Response of a fractured bedrock aquifer to recharge from heavy rainfall events

Abstract

Climate change is predicted to lead to an increased frequency of high intensity rainfall events. The impact on groundwater resources, particularly in areas of fractured bedrock aquifer systems, requires knowledge of how recharge is affected. In this study, the response of a fractured bedrock aquifer to heavy rainfall events is characterized. The study area is the Gulf Islands, a temperate region in British Columbia, Canada. Of the 14 provincial observation wells with hourly groundwater level data, only three showed pronounced responses to heavy rain events and were analyzed in detail. A detailed analysis at one well suggests rises in groundwater level were better correlated to all rain events than exclusively heavy rain during summer. The strength of the correlation decreased as the rainfall intensity increased. Solution of the Green-Ampt equation for rainfall events of varying magnitude indicated that an increase in rainfall intensity leads to more surface ponding and overland flow. Thus, the occurrence of more frequent heavy rain events in the future may result in lower net recharge. Thermal infrared images in seepage areas and the δ18O and δ2H composition for precipitation and seepage suggest rapid seepage responses to heavy rain events and rapid seepage dissipation rates. The overall rapid response of the fractured bedrock was advantageous for studying the response of the system to heavy rain events.