Addressing drivers and data gaps in Spain's non-compliance of drinking water quality standards

In 2021 Spain passed a new law regarding the quality of drinking water, increasing the transparency and introducing a risk assessment approach to the catchment area, which ascribe to the shift in recent years in how drinking water management is understood in the European Union legislation. Good data quality is important to ensure the correct implementation of policies. We used the drinking quality data uploaded to Spain's National Drinking Water Information System to gauge the state of the drinking water reporting in Spain, the differences between rural urban and rural areas in both quality and reporting and identify which variables at catchment level influence the probability of a municipality incurring in drinking water quality non-compliance. Random forest modeling was used to assess the drivers of non-compliance, including environmental (e.g., land cover, lithology, climate, state of the water supply source) and demographic (e.g., tap water expenditure, population density) data. We found that rural municipalities are more vulnerable both because of a lack of reporting but also because they have higher non-compliance rates for arsenic, microbiological and contaminants and nitrogen compounds (e.g. nitrate). We also found different spatial patterns of non-compliance according to each group of contaminants (e.g., microbiological violations are widespread in the northern half of Spain). The random forest model suggests that agriculture and confined livestock farming are behind nitrogen and microbiological non-compliances. Climate drivers have also emerged for all the groups of contaminants, which underscores the importance of studying drinking water quality non-compliance on a case-by-case basis in order to properly adapt to local realities and enhance compliance across Spain.