SUPERVISOR: Thomas HEIN

PROJECT ASSIGNED TO: Nadija CEHAJIC

Rivers are not simple canals transporting water and solutes from land to sea, but complex biogeochemical reactors in which a large fraction of terrestrial carbon and nutrients is actively transformed before reaching 
downstream ecosystems (Aufdenkampe et al., 2011). Central to this processing is the hyporheic zone (HZ), defined as the saturated subsurface region beneath and alongside river channels where surface water and groundwater mix along flow paths that originate from and return to the stream within timescales relevant to biogeochemical and ecological processes (Boulton et al., 1998, 2010). Through these exchanges, the hyporheic zone has a disproportionate influence on stream metabolism, nutrient retention, greenhouse gas production, and ecosystem resilience across spatial scales.

Physically, the hyporheic zone is described as a temporally and spatially dynamic saturated transition zone between surface water and groundwater (Krause et al., 2011). Its vertical and lateral extent can range from centimeters beneath the streambed to several meters into alluvial aquifers, riparian soils, and paleochannels, depending on geomorphology, sediment structure, and hydrological gradients (Boulton et al., 1998; Krause et al., 2011).

Ecologically and biogeochemically, the hyporheic zone functions as a dynamic ecotone and an ecosystem control point, where localized hot spots and hot moments exert landscape-scale influence on material fluxes 
(Burrows et al., 2017). Strong spatial heterogeneity in sediment properties and flow paths produce steep chemical and redox gradients over centimeter scales, allowing aerobic and anaerobic processes to coexist in close proximity (Roy Chowdhury et al., 2020). As a result, the hyporheic zone simultaneously supports carbon mineralization, nutrient transformation, greenhouse gas production and consumption, and biological refugia during hydrological extremes such as floods and droughts (DelVecchia et al., 2022).

This research will look at the functionality of the hyporheic zone comparing a restored and unrestored floodplain system. Following questions will be answered:
1. How does the degree of hydrologic connectivity in restored vs. unrestored floodplains alter the net flux of carbon and nutrients within the hyporheic zone?
2. How does restoration shift the molecular composition, thermodynamic quality, and origin of organic matter across the surface water-groundwater interface?
3. How does the depth of surface-water infiltration influence the vertical stratification of redox processes (aerobic respiration, denitrification, and methanogenesis)?

Understanding the shift in hyporheic functionality following restoration is critical for human-river interactions, as these 'biogeochemical reactors' serve as cost-effective, nature-based solutions for water filtration, flood mitigation, and the long-term sequestration of nutrients and organic matter that would otherwise compromise drinking water supplies.

References:
Aufdenkampe, A.K. et al. (2011) ‘Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere’, Frontiers in Ecology and the Environment, 9(1), pp. 53–60. Available at: 
doi.org/10.1890/100014.
Boulton, A.J. et al. (1998) ‘The Functional Significance of the Hyporheic Zone in Streams and Rivers’, Annual Review of Ecology and Systematics, 29, pp. 59–81.
Boulton, A.J. et al. (2010) ‘Ecology and management of the hyporheic zone: stream–groundwater interactions of running waters and their floodplains’, Journal of the North American Benthological Society, 29(1), pp. 26–40. Available at: doi.org/10.1899/08-017.1.
Krause, S. et al. (2011) ‘Inter‐disciplinary perspectives on processes in the hyporheic zone’, Ecohydrology, 4(4), pp. 481–499. Available at: doi.org/10.1002/eco.176.
Burrows, R.M. et al. (2017) ‘High rates of organic carbon processing in the hyporheic zone of intermittent streams’, Scientific Reports, 7(1), p. 13198. Available at: doi.org/10.1038/s41598-017-12957-5.
Roy Chowdhury, S. et al. (2020) ‘Formation Criteria for Hyporheic Anoxic Microzones: Assessing Interactions of Hydraulics, Nutrients, and Biofilms’, Water Resources Research, 56(3), p. e2019WR025971. Available 
at: doi.org/10.1029/2019WR025971.
DelVecchia, A.G. et al. (2022) ‘Reconceptualizing the hyporheic zone for nonperennial rivers and streams’, Freshwater Science, 41(2), pp. 167–182. Available at: doi.org/10.1086/720071.