the BGS soil gas techniques as a result of the training and equipment given during this work.

It is hoped that BGS can extend and develop the techniques and equipment used in the Philippines with the aim of improving the rapid determination of radon and other gases at volcanoes. This could help with the prediction of volcanic activity, in important consideration in countries such as the Philippines with a high risk from volcanoes.

7 ACKNOWLEDGEMENTS

The author wishes to thank the Directors and staff from MGB, Philvolcs, PNRI and the National Power Corporation for their assistance with this work. In particular, Dr Edwin Doming0 and Zenaida Amante of MGB for organising the office facilities and transport;

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Erlinda de Luna and Resureccion Yabut of Philvolcs for their valuable assistance during fieldwork.

8 REFERENCES

BALL, T K, NICHOLSON, R A & PEACHEY, D, 1983a. Effects of meteorological variables on certain soil gases used to detect buried ore deposits. Institution of Mining and Metallurgy, Transactions Section B, 92, B 183-190.

BALL, T K , HEXTALL, J P, NICHOLSON, R A & PEACHEY, D, 1983b. Application of Orsat stack-gas analyser to geochemical exploration research. Institution of Mining and Metallurgy, Transactions Section B, 92, B49-5 1.

BALL, T K, PEACHEY, D & VICKERS, B P, 1989. Soil gas techniques for use in geochemical surveys overseas. BGS Technical Report W 1/89/8.

BALL, T K, 1989. Radon measurements in Kenya, 1989 (unpublished).

BALL, T K, CAMERON, D G, COLMAN, T B, ROBERTS, P D, 1991. Behaviour of radon in the geological environment. Quarterly Journal Engineering Geology, 24, 169- 182.

BALL, T K, CLARK, M C G, 1992. Gas geochemistry in exploration for geothermal energy resources. M6m. Expl. Cartes G6ologiques et Minikres de la Belgique 32, 191- 204.

COX, M E, 1983. Summit outgassing as indicated by radon, mercury and pH mapping, Kilauea volcano, Hawaii. Jour. of Volcanology and Geothermal Research, 16, 131-151.

DE LUNA, E U, ROBERTS, P D, YABUT, R P, RAMOS, A, BUGAOISAN, N, AMANTE, Z, 1991. Radon level measurements as monitoring tool in active volcanoes and active faults. Geol. Soc. Phil. 4th Ann. Cong. 1991 (in press).

NICHOLSON, R A, HOWELLS M F, ROBERTS P D, BAXTER P J, 1992. Gas geochemistry at Poas volcano, Costa Rica. BGS Technical Report WC/92/10.

ROBERTS, P D, 1990. Radon - Mapping the Hazard. GEOCON-90, Geol. Soc. Philippines, Manila, Dec. 1990.

ROBERTS, P D, 1991. Report on a visit to the Philippines. November 1990 - January 1991. Report to ODA, BGS. Technical Report WC/91/10/R 21pp.

ROBERTS, P D, 1992. Report on visits to Philippines May and July 1991. Report to ODA, BGS. Technical Report WC/92/13/R 8pp.

THOMAS, D M & CUFF, E, 1986. The association between ground gas radon variations and geologic activity in Hawaii. Jour. of Geophys. Res. 91, B12, 186-198, 1986.

WOLFE, J A & SELF, S, 1982. Structural and neogene volcanism in south-western Luzon. Amer. Geophys. Union Monog. 27, 157-172.

12

0 0

0 0

8 0

0

8 0 0

0 0

0

8 8 8 I)

0

0 0

a 8

m I)

0 0 0 0

8

9 FIGURES

1 2 3 4

5 6

7 8 9 10 11 12 13 14 15 16 17

10

1 2 3 4

5 6

Philippines and central Luzon. Luzon with Bataan lineament. Phi1 i pp ines , tectonic setting Taal lake and Island, showing crater, vents, traverse and Pira-Piraso Taal traverse - radon Taal traverse - carbon dioxide Taal traverse - thoron Taal traverse - radon/thoron ratios Taal traverse - radon/C02 ratios Taal vents and Pira-Piraso station - radon Taal vents and Pira-Piraso station - carbon dioxide Taal vents and Pira-Piraso station - thoron Taal vents and Pira-Piraso station - radon/thoron ratios Taal - radon and thoron comparison PNRI - BGS Mayon - radon, thoron, CO2 Tiwi - radon Taal - seismic activity March - October 1992

TABLES

Taal traverse Taal vents Taal Drager tube Taal Pira-Piraso station Mayon Tiwi geothermal field

13

P H I L I P P I N E S D

W 0 - 0

Figure 1 Philippines, general location

Mt

Mt

CENTRAL PHILIPPINES - LUZON ISLAND

100 km I

Figure 2 Luzon with Bataan Lineament

PHILIPPINES - TECTONIC SETTING

SOUTH CHINA SEA

LEGEND Subduction

0-

*,- Fault

y/?i Zone of collision

(After Punongbayan et al., 1989:

