National-scale river water quality in the Philippines: establishing baselines in a mineralised megadiverse nation

Mining underpins the global economy, is crucial to achieving net-zero targets yet can cause significant degradation of river health. Critical to managing this risk are national river monitoring programmes ideally incorporating pre-mining baseline water quality data to evaluate the impacts of mining and the success of mine site rehabilitation. We obtained metal(loid) flux, concentration, and bioavailability baseline data across the Philippines, a country with limited environmental datasets, globally significant biodiversity, and considerable mineral wealth, to establish nationwide benchmarks of water quality prior to an industrialisation of mining operations. Through sampling 10 major catchments, we show the Philippines contributes proportionally large volumes of metal(loid)s to the oceans (3 % Co, 1.7 % Ni, 1.2 % Cu, 0.6 % Cr). These large fluxes are not solely from mining-impacted catchments but also from mineralised catchments with no large-scale mining. This suggests that an apportionment approach, where contaminant contributions are allocated to baseline and anthropogenic sources, would be beneficial to catchment managers and help identify and prioritise remedial interventions. Additionally, C_DGT (bioavailable) metal(loid) concentrations varied substantially across the Philippines, demonstrating the need for water quality guidelines based on ecologically-relevant metal(loid) concentrations. This study establishes the first nationwide ground-truthed baselines of hydrology and water quality in the Philippines, providing a reference point for future management and a statement of current condition. Beyond the Philippines, our monitoring approach can be adopted in other mineralised nations so that the future impacts of mining and the efficacy of sustainable mining approaches can be effectively monitored and reported on.