The usefulness of CPTs for deterministic, spatially heterogeneous, large-scale aquitard parametrisation

in Oswald, S.E.; Kolditz, O.; Attinger, S. (Eds.) Models - Repositories of Knowledge (2012, December)

Aquitards can be effectively parameterised and incorporated in a groundwater flow model by using standard cone penetration tests (CPTs). Several conceptually different realizations of an aquitard’s ... [more ▼]

Aquitards can be effectively parameterised and incorporated in a groundwater flow model by using standard cone penetration tests (CPTs). Several conceptually different realizations of an aquitard’s hydraulic conductivity field were evaluated based on: (i) conventional methods of soil behaviour type classification, (ii) recent relationships from the literature, and (iii) novel site-specific relations with hydraulic conductivity. We show that use of most of these CPT-based hydraulic conductivity estimations in groundwater flow modelling effectively enhance model performance based on absolute head values and gradients across the aquitard. Conceptual models that considered a spatially heterogeneous hydraulic conductivity for the aquitard performed better than the reference case with a uniform aquitard hydraulic conductivity. However, the hydraulic conductivity of thin heavy clay lenses, characteristic of the aquitard present in our study area, cannot be captured using these continuum approaches. The latter leads to a bias in the direct hydraulic conductivity predictions; an alternative is to invoke inverse modelling with the heterogeneous parameter fields. To address this issue, the concept of the boundary energy associated with the CPT signal is also introduced for characterising the presence of heavy clay lenses. Overall, the CPT-based concepts provide more accurate, robust, and high-resolution data-based parameterisation of the studied aquitard. [less ▲]

Relationship between sedimentary features and permeability at different scales in the Brussels Sands

in Geologica Belgica (2012), 15(3), 156-164

The Brussels Sands display a complex three-dimensional subsurface architecture. This sedimentological heterogeneity induces a highly heterogeneous spatial distribution of hydrogeological parameters at ... [more ▼]

The Brussels Sands display a complex three-dimensional subsurface architecture. This sedimentological heterogeneity induces a highly heterogeneous spatial distribution of hydrogeological parameters at different scales and may consequently influence subsurface fluid flow and solute migration. This study aims at characterizing spatial variability of permeability at different scales in the Brussels Sands. Firstly, a literature review on the permeability distribution of the Brussels Sands was performed. Secondly, a field campaign was carried out consisting of field observations of the small-scale sedimentary structures and in situ measurements of air permeability. A total of 6550 cm-scale air permeability measurements were carried out in situ in three Brussels Sands quarries in the central part of Belgium: Bierbeek, Mont Saint Guibert and Chaumont Gistoux. On the large basin scale, substantial differences in permeability are observed. A literature data analysis shows that there is no clear correlation between hydraulic conductivity and sedimentary facies. At the small scale, results show that permeability heterogeneity and anisotropy are strongly influenced by sedimentary heterogeneity in all three quarries. Clay-rich sedimentary features such as bottomsets and distinct mud drapes exhibit a different statistical and geostatistical permeability distribution compared to the cross-bedded lithofacies, where the permeability anisotropy is dominated by the foreset lamination orientation. [less ▲]

Flux-based risk assessment of the impact of contamnants on water ressources and ecosystems - FRAC-WECO.

Report (2012)

Groundwater model parameter identification using a combination of cone-penetration tests and borehole data

in International Groundwater Symposium 2010, IAHR (2010, September)

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterisation has been ... [more ▼]

In the framework of the disposal of short-lived low- and intermediate-level radioactive waste in a near-surface disposal facility in Dessel, Belgium, additional extensive site characterisation has been performed in 2008. The gathered data now include 388 hydraulic conductivity measurements on samples from 8 cored boreholes. Detailed characterisation of these cored boreholes, together with geophysical logging, enabled to identify various hydrostratigraphical units at 8 discrete locations in the research area. Various analyses were performed on the cores, yielding information on grain size, mineralogy, density and total porosity. Geophysical logging parameters were derived from gamma-ray and resistivity measurements. Subsequently, an extensive geotechnical logging campaign was performed in order to establish a 3D-model of the hydrostratigraphical units, based on a dense network of investigation points. About 180 cone penetration tests (CPTs) were executed and lithology was deduced in detail based on existing soil classi cation charts. As such, a description of the regional subsurface up to depths of nearly 50 m was established, and this information was integrated with the borehole data. Most importantly, the lateral extent, depth and thickness of a hydrogeologically important aquitard was identi fied. Based on the 2008 site characterisation results and their interpretation, an update of a ground- water fl ow model used in safety assessments was made. The CPT-based stratigraphic model and the hydraulic conductivity data determined at different scales were combined into a new 3D hydrostratigraphical model. The small-scale measurements (on 100 cm³ core samples) are compared with hydraulic conductivity values obtained from pumping tests and the large-scale parameters derived by inverse modelling. The performance of the original and the updated flow model are compared. The presented approach was succesfull in substantially decreasing the conceptual model and parameter uncertainty and resulted in an improved calibration of the groundwater flow model. [less ▲]

