USE OF GEOCHEMICAL MASS-BALANCE MODELING TO EVALUATE THE ROLE OF WEATHERING IN DETERMINING STREAM CHEMISTRY IN 5 MID-ATLANTIC WATERSHEDS ONDIFFERENT LITHOLOGIES

The importance of mineral weathering was assessed and compared for fiv e mid-Atlantic watersheds receiving similar atmospheric inputs but und erlain by differing bedrock. Annual solute mass balances and volume-we ighted mean solute concentrations were calculated for each watershed f or each year of record. In addition, primary and secondary mineralogy were determined for each of the watersheds through analysis of soil sa mples and thin sections using petrographic, scanning electron microsco pe, electron microprobe and X-ray diffraction techniques. Mineralogica l data were also compiled from the literature. These data were input t o NETPATH, a geochemical program that calculates the masses of mineral s that react with precipitation to produce stream water chemistry. The feasibilities of the weathering scenarios calculated by NETPATH were evaluated based on relative abundances and reactivities of minerals in the watershed. In watersheds underlain by reactive bedrocks, weatheri ng reactions explained the stream base cation loading. In the acid-sen sitive watersheds on unreactive bedrock, calculated weathering scenari os were not consistent with the abundance of reactive minerals in the underlying bedrock, and alternative sources of base cations are discus sed. (C) 1997 John Wiley & Sons, Ltd.