Imaging and characterization of solute transport during two tracer tests in a shallow aquifer using electrical resistivity tomography and multilevel groundwater samplers

Abstract

[1] The relevance of aquifer heterogeneity for flow and transport is recognized broadly; however, its characterization is hampered by the inaccessibility of the subsurface. Time-lapse electrical resistivity tomography (ERT) offers the possibility of imaging noninvasively subsurface transport. We present results of two tracer tests that were carried out successively in a shallow aquifer at the Krauthausen test site (Germany). The breakthroughs of an electrically conductive and a resistive tracer were monitored with ERT and local multilevel groundwater samplers (MLS) along two cross sections perpendicular to the mean flow direction. Sinking of the conductive salt tracer due to density effects was observed with ERT. We applied a stream tube model to characterize the spatially variable transport. ERT-derived stream tube parameters showed similar patterns for the two tracer experiments, reflecting the effect of aquifer heterogeneity on transport. MLS data did not show similar spatial patterns, which indicates that these measurements may be prone to subtle changes of the flow field in the small sampling volume and mixing within screened wells. Between 50% and 10% of the tracer was recovered in the ERT-derived breakthrough curves. Compared with transport simulations in a homogeneous aquifer, ERT-derived time-integrated changes in electrical conductivity were locally larger but focused in a smaller area. MLS data indicated that in this area, ERT did not underestimate the tracer recovery. The relatively low tracer recovery was attributed to undetected tracer breakthrough in regions with low ERT sensitivity and in regions where the length of the tracer plume and the electrical conductivity contrast were small.

A salt tracer test monitored with surface ERT to detect preferential flow and transport paths in fractured/karstified limestones

“…In the past two decades, ERT has been used to monitor salt tracer tests or water infiltration through the vadose zone in relatively homogeneous or stratified hydrogeological systems either in the laboratory (Binley et al, 1996a;Slater et al, , 2002Koestel et al, 2008) or in test sites (Slater et al, 1997a;al Hagrey and Michaelsen, 1999;Day-Lewis et al, 2003;Singha and Gorelick, 2005;Cassiani et al, 2006;Müller et al, 2010). Table 1 presents a nonexhaustive list of surveys that were designed to monitor water infiltration or salt tracer propagation in the laboratory and in the field.…”

Characterization of deep aquifer dynamics using principal component analysis of sequential multilevel data

“…Multilevel sampling (MLS) has been found useful for improving hydrochemical and hydraulic characterizations of aquifers relative to other sampling methods (Ronen et al, 1986;Cherry et al, 2007;Müller et al, 2010;. Depth-dependent samples from deep aquifers are usually obtained during drilling (Williams and Chou, 2007;Hendry et al, 2011); hence interpretation of aquifer dynamics from these single-time data sets is limited.…”

Geophysical Methods for Monitoring Temperature Changes in Shallow Low Enthalpy Geothermal Systems

“…ERT proved its ability to detect temperature changes below 1.5 °C and then to follow lower incremental changes. The design of cross-borehole experiment allows more flexibility regarding the depth of the reservoir and the resolution since a major drawback of surface ERT is the loss of resolution/sensitivity with increasing depth [51,107]. Firmbach et al [108] studied heat transport with ERT in an experimental box (1 m × 0.4 m × 0.4 m) with various levels of saturation.…”

“…However, plenty of studies showed the ability of ERT in general to follow dynamic processes in less favourable conditions. As an example, Kemna et al [50] and then Müller et al [107] successfully used ERT in the Krauthausen test site in Germany to follow different types of tracer migration within a 10m thick heterogeneous and layered sandy aquifer. Supper et al [109] demonstrated the ability of the method to study seasonal variations in permafrost.…”