Nutrient inputs from natural and anthropogenic sources to the Guapi-Macacu River Basin (Rio de Janeiro, Brazil): A region under the influence of peri-urban expansion processes

Abstract

Increased anthropogenic nitrogen (N) and phosphorus (P) inputs to river basins are recognised as one of the main drivers of water quality deterioration. Identifying spatial patterns of N and P inputs is therefore crucial for appropriate management of river basins. This study investigated the spatial variation of natural and anthropogenic N and P inputs to the Guapi-Macacu River Basin (GMB) by integrating emission factor methodology with geoprocessing analysis. The N and P inputs from natural sources were 2170 and 10 kg km⁻² yr⁻¹, respectively. For anthropogenic sources, the N and P inputs were 1888 and 1009 kg km⁻² yr⁻¹, with livestock activity being the main contributor for both elements (53 % of N and 67 % of P). According to the geoprocessing analysis: (i) N input derived from biological fixation in the Atlantic Forest occur predominantly at steepest surfaces; (ii) livestock activity is distributed across flat and medium-slope surfaces; and (iii) inputs associated with agricultural and urban uses are concentrated in river plains. Despite the low relative importance of urban (35 and 21 % of anthropogenic N and P) and agricultural (12 and 11 % of anthropogenic N and P) inputs, spatial patterns suggest a high potential impact of these uses on water quality. Elevated N and P river flows in peri-urban areas, which are considerably higher than in rural areas, confirm this hypothesis. Mitigation strategies, such as the implementation of manure management and the construction of sewage treatment plants, are crucial to prevent water quality deterioration in the GMB.