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

Abstract

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.