The Effect of Initial Saturation on Solute Transport and Fracture–Matrix Exchange Rates in Chalk

Chalk, a high-porosity carbonate rock, is often intersected by fractures, allowing an increase in permeability by orders of magnitude and solute bypass of the matrix, which induces rapid water flow and contaminant migration. However, depending on the level of saturation at the fracture–matrix interface, mass exchange may occur. Consequently, the matrix can store a significant fraction of infiltrating water and solutes, thereby controlling hydrological dynamics. Despite its importance, our understanding of the sensitivity and variability of exchange rates to the initial level of saturation remains limited. Therefore, this study implemented a unique experimental setup to quantify the effects of initial saturation variation on the transport using Rhenium (Re) as a conservative tracer. The system encloses a chalk core drilled from the Eocene-age Avdat Group in the northwestern Negev Desert, containing a 1 mm artificial vertical fracture along its longitudinal axis to mimic preferential flow pathways observed in fractured chalk formations. Three initial saturation levels were considered: nearly saturated conditions (≈ 95%), and unsaturated conditions (40% and 60%). Controlled Re tracer injection, followed by artificial rainwater infiltration, was performed, and outlet concentrations were collected under controlled boundary conditions and analyzed using inductively coupled plasma mass spectrometry (ICP–MS).

 The Re breakthrough curve (BTC) results, under unsaturated conditions, show a higher peak and lower dispersion compared to those under nearly saturated conditions.  These results were further validated by a dual-porosity model (DPM) that was solved using the Hydrus 1D code. The Latin hypercube sampling method was used to generate multiple combinations of hydraulic parameters and longitudinal dispersivity for the DPM. Any simulation that produced an NSE larger than 0.9 was identified as a behavioral simulation. The relationship between solute transfer and initial saturation conditions exhibits pronounced nonlinear behavior. At relatively wet initial conditions (low pressure head —h—), solute transfer remains very limited, indicating weak fracture–matrix exchange. As the system becomes progressively drier, solute transfer increases sharply over a relatively narrow range of pressure heads, reflecting enhanced exchange between mobile and immobile water regions. Beyond this transition zone, a further decrease in initial pressure head results in only minor changes in solute transfer due to intrinsic storage limitations.