Identification of the Hydrological System of Kettle Holes of Northeastern Brandenburg, Germany based on their Geochemical Characteristics

Small postglacial depressions, such as kettle holes in northeastern Germany, are distributed across glacially embossed areas of the world. Water bodies forming in these depressions are distinct hydrological systems. Depending on the exchange fluxes with adjoining groundwater system, kettle holes are classified as recharge, flow-through-, and discharge-dominant systems. This classification is a result of their topographical position over an undulating landscape. The upland and lowland kettle holes across the undulating landscape are expected to represent recharge- and discharge-dominant systems, respectively. Hence, those located between these two are expected to be flow-through kettle holes. Nonetheless, due to the complexity of the geological setting of undulating postglacial landscapes, this topography-based classification may be wrong. Furthermore, the hydrological system of kettle holes varies in both time and space. Dynamic boundary conditions of kettle holes, resulting from extreme weather conditions such as severe or prolonged droughts and heavy storm events, may cause a discharge-dominant kettle hole to temporarily shift to a recharge-dominant or a flow-through system.

Many kettle holes of northeastern Brandenburg, Germany are scattered throughout croplands. As a result, fertilizers are transported via the surface runoff and/or groundwater to the kettle holes. Thus, distribution and redistribution of water and solute from each of the kettle hole types and their adjoining groundwater domain and vice versa would likely be different. As these three types of kettle holes have different roles in the context of the hydrological cycle, differentiation of them based on the aforementioned classification would be of paramount importance for their proper characterization and role within a landscape hydrological system. A better characterization will also help to reduce the uncertainty in tracing water and solutes in these hydrogeologically complex systems. An extensive monitoring network of piezometers, installed within kettle holes and around them, is probably the most accurate method to characterize their hydrological system. However, its implementation is expensive, labor-intensive, and time-consuming. Therefore, it cannot be used for the determination of a landscape scale hydrological system containing a great number of kettle holes. The evaporation-to-inflow ratio (E/I) — derived from the stable isotopes of water (H and O) — has been demonstrated to be a viable alternative. We will present a new approach to determine the hydrological system of kettle holes based on geochemistry. To that end, eight chemical species (Ca, Mg, K, Na, Br, Cl, NO3, and SO4), and four in-situ parameters (temperature, pH, electrical conductivity, and redox potential) were monitored for 36 kettle holes over a 17 months period. Based on this dataset, the geochemical characteristics of the kettle holes will be identified using an advanced multivariate statistical algorithm, i.e. Gaussian finite mixture modelling (GFMM) and these will be compared to the hydrological classification of the kettle holes based on the E/I ratios.

 

Keywords: Kettle Holes, Geochemical Characteristics, Groundwater System, Stable Water Isotopes, Germany