Population size, income and poor sanitation interact to explain widespread streamwater contamination by antidepressants in the Metropolitan Region of São Paulo.

The expansion of urban settlements over native environments may expose biodiversity to a host of emerging contaminants, with unintended ecological effects. This study evaluated patterns of contamination of streamwater by antidepressants in the Upper Tietê River Basin, a watershed of high social, economic and environmental relevance for comprising both the largest urban settlement in South America (the Metropolitan Region of São Paulo) and remnants of a globally important biodiversity hotspot (the Atlantic Rainforest). We sampled 53 third-order streams draining catchments regularly distributed across a gradient in urban cover. Antidepressant contamination was found to be widespread. Whereas no antidepressants were detected in any of the 11 streams draining entirely forested catchments, 39 of 42 remaining streams were contaminated with one to eight antidepressant molecules. Concentrations increased monotonically with urban cover and bracketed the entire range found in global freshwaters. Concentrations increased with the number of inhabitants in the catchment and with number of households with no sanitation, but only in catchments with higher mean per capita income. Although concentrations in the range of tens to hundreds of nanograms-per-liter as found may appear to be low, literature data demonstrate effects on individual performance, population growth rates, and even transgenerational effects in which short-term exposure at the embryonic stage may affect life history traits over three generations of descendants. These findings highlight the need to expand sanitation infrastructure and to adopt policies of urban planning that reconcile human settlement with biodiversity conservation in Latin America.