Declining Alkalinity Sink in the Potomac River Estuary

Strong evidence for the alkalinization of rivers has been presented in a number of recent studies, with changes in land use, decreased acid deposition, and recovery from acid mine drainage proposed as some of the potential drivers. However, studies of long-term changes in alkalinity in estuarine waters are less common. In the present study we analyze a multi-decadal alkalinity time series in the Potomac River estuary - a major tributary to the Chesapeake Bay. The 30-year trends at nine tidal locations vary from 10 to 12 mmol m^-3 yr^-1, substantially exceeding the observed trend in the non-tidal Potomac (4 mmol m^-3 yr^-1). Mean alkalinity in the estuary is about 300 mmol m^-3 yr^-1 below the mean non-tidal value, which suggests that there must be an alkalinity sink in the tidal Potomac. Given the observed increasing temporal trend in alkalinity, we infer that this sink must be declining with time. The nature of the alkalinity sink as well as the potential causes for its decline are yet to be determined.