Differentiate sorption and dissolution in soil for contaminant mobility, and state how each affects bioavailability.

• A. Sorption binds contaminants to soil particles reducing mobility; dissolution releases contaminants into pore water affecting bioavailability and transport; both influence exposure.
• B. Sorption increases mobility; dissolution reduces mobility; neither affects bioavailability.
• C. Sorption is dissolution in water; dissolution is binding to soil; both have the same effect on mobility.
• D. Sorption only affects metals; dissolution only affects organics.

Answer: (A)

The main idea being tested is how two soil processes—sorption and dissolution—shape how contaminants move and how available they are to organisms.

Sorption binds contaminants to soil particles, such as clay minerals and organic matter. This attachment keeps most of the contaminant in the solid phase, which slows or limits its movement with water through soil and into groundwater. Because the contaminant is tied up in the solid phase, the immediate amount in pore water—and thus the portion that can cross biological membranes—tends to be lower, reducing bioavailability in the short term.

Dissolution, by contrast, is the process of contaminants entering the pore water from the solid phase. When more contaminant is dissolved, it becomes mobile with flowing water and more readily accessible for uptake by organisms, increasing both transport potential and bioavailability.

The effect on exposure comes from the balance between these two processes. Strong sorption reduces mobility and lowers dissolved concentrations, which can limit bioavailability, while dissolution increases dissolved concentrations, enhancing mobility and bioavailability. Over time, desorption can occur, so the two processes are interconnected and together determine how much contaminant exposure may occur.

Other statements that imply sorption increases mobility, or that dissolution always reduces mobility, or that these processes apply only to specific contaminant types, don’t fit because sorption generally limits movement while dissolution enhances it, and both processes can affect metals and organics alike.