Land use decisions deteriorate stream water quality jeopardizing water supply in a densely populated tropical region

Pedro Ribeiro Piffer, Ecology, Evolution and Environmental Biology, Columbia University, New York, NY, Leandro R. Tambosi, Center for Engineering, Modeling and Applied Social Sciences, Federal University of ABC, Santo Andre, Brazil and Maria Uriarte, Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY

Background/Question/Methods
One of the most pressing challenges today is ensuring an adequate supply and quality of water in light of burgeoning and often conflicting human and ecosystem needs. Land use decisions can influence soil erosion and discharge of nutrients and pollutants into streams. Over time, these effects can severely degrade stream water quality, compromising water supply and increasing water treatments costs. Here we examine changes in water quality between 2000 and 2019 and their association with land use in the state of São Paulo, Brazil. The growing costs of water treatment in the region, home to over 40 million people, call for the development of integrated water and land management plans. We analyzed water quality data for a number of water quality metrics collected bi-monthly between 2000 and 2019 from 230 monitoring stations together with detailed land use cover maps. To assess trends in water quality for each monitoring station, we calculated the number of times each metric exceeded the legal threshold between 2000 and 2019 and asked if metrics were improving or deteriorating over time. We then investigated the effects of watershed landscape composition on temporal trends in water quality.

Results/Conclusions
Across all stations, a large proportion of stream water samples exceeded legal thresholds for at least one water quality metric. The proportion was highest for total dissolved solids and total phosphorus, with fewer samples exceeding the threshold for turbidity and dissolved oxygen. For over 30% of monitoring stations, water quality deteriorated between 2000 and 2019 while it improved for less than 15%. Deteriorating trends prevailed for dissolved solids and total nitrogen, while total phosphorus, turbidity and fecal coliforms exhibited improving trends. Monitoring stations in watersheds with high native vegetation cover were less likely to exceed turbidity thresholds, while a high proportion of pasture was associated with lower compliance for turbidity. Watersheds with a high proportion of urban cover had low compliance for dissolved solids, total phosphorus and dissolved oxygen. Total nitrogen increased and dissolved oxygen decreased over time in watersheds with high urban cover. For watersheds dominated by native vegetation, dissolved oxygen increased while turbidity decreased. The deterioration in water quality observed in the region highlights the need to develop land use management strategies to protect water quality and reduce growing costs of water treatment in the state.