Figure 3 Philippines, tectonic setting

T A A L VOLCANO

Scale - 0 1000 m

TAAL LAKE

KEY

0 Crater

Figure 4 Taal lake and Island, showing crater, vents, traverse and Pira-Piraso station

Ridge traverse Radon

50mW 25mW Ridgetop 25mE 50mE

Figure 5 Taal traverse - radon values

Ridge traverse Carbon Dioxide

35

30

25

20

15

10

YO

August 5 ly

0 50mW 25mW Ridgetop 25mE 50mE

Figure 6 Taal traverse - Carbon Dioxide

Ridge traverse Thoron

15

Bq/l 10

5

0 5OmW 25mW R-top 25mE 5OmE

Figure 7 Taal traverse - Thoron

Radon/Thoron ratios

50

45

40

35

30

25

20

15

10

5

t

v

6th August

w10thMay 1 0 5th July 1

25th July

6th August

50mW 25m W Ridge top 25m E Traverse position

Figure 8 Taal traverse - RadodThoron ratios

50m E

7 :i RadonICarbon Dioxide ratios

-I _______

0 10th May

5th July

H 25th July

7th August

50m W 25m W Ridge top 25m E 50m E

Figure 9 Taal traverse - RadodCarbonDioxide ratios

Taal - Radon Vents and Pira-Piraso station soil

70 -

60 --

50 --

40 --

30 -- 8 :

20 --

10 --

0 ND t

10th May 5th July 25th July 6th August

Figure 10 Taal vents and Pira Piraso station - Radon

0 Crater vent

Vent 50m N of crater I H Vent 400m N of crater j

Pira-Piraso station I

100

90

80

70

60

% 5 0

40

30

20

10

0

40

35 1-

30 ;-

25 ;-

$20 I- m

15 1-

10 1'

5 1'

0 -

Taal - Carbon Dioxide Vents and Pira-Piraso station soil

10th May

1

5th July 25th July 6th August

Figure 1 1 Taal vents and Pira Piraso station - Carbon Dioxide

Taal - Thoron Vents and Pira-Piraso station soil

1 10th May 5th July 25th July 6th August

Figure 12 Taal vents and Pira Piraso station - Thoron

CI Crater vent

Vent 50rn N of crater

Vent 400rn N of crater

Pira-Piraso station

0 Crater vent

Vent 50m N of crater

EZi Vent 400m N of crater

H Pira-Piraso station

l 2

10 I a --

5 6:- m

4 --

2 --

Taal - Radon/Thoron ratios Vents and Pira-Piraso station soil

I Crater vent

Vent 50m N of crater

Vent 400m N of crater

Pira-Piraso station

0 10th May 5th July 25th July 6th August

Figure 13 Taal vents and Pira Piraso station - Ratios

Taal traverse - 7th August 1991 Comparison PNRl EDA - BGS Pylon

+ + +

0 EDA - PNRl spike

EDA - BGS spike

Pylon - BGS spike

50m W 25m W Ridge top 25m E 50m E

Figure 14 Taal traverse - Radon comparison BGS - PNRI data

e

Mayon Radon, Thoron and Carbon Dioxide

5 T

$ g 3.5 g 3

2.5 '13 5 2

8 1.5

2 0.5 0

3 1

1 n A B C D E F G

Figure 15 Mayon volcano - Radon, Thoron and Carbon Dioxide

Tiwi geothermal plant and environs Radon

90

80

70

60

50 B 4 l *

30

20

10

0 S N N S N

0 Radon

Thoron

Carbon Dioxide

SW of plant

'B'