On the value of conditioning data to reduce conceptual model uncertainty in groundwater modeling

in Water Resources Research (2010), 46(8), 08520

Recent applications of multi-model methods have demonstrated their potential in quantifying conceptual model uncertainty in groundwater modeling applications. To date, however, little is known about the ... [more ▼]

Recent applications of multi-model methods have demonstrated their potential in quantifying conceptual model uncertainty in groundwater modeling applications. To date, however, little is known about the value of conditioning to constrain the ensemble of conceptualizations, to differentiate among retained alternative conceptualizations, and to reduce conceptual model uncertainty. We address these questions by conditioning multi-model simulations on measurements of hydraulic conductivity and observations of system-state variables and evaluating the e ffects on (i) the posterior multi-model statistics and (ii) the contribution of conceptual model uncertainty to the predictive uncertainty. Multi-model aggregation and conditioning is performed by combining the generalized likelihood uncertainty estimation (GLUE) method and Bayesian model averaging (BMA). As an illustrative example we employ a 3-dimensional hypothetical system under steady-state conditions, for which uncertainty about the conceptualization is expressed by an ensemble (M) of 7 models with varying complexity. Results show that conditioning on heads allowed for the exclusion of the two simplest models, but that their information content is limited to further diff erentiate among the retained conceptualizations. Conditioning on increasing numbers of conductivity measurements allowed for a further reffinement of the ensemble M and resulted in an increased precision and accuracy of the multi-model predictions. For some groundwater flow components not included as conditioning data, however, the gain in accuracy and precision was partially o ffset by strongly deviating predictions of a single conceptualization. Identifying the conceptualization producing the most deviating predictions may guide data collection campaigns aimed at acquiring data to further eliminate such conceptualizations. Including groundwater flow and river discharge observations further allowed for a better diff erentiation among alternative conceptualizations and drastic reductions of the predictive variances. Results strongly advocate the use of observations less commonly available than groundwater heads to reduce conceptual model uncertainty in groundwater modeling. [less ▲]

Assessment of conceptual model uncertainty for the regional aquifer Pampa del Tamarugal – North Chile

in Hydrology & Earth System Sciences (2010), 14

In this work we assess the uncertainty in modelling the groundwater flow for the Pampa del Tamarugal Aquifer (PTA) – North Chile using a novel and fully integrated multimodel approach aimed at explicitly ... [more ▼]

In this work we assess the uncertainty in modelling the groundwater flow for the Pampa del Tamarugal Aquifer (PTA) – North Chile using a novel and fully integrated multimodel approach aimed at explicitly accounting for uncertainties arising from the definition of alternative conceptual models. The approach integrates the Generalized Likelihood Uncertainty Estimation (GLUE) and Bayesian Model Averaging (BMA) methods. For each member of an ensemble M of potential conceptualizations, model weights used in BMA for multi-model aggregation are obtained from GLUE-based likelihood values. These model weights are based on model performance, thus, reflecting how well a conceptualization reproduces an observed dataset D. GLUE-based cumulative predictive distributions for each member of M are then aggregated obtaining predictive distributions accounting for conceptual model uncertainties. For the PTA we propose an ensemble of eight alternative conceptualizations covering all major features of groundwater flow models independently developed in past studies and including two recharge mechanisms which have been source of debate for several years. Results showed that accounting for heterogeneities in the hydraulic conductivity field (a) reduced the uncertainty in the estimations of parameters and state variables, and (b) increased the corresponding model weights used for multi-model aggregation. This was more noticeable when the hydraulic conductivity field was conditioned on available hydraulic conductivity measurements. Contribution of conceptual model uncertainty to the predictive uncertainty varied between 6% and 64% for ground water head estimations and between 16% and 79% for ground water flow estimations. These results clearly illustrate the relevance of conceptual model uncertainty. [less ▲]

Identification and quantification of sources of major solutes in a sandy, phreatic aquifer in Central Belgium through ionic ratios and geochemical mass-balance modelling

in Groundwater and Ecosystems, Proc. of the XXXV IAH Congress (2007)

In this study the processes affecting groundwater chemistry in the Eocene Brussels sands aquifer in Central Belgium are identified based on evaluation of ionic ratios of major solutes. Based on these ... [more ▼]

