Team Gas1 and Team Environmental2, Montserrat Volcano Observatory
1Team Environmental: S. Young, P. Baxter, A-M. Lejeune, J. Barclay, E. Calder, G. Norton, A. Nicholl, B. Tonge
2Team Gas: B. Darroux, S. Young, R. Robertson, G. Norton
The monitoring of the potential effect of active volcanoes on the environment and on the health of the population living near volcanoes should be an integral part of any monitoring programme. Environmental monitoring at Soufriere Hills Volcano, Montserrat has been undertaken by MVO since November 1995 and consists of regular sampling of rain, surface and ground water, gases, and ash.
Four rainwater samples are collected from west and north of the volcano on a weekly basis or after heavy rainfall, and analyzed locally for pH, conductivity, sulphate, chloride, fluoride and total dissolved solids. Sub-samples are now sent on a regular basis to the British Geological Survey where they are analyzed for a range of elements. The rain water is acidic (usually between pH 3 and 4), and shows enhanced levels of the analyzed species. The site to the north of the volcano (Weekes) shows consistently higher pH values and lower values of the other analytes. This is due to the prevailing wind and hence plume direction from the east increasing the acidity of the rainfall over Plymouth. The lowest pH values are in the Upper Amersham area, and the effect decays with distance away from the volcano.
COSPEC is used routinely to monitor the output of sulphur dioxide from the volcano, and shows a general background output of 200-300 tonnes/day. However, high levels were noted in July and October, 1996. Unfortunately the machine was out of operation between end August and mid October.
An additional measurement of sulphur dioxide is given by the analysis of diffusion tubes. These tubes are situated at five sites to the north and west of the volcano for exposure times of between 12 and 16 days. The contents of the tube are then analyzed and the concentration is averaged over the exposure period. These data show good correlation with the COSPEC data, particularly through July and August. High levels from the diffusion tubes in September and early October suggests that there was high gas flux from the volcano during this period. The levels at Upper Amersham are above the WHO guidelines for long term exposure to sulphur dioxide and thus have direct implications for the welfare of the population in this area.
Ash collection and sampling occurs during periods of heavy ashfall. The mass of ash deposited in Plymouth during 17/18 September is estimated at 600,000 tonnes. Using the method of Pyle (1989), the total volume of ash erupted is approximately 500,000 m3. However this is likely to be an underestimate since the capacity of the collection trays was exceeded during the night of the eruption. This eruption was by far the most voluminous since the start of the activity. The volume estimates are used for volcanological interpretations of eruptions. Ash samples are now routinely sent to BGS for chemical analysis, and these results are then fed back into the environmental monitoring.
As a result of the continuing monitoring programme, several new hazards have been identified. High levels of cristobalite in ash are thought to be a problem and may result in silicosis of individuals living or working in ashy environments over long time periods. High fluorine in ash samples suggest that fluorosis may become a problem in cattle grazing in areas west of the volcano. Sampling programmes for both these hazards are currently under way.
Other groups have been involved in collecting ash and gas samples to study their environmental impact.
This ongoing environmental monitoring programme sends regular feedback to the local authorities to enable them to assess the potential health hazard for the human and animal population.