Experimental study of particle transport and deposition in fractured chalk under abrupt pH variations

Groundwater chemistry plays a pivotal role in governing colloidal transport, particularly in fractured porous media. This study systematically examines the impact of pH on kaolinite particle dynamics within saturated fractured chalk, spanning a pH range from 5.0 to 12.3. Controlled laboratory experiments, coupled with a one-dimensional advection–dispersion model incorporating fracture roughness via the joint roughness coefficient (JRC), reveal pronounced pH-dependent transport behavior. Elevated pH enhances particle mobility due to increased electrostatic repulsion, reducing attachment and promoting detachment via augmented shear forces. The attachment coefficient ranged from 2.3 × 10−3 to 3.0 × 10−4 (1/s), and the detachment coefficient ranged from 7.5 × 10−6 to 3.0 × 10−55 (1/s), respectively for the lowest and highest pH at low flow velocity (U1 = 0.0128 cm/s). Conversely, acidic conditions favor particle retention, where the recovery rate was 15% at pH 5.0 under low flow velocity (U1) and reached 63% at pH 12.3. The observed trends align with XDLVO theory, elucidating pH-mediated energy barriers. These findings advance mechanistic understanding of geochemically driven colloid mobility in subsurface fractured systems.