Solute dynamics in the hyporheic zone of a headwater stream in Watershed 1 at the Andrews Experimental Forest, 2016-2018

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.

本研究聚焦于溪流地表水与地下沉积物之间的相互作用，旨在量化其对美国俄勒冈州H.J.安德鲁斯实验森林（H. J. Andrews Experimental Forest）内汇水区域为森林的高坡度源头山地溪流潜流带（hyporheic zone）中有机碳呼吸与溶解无机碳（DIC）生成的影响。本研究于2016年春季搭建了6组长度为2米的潜流带人工模拟生态系统（hyporheic mesocosms），装置内部填充有河床沉积物。该模拟系统设置于1号流域（WS1）的溪流测流断面处，自2016年春季填充完成后，便持续通入取自WS1的溪流地表水，直至2018年秋季本研究结束（甚至延续至研究结束之后）。该模拟装置以1米长、直径20厘米的铝管段搭建而成，沿每个装置的水流路径每1米设置一个采样端口，因此可分别在进水口、1米处以及出水口完成采样，覆盖完整的2米水流路径。采样工作于2016年10月23日至2018年8月27日期间的7个采样日开展。其中2个采样日仅采集背景样品，剩余5个采样日均围绕连续注入示踪剂的实验展开，同时使用保守示踪剂（盐类）与反应性示踪剂（多种溶解有机底物）。在背景采样事件中，样品通常仅采集一次。示踪剂实验包含4个明确的采样时段：1. 注入前（处于背景条件下）；2. 早期平台期；3. 晚期平台期；4. 注入后（其中一次注入实验额外增设了第5个采样时段，即晚期注入后阶段）。每一轮采样中，首先通过小型流通池内的传感器测定装置内水体的温度、pH值、电导率（EC）与溶解氧（DO）；随后采集水样送至实验室，分析溶解有机碳（DOC）与溶解无机碳（DIC）的含量。此外，本研究还基于保守示踪剂的突破曲线，计算了水流在2米长模拟装置各管段中的中位停留时间。