Which anion is considered the least reactive and most persistent tracer of a geochemical contaminant plume?

Chloride ions (Cl-) are considered the least reactive and most persistent tracers of a geochemical contaminant plume due to several key characteristics. Chloride is a relatively stable and non-reactive ion in the environment, meaning it does not readily participate in chemical reactions that could alter its form or concentration. This persistence makes chloride an excellent tracer for identifying the movement and extent of contaminants in groundwater and other environments.

Chloride tends to remain in solution without significant degradation or transformation over time, allowing it to travel with the water flow while serving as a reliable indicator of contamination pathways. Its behavior can help hydrologists and environmental scientists track the source and spread of contaminants in a plume.

In contrast, other ions like sulfate (SO4-2), bicarbonate (HCO3-1), and nitrate (NO3-1) have more reactive properties and can undergo various geochemical processes such as precipitation, adsorption, or biological uptake. These processes can change their concentrations and distributions, making them less effective as long-term tracers compared to chloride. Therefore, chloride's low reactivity and persistence make it particularly useful in environmental studies focused on contaminant transport.