Track 10m W

Figure 16 Tiwi geothermal station - Radon

TAAL VOLCANO

v) c Q) > a, 0

c,

cc

0 z

i i i I 3 I

26-.Jun-4 1 19-Aug-91 12-dct-3 1 03-May-9 1

Figure 17 Taal - seismic activity March - October 1992

TABLE 1

TAAL TRAVERSE

Date Position

10th May 50m W 25m W Ridge top 25m E 50m E

5th July 50m W 25m W Ridge top 25m E 50m E

25th July 50m W 25m W Ridge top 25m E 50m E

7thAugust 50m W 25m W Ridge top 25m E 50m E

ND = not determined

TABLE 2

TAAL VENTS

Date

10th May

11 th May

5th July

6th July

25t h July

7th August

Position

50m N crater 400m N of crater Crater N side

50m N crater 400m N of crater Crater N side Crater N side

50m N crater

50m N crater 400m N of crater Soil nearbv

Rn Bq/l

0.6 11.0 52.7 5.9 3.5

3.7 17.7

155.5 23.2 2.9

5.4 19.1

1 39.4 26.2 5.1

3.7 13.7 72.7 7.1 2.0

Rn Bq/l

52.9 38.2

2.3

58.8 37.0

15.2 9.1

69.5

58.4 31.2

Tn Bq/I

5.0 7.2

12.9 4.3 6.5

4.3 8.3

Very low 7.1 6.6

8.5 9.7 4.9 8.3 6.9

6.5 7.1

15.6 6.6 7.1

Tn Bq/l

9.0 7.4

39.4

11.9 3.5

2.8 7.4

15.1

8.6 5.0

J 34.7 1.8

CO,%

0.5 7.0 6.0 1 .o I .O

2.5 13.0 25.5 9.5 2.0

7.0 12.5 32.5 14.0 4.5

5.0 11.0 24.0 5.5 1.5

CO,%

6.5 5.5

95.5

15.0 6.0

77.0 77.0

24.0

22.5 7.5

7.0

02% 15.5 13.0 15.0 14.5 14.5

15.0 13.0 12.0 13.0 13.5

16.0 15.5 12.5 17.0 14.5

18.5 17.0 16.0 18.0 19.5

02% 14.0 15.0

0.0

13.0 16.5

5.0 5.0

9.5

14.5 17.0

17.5

RnfTn

0.1 1.5 4.1 1.4 0.5

0.9 2.2

Large 3.3 0.4

0.6 2.0

28.7 3.2 0.7

0.6 1.9 4.7 1.1 0.3

R f l n

5.9 5.2

0.1

5.0 10.7

5.4 1.2

4.6

6.8 6.2

RdC02

1.1 1.6 8.8 5.9 3.5

1.5 1.4 6.1 2.4 1.4

0.8 1.5 4.3 1.9 1.1

0.7 1.2 3.0 1.3 1.3

RltlC02

8.1 6.9

0.02

3.9 6.2

0.2 0.1

2.9

2.6 4.2

19.5 5.0

TABLE 3

TAAL VOLCANO

DRAGERTUBE GAS DATA

Steaming vent on S side of crater lake

ANALWE

Gas Temperature

H2SO

HCI

H2S

so2 HF

CO

HCN

Hg M ERCAPTANS

HYDROCARBONS

below det. = below detection limit

TABLE 4

TAAL PlRA-PlRASO STATION

Date Position Rn Ba/l

10thMay Tree

5th July Tree Tree

25th July Tree Grass

After rain Grass Grass

26th July Tree Grass

6th August Tree Tree

1.2

3.7 2.5

3.5 1.8 6.0 6.8

4.9 5.5

5.0 3.1

UNITS

(deg. C)

mgm3

PPm

PPm

PPm

PPm

PPm

PPm

PPm

PPm

PPm

Tn Bq/l

4.5

4.1 5.1

6.0 6.9 5.2 3.3

4.7 3.2

3.7 4.4

11-May-91

97

-2

4 0

<I

<50 (-20)

<I .5

nd

-25

below det.

?>loo ?

trace

C02% 02%

0.5 17.5

3.0 14.0 2.5 14.5

3.0 15.0 2.5 16.5 2.5 16.5 2.5 16.5

2.0 17.0 2.5 18.0

3.0 18.0 3.0 18.0

Grass 0.9 5.1 2.0 19.0

64~1-91

97.5

<1

be low det . >20

nd

nd

below det.

>30

nd

below det.

trace

R M n RNC02

0.3 2.3

0.9 1.2 0.5 1 .o

0.6 1.2 0.3 0.7 1.2 2.4 2.0 2.7

1 .o 2.5 1.7 2.2

1.3 1.7 0.7 1.0 0.2 0.5

TABLE 5

MAYON VOLCANO, S LUZON

Date Position RnBq/l TnBq/l CO296

16th July Philvolcs stn. Grass W of stn. A 3.4 1.1 1.5

Grass S of stn. B 1.1 1.9 4.0 Grass E of stn C 1.1 0 3.5

18th July NEofsummit 1 20mSof D 0.8 0.1 2.0

2 2mSof1 E 0.1 0 5.0 3 3mSof2 F 0 1 .o 0.5

gulley

Low, in forest On ridge G 0.1 1.5 5.0

TABLE 6

TlWl GEOTHERMAL FIELD, S LUZON

17th July 1991

Area Position

Plant A S end N end

Plant C N end S end

Plant B N end N end

Naglagbong SW of plant Near track

10m to W

ND = not determined

RnBq/l TnBq/l C02% 02%

65.8 0 98.0 ND 64.1 0 95.5 ND

52.0 3.0 67.2 0 99.0 ND

37.4 0 99.0 ND 35.8 0 99.0 ND

80.3 0 52.0 6.0 83.0 0 95.0 ND

02%

17.0 15.0 16.0

19.0

15.5 20.0

14.0

Rnnn Rn/C02

3.2 2.30 0.6 0.30

0.30

14.1 0.40

0.03

0.05 0.01

W C 0 2

6.3 6.3

6.3

3.5 3.4

14.4 8.2