Sewage contamination assessment in an urbanized tropical estuary in Northeast Brazil using elemental, isotopic and molecular proxies

The Capibaribe Estuarine System (CES) is a tropical estuary located within a densely populated metropolitan region in the Brazilian Northeast, providing multiple ecosystem services including artisanal fisheries, tourism, and navigation. A significant load of domestic and industrial effluents is causing visible environmental degradation that needs to be characterized. This study evaluated the degree of contamination by domestic sewage and the relative contribution of organic carbon (OC) sources in sediments, measuring carbon-to-nitrogen atomic (C/N)(a) ratios, delta C-13 of OC, and linear alkylbenzene concentrations (LAB). The SIAR mixing model indicated that estuarine phytoplankton and sewage contribute 73% and 22% to OC, respectively, based on sedimentary (C/N)(a) ratios that varied from 8.5 to 13, and delta C-13 ranging from -25.21 to -21.63 parts per thousand. This suggests that eutrophication of CES is triggered by sewage inflow. The CES was moderately contaminated by LAB, and the highest concentrations observed in the inner portion, from 287 to 1349 ng g(-1) dry weight, suggests the retention of sewage. The mixing isotopic model indicated a significant dilution process of sewage-derived OC by estuarine phytoplankton. The lowest LAB in the lower estuary (317-320 ng g(-1) dw) probably resulted from marine dilution. Principal component analysis demonstrated that mud regulates sedimentary OC content, and that the sewage plume regulates sedimentary LAB. High degradation of LAB isomers probably reflects the net heterotrophic condition of this system, also suggesting these compounds are introduced into CES partially degraded. Our results pointed out that degraded OM is a result of chronic and long-term sewage input. Considering the implications of sewage pollution to environmental and public health, there is an urgent need of better policies to improve domestic effluent treatment capacity, reducing contamination of estuarine environments.

Automated summary

The Capibaribe Estuarine System (CES) located in the Brazilian Northeast is a tropical estuary that provides multiple ecosystem services but is facing visible environmental degradation due to domestic and industrial effluents. This study evaluated the contamination level of domestic sewage and the contribution of organic carbon sources in sediments. The results indicate that sewage inflow triggers eutrophication of CES, and the system is moderately contaminated by linear alkylbenzene (LAB), suggesting retention of sewage. Improved policies are urgently needed to reduce contamination of estuarine environments and protect public health.