A simple framework for attribution of changes in concentration-load-discharge relationships and estimating the longevity of biosolids legacy phosphorus

Legacy phosphorus (P) is a key driver of freshwater eutrophication, additionally limiting water quality improvements after implementing best managing practices. Recycled wastewater biosolids used as fertilizers are often significant contributors of legacy P in agricultural landscapes, leading to an urgent need to understand the timeframe of P releases from impacted soils. Therefore, this study aims to estimate the longevity of the effects of biosolids-derived P on water quality. We employed a parsimonious data-driven approach (two parameters estimated through linear least squares regression) to evaluate the drivers of P exports and their change over time using more than 20 years of discharge (Q) and P concentration (C) data from 9 catchments of the St. Johns River Basin, FL. We further developed novel elasticity coefficients to evaluate changes in C and exported loads (L) in response to changes in Q and the two regression parameters. To forecast the depletion of biosolids P, we implemented zero- and first-order mass balance models. We found the ratio of the variances of ln C and ln Q to be an effective metric to estimate solute exports drivers, and highlighted limitations with the commonly used metric of coefficients of variation, providing evidence to question whether commonplace statistics are the most appropriate. The novel elasticity coefficients identified the expansion of solute sources as the dominant driver of increased P exports, followed by changes in source spatial distribution, and negligible effects from changes in discharge. The longevity of biosolids P pools was estimated in the order of a hundred to a thousand years, providing valuable information for managers and policy makers to estimate times of recovery and to establish intervention priorities.