Enhanced copper release from pipes by alternating stagnation and flow events

Traditional studies of copper release in plumbing systems assume that the water extracted from a pipe follows a plug-type flow and that the pipe surface does not interact with the bulk water under flow conditions. We characterized actual stagnation-flushing cycles in a household pipe undergoing corrosion in the presence of a microbial biofilm. The mass of copper released in 10 experiments was on average 8 times the value estimated by using the plug-flow assumption. The experimental copper release pattern was explained by an advection-diffusion model only if a high copper concentration occurs near the pipe surface after stagnation. Microscopic examination of the pipe surface showed a complex assemblage of biotic and abiotic features. X-ray diffraction analyses identified only malachite, while X-ray absorption spectroscopy also revealed cupric hydroxide and cuprite. These results indicate that the surface serves as a storage compartment of labile copper that may be released under flow conditions. Thus, the diffusive transport from the pipe surface to the bulk during stagnation is not the only control of the flux of copper to the tap water when porous reactive microstructures cover the pipe. Our results highlight the need for models that consider the interaction between the hydrodynamics, chemistry, and structure at the solid-water interface to predict the release of corrosion byproducts into drinking water.