Hydrogeology of Montserrat review and new insights

Abstract

a b s t r a c tStudy region: The tropical, active volcanic arc island of Montserrat, Lesser Antilles, Caribbean. Study focus: New insights into hydrological recharge distribution, measurements of aquifer permeability, and geological and hydrological field observations from Montserrat are combined with a review of the current understanding of volcanic island hydrology. The aim is to begin to develop a conceptual model for the hydrology of Montserrat, and to inform and stimulate further investigation into the hydrology of volcanic arc islands, by combining a review of the current understanding of essential components of the hydrological system with fresh analysis of existing data, and new observations, data collection and analysis. This study provides new insights into hydrological recharge distribution, measurements of aquifer permeability, and geological and hydrological field observations from Montserrat. New hydrological insights for the region: A new groundwater recharge model predicts whole island recharge of 266 mm/year, between 10% and 20% of annual rainfall. Core scale permeability tests reveal ranges from 10 −14 to 10 −12 m 2 for volcaniclastic rocks with coarse matrix, to a minimum of 10 −18 m 2 for andesitic lavas and volcaniclastics with fine or altered matrix. Analysis of historical pumping tests on aquifers in reworked, channel and alluvial sediment indicate permeabilities ∼10 −10 m 2 . Springs at elevations between 200 and 400 m above mean sea level on Centre Hills currently discharge over 45 L/s. High discharge require a reasonably laterally continuous low permeability body. Contrasting conceptual models are presented to illustrate two potential hydrogeological scenarios. New field observations also reveal systematic spatial variations in spring water temperature and specific electrical conductivity indicating that meteoric waters supplying the springs are mixed with a deeper groundwater source at some sites.

Groundwater flow and volcanic unrest

“…Decline in non-thermal spring discharge on Centre Hills, Montserrat, were observed prior to the onset of volcanic activity at the adjacent Soufrière Hills Volcano in 1995. This was followed by an increase after the cessation of the second eruptive phase in 2004 (Hemmings et al 2015a). The mechanism behind such fluctuation is unclear, it may relate to fracture dynamics associated with magmatic pressurisation (and depressurisation).…”

“…Many studies suggest the presence of high elevation springs, however it is not clear whether they are fed by the regional water table or by perched saturated layers high up on the cone (Cabrera and Custodio 2004;Custodio 2007;Cruz and Oliveira Silva 2001;Hemmings et al 2015a;Ingebritsen and Scholl 1993;Join et al 2005;Peterson 1972). Access to wells on the flank of volcanoes and geophysical imaging methods can help resolve the hydrogeology behind such springs (Finn et al 1987(Finn et al , 2001Aizawa et al 2008).…”

“…However, the earth's surface could equally be considered part of the system. This adds the further complexity of flow within the unsaturated (vadose) zone (Hemmings et al 2015a). Furthermore, the upper limit of the hydrological system closely depends on the precipitation regime.…”

A new record of the freshwater polychaete Namanereis hummelincki <br />(Polychaeta: Nereididae) from epigean waters of Montserrat

“…This river is fed by springs, the largest of which produces 12 L/s located at 255 m above sea level (Hemmings et al 2015). There is a seasonal pattern to rainfall on the island, and the local wet season spans from July to November (Hemmings et al 2015). In January 2015, the Nantes River was flowing, though it was shallow and no more than 1 m wide.…”

“…The Nantes River is about 3 km long and drops about 550 m in altitude (Montserrat Tourist Board 1983). This river is fed by springs, the largest of which produces 12 L/s located at 255 m above sea level (Hemmings et al 2015). There is a seasonal pattern to rainfall on the island, and the local wet season spans from July to November (Hemmings et al 2015).…”