We use 3‐D high‐resolution reactive transport modeling to investigate whether the spatial distribution of organic‐carbon‐rich and chemically reduced sediments located in the riparian zone and ...temporal variability in groundwater flow direction impact the formation and distribution of nitrogen hot spots (regions that exhibit higher reaction rates when compared to other locations nearby) and hot moments (times that exhibit high reaction rates as compared to longer intervening time periods) within the Rifle floodplain in Colorado. Groundwater flows primarily toward the Colorado River from the floodplain but changes direction at times of high river stage. The result is that oxic river water infiltrates the Rifle floodplain during these relatively short‐term events. Simulation results indicate that episodic rainfall in the summer season leads to the formation of nitrogen hot moments associated with Colorado River rise and resulting river infiltration into the floodplain. The results further demonstrate that the naturally reduced zones (NRZs) present in sediments of the Rifle floodplain have a higher potential for nitrate removal, approximately 70% greater than non‐NRZs for typical hydrological conditions. During river water infiltration, nitrate reduction capacity remains the same within the NRZs, however, these conditions impact non‐NRZs to a greater extent (approximately 95% less nitrate removal). Model simulations indicate chemolithoautotrophs are primarily responsible for the removal of nitrate in the Rifle floodplain. These nitrogen hot spots and hot moments are sustained by microbial respiration and the chemolithoautotrophic oxidation of reduced minerals in the riparian zone.
Plain Language Summary
Riverine floodplains play a significant role in the cycling of nitrogen. Reliable predictions of nitrogen dynamics in the floodplain and its export to the river depend on accurate representation of hydrologic flow paths and biogeochemical processes in the subsurface. The objective of this study was to quantify the transport and distribution of nitrogen at a floodplain site in Rifle, CO, using a high‐resolution, 3‐D flow and reactive transport model. Groundwater flows primarily toward the Colorado River from the floodplain but changes direction occasionally. The simulations demonstrate that nitrogen hot spots are both flow‐related and microbially driven in the Rifle floodplain. Overall, 3‐D simulations were able to capture the significant spatial and temporal variability associated with nitrogen fluxes in the floodplain environment.
Key Points
We investigate the formation and distribution of nitrogen hot spots and hot moments using a high‐resolution, 3‐D reactive transport model
Results indicate that nitrogen hot spots occur because of complex fluid mixing, localized reduced zones, and biogeochemical variability
Chemically reduced sediments of the Rifle floodplain have 70% greater potential for nitrate removal than nonreduced zones
Hyporheic exchange has been hypothesized to have basin‐scale consequences; however, predictions throughout river networks are limited by available geomorphic and hydrogeologic data and by models that ...can analyze and aggregate hyporheic exchange flows across large spatial scales. We developed a parsimonious but physically based model of hyporheic flow for application in large river basins: Networks with EXchange and Subsurface Storage (NEXSS). We applied NEXSS across a broad range of geomorphic diversity in river reaches and synthetic river networks. NEXSS demonstrates that vertical exchange beneath submerged bed forms rather than lateral exchange through meanders dominates hyporheic fluxes and turnover rates along river corridors. Per kilometer, low‐order streams have a biogeochemical potential at least 2 orders of magnitude larger than higher‐order streams. However, when biogeochemical potential is examined per average length of each stream order, low‐ and high‐order streams were often found to be comparable. As a result, the hyporheic zone's intrinsic potential for biogeochemical transformations is comparable across different stream orders, but the greater river miles and larger total streambed area of lower order streams result in the highest cumulative impact from low‐order streams. Lateral exchange through meander banks may be important in some cases but generally only in large rivers.
Key Points
Physically based model predicts hyporheic exchange in any lowland riverVertical exchange rather than lateral exchange dominates hyporheic flow in river networksReaction potential is comparable for small and large streams
Pharmaceuticals and Personal Care Compounds (PPCPs) are contaminants present in wastewater and in the receiving surface waters, which have no regulations and can bring on environmental risks. In this ...study, we evaluated the presence of six PPCPs in the Oro River Sub-basin (Colombia) and the environmental risk associated with them. We have verified that the monitored rivers show the presence of Ibuprofen, Cephalexin and Carbamazepine; the first ones (Ibuprofen and cephalexin) were those that presented higher concentrations since they are widely prescribed in Colombia. Pharmaceutical compound concentrations in the rivers downstream of the wastewater treatment plants from Floridablanca were higher than in other monitoring sites being a significant point source of contamination. This wastewater treatment plant receives hospital discharges from the city, including internationally recognized clinics accepting patients from different parts of the country. The environmental risk assessment showed that ibuprofen and Cephalexin have a higher impact on aquatic organisms.
•Cephalexin has higher concentrations in the Frio River.•Ibuprofen and cephalexin presented high environmental risk in the Frio and Oro rivers.•Pharmaceutical pollutants downstream the FWTP were higher due to large hospital areas.
