Both globally and locally on the island of Curaçao, coral reef degradation is a pressing issue. These ecosystems are threatened by climate change and pollution due to human activities. The latter mostly occurs through pollution of groundwater, elevating the groundwater concentration of nutrients such as Nitrogen (N) and Phosphorus (P). A possible route of this polluted groundwater towards the coastal areas is through submarine groundwater discharge (SGD). One of the main groundwater polluting factors in Curaçao is the leakage of N and P from onsite sewage disposal systems (OSDS). Limited data is available on these leakage processes, and how the increased nutrient concentrations reach the sea through SGD. With the reef degradation furthering, it is of high importance to gain insight into this pollution process.

The goal of this thesis was to determine the behaviour of nutrient fluxes towards the sea and the bays through SGD on Curaçao. This was done by creating a numerical model in MODFLOW6, which first determined the groundwater head distribution and flow field on Curaçao based on a variable-density model. In this model, simplifications were done and fractured flow was ignored. A nutrient model was used to simulate the current state of the nutrient levels of N and P in the groundwater resulting from leakage from OSDS, and their progression towards the coastal area. The current state and three future scenarios were performed, consisting of a pollution increase due to population growth, a pollution stop and constant pollution similar to the current state.

The scenario simulations highlighted the immediate and delayed impacts of population growth and stopped pollution on nutrient fluxes. While N-fluxes responded quickly to changes in input, P, due to its sorptive behaviour, showed a delayed response. Although the SGD is equally distributed between the bays and the sea, the nutrient fluxes are relatively higher towards the bays for the current situation (83% for N and 61% for P). In both current and future states, N-concentrations were higher than P-concentrations, due to a higher N-input rate and the sorptive behaviour of P. If leakage from OSDS stops, N as decaying is almost completely flushed out within 25 years. For N with conservative behaviour, the N flux will be reduced by 75% in 2050. For P, it will take much longer to flush out, as after 30 years the outflow is still increasing. If current pollution increases due to population growth, the N-flux increase by 13% and 12% for respectively the conservative and decay simulations. In 2050, or quickly after, the flux will reach a new constant level. Contrarily, the P-flux will continue to increase for the foreseeable future, decreasing the N:P mass ratio.

The model results provided first insights into nutrient fluxes, current pollution, and future scenarios. However, the results are subject to uncertainties because fractured flow and dual-porosity were not included, requiring careful consideration of the results. In addition, future research should refine parameters and include an uncertainty analysis. Despite these limitations, the model offered valuable insights into nutrient behaviour and the model could be employed in more realistic simulations, allowing assessment of the future of groundwater pollution and its effect on Curaçao’s coral reefs.