If we want to talk about concentrations, solutes and contaminant transport in groundwater, we need to mention the Source/Sink concepts and natural concentrations in groundwater. Sources represent solute mass entering the system, which could be representing spills or natural lixiviation processes. Sinks are the zones where water with solute concentrations leave the system.
In almost every zone determined as Sinks (drains, rivers, lakes, sea), groundwater exits with a defined concentration and proceeds to mix with superficial water. The exception is the evapotranspiration process. where water consumed by plants is discharged “pure” to the atmosphere, in other words, evapotranspirated water is clean because solutes stay in the soil.
Over thousands of years, the components in the soil solution tend to accumulate in wetlands, making them “kidnapping zones” of major ions, minor ions and metals.
This tutorial shows the process of cooper bioaccumulation in an aquifer over a period of 1000 years. According to the calculations, the metal concentration can increase from 3 to 4 times in comparison to original concentration values over the simulated time.

Wetlands and evapotranspiration

Wetlands do not appear and disappear spontaneously. Wetlands are ecosystems formed when the water table is too close to the surface and plants are able to take groundwater through their roots. By consuming groundwater, these plants remain green even during the dry times of the year.
The following image shows two groundwater discharge zones: one related to a river and the other one to a wetland. It is important to notice that the wetland exists when the water table is close to the surface.

The location and extension of wetlands are related to the distribution and oscilations of the groundwater system. The water consumed by vegetation is discharged as steam through the leaves’ stomas in a process called transpiration; water from soil humidity is evaporated in a process called evaporation. Both processes can be joined in a component called evapotranspiration, which discharges groundwater to the environment.

This water is conceptualized as “pure” because solutes remain in the soil, making the wetlands a “kidnapping” zone of dissolved components such as principal and secondary ions and metals.