Long‐term effects of orthophosphate treatment on copper concentration

Abstract

In laboratory, pilot, and Lead and Copper Rule (LCR) monitoring studies of copper solubility, cupric hydroxide or a highly microcrystalline tenorite appears to be the dominant solid phase, supporting the theoretical cupric hydroxide copper solubility model. This model predicts that as pipe ages, tenorite or malachite will form and predominate as the surface phase, lowering copper levels. Orthophosphate treatment initially may lower copper levels and yield temporally stable copper concentrations instead of displaying this aging phenomenon. Given enough time (i.e., years to decades), systems without orthophosphate likely will experience copper concentrations lower than what would be achieved with orthophosphate. Because LCR‐compliance monitoring structure is intentionally biased toward sites that may exhibit elevated lead rather than elevated copper, water utilities should be aware that copper concentrations above the action level and several times the 90th percentile value can be prevalent in parts of their distribution systems. To achieve maximum health protection and strict regulatory compliance, systems with higher percentages of newer construction with copper water lines may need to consider the tradeoffs between pH and/or alkalinity adjustment and orthophosphate addition for their optimal corrosion‐control treatment strategy.