Explain how the phenomenon of sorption affects the observed migration rate of hydrophobic organic contaminants in groundwater compared to the actual groundwater flow velocity.

The actual groundwater flow velocity refers to the speed at which water moves through the interconnected pore spaces within an aquifer. Hydrophobic organic contaminants, or HOCs, are chemical compounds that have a low affinity for water and a high affinity for organic matter. Examples include polychlorinated biphenyls (PCBs) or polycyclic aromatic hydrocarbons (PAHs). The phenomenon of sorption significantly affects how these HOCs move through groundwater environments, causing their observed migration rate to be slower than the actual groundwater flow velocity. Sorption is the general process by which a substance transfers from the fluid phase, in this case groundwater, to a solid phase, which are the aquifer materials like soil and sediment particles. This process includes both adsorption, where the contaminant adheres to the surface of the solid material, and absorption, where the contaminant penetrates into the bulk of the solid material. For HOCs in groundwater, the primary mechanism of sorption is their partitioning from the water into the natural organic carbon present within the aquifer solids. Because HOCs are hydrophobic, meaning water-fearing, they prefer to associate with the organic carbon in the soil matrix rather than remaining dissolved in the groundwater. When an HOC molecule is sorbed onto the solid phase, it is temporarily immobilized and therefore not moving with the flowing groundwater. Only the HOCs that are currently dissolved in the water phase are transported. As the groundwater flows, there is a continuous exchange between the sorbed and dissolved phases: HOCs desorb from the solids back into the water, and dissolved HOCs sorb onto available solid surfaces. This constant but temporary retention on the solid matrix means that the overall movement of the contaminant plume, which is the zone of contaminated groundwater, is retarded relative to the water itself. This retardation is quantified by the retardation factor, which is the ratio of the groundwater velocity to the contaminant migration velocity. A retardation factor greater than one indicates that the contaminant moves slower than the water. The extent of this retardation depends on several factors, primarily the hydrophobicity of the contaminant and the organic carbon content of the aquifer material. Highly hydrophobic contaminants, characterized by a large octanol-water partition coefficient (Kow), have a stronger tendency to sorb and thus experience greater retardation. Similarly, aquifer materials with a higher fraction of organic carbon (foc) provide more sorption sites, leading to increased sorption and a slower observed contaminant migration rate. This dynamic process of sorption and desorption effectively slows down the contaminant's progress, meaning the observed migration rate of hydrophobic organic contaminants in groundwater is consistently slower than the actual groundwater flow velocity.