Groundwater is often an essential natural water resource on active volcanic islands. It can also interact with the volcanic system to generate hazards and exploitable geothermal reservoirs. Unfortunately, in volcanic island arc settings an understanding of the fundamental hydrology is lacking. Using field observation and numerical models, this study explores the hydrology of the active Caribbean volcanic arc island of Montserrat and aims to improve the current understanding of hydrology on Montserrat and island arc volcanoes in general. Recharge models for Montserrat, presented here, suggest that 10%-20% of annual rainfall is recharged to the groundwater system. Predicted recharge is strongly seasonal and spatially distributed, associated with rainfall and land use variations. Permeability measurements of core samples reveal a great range of permeabilities, between 1 x 10-18 and 1 x 10-12 m2, for lavas and volcaniclastic material. A lack of surface water is suggestive of relatively high permeability across the island. However, > 20 springs, with yields up to 19 L/s at elevations between 200 and 400 m, on the extinct volcanic complex of Centre Hills indicate the presence low permeability material. With recharge on the order of 0.24 mlyr such low permeability units must be reasonably laterally continuous. A suite of 250 TOUGH2 3D flow simulations, together with observations from Montserrat, suggest that the high elevation springs are supported by a low permeability central core of intrusive volcanics and associated (hydro ) thermally altered material. The numerical models generally struggle to reproduce high total spring discharge, suggesting that additional processes contribute to the observed high spring yields. A contribution of deeper waters to the Centre Hills springs has the potential to explain the observed high yields. This hypothesis is supported by field observations; an increase in temperature (up to 29°C) correlates with elevated specific conductivity and a decline in chlorofluorocarbon (CFC) concentrations in the southern Centre Hills springs, indicative of the presence of a deeper, older groundwater component. A final suite of more generic numerical models, applying the insights gained elsewhere in this study, reveal that temporal variations in recharge can generate gravity changes > 100 μGal. These simulations demonstrate the importance of understanding hydrology when performing and interpreting geophysical surveys that monitor active volcanoes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:682336 |
| Date | January 2014 |
| Creators | Hemmings, Brioch |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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