Beaver dam analogues (BDAs) are a cost‐effective stream restoration approach that leverages the recognized environmental benefits of natural beaver dams on channel stability and local hydrology. Although natural beaver dams are known to exert considerable influence on the hydrologic conditions of a stream system by mediating geomorphic processes, nutrient cycling, and groundwater–surface water interactions, the impacts of beaver‐derived restoration methods on groundwater–surface water exchange are poorly characterized. To address this deficit, we monitored hyporheic exchange fluxes and streambed porewater biogeochemistry across a sequence of BDAs installed along a central Wyoming stream during the summer of 2019. Streambed fluxes were quantified by heat tracing methods and vertical hydraulic gradients. Biogeochemical activity was evaluated using major ion porewater chemistry and principal component analysis. Vertical fluxes of approximately 1.0 m/day were observed around the BDAs, as was the development of spatially heterogeneous zones of nitrate production, groundwater upwelling, and anaerobic reduction. Strong contrasts in hyporheic zone processes were observed across BDAs of differing sizes. This suggests that structures may function with size‐dependent behaviour, only altering groundwater–surface water interactions after a threshold hydraulic step height is exceeded. Patterns of hyporheic exchange and biogeochemical cycling around the studied BDAs resemble those around natural beaver dams, suggesting that BDAs may provide comparable benefits to channel complexity and near‐stream function over a 1‐year period.
This study presents the groundwater flow and salinity dynamics along a river estuary, the Werribee River in Victoria, Australia, at local and regional scales. Along a single reach, salinity across a transverse section of the channel (~80 m long) with a point bar was monitored using time‐lapse electrical resistivity (ER) through a tidal cycle. Groundwater fluxes were concurrently estimated by monitoring groundwater levels and temperature profiles. Regional porewater salinity distribution was mapped using 6‐km long longitudinal ER surveys during summer and winter. The time‐lapse ER across the channel revealed a static electrically resistive zone on the side of the channel with a pronounced cut bank. Upward groundwater flux and steep vertical temperature gradients with colder temperatures deeper within the sediment suggested a stable zone of fresh groundwater discharge along this cut bank area. Generally, less resistive zones were observed at the shallow portion of the inner meander bank and at the channel center. Subsurface temperatures close to surface water values, vertical head gradients indicating both upward and downward groundwater flux, and higher porewater salinity closer to that of estuary water suggest strong hyporheic circulation in these zones. The longitudinal surveys revealed higher ER values along deep and sinuous segments and low ER values in shallow and straighter reaches in both summer and winter; these patterns are consistent with the local channel‐scale observations. This study highlights the interacting effects of channel morphology, broad groundwater–surface water interaction, and hyporheic exchange on porewater salinity dynamics underneath and adjacent to a river estuary.
more » « less- NSF-PAR ID:
- 10367358
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Hydrological Processes
- Volume:
- 35
- Issue:
- 10
- ISSN:
- 0885-6087
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
null (Ed.)Fluvio-deltaic aquifers are the primary source of drinking water for the people of Bangladesh. Such aquifers, which comprise the Ganges-Brahmaputra-Meghna Delta, are extremely hydrogeologically heterogeneous. Because of widespread groundwater quality issues in Bangladesh, it is crucial to know the hydrostratigraphic architecture and hydrochemistry of the aquifers as some units are contaminated whereas others are safe. Geophysical methods provide a potentially effective and non-invasive method for extensive characterization of these aquifers. This study applies and investigates the limitations of using electrical resistivity imaging (ERI) for mapping the hydrostratigraphy and salinity of an aquifer-aquitard system adjacent to the Meghna River. Some ER sections showed excellent correlation between resistivity and grain size. These suggest that ERI is a powerful tool for mapping internal aquifer architecture and their boundaries with finer-grained aquitards which clearly appear as low ER zones. However, in parts of some ER sections, variations in electrical properties were determined by porewater resistivity. In these cases, low ER was indicative of brine and did not indicate the presence of finer-grained materials such as silt or clay. Accordingly, the following hydrostratigraphic zones with different resistivities were detected: (1) aquifers saturated with fresh ground water, (2) a regional silt/clay aquitard, and (3) a deeper brine-saturated formation. In addition, shallow silt/clay pockets were detected close to the river and below the vadose zone. ERI is thus a promising technique for mapping aquifers versus aquitards. However, the observations are easily confounded by porewater salinity. In such cases, borehole information and groundwater salinity measurements are necessary for ground-truthing.more » « less
-
