Numerical Investigation of the Hydrology of Volcaniclastic Aquifers on Montserrat and Their Response to Volcanic Unrest

An understanding of hydrological processes on volcanic islands is vital for both resource and hazard management. A hydrological system can both modify the volcanic hazard and react to volcanic perturbations. Understanding this interaction is essential for the development of a truly multi-parameter, hazard monitoring dataset. The Caribbean island of Montserrat, in the Lesser Antilles volcanic island arc, hosts a number of productive perched springs as well as confined aquifers. The presence of both active volcanic and hydrological systems means that Montserrat provides a unique environment to study their interaction. Although there is circumstantial evidence for a volcanic influence on spring yield in Montserrat, the relationship between the hydrology of volcaniclastic aquifers and sub-volcanic processes is poorly understood. Using the TOUGH2 numerical simulator for fluid flow in porous and fractured media, we explore the fundamental hydrogeology of the extinct Centre Hills (CH) complex (0.5 -1 Ma) as a basis for better understanding the nature of volcanic and hydrological interactions. Our generalised conceptual hydrological model for Montserrat, is one of steep-sided, central cores of variably brecciated and fractured remnant lava domes and plugs, surrounded by aprons of relatively permeable volcaniclastic deposits. Low permeability, indurated surfaces and clay rich debris avalanche deposits are capable of perching aquifers on the volcanic flanks and confining them at lower elevations. We explore and refine this conceptual hydrological model, and compare modeled perched aquifer response to seasonal recharge fluctuations with recorded spring discharge variation. By comparing hydrological and volcanic monitoring data, we identify potential volcanological perturbations to spring discharge and well water level, to inform further THOUGH2 simulations exploring spring production behavior and temperature anomalies potentially associated with volcanic activity.