Debates—Stochastic subsurface hydrology from theory to practice: why stochastic modeling has not yet permeated into practitioners?

This is the peer reviewed version of the following article: [Sanchez-Vila, X., and D. Fernàndez-Garcia (2016), Debates—Stochastic subsurface hydrology from theory to practice: Why stochastic modeling has not yet permeated into practitioners?, Water Resour. Res., 52, 9246–9258, doi:10.1002/2016WR019302], which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/2016WR019302/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-ArchivingWe address modern topics of stochastic hydrogeology from their potential relevance to real modeling efforts at the field scale. While the topics of stochastic hydrogeology and numerical modeling have become routine in hydrogeological studies, nondeterministic models have not yet permeated into practitioners. We point out a number of limitations of stochastic modeling when applied to real applications and comment on the reasons why stochastic models fail to become an attractive alternative for practitioners. We specifically separate issues corresponding to flow, conservative transport, and reactive transport. The different topics addressed are emphasis on process modeling, need for upscaling parameters and governing equations, relevance of properly accounting for detailed geological architecture in hydrogeological modeling, and specific challenges of reactive transport. We end up by concluding that the main responsible for nondeterministic models having not yet permeated in industry can be fully attributed to researchers in stochastic hydrogeology.Peer ReviewedPostprint (author's final draft

Stochastic estimation of hydraulic transmissivity fields using flow connectivity indicator data

This is the peer reviewed version of the following article: [Freixas, G., D. Fernàndez-Garcia, and X. Sanchez-Vila (2017), Stochastic estimation of hydraulic transmissivity fields using flow connectivity indicator data, Water Resour. Res., 53, 602–618, doi:10.1002/2015WR018507], which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/2015WR018507/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.Most methods for hydraulic test interpretation rely on a number of simplified assumptions regarding the homogeneity and isotropy of the underlying porous media. This way, the actual heterogeneity of any natural parameter, such as transmissivity ( math formula), is transferred to the corresponding estimates in a way heavily dependent on the interpretation method used. An example is a long-term pumping test interpreted by means of the Cooper-Jacob method, which implicitly assumes a homogeneous isotropic confined aquifer. The estimates obtained from this method are not local values, but still have a clear physical meaning; the estimated math formula represents a regional-scale effective value, while the log-ratio of the normalized estimated storage coefficient, indicated by math formula, is an indicator of flow connectivity, representative of the scale given by the distance between the pumping and the observation wells. In this work we propose a methodology to use math formula, together with sampled local measurements of transmissivity at selected points, to map the expected value of local math formula values using a technique based on cokriging. Since the interpolation involves two variables measured at different support scales, a critical point is the estimation of the covariance and crosscovariance matrices. The method is applied to a synthetic field displaying statistical anisotropy, showing that the inclusion of connectivity indicators in the estimation method provide maps that effectively display preferential flow pathways, with direct consequences in solute transport.Peer ReviewedPostprint (published version

The role of advection and dispersion in the rock matrix on the transport of leaking CO2-saturated brine along a fractured zone

CO2 that is injected into a geological storage reservoir can leak in dissolved form because of brine displacement from the reservoir, which is caused by large-scale groundwater motion. Simulations of the reactive transport of leaking CO2aq along a conducting fracture in a clay-rich caprock are conducted to analyze the effect of various physical and geochemical processes. Whilst several modeling transport studies along rock fractures have considered diffusion as the only transport process in the surrounding rock matrix (diffusive transport), this study analyzes the combined role of advection and dispersion in the rock matrix in addition to diffusion (advection-dominated transport) on the migration of CO2aq along a leakage pathway and its conversion in geochemical reactions. A sensitivity analysis is performed to quantify the effect of fluid velocity and dispersivity. Variations in the porosity and permeability of the medium are found in response to calcite dissolution and precipitation along the leakage pathway. We observe that advection and dispersion in the rock matrix play a significant role in the overall transport process. For the parameters that were used in this study, advection-dominated transport increased the leakage of CO2aq from the reservoir by nearly 305%, caused faster transport and increased the mass conversion of CO2aq in geochemical reactions along the transport pathway by approximately 12.20% compared to diffusive transport.Peer ReviewedPostprint (author's final draft

