Mapping fire-induced water contamination risk in Portugal

Abstract

Wildfires can have severe impacts on humans and nature. One such impact pertains to changes in water quality resulting from the generation and transportation of ash into bodies of water. The quantity and content of this ash can affect the water quality, and thereby the drinking water supply. The Mediterranean region and Portugal specifically, have been, and are increasingly expected to be affected by fire-induced water contamination. Current research has focused on local evaluation of impacts, as well as the global identification of areas at risk. What lacks is the assessment of fire-induced water contamination risk at an intermediate scale, which can be used as a basis for the efficient allocation of efforts mitigating this risk. This study performed a risk assessment of Portuguese reservoirs based on their historical exposure to wildfires, their drinking water dependency and their vulnerability to fire-induced water contamination. A general risk index was established through the creation and combination of hazard and exposure indices, reflecting a reservoirs’ drinking water dependency and historical exposure to wildfire respectively. The former was based on data on the yearly water uptake from drinking water facilities at a reservoir. The latter was based on the processing of 32 years of historical data on wildfires and reflects to which extent and frequency reservoirs’ watershed areas have been burned. This was followed by further analysis at local scale, consisting of (1) a more relevant method of measuring exposure that includes historic reservoir volumes, as well as (2) an exploration of the vulnerability of several water supply systems. This study has produced a nationwide overview of fire-induced water contamination risk as well as of the degree to which the hazard and exposure concepts contribute to this risk. Numerous differences were found regarding the degree to which individual reservoirs are at risk. These can be ascribed to various regional differences, concerning vegetational, meteorological and land use conditions, as well as local differences as a consequence of water infrastructure design, mitigation measures and the availability of alternative sources. Hence, these results can be used to identify areas at risk, and to evaluate what type of mitigation measures should be implemented and to what extent. Further analysis on exposure has provided an improved method of estimating the exposure of a reservoir to fire-induced water contamination. This measure estimates the yearly probability of the occurrence of significant changes in total suspended sediment concentrations for individual reservoirs. Further analysis on vulnerability has most importantly shown that increasing the size and interconnectivity of water supply systems decreases their vulnerability to fire-induced water contamination. These findings contribute to the understanding of fire-induced water contamination risk in Portugal. Further studies can improve on this work by (1) applying the improved exposure measure and exploring the correlation between reservoir volume and fire size, (2) creating a more detailed hazard measure that possibly includes the dependence on other water sources and (3) assessing and including projections on the possible impact of climate change and its effects on meteorological and vegetational conditions.