Regionalized phosphorus fate factors for freshwater eutrophication in Bahia, Brazil: an analysis of spatial and temporal variability

Abstract

Fate factors, for freshwater eutrophication, represent the route of a limiting-nutrient, phosphorus or nitrogen, and their degradation in the environment. Their value may vary according to the location and emission season; that is, they are site and temporal dependent. In this study, phosphorus fate factors for the freshwater eutrophication impact category were estimated in a native scale of state hydrographic units (SHU), considering its variability and applied in a case study for one hydrographic basin in the state of Bahia, Brazil. The fate model considered the phosphorus removal processes of advection, retention, and agricultural and domestic water use, based on national databases (Geonetwork, HidroWeb, SAR, and SNIS). Heat maps and correlation networks allowed the assessment of fate factors’ spatiotemporal variability. The new generated fate factors were qualitatively and quantitatively assessed, then further applied in a case study of buffalo milk production at Leste SHU to determine the regionalization effects on the freshwater eutrophication potential impact. This study reconfirmed the spatial and temporal variability of the fate factors. The fate factor values found, in this study, for Pardo SHU are similar to those estimated with the non-regionalized data used by the adopted model, being within the range of 0 to 20 days. However, the fate factors for De Contas and, mainly, for Leste were underestimated, reaching a value up to five times higher, indicating that the model’s proposed changes impacted the final result. The temporal differentiation was mainly determined by periods of higher and lower water availability. Advection processes inversely determined the fate factors, and the phosphorus removal by retention processes became more prominent when the water availability was lower. On the other hand, the role of phosphorus removal through water use was minimum. Water availability was the largest contributor to the sensitivity analysis, followed by the total freshwater volume. The fate modeling adopted in this study, with the use of data from national databases and native resolution, gave a more precise and detailed analysis perspective of the phosphorus fate in Brazilian SHUs, setting up site- and temporal-specific fate factors. Besides, this study provides a further understanding of the fate factors establishment by analyzing their relationship with the respective input parameters and highlighting one of the used model’s weaknesses: the estimation of phosphorus removals instead of phosphorus load underestimates the true freshwater eutrophication potential.