Constraining spatial variability in recharge and discharge in an arid environment through modeling carbon‐14 with improved boundary conditions

Wood, Cameron; Cook, Peter G.; Harrington, Glenn A.; Knapton, Anthony

doi:10.1002/2015wr018424

Cited by 9 publications

(10 citation statements)

References 56 publications

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“…If unconventional observations are used alongside classical observations for flow model calibration, the calibration routine should be based on (regularized) weighted multivariate objective functions that allow the simultaneous calibration of all model parameters against all observation types. The benefit of this procedure in relation to observations of C was, for example, demonstrated by Sanford et al (), Hunt et al (), Rasa et al (), Carniato et al (), and Wood et al (). In their guideline, Doherty and Hunt () provided specific suggestions for the weighting of observations of C if used alongside observations of H: They suggest (1) ensuring that the total weight of all observations of C is similar to the total weight of all observations of H so that both groups are equally important in the calibration objective function and (2) setting the weights among contamination plume concentration observations so that small concentrations marking the outlines of the contamination plume are more strongly visible in the calibration objective function, as these observations inform most about local subsurface heterogeneity.…”

Section: Review Of the Use Of Unconventional Observation Typesmentioning

confidence: 80%

“…Unfortunately, the manual calibration approach was not well documented. In a more recent, systematic and well‐documented calibration study, Wood et al () calibrated a flow and transport IFM against observations of H and C obtained from 14 C measurements. They simultaneously calibrated K aq and recharge of 18 different zones using a weighted multivariate objective function and an automated calibration procedure.…”

Section: Review Of the Use Of Unconventional Observation Typesmentioning

confidence: 99%

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Beyond Classical Observations in Hydrogeology: The Advantages of Including Exchange Flux, Temperature, Tracer Concentration, Residence Time, and Soil Moisture Observations in Groundwater Model Calibration

Schilling

Cook

Brunner

2019

Reviews of Geophysics

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Traditionally, groundwater and surface water flow models have been calibrated against two observation types: hydraulic heads and surface water discharge. It has repeatedly been demonstrated, however, that these classical observations do not contain sufficient information to calibrate flow models. To reduce the predictive uncertainty of flow models, the consideration of other observation types constitutes a promising way forward. Despite the ever-increasing availability of other observation types, however, they are still unconventional when it comes to flow model calibration. By reviewing studies that included nonclassical observations in flow model calibration, benefits and challenges associated with their integration in flow model calibration were identified, and their information content was analyzed. While explicit simulation of mass transport processes in flow models poses challenges, even simplified approaches to integrate tracer concentrations yield significantly better calibration results than using only classical observations. For a majority of calibrated flow models, observations of tracer concentrations and of exchange fluxes were beneficial. Temperature observations improved the simulation of heat transport but often worsened all other model outcomes. Only when temperature observations were made within 2 m of the surface water-groundwater interface did they have the potential to also improve flow and mass transport simulations. Surprisingly, many models were calibrated manually rather than with the widely available, mathematically robust and automated tools. There is a clear need for more systematic implementation of unconventional observations and automated flow model calibration as well as for more systematic quantification of the information content of unconventional observations. Plain Language Summary Traditionally, groundwater and surface water flow models, which are critical for water resources assessment, have been calibrated against only two classical observation types: groundwater levels and surface water discharge. In the past, it has repeatedly been demonstrated that these classical observations do not contain sufficient information to calibrate the parameters required for the simulation of groundwater and surface water flow systems. Owing to the rapid development of measurement techniques throughout the last three decades, however, many other observations of hydrological systems have become widely available. Despite this, observation types other than the classical ones are still unconventional when it comes to flow model calibration. The overall goal of this review is to identify optimal observation types and procedures for flow model calibration and hydrological predictions. We found that observations of tracer concentrations and exchange fluxes are beneficial for most flow models. Temperatures improve the simulation of heat transport but often worsen other flow model outcomes, unless temperatures are measured within 2 m of the surface water-groundwater interface. We identified a need for...

