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 end points. Unfortunately, concentrations of phosphorus (P) and nitrogen (N) remain high in many U.S. waterbodies and may be increasing in remote, relatively pristine watersheds. Several hypotheses have been advanced to explain this increase, including warming temperatures, increased dust levels, 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 U.S. Remarkably, when expressed on a per-area basis, retardant P and N are applied at rates 4 to 44 times and 0.3 to 16 times 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 in the future.

Impact/Purpose

This journal article estimates the amount of nitrogen (N) and phosphorus (P) delivered to National Forest Land in the western US between 2012 and 2019 via fire retardants.  Excessive nutrients can cause a myriad of water quality problems.  Fire-retardant nutrient estimates were derived using data on fire retardant usage from the US Forest Service and the known chemistry of these fire retardants.  For context, we then compared the amount of these nutrients to fertilizer averages for row crops and to estimates of deposition at the subbasin scale.  In all, we showed that nutrients from retardants are applied at rates that generally exceed that of row crops.  When expressed at the subbasin scale, P deposition from retardants is equivalent in many subbasins to the magnitude of P deposition, while N from fire retardants is much smaller than depositional N.  In all, we find that P from fire retardants is likely an important contributor to nutrient budgets for western US forests and waterways, with the potential to affect terrestrial ecosystems and water quality.  We think resource managers and other researchers will be interested in these findings. 

Citation

Moorhead, Leigh C., Michael J. Pennino, Robert D. Sabo, AND Stephen D. LeDuc. Fire Retardants Are an Overlooked Source of Phosphorus to Western US Ecosystems. American Chemical Society, Washington, DC, 5(4):1620-1627, (2025). [DOI: 10.1021/acsestwater.4c00966]

DOI: Fire Retardants Are an Overlooked Source of Phosphorus to Western US Ecosystems