The self-purification capacity of an urban stream and the loss of ecosystem services

Abstract

The hypothesis was that all streams, even those highly transformed, retain some of their capacity to provide ecosystem services. The Salgado stream is an urban stream located in the city of Lobos; it is channelled to prevent flooding and promote faster flow of its waters. In its middle section, this stream receives the drainage of the treated effluent from Lobos. Its purification capacity was analysed using two methods, and in two adjacent reaches. In one approach, the wastewater treatment plant effluent was considered a continuous addition of nutrients, and stations were established downstream to analyse the potential for retention of these materials and selfpurification of the stream. In the other approach, sewage effluent was added using a peristaltic pump. After the entire water body was mixed and stabilized, samples were taken downstream to evaluate the nutrient uptake and retention capacity of other materials. In both cases, metrics were used to estimate the length of the nutrient spiral as a measure of self-purification capacity for phosphates, nitrates, ammonium and particulate matter. In cases where the spiral length could be estimated, it was up to 15 and 20 times shorter upstream of the wastewater treatment plant; its efficiency and capture rate also decreased significantly. These differences are attributed to the intensity and persistence of the impact after the wastewater treatment plant. It is concluded that the canal continues to function —at least partially— as a stream, providing the self-purification service, albeit with reduced efficiency. Furthermore, the differences in retention capacity in each section can be considered a way of measuring the loss of the self-purification ecosystem service downstream of a point source of pollution.