Analyzing wastewater management impacts on water quality at coral reef sites around the Colombian island San Andrés with a mass balance model in combination with a Bayesian belief network

The Caribbean Island San Andrés, like many other tropical islands, strongly relies on its tourism
industry, which has seen a sharp increase during the last decades. However, this increase has
not only had positive effects. Wastewater generated by tourism, combined with a lack of
sufficient treatment infrastructure and management, led to groundwater contamination and
nutrient enrichment of the surrounding coastal waters, including coral reefs. Although limited
monitoring data on the coastal water quality is available, no study has yet investigated the
dominant wastewater pathways and potential intervention points. In this work, a mass balance
model and a Bayesian network were developed to track the nutrient fluxes (inorganic nitrogen
and phosphate) from their generation in domestic and tourist wastewater to their final disposal
in the ocean with a focus on my case study site “Alcantarillado”. To develop the models, I
applied quantitative and qualitative research methods with data collected from several sources,
ranging from literature reviews, and existing empirical data, to semi-structured and structured
expert interviews. This exploratory study allowed me to gain a better system understanding of
the sanitation practices on the island. The treatment pathways differ in how many nutrients are
released and where they enter the environment. I analyzed key factors leading to nutrient
pollution and the influence of tourism relative to population growth. With the model, I found
that tourism contributes comparatively small amounts of nutrients to the study site relative to
the general population. However, with the model structure, I can only examine the direct impact
of tourism. The indirect impact of the tourism industry leading to a steep growth of population
on the island cannot be quantified. Since this modeling approach is the first of its kind for this
island, it would be beneficial to update and further validate it as more data become available.
These models can support decision-makers in identifying drivers of nutrient pollution. In the
future, the approach can be adapted to further study the impacts of different wastewater
management actions in form of scenarios. Additionally, the developed framework can be
adapted to other tropical islands facing similar issues. Excess nutrients pose a serious threat to
marine ecosystems, such as coral reefs, which act as the main tourist attraction on tropical
islands and provide a range of other ecosystem services. Improving wastewater management,
therefore, is crucial to maintaining the socio-ecological system of these islands.