## 2 Materials and Methods
### Methods  
The collected samples were submitted to particle size, mineralogical, and chemical analyses. The grain-size distribution was performed using a laser analyzer (Malvern Mas  
Figure 1: Location map of phosphate laundries in the Gafsa-Metlaoui Basin. tersizer 2000, Malvern Panalytical, Malvern, UK). The total calcium carbonate content (%CaCO\({}_{3}\)) was carried out by means of a Bernard calcimeter (Gabbrielli Technology, Calendzano, Italy). The organic carbon (C\({}_{\text{org}}\)) and nitrogen (N) rates were successively determined by an elementary micro analyzer carbon/sulfur ([PERSON], Yvon, Edison, NJ, USA), and the nitrogen was assessed by means of a UDK 132 semi-automatic distillation unit, VELP SCIENTIFICA, Usmate velate (MB), Italy. The organic matter (OM) content was determined by multiplying Corg by 1.72 (Van Bemmelen factor, [11]). The mineralogical analyzes were carried out by a PANalyticalX'Pert Pro X-ray diffractometer (XRD, PANalytical, Almelo, The Netherland). The chemical analysis of the major elements (Ca, Al, Mg, Fe, Si, K, P, and Na) was carried out by fluorescent spectroscopy (XRF, PANalyticalMagix PW2403, PANalytical, Almelo, The Netherland) on pearls obtained by fusion. The detection limits (DL) for the analyzed elements were 0.6 mg/kg, 0.04 mg/kg, 0.01 mg/kg, 0.08 mg/kg, 0.16 mg/kg, 0.01 mg/kg, 0.4 mg/kg, and 0.01 mg/kg, respectively. For trace (Cd, Cr, Pb, Zn, U, and Sr) and rare earth (La, Ce, Sm, Nd, Eu, Gd, Tb, and Yb) elements analysis, samples were sieved to \(-\)2 mm, then finely grounded in an agate mortar and dissolved in a triacid solution (HCIO\({}_{4}\), HF, and HNO\({}_{3}\)). The measurements were carried out by inductively coupled plasma-atomic emission spectroscopy, (ICP-AES, Activa of Horiba-Jobin Yvon, Horiba scientific, Edison, NJ, USA). For the trace elements, the DLs were 0.2 mg/kg, 3.4 mg/kg, 0.53 mg/kg, 1.6 mg/kg, 1 mg/kg, and 8 mg/kg, respectively, and for the rare earth elements, the DLs were 2 mg/kg, 4 mg/kg, 0.8 mg/kg, 8 mg/kg, 0.2 mg/kg, 0.2 mg/kg, 0.2 mg/kg, and 0.1 mg/kg, respectively. The distribution of rare-earth elements (REEs) is displayed as a semi-logarithmic graphical representation where concentrations are normalized to the upper continental crust (UCC) [12]. For the analysis of water samples, the concentrations of anions (NO\({}_{2}\)-, NO\({}_{3}\)-, F\({}^{-}\), PO\({}_{4}\)\({}^{3}\)-, and SO\({}_{4}\)\({}^{2}\)-) were determined by ion chromatography (Shimadzu, Kyoto, Japan). The DLs were 0.01 mg/L; 0.1 mg/L, 0.1 mg/L, 0.1 mg/L, and 0.1 mg/L, respectively. The contents of trace elements (Cd, Fe, Pb, Zn, and U) were determined by ICP-AES. The DLs were 0.0002 mg/L, 0.0023 mg/L, 0.0005 mg/L, 0.0016 mg/L, and 0.001 mg/L, respectively. The chemical analysis, conducted on the certified reference material (BCR-32), showed that the error margin did not exceed 5%. In most cases, mobility designates the ability of an element to migrate in space, to shift from one chemical form to another, or to undergo a phase change (mainly from a solid phase to a liquid one) [13]. The transfer processes between the two phases (solid and liquid) are multiple; they strongly depend on the nature of the link between the elements being considered, in addition to the solid phase, which is also known as the carrier or retention phase, and the physico-chemical conditions prevalent in the environment. Several physical and chemical processes at the solid-liquid interface generally allow the trapping of metals, such as (physical and chemical) adsorption [14; 15], precipitation [16; 17; 18], substitution in the crystal lattice, and mechanical trapping. Leaching is a method for removing soluble components from a solid matrix [19; 20]. The batch extraction procedure is a short-term leaching test intended to estimate the release potential of constituents from waste materials over a range of environmental conditions [21], and to evaluate potential risks to human and/or groundwater [22].