Phosphorus in the Susquehanna River may be tied to legacy sediment and changing pH

Issue

In addition to nutrient pollution from agricultural and urban areas, this river has historically had a slightly acidic pH (<7) due to acid drainage from widespread coal mining and atmospheric deposition. Despite implementation of pollution-control measures and reductions of phosphorus (P) inputs across the watershed, total phosphorus (TP) and dissolved orthophosphate (PO₄) loads and concentrations near the river mouth have not decreased. 

There is a need to identify and track sources of TP and PO₄, as well as storage and transformation pathways from headwaters to the Chesapeake Bay. Increases in pH, due to long-term decreases in acidity from coal-mine discharges and atmospheric deposition, may be contributing to the release of dissolved (bioavailable) PO₄ from legacy sediment that has filled major impoundments in lower reaches of the Susquehanna River. At pH >8, which is the current baseline pH in the river, PO₄ may be mobilized from river sediment to the water column.

USGS Study

The USGS created a geochemical model that demonstrates the potential release of PO₄ bound to sediment with increasing pH. Empirical data from laboratory experiments corroborate model results. 

Model features include:

• A user-friendly interface that facilitates input of initial solute concentrations and adjustment to system variables, such as sorbent properties and/or potential for mineral precipitation, without changing the underlying coding.
• Potential to evaluate distributions of PO₄ among aqueous and solid phases as functions of pH, solubility, and sorbent availability.

Primary Findings

• Legacy sediment in the Susquehanna River is a tenable source of bioavailable PO₄.
• Since the 1950s, baseline pH of the Susquehanna River has increased from ~6.5 to ~8.
• Model simulations demonstrate that the release of PO₄ from sediment to the water column increases dramatically from pH 6.5 to higher values, with nearly all PO₄ dissolved at pH 9 (Fig. 1). 

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Implications for Management

Legacy sediment is a currently unquantified source of PO₄ that warrants consideration by resource managers and programs to reverse and prevent degradation of the Chesapeake Bay:

• The desorption of PO₄ may be particularly applicable for shallow conditions where water and sediment are closely associated, and where algal growth and decay can produce dynamic extremes in pH, with maximum values exceeding 9. 
• The transfer of PO₄ into the water column may increase algae growth, which removes CO₂ and drives pH to higher values, facilitating additional PO₄ release and exacerbating the potential for harmful algal blooms.
• There are significant knowledge gaps in terms of how current watershed-scale nutrient load models represent sources, transport, and transformation of P within the watershed and river network. 
• The study results are highly relevant to understanding the origin of PO₄ in the water column and potential interactions with other physical, chemical, and biological parameters. 

Publication Details

Charles A. Cravotta, Travis L. Tasker, Peter M. Smyntek, Joel D. Blomquist, John W. Clune, Qian Zhang, Noah M. Schmadel, Natalie K. Schmer, Legacy sediment as a potential source of orthophosphate: Preliminary conceptual and geochemical models for the Susquehanna River, Chesapeake Bay watershed, USA, Science of The Total Environment, Volume 912. https://doi.org/10.1016/j.scitotenv.2023.169361

For more information, please contact the study leaders

Charles A. Cravotta, Research Hydrologist
Retired USGS, Pennsylvania Water Science Center
Email: cravottageochemical@gmail.com

Joel D. Bloomquist, Supervisory Hydrologist
USGS, Water Mission Area, Baltimore, MD 
Email: jdblomqu@usgs.gov