Ocean acidification due to uptake of atmospheric CO₂ is a concern in the open ocean. In contrast, pH in coastal systems has shown both decreasing and increasing long-term trends. A number of global and regional processes drive these diverse trends, including changes in nutrient loading, human-accelerated chemical weathering in watersheds, changes in acid-rain and land-use, as well as atmospheric CO₂ increases. Using the Chesapeake Bay as a model coastal system, we conducted a 60-year model simulation and scenario analysis of carbonate chemistry between 1951 and 2010. We found pH increased in the upper Bay, decreased in the bottom waters of the mid-and lower Bay, and displayed no trend in the surface waters of the mid- and lower Bay. River alkalinization drove the pH increase in the upper Bay, whereas ocean acidification drove the pH decline in the lower Bay. The two drivers canceled each other in the surface waters of the mid- and lower Bay and resulted in no long-term trends in pH there. As ocean alkalinity enhancement (OAE) is being considered as an approach for marine carbon dioxide removal, results of this analysis strongly suggest that OAE could be an effective way to mitigate ocean acidification in coastal waters.