Hydrogeology

 

Research Topics

The focus of the Hydrogeology Group is to improve the understanding of heterogeneous and dynamic water flow and transport in the subsurface. The main research topics and objectives of the group are:

  • understanding dynamic and heterogeneous water and matter fluxes at the interface of the unsaturated and saturated zone.

  • quantifying spatially heterogeneous and temporally dynamic groundwater recharge rates and water transit time distributions using stable isotopes of water (2H und 18O).

  • elucidating the transport and fate of bacteria and viruses in porous aquifers under different hydrogeological and ecological conditions.

The group provides knowledge on dynamic and heterogeneous water fluxes and fate of contaminants in the subsurface. We will contribute to a better understanding of dynamic processes in ecohydrology which is fundamental for a common holistic and integrative understanding of groundwater ecosystem functioning.

Dynamic interface unsaturated - saturated zone

Although hydrological processes and mass fluxes in the unsaturated and saturated zone have been well studied separately, little is known about the spatial and temporal dynamics of transition processes between these zones. Water and mass fluxes at the interface have been ignored or simplified assuming, for example, a static groundwater table and a static capillary fringe. However, the interface at the groundwater table is highly dynamic and the extent of water table fluctuations exceeds the dimension of capillary fringes by far. Irregular rain events and subsequent recharge as well as the hydrology of nearby surface waters considerably trigger fluctuations of the hydraulic head. These dynamics may become even more important in the context of global change. Increasing frequencies of extreme hydrological events may impair the filter functions of soils. Therefore, groundwater recharge rates as well as mass fluxes (e.g., nutrients, pollutants) into groundwater systems are expected to increase. A direct, negative effect to groundwater quality is suggested. Furthermore, we expect variable vertical and lateral fluxes and redox conditions over time and space in the vicinity of the water table due to dynamic water table fluctuations. These variable conditions can dynamically trigger dissolution and precipitation of minerals and the fate of pollutants endangering the environmental health. We study hydrological and chemical processes at the interface of the unsaturated and saturated zone under controlled, known boundary conditions in flow-through laboratory experiments with and without a fluctuating water table (F. Rühle). Observational hydrogeology will be combined with mathematical modeling to identify key processes and quantify mass fluxes. This will enable a qualitative and quantitative assessment of dynamic water table fluctuations and its impact on transport, geochemical processes and biodegradation. Based on the findings from laboratory experiments, we will go into the field and study water fluxes in situ in the vicinity of fluctuating water tables. We intend to evaluate how these fluxes impact transport and transformation of chemicals.

Heterogeneity in subsurface water flow

One major obstacle in hydrogeology is the appropriate assessment of hydraulic heterogeneities like preferential flow or stagnant water zones affecting the water flow and transport in porous media. Reasons for this knowledge gap are the restricted accessibility of subsurface water systems and the mostly ignored need for a high spatial and temporal resolution. Hydraulic data (e.g., hydraulic heads and conductivities) are important parameters which however may not provide information about water flow velocity distributions. Besides, they do not deliver any information about transport parameters such as diffusion and dispersion. We will overcome these obstacles by the use of stable isotopes of water as ideal tracer. These environmental tracers provide integrative information about heterogeneous and dynamic flow and transport processes in the subsurface (e.g., Stumpp et al. 2007, 2009c,d). We identified the impact of different soils and land use on the quantity of matrix and preferential flow as well as on their transit times (Stumpp et al., 2007; Stumpp and Maloszewski, 2010). For bare, sandy soils it was found that measured isotope contents in precipitation can directly be taken as system input function as most of the precipitation resulted in groundwater recharge (Maloszewski et al., 2006). We found that the amounts of preferential flow varied between 15-30%. Here, the vulnerability increased with increasing grain size and saturated hydraulic conductivity of the soil (Stumpp et al., 2007). For cropped soils, the application of our conceptual model was accomplished by developing a new system input function, i.e. estimating the isotope concentrations in the recharging water, which is not equal to the concentration in precipitation and thus not measurable (Stumpp et al. 2009b). An input function was developed considering actual evapotranspiration rates and thus, the isotopic ratios measured in precipitation were weighted according to their actual contribution in the groundwater recharge. With this pioneering work we identified that preferential flow varied not only depending on vadose zone structure, but also according to land use. These important parameters can now be incorporated into future groundwater management and protection strategies. In collaboration with the Hydrological Modeling Group (P. Maloszewski, C. Kübeck), we use water isotopes and other tracers to characterize heterogeneous flow paths in the framework of the EU project GENESIS (Groundwater and Dependent Ecosystems: New Scientific and Technological Basis for Assessing Climate Change and Land-use Impacts on Groundwater). This project is funded by the Seventh Framework Program of the European Community with the objective to fill knowledge gaps at the time of the first revision of the EU Ground Water Directive. It further attempts to improve the management of groundwater resources.  With our contribution, we will identify the spatial and temporal heterogeneity of groundwater flow as well as of groundwater-surface water interactions leading to improved conceptual models of hydrological processes on the regional scale. Finally, we will identify major threats to groundwater-dependent ecosystems from land use and climate change based on estimation of water transit time distributions in these ecosystems.

Transport of microorganisms

Pathogenic microorganisms and viruses are wide spread pollutants of drinking water resources and are regularly introduced to the subsurface during riverbank infiltration or due to land application of waste water effluents, sewage injection or animal manure. Moreover, similar to other colloidal particles, bacteria can enhance facilitated transport and therefore increase the mobility of inorganic and organic toxic pollutants. Accurate prediction of bacterial transport and thus behavior is valuable when applying in-situ bioremediation techniques (e.g., bioaugmentation) or natural attenuation to control the degradation of groundwater contamination. Both, the efficiency of remediation strategies and the protection of groundwater resources can be improved through a better understanding of the complexity of bacterial transport. Bacterial transport mechanisms and sorption rates linked to various biological, chemical and physical factors have been well studied for individual species. However, we recently demonstrated that transport parameters (sorption rates, dispersivities) from studies performed with only one bacterial strain should not be generalized and applied to more complex and/or field situations (Stumpp et al. 2011). For the first time, bacteria-bacteria interactions and their impact on transport processes were investigated conducting individual, simultaneous and successive bacterial breakthrough experiments. The results of this pioneering work showed that different bacterial strains exhibit a strong variability in sorption when transported in the presence of other strains.