Chemical weathering and land use control river alkalinization and dissolved inorganic carbon in the Potomac River, USA

Authors: Duan, Shuiwang; KAUSHAL, SUJAY - University Of Maryland; ROSENFELDT, ERIK - Hazen And Sawyer, Dpc; MURTHY, SUDHIR - District Of Columbia Water & Sewer Authority (DCWASA); Fischel, Matthew

Submitted to: Applied Geochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2025
Publication Date: 8/18/2025
Citation: Duan, S., Kaushal, S.S., Rosenfeldt, E.J., Murthy, S., Fischel, M.H. 2025. Chemical weathering and land use control river alkalinization and dissolved inorganic carbon in the Potomac River, USA. Applied Geochemistry. 191. Article 106530. https://doi.org/10.1016/j.apgeochem.2025.106530.
DOI: https://doi.org/10.1016/j.apgeochem.2025.106530

Interpretive Summary: Dissolved inorganic carbon is present in river water predominantly as carbon dioxide and bicarbonate, which can be released from natural rocks or human development from concrete. The dissolved inorganic carbon in these waterways is an essential indicator of stream health and reflects the watershed land use, agricultural practices, and geology. However, the current concentration of dissolved inorganic carbon in the Potomac River and its watersheds is unknown. This study investigates the dissolved inorganic carbon concentration in the Potomac River and its major tributaries to determine if land use impacts carbonate concentration in the rivers. The results show that agricultural practices such as liming and land use changes such as destroying riparian buffers around streams alter the dissolved inorganic carbon content; however, the major controlling factor is still the predominant geology. This information is essential for scientists and policymakers to determine how human activities alter the chemical balance of natural waterways and if steps are needed to limit the amount of dissolved inorganic carbon in specific waterways.

Technical Abstract: Many rivers in the United States (including the Potomac River) have experienced noticeable increases in dissolved inorganic carbon (DIC) and alkalinity over the last few decades partially due to land use changes and other human activities in their watersheds. This study examined hotspots and possible controlling factors on DIC concentrations in the watershed of the Potomac River. Results showed that the hotpots of the DIC matched best with those of the carbonate rocks, rather than any types of land use including urban and agricultural, suggesting watershed lithology is still the primary control on DIC concentrations. We observed rapid DIC increases in the Potomac main stem as the river flowed through the sub-watersheds underlain by carbonate rocks. Many rivers in the United States have experienced noticeable increases in dissolved inorganic carbon (DIC) and alkalinity over the last few decades. This study examines possible controlling factors on DIC in the Potomac River watershed using combined spatial “hot spot” analyses and correlation analyses with data from DIC, geochemical tracers, land use, and lithology. Results showed that the DIC hotpots matched best with those of carbonate rocks, better than any land use type. Mean daily DIC flux ranged from 21.6 kg day-1 km-1 in carbonate sub-watersheds, higher than those from forest sub-watersheds with siliciclastic lithology (9.7 kg day-1 km-1), crystalline lithology in agricultural and urban watersheds (8.2 and 7.2 kg day-1 km-1). Although DIC concentrations in tributaries were positively correlated with calcium (Ca2+) plus magnesium (Mg2+), the slopes of the regressions showed high depletion of DIC relative to Ca2++Mg2+ in two upper sub-watersheds affected by acid mine drainage (DIC/Ca2++Mg2+ = 0.17) and moderate depletion in the lower urban sub-watersheds (DIC/Ca2++Mg2+ = 0.52). A positive correlation between DIC and nitrate in agricultural sub-watersheds indicates additional DIC inputs from agricultural liming or weathering of native carbonate minerals in response to acidification from the nitrification of ammonium fertilizer. This study suggests that carbonate lithology is still the primary control on DIC concentrations and fluxes in the Potomac River watershed, but more research is needed on the effects of lime dosers, urbanization, agricultural liming, and ammonium fertilizer application on human-accelerated weathering of carbonate lithology to understand long-term trends in river alkalinization and shifting alkalinity transport to the estuary.