Entwicklung und Anwendung reaktiver Tracerverfahren zur Bestimmung der Aquifer-Oxidationskapazität
Microbial reduction of Fe(III) in sediments can play an important role as a terminal electron accepting process for oxidation of aromatic hydrocarbons in groundwater. Traditionally, quantification of total, extractable or so called bioavailable Fe(III) is assessed by various wet chemical extractions. The main disadvantage of all these methods derives from aquifer heterogeneities, which cannot be taken into account adequately. Furthermore, chemical extraction techniques determine total mass of the (bioavailable) Fe(III) minerals only and limiting factors like decreasing degradation rate due to diminishing mineral surface or passivation by precipitation of reaction products are neglected. Reactive tracers to determine aquifer Fe(III) content or aquifer oxidation capacity (OXC) are being developed to overcome these disadvantages. Reactive tracer tests offer an integral approach to assess aquifer OXC, because tons of aquifer material rather than grams as in chemical extractions are considered here. Tracer solutions use the same groundwater flow paths as contaminants. In this way, geochemical and hydraulic heterogeneities are not „mapped“ in detail, but considered in its completeness. Different approaches exist to use reactive tracers. Sorptive reactive tracers like phosphate interact with the surface of Fe(III) minerals and retardation of the tracer substance allows to determine reactive surface areas and consequently the reducible part of Fe(III) minerals. Redox reactive tracers like sulphide directly reduce Fe(III) present in the aquifer. Sulphur electron balances of the consumed sulphide tracer enables to calculate the mass of the mineral phase Fe(III). The sorptive reactive tracer phosphate was successfully tested with artificially composed sediments and the redox reactive tracer sulphide with artificially composed and natural sediments in column experiments. A field push-pull tracer test was performed with sulphide tracer to assess the sedimentary OXC.