Browsing by Author "Haslauer, Claus"

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Open Access
Analysis of real-world spatial dependence of subsurface hydraulic properties using copulas with a focus on solute transport behaviour
(2011) Haslauer, Claus; Bárdossy, András (Prof. Dr. rer.nat. Dr.-Ing.)
Copulas are a novel tool in geostatistics that allows modelling of pure spatial dependence independently of the marginal distribution and without an assumption of multivariate Gaussian dependence. By using a transformation via the marginal distribution, the effect of extreme values is substantially decreased compared to traditional Gaussian based geostatistical measures such as Kriging. Additionally, the dependence is not described as an average variance as in Kriging, but a different degree of dependence can be modelled for different quantiles of the marginal distribution. Two data-sets from field sites at Borden and North Bay, both in Ontario, Canada, were used to test the performance of copulas as stochastic models for spatial dependence. Furthermore, this thesis explores possible effects of modelling spatial dependence using non-Gaussian copulas on physical properties that are based on such heterogeneous fields. For comparison, the effects of Gaussian structures are evaluated. The Gaussian- and non-Gaussian structures can not be distinguished by their variograms. It was shown that neither of the two data-sets exhibits Gaussian dependence – despite the fact that the Borden aquifer is commonly thought of as a relatively homogeneous porous medium with a small variance of hydraulic conductivity. Two non-Gaussian copula models, v-copulas and maximum Gaussian copulas were fitted to the hydraulic conductivity data, to be compared with a Gaussian copula model. The theoretical copula models were subsequently used for spatial interpolation and simulation. In addition to evaluating the spatial dependence structure of the hydraulic conductivity data-sets, fitting theoretical copula models and using them for interpolation and simulation, the goal of this thesis is to explore if the structure of the hydraulic conductivity field influences a physical property, such as plume evolution as evaluated by second central moments of concentration fields. Despite the fact that Borden is a relatively homogeneous porous medium, and despite the fact that both types of spatial fields are not distinguishable by their variograms, the solute transport characteristics based on these two types of fields differ significantly in two- dimensional settings. The difference is less pronounced in three-dimensions. Non-Gaussian dependence can lead to a non-symmetric distribution of variance of concentration along the main direction of flow. Increasing the variance of a marginal distribution by a certain factor does not necessarily lead to a dispersivity increased by the same factor in the case of non-Gaussian fields. It is postulated that non-Gaussian spatial dependence of hydraulic conductivity and a more skewed marginal distribution of hydraulic conductivity will have significant implications in the other more heterogeneous aquifers.
Open Access
Comparison of multivariate spatial dependence structures of DPIL and flowmeter hydraulic conductivity data sets at the MADE site
(2019) Xiao, Bo; Haslauer, Claus; Bohling, Geoffrey
Open Access
Heat transport from atmosphere through the subsurface to drinking‐water supply pipes
(2023) Nissler, Elisabeth; Scherrer, Samuel; Class, Holger; Müller, Tanja; Hermannspan, Mark; Osmancevic, Esad; Haslauer, Claus
Drinking‐water quality in supply pipe networks can be negatively affected by high temperatures during hot summer months due to detrimental bacteria encountering ideal conditions for growth. Thus, water suppliers are interested in estimating the temperature in their distribution networks. We investigate both experimentally and by numerical simulation the heat and water transport from ground surface into the subsurface, (i.e., above drinking‐water pipes). We consider the meteorological forcing functions by a sophisticated approach to model the boundary conditions for the heat balance at the soil-atmosphere interface. From August to December 2020, soil temperatures and soil moisture were measured dependent on soil type, land‐use cover, and weather data at a pilot site, constructed specifically for this purpose at the University of Stuttgart with polyethylene and cast‐iron pipes installed under typical in situ conditions. We included this interface condition at the atmosphere-subsurface boundary into an integrated non‐isothermal, variably saturated (Richards') the numerical simulator DuMux 3. This allowed, after calibration, to match measured soil temperatures with ±2°C accuracy. The land‐use cover influenced the soil temperature in 1.5 m more than the soil material used for back‐filling the trench above the pipe.
Open Access
Investigating rate-limited sorption, sorption to air-water interfaces, and colloid-facilitated transport during PFAS leaching
(2023) Bierbaum, Thomas; Hansen, Scott K.; Poudel, Bikash; Haslauer, Claus
Various sorption processes affect leaching of per- and polyfluoroalkyl substances (PFAS) such as PFOA and PFOS. The objectives of this study are to (1) compare rate-limited leaching in column and lysimeter experiments, (2) investigate the relevance of sorption to air-water interfaces (AWI), and (3) examine colloid-facilitated transport as a process explaining early experimental breakthrough. A continuum model (CM) with two-domain sorption is used to simulate equilibrium and rate-limited sorption. A random walk particle tracking (PT) model was developed and applied to analyze complex leaching characteristics. Results show that sorption parameters derived from column experiments underestimate long-term PFOA leaching in lysimeter experiments due to early depletion, suggesting that transformation of precursors contributes to the observed long-term leaching in the lysimeters (approximately 0.003 µg/kg/d PFOA). Both models demonstrate that sorption to AWI is the dominant retention mechanism for PFOS in lysimeter experiments, with retardation due to AWI being 3 (CM) to 3.7 (PT) times higher than retardation due to solid phase sorption. Notably, despite a simplified conception of AWI sorption, the PT results are closer to the observations. The PT simulations demonstrate possible colloid-facilitated transport at early time; however, results using substance-specific varying transport parameters align better with the observations, which should be equal if colloid-facilitated transport without additional kinetics is the sole mechanism affecting early breakthrough. Possibly, rate-limited sorption to AWI is relevant during the early stages of the lysimeter experiment. Our findings demonstrate that rate-limited sorption is less relevant for long-term leaching under field conditions compared to transformation of precursors and that sorption to AWI can be the dominant retention mechanism on contaminated sites. Moreover, they highlight the potential of random walk particle tracking as a practical alternative to continuum models for estimating the relative contributions of various retention mechanisms.