Mechanistic Study on Deposition Characteristics of Microorganisms in Layered Sandy Soil During Water-Source Heat Pump Reinjection
Clogging of reinjection wells by microbial deposition in heterogeneous layered aquifers severely limits the operational lifespan of water-source heat pump (WSHP) systems. This study investigates the migration and deposition behavior of Escherichia coli in stratified porous media using a one-dimensional sand column apparatus. Three distinct sand fractions—coarse gravel (T1, d50 = 2.67 mm, k = 3.65 × 10⁻¹ cm/s), medium gravel (T2, d50 = 2.59 mm, k = 3.33 × 10⁻¹ cm/s), and fine gravel (T3, d50 = 1.98 mm, k = 2.98 × 10⁻¹ cm/s)—were packed in a controlled sequence to simulate layered aquifer configurations. A bacterial suspension of 8 × 10⁹ CFU/mL was injected, and breakthrough curves along with pore-water pressure were monitored at four ports spaced 15 cm apart. Results demonstrate that E. coli deposition is predominantly concentrated at the surface layer, with concentration declining exponentially with migration distance. Lower permeability media (fine gravel) shift the deposition front closer to the inlet. The layered sequence exerts a pronounced effect on transport: a coarse-to-fine packing order causes earlier breakthrough peak arrival compared to homogeneous or fine-to-coarse arrangements. A permeability decay model was validated against experimental data, showing strong agreement between predicted and measured permeability reduction over time. These findings provide a mechanistic basis for optimizing reinjection well design and filter material grading to mitigate microbial clogging in WSHP systems.