Fire retardants are an overlooked source of phosphorus to western US ecosystems.

Excessive nutrient loading to surface waters endangers drinking water supplies, recreation, aquatic life, and many other water quality endpoints. Unfortunately, concentrations of phosphorus (P) and nitrogen (N) remain high in many US waterbodies and may be increasing in remote, relatively pristine watersheds. Several hypotheses have been advanced to explain this increase, including warming temperatures, increased dust, and wildfire-associated smoke and ash deposition. Notably, nutrients from fire retardants have been heretofore overlooked. For the first time, we estimate P and N inputs from ammonium phosphate-based fire retardants in the western US. Remarkably, when expressed on a per area basis, retardant P and N are applied at rates 4-44x and 0.3-16x greater, respectively, than agriculture fertilizer rates for corn, wheat, and other row crops. Moreover, aggregated across subbasins, retardant P-but not N-is comparable to estimated atmospheric deposition rates. Fire retardants help protect human lives and property, and measures are taken to avoid application directly to waterbodies and riparian areas. Nevertheless, the potential for runoff exists, and, even if this does not occur, nutrients from retardants may alter terrestrial ecosystem productivity and carbon cycling, particularly given their usage is only likely to increase under future climate change.