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Section: Review Of the Use Of Unconventional Observation Typesmentioning

confidence: 80%

Section: Review Of the Use Of Unconventional Observation Typesmentioning

confidence: 99%

Beyond Classical Observations in Hydrogeology: The Advantages of Including Exchange Flux, Temperature, Tracer Concentration, Residence Time, and Soil Moisture Observations in Groundwater Model Calibration

Schilling

Cook

Brunner

2019

Reviews of Geophysics

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“…An alternative approach could be to estimate the DTW in the recharge zone, particularly in cases where there may be significant differences between the recharge zone and sampling location. For example, Wood et al (2017) used the relationship between DTW and 14 Cuz for the Ti Tree Basin (central Australia) to generate spatially variable 14 C inputs in a regional scale solute transport model. 270…”

Section: Discussion 245mentioning

confidence: 99%

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Irvine¹,

Wood²,

Cartwright³

et al. 2021

Preprint

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Abstract. Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C-depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling, or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C-depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of 14Cuz to MRTs derived from 14C data.

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“…A spatially uniform recharge distribution is the basic assumption of several age dating methods (Cook & Böhlke, 2000) and also a commonly made assumption in numerical modeling studies (e.g., Salmon et al, 2015;Stauffer et al, 2002;Weissmann et al, 2002). However, in most aquifer settings, recharge varies spatially across a catchment due to variations in soil and vegetation cover (Cook et al, 1998), topographic relief (McGuire et al, 2005), localized recharge sources such as surface waters, wetlands or mountain front (Fulton et al, 2012;Siade et al, 2015;Wilson & Guan, 2004), and the variability of vadose zone processes (Scanlon et al, 2002;Wood et al, 2015Wood et al, , 2017. This can result in adjacent groundwater flow paths of very different residence times, which could be expected to create substantial mixing of tracer concentrations indicating distinctively different ages where these flow paths converge, such as in long-screen wells (Wang et al, 2016).…”

Section: Scenario 3: Spatially Variable Rechargementioning

confidence: 99%

Physical and Chemical Controls on the Simultaneous Occurrence of Young and Old Groundwater Inferred From Multiple Age Tracers

Underwood

McCallum

Cook

et al. 2018

Water Resources Research

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Understanding groundwater ages within an aquifer system has the potential to better constrain estimates of groundwater recharge and flow rates and therefore increase the reliability of groundwater models. Groundwater ages are generally interpreted from field‐observed environmental tracer concentrations, but in many cases in which multiple groundwater age tracers have been analyzed simultaneously the results show significant disparities among tracer‐specific estimated ages. The disparities are generally attributed to physical mixing between waters of different ages. However, especially in the geochemical literature environmental tracer concentrations are often analyzed with simplistic models in which the degree of the simulated mixing might be considered unrealistic for natural heterogeneous geologic media. In this study we use numerical experiments to examine under which physical conditions measured concentrations of selected environmental tracers (CFC‐12, 39Ar, and 14C) may return discrepant ages. Our model simulations suggest that matrix diffusion has the greatest potential to cause mixing of different‐aged water and to generate age biases between tracers. The multitracer simulations also suggest that there is a limit to the magnitude of the discrepancies that can be attributed to physical processes. When comparing data collected from the Pilbara region of Western Australia with our numerical modeling studies, it was found that a dual‐domain mass transfer model was required to explain the field‐observed age discrepancies.

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Constraining spatial variability in recharge and discharge in an arid environment through modeling carbon‐14 with improved boundary conditions

Cited by 9 publications

References 56 publications

Beyond Classical Observations in Hydrogeology: The Advantages of Including Exchange Flux, Temperature, Tracer Concentration, Residence Time, and Soil Moisture Observations in Groundwater Model Calibration

Beyond Classical Observations in Hydrogeology: The Advantages of Including Exchange Flux, Temperature, Tracer Concentration, Residence Time, and Soil Moisture Observations in Groundwater Model Calibration

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Physical and Chemical Controls on the Simultaneous Occurrence of Young and Old Groundwater Inferred From Multiple Age Tracers

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