From pipes to streams: The hidden influence of orthophosphate additions on urban waterways

We present clear evidence that orthophosphate (PO 4 3- ), commonly used by drinking water treatment plant (DWTP) operators to reduce lead pipe corrosion, can move beyond its intended domain and enter urban stream networks. This migration influences stream biogeochemistry in ways previously undocumented. In collaboration with a DWTP in Pittsburgh, Pennsylvania, we conducted a study across five urban streams, capturing pre- and post-implementation phases of PO 4 3- -based corrosion control. Through comprehensive chemical analyses of nutrients, anions, metals, and nitrate isotopes, as well as nutrient limitation bioassays with Cylindrospermopsis sp. and Raphidocelis subcapitata , we demonstrate statistically robust increases in streamwater phosphorous concentrations (total dissolved phosphorus: p < 0.0001; total reactive phosphorus and total phosphorus: p < 0.05). These shifts coincide with elevated dissolved metal concentrations, implicating corrosion control byproducts as co-transported constituents. Principal Component Analysis reveals urban stream chemistry is governed by a complex interplay of solutes derived from PO 4 3- -treated drinking water, pipe corrosion, mineral weathering, and wastewater. Bioassays confirm that nutrient additions, particularly P and NP, significantly stimulate algal biomass (p < 0.05) with Cylindrospermopsis sp. showing heightened responsiveness to N additions shortly after PO 4 3- deployment. These findings expose a critical, underrecognized pathway by which drinking water infrastructure contributes to nutrient enrichment and eutrophication in urban aquatic systems. The implications are clear: subsurface infrastructure is not a closed system, and its chemical footprint extends into the urban hydroscape with ecological consequences that warrant closer attention.