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Abstract Riverbank groundwater discharge faces are spatially extensive areas of preferential seepage that are exposed to air at low river flow. Some conceptual hydrologic models indicate discharge faces represent the spatial convergence of highly variable age and length groundwater flowpaths, while others indicate greater consistency in source groundwater characteristics. Our detailed field investigation of preferential discharge points nested across mainstem riverbank discharge faces was accomplished by: (1) leveraging new temperature‐based recursive estimation (extended Kalman Filter) modelling methodology to evaluate seasonal, diurnal, and event‐driven groundwater flux patterns, (2) developing a multi‐parameter toolkit based on readily measured attributes to classify the general source groundwater flowpath depth and flowpath length scale, and, (3) assessing whether preferential flow points across discharge faces tend to represent common or convergent groundwater sources. Five major groundwater discharge faces were mapped along the Farmington River, CT, United States using thermal infrared imagery. We then installed vertical temperature profilers directly into 39 preferential discharge points for 4.5 months to track vertical discharge flux patterns. Monthly water chemistry was also collected at the discharge points along with one spatial synoptic of stable isotopes of water and dissolved radon gas. We found pervasive evidence of shallow groundwater sources at the upstream discharge faces along a wide valley section with deep bedrock, as primarily evidenced by pronounced diurnal discharge flux patterns. Discharge flux seasonal trends and bank storage transitions during large river flow events provided further indication of shallow, local sources. In contrast, downstream discharge faces associated with near surface cross cutting bedrock exhibited deep and regional source flowpath characteristics such as more stable discharge patterns and temperatures. However, many neighbouring points across discharge faces had similar discharge flux patterns that differed in chloride and radon concentrations, indicating the additional effects of localized flowpath heterogeneity overprinting on larger scale flowpath characteristics.
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Impacts of Evaporation‐Induced Groundwater Upwelling on Mixing Dynamics in Shallow Wetlands
Abstract Groundwater mixing dynamics play a crucial role in the biogeochemical cycling of shallow wetlands. In this paper, we conducted groundwater simulations to investigate the combined effects of evaporation and local heterogeneity on mixing dynamics in shallow wetland sediments. The results show that evaporation causes groundwater and solutes to upwell from deep sediments to the surface. As the solute reaches the surface, evaporation enhances the accumulation of the solute near the surface, resulting in a higher solute concentration than in deep sediments. Mapping of flow topology reveals that local heterogeneity generates spatially varied mixing patterns mainly along preferential flow pathways. The upwelling of groundwater induced by surface evaporation through heterogeneous sediments is likely to create distinct mixing hotspots that differ spatially from those generated by lateral preferential flows driven by large‐scale hydraulic gradients, which enhances the overall mixing in the subsurface. These findings have strong implications for biogeochemical processing in wetlands.
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- Award ID(s):
- 2130595
- PAR ID:
- 10495792
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 15
- ISSN:
- 0094-8276
- Subject(s) / Keyword(s):
- Evaporation groundwater flow contaminant transport heterogeneity numerical modeling preferential flow
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023GL104642
