Tracking nutrients from sewage

Posted on December 16, 2014

Sizeable quantities of nutrients, organic matter, micro-pollutants and pathogens routinely enter stream systems from wastewater treatment plants and from sewer overflows. A new way to detect and track the ecological impacts of discharged sewage relies on the use of stable isotope profiling. Treated sewage is enriched in the heavy nitrogen isotope 15N because bacteria in wastewater treatment plants preferentially deplete the lighter isotope 14N. As a result, the high content of 15N in treated sewage (10-15 o/oo) distinguishes it from raw sewage (0-5 o/oo) and other sources of nitrogen such as inorganic fertilisers (0 o/oo). Working in 11 British rivers, researchers collected macroinvertebrates from riffles and pools within 500–1000 m of wastewater treatment plants. Animal samples were ground into fine powder for stable isotope analysis. Differences in invertebrate community composition between sections of stream above and below the treatment plants were examined using multivariate ordination techniques. Downstream macroinvertebrate 15N concentrations were significantly higher than those upstream, and were proportional to independently-calculated flows of sewage from the plant. There were corresponding changes in the structure of macroinvertebrate communities, driven mainly by downstream increases in total abundance, decreases in biodiversity and shifts towards pollution-tolerant species such as gammarid shrimps and midge larvae. Nitrogen isotope analysis promises to be a practical and sensitive way of tracing sewage-derived nutrients through freshwater food webs.

Reference: Morrisey, C.A. et al. 2013. Stable isotopes as indicators of wastewater effects on the macroinvertebrates of urban rivers. Hydrobiologia 700, 231–244.