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1. A characterization of Colorado Front Range and Denver Basin Aquifer System wateer stable isotope signatures

   Ulate, Isabella (August 2024, ProQuest)

   The Denver Basin Aquifer System (DBAS) is an important groundwater resource for Front Range communities and is currently experiencing increasing demand as populations grow and surface water supplies remain limited. It is necessary to better constrain aquifer recharge mechanisms to enable sustainable use of this resource. In other sedimentary basin aquifer systems, mountain front recharge has been shown to be a significant contributor to local basin groundwater recharge. In the DBAS, inputs from the mountain block are poorly understood, and previous numerical models have treated large segments of the mountain-front boundary as impermeable. However, there exist potential connections between the mountain block and the DBAS, either by direct contact of permeable units, which would facilitate underflow recharge into the basin, or by surface water infiltration to the aquifer units where they outcrop near the mountain front. To observe spatial and temporal relationships between mountain block water and DBAS water, we use water stable isotopes and characterize the d2H and d18O of monthly precipitation, seasonal surface waters, and groundwaters in and around the Front Range and Denver Basin. The goal of this study is to determine if differences in the isotopic composition of waters across the Front Range permit the use of d18O and d2H as tracers of water flow between Front Range streams and groundwater and the DBAS. We analyzed the unique signature of mountain-block water to compare with DBAS water stable isotope data collected from Castle Rock Water municipal wells. Stable isotope ratios varied spatially and temporally, with the greatest temporal variance observed in precipitation. Streams showed great spatial variance, and less significant seasonal variance between the three seasonal sampling events conducted. Groundwaters showed very little temporal variance but had great spatial variance both between the aquifer units of the DBAS and between different locations within the mountain block crystalline aquifer. The lowest d2H and d18O ratios were measured in winter precipitation, winter streams, and groundwater samples collected from the high-elevation Front Range. Samples of DBAS groundwaters with the lowest d2H and d18O ratios indicate potential hydrogeologic connection to the mountain block. Interpreted mixing lines on a d-excess versus d18O plot support the potential DBAS-mountain block connection. The deepest aquifer units of the DBAS (Arapahoe and Laramie-Fox Hills) show the least relationship with meteoric or surface waters on both a d2H and d18O plot and the d-excess versus d18O plot and have higher d18O values than would be predicted based on their previously measured recharge ages and paleoclimate data from the region. Characterizing the spatial and temporal variations in water stable isotope signatures of the Front Range and DBAS region enhances understanding of the region’s hydrology and hydrogeology. Additionally, these results help to better inform models of aquifer recharge and promote sustainable use of the DBAS resource. 

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2. Combined impacts of uncertainty in precipitation and air temperature on simulated mountain system recharge from an integrated hydrologic model

   https://doi.org/10.5194/hess-26-1145-2022  

   Schreiner-McGraw, Adam P.; Ajami, Hoori (January 2022, Hydrology and Earth System Sciences)

   Abstract. Mountainous regions act as the water towers of the worldby producing streamflow and groundwater recharge, a function that isparticularly important in semiarid regions. Quantifying rates of mountainsystem recharge is difficult, and hydrologic models offer a method toestimate recharge over large scales. These recharge estimates are prone touncertainty from various sources including model structure and parameters.The quality of meteorological forcing datasets, particularly in mountainousregions, is a large source of uncertainty that is often neglected ingroundwater investigations. In this contribution, we quantify the impact ofuncertainty in both precipitation and air temperature forcing datasets onthe simulated groundwater recharge in the mountainous watershed of theKaweah River in California, USA. We make use of the integrated surface water–groundwater model, ParFlow.CLM, and several gridded datasets commonly usedin hydrologic studies, downscaled NLDAS-2, PRISM, Daymet, Gridmet, andTopoWx. Simulations indicate that, across all forcing datasets, mountain front recharge is an important component of the water budget in themountainous watershed, accounting for 9 %–72 % of the annual precipitation and ∼90 % of the total mountain system recharge to theadjacent Central Valley aquifer. The uncertainty in gridded air temperatureor precipitation datasets, when assessed individually, results in similarranges of uncertainty in the simulated water budget. Variations in simulatedrecharge to changes in precipitation (elasticities) and air temperature(sensitivities) are larger than 1 % change in recharge per 1 % change inprecipitation or 1 ∘C change in temperature. The total volume ofsnowmelt is the primary factor creating the high water budget sensitivity, and snowmelt volume is influenced by both precipitation and air temperatureforcings. The combined effect of uncertainty in air temperature andprecipitation on recharge is additive and results in uncertainty levels roughly equal to the sum of the individual uncertainties depending on thehydroclimatic condition of the watershed. Mountain system recharge pathwaysincluding mountain block recharge, mountain aquifer recharge, and mountainfront recharge are less sensitive to changes in air temperature than changesin precipitation. Mountain front and mountain block recharge are moresensitive to changes in precipitation than other recharge pathways. Themagnitude of uncertainty in the simulated water budget reflects theimportance of developing high-quality meteorological forcing datasets in mountainous regions. 

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3. Groundwater Circulation Within the Mountain Block: Combining Flow and Transport Models With Magnetotelluric Observations to Untangle Its Nested Nature

   https://doi.org/10.1029/2023WR035906  

   Gonzalez‐Duque, D; Gomez‐Velez, J D; Person, M A; Kelley, S; Key, K; Lucero, D (April 2024, Water Resources Research)

   Abstract Mountains are vital water sources for humans and ecosystems, continuously replenishing lowland aquifers through surface runoff and mountain recharge. Quantifying these fluxes and their relative importance is essential for sustainable water resource management. However, our mechanistic understanding of the flow and transport processes determining the connection between the mountain block and the basin aquifer remains limited. Traditional conceptualizations assume groundwater circulation within the mountain block is predominantly shallow. This view neglects the role of deep groundwater flowpaths significantly contributing to the water, solute, and energy budgets. Overcoming these limitations requires a holistic characterization of the multiscale nature of groundwater flow along the mountain‐to‐valley continuum. As a proof‐of‐concept, we use a coupled groundwater flow and transport model to design a series of numerical experiments that explore the role of geology, topography, and weathering rates in groundwater circulation and their resulting resistivity patterns. Our results show that accumulating solutes near stagnation zones create contrasting electrical resistivity patterns that separate local, intermediate, and regional flow cells, presenting a target for magnetotelluric observations. To demonstrate the sensitivity of magnetotelluric data to features in our resistivity models, we use the MARE2DEM electromagnetic modeling code to perform forward and inverse simulations. This study highlights the potential of magnetotelluric surveys to image the resistivity structure resulting from multiscale groundwater circulation through relatively impervious crystalline basement rocks in mountainous terrains. This capability could change our understanding of the critical zone, offering a holistic perspective that includes deep groundwater circulation and its role in conveying solutes and energy. 

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4. 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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5. 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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