Integrating the influence of untreated sewage into our understanding of the urban stream syndrome

Abstract

The process of urbanization profoundly changes aquatic ecosystems. Urban streams often have flashier hydrology, increased nutrient and contaminant concentrations, and reduced biodiversity when compared with nonurban streams. Collectively, these characteristics are described as the urban stream syndrome (USS). The USS is traditionally attributed to the discharge of large volumes of stormwater into streams, thus pointing to stormwater as the primary driver of urban stream degradation. However, the USS was developed using evidence from studies in streams in high-income economies in the Global North, and the extent to which it applies to low- and mid-income countries in the Global South remains unclear. Urban expansion in the Global South is often not concomitant with the expansion of sanitation infrastructure, which can lead to large volumes of untreated sewage being discharged into urban streams. In this paper we explore the role of untreated sewage in the application of the USS concept in Latin America. First, we collected and quantified studies that consider the effect of untreated sewage on urban streams in Latin America. We then use the existing evidence to assess if untreated sewage has the potential to affect urban streams in Latin America in similar ways as stormwater affects urban streams in the Global North. Although untreated sewage pollution is widespread in Latin America, our search documented only 57 studies that consider the effects of untreated sewage in local streams. Despite the lack of empirical studies, it is likely that untreated sewage can be as important as stormwater in driving the USS in Latin America. We assert that to support effective management and conservation of urban streams in Latin America, untreated sewage pollution needs to be explicitly considered in the application of the USS concept.