## 4 Metal Ion Leaching and Solute Transport  
### Soil Medium  
The geochemical PHEREQC code [177] performs sufficient numerical simulations incorporating mixing transport models both in the aqueous and gaseous phases, taking into account geochemical reactions such as precipitation, dissolution, complexation phenomena involving metal oxides and microbial activity. The reactive transport model incorporates reaction kinetics for chemical processes, including ion association and specific ion interaction theory for solute activity calculations [176]. In a publication, it was employed to calculate saturation indices (SIs) [178]. The simulation code was subsidized by predicting/estimating tools for model parameters, such as parameter estimation (PEST) software (version 5.0) [179]. Sorption phenomena are simulated by employing a two-layer model introduced by [180]. Organic matter sorption was modeled by implementing the WHAM model, an acronym of Windermere humic aqueous model [181], which is based upon humic acid behavior according to [PERSON] theory for electrondonors. Mine tailings and heavy metal leaching, e.g., Pb, were encountered by activating reclamation measurements to mitigate negative leaching effects. Mining slurry combined with manure leads to metal sorption, reducing any leaching processing. A new biogeochemical model was developed taking into account certain phenomena, i.e., kinetically controlled dissolution/precipitation, adsorption, water gas exchanges, surface complexation reactions, microbial respiration and growth, dissolution and precipitation. The amended model more accurately predicts Pb reactivity and soil tailings [182]. Heavily contaminated soils (brownfield) by potentially toxic elements (PTEs) are a permanent threat to the local ecology via toxicity and bioaccumulation. Human wellbeing is put in peril by leaching and runoff phenomena. Threats are growing even bigger considering the pH change in the soil cap. Therefore, chemically burdened sites have tobe under constant monitoring in terms of heavy metal quantification and ion mobility checking indices to ensure that surface watersheds and aquifers are not threatened. It is acknowledged that there is a lack of detailed knowledge of minerals which exert control over toxic element leaching. Thus, it is of great importance to investigate soil mineralogy in PTE release and predict leaching [183]. The geochemical modeling program PHREEQC (version 3.1) was deployed to calculate SIs, which indicate whether a specific mineral is likely to dissolve or precipitate in groundwater, in a case study in Pakistan of monitoring groundwater areas suffering from arsenic contamination [178]. Geochemical simulations were conducted making use of PHREEQC software (version 3) [177], MINTEQ's two thermodynamic databases [184] and Lawrence Livermore national laboratory (LLNL). Sampling investigations conducted at a brownfield site served for fertilizer production. Jarosite rich in Pb, hematite, and gypsum were the most abundant mineralogical phases/stratifications, with zinc sulfate, kintoreite, and anglesite. Pb and Zn phases were identified as the dominant ones [183]. pH-dependent leaching tests were applied incl. HNO\({}_{3}\) and NaOH as the pH control agents in combination with geochemical models. The testing target was the reveal of leaching mechanisms and contaminant solubility in a pH ranging from 1 to 12. All information gained advances scientific knowledge, facilitates future remediation strategies, and promotes integrated sustainable management and strategy design over brownfields [183]. Barren piles of mining undergo bio-oxidation, giving rise to metal sulfide oxidation and finally acidification phenomena that motivate ion mobility, well known in the scientific community as acid mine drainage (AMD). AMD leachate mitigation exerts great pressure on bedrock, surface waters, and local aquifers since metal ions' active plume puts at risk the overall ecological quality of the region in the vicinity of the mines.