This project examined the interactions between stream water and subsurface sediment to quantify how these interactions influenced organic C respiration and dissolved inorganic C (DIC) production in the hyporheic zone of a high-gradient headwater mountain stream draining a forested catchment at the H. J. Andrews Experimental Forest, Oregon, USA. The study used six 2-m long hyporheic mesocosms which were packed with streambed sediment in the spring of 2016. The mesocosms are located at the Watershed 1 (WS1) stream gage and stream water from WS1 has been pumped through the mesocosms continuously since they were first packed through the end of (and beyond) this study in autumn of 2018. The mesocosms were designed around 1-m long 20-cm diameter aluminum pipe segments with sample ports located each meter along the flowpath through each mesocosm – thus sampling at the inlet, at 1 m, and at the outlet which represents the full 2-m long flow path. Sampling was conducted on seven dates between Oct 23 2016 and Aug 27 2018. On two of these dates, only background samples were collected. On the remaining 5 dates, sampling was designed around continuous-injection tracer experiments using both a conservative tracer (salt) and a reactive tracer (various dissolved organic substrates). For background sampling events, samples were generally only collected once. The tracer experiments involved 4 discreet sampling times: 1. pre-injection (under background conditions); 2. early plateau; 3. late plateau, and 4. post-injection (and in one injection experiment, a 5th sample at late-post-injection time). For each round of samples, the mesocosm water temperature, pH, EC, and DO were measured with sensors in a small flow-through cell. Then water samples were collected for laboratory analysis for both DOC and DIC. The median travel time of water through each pipe segment of the 2-m mesocosms was also calculated from the conservative tracer break-through curves.
Angelo Sanfilippo, Kevin Feris, Robert S. Pennington, Roy Haggerty, Satish Prasad Serchan, Steven M. Wondzell
The focus of this study was to investigate microbial processing of organic carbon in the hyporheic zones of headwater mountainous streams draining forested catchments. However, biogeochemical processes in the hyporheic zone are open to multiple influences, from hillslope soil water and ground water as well as leachate from overlying soils. Mixing of water from these various sources can obscure the changes in water chemistry that occur simply from the passage of stream water through the sediment comprising the streambed and floodplain. To isolate just those biogeochemical processes occurring between stream water and the streambed sediment, “hyporheic mesocosms” were designed and built. These mesocosms are essentially a system of pipes packed with streambed sediment with continuous through flow of stream water. Water samples were collected from inlets, intermediate, and outlet sampling ports of the hyporheic mesocosms to characterize the influence of biogeochemical processes on cycling of carbon in the stream corridor. In addition, 5 injection experiments were conduction in which DOC entering the mesocosm was slightly elevated to examine how different sources of DOC were processed in this simulated hyporheic zone.