10 research outputs found
Monitoring of saline tracer movement with vertically distributed self-potential measurements at the HOBE agricultural test site, Voulund, Denmark
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.Comment: 45 pages, 9 figures, 2 table
Comparison of time-lapse GPR data collected under natural and forced infiltration conditions to estimate vadose zone hydraulic parameters
Time-lapse crosshole ground-penetrating radar (GPR) data, collected
while infiltration occurs, can provide valuable information regarding
the hydraulic properties of the unsaturated zone. In particular,
the stochastic inversion of such data provides estimates of parameter
uncertainties, which are necessary for hydrological prediction and
decision making. Here, we investigate the effect of different infiltration
conditions on the stochastic inversion of time-lapse, zero-offset-profile,
GPR data. Inversions are performed using a Bayesian Markov-chain-Monte-Carlo
methodology. Our results clearly indicate that considering data collected
during a forced infiltration test helps to better refine soil hydraulic
properties compared to data collected under natural infiltration
condition
Examining the information content of time-lapse crosshole GPR data collected under different infiltration conditions to estimate unsaturated soil hydraulic properties
Time-lapse geophysical data acquired during transient hydrological
experiments are being increasingly employed to estimate subsurface
hydraulic properties at the field scale. In particular, crosshole
ground-penetrating radar (GPR) data, collected while water infiltrates
into the subsurface either by natural or artificial means, have been
demonstrated in a number of studies to contain valuable information
concerning the hydraulic properties of the unsaturated zone. Previous
work in this domain has considered a variety of infiltration conditions
and different amounts of time-lapse GPR data in the estimation procedure.
However, the particular benefits and drawbacks of these different
strategies as well as the impact of a variety of key and common assumptions
remain unclear. Using a Bayesian Markov-chain-Monte-Carlo stochastic
inversion methodology, we examine in this paper the information content
of time-lapse zero-offset-profile (ZOP) GPR traveltime data, collected
under three different infiltration conditions, for the estimation
of van Genuchten-Mualem (VGM) parameters in a layered subsurface
medium. Specifically, we systematically analyze synthetic and field
GPR data acquired under natural loading and two rates of forced infiltration,
and we consider the value of incorporating different amounts of time-lapse
measurements into the estimation procedure. Our results confirm that,
for all infiltration scenarios considered, the ZOP GPR traveltime
data contain important information about subsurface hydraulic properties
as a function of depth, with forced infiltration offering the greatest
potential for VGM parameter refinement because of the higher stressing
of the hydrological system. Considering greater amounts of time-lapse
data in the inversion procedure is also found to help refine VGM
parameter estimates. Quite importantly, however, inconsistencies
observed in the field results point to the strong possibility that
posterior uncertainties are being influenced by model structural
errors, which in turn underlines the fundamental importance of a
systematic analysis of such errors in future related studies
Estimation of recharge from long-term monitoring of saline tracer transport using electrical resistivity tomography
We conducted a field experiment at the agricultural field site Voulund within the Danish hydrological observatory, HOBE, with the purpose of estimating recharge using geophysical methods. In September 2011, a saline tracer was added across a 142-m2 area at the surface at an application rate mimicking natural infiltration. The movement of the saline tracer front was monitored using cross-borehole electrical resistivity tomography (ERT); data were collected on a daily to weekly basis and continued for 1 yr after tracer application. The ERT data were inverted and corrected for temperature changes in the subsurface, and spatial moment analysis was used to calculate the tracer mass, position of the center of mass, and thereby the downwardly recharging flux. The recovered mass was underestimated by the ERT data by up to 50%. Mass balance errors are widely recognized and are a result of variable resolution of the tomographic models and smoothing applied in the inversion routine. The results were nonetheless in very good agreement with pore water samples collected and analyzed from five cores extracted within the tracer application area during the same period. Recharge during the 7.5 mo from September 2011 to the end of April 2012 was estimated to be about 500 mm using the ERT data. This value is in good accord with recharge estimates made based on drainage data from buried lysimeters located only meters away from the cross-borehole ERT array. This suggests that long-term automated ERT monitoring of a surface-applied tracer is a promising technique for estimating groundwater recharge