skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Nitrous oxide processing in carbonate karst aquifers
The increased environmental abundance of anthropogenic reactive nitrogen species (Nr = ammonium [NH4+], nitrite [NO2􀀀 ] and nitrate [NO3􀀀 ]) may increase atmospheric nitrous oxide (N2O) concentrations, and thus global warming and stratospheric ozone depletion. Nitrogen cycling and N2O production, reduction, and emissions could be amplified in carbonate karst aquifers because of their extensive global range, susceptibility to nitrogen contamination, and groundwater-surface water mixing that varies redox conditions of the aquifer. The magnitude of N2O cycling in karst aquifers is poorly known, however, and thus we sampled thirteen springs discharging from the karstic Upper Floridan Aquifer (UFA) to evaluate N2O cycling. The springs can be separated into three groups based on variations in subsurface residence times, differences in surface–groundwater interactions, and variable dissolved organic carbon (DOC) and dissolved oxygen (DO) concentrations. These springs are oxic to sub-oxic and have NO3􀀀 concentrations that range from < 0.1 to 4.2 mg N-NO3􀀀 /L and DOC concentrations that range from < 0.1 to 50 mg C/L. Maximum spring water N2O concentrations are 3.85 μg N-N2O/L or ~ 12 times greater than water equilibrated with atmospheric N2O. The highest N2O concentrations correspond with the lowest NO3􀀀 concentrations. Where recharge water has residence times of a few days, partial denitrification to N2O occurs, while complete denitrification to N2 is more prominent in springs with longer subsurface residence times. Springs with short residence times have groundwater emission factors greater than the global average of 0.0060, reflecting N2O production, whereas springs with residence times of months to years have groundwater emission factors less than the global average. These findings imply that N2O cycling in karst aquifers depends on DOC and DO concentrations in recharged surface water and subsequent time available for N processing in the subsurface.  more » « less
Award ID(s):
1740481
PAR ID:
10219238
Author(s) / Creator(s):
Date Published:
Journal Name:
Journal of hydrology
Volume:
594
ISSN:
0022-1694
Page Range / eLocation ID:
1-13
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; (, Journal of hydrology)
    null (Ed.)
    The increased environmental abundance of anthropogenic reactive nitrogen species (Nr = ammonium [NH4+], nitrite [NO2−] and nitrate [NO3−]) may increase atmospheric nitrous oxide (N2O) concentrations, and thus global warming and stratospheric ozone depletion. Nitrogen cycling and N2O production, reduction, and emissions could be amplified in carbonate karst aquifers because of their extensive global range, susceptibility to nitrogen contamination, and groundwater-surface water mixing that varies redox conditions of the aquifer. The magnitude of N2O cycling in karst aquifers is poorly known, however, and thus we sampled thirteen springs discharging from the karstic Upper Floridan Aquifer (UFA) to evaluate N2O cycling. The springs can be separated into three groups based on variations in subsurface residence times, differences in surface–groundwater interactions, and variable dissolved organic carbon (DOC) and dissolved oxygen (DO) concentrations. These springs are oxic to sub-oxic and have NO3− concentrations that range from < 0.1 to 4.2 mg N-NO3−/L and DOC concentrations that range from < 0.1 to 50 mg C/L. Maximum spring water N2O concentrations are 3.85 μg N-N2O/L or ~ 12 times greater than water equilibrated with atmospheric N2O. The highest N2O concentrations correspond with the lowest NO3− concentrations. Where recharge water has residence times of a few days, partial denitrification to N2O occurs, while complete denitrification to N2 is more prominent in springs with longer subsurface residence times. Springs with short residence times have groundwater emission factors greater than the global average of 0.0060, reflecting N2O production, whereas springs with residence times of months to years have groundwater emission factors less than the global average. These findings imply that N2O cycling in karst aquifers depends on DOC and DO concentrations in recharged surface water and subsequent time available for N processing in the subsurface. 
    more » « less
  2. ; ; (, Earth Surface Processes and Landforms)
    Abstract The high primary porosity and permeability of eogenetic karst aquifers permit water recharged through secondary dissolution features to be temporarily stored in aquifer matrix porosity. The recharged water contains elevated dissolved organic carbon (DOC) concentrations that, when oxidized, enhance limestone dissolution and impact carbon cycling. We evaluate the relationship between DOC oxidation and limestone dissolution using observations at a stream sink‐rise system and reversing spring in the Floridan aquifer, north‐central Florida, USA, where subsurface residence times of recharged water are days and months, respectively. We estimate water chemical compositions during surface water‐groundwater interactions at these two systems with mixing models of surface water and groundwater compositions and compare them with measured DOC, dissolved inorganic carbon (DIC), Ca2+and dissolved organic nitrogen (DON) concentrations. Differences between measured and modelled concentrations represent net changes that can be attributed to calcite dissolution and redox reactions, including DOC oxidation. DOC losses and Ca2+gains exhibit significant (p < 0.01) inverse linear correlations at both the reversing spring (slope = −0.9, r2 = 0.99) and the sink‐rise system (slope = −0.4, r2 = 0.72). DOC oxidation in both systems was associated with decreases in the molar C:N ratio (DOC:DON). Significant (p < 0.01) positive linear correlations between increases in Ca2+and DIC concentrations after correcting for DIC derived from calcite dissolution occurred at both the reversing spring (slope = 1.3, r2 = 0.99) and the sink‐rise system (slope = 1.61, r2 = 0.75). Greater deviations from the expected slope of −1 or +1 at the sink‐rise system than at the reversing spring indicate DOC oxidation contributes less dissolution at the sink‐rise system than at the reversing spring, likely from shorter storage in the subsurface. A portion of the deviation from expected slope values can be explained by the dissolution of Mg‐rich carbonate or dolomite rather than pure calcite dissolution. Despite this, slope values reflect kinetic effects controlling incomplete consumption of carbonic acid during dissolution reactions. 
