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1. Nitrous oxide processing in carbonate karst aquifers

   https://doi.org/https://doi.org/10.1016/j.jhydrol.2020.125936  

   Flint, Madison K ; Martin, Jonathan B. ; Summerall, Tatiana I. ; Barry-Sosa, Adrian ; Christner, Brent C. ( March 2021 , 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. 

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2. Nitrous oxide processing in carbonate karst aquifers

   https://doi.org/doi.org/10.1016/j.jhydrol.2020.125936  

   Flint, M.k. ( January 2021 , 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. 

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3. Geochemistry and Provenance of Springs in a Baja California Sur Mountain Catchment

   https://doi.org/10.1111/gwat.13177  

   Lerback, Jory C. ; Bowen, Brenda B. ; Humphrey, C. E. ; Fernandez, Diego. P. ; Bernau, Jeremiah A. ; Macfarlan, Shane J. ; Schniter, Eric ; Garcia, J. J. ( February 2022 , Groundwater)

   Fractured rock aquifers cover much of Earth's surface and are important mountain sites for groundwater recharge but are poorly understood. To investigate groundwater systematics of a fractured‐dominated aquifer in Baja California Sur, Mexico, we examined the spatial patterns of aquifer recharge and connectivity using the geochemistry of springs. We evaluate a range of geochemical data within the context of two endmember hypotheses describing spatial recharge patterns and fracture connectivity. Hypothesis 1 is that the aquifer system is segmented, and springs are fed by local recharge. Hypothesis 2 is that the aquifer system is well connected, with dominant recharge occurring in the higher elevations. The study site is a small <15 km²catchment. Thirty‐four distinct springs and two wells were identified in the study area, and 24 of these sites were sampled for geochemical analyses along an elevation gradient and canyon transect. These analyses included major ion composition, trace element and strontium isotopes, δ¹⁸O and δ²H isotopes, radiocarbon, and tritium. δ¹⁸O and δ²H isotopes suggest that the precipitation feeding the groundwater system has at least two distinct sources. Carbon isotopes showed a change along the canyon transect, suggesting that shorter flowpaths feed springs in the top of the transect, and longer flowpaths discharge near the bottom. Geochemical interpretations support a combination of the two proposed hypotheses. Understanding of the connectivity and provenance of these springs is significant as they are the primary source of water for the communities that inhabit this region and may be impacted by changes in recharge and use.

    

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4. An Integrated Approach Toward Sustainability via Groundwater Banking in the Southern Central Valley, California

   https://doi.org/10.1029/2018WR024069  

   Ghasemizade, Mehdi ; Asante, Kwabena O. ; Petersen, Christian ; Kocis, Tiffany ; Dahlke, Helen E. ; Harter, Thomas ( April 2019 , Water Resources Research)

   Intensive groundwater withdrawals in California have resulted in depletion of streams and aquifers in some regions. Agricultural managed aquifer recharge (Ag‐MAR) initiatives have recently been piloted in California to mitigate the effects of unsustainable groundwater withdrawals. These initiatives rely on capturing wet‐year water and spreading it on large areas of irrigated agricultural lands to enhance recharge to aquifers. While recharge studies typically consider local effects on aquifer storage, few studies have investigated Ag‐MAR benefits and challenges at a regional scale. Here we used the Integrated Water Flow Model, to evaluate how Ag‐MAR projects can affect streamflows, diversions, pumping, and unsaturated zone flows in the southern Central Valley, California. We further tested the sensitivity of three different spatial patterns of Ag‐MAR, each chosen based on different thresholds of soil suitability, on the hydrologic system. This study investigates how the distribution of Ag‐MAR lands benefit the regional groundwater system and other water balance components. The results suggest that Ag‐MAR benefits vary as a function of the location of Ag‐MAR lands. Stream‐aquifer interactions play a crucial factor in determining the ability to increase groundwater storage in overdrafted basins. The results also indicate that Ag‐MAR projects conducted during the November–April recharge season have implications for water rights outside of the Ag‐MAR season. If not properly monitored, Ag‐MAR can cause a rise of groundwater table into the root zone, negatively impacting sensitive crops. Our work also highlights the benefits of using an integrated hydrologic and management model to evaluate Ag‐MAR at a regional scale.

    

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5. Using δ18O and δ2H to Detect Hydraulic Connection Between a Sinkhole Lake and a First‐Magnitude Spring

   https://doi.org/10.1111/gwat.13105  

   Ahmed, Nur ; Ye, Ming ; Wang, Yang ; Greenhalgh, Tom ; Fowler, Karlee ( April 2021 , Groundwater)

   Oxygen and hydrogen isotopes were used in this study to detect a hydraulic connection between a sinkhole lake and a karst spring. In karst areas, surface water that flows to a lake can drain through sinkholes in the lakebed to the underlying aquifer, and then flows in karst conduits and through aquifer matrix. At the study site located in northwest Florida, USA, Lake Miccosukee immediately drains into two sinkholes. Results from a dye tracing experiment indicate that lake water discharges at Natural Bridge Spring, a first‐magnitude spring 32 km downgradient from the lake. By collecting weekly water samples from the lake, the spring, and a groundwater well 10 m away from the lake during the dry period between October 2019 and January 2020, it was found that, when rainfall effects on isotopic signature in spring water are removed, increased isotope ratios of spring water can be explained by mixing of heavy‐isotope‐enriched lake water into groundwater, indicating hydraulic connection between the lake and the spring. Such a detection of hydraulic connection at the scale of tens of kilometers and for a first‐magnitude spring has not been previously reported in the literature. Based on the isotope ratio data, it was estimated that, during the study period, about 8.5% the spring discharge was the lake water that drained into the lake sinkholes.

    

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