Travis L. Tasker Peter M. Smyntek Joel D. Blomquist John W. Clune Qian Zhang Noah Schmadel Natalie Katrina Schmer Charles A. Cravotta III 2024 <div id="ab0005" class="abstract author" lang="en"><div id="as0005"><p id="sp0030"><span>Nutrient pollution from agriculture and urban areas plus&nbsp;acid mine drainage&nbsp;(AMD) from legacy coal mines are primary causes of water-quality impairment in the Susquehanna River, which is the predominant source of freshwater and nutrients entering the Chesapeake Bay. Recent increases in the delivery of dissolved&nbsp;orthophosphate&nbsp;(PO</span>₄) from the river to the bay may be linked to long-term increases in pH, decreased acidity of precipitation, and decreased acidity, iron, and aluminum loading from widespread AMD. Since the 1950s, baseline pH increased from ~6.5 to ~8 in the West Branch and “North Branch” of the Susquehanna River, which drain bituminous and anthracite coalfields of Pennsylvania. A current baseline pH of ~8 and daily maxima exceeding 9 have been documented along the lower Susquehanna River. In response to improved river quality, bioavailable PO₄<span>&nbsp;</span>now may be released into solution from legacy sediment that has filled major impoundments in lower reaches of the river. At typical pH (5–8) of natural water, aqueous PO₄<span>&nbsp;species tend to be adsorbed by hydrous iron, aluminum, and&nbsp;manganese oxides&nbsp;that coat soil and sediment particles; however, PO</span>₄<span>&nbsp;</span>may be substantially desorbed at pH &gt;8. We created a geochemical model that simulates equilibrium aqueous/solid distributions of PO₄<span>&nbsp;</span>as pH and other solution characteristics change. Considering current conditions in the lower Susquehanna River, the model demonstrates potential for extensive release of adsorbed PO₄<span>&nbsp;</span>at pH &gt;8. Empirical data from laboratory experiments corroborate model results. The transfer of PO₄<span>&nbsp;</span>into the water column may increase algae growth, which removes CO₂<span>&nbsp;</span>and drives pH to higher values, facilitating additional PO₄<span>&nbsp;release and exacerbating the potential for&nbsp;harmful algal blooms. Thus, legacy sediment is a currently unquantified source of PO</span>₄<span>&nbsp;</span>that warrants consideration by resource managers and programs collaborating to reduce phosphorus loads to the bay and similar settings worldwide.</p></div></div> application/pdf 10.1016/j.scitotenv.2023.169361 en Elsevier Legacy sediment as a potential source of orthophosphate: Preliminary conceptual and geochemical models for the Susquehanna River, Chesapeake Bay watershed, USA article