SEASONAL CYCLES OF DISSOLVED CONSTITUENTS IN STREAMWATER IN 2 FORESTED CATCHMENTS IN THE MID-ATLANTIC REGION OF THE EASTERN USA

Streamwater discharge and chemistry of two small catchments on Catocti n Mountain in north-central Maryland have been monitored since 1982. R epetitive seasonal cycles in streamwater chemistry have been observed each year, along with seasonal cycles in the volume of stream discharg e and in groundwater levels. The hypothesis that the observed streamwa ter chemical cycles are related to seasonal changes in the hydrologica l flow paths that contribute to streamflow is examined using a combina tion of data on groundwater levels, shallow and deep groundwater chemi stry, streamwater discharge, streamwater chemistry, soil-water. chemis try, and estimates of water residence times. The concentrations of con stituents derived from rock weathering, particularly bicarbonate and s ilica, increase in streamwater during the summer when the water table is below the regolith-bedrock interface and stream discharge consists primarily of deep groundwater from the fractured-bedrock aquifer. Conv ersely, the concentrations in streamwater of atmospherically derived c omponents, particularly sulfate, increase in winter when the water tab le is above the regolith-bedrock interface and stream discharge consis ts primarily of shallow groundwater from the regolith. Tritium and chl orofluorocarbon (CFC) measurements suggest that the groundwater in the se systems is young, with a residence time of less than several years. The results of this study have implications for the design of large-s cale water-quality monitoring programs.