•An applicable and automatic model was proposed for surface water mapping.•We built an earth surface water dataset for the surface water features learning.•The proposed deep leaning method remarkably ...outperforms existing methods.
Earth’s surface water plays an important role in the global water cycle, environmental processes, and human society, and it is necessary to dynamically capture the distribution and extent of surface water on Earth. However, due to the high complexity of the surface environment of Earth, the current surface water mapping methods are limited in applicability and precision. In this study, to explore an automatic and applicable model for surface water mapping, particularly for the regions with highly heterogenous backgrounds, we adopted state-of-the-art deep learning techniques and structured a new model, namely, WatNet, for surface water mapping. Specifically, we combined a state-of-the-art image classification model and a semantic segmentation model into an improved deep learning model. For the fine-scale identification of small water bodies, the combined model was further improved with surface water mapping-tailored design. To learn the surface water features of worldwide regions, a surface water knowledge base that consists of worldwide satellite images was built in this study. The newly structured WatNet model was tested on three highly heterogeneous regions, and as demonstrated by the results, 1) the trained WatNet model achieved the highest accuracies, which were above 95%, for all the selected test regions; 2) the new structured WatNet model yields significant improvements through state-of-the-art model combinations and the surface water-tailored design; and 3) unlike conventional methods, which usually require parameterization in accordance with the specific surface environment, trained WatNet can be directly applied for highly accurate surface water mapping, and, thus, no human labor is required.
The hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact. The hyporheic zone presents unique conditions for reaction of solutes from ...both surface water and groundwater, including reactions which depend upon mixing of source waters. Some models assume that hyporheic zones are well‐mixed and conceptualize the hyporheic zone as a surface water‐groundwater mixing zone. But what are the controls on and effects of hyporheic mixing? What specific mechanisms cause the relatively large (>∼1 m) mixing zones suggested by subsurface solute measurements? In this commentary, we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater, and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multiscale physical and chemical spatial heterogeneity. We encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations.
Plain Language Summary
The hyporheic zone is the area beneath and adjacent to streams and rivers where surface water and groundwater interact. The hyporheic zone presents unique conditions for reaction of pollutants from both surface water and groundwater, including reactions which depend upon mixing of these different source waters. This type of mixing is not well understood, yet potentially important for pollutant mitigation in watersheds. In this commentary, we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater, and pose the question whether the substantial mixing observed by field studies may be due to the combination of fluctuating boundary conditions and multi‐scale physical and chemical spatial heterogeneity. We encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations.
Key Points
Field studies show large hyporheic mixing zones
Current theory cannot explain large field mixing zones
Sediment heterogeneity and surface water dynamics may explain mixing
The permeability heterogeneity of cross-bedded sediment increases path lengths of river-groundwater mixing (hyporheic exchange) in riverbeds and modifies the distribution of residence times. For two ...case studies, we numerically simulated fluid flow and solute transport through immobile bed forms composed of heterogeneous sediment and equivalent homogeneous sediment in order to clarify how cross-bedded permeability structures impact hyporheic exchange. The two permeability fields are from the cross-bedded Massillon Sandstone and modern climbing ripple deposits of the Brazos River (Texas). In both cases, permeability heterogeneity creates long hyporheic exchange paths but only slightly increases the depth of exchange relative to equivalent homogeneous sediment. In the Massillon example, permeability heterogeneity increases the proportion of long hyporheic residence times (>3 days). In the Brazos example, permeability heterogeneity increases the proportion of short residence times (<17 h). We attribute the different responses in residence time distributions to differences in permeability patterns near the sediment-water interface. The tails of residence time distributions extend for tens of years and conform to a power law in both heterogeneous and homogeneous sediment. Current-bed form interactions are responsible for the long tails, as opposed to permeability heterogeneity.
•Properties and processes of the GW–SW interface are variable in space and time.•Revealing hydrological and biogeochemical heterogeneity remains a challenge.•Geophysics offer useful tools for ...addressing variability across multiple scales.•Future studies should incorporate geophysical progress gained in parallel fields.
Interactions between groundwater (GW) and surface water (SW) have important implications for water quantity, water quality, and ecological health. The subsurface region proximal to SW bodies, the GW–SW interface, is crucial as it actively regulates the transfer of nutrients, contaminants, and water between GW systems and SW environments. However, geological, hydrological, and biogeochemical heterogeneity in the GW–SW interface makes it difficult to characterise with direct observations. Over the past two decades geophysics has been increasingly used to characterise spatial and temporal variability throughout the GW–SW interface. Geophysics is a powerful tool in evaluating structural heterogeneity, revealing zones of GW discharge, and monitoring hydrological processes. Geophysics should be used alongside traditional hydrological and biogeochemical methods to provide additional information about the subsurface. Further integration of commonly used geophysical techniques, and adoption of emerging techniques, has the potential to improve understanding of the properties and processes of the GW–SW interface, and ultimately the implications for water quality and environmental health.
PDF
