  The self-potential (SP) method is sensitive to water fluxes in saturated and
partially saturated porous media, such as those associated with rainwater
infiltration and groundwater recharge. We present a field-based study at the
Voulund agricultural test site, Denmark, that is, to the best of our knowledge,
the first to focus on the vertical self-potential distribution prior to and
during a saline tracer test. A coupled hydrogeophysical modeling framework is
used to simulate the SP response to precipitation and saline tracer
infiltration. A layered hydrological model is first obtained by inverting
dielectric and matric potential data. The resulting model that compares
favorably with electrical resistance tomography models is subsequently used to
predict the SP response. The electrokinetic contribution (caused by water
fluxes in a charged porous soil) is modeled by an effective excess charge
approach that considers both water saturation and pore water salinity. Our
results suggest that the effective excess charge evolution prior to the tracer
injection is better described by a recent flux-averaged model based on soil
water retention functions than by a previously proposed volume-averaging model.
This is the first time that raw vertically distributed SP measurements have
been explained by a physically based model. The electrokinetic contribution
cannot alone reproduce the SP data during the tracer test and an
electro-diffusive contribution (caused by concentration gradients) is needed.
The predicted amplitude of this contribution is too small to perfectly explain
the data, but the shape is in accordance with the field data. This discrepancy
is attributed to imperfect descriptions of electro-diffusive phenomena in
partially saturated soils, unaccounted soil heterogeneity, and discrepancies
between the measured and predicted electrical conductivities in the tracer
infiltration area.
