Quantifying Spatiotemporal Variations in Salt Leaching Effects of Subsurface Pipe Drainage Using Multi-Frequency Electromagnetic Induction Method

The remediation of saline-alkali land is jointly influenced by stratigraphic structures, engineering interventions, and temperature fluctuations. In this study, apparent electrical conductivity (ECa) dynamics were monitored across pre-, mid-, and post-remediation stages in the Jingtai Irrigation District, Gansu Province, using the GEM-2 multi-frequency electromagnetic induction technique with temperature calibration. Results showed that spring leaching driven by irrigation and drainage significantly reduced soil ECa, with reduction proportions exceeding 70% across frequencies. During the growing season, a pattern of near-surface salt accumulation coupled with deeper desalination emerged; subsurface drainage effectively suppressed capillary salt return, achieving 84.1% lower-depth desalination despite an increase in near-surface responses across 63.3% of the area. One year post-remediation, substantial apparent desalination was achieved, with the 5325 Hz frequency response (representing deeper cumulative induction) exhibiting the highest reduction in intensity (net ECa decrease of 29.7 mS/m; 81.6% high-efficiency area). Crucially, the subsurface pipe drainage system substantially outperformed non-drained treatments across all monitored frequencies. Annually, the 5325 Hz band (associated with deeper cumulative responses) showed maximum drainage benefit yielding a mean ECa decrease of 33.09 mS/m and an 84.9% high-efficiency proportion, compared to 27.94 mS/m and 75.8% in non-drained sections. A paired t-test indicated that this difference is statistically significant (t = 4.1231, p = 4.16 × 10−5). This network also enhanced leaching represented by higher-to-intermediate frequency responses, yielding average ECa drops of 24.34 mS/m at 63,025 Hz and 21.68 mS/m at 18,325 Hz (versus 19.44 mS/m and 17.94 mS/m without drainage, respectively). These findings indicate that subsurface drainage systems successfully reshape local water-salt dynamics, providing robust data support for precise salinization regulation.