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

The chloride ion (Cl-) is considered the least reactive and most persistent tracer of a geochemical contaminant plume due to its chemical stability and resistance to various environmental processes. Unlike other anions such as sulfate (SO4-2), bicarbonate (HCO3-1), and nitrate (NO3-1), chloride does not readily participate in complex chemical reactions or significant biological uptake in typical environmental conditions.

Chloride is often used in hydrology and environmental science as a conservative tracer because it tends to follow the movement of water without undergoing substantial changes in its concentration due to reactions with sediments, microorganisms, or other chemical processes. This makes Cl- particularly useful for tracking the movement of water and contaminants through subsurface environments, allowing scientists and researchers to delineate the extent of contamination and understand the dynamics of contaminant transport.

In contrast, the other anions can engage in various transformations. For example, sulfate can be reduced by bacteria in anoxic conditions, and nitrate can be subject to denitrification, where it is converted into nitrogen gas. Bicarbonate can also react and participate in carbonate equilibria or be involved in biological processes. Therefore, while each of these anions serves as a useful indicator in certain contexts, chloride's