Avaluació de riscos ambientals mitjançant modelació geoquímica

L'interés sobre la recàrrega artificial d'aqüífers s'ha incrementat en els últims anys. La recerca del Grup d'Hidrologia Subterrània (GHS) de l'ETSECCPB vol donar una visió integrada i multidisciplinar sobre el risc (enginyeril i sanitari) associat a les pràctiques de recàrrega en basses d'infiltració. Per això és necessari avançar en paral·lel tant en l'estudi de la reducció en l'infiltració amb el temps, com en diferents aspectes relacionats amb les variacions hidrogeoquímiques durant el procés d'infiltració de l'aigua pel sòl i un cop a l'aqüífer. La incertesa lligada al desconeixement del medi natural implica haver de tractar amb el concepte risc des d'un punt de vista probabilístic. Els estudis realitzats o en marxa integren aspectes teòrics i numèrics amb assajos de laboratori i treball de camp en una zona pilot ja parcialment instrumentada

Linking biofilm spatial structure to real-time microscopic oxygen decay imaging

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Biofouling on 2018, available online at: http://www.tandfonline.com/10.1080/08927014.2017.1423474Two non-destructive techniques, confocal laser scanning microscopy (CLSM) and planar optode (VisiSens imaging), were combined to relate the fine-scale spatial structure of biofilm components to real-time images of oxygen decay in aquatic biofilms. Both techniques were applied to biofilms grown for seven days at contrasting light and temperature (10/20°C) conditions. The geo-statistical analyses of CLSM images indicated that biofilm structures consisted of small (~100 µm) and middle sized (~101 µm) irregular aggregates. Cyanobacteria and EPS (extracellular polymeric substances) showed larger aggregate sizes in dark grown biofilms while, for algae, aggregates were larger in light-20°C conditions. Light-20°C biofilms were most dense while 10°C biofilms showed a sparser structure and lower respiration rates. There was a positive relationship between the number of pixels occupied and the oxygen decay rate. The combination of optodes and CLMS, taking advantage of geo-statistics, is a promising way to relate biofilm architecture and metabolism at the micrometric scale.Peer ReviewedPostprint (author's final draft

Bayesian estimation of the transmissivity spatial structure from pumping test data

Estimating the statistical parameters (mean, variance, and integral scale) that define the spatial structure of the transmissivity or hydraulic conductivity fields is a fundamental step for the accurate prediction of subsurface flow and contaminant transport. In practice, the determination of the spatial structure is a challenge because of spatial heterogeneity and data scarcity. In this paper, we describe a novel approach that uses time drawdown data from multiple pumping tests to determine the transmissivity statistical spatial structure. The method builds on the pumping test interpretation procedure of Copty et al. (2011) (Continuous Derivation method, CD), which uses the time-drawdown data and its time derivative to estimate apparent transmissivity values as a function of radial distance from the pumping well. A Bayesian approach is then used to infer the statistical parameters of the transmissivity field by combining prior information about the parameters and the likelihood function expressed in terms of radially-dependent apparent transmissivities determined from pumping tests. A major advantage of the proposed Bayesian approach is that the likelihood function is readily determined from randomly generated multiple realizations of the transmissivity field, without the need to solve the groundwater flow equation. Applying the method to synthetically-generated pumping test data, we demonstrate that, through a relatively simple procedure, information on the spatial structure of the transmissivity may be inferred from pumping tests data. It is also shown that the prior parameter distribution has a significant influence on the estimation procedure, given the non-uniqueness of the estimation procedure. Results also indicate that the reliability of the estimated transmissivity statistical parameters increases with the number of available pumping tests.Peer ReviewedPostprint (author's final draft

