Are Visible Fractures Accurate Predictors of Flow and Mass Transport in Fractured Till?

Abstract

Tracer experiments conducted in the laboratory on undisturbed core samples (<7.3‐cm‐diameter) have been a standard method for estimating hydraulic and transport properties of fractured till since the 1980s. This study assesses the relationship between visible fractures on the top and bottom of core samples and the resulting hydraulic and mass transport properties of the core. We hypothesized that more visible fractures would indicate the presence of a well‐connected fracture network, leading to greater hydraulic conductivity (K) values and earlier chemical breakthrough times. To test this hypothesis, water flow and bromide (Br‐) tracer experiments were performed on 10, 16‐cm diameter, 16‐cm‐tall samples of fractured Dows Formation till from central Iowa. Visually identifiable fractures were present on the top and bottom of every sample. Results indicate that the visual identification of fractures does not predict a connected fracture network, as some samples produced breakthrough curves showing rapid first arrival times and shapes characteristic of solute transport in a fractured medium, while others appeared similar to an unfractured medium. No correlation was found between the number of visible fractures and K (Pearson's r = 0.25), or Br‐ first arrival time (r = −0.33), but a strong negative correlation between K and first arrival time (r = −0.92). Results indicate that the sample volume was not large enough to reliably contain a connected fracture network. Thus, testing large volumes of till at the field scale coupled with fracture‐flow modeling likely represents the best approach for estimating hydraulic and mass transport properties for fractured till.

Deep Hydraulically‐Active Fractures in Sensitive Clay Deposits: Implications for Groundwater Flow and Slope Stability

“…(1989). This method provides a convenient and cost‐effective means for assessing the approximate depth of active fractures, in contrast to the more intensive methods commonly used to characterize fractured till and clay, such as trenching, field and laboratory tracer tests, and numerical modeling (Helmke et al., 2005; Keller et al., 1985; McKay et al., 1993; Mosthaf et al., 2021; Ruland et al., 1991; Young et al., 2019, 2020). The area monitored by the network presented here represents a significant portion of the postglacial marine‐clay deposits in Eastern Canada, and the subsequent analysis enables a formation‐wide characterization of groundwater flow dynamics.…”

“…Cross-correlation of these time series was used to determine whether local groundwater flow systems were characterized by the slow, matrix-driven flow assumed in Lafleur and Lefebvre (1980), or whether the variations in hydraulic head could only be explained by rapid flow within a fracture network, as first proposed by Keller et al (1989). This method provides a convenient and cost-effective means for assessing the approximate depth of active fractures, in contrast to the more intensive methods commonly used to characterize fractured till and clay, such as trenching, field and laboratory tracer tests, and numerical modeling (Helmke et al, 2005;Keller et al, 1985;McKay et al, 1993;Mosthaf et al, 2021;Ruland et al, 1991;Young et al, 2019Young et al, , 2020. The area monitored by the network presented here represents a significant portion of the postglacial marine-clay deposits in Eastern Canada, and the subsequent analysis enables a formation-wide characterization of groundwater flow dynamics.…”

Transport of Tracers and Pesticides Through Fractured Clayey Till: Large Undisturbed Column Experiments and Model‐Based Interpretation

“…Glacial deposits such as clayey tills are often found in the upper geological layers in many countries of the Northern hemisphere (Høyer et al., 2019; Jørgensen et al., 2002; Kim et al., 2017; McKay, Gillham, et al., 1993; Parker et al., 1994; Young et al., 2021). Usually, clayey tills have a low hydraulic conductivity and can slow down solute migration, or even act as a barrier for contaminant leaching toward groundwater bodies (Chapman et al., 2018).…”