Reference, completed project

Reunion Island: conducting predictive simulations of nitrate levels in the Cocos aquifer in order to restore groundwater quality

The main catchments of the Cocos aquifer on Reunion Island are affected by groundwater nitrate levels exceeding the alert threshold. In order to draw up an action plan based on predictive scenarios, the first step was to numerically simulate the relationships between changes in agricultural and/or urban practices and nitrate concentrations.

25 November 2024

Comparison of the results of nitrate concentration simulations between 1985 and 2050 for the theoretical scenario “without agricultural nitrogen” and for the theoretical scenario “without urban nitrogen” on Coco borehole 1

The need

The main catchments of the Cocos aquifer in Saint-Louis (Reunion Island) have high nitrate concentrations in the Coco 1, Coco 2 and Coco CGE boreholes (around 30 to 50 mg/L) that are steadily increasing (+0.5 to +1 mg/L/year). An analysis of the hydrochemical data from the Cocos aquifer suggests that both agricultural and urban practices are contributing to these high concentrations.

Based on this observation, we had to assess the relationship between pressure and impact in order to see how changes in agricultural and/or urban practices in the area could contribute to improving groundwater quality.

The results

Using the BICHE global model developed by BRGM (software dedicated to the global modelling of nitrate transfers in a hydrogeological basin), the study sought to simulate and predict changing nitrate concentrations in the catchments of the Cocos aquifer, based on various agronomic and urban scenarios. The models were calibrated using hydrological data covering a period of 35 years (1985-2020) and 20 years (2000-2020) in the case of the chemical data.

Predictive simulations of concentrations were carried out up to 2050. A number of scenarios were defined for changes in local agricultural and urban practices and their impact simulated. Extreme theoretical scenarios based on sudden but unrealistic changes in land use were used in the first instance to assess the relationship between usage and the transfer of nitrates to the aquifers over time, and to put forward two realistic scenarios for action developed by local experts (DAAF, Chamber of Agriculture, CIVIS).

According to the two realistic scenarios simulated, a reversal of the trend is observed around ten years after the implementation of actions. The results show that the “urban lever” for changing practices appears to be more important than the “agricultural lever”. However, it is important to implement both levers in order to restore water quality more quickly.

This BRGM survey is a fundamental decision-support tool for the French Overseas Departments Water Plan. It enables government and water services to support the local authority concerned (CIVIS) in managing the surface areas supplying a number of catchments that are essential for providing drinking water to the south of Reunion Island. The study also reinforces awareness of the fact that on-site wastewater treatment is a long-term source of nitrogen pollution of groundwater. So we're purifying tomorrow's water today.

Matthieu Saliman, Head of the French Overseas Department Water Plan, Water and Biodiversity Department, DEAL, Reunion Island

BICHE model: operating principle of mass transfer (according to Thiéry, 1990)

Using the results

The results obtained from predictive simulations provide decision-makers with objective information to guide their strategy and draw up an action plan for (i) agriculture, to implement the split method of fertiliser application, and (ii) urban areas, to develop public sewage treatment systems.