Esther Bloem

Research Scientist

(+47) 464 44 223
esther.bloem@nibio.no

Place
Ås F20

Visiting address
Fredrik A. Dahls vei 20, 1430 Ås

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Abstract

Degradation of organic chemicals in natural soils depends on oxidation-reduction conditions. To protect our groundwater resources we need to understand the degradation processes under anaerobic conditions. Available iron and manganese oxides are used as electron acceptors for anaerobic degradation and are reduced to the dissolved form of metallic cations in pore water. To monitor this process is a challenge, because anaerobic conditions are difficult to sample directly without introducing oxygen. A few studies have shown an impact of iron reduction on spectral induced polarisation (SIP) signature, often associated with bacterial growth. Our objective is to study the impact of iron and manganese oxide dissolution, caused by degradation of an organic compound, with spectral induced polarisation signatures. Twenty-six vertical columns (30 cm high, inner diameter 4.6 cm) were filled with a sand rich in oxides (manganese and iron) with a static water table in the middle. In half of the columns, a 2 cm high contaminated layer was installed just above the water table. As the contaminant degrades, the initial oxygen is consumed and anaerobic conditions form Every three days over a period of one month, spectral induced polarisation (twenty frequencies between 5mHz and 10 kHz) data were collected on six columns: three contaminated replicates and three control replicates. Chemical analysis was done on twenty columns assigned for destructive water sampling, ten contaminated columns and ten control. The results show an increase of the real conductivity associated with the degradation processes, independent of frequency. Compared with the pore water electrical conductivity in the saturated zone, the real conductivity measurement revealed the formation of surface conductivity before iron was released in the pore water. In parallel, we also observed an evolution of the imaginary conductivity in both saturated and unsaturated zones at frequencies below 1 Hz. Overall, the anaerobic reduction of iron and manganese oxide during the organic degradation increased both the conductive and polarisation component of the complex conductivity.

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Abstract

For subsurface solute transport, flux concentrations are key, while usually resident concentrations are measured. Flux concentrations are frequently estimated from resident concentrations by temporal moment analysis. We tested this approach by simulating transport of an injected tracer during steady flow in an aquifer with a heterogeneous saturated hydraulic conductivity. We constructed grid-cell scale breakthrough curves (BTCs) from flux concentrations and approximate BTCs from resident concentrations and estimated flux concentrations. We assembled these BTCs into spatio-temporal leaching surfaces at various aquifer cross-sections for subsequent analysis. Resident concentrations were unsuitable to assess solute movement in the aquifer. Temporal moment analysis worked well when the entire aquifer cross-section was considered, but performed poorer at the grid-cell scale because it approximates the local velocity by the trajectory average. The leaching surfaces served as valuable tools to demonstrate and quantify the limitations of temporal moment analysis.