Effects of Flow Rate and Initial Saturation on Solute Transport and Flow Dynamics in Fractured Chalk

Flow and transport in fractured systems are a major challenge in understanding contaminant transport in the vadose zone. Chalk, a carbonate rock, is characterized by a high porosity and very low hydraulic conductivity. However, this rock can be intersected by fractures that may act as highly permeable pathways that dominate the migration of water, solutes, and particulate matter.  Unsaturated conditions further introduce additional complexities, as chalk systems are characterized by heterogeneity, capillary effects, and changes in saturation. Despite significant progress in this field, our understanding of the interaction between transport processes and dynamic flow behavior at different initial water saturation and across different flow rates remains limited. Therefore, this work studies the transport and dynamics of a dyed conservative tracer in a fractured chalk system under two saturation conditions (98% and 40%) and at two flow rates (0.1 and 1.1 mL/min). Laboratory experiments were conducted using a novel system containing a half-cylindrical fractured chalk core, drilled from the Eocene-age Avdat Group (northwestern Negev Desert), with a 5 mm artificial vertical fracture and a transparent wall enabling direct visualization of flow patterns. Time-lapse images of the tracer migration along the fracture surface were acquired using a digital camera, and flow and transport behavior were investigated under controlled laboratory conditions using a combination of traditional breakthrough curves (BTCs) and time-resolved image processing in Python to characterize tracer movement along the fracture surface.

Results show that, under near-saturated conditions, the flow rate has no effect on the mass balance: the recovered mass is similar at both low and high flow rates, with an average of 51%. BTCs obtained under these conditions show early tracer arrival and a higher peak at both flow rates. However, the effect of initial saturation level at low flow rate is observed: the average recovered mass under near-saturated conditions is approximately 2.5 times higher than under unsaturated conditions, where the BTCs show delayed tracer arrival and lower peak concentrations. Image-based analysis indicates that increasing the flow rate from 0.1 mL/min to 1.1 mL/min at near-saturated conditions significantly affects the tracer distribution on the fracture surface. At low rates, narrow channels covering ~20% of the fracture surface developed. However, at higher rates, flow channels covered ~50% of the fracture surface. Under unsaturated conditions (low rate), the flow is characterized by an initial wetting front, followed by the formation of channels that cover up to 20% of the fracture surface.