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.

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.

ABSTRACT:

Microbial processing of reactive nitrogen in stream sediments and connected aquifers can remove and transform nitrogen prior to its discharge into coastal waters, decreasing the likelihood of harmful algal blooms and low oxygen levels in estuaries. Canonical wisdom points to the decreased capacity of rivers to retain nitrogen as they flow towards the coast. However, how tidal freshwater zones, which often extend hundreds of kilometers inland, process and remove nitrogen remains unknown. Using geochemical measurements and numerical models, we show that tidal pumping results in the rapid cycling of nitrogen within distinct zones throughout the riparian aquifer. Near the fluctuating water table nitrification dominates, with high nitrate concentrations (>10 mg N L-1) and consistent isotopic composition. Beneath this zone, isotopes reveal that nitrate is both denitrified and added over the tidal cycle, maintaining nitrate concentrations >3-4 mg N L-1. In most of the riparian aquifer and streambed, nitrate concentrations are <0.5 mg N L-1, suggesting denitrification dominates. Model results reveal that oxygen delivery to groundwater from the overlying unsaturated soil fuels mineralization and nitrification, with subsequent denitrification in low oxygen, high organic matter regions. Depending on flow paths, tidal freshwater zones could be sources of nitrate in regions with permeable sediment and low organic matter content.

ABSTRACT:

Groundwater is a primary source of drinking water worldwide, but excess nutrients and emerging contaminants could compromise groundwater quality and limit its usage as a drinking water source. As such contaminants become increasingly prevalent in the biosphere, a fundamental understanding of their fate and transport in groundwater systems is necessarily to implement successful remediation strategies. The dynamics of surface water-groundwater (hyporheic) exchange within a glacial, buried-valley aquifer systems are examined in the context of their implications for subsurface transport of nutrients and contaminants. High permeability facies act as preferential flow pathways which enhance nutrient and contaminant delivery, especially during storm events, but transport throughout the aquifer also depends on subsurface sedimentary architecture (e.g. interbedded high and low permeability facies). Sediment analyses reveal high stratigraphic heterogeneity, with cross-stratified open-framework gravel facies throughout the aquifer. Temperature and specific conductivity measurements indicate extensive hyporheic mixing near the river, but surface water influence was also observed far from the stream-aquifer interface. Measurements of river stage and hydraulic head indicate that significant flows during storms alter groundwater flow patterns, even between consecutive storm events, as riverbed conductivity and hydraulic connectivity between the river and aquifer change. Given the similar mass transport characteristics of buried-valley aquifers, these findings are likely representative of glacial aquifer systems worldwide. Our results suggest that water resources management decisions based on average (base) flow conditions may inaccurately represent the system being evaluated, and could reduce the effectiveness of remediation strategies for nutrients and emerging contaminants.

ABSTRACT:

Water table elevation, specific conductivity, temperature, and oxidation-reduction (redox) data collected at the Theis Environmental Monitoring and Modeling Site.

ABSTRACT:

Supplementary metadata for "Spatiotemporal dynamics of nitrous oxide emission hotspots in heterogeneous riparian sediments", which includes time-series of river stage, water table elevation, groundwater temperature, and groundwater specific conductivity at the Theis Environmental Monitoring and Modeling Site (TEMMS). Chemical data was performed on samples collected at sampling ports throughout the TEMMS floodplain.