Trends in phosphorus fluxes are driven by intensification of biosolids applications in the Upper St. Johns River Basin (Florida, United States)

Abstract Canion A, Hoge V, Hendrickson J, Jobes T, Dobberfuhl D. 2022. Trends in phosphorus fluxes are driven by intensification of biosolids applications in the Upper St. Johns River Basin (Florida, United States). Lake Reserv Manage. 38:215–227. Biosolids are beneficially used in agricultural production, but the potential for nutrient enrichment, primarily phosphorus (P), in runoff water remains a concern. This study provides strong correlative evidence that intensified Class B biosolids applications led to increases in total P (TP) and total nitrogen (TN) fluxes in the Upper St. Johns River Basin (USJRB). In 2013, new state regulations resulted in the elimination of Class B biosolids applications in 3 watersheds encompassing most of southern Florida. Most of the applications from these watersheds were shifted into the USJRB, which received 78% of statewide Class B biosolids applications by 2019. Weighted regressions on time, discharge, and season (WRTDS) were used to evaluate the relationship between long-term (1995–2020) trends in tributary TP and TN concentrations and fluxes and the timing and magnitude of biosolids applications in 8 USJRB watersheds. No significant land use change occurred that could account for water quality trends. Flow-normalized concentrations and fluxes were generally stable from 1995 to 2012, but after intensification of applications in 2013, significant increases occurred in 6 and 4 watersheds for TP and TN, respectively. P fluxes increased by 0.9–16.4 metric tons (MT; 40–200%) and N fluxes increased by 1.6–19.7 MT (5–20%). The magnitude of P and N flux increases were between 0.5% and 2.0% of land-applied biosolids P and N, which suggests that small losses of P and N from the landscape were required to produce the observed trends.