null (Ed.)Biogeochemical processes capable of altering global carbon systems occur frequently in Earth’s Critical Zone–the area spanning from vegetation canopy to saturated bedrock–yet many of these phenomena are difficult to detect. Observation of these processes is limited by the seasonal inaccessibility of remote ecosystems, such as those in mountainous, snow- and ice-dominated areas. This isolation leads to a distinct gap in biogeochemical knowledge that ultimately affects the accuracy and confidence with which these ecosystems can be computationally modeled for the purpose of projecting change under different climate scenarios. To examine a high-altitude, headwater ecosystem’s role in methanogenesis, sulfate reduction, and groundwater-surface water exchange, water samples were continuously collected from the river and hyporheic zones (HZ) during winter isolation in the East River (ER), CO watershed. Measurements of continuously collected ER surface water revealed up to 50 μM levels of dissolved methane in July through September, while samples from 12 cm deep in the hyporheic zone at the same location showed a spring to early summer peak in methane with a strong biogenic signature (<65 μM, δ13C-CH4, −60.76‰) before declining. Continuously collected δ 18 O-H 2 O and δ 2 H-H 2 O isotopes from the water column exhibited similar patterns to discrete measurements, while samples 12 cm deep in the hyporheic zone experienced distinct fluctuations in δ 18 O-H 2 O, alluding to significant groundwater interactions. Continuously collected microbial communities in the river in the late fall and early winter revealed diverse populations that reflect the taxonomic composition of ecologically similar river systems, including taxa indicative of methane cycling in this system. These measurements captured several biogeochemical components of the high-altitude watershed in response to seasonality, strengthening our understanding of these systems during the winter months.more » « less
-
more » « less
Abstract The hyporheic zone is the ecotone between stream and river channel flow and groundwater that can process nutrients and improve water quality. Transient hyporheic zones occur in the riparian zone (bank storage or “lung model” exchange) during channel stage fluctuations. Recent studies show that soil pipes are widespread in stream banks and beneath floodplains, creating highly preferential flow between channel and riparian groundwater such that the traditional Darcy model of flow does not apply. We used MODFLOW with the conduit flow package to model a series of stream bank soil pipes and examined soil pipe density (number per m), length, diameter, height above baseflow water surface, connectivity, and matrix hydraulic conductivity on transient particle flow paths and total hyporheic exchange volume (i.e., bank storage) over the course of a peak flow (e.g., storm) event. We found that adding five soil pipes per meter more than doubled hyporheic volume. Soil pipe length was the most important control; adding one 1.5‐m‐long soil pipe caused a 73.4% increase in hyporheic volume. The effect of increasing soil pipe diameter on hyporheic volume leveled off at ~1 cm, as flow limitation switched from pipe flow to pipe‐matrix exchange. To validate our approach, we used the model to successfully reproduce trends from field studies. Our results highlight the need to consider soil pipes when modeling, monitoring, or managing bank storage, floodplain connectivity, or hyporheic exchange.
-
more » « less
Abstract Hyporheic exchange influences water quality and controls numerous physical, chemical, and biological processes. Despite its importance, hyporheic exchange and the associated dynamics of solute mixing are often difficult to characterize due to spatial (e.g., sedimentary heterogeneity) and temporal (e.g., river stage fluctuation) variabilities. This study coupled geophysical techniques with physical and chemical sediment analyses to map sedimentary architecture and quantify its influence on hyporheic exchange dynamics within a compound bar deposit in a gravel‐dominated river system in southwestern Ohio. Electromagnetic induction (EMI) was used to quantify variability in electrical conductivity within the compound bar. EMI informed locations of electrode placement for time‐lapse electrical resistivity imaging (ERI) surveys, which were used to examine changes in electrical resistivity driven by hyporheic exchange. Both geophysical methods revealed a zone of high electrical conductivity in the center of the bar, identified as a fine‐grained cross‐bar channel fill. The zone acts as a baffle to flow, evidenced by stable electrical conditions measured by time‐lapse ERI over the study period. Large changes in electrical resistivity throughout the survey period indicate preferential flowpaths through higher permeability sands and gravels. Grain size analyses confirmed sedimentological interpretations of geophysical data. Loss on ignition and x‐ray fluorescence identified zones with higher organic matter content that are locations for potentially enhanced geochemical activity within the cross‐bar channel fill. Differences in the physical and geochemical characteristics of cross‐bar channel fills play an important role in hyporheic flow dynamics and nutrient processing within riverbed sediments. These findings enhance our understanding of the applications of geophysical methods in mapping riverbed heterogeneity and highlight the importance of accurately representing geomorphologic features and heterogeneity when studying hyporheic exchange processes.