In this study the processes affecting groundwater chemistry in the Eocene Brussels sands aquifer in Central Belgium are identified based on evaluation of ionic ratios of major solutes. Based on these results, in combination with mineralogical and hydrogeological information of the aquifer, a geochemical mass-balance model is created to quantify the contribution of each of the processes to the observed composition of groundwater. After a rigorous validation process, a dataset of 99 groundwater samples is obtained from observation and pumping wells in the Eocene Brussels sands aquifer, which is one of the main aquifers for drinking water production in Belgium. The aquifer consists of heterogeneous alteration of calcified and silicified coarse sands, with local presence of clay drapes and glauconite-rich zones (Laga et al. 2001). The entire aquifer is overlain by Quaternary eolian deposits, mainly consisting of loam with the exception of the north east, where the Quaternary deposits are sandy loam. The groundwater in this aquifer is of Ca-Mg-HCO3-type with locally elevated nitrate concentrations. Based on the evaluation of ionic ratios and the mineralogy of the aquifer, a conceptual geochemical model is developed for mass-balance modeling, including (1) concentration of precipitation by a factor 1 to 5 due to evaporation, (2) dissolution of a pure calcite phase and a calcite phase containing 25 % magnesium by both carbonic acid and sulfuric acid, (3) anthropogenic inputs for all major cations and anions except bicarbonate, (4) dissolution of glauconite, (5) cation exchange of sodium and potassium for calcium and magnesium. The two calcite phases can be thought of as end-members of a solid solution of magnesium in calcite. The mass-balance modeling consists of a mole-balance equation for each considered element according to: [Obs] = p[Prec] + p1[Phase 1] + ... + pi[Phase i] + a [Anthropogenic] +/- c[Cation Exchange] This set of linear equations is additionally constrained by (1) defining a range for concentration factors p based on measured and calculated evaporation rates, (2) charge balance for the anthropogenic sources and (3) pi being positive or negative according to whether the phase dissolves or precipitates. The set of linear equations with the given constraints is solved using a least squares optimization. Based on the possible processes and reactions several geochemical models are tested for each sample and a model is considered adequate if the root mean squared error (RMSE) between observed and calculated concentrations is less than 10-10 mol/L and the charge balance of the calculated composition is less than 5 %. If several models are able to explain the observed concentrations, the RMSE provides an objective measure to compare the quality of the models. The best model for each sample is selected and the spatial distribution of these models is compared to the spatial variations in lithology and land-use to asses the feasibility of the proposed models. [less ▲]

Study of cavernous underground conduits in Nam La (Northwest Vietnam) by an integrative approach

in Hydrogeology Journal (2005), 13(5-6), 675-689

This paper presents the result of an investigation of underground conduits, which connect the swallow holes and the resurgence of a blind river in the tropical, highly karstified limestone Nam La ... [more ▼]

This paper presents the result of an investigation of underground conduits, which connect the swallow holes and the resurgence of a blind river in the tropical, highly karstified limestone Nam La catchment in the NW of Vietnam. The Nam La River disappears underground in several swallow holes near the outlet of the catchment. In the rainy season this results in flooding upstream of the sinkholes. A hypothesis is that the Nam La River resurges at a large cavern spring 4.5 km east of the catchment outlet. A multi-thematic study of the possible connections between the swallow holes and the resurgence was carried out to investigate the geological structure, tectonics, cave structure analysis and discharge time series. The existence of the underground conduits was also tested and proven by tracer experiments. On the basis of a lineament analysis the location of the underground conduits were predicted. A remote sensing derived lineament-length density map was used to track routes from the swallow holes to the resurgence, having the shortest length but highest lineament density. This resulted in a plan-view prediction of underground conduits that matches with the cave and fault development. The functioning of the conduits was further explained by analysing flooding records of a nearby doline, which turns out to act as a temporary storage reservoir mitigating flooding of the catchment outlet area. [less ▲]

Characterization of a cavern conduit system in Vietnam by time series correlation, cross-spectrum and wavelet analyses

in Hydrological Sciences Journal = Journal des Sciences Hydrologiques (2004), 49(5), 879-900

Time series analyses are applied to characterize the transient flow regimes of the Nam La cavern conduit, northwest Vietnam. The conduit transforms the input signal to an output signal, and the degree of ... [more ▼]

Time series analyses are applied to characterize the transient flow regimes of the Nam La cavern conduit, northwest Vietnam. The conduit transforms the input signal to an output signal, and the degree of transformation provides information on the nature of the flow system. The input for the analysis is net precipitation and the flow hydrograph at the cave entrance, while the output series is the flow hydrograph at the resurgence. Cross-correlation and cross-spectrum analysis are used to investigate the stationarity and linearity of the input-output transformation of the system, resulting in hydrodynamic properties such as system memory, response time, and mean delay between input and output. It is shown that during high flow periods, the flow in the conduit is pressurized. Consequently, the linear input-output assumption holds only for low flows. To highlight the hydrodynamics of the cavern conduit for the high flow periods, wavelet spectrum and wavelet cross-spectrum analyses are applied. [less ▲]