 |
| Titel |
Spectral induced polarization as a tool to map subsurface biogeochemical hot spots: a first laboratory evaluation in the Fe-S system |
| VerfasserIn |
Sven Nordsiek, Ben Gilfedder, Sven Frei |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250149530
|
| Publikation (Nr.) |
 EGU/EGU2017-13886.pdf |
|
|
|
|
|
| Zusammenfassung |
| Zones of intense biogeochemical reactivity (hot spots) arise in the saturated subsurface at the
interface between regions with oxidizing and reducing conditions. Hot spots are both sinks
and sources of different chemical compounds, thus they are of particular importance for
element cycling in the subsurface. However, the investigation of hot spot structures is
difficult, because they are not directly identifiable from the surface and can only
be investigated by invasive methods in the subsurface. Additionally, they often
form in sensitive wetland ecosystems where only non-destructive measurements are
applicable to avoid significant degradation of these sensitive environments. Under
these circumstances, geophysical methods may provide useful tools to identify
biogeochemically active regions. One of the most important biogeochemical reactions in
wetlands is the reduction of sulphate and formation and accumulation of FexSy
minerals (where x and y delineate mineral stoichiometry). These reactions only
occur in specific hot spots where specific chemical and microbial conditions are
met.
Within a research project concerning biogeochemical transformations and turnover in
wetlands, we investigate the applicability of the geoelectrical method of spectral induced
polarization (SIP) to locate and monitor regions containing polarizing FexSy particles as
indicator for biogeochemical hot spots. After developing and testing a sample holder and a set
of non-polarizing electrodes for laboratory SIP measurements, we performed experiments on
natural soil samples taken from the hyporheic zone of a local river channel. The collected
material originates from a location known for biogeochemical activity. The sample contains a
high percentage of dark grayish/black sediment interpreted as FexSy, and possibly pyrite
(FeS2). The material was homogenized and split into four samples. The FexSy concentration
was adjusted to three different levels by oxidation using H2O2. For all samples
we recorded the SIP spectra in the frequency range between 0.26 Hz and 25 Hz
repeatedly.
The sensitivity of SIP measurements to the FexSy content is promising. For a sample of
the sediment saturated with its natural fluid (conductivity > 600 μS/cm), we observed phase
shifts of more than 7 mrad at frequencies between 1 and 10 Hz. We expect a lower fluid
conductivity and thus a larger maximum phase shift for measurements in wetlands. The
application of the SIP method in field surveys at places with biogeochemical activity will be
the next step in our study. |
|
|
|
|
|
|