    more » « less
  3. ; ; ; ; ; ; ; ; (, Water Resources Research)
    Coastal aquifers experience water table fluctuations that push and pull water and air through organic‐rich soils. This exchange affects the supply of oxygen, dissolved organic carbon (DOC), and nitrogen (N) to shallow aquifers and influences groundwater quality. To investigate the fate of N species, we used a meter‐long column containing a sequence of natural organic topsoil and aquifer sediments. A fluctuating head was imposed at the column bottom with local, nitrate‐rich groundwater (16.5 mg/L NO3‐N). We monitored in‐situ redox potential and collected pore water samples for analysis of inorganic N species and DOC over 16 days. Reactive processes were more complex than anticipated. The organic‐rich topsoil remained anaerobic, while mineral sediments beneath alternated between aerobic, when the water table dropped and sucked air across preferential flow paths, and anaerobic conditions, when the water table was high. A fluid flow and reactive transport model shows that when the water table rises into organic‐rich soils, it limits the flow of oxygen, while the soils release DOC, which stimulates the removal of nitrate from groundwater by denitrification. At the end of the experiment, we introduced seawater to the column to mimic a storm surge. Seawater mobilized N and DOC from shallow soil horizons, which could reach the aquifer if the surge is long enough. These processes are relevant for groundwater quality in developed coastal areas with anthropogenic N sources, as climate change and rising seas will drive changes in water table and flood dynamics. 
    more » « less
  4. ; ; ; ; ; ; ; (, Annual Review of Earth and Planetary Sciences)
    The Grand Canyon provides a deeply dissected view of the aquifers of the Colorado Plateau and its public and tribal lands. Stacked sandstone and karst aquifers are vertically connected by a network of faults and breccia pipes creating a complex groundwater network. Hydrochemical variations define structurally controlled groundwater sub-basins, each with main discharging springs. North Rim (N-Rim), South Rim (S-Rim), and far-west springs have different stable isotope fingerprints, reflecting different mean recharge elevations. Variation within each region reflects proportions of fast/slow aquifer pathways. Often considered perched, the upper Coconino (C) aquifer has a similar compositional range as the regional Redwall-Muav (R-M) karst aquifer, indicating connectivity. Natural and anthropogenic tracers show that recharge can travel 2 km vertically and tens of kilometers laterally in days to months via fracture conduits to mix with older karst baseflow. Six decades of piping N-Rim water to S-Rim Village and infiltration of effluent along the Bright Angel fault have sustained S-Rim groundwaters and likely induced S-Rim microseismicity. Sustainable groundwater management and uranium mining threats require better monitoring and application of hydrotectonic concepts. ▪ Hydrotectonic concepts include distinct structural sub-basins, fault fast conduits, confined aquifers, karst aquifers, upwelling geothermal fluids, and induced seismicity. ▪ N-Rim, S-Rim, and far-west springs have different stable isotope fingerprints reflecting different mean recharge elevations and residence times. ▪ The upper C and lower R-M aquifers have overlapping stable isotope fingerprints in a given region, indicating vertical connectively between aquifers. ▪ S-Rim springs and groundwater wells are being sustained by ∼60 years of piping of N-Rim water to S-Rim, also inducing seismicity. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 52 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 
    more » « less
  5. ; ; ; ; ; ; (, Journal of Geophysical Research: Biogeosciences)
    Abstract Nitrate legacy is affecting groundwater sources across the tropics. This study describes isotopic and ionic spatial trends across a tropical, fractured, volcanic multi‐aquifer system in central Costa Rica in relation to land use change over four decades. Springs and wells (from 800 to 2,400 m asl) were sampled for NO3−and Cl−concentrations, δ18Owater, δ15NNO3, and δ18ONO3. A Bayesian isotope mixing model was used to estimate potential source contributions to the nitrate legacy in groundwater. Land use change was evaluated using satellite imagery from 1979 to 2019. The lower nitrate concentrations (<1 mg/L NO3−N) were reported in headwater springs near protected forested areas, while greater concentrations (up to ∼63 mg/L) were reported in wells (mid‐ and low‐elevation sites in the unconfined unit) and low‐elevation springs. High‐elevation springs were characterized by low Cl−and moderate NO3−/Cl−ratios, indicating the potential influence of soil nitrogen (SN) inputs. Wells and low‐elevation springs exhibited greater NO3−/Cl−ratios and Cl−concentrations above 100 μmol/L. Bayesian calculations suggest a mixture of sewage (domestic septic tanks), SN (forested recharge areas), and chemical fertilizers (coffee plantations), as a direct result of abrupt land use change in the last 40 years. Our results confirm the incipient trend in increasing groundwater nitrogen and highlight the urgent need for a multi‐municipal plan to transition from domestic septic tanks to regional sewage treatment and sustainable agricultural practices to prevent future groundwater quality degradation effectively. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email