Estimation of Recharge from Long‐Term Monitoring of Saline Tracer Transport Using Electrical Resistivity Tomography

Abstract

We conducted a ield experiment at the agricultural ield 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-m 2 area at the surface at an application rate mimicking natural iniltration. 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 lux. 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 ive 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.

Impact of small-scale saline tracer heterogeneity on electrical resistivity monitoring in fully and partially saturated porous media: Insights from geoelectrical milli-fluidic experiments

“…[8]). Time-lapse ERT has been widely applied under both saturated (e.g., [9,10,11]) and partially saturated conditions (e.g., [12,13,14,15]) using electrodes placed on the ground or in boreholes.…”

Mapping sand layers in clayey till using crosshole ground-penetrating radar

“…The recorded EM signal provides information on the electrical permittivity and the electrical conductivity of the investigated formation, that can be related to velocity and the attenuation of the EM wave, respectively (Annan, 2005). To date, published field studies using crosshole GPR only include sediments with low electrical conductivity, e.g., unconsolidated sands (Cassiani et al, 2006;Irving et al, 2007;Haarder et al, 2015;Looms et al, 2008;Klotzsche et al, 2010;Gueting et al, 2015), sandstone (Binley et al, 2002), and chalk (Keskinen et al, 2017). In general, clayey environments have been avoided as the attenuation of the EM signal reduces the signal strength and thereby the possible distance between boreholes.…”

Advancing quantitative understanding of self-potential signatures in the critical zone through long-term monitoring

“…The tracer experiment was described and analyzed by Haarder et al (2015) and the SP Jougnot et al, 2015). Electrode SP-0.5B was installed several meters away from the area depicted in (b).…”

“…The spatio-temporal variations of the simulated solute concentration of pore water assuming a dispersivity of 0.1 m. The dashed line corresponds to the depth of maximum concentration, the black triangles indicate the center of solute mass determined by drilled core data and the red diamonds are the center of solute mass inferred byHaarder et al (2015) using ERT data.…”