Mapping regions of equifinality in the parameter space: A method to evaluate inverse estimates and to plan experimental boundary conditions

Breakthrough curves of column outflow experiments are frequently used to identify parameters of reactive transport in porous media. However, inverse techniques coincide often with uniqueness problems, i.e. more than a single parameter set can reproduce the breakthrough curve equally well – a problem, which is termed equifinality. In this study, maps of regions with reliable parameter estimates were created applying numerical case studies with systematically varied parameters and boundary conditions. This was carried out by comparing the numerically produced breakthrough curves from forward simulations of the advection dispersion equation including equilibrium and non-equilibrium sorption (two-site/two-region model). Mapping of regions of uniqueness in the parameter space can help to improve the reliability of inverse simulations in two ways. On the one hand, regions of uniqueness depend on the boundary conditions of the experiment. Consequently, if the correct experimental boundary conditions – such as flow velocity, number and duration of percolation intervals and flow interruptions – are chosen, uniqueness problems can be avoided a priori. All what is required are some a priori information on the target compounds and materials and maps of regions of uniqueness for different experimental designs. On the other hand parameter estimates can be plotted into such maps a posteriori, to be able to evaluate whether equifinal solutions exist. are required, then an appropriate experimental design can be selected using maps of uniqueness

Characterization of reactive transport by 3-D electrical resistivity tomography (ERT) under unsaturated conditions

The leaching of nitrate from intensively used arable soil is of major concern in many countries. In this study, we show how time lapse electrical resistivity tomography (ERT) can be used to characterize spatially heterogeneous processes of ion production, consumption, and transport in soils. A controlled release fertilizer was introduced into an undisturbed soil core in a laboratory lysimeter and subjected to infiltration events. The production of ions resulting from processes associated with nitrification and their transport through the soil core was observed by time lapse ERT and analysis of seepage water samples from a multicompartment sampler. ERT images show development and propagation of a high-conductivity plume from the fertilizer source zone. Molar amounts of nitrate produced in and exported from the soil core could be well reproduced by time lapse ERT using a spatial moment analysis. Furthermore, we observed that several shape measures of local breakthrough-curves (BTCs) of seepage water conductivity and nitrate derived by effluent analyses and BTCs of bulk conductivity derived by ERT are highly correlated, indicating the preservation of spatial differences of the plume breakthrough in the ERT data. Also differences between nitrate breakthrough and a conservative tracer breakthrough can be observed by ERT. However, the estimation of target ion concentrations by ERT is error bound and the smoothing algorithm of the inversion masks spatial conductivity differences. This results in difficulties reproducing spatial differences of ion source functions and variances of travel times. Despite the observed limitations, we conclude that time lapse ERT can be qualitatively and quantitatively informative with respect to processes affecting the fate of nitrate in arable soils

Characterization of reactive transport by 3-D electrical resistivity tomography (ERT) under unsaturated conditions

The leaching of nitrate from intensively used arable soil is of major concern in many countries. In this study, we show how time lapse electrical resistivity tomography (ERT) can be used to characterize spatially heterogeneous processes of ion production, consumption, and transport in soils. A controlled release fertilizer was introduced into an undisturbed soil core in a laboratory lysimeter and subjected to infiltration events. The production of ions resulting from processes associated with nitrification and their transport through the soil core was observed by time lapse ERT and analysis of seepage water samples from a multicompartment sampler. ERT images show development and propagation of a high-conductivity plume from the fertilizer source zone. Molar amounts of nitrate produced in and exported from the soil core could be well reproduced by time lapse ERT using a spatial moment analysis. Furthermore, we observed that several shape measures of local breakthrough-curves (BTCs) of seepage water conductivity and nitrate derived by effluent analyses and BTCs of bulk conductivity derived by ERT are highly correlated, indicating the preservation of spatial differences of the plume breakthrough in the ERT data. Also differences between nitrate breakthrough and a conservative tracer breakthrough can be observed by ERT. However, the estimation of target ion concentrations by ERT is error bound and the smoothing algorithm of the inversion masks spatial conductivity differences. This results in difficulties reproducing spatial differences of ion source functions and variances of travel times. Despite the observed limitations, we conclude that time lapse ERT can be qualitatively and quantitatively informative with respect to processes affecting the fate of nitrate in arable soils

Non-equilibrium conditions during organic contaminant mobilization, transport and degradation in the subsurface

