THE HYPORHEIC ZONE AS A SOURCE OF DISSOLVED ORGANIC-CARBON AND CARBONGASES TO A TEMPERATE FORESTED STREAM

The objective of this study was to examine chemical changes in porewat ers that occur over small scales (cm) as groundwater flows through the hyporheic zone and discharges to a stream in a temperate forest of no rthern Wisconsin. Hyporheic-zone porewaters were sampled at discrete d epths of 2, 10, 15, 61, and 183 cm at three study sites in the study b asin. Chemical profiles of dissolved organic carbon (DOC), CO2, CH4, a nd pH show dramatic changes between 61 cm sediment depth and the water -sediment interface. Unless discrete samples at small depth intervals are taken, these chemical profiles are not accounted for. Similar tren ds were observed at the three study locations, despite each site havin g very different hydraulic-flow regimes. Increases in DOC concentratio n by an order of magnitude from 61 to 15 cm depth with a corresponding decrease in pH and rapid decreases in the molecular weight of the DOC suggest that aliphatic compounds (likely organic acids) are being gen erated in the hyporheic zone. Estimated efflux rates of DOG, CO2, and CH4 to the stream are 6.2, 0.79, 0.13 moles m(2) d(-1), respectively, with the vast majority of these materials produced in the hyporheic zo ne. Very little of these materials are accounted for by sampling strea m water, suggesting rapid uptake and/or volatilization.