Groundwater conditions the effectiveness of surface water diversion in the remediation of the eutrophicated volcanic lake of Furnas, Azores archipelago.

Volcanic lakes in oceanic islands represent extremely important areas for biodiversity and offer exceptional conditions for nature-based tourism as one of the main pillars of economic growth in these regions. Nitrogen and phosphorus fertilizers are being used extensively at the Azores archipelago, similarly to other places in the world, to increase agricultural production and is causing severe pollution and eutrophication of surface freshwater reservoirs. This work concentrates on the evaluation of the efficiency of surface water diversion as a remediation measure to reduce nutrient loading and reverse eutrophication of Furnas crater lake on the island of São Miguel. Nutrient loading was monitored using an extensive water quality monitoring program in the main watershed of the eutrophicated lake that, together with watershed-scale mass balance methods and groundwater and solute transport models, allowed us to identify an average 98% efficiency in the reduction of nitrate loading. However, phosphorus total load in the discharged water to Furnas lake was observed to only be decreased by 33% due to the groundwater origin of phosphorus. Results from modelling suggested that nutrients were emitted from both point (nitrates) and diffuse (phosphorus) sources as surface runoff and as groundwater seepage, respectively. The results obtained recognized a partially successful surface water diversion, since groundwater path was not initially identified, thus highlighting the importance of groundwater flow regime in the design of such remediation measures. This work also provides a perspective on surface water diversion to revert eutrophication under a volcanic formation, where lakes can be naturally more nutrient rich.