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  1. Abstract

    Tides in coastal rivers drive river‐groundwater (hyporheic) exchange and provide opportunities for nitrate removal that may improve coastal water quality. Silt and sand layers in coastal floodplain sediments can alter the flow and transformation of nitrate. Our goal was to understand how sediment heterogeneity influences nitrogen dynamics near tidal rivers. Numerical simulations show that oxic, variably saturated sand layers and anoxic, organic‐rich silt layers are sites of nitrification and denitrification, respectively. The exchange of river water and nitrate through heterogeneous sediments increases with sand fraction, as sand lenses become longer and more connected. The amount of nitrate removed from river water also increases but represents a smaller portion of total nitrate exchange through the hyporheic zone, causing removal efficiency to decline. Our results suggest that accurate characterization of aquifer heterogeneity leads to an improved understanding of sites of nutrient transformation within floodplain sediments.

     
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  2. Abstract

    In coastal rivers, tides facilitate surface water‐groundwater exchange and strongly coupled nitrification‐denitrification near the fluctuating water table. We used numerical fluid flow and reactive transport models to explore hydrogeologic and biogeochemical controls on nitrogen transport along an idealized tidal freshwater zone based on field observations from White Clay Creek, Delaware, USA. The capacity of the riparian aquifer to remove nitrate depends largely on nitrate transport rates, which initially increase with increasing tidal range but then decline as sediments become muddier and permeability decreases. Over the entire model reach, local nitrification provides a similar amount of nitrate as surface and groundwater contributions combined. More than half (~66%) of nitrate removed via denitrification is produced in situ, while the vast majority of remaining nitrate removed comes from groundwater sources. In contrast, average nitrate removal from surface water due to tidal pumping amounts to only ~1% of the average daily in‐channel riverine nitrate load or 1.77 kg of nitrate along the reach each day. As a result, tidal bank storage zones may not be major sinks for nitrate in coastal rivers but can act as effective sinks for groundwater nitrate. By extension, tidal bank storage zones provide a critical ecosystem service, reducing contributions of groundwater nitrate, which is often derived from septic tanks and fertilizers, to coastal rivers.

     
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  3. Abstract

    Changes in streamflow and water table elevation influence oxidation–reduction (redox) conditions near river–aquifer interfaces, with potentially important consequences for solute fluxes and biogeochemical reaction rates. Although continuous measurements of groundwater chemistry can be arduous, in situ sensors reveal chemistry dynamics across a wide range of timescales. We monitored redox potential in an aquifer adjacent to a tidal river and used spectral and wavelet analyses to link redox responses to hydrologic perturbations within the bed and banks. Storms perturb redox potential within both the bed and banks over timescales of days to weeks. Tides drive semidiurnal oscillations in redox potential within the streambed that are absent in the banks. Wavelet analysis shows that tidal redox oscillations in the bed are greatest during late summer (wavelet magnitude of 5.62 mV) when river stage fluctuations are on the order of 70 cm and microbial activity is relatively high. Tidal redox oscillations diminish during the winter (wavelet magnitude of 2.73 mV) when river stage fluctuations are smaller (on the order of 50 cm) and microbial activity is presumably low. Although traditional geochemical observations are often limited to summer baseflow conditions, in situ redox sensing provides continuous, high‐resolution chemical characterization of the subsurface, revealing transport and reaction processes across spatial and temporal scales in aquifers.

     
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  4. Online modes of teaching and learning have gained increased attention following the COVID-19 pandemic, resulting in education delivery trends likely to continue for the foreseeable future. It is therefore critical to understand the implications for student learning outcomes and their interest in or affinity towards the subject, particularly in water science classes, where educators have traditionally employed hands-on outdoor activities that are difficult to replicate online. In this study, we share our experiences adapting a field-based laboratory activity on groundwater to accommodate more than 700 students in our largest-enrollment general education course during the pandemic. As part of our adaptation strategy, we offered two versions of the same exercise, one in-person at the Mirror Lake Water Science Learning Laboratory, located on Ohio State University’s main campus, and one online. Although outdoor lab facilities have been used by universities since at least the 1970s, this research is novel in that 1) it considers not only student achievement but also affinity for the subject, 2) it is the first of its kind on The Ohio State University’s main campus, and 3) it was conducted during the COVID-19 pandemic, at a time when most university classes were unable to take traditional field trips. We used laboratory grades and a survey to assess differences in student learning and affinity outcomes for in-person and online exercises. Students who completed the in-person exercise earned better scores than their online peers. For example, in Fall 2021, the median lab score for the in-person group was 97.8%, compared to 91.7% for the online group. The in-person group also reported a significant ( p < 0.05) increase in how much they enjoyed learning about water, while online students reported a significant decrease. Online students also reported a significant decrease in how likely they would be to take another class in water or earth sciences. It is unclear whether the in-person exercise had better learning and affinity outcomes because of the hands-on, outdoor qualities of the lab or because the format allowed greater interaction among peers and teaching instructors (TAs). To mitigate disparities in student learning outcomes between the online and in-person course delivery, instructors will implement future changes to the online version of the lab to enhance interactions among students and TAs. 
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