Tracing nitrogen pollution from wastewater on an oceanic, touristic island: Integrating local knowledge and uncertainty into a mass balance modeling approach

Nutrient pollution from wastewater endangers coastal and marine ecosystems and contributes to health risks in many small islands in the Wider Caribbean Region. Identifying promising intervention points for management requires understanding the flows of wastewater and contained nutrients. However, on such islands data about wastewater flows is typically scarce, and knowledge is distributed among various actors and institutions. Additionally the system ' s complexity is high, as nutrient fate is determined by an interplay of technical, social, environmental, and ecological factors. Using multiple methods, this paper addresses these complexities, focusing on San Andre <acute accent>s Island, Colombia, which is subject to strong population and tourism pressure. Based on interviews with local actors, we develop a conceptual model to map the total inorganic nitrogen (TIN) fate from touristic and domestic sources to sinks in the environment. With a mass balance model that integrates diverse data sources and elicitation from local experts, we quantify TIN pathways taking into account uncertainty with parameter distributions. Based on the model, we estimate that on an average day in the dry season about 56 % of wastewater -derived TIN ends up in the ocean and 44 % in the island ' s land sinks. Tourists directly contribute about 13 % of TIN, though their indirect contribution is likely much higher. The main pathway of TIN to the ocean is via a centralized sewer network and to the land sink via distributed soak pits. Without additional treatment or nutrient cycling, the potential management scenarios we investigate merely shift pollution issues between environmental compartments. This research underlines the need for a systems perspective on wastewater management for small islands.