Early Hydrochemical Evolution of Groundwater Under Landfill Leachate Influence: Case of the Tangier Municipal Site

This study evaluates short-term changes in groundwater quality around the new municipal landfill of Tangier, Morocco . In total, 10 sampling sites (8 wells and 2 surface waters) within 2 km were sampled in late summer 2023 (S-23) and late winter 2025 (F-25), alongside the flow gradient. Major ions (ICs), trace metals (ICP-OES) and field parameters were analyzed; data were converted to milliequivalents and quality-checked by ionic balance error (IBE).

S-23 samples were mostly fresh Ca–HCO₃/Cl waters (EC < 1,000 µS cm⁻¹; TDS < 500 mg L⁻¹). By F-25, downgradient wells evolved from Ca–Cl to mixed Ca–Na–Cl–SO₄ facies, with Cl⁻ accounting for > 70% of anions and Ca²⁺ contributing to 20–50% of cations; the Piper diamond collapses toward the strong acid corner. A PCA on major ions (meq L⁻¹) shows PC1 (80.9%) as a salinity/leachate axis, clearly separating F-25 downgradient waters (notably P3–P4–P8) from upgradient S-23 points. PC2 (10.8%) represents hardness vs. alkalinity (SO₄/Mg/Ca vs HCO₃). A simple two-endmember chloride mixing model estimates the leachate fraction f_L​ increasing from (0.0–0.1 upgradient) to (0.2–0.5 at P3–P4) and (0.9–1.0 at P8). Paired Wilcoxon tests (n = 10) on Cl⁻/SO₄²⁻ show non-significant p-values (0.19–0.38), reflecting limited power, yet large effect sizes are captured by PCA, medians and f_L. Heavy metal enrichments (e.g., Fe, Mn and Zn) co-locate with high f_L and strong acid fractions.

Together, these lines of evidence indicate rapid salinization and leachate intrusion in less than 2 years along the downgradient axis. We outline a monitoring framework (quarterly sampling, sentinel wells P3–P4–P8, trigger thresholds on EC/Cl/SO₄ and IBE), and propose MODFLOW transport modeling constrained by the local stratigraphy and measured heads to forecast plume migration and guide mitigation (drainage capture, liner integrity checks and wellhead protection).