Making profiles in boys and girls playing basketball

Decision-making in players of sports such as basketball is essential to the sport activity itself. For this reason, decision-making is being researched in sports from a range of approaches and angles. The focus of this study is on understanding the decision-making profiles of basketball playing boys and girls (n=63), aged between 10 and 12. They belong to a basketball school with a distinct educational philosophy and an internal competition style that favour educational values required in sports at the initial stages. A descriptive study was conducted by applying a questionnaire on decision-making styles in sports (CETD). The questionnaire results according to the three analysed parameters are as follows: perceived decisional competence, M=2.26 ± 0.2. Anxiety and feeling overwhelmed when making decisions, M=2.36 ± 0.3. Tactical learning commitment, M=3.43 ± 0.3. These results have allowed the acquisition of greater knowledge about the players being analysed in terms of their perception on how they make decisions in training and competition, thus allowing the application of certain teaching strategies, enabling them to progress in basketball skills.La toma de decisiones de los jugadores en deportes como el baloncesto, es consustancial al propio deporte. Por este motivo, la decisión es objeto de investigación en el ámbito deportivo, abordándola desde diferentes enfoques y perspectivas. El objetivo de este estudio ha sido conocer el perfil decisional de un grupo de niños y niñas que practican baloncesto (n=63), con edades comprendidas entre los 10 y 12 años. Pertenecen a una escuela de baloncesto con una marcada filosofía educativa y un tipo de competición interna favorecedora de los valores educativos necesarios para la práctica deportiva en las etapas de iniciación. Se realizó un estudio descriptivo aplicando el cuestionario de estilos al decidir en el deporte (CETD). Los resultados obtenidos según las tres dimensiones analizadas del cuestionario fueron: competencia decisional percibida, M=2.26 ± 0.2. Ansiedad y agobio al decidir, M=2.36 ± 0.3. Compromiso en el aprendizaje táctico, M=3.43 ± 0.3. Estos resultados han permitido obtener mayor conocimiento de los jugadores y jugadoras analizados en cuanto a su percepción sobre cómo deciden en el entrenamiento y la competición y, de este modo, poder aplicar determinadas estrategias didácticas que les permita progresar en el aprendizaje del baloncesto

Infilitration tests at the Sant Vicenç dels Horts artificial recharge experimental site

Infiltration capacity is the key parameter in an artificial recharge operation site. Infiltration capacity is spatially variable, and during operation it is also temporally variable due to surface clogging processes. Double-ring infiltrometer tests were performed at an experimental site close to Barcelona city (Spain). The site is located on alluvial deposits from the Llobregat River and comprises two half hectare ponds. River water collected upstream traveled through a two km pipe before entering the settling pond. Once the pond is filled water flows to the infiltration pond. Tests were performed only in the latter, prior to and after recharging the ponds. Prior to recharge, six points were selected to estimate infiltration capacity Points were evenly distributed and chosen considering apparent soil texture at the site (coarse, medium and fine grains). All tests were performed allowing water to infiltrate for two hours and data was interpreted using the modified Kostiakov equation. Ponds were then flooded for about two months. The average infiltration rate values for the full infiltration pond before and after the flooding campaign were 5.8 m/day and 2.2 m/day, respectively. The double ring tests were then repeated at the same points, showing a reduction of the infiltration rate that varied between 7 and 90%. Control points with the initial highest infiltration rates presented the highest reduction in infiltration. Physical clogging due to particles settling appears to be the most likely cause of the diminished infiltration rates. This result is confirmed by other independent measurements during the flooding test. There is a clear tendency towards a lower infiltration rates when observing the relation through time of flow entering per volume of water on the infiltration pond at a given time.Peer ReviewedPostprint (published version

Conditional stochastic mapping of transport connectivity

We present a method for the stochastic simulation of point-to-point transport connectivity honoring data from three types of information: (a) travel time estimates obtained from field tracer tests; (b) estimates of flow connectivity indicators obtained from the relatively fast or slow flow response that is observed at a point location given the flow impulse at another location; and (c) measurements of transmissivity at a local scale. The method thus efficiently integrates data obtained from different hydraulic tests, each one sampling different areas within the aquifer. To achieve this, we first extend the concept of point-to-point flow connectivity and transport connectivity, mathematically formulated by Trinchero et al. (2008) for pumping conditions, to support a more general flow configuration. Interestingly, point-to-point flow connectivity can be generally seen as a weighted integral of transmissivity over the entire domain, the weighting function being proportional to the sensitivity of heads with respect to the natural log of transmissivity per unit of aquifer volume. On the contrary, point-to-point transport connectivity is a weighted integral along the particle path of the solute mass that involves two variables: transmissivity and flow connectivity. Each variable has its own distinct weighting function. The weighting function of transmissivity is inversely proportional to both the homogeneous travel time and the point velocity sampled along the travel path. Based on this, we show how to generate conditional point-to-point transport connectivity maps. The method avoids the inference of cross-covariance functions between variables measured over different scales and sampled areas (which cannot be otherwise estimated with a few data measurements), by expressing them as a function of the local transmissivity covariance function. An example of the method is provided to evaluate the worth of including tracer data to delineate capture zones of abstraction wells originally defined from local transmissivity measurements. Monte Carlo simulations reveal that the impact of including tracer data is maximum when the travel time data is obtained at a different location than that of transmissivity measurements. The reason is that weighting functions gives larger weights to the injection location, and so introducing tracer test data at points where transmissivity is already known is somewhat redundant.Peer ReviewedPostprint (published version