Processes determining the fate of organic contaminants in the subsurface are often characterized by non-equilibrium conditions. This habilitation treatise explores the kinetic characteristics of the processes influencing the fate of organic contaminants in the subsurface employing computational-, laboratory- and field experiments. In the first part, release and transport processes of hydrophobic contaminants from multicomponent non-aqueous phase liquids (NAPLs) are examined in 5 individual studies. These comprise a review of the literature on the fate of contaminants at manufactured gas plants, laboratory experiments and modeling of the release kinetics of polycyclic aromatic hydrocarbons (PAHs) from aged and fresh tar phases, and a field study of a crude oil contaminated aquifer employing geophysical, hydrogeochemical and microbiological methods. The second part comprises three studies, which explore transport and degradation kinetics of hydrophilic compounds, which are used as deicing chemicals at airports. In this part, a laboratory percolation experiment was evaluated with a numerical model and the degradation of the deicing chemicals propylene glycol and formate under field conditions was characterized in lysimeters, employing, among other methods, electrical resistivity tomography (ERT). The studies show that multiple interdependencies exist between contaminant transport time-scales and time-scales of mobilization and degradation. Solubilization of PAHs from tar oils was diffusionally highly restricted with extremely long equilibration times. Hydrophobic compounds are often subject to mobilization and transport as colloids, which was observed in all experimental studies with aged NAPLs. Degradation kinetics of deicing chemicals were found to be dependent on a dynamic reaction of the biomass in column experiments and did not agree with first-order kinetics. The field lysimeter experiments showed that the ratio of degradation time-scale to transport time-scale determines the fate of propylene glycol. As a consequence, non-equilibrium transport conditions resulted in rapid discharge of large amounts of propylene glycol from the topsoil. The results have consequences for experimental procedures aiming for process identification and parameter quantification of mobilization, transport and degradation of organic contaminants. For a realistic estimation of risk of exposure, natural attenuation and biological remediation, experimental and numerical evaluation including transport conditions is inevitable

Integration of geophysical, geochemical and microbiological data for a comprehensive small-scale characterization of an aged LNAPL-contaminated site

Characterization of aged hydrocarbon-contaminated sites is often a challenge due to the heterogeneity of subsurface conditions. Geoelectrical methods can aid in the characterization of such sites due to their non-invasive nature, but need to be supported by geochemical and crobiological data. In this study, a combination of respective methods was used to characterize an aged light non-aqueous phase liquid-contaminated site, which was the scene of a crude oil blow-out in 1994. As a consequence, a significant amount of crude oil was released into the subsurface. Complex resistivity has been acquired, both along single boreholes and in cross-hole configuration, in a two-borehole test site addressed with electrodes, to observe the electrical behaviour at the site over a two-year period (2010–2011). Geoelectrical response has been compared to results of the analysis of hydrocarbon contamination in soil and groundwater samples. Geochemical parameters of groundwater have been observed by collecting samples in a continuous multi-channel tubing (CMT) piezometer system. We have also performed a biological characterization on soil samples by drilling new boreholes close to the monitoring wells. Particular attention has been given to the characterization of the smear zone that is the sub-soil zone affected by the seasonal groundwater fluctuations. In the smear zone, trapped hydrocarbons here present, serving as organic substrate for chemical and biological degradation, as was indicated by an increase of microbial biomass and activity as well as ferrogenicsulfidogenic conditions in the smear zone. The results show a good agreement between the intense electrical anomaly and the peaks of total organic matter and degradation by-products, particularly enhanced in the smear zone

Non-invasive characterization of the Trecate (Italy) crude-oil contaminated site:links between contamination and geophysical signals

The characterization of contaminated sites can benefit from the supplementation of direct investigations with a set of less-invasive, and more extensive, measurements. A combination of geophysical methods and direct push techniques for contaminated land characterization has been proposed within the EU FP7 project ModelPROBE and the affiliated project SoilCAM. In this paper we present results of the investigations conducted at the Trecate field site (NW Italy), which was affected in 1994 by crude oil contamination. The less-invasive investigations include ground-penetrating radar (GPR), electrical resistivity tomography (ERT) and electromagnetic induction (EMI) surveys, together with direct push sampling and soil electrical conductivity (EC) logs. Many of the geophysical measurements were conducted in time-lapse mode in order to separate static and dynamic signals, the latter being linked to strong seasonal changes in water table elevations. The main challenge was to extract significant geophysical signals linked to contamination from the mix of geological and hydrological signals present at the site. The most significant aspects of this characterization are: (a) the geometrical link between the distribution of contamination and the site's heterogeneity, with particular regard to the presence of less permeable layers, as evidenced by the extensive surface geophysical measurements; and (b) the link between contamination and specific geophysical signals, particularly evident from cross-hole measurements. The extensive work conducted at the Trecate site shows how a combination of direct (e.g. chemical) and indirect (e.g. geophysical) investigations can lead to a comprehensive and solid understanding of a contaminated site's functioning