On the possibility of using organic molecules in the characterization of subsurface processes

Nowadays, tracer tests represent an important part in the repertoire of hydro(geo)logical characterization techniques. For determining hydromechanical parameters of and flow properties within a reservoir or an aquifer, conservative tracer compounds are usually the applied chemicals of choice. This means that these compounds behave inert and, thus, interact only negligible during their residence time in the investigated geosystem. In contrast, the use of non-conservative organic molecules as tracers is a novel approach. Here, additional information on subsurface processes may be provided when the molecules interact in a known way with certain components of the considered system. However, the interpretation of the tracer behavior requires a sufficient a priori knowledge on processes and influence factors affecting the tracer transport. Since sorption is probably the most relevant physicochemical transport process for numerous organic solutes in the subsurface, the systematic investigation of sorption processes including their relation to molecular properties and boundary conditions was a main focus of the presented cummulative thesis. In addition, the derived implications on tracer transport were directly incorporated into the design of a new reactive reservoir tracer for geological storage of CO2. In order to get a general overview on the sorption of organic compounds with different functionalities, the pH-dependent transport of several ionizable pharmaceuticals (acids, bases, zwitter) was investigated by using sediment column experiments. The sorption coefficients were determined and compared with two correlation approaches. As a result, a strong pH-dependence of sorption was found for all compounds with pKa values in or near the studied pH range. However, an appropriate prediction of sorption was only feasible for neutral and anionic (acidic) compounds. In contrast, the sorption of cationic (bases) and zwitter-ionic compounds was stronger than expected indicating the occurrence of additional electrostatic sorption mechanisms, which are not considered in the conventional concepts. Consequently, cation exchange processes were expected to have a significant influence on the retardation of organic cations in the subsurface. The dominating role of cation exchange processes was demonstrated for two cationic beta-blockers in column experiments. Higher concentrations of inorganic cations in solution led to lower sorption of the organic cation due to increased competition for the negatively charged sorbent exchange sites. The contribution of non-hydrophobic interactions to the overall sorption was estimated with >99%. Due to the importance of cation exchange, more influence factors have to be considered for the successful prediction of the transport of organic cations in the subsurface. As a first step towards the systematic consideration of further influence factors, the role of competing inorganic cations on the sorption of organic cations was studied in batch experiments. Sorption isotherms for the cationic beta-blocker metoprolol on sediment were obtained for varying background concentrations of competing inorganic ions. The obtained data showed that the influence of competing Ca2+ on the sorption of metoprolol is stronger than for Na+. The found correlation between the Freundlich sorption coefficient and the concentration of competing inorganic ions allows the prediction of metoprolol sorption within the investigated range of boundary conditions. Furthermore, the contribution of sorptive interactions to the organic carbon of the sediment was determined to be negligible. Therefore, the results improve the process understanding and help to improve predicting the subsurface transport of organic cations. Eventually, the concept and the design of a new reactive tracer (KIS tracer) intended for the characterization of interfacial areas during injections of supercritical CO2 into deep saline aquifers is presented. Due to an interface-sensitive hydrolysis reaction with known kinetics the temporal study of the CO2/brine interfacial area is feasible. Based on the findings from previous investigations, the conservative organic anion naphthalenesulfonate was selected as base structure for the ester synthesis (esterification) and, thus, also as hydrolysis product. Beside the definition of required tracer properties, first promising compounds were synthesized, tested in the laboratory, and evaluated with a newly developed macroscopic model. Furthermore, the numerical forward modeling of the tracer behavior is exemplarily described for several hypothetical functions of the interfacial area change. The first results are promising and provide first insight into the potential of KIS tracers.