Technical assessment combined with an extended cost–benefit analysis for the restoration of groundwater and forest ecosystem services – an application for Grand Bahama

Abstract. A large storm surge caused by Hurricane Dorian in 2019 resulted in extensive flooding and saltwater intrusion into the aquifers of the island of Grand Bahama. This caused about 40 % of the island's water supply to become brackish with no or slow recovery to date and damage to more than 70 % of mangroves and forests on Grand Bahama. Managed aquifer recharge (MAR) and reforestation were considered nature-based solutions (NBS) to mitigate the impacts of Hurricane Dorian. First, a technical assessment of MAR investigated (hydro)geological aspects. As a result, potential locations for a MAR scheme are proposed. Further, a financial and an extended cost–benefit analysis (CBA) integrating ecosystem service (ES) assessments are conducted for proposed MAR and reforestation measures. Based on the current data availability, results indicate that the MAR scheme of rooftop rainwater harvesting is technically feasible. However, based on our first estimate with limited data, this MAR scheme will be able to provide only about 10 % of the water demand in the study area and thus would not be favorable from a financial perspective. Since MAR has a range of positive aspects (including potential reduction of desalinization efforts and improvement freshwater-dependent ecosystems), we recommend reassessment with more detailed hydrogeological data. On the other hand, reforestation measures are assessed to be financially profitable. The results of this study not only prove the technical feasibility and the added value of restoring groundwater and the forest ecosystem on Grand Bahama, but also highlight the associated high costs. The developed methods for investigating ecosystem services from an economic perspective was proven to allow for a systematic comparison of NBSs and reverse osmosis costs and benefits helping, e.g., policy- and decision-makers